Apert syndrome is characterized by the presence of multisuture craniosynostosis, midface retrusion, and syndactyly of the hands with fusion of the second through fourth nails. Almost all affected individuals have coronal craniosynostosis, and a majority also have involvement of the sagittal and lambdoid sutures. The midface in Apert syndrome is underdeveloped as well as retruded; a subset of affected individuals have cleft palate. The hand in Apert syndrome always includes fusion of the middle three digits; the thumb and fifth finger are sometimes also involved. Feeding issues, dental abnormalities, hearing loss, hyperhidrosis, and progressive synostosis of multiple bones (skull, hands, feet, carpus, tarsus, and cervical vertebrae) are also common. Multilevel airway obstruction may be present and can be due to narrowing of the nasal passages, tongue-based airway obstruction, and/or tracheal anomalies. Nonprogressive ventriculomegaly is present in a majority of individuals, with a small subset having true hydrocephalus. Most individuals with Apert syndrome have normal intelligence or mild intellectual disability; moderate-to-severe intellectual disability has been reported in some individuals. A minority of affected individuals have structural cardiac abnormalities, true gastrointestinal malformations, and anomalies of the genitourinary tract.

Coloboma is an eye abnormality that occurs before birth. Colobomas are missing pieces of tissue in structures that form the eye. They may appear as notches or gaps in one of several parts of the eye, including the colored part of the eye called the iris; the retina, which is the specialized light-sensitive tissue that lines the back of the eye; the blood vessel layer under the retina called the choroid; or the optic nerves, which carry information from the eyes to the brain.\n\nColobomas may be present in one or both eyes and, depending on their size and location, can affect a person's vision. Colobomas affecting the iris, which result in a "keyhole" appearance of the pupil, generally do not lead to vision loss. Colobomas involving the retina result in vision loss in specific parts of the visual field. Large retinal colobomas or those affecting the optic nerve can cause low vision, which means vision loss that cannot be completely corrected with glasses or contact lenses.\n\nSome people with coloboma also have a condition called microphthalmia. In this condition, one or both eyeballs are abnormally small. In some affected individuals, the eyeball may appear to be completely missing; however, even in these cases some remaining eye tissue is generally present. Such severe microphthalmia should be distinguished from another condition called anophthalmia, in which no eyeball forms at all. However, the terms anophthalmia and severe microphthalmia are often used interchangeably. Microphthalmia may or may not result in significant vision loss.\n\nPeople with coloboma may also have other eye abnormalities, including clouding of the lens of the eye (cataract), increased pressure inside the eye (glaucoma) that can damage the optic nerve, vision problems such as nearsightedness (myopia), involuntary back-and-forth eye movements (nystagmus), or separation of the retina from the back of the eye (retinal detachment).\n\nSome individuals have coloboma as part of a syndrome that affects other organs and tissues in the body. These forms of the condition are described as syndromic. When coloboma occurs by itself, it is described as nonsyndromic or isolated.\n\nColobomas involving the eyeball should be distinguished from gaps that occur in the eyelids. While these eyelid gaps are also called colobomas, they arise from abnormalities in different structures during early development.

Lowe syndrome (oculocerebrorenal syndrome) is characterized by involvement of the eyes, central nervous system, and kidneys. Dense congenital cataracts are found in all affected boys and infantile glaucoma in approximately 50%. All boys have impaired vision; corrected acuity is rarely better than 20/100. Generalized hypotonia is noted at birth and is of central (brain) origin. Deep tendon reflexes are usually absent. Hypotonia may slowly improve with age, but normal motor tone and strength are never achieved. Motor milestones are delayed. Almost all affected males have some degree of intellectual disability; 10%-25% function in the low-normal or borderline range, approximately 25% in the mild-to-moderate range, and 50%-65% in the severe-to-profound range of intellectual disability. Affected males have varying degrees of proximal renal tubular dysfunction of the Fanconi type, including low molecular-weight (LMW) proteinuria, aminoaciduria, bicarbonate wasting and renal tubular acidosis, phosphaturia with hypophosphatemia and renal rickets, hypercalciuria, sodium and potassium wasting, and polyuria. The features of symptomatic Fanconi syndrome do not usually become manifest until after the first few months of life, except for LMW proteinuria. Glomerulosclerosis associated with chronic tubular injury usually results in slowly progressive chronic renal failure and end-stage renal disease between the second and fourth decades of life.

Prader-Willi syndrome (PWS) is characterized by severe hypotonia and feeding difficulties in early infancy, followed in later infancy or early childhood by excessive eating and gradual development of morbid obesity (unless eating is externally controlled). Motor milestones and language development are delayed. All individuals have some degree of cognitive impairment. A distinctive behavioral phenotype (with temper tantrums, stubbornness, manipulative behavior, and obsessive-compulsive characteristics) is common. Hypogonadism is present in both males and females and manifests as genital hypoplasia, incomplete pubertal development, and, in most, infertility. Short stature is common (if not treated with growth hormone); characteristic facial features, strabismus, and scoliosis are often present.

Sotos syndrome is characterized by a distinctive facial appearance (broad and prominent forehead with a dolichocephalic head shape, sparse frontotemporal hair, downslanting palpebral fissures, malar flushing, long and narrow face, long chin); learning disability (early developmental delay, mild-to-severe intellectual impairment); and overgrowth (height and/or head circumference =2 SD above the mean). These three clinical features are considered the cardinal features of Sotos syndrome. Major features of Sotos syndrome include behavioral problems (most notably autistic spectrum disorder), advanced bone age, cardiac anomalies, cranial MRI/CT abnormalities, joint hyperlaxity with or without pes planus, maternal preeclampsia, neonatal complications, renal anomalies, scoliosis, and seizures.

A benign form of macrocephaly, sometimes identified with Sotos syndrome, with normal or near-normal birth weight and length with subsequent obesity, variable developmental delay, and typical square facies with frontal bossing, dished-out midface, biparietal narrowing, and long philtrum.

An autosomal recessive subtype of cerebrooculofacioskeletal syndrome caused by mutation(s) in the ERCC6 gene, encoding DNA excision repair protein ERCC-6.

A rare autosomal recessive inherited syndrome. It is characterized by xeroderma pigmentosum, mental retardation, dwarfism, hypogonadism, and neurologic abnormalities.

EZH2-related overgrowth includes EZH2-related Weaver syndrome at one end of the spectrum and tall stature at the other. Although most individuals diagnosed with a heterozygous EZH2 pathogenic variant have been identified because of a clinical suspicion of Weaver syndrome, a minority have been identified through molecular genetic testing of family members of probands or individuals with overgrowth who did not have a clinical diagnosis of Weaver syndrome. Thus, the extent of the phenotypic spectrum associated with a heterozygous EZH2 pathogenic variant is not yet known. Weaver syndrome is characterized by tall stature, variable intellect (ranging from normal intellect to severe intellectual disability), characteristic facial appearance, and a range of associated clinical features including advanced bone age, poor coordination, soft doughy skin, camptodactyly of the fingers and/or toes, umbilical hernia, abnormal tone, and hoarse low cry in infancy. Brain MRI has identified abnormalities in a few individuals with EZH2-related overgrowth. Neuroblastoma occurs at a slightly increased frequency in individuals with a heterozygous EZH2 pathogenic variant but data are insufficient to determine absolute risk. There is currently no evidence that additional malignancies (including hematologic malignancies) occur with increased frequency.

The Marshall-Smith syndrome (MRSHSS) is a malformation syndrome characterized by accelerated skeletal maturation, relative failure to thrive, respiratory difficulties, mental retardation, and unusual facies, including prominent forehead, shallow orbits, blue sclerae, depressed nasal bridge, and micrognathia (Adam et al., 2005).

Schinzel-Giedion syndrome is a highly recognizable syndrome characterized by severe mental retardation, distinctive facial features, and multiple congenital malformations including skeletal abnormalities, genitourinary and renal malformations, and cardiac defects, as well as a higher-than-normal prevalence of tumors, notably neuroepithelial neoplasia (summary by Hoischen et al., 2010).

Coffin-Lowry syndrome (CLS) is usually characterized by severe-to-profound intellectual disability in males; less severely impaired individuals have been reported. Neuropsychiatric concerns can include behavioral problems, loss of strength, progressive spasticity or paraplegia, sleep apnea, or stroke. Stimulus-induced drop attacks (SIDAs) in which unexpected tactile or auditory stimuli or excitement triggers a brief collapse but no loss of consciousness are present in approximately 20% of affected individuals. Typically SIDAs begin between mid-childhood and the teens. Characteristic facial features may be more apparent with age. Upper-extremity differences may be subtle and include short, soft, fleshy hands with tapered fingers as well as fleshy forearms. Progressive kyphoscoliosis is one of the most difficult aspects of long-term care. Affected females tend to have intellectual disability in the mild-to-moderate range and may also have the typical facial, hand, and skeletal findings noted in males.

Femoral-facial syndrome (FFS), also known as femoral hypoplasia-unusual facies syndrome (FHUFS), is a rare and sporadic multiple congenital anomaly syndrome comprising bilateral femoral hypoplasia and characteristic facial features, such as long philtrum, thin upper lip, micrognathia with or without cleft palate, upward-slanting palpebral fissures, and a short nose with broad tip. Other features, such as renal anomalies, are more variable (summary by Nowaczyk et al., 2010).

Typical Greig cephalopolysyndactyly syndrome (GCPS) is characterized by macrocephaly, widely spaced eyes associated with increased interpupillary distance, preaxial polydactyly with or without postaxial polydactyly, and cutaneous syndactyly. Developmental delay, intellectual disability, or seizures appear to be uncommon manifestations (~<10%) of GCPS and may be more common in individuals with large (>300-kb) deletions that encompass GLI3. Approximately 20% of individuals with GCPS have hypoplasia or agenesis of the corpus callosum.

Pallister-Killian syndrome (PKS) is a dysmorphic condition involving most organ systems, but is also characterized by a tissue-limited mosaicism; most fibroblasts have 47 chromosomes with an extra small metacentric chromosome, whereas the karyotype of lymphocytes is normal. The extra metacentric chromosome is an isochromosome for part of the short arm of chromosome 12: i(12)(p10) (Peltomaki et al., 1987; Warburton et al., 1987).

Xeroderma pigmentosum (XP) is characterized by: Acute sun sensitivity (severe sunburn with blistering, persistent erythema on minimal sun exposure) with marked freckle-like pigmentation of the face before age two years; Sunlight-induced ocular involvement (photophobia, severe keratitis, atrophy of the skin of the lids, ocular surface neoplasms); Greatly increased risk of sunlight-induced cutaneous neoplasms (basal cell carcinoma, squamous cell carcinoma, melanoma) within the first decade of life. Approximately 25% of affected individuals have neurologic manifestations (acquired microcephaly, diminished or absent deep tendon stretch reflexes, progressive sensorineural hearing loss, progressive cognitive impairment, and ataxia). The most common causes of death are skin cancer, neurologic degeneration, and internal cancer. The median age at death in persons with XP with neurodegeneration (29 years) was found to be younger than that in persons with XP without neurodegeneration (37 years).

Initial symptoms of multiple sulfatase deficiency (MSD) can develop from infancy through early childhood, and presentation is widely variable. Some individuals display the multisystemic features characteristic of mucopolysaccharidosis disorders (e.g., developmental regression, organomegaly, skeletal deformities) while other individuals present primarily with neurologic regression (associated with leukodystrophy). Based on age of onset, rate of progression, and disease severity, several different clinical subtypes of MSD have been described: Neonatal MSD is the most severe with presentation in the prenatal period or at birth with rapid progression and death occurring within the first two years of life. Infantile MSD is the most common variant and may be characterized as attenuated (slower clinical course with cognitive disability and neurodegeneration identified in the 2nd year of life) or severe (loss of the majority of developmental milestones by age 5 years). Juvenile MSD is the rarest subtype with later onset of symptoms and subacute clinical presentation. Many of the features found in MSD are progressive, including neurologic deterioration, heart disease, hearing loss, and airway compromise.

GLB1-related disorders comprise two phenotypically distinct lysosomal storage disorders: GM1 gangliosidosis and mucopolysaccharidosis type IVB (MPS IVB). The phenotype of GM1 gangliosidosis constitutes a spectrum ranging from severe (infantile) to intermediate (late-infantile and juvenile) to mild (chronic/adult). Type I (infantile) GM1 gangliosidosis begins before age 12 months. Prenatal manifestations may include nonimmune hydrops fetalis, intrauterine growth restriction, and placental vacuolization; congenital dermal melanocytosis (Mongolian spots) may be observed. Macular cherry-red spot is detected on eye exam. Progressive central nervous system dysfunction leads to spasticity and rapid regression; blindness, deafness, decerebrate rigidity, seizures, feeding difficulties, and oral secretions are observed. Life expectancy is two to three years. Type II can be subdivided into the late-infantile (onset age 1-3 years) and juvenile (onset age 3-10 years) phenotypes. Central nervous system dysfunction manifests as progressive cognitive, motor, and speech decline as measured by psychometric testing. There may be mild corneal clouding, hepatosplenomegaly, and/or cardiomyopathy; the typical course is characterized by progressive neurologic decline, progressive skeletal disease in some individuals (including kyphosis and avascular necrosis of the femoral heads), and progressive feeding difficulties leading to aspiration risk. Type III begins in late childhood to the third decade with generalized dystonia leading to unsteady gait and speech disturbance followed by extrapyramidal signs including akinetic-rigid parkinsonism. Cardiomyopathy develops in some and skeletal involvement occurs in most. Intellectual impairment is common late in the disease with prognosis directly related to the degree of neurologic impairment. MPS IVB is characterized by skeletal dysplasia with specific findings of axial and appendicular dysostosis multiplex, short stature (below 15th centile in adults), kyphoscoliosis, coxa/genu valga, joint laxity, platyspondyly, and odontoid hypoplasia. First signs and symptoms may be apparent at birth. Bony involvement is progressive, with more than 84% of adults requiring ambulation aids; life span does not appear to be limited. Corneal clouding is detected in some individuals and cardiac valvular disease may develop.

GLB1-related disorders comprise two phenotypically distinct lysosomal storage disorders: GM1 gangliosidosis and mucopolysaccharidosis type IVB (MPS IVB). The phenotype of GM1 gangliosidosis constitutes a spectrum ranging from severe (infantile) to intermediate (late-infantile and juvenile) to mild (chronic/adult). Type I (infantile) GM1 gangliosidosis begins before age 12 months. Prenatal manifestations may include nonimmune hydrops fetalis, intrauterine growth restriction, and placental vacuolization; congenital dermal melanocytosis (Mongolian spots) may be observed. Macular cherry-red spot is detected on eye exam. Progressive central nervous system dysfunction leads to spasticity and rapid regression; blindness, deafness, decerebrate rigidity, seizures, feeding difficulties, and oral secretions are observed. Life expectancy is two to three years. Type II can be subdivided into the late-infantile (onset age 1-3 years) and juvenile (onset age 3-10 years) phenotypes. Central nervous system dysfunction manifests as progressive cognitive, motor, and speech decline as measured by psychometric testing. There may be mild corneal clouding, hepatosplenomegaly, and/or cardiomyopathy; the typical course is characterized by progressive neurologic decline, progressive skeletal disease in some individuals (including kyphosis and avascular necrosis of the femoral heads), and progressive feeding difficulties leading to aspiration risk. Type III begins in late childhood to the third decade with generalized dystonia leading to unsteady gait and speech disturbance followed by extrapyramidal signs including akinetic-rigid parkinsonism. Cardiomyopathy develops in some and skeletal involvement occurs in most. Intellectual impairment is common late in the disease with prognosis directly related to the degree of neurologic impairment. MPS IVB is characterized by skeletal dysplasia with specific findings of axial and appendicular dysostosis multiplex, short stature (below 15th centile in adults), kyphoscoliosis, coxa/genu valga, joint laxity, platyspondyly, and odontoid hypoplasia. First signs and symptoms may be apparent at birth. Bony involvement is progressive, with more than 84% of adults requiring ambulation aids; life span does not appear to be limited. Corneal clouding is detected in some individuals and cardiac valvular disease may develop.

The findings in X-linked chondrodysplasia punctata 2 (CDPX2) range from fetal demise with multiple malformations and severe growth retardation to much milder manifestations, including females with no recognizable physical abnormalities. At least 95% of live-born individuals with CDPX2 are female. Characteristic features include growth deficiency; distinctive craniofacial appearance; chondrodysplasia punctata (stippling of the epiphyses of the long bones, vertebrae, trachea, and distal ends of the ribs); often asymmetric rhizomelic shortening of limbs; scoliosis; linear or blotchy scaling ichthyosis in the newborn; later appearance of linear or whorled atrophic patches involving hair follicles (follicular atrophoderma); coarse hair with scarring alopecia; and cataracts.

IPEX (immune dysregulation, polyendocrinopathy, enteropathy, X-linked) syndrome is characterized by systemic autoimmunity, typically beginning in the first year of life. Presentation is most commonly the clinical triad of watery diarrhea, endocrinopathy (most commonly insulin-dependent diabetes mellitus), and eczematous dermatitis. Most children have other autoimmune phenomena including cytopenias, autoimmune hepatitis, or nephropathy; lymphadenopathy, splenomegaly, alopecia, arthritis, and lung disease related to immune dysregulation have all been observed. Fetal presentation of IPEX includes hydrops, echogenic bowel, skin desquamation, IUGR, and fetal akinesia. Without aggressive immunosuppression or bone marrow transplantation, the majority of affected males die within the first one to two years of life from metabolic derangements, severe malabsorption, or sepsis; a few with a milder phenotype have survived into the second or third decade of life.

Fumarate hydratase (FH) deficiency results in severe neonatal and early infantile encephalopathy that is characterized by poor feeding, failure to thrive, hypotonia, lethargy, and seizures. Dysmorphic facial features include frontal bossing, depressed nasal bridge, and widely spaced eyes. Many affected individuals are microcephalic. A spectrum of brain abnormalities are seen on magnetic resonance imaging, including cerebral atrophy, enlarged ventricles and generous extra-axial cerebral spinal fluid (CSF) spaces, delayed myelination for age, thinning of the corpus callosum, and an abnormally small brain stem. Brain malformations including bilateral polymicrogyria and absence of the corpus callosum can also be observed. Development is severely affected: most affected individuals are nonverbal and nonambulatory, and many die during early childhood. Less severely affected individuals with moderate cognitive impairment and long-term survival have been reported.

D-bifunctional protein deficiency is a disorder of peroxisomal fatty acid beta-oxidation. See also peroxisomal acyl-CoA oxidase deficiency (264470), caused by mutation in the ACOX1 gene (609751) on chromosome 17q25. The clinical manifestations of these 2 deficiencies are similar to those of disorders of peroxisomal assembly, including X-linked adrenoleukodystrophy (ALD; 300100), Zellweger cerebrohepatorenal syndrome (see 214100) and neonatal adrenoleukodystrophy (NALD; see 601539) (Watkins et al., 1995). DBP deficiency has been classified into 3 subtypes depending upon the deficient enzyme activity. Type I is a deficiency of both 2-enoyl-CoA hydratase and 3-hydroxyacyl-CoA dehydrogenase; type II is a deficiency of hydratase activity alone; and type III is a deficiency of dehydrogenase activity alone. Virtually all patients with types I, II, and III have a severe phenotype characterized by infantile-onset of hypotonia, seizures, and abnormal facial features, and most die before age 2 years. McMillan et al. (2012) proposed a type IV deficiency on the basis of less severe features; these patients have a phenotype reminiscent of Perrault syndrome (PRLTS1; 233400). Pierce et al. (2010) noted that Perrault syndrome and DBP deficiency overlap clinically and suggested that DBP deficiency may be underdiagnosed.

GAPO syndrome is the acronymic designation for a complex of growth retardation, alopecia, pseudoanodontia (failure of tooth eruption), and progressive optic atrophy (Tipton and Gorlin, 1984). Ilker et al. (1999) and Bayram et al. (2014) noted that optic atrophy is not a consistent feature of the disorder.

Kohlschutter-Tonz syndrome (KTZS) is an autosomal recessive disorder characterized by severe global developmental delay, early-onset intractable seizures, spasticity, and amelogenesis imperfecta affecting both primary and secondary teeth and causing yellow or brown discoloration of the teeth. Although the phenotype is consistent, there is variability. Impaired intellectual development is related to the severity of seizures, and the disorder can thus be considered an epileptic encephalopathy. Some infants show normal development until seizure onset, whereas others are delayed from birth. The most severely affected individuals have profound mental retardation, never acquire speech, and become bedridden early in life (summary by Schossig et al., 2012 and Mory et al., 2012). See also Kohlschutter-Tonz syndrome-like (KTZSL; 619229), caused by heterozygous mutation in the SATB1 gene (602075) on chromosome 3p23.

Gillespie syndrome (GLSP) is usually diagnosed in the first year of life by the presence of fixed dilated pupils in a hypotonic infant. Affected individuals have a characteristic form of iris hypoplasia in which the pupillary border of the iris exhibits a scalloped or 'festooned' edge, with iris strands extending onto the anterior lens surface at regular intervals. The key extraocular features of Gillespie syndrome are congenital hypotonia, progressive cerebellar hypoplasia, and ataxia, as well as variable cognitive impairment that is usually mild (summary by Gerber et al., 2016 and McEntagart et al., 2016).

Lenz-Majewski hyperostotic dwarfism is a rare condition characterized by intellectual disability, sclerosing bone dysplasia, distinct craniofacial and dental anomalies, loose skin, and distal limb anomalies, particularly brachydactyly and symphalangism. Patients have multiple radiographic abnormalities due to progressive generalized hyperostosis that affects the cranium, vertebrae, and diaphyses of tubular bones, leading to severe growth retardation (summary by Sousa et al., 2014).

Monosomy 18q is a partial deletion of the long arm of chromosome 18 characterized by highly variable phenotype, most commonly including hypotonia, developmental delay, short stature, growth hormone deficiency, hearing loss and external ear anomalies, intellectual disability, palatal defects, dysmorphic facial features, skeletal anomalies (foot deformities, tapering fingers, scoliosis) and mood disorders.

Mulibrey nanism (MUL) is a rare autosomal recessive growth disorder with prenatal onset, including occasional progressive cardiomyopathy, characteristic facial features, failure of sexual maturation, insulin resistance with type 2 diabetes, and an increased risk for Wilms tumor (summary by Hamalainen et al., 2006).

While the majority of individuals with Costello syndrome share characteristic findings affecting multiple organ systems, the phenotypic spectrum is wide, ranging from a milder or attenuated phenotype to a severe phenotype with early lethal complications. Costello syndrome is typically characterized by failure to thrive in infancy as a result of severe postnatal feeding difficulties; short stature; developmental delay or intellectual disability; coarse facial features (full lips, large mouth, full nasal tip); curly or sparse, fine hair; loose, soft skin with deep palmar and plantar creases; papillomata of the face and perianal region; diffuse hypotonia and joint laxity with ulnar deviation of the wrists and fingers; tight Achilles tendons; and cardiac involvement including: cardiac hypertrophy (usually typical hypertrophic cardiomyopathy), congenital heart defect (usually valvar pulmonic stenosis), and arrhythmia (usually supraventricular tachycardia, especially chaotic atrial rhythm/multifocal atrial tachycardia or ectopic atrial tachycardia). Relative or absolute macrocephaly is typical, and postnatal cerebellar overgrowth can result in the development of a Chiari I malformation with associated anomalies including hydrocephalus or syringomyelia. Individuals with Costello syndrome have an approximately 15% lifetime risk for malignant tumors including rhabdomyosarcoma and neuroblastoma in young children and transitional cell carcinoma of the bladder in adolescents and young adults.

Cockayne syndrome (referred to as CS in this GeneReview) spans a continuous phenotypic spectrum that includes: CS type I, the "classic" or "moderate" form; CS type II, a more severe form with symptoms present at birth; this form overlaps with cerebrooculofacioskeletal (COFS) syndrome; CS type III, a milder and later-onset form; COFS syndrome, a fetal form of CS. CS type I is characterized by normal prenatal growth with the onset of growth and developmental abnormalities in the first two years. By the time the disease has become fully manifest, height, weight, and head circumference are far below the fifth percentile. Progressive impairment of vision, hearing, and central and peripheral nervous system function leads to severe disability; death typically occurs in the first or second decade. CS type II is characterized by growth failure at birth, with little or no postnatal neurologic development. Congenital cataracts or other structural anomalies of the eye may be present. Affected children have early postnatal contractures of the spine (kyphosis, scoliosis) and joints. Death usually occurs by age five years. CS type III is a phenotype in which major clinical features associated with CS only become apparent after age two years; growth and/or cognition exceeds the expectations for CS type I. COFS syndrome is characterized by very severe prenatal developmental anomalies (arthrogryposis and microphthalmia).

Recombinant chromosome 8 syndrome (Rec8 syndrome) is a chromosomal disorder found among individuals of Hispanic descent with ancestry from the San Luis Valley of southern Colorado and northern New Mexico. Affected individuals typically have impaired intellectual development, congenital heart defects, seizures, a characteristic facial appearance with hypertelorism, thin upper lip, anteverted nares, wide face, and abnormal hair whorl, and other manifestations (Sujansky et al., 1993, summary by Graw et al., 2000).

Smith-Magenis syndrome (SMS) is characterized by distinctive physical features (particularly coarse facial features that progress with age), developmental delay, cognitive impairment, behavioral abnormalities, sleep disturbance, and childhood-onset abdominal obesity. Infants have feeding difficulties, failure to thrive, hypotonia, hyporeflexia, prolonged napping or need to be awakened for feeds, and generalized lethargy. The majority of individuals function in the mild-to-moderate range of intellectual disability. The behavioral phenotype, including significant sleep disturbance, stereotypies, and maladaptive and self-injurious behaviors, is generally not recognized until age 18 months or older and continues to change until adulthood. Sensory issues are frequently noted; these may include avoidant behavior, as well as repetitive seeking of textures, sounds, and experiences. Toileting difficulties are common. Significant anxiety is common as are problems with executive functioning, including inattention, distractibility, hyperactivity, and impulsivity. Maladaptive behaviors include frequent outbursts / temper tantrums, attention-seeking behaviors, opposition, aggression, and self-injurious behaviors including self-hitting, self-biting, skin picking, inserting foreign objects into body orifices (polyembolokoilamania), and yanking fingernails and/or toenails (onychotillomania). Among the stereotypic behaviors described, the spasmodic upper-body squeeze or "self-hug" seems to be highly associated with SMS. An underlying developmental asynchrony, specifically emotional maturity delayed beyond intellectual functioning, may also contribute to maladaptive behaviors in people with SMS.

Curry-Jones syndrome (CRJS) is a multisystem disorder characterized by patchy skin lesions, polysyndactyly, diverse cerebral malformations, unicoronal craniosynostosis, iris colobomas, microphthalmia, and intestinal malrotation with myofibromas or hamartomas (summary by Twigg et al., 2016).

TBC1D24-related disorders comprise a continuum of features that were originally described as distinct, recognized phenotypes: DOORS syndrome (deafness, onychodystrophy, osteodystrophy, mental retardation, and seizures). Profound sensorineural hearing loss, onychodystrophy, osteodystrophy, intellectual disability / developmental delay, and seizures. Familial infantile myoclonic epilepsy (FIME). Early-onset myoclonic seizures, focal epilepsy, dysarthria, and mild-to-moderate intellectual disability. Progressive myoclonus epilepsy (PME). Action myoclonus, tonic-clonic seizures, progressive neurologic decline, and ataxia. Early-infantile epileptic encephalopathy 16 (EIEE16). Epileptiform EEG abnormalities which themselves are believed to contribute to progressive disturbance in cerebral function. Autosomal recessive nonsyndromic hearing loss, DFNB86. Profound prelingual deafness. Autosomal dominant nonsyndromic hearing loss, DFNA65. Slowly progressive deafness with onset in the third decade, initially affecting the high frequencies.

Hereditary motor and sensory neuropathy with agenesis of the corpus callosum (HMSN/ACC), a neurodevelopmental and neurodegenerative disorder, is characterized by severe progressive sensorimotor neuropathy with resulting hypotonia, areflexia, and amyotrophy, and by variable degrees of dysgenesis of the corpus callosum. Mild-to-severe intellectual disability and "psychotic episodes" during adolescence are observed. Sensory modalities are moderately to severely affected beginning in infancy. The average age of onset of walking is 3.8 years; the average age of loss of walking is 13.8 years; the average age of death is 33 years.

L1 syndrome involves a phenotypic spectrum ranging from severe to mild and includes three clinical phenotypes: X-linked hydrocephalus with stenosis of the aqueduct of Sylvius (HSAS). MASA (mental retardation [intellectual disability], aphasia [delayed speech], spastic paraplegia [shuffling gait], adducted thumbs) syndrome including X-linked complicated hereditary spastic paraplegia type 1. X-linked complicated corpus callosum agenesis. Males with HSAS are born with severe hydrocephalus, adducted thumbs, and spasticity; intellectual disability is severe. In less severely affected males, hydrocephalus may be subclinically present and documented only because of developmental delay; intellectual disability ranges from mild (IQ: 50-70) to moderate (IQ: 30-50). It is important to note that all phenotypes can be observed in affected individuals within the same family.

Peters plus syndrome is characterized by anterior chamber eye anomalies, short limbs with broad distal extremities, characteristic facial features, cleft lip/palate, and variable developmental delay / intellectual disability. The most common anterior chamber defect is Peters' anomaly, consisting of central corneal clouding, thinning of the posterior cornea, and iridocorneal adhesions. Cataracts and glaucoma are common. Developmental delay is observed in about 80% of children; intellectual disability can range from mild to severe.

A very rare syndrome described in two siblings with manifestation of prenatal onset of growth deficiency, microcephaly, hypoplastic genitalia, and birth onset of convulsions.

Primrose syndrome is characterized by macrocephaly, hypotonia, developmental delay, intellectual disability with expressive speech delay, behavioral issues, a recognizable facial phenotype, radiographic features, and altered glucose metabolism. Additional features seen in adults: sparse body hair, distal muscle wasting, and contractures. Characteristic craniofacial features include brachycephaly, high anterior hairline, deeply set eyes, ptosis, downslanted palpebral fissures, high palate with torus palatinus, broad jaw, and large ears with small or absent lobes. Radiographic features include calcification of the external ear cartilage, multiple Wormian bones, platybasia, bathrocephaly, slender bones with exaggerated metaphyseal flaring, mild epiphyseal dysplasia, and spondylar dysplasia. Additional features include hearing impairment, ocular anomalies, cryptorchidism, and nonspecific findings on brain MRI.

Kumar-type X-linked syndromic intellectual developmental disorder (MRXSK) is an X-linked recessive disorder that shows phenotypic variability and multisystem involvement apparent from birth or early infancy. Most affected individuals are male, although 1 severely affected girl with a de novo THOC2 mutation has been reported. At the severe end of the spectrum, affected individuals have hypotonia, neonatal difficulties, failure to thrive with poor overall growth, feeding difficulties, respiratory insufficiency, visual impairment, profoundly impaired intellectual development with poor or absent speech, and motor abnormalities, such as inability to walk and hyperkinetic movements. Less severely affected individuals have mildly to moderately impaired intellectual development and speech delay. Additional features include behavioral abnormalities, hearing or visual defects, seizures, dysmorphic facial features, and brain imaging abnormalities (Kumar et al., 2015; Kumar et al., 2018; Kumar et al., 2020).

X-linked Dandy-Walker malformation with intellectual disability, basal ganglia disease and seizures (XDIBS), or Pettigrew syndrome is a central nervous system malformation characterized by severe intellectual deficit, early hypotonia with progression to spasticity and contractures, choreoathetosis, seizures, dysmorphic face (long face with prominent forehead), and brain imaging abnormalities such as Dandy-Walker malformation, and iron deposition. (From Mondo:0010574)

Thanatophoric dysplasia (TD) is a short-limb skeletal dysplasia that is usually lethal in the perinatal period. TD is divided into subtypes: TD type I is characterized by micromelia with bowed femurs and, uncommonly, the presence of craniosynostosis of varying severity. TD type II is characterized by micromelia with straight femurs and uniform presence of moderate-to-severe craniosynostosis with cloverleaf skull deformity. Other features common to type I and type II include: short ribs, narrow thorax, relative macrocephaly, distinctive facial features, brachydactyly, hypotonia, and redundant skin folds along the limbs. Most affected infants die of respiratory insufficiency shortly after birth. Rare long-term survivors have been reported.

Saul-Wilson syndrome (SWS) is a skeletal dysplasia characterized by profound short stature, distinctive craniofacial features, short distal phalanges of fingers and toes, and often clubfoot. Early development (primarily speech and motor) is delayed; cognition is normal. Other findings can include hearing loss (conductive, sensorineural, and mixed), lamellar cataracts, and/or rod-cone retinal dystrophy. To date, 16 affected individuals have been reported.

Transient neonatal hyperparathyroidism is characterized by interference with placental maternal-fetal calcium transport, causing fetal calcium deficiency resulting in hyperparathyroidism and metabolic bone disease. Because 80% of calcium is transferred during the third trimester, abnormalities may not be detected on second-trimester ultrasounds. Affected infants present at birth with prenatal fractures, shortened ribs, and bowing of long bones, as well as respiratory and feeding difficulties. Postnatal recovery or improvement is observed once calcium is provided orally, with most patients showing complete resolution of skeletal abnormalities by 2 years of age (Suzuki et al., 2018).

Cornelia de Lange syndrome (CdLS) encompasses a spectrum of findings from mild to severe. Severe (classic) CdLS is characterized by distinctive facial features, growth restriction (prenatal onset; <5th centile throughout life), hypertrichosis, and upper-limb reduction defects that range from subtle phalangeal abnormalities to oligodactyly (missing digits). Craniofacial features include synophrys, highly arched and/or thick eyebrows, long eyelashes, short nasal bridge with anteverted nares, small widely spaced teeth, and microcephaly. Individuals with a milder phenotype have less severe growth, cognitive, and limb involvement, but often have facial features consistent with CdLS. Across the CdLS spectrum IQ ranges from below 30 to 102 (mean: 53). Many individuals demonstrate autistic and self-destructive tendencies. Other frequent findings include cardiac septal defects, gastrointestinal dysfunction, hearing loss, myopia, and cryptorchidism or hypoplastic genitalia.

Carnitine palmitoyltransferase II (CPT II) deficiency is a disorder of long-chain fatty-acid oxidation. The three clinical presentations are lethal neonatal form, severe infantile hepatocardiomuscular form, and myopathic form (which is usually mild and can manifest from infancy to adulthood). While the former two are severe multisystemic diseases characterized by liver failure with hypoketotic hypoglycemia, cardiomyopathy, seizures, and early death, the latter is characterized by exercise-induced muscle pain and weakness, sometimes associated with myoglobinuria. The myopathic form of CPT II deficiency is the most common disorder of lipid metabolism affecting skeletal muscle and the most frequent cause of hereditary myoglobinuria. Males are more likely to be affected than females.

Most characteristically, Aicardi-Goutières syndrome (AGS) manifests as an early-onset encephalopathy that usually, but not always, results in severe intellectual and physical disability. A subgroup of infants with AGS present at birth with abnormal neurologic findings, hepatosplenomegaly, elevated liver enzymes, and thrombocytopenia, a picture highly suggestive of congenital infection. Otherwise, most affected infants present at variable times after the first few weeks of life, frequently after a period of apparently normal development. Typically, they demonstrate the subacute onset of a severe encephalopathy characterized by extreme irritability, intermittent sterile pyrexias, loss of skills, and slowing of head growth. Over time, as many as 40% develop chilblain skin lesions on the fingers, toes, and ears. It is becoming apparent that atypical, sometimes milder, cases of AGS exist, and thus the true extent of the phenotype associated with pathogenic variants in the AGS-related genes is not yet known.

Congenital disorders of glycosylation (CDGs) are metabolic deficiencies in glycoprotein biosynthesis that usually cause severe mental and psychomotor retardation. Different forms of CDGs can be recognized by altered isoelectric focusing (IEF) patterns of serum transferrin. For a general discussion of CDGs, see CDG Ia (212065) and CDG Ib (602579).

Emanuel syndrome is characterized by pre- and postnatal growth deficiency, microcephaly, hypotonia, severe developmental delays, ear anomalies, preauricular tags or pits, cleft or high-arched palate, congenital heart defects, kidney abnormalities, and genital abnormalities in males.

ASPM primary microcephaly (ASPM-MCPH) is characterized by: (1) significant microcephaly (below -3 SD for age) usually present at birth and always present before age one year and (2) the absence of other congenital anomalies. While developmental milestones are usually normal in young children, older children have variable levels of intellectual disability. Neurologic examination is usually normal except for mild spasticity. Seizures are not common.

CODAS is an acronym for cerebral, ocular, dental, auricular, and skeletal anomalies. CODAS syndrome is a rare disorder characterized by a distinctive constellation of features that includes developmental delay, craniofacial anomalies, cataracts, ptosis, median nasal groove, delayed tooth eruption, hearing loss, short stature, delayed epiphyseal ossification, metaphyseal hip dysplasia, and vertebral coronal clefts (summary by Strauss et al., 2015).

Genetic defects in the pyruvate dehydrogenase complex are one of the most common causes of primary lactic acidosis in children. Most cases are caused by mutation in the E1-alpha subunit gene on the X chromosome. X-linked PDH deficiency is one of the few X-linked diseases in which a high proportion of heterozygous females manifest severe symptoms. The clinical spectrum of PDH deficiency is broad, ranging from fatal lactic acidosis in the newborn to chronic neurologic dysfunction with structural abnormalities in the central nervous system without systemic acidosis (Robinson et al., 1987; Brown et al., 1994). Genetic Heterogeneity of Pyruvate Dehydrogenase Complex Deficiency PDH deficiency can also be caused by mutation in other subunits of the PDH complex, including a form (PDHXD; 245349) caused by mutation in the component X gene (PDHX; 608769) on chromosome 11p13; a form (PDHBD; 614111) caused by mutation in the PDHB gene (179060) on chromosome 3p14; a form (PDHDD; 245348) caused by mutation in the DLAT gene (608770) on chromosome 11q23; a form (PDHPD; 608782) caused by mutation in the PDP1 gene (605993) on chromosome 8q22; and a form (PDHLD; 614462) caused by mutation in the LIAS gene (607031) on chromosome 4p14.

L1 syndrome involves a phenotypic spectrum ranging from severe to mild and includes three clinical phenotypes: X-linked hydrocephalus with stenosis of the aqueduct of Sylvius (HSAS). MASA (mental retardation [intellectual disability], aphasia [delayed speech], spastic paraplegia [shuffling gait], adducted thumbs) syndrome including X-linked complicated hereditary spastic paraplegia type 1. X-linked complicated corpus callosum agenesis. Males with HSAS are born with severe hydrocephalus, adducted thumbs, and spasticity; intellectual disability is severe. In less severely affected males, hydrocephalus may be subclinically present and documented only because of developmental delay; intellectual disability ranges from mild (IQ: 50-70) to moderate (IQ: 30-50). It is important to note that all phenotypes can be observed in affected individuals within the same family.

Any holoprosencephaly in which the cause of the disease is a mutation in the SHH gene.

Gaucher disease (GD) encompasses a continuum of clinical findings from a perinatal lethal disorder to an asymptomatic type. The identification of three major clinical types (1, 2, and 3) and two other subtypes (perinatal-lethal and cardiovascular) is useful in determining prognosis and management. GD type 1 is characterized by the presence of clinical or radiographic evidence of bone disease (osteopenia, focal lytic or sclerotic lesions, and osteonecrosis), hepatosplenomegaly, anemia and thrombocytopenia, lung disease, and the absence of primary central nervous system disease. GD types 2 and 3 are characterized by the presence of primary neurologic disease; in the past, they were distinguished by age of onset and rate of disease progression, but these distinctions are not absolute. Disease with onset before age two years, limited psychomotor development, and a rapidly progressive course with death by age two to four years is classified as GD type 2. Individuals with GD type 3 may have onset before age two years, but often have a more slowly progressive course, with survival into the third or fourth decade. The perinatal-lethal form is associated with ichthyosiform or collodion skin abnormalities or with nonimmune hydrops fetalis. The cardiovascular form is characterized by calcification of the aortic and mitral valves, mild splenomegaly, corneal opacities, and supranuclear ophthalmoplegia. Cardiopulmonary complications have been described with all the clinical subtypes, although varying in frequency and severity.

Congenital disorder of glycosylation type Ii (CDG1I) is a rare autosomal recessive disorder characterized by neurologic involvement, including a convulsive syndrome of unknown origin, axial hypotonia, and mental and motor regression (summary by Papazoglu et al., 2021). For a general discussion of CDGs, see CDG1A (212065).

Fanconi anemia (FA) is characterized by physical abnormalities, bone marrow failure, and increased risk for malignancy. Physical abnormalities, present in approximately 75% of affected individuals, include one or more of the following: short stature, abnormal skin pigmentation, skeletal malformations of the upper and/or lower limbs, microcephaly, and ophthalmic and genitourinary tract anomalies. Progressive bone marrow failure with pancytopenia typically presents in the first decade, often initially with thrombocytopenia or leukopenia. The incidence of acute myeloid leukemia is 13% by age 50 years. Solid tumors – particularly of the head and neck, skin, and genitourinary tract – are more common in individuals with FA.

X-linked intellectual deficit-cerebellar hypoplasia, also known as OPHN1 syndrome, is a rare syndromic form of cerebellar dysgenesis characterized by moderate to severe intellectual deficit and cerebellar abnormalities.

The Cabezas type of X-linked syndromic intellectual developmental disorder is characterized primarily by short stature, hypogonadism, and abnormal gait, with other more variable features such as speech delay, prominent lower lip, and tremor (Cabezas et al., 2000).

X-linked lissencephaly-2 (LISX2) is a developmental disorder characterized by structural brain anomalies, early-onset intractable seizures, severe psychomotor retardation, and ambiguous genitalia. Males are severely affected and often die within the first days or months of life, whereas females may be unaffected or have a milder phenotype (Bonneau et al., 2002). LISX2 is part of a phenotypic spectrum of disorders caused by mutation in the ARX gene comprising a nearly continuous series of developmental disorders ranging from hydranencephaly and lissencephaly to Proud syndrome (300004) to infantile spasms without brain malformations (DEE1; 308350) to syndromic (309510) and nonsyndromic (300419) mental retardation (Kato et al., 2004; Wallerstein et al., 2008). For a general phenotypic description and a discussion of genetic heterogeneity of lissencephaly, see LIS1 (607432).

Simpson-Golabi-Behmel syndrome type 2 (SGBS2) is an X-linked recessive disorder in which affected males have severely impaired intellectual development, ciliary dyskinesia, and macrocephaly (summary by Budny et al., 2006). For a general phenotypic description and a discussion of genetic heterogeneity of Simpson-Golabi-Behmel syndrome, see 312870.

MEHMO syndrome is a rare intellectual disability disorder that exhibits phenotypic heterogeneity and is variably characterized by mental retardation, epileptic seizures, hypogonadism with hypogenitalism, microcephaly, and obesity. Life expectancy ranges from less than 1 year to adulthood, and the condition is associated with significant morbidity and mortality (summary by Gregory et al., 2019).

Complex cortical dysplasia with other brain malformations-14A (CDCBM14A) is an autosomal recessive neurologic disorder characterized by global developmental delay with impaired intellectual development, motor delay, poor speech development, and early-onset seizures, often focal or atypical absence. Additional features may include strabismus, nystagmus, exo- or esotropia, axial hypotonia, and spasticity. Brain imaging shows bilateral frontoparietal polymicrogyria, a frontal-predominant cobblestone malformation of the cortex, scalloping of the cortical/white matter junction, enlarged ventricles, and hypoplasia of the pons, brainstem, and cerebellum. The disorder can be classified as a malformation of cortical development (summary by Parrini et al., 2009; Luo et al., 2011; Zulfiqar et al., 2021). For a discussion of genetic heterogeneity of CDCBM, see CDCBM1 (614039).

MDDGB5 is an autosomal recessive congenital muscular dystrophy with impaired intellectual development and structural brain abnormalities (Brockington et al., 2001). It is part of a group of similar disorders resulting from defective glycosylation of alpha-dystroglycan (DAG1; 128239), collectively known as 'dystroglycanopathies' (Mercuri et al., 2006). For a discussion of genetic heterogeneity of congenital muscular dystrophy-dystroglycanopathy type B, see MDDGB1 (613155).

Developmental and epileptic encephalopathy-9 (DEE9) is an X-linked disorder characterized by seizure onset in infancy and mild to severe intellectual impairment. Autistic and psychiatric features have been reported in some individuals. The disorder affects heterozygous females only; transmitting males are unaffected (summary by Jamal et al., 2010). For a general phenotypic description and a discussion of genetic heterogeneity of developmental and epileptic encephalopathy, see 308350.

Short-rib thoracic dysplasia (SRTD) with or without polydactyly refers to a group of autosomal recessive skeletal ciliopathies that are characterized by a constricted thoracic cage, short ribs, shortened tubular bones, and a 'trident' appearance of the acetabular roof. SRTD encompasses Ellis-van Creveld syndrome (EVC) and the disorders previously designated as Jeune syndrome or asphyxiating thoracic dystrophy (ATD), short rib-polydactyly syndrome (SRPS), and Mainzer-Saldino syndrome (MZSDS). Polydactyly is variably present, and there is phenotypic overlap in the various forms of SRTDs, which differ by visceral malformation and metaphyseal appearance. Nonskeletal involvement can include cleft lip/palate as well as anomalies of major organs such as the brain, eye, heart, kidneys, liver, pancreas, intestines, and genitalia. Some forms of SRTD are lethal in the neonatal period due to respiratory insufficiency secondary to a severely restricted thoracic cage, whereas others are compatible with life (summary by Huber and Cormier-Daire, 2012 and Schmidts et al., 2013). There is phenotypic overlap with the cranioectodermal dysplasias (Sensenbrenner syndrome; see CED1, 218330). For a discussion of genetic heterogeneity of short-rib thoracic dysplasia, see SRTD1 (208500).

A rare genetic neurological disease characterized by progressive encephalopathy, early-onset seizures with a hypsarrhythmic pattern, facial and limb edema, severe hypotonia, early arrest of psychomotor development and craniofacial dysmorphism (evolving microcephaly, narrow forehead, short nose, prominent auricles, open mouth, micrognathia), in the absence of neuro-ophthalmic or neuroradiologic findings. Poor visual responsiveness, growth failure and tapering fingers are also associated. There is evidence the disease is caused by homozygous mutation in the CCDC88A gene on chromosome 2p16.

Mosaic variegated aneuploidy (MVA) syndrome is a rare disorder in which some cells in the body have an abnormal number of chromosomes instead of the usual 46 chromosomes, a situation known as aneuploidy. Most commonly, cells have an extra chromosome, which is called trisomy, or are missing a chromosome, which is known as monosomy. In MVA syndrome, some cells are aneuploid and others have the normal number of chromosomes, which is a phenomenon known as mosaicism. Typically, at least one-quarter of cells in affected individuals have an abnormal number of chromosomes. Because the additional or missing chromosomes vary among the abnormal cells, the aneuploidy is described as variegated.\n\nIn MVA syndrome, growth before birth is slow (intrauterine growth restriction). After birth, affected individuals continue to grow at a slow rate and are shorter than average. In addition, they typically have an unusually small head size (microcephaly). Another common feature of MVA syndrome is an increased risk of developing cancer in childhood. Cancers that occur most frequently in affected individuals include a cancer of muscle tissue called rhabdomyosarcoma, a form of kidney cancer known as Wilms tumor, and a cancer of the blood-forming tissue known as leukemia.\n\nThere are at least three types of MVA syndrome, each with a different genetic cause. Type 1 is the most common and displays the classic signs and symptoms described above. Type 2 appears to have slightly different signs and symptoms than type 1, although the small number of affected individuals makes it difficult to define its characteristic features. Individuals with MVA syndrome type 2 grow slowly before and after birth; however, their head size is typically normal. Some people with MVA syndrome type 2 have unusually short arms. Individuals with MVA syndrome type 2 do not seem to have an increased risk of cancer. Another form of MVA syndrome is characterized by a high risk of developing Wilms tumor. Individuals with this form may also have other signs and symptoms typical of MVA syndrome type 1.\n\nLess commonly, people with MVA syndrome have eye abnormalities or distinctive facial features, such as a broad nasal bridge and low-set ears. Some affected individuals have brain abnormalities, the most common of which is called Dandy-Walker malformation. Intellectual disability, seizures, and other health problems can also occur in people with MVA syndrome.

STAC3 disorder is characterized by congenital myopathy, musculoskeletal involvement of the trunk and extremities, feeding difficulties, and delayed motor milestones. Most affected individuals have weakness with myopathic facies, scoliosis, kyphosis or kyphoscoliosis, and contractures. Other common findings are ptosis, abnormalities of the palate (including cleft palate), and short stature. Risk for malignant hyperthermia susceptibility and restrictive lung disease are increased. Intellect is typically normal. Originally described in individuals from the Lumbee Native American tribe (an admixture of Cheraw Indian, English, and African American ancestry) in the state of North Carolina and reported as Native American myopathy, STAC3 disorder has now been identified in numerous other populations worldwide.

Cerebrooculofacioskeletal syndrome is a severe, progressive neurologic disorder characterized by prenatal onset of arthrogryposis, microcephaly, and growth failure. Postnatal features include severe developmental delay, congenital cataracts (in some), and marked UV sensitivity of the skin. Survival beyond 6 years of age is rare. COFS represents the severe end of the spectrum of disorders caused by mutations in nucleotide excision repair (NER) genes, with Cockayne syndrome and xeroderma pigmentosum being milder NER-related phenotypes (summary by Drury et al., 2014). For a phenotypic description and a discussion of genetic heterogeneity of cerebrooculofacioskeletal syndrome, see COFS1 (214150).

Beare-Stevenson cutis gyrata syndrome (BSTVS) is an autosomal dominant condition characterized by the furrowed skin disorder of cutis gyrata, acanthosis nigricans, craniosynostosis, craniofacial dysmorphism, digital anomalies, umbilical and anogenital abnormalities, and early death (summary by Przylepa et al., 1996).

Phelan-McDermid syndrome is characterized by neonatal hypotonia, absent to severely delayed speech, developmental delay, and minor dysmorphic facial features. Most affected individuals have moderate to profound intellectual disability. Other features include large fleshy hands, dysplastic toenails, and decreased perspiration that results in a tendency to overheat. Normal stature and normal head size distinguishes Phelan-McDermid syndrome from other autosomal chromosome disorders. Behavior characteristics include mouthing or chewing non-food items, decreased perception of pain, and autism spectrum disorder or autistic-like affect and behavior.

A multisystem malformation syndrome that has been reported in about 10 patients. The clinical features include bilateral anophthalmia, abnormal nares, central nervous system anomalies, and neurodevelopmental delay. Additional features include brachycephaly and other facial anomalies. Non-facial anomalies have also been reported: postaxial polydactyly, genital hypoplasia. All cases reported so far have been sporadic, suggesting that the syndrome may be due to a new dominant mutation.

Molybdenum cofactor deficiency (MoCD) represents a spectrum, with some individuals experiencing significant signs and symptoms in the neonatal period and early infancy (termed early-onset or severe MoCD) and others developing signs and symptoms in childhood or adulthood (termed late-onset or mild MoCD). Individuals with early-onset MoCD typically present in the first days of life with severe encephalopathy, including refractory seizures, opisthotonos, axial and appendicular hypotonia, feeding difficulties, and apnea. Head imaging may demonstrate loss of gray and white matter differentiation, gyral swelling, sulci injury (typically assessed by evaluating the depth of focal lesional injury within the sulci), diffusely elevated T2-weighted signal, and panlobar diffusion restriction throughout the forebrain and midbrain with relative sparring of the brain stem. Prognosis for early-onset MoCD is poor, with about 75% succumbing in infancy to secondary complications of their neurologic disability (i.e., pneumonia). Late-onset MoCD is typically characterized by milder symptoms, such as acute neurologic decompensation in the setting of infection. Episodes vary in nature but commonly consist of altered mental status, dystonia, choreoathetosis, ataxia, nystagmus, and fluctuating hypotonia and hypertonia. These features may improve after resolution of the inciting infection or progress in a gradual or stochastic manner over the lifetime. Brain imaging may be normal or may demonstrate T2-weighted hyperintense or cystic lesions in the globus pallidus, thinning of the corpus callosum, and cerebellar atrophy.

Molybdenum cofactor deficiency (MoCD) represents a spectrum, with some individuals experiencing significant signs and symptoms in the neonatal period and early infancy (termed early-onset or severe MoCD) and others developing signs and symptoms in childhood or adulthood (termed late-onset or mild MoCD). Individuals with early-onset MoCD typically present in the first days of life with severe encephalopathy, including refractory seizures, opisthotonos, axial and appendicular hypotonia, feeding difficulties, and apnea. Head imaging may demonstrate loss of gray and white matter differentiation, gyral swelling, sulci injury (typically assessed by evaluating the depth of focal lesional injury within the sulci), diffusely elevated T2-weighted signal, and panlobar diffusion restriction throughout the forebrain and midbrain with relative sparring of the brain stem. Prognosis for early-onset MoCD is poor, with about 75% succumbing in infancy to secondary complications of their neurologic disability (i.e., pneumonia). Late-onset MoCD is typically characterized by milder symptoms, such as acute neurologic decompensation in the setting of infection. Episodes vary in nature but commonly consist of altered mental status, dystonia, choreoathetosis, ataxia, nystagmus, and fluctuating hypotonia and hypertonia. These features may improve after resolution of the inciting infection or progress in a gradual or stochastic manner over the lifetime. Brain imaging may be normal or may demonstrate T2-weighted hyperintense or cystic lesions in the globus pallidus, thinning of the corpus callosum, and cerebellar atrophy.

Kaufman oculocerebrofacial syndrome (KOS) is characterized by developmental delay, severe intellectual disability, and distinctive craniofacial features. Most affected children have prenatal-onset microcephaly, hypotonia, and growth deficiency. Feeding issues, ocular abnormalities, hearing impairment, and respiratory tract abnormalities are common. Ocular abnormalities can include structural abnormalities (microcornea or microphthalmia, coloboma, optic nerve hypoplasia), refractive errors (myopia ± astigmatism, hyperopia), strabismus, and entropion. Both conductive and sensorineural hearing loss have been reported as well as mixed conductive-sensorineural hearing loss of variable severity. Breathing problems can lead to prolonged hospitalization after birth in more than half of individuals. Less common findings include ectodermal abnormalities, cardiac manifestations, urogenital abnormalities, seizures, and skeletal abnormalities.

Baraitser-Winter cerebrofrontofacial (BWCFF) syndrome is a multiple congenital anomaly syndrome characterized by typical craniofacial features and intellectual disability. Many (but not all) affected individuals have pachygyria that is predominantly frontal, wasting of the shoulder girdle muscles, and sensory impairment due to iris or retinal coloboma and/or sensorineural deafness. Intellectual disability, which is common but variable, is related to the severity of the brain malformations. Seizures, congenital heart defects, renal malformations, and gastrointestinal dysfunction are also common.

Hypoparathyroidism-retardation-dysmorphism syndrome (HRDS) is an autosomal recessive multisystem disorder characterized by intrauterine and postnatal growth retardation, infantile-onset hypoparathyroidism that can result in severe hypocalcemic seizures, dysmorphic facial features, and developmental delay (summary by Padidela et al., 2009 and Ratbi et al., 2015).

Mowat-Wilson syndrome (MWS) is characterized by distinctive facial features (widely spaced eyes, broad eyebrows with a medial flare, low-hanging columella, prominent or pointed chin, open-mouth expression, and uplifted earlobes with a central depression), congenital heart defects with predilection for abnormalities of the pulmonary arteries and/or valves, Hirschsprung disease or chronic constipation, genitourinary anomalies (particularly hypospadias in males), and hypogenesis or agenesis of the corpus callosum. Most affected individuals have moderate-to-severe intellectual disability. Speech is typically limited to a few words or is absent, with relative preservation of receptive language skills. Growth restriction with microcephaly and seizure disorder are also common. Most affected people have a happy demeanor and a wide-based gait that can sometimes be confused with Angelman syndrome.

A rare genetic disease characterized by the presence of Müllerian duct derivatives (rudimentary uterus, fallopian tubes, and atretic vagina) and other genital anomalies (cryptorchidism, micropenis) in male newborns, intestinal and pulmonary lymphangiectasia, protein-losing enteropathy, hepatomegaly, and renal anomalies. Postaxial polydactyly, facial dysmorphism (including broad nasal bridge, bulbous nasal tip, long and prominent upper lip with smooth philtrum, hypertrophic alveolar ridges, and mild retrognathia, among other features), and short limbs have also been described. The syndrome is fatal in infancy.

The proliferative vasculopathy and hydranencephaly-hydrocephaly syndrome is a rare, autosomal recessive, usually prenatally lethal disorder characterized by hydranencephaly, a distinctive glomerular vasculopathy in the central nervous system and retina, and diffuse ischemic lesions of the brain stem, basal ganglia, and spinal cord with calcifications. It is usually diagnosed by ultrasound between 26 and 33 weeks' gestation (summary by Meyer et al., 2010). Rarely, affected individuals may survive, but are severely impaired with almost no neurologic development (Kvarnung et al., 2016).

2,4-Dienoyl-CoA reductase deficiency (DECRD) is a rare autosomal recessive inborn error of metabolism resulting in mitochondrial dysfunction due to impaired production of NADPH, which is an essential cofactor for several mitochondrial enzymes. Affected individuals have a variable phenotype: some may have severe neurologic symptoms and metabolic dysfunction beginning in early infancy, whereas others may present with more subtle features, such as childhood-onset optic atrophy or intermittent muscle weakness. The variable severity is putatively dependent on the effect of the mutation on the NADK2 enzyme. Biochemical analysis typically shows hyperlysinemia, due to defective activity of the mitochondrial NADP(H)-dependent enzyme AASS (605113), which is usually a benign finding. More severe cases have increased C10:2-carnitine levels, due to defective activity of the enzyme DECR (DECR1; 222745) (summary by Houten et al., 2014 and Pomerantz et al., 2018).

Ventriculomegaly with cystic kidney disease (VMCKD) is a severe autosomal recessive developmental disorder characterized by onset in utero of dilated cerebral ventricles and microscopic renal tubular cysts. The pregnancies of affected individuals are associated with increased alpha-fetoprotein (AFP). Most affected pregnancies have been terminated (summary by Slavotinek et al., 2015). See also 602200 for a disorder characterized by ventriculomegaly and defects of the radius and kidney.

A rare cranial malformation syndrome characterized by the premature closure of both lambdoid sutures and the posterior sagittal suture, resulting in abnormal skull contour (frontal bossing, anterior turricephaly with mild brachycephaly, biparietal narrowing, occipital concavity) and dysmorphic facial features (low-set ears, midfacial hypoplasia). Short stature, developmental delay, epilepsy, and oculomotor dyspraxia have also been reported. Associated anomalies include enlargement of the cerebral ventricles, agenesis of the corpus callosum, Arnold-Chiari malformation type I, venous anomalies of skull, and hydrocephalus.

Temtamy syndrome is a mental retardation/multiple congenital anomaly syndrome characterized by variable craniofacial dysmorphism, ocular coloboma, seizures, and brain abnormalities, including abnormalities of the corpus callosum and thalamus (summary by Akizu et al., 2013).

7q11.23 duplication syndrome is characterized by delayed motor, speech, and social skills in early childhood; neurologic abnormalities (hypotonia, adventitious movements, and abnormal gait and station); speech sound disorders including motor speech disorders (childhood apraxia of speech and/or dysarthria) and phonologic disorders; behavior problems including anxiety disorders (especially social anxiety disorder [social phobia]), selective mutism, attention-deficit/hyperactivity disorder, oppositional disorders, physical aggression, and autism spectrum disorder; and intellectual disability in some individuals. Distinctive facial features are common. Cardiovascular disease includes dilatation of the ascending aorta. Approximately 30% of individuals have one or more congenital anomalies.

Microhydranencephaly (MHAC) is a severe neurodevelopmental defect characterized by extreme microcephaly, profound motor and mental retardation, spasticity, and incomplete cerebral formation. Radiologic studies show gross dilation of the ventricles resulting from the absence of cerebral hemispheres or severe delay in their development, as well as hypoplasia of the corpus callosum, cerebellum, and brainstem (summary by Guven et al., 2012).

Chudley-McCullough syndrome is an autosomal recessive neurologic disorder characterized by early-onset sensorineural deafness and specific brain anomalies on MRI, including hypoplasia of the corpus callosum, enlarged cysterna magna with mild focal cerebellar dysplasia, and nodular heterotopia. Some patients have hydrocephalus. Psychomotor development is normal (summary by Alrashdi et al., 2011).

Restrictive cardiomyopathy (RCM) is a myocardial disease characterized by impaired ventricular filling and reduced diastolic volume in the presence of normal systolic function and normal or near-normal myocardial thickness. The disease is characterized by symptoms of progressive left- and right-sided heart failure. The overall prognosis is poor, especially when onset is in childhood, and patients often require cardiac transplantation (Mogensen et al., 2003). Genetic Heterogeneity of Familial Restrictive Cardiomyopathy Other forms of familial restrictive cardiomyopathy include RCM2 (609578), mapped to chromosome 10q23; RCM3 (612422), caused by mutation in the TNNT2 gene (191045) on chromosome 1q32; RCM4 (see 615248), caused by mutation in the MYPN gene (608517) on chromosome 10q21; RCM5 (see 617047), caused by mutation in the FLNC gene (102565) on chromosome 7q32; and RCM6 (619433), caused by mutation in the KIF20A gene (605664) on chromosome 5q31.

Verloes et al. (1992) described a rare variant of frontonasal dysplasia (see FND1, 136760), designated acromelic frontonasal dysplasia (AFND), in which similar craniofacial anomalies are associated with variable central nervous system malformations and limb defects including tibial hypoplasia/aplasia, talipes equinovarus, and preaxial polydactyly of the feet.

Neuronal intranuclear inclusion disease (NIID) is an autosomal dominant, slowly progressive neurodegenerative disorder characterized by a wide range of clinical manifestations, including pyramidal and extrapyramidal symptoms, cerebellar ataxia, cognitive decline and dementia, peripheral neuropathy, and autonomic dysfunction. The age at onset varies, but most individuals present as adults between about 30 and 70 years of age. Pathologic investigation shows eosinophilic intranuclear inclusions in almost all cell types, including neurons, skin cells, fibroblasts, and skeletal muscle. Brain imaging shows a characteristic leukoencephalopathy with high intensity signals in the corticomedullary junction on diffusion-weighted imaging (DWI), as well as white matter abnormalities in subcortical and brainstem regions. Skin biopsy combined with brain imaging is useful for diagnosis (summary by Sone et al., 2016). The phenotype in some cases is suggestive of Parkinson disease (see 168600) and/or Alzheimer disease (see 104300), consistent with an evolving phenotypic spectrum of adult-onset NIID (summary by Tian et al., 2019).

This syndrome has characteristics of anophthalmia or microphthalmia, retinal dystrophy, and/or myopia, associated in some cases with cerebral anomalies. It has been described in two families. Polydactyly may also be present. Linkage analysis allowed identification of mutations in the BMP4 gene, which has already been shown to play a role in eye development.

Holoprosencephaly associated with mutations in the ZIC2 gene.

Combined oxidative phosphorylation deficiency type 3 is an extremely rare clinically heterogenous disorder described in about 5 patients to date. Clinical signs included hypotonia, lactic acidosis, and hepatic insufficiency, with progressive encephalomyopathy or hypertrophic cardiomyopathy.

Koolen-de Vries syndrome (KdVS) is characterized by developmental delay / intellectual disability, neonatal/childhood hypotonia, dysmorphisms, congenital malformations, and behavioral features. Psychomotor developmental delay is noted in all individuals from an early age. The majority of individuals with KdVS function in the mild-to-moderate range of intellectual disability. Other findings include speech and language delay (100%), epilepsy (~33%), congenital heart defects (25%-50%), renal and urologic anomalies (25%-50%), and cryptorchidism (71% of males). Behavior in most is described as friendly, amiable, and cooperative.

Spondyloepiphyseal dysplasia of the Genevieve type (SEMDG) is characterized by infantile-onset severe developmental delay and skeletal dysplasia, including short stature, premature carpal ossification, platyspondyly, longitudinal metaphyseal striations, and small epiphyses (summary by van Karnebeek et al., 2016).

PIK3CA-related overgrowth spectrum (PROS) encompasses a range of clinical findings in which the core features are congenital or early-childhood onset of segmental/focal overgrowth with or without cellular dysplasia. Prior to the identification of PIK3CA as the causative gene, PROS was separated into distinct clinical syndromes based on the tissues and/or organs involved (e.g., MCAP [megalencephaly-capillary malformation] syndrome and CLOVES [congenital lipomatous asymmetric overgrowth of the trunk, lymphatic, capillary, venous, and combined-type vascular malformations, epidermal nevi, skeletal and spinal anomalies] syndrome). The predominant areas of overgrowth include the brain, limbs (including fingers and toes), trunk (including abdomen and chest), and face, all usually in an asymmetric distribution. Generalized brain overgrowth may be accompanied by secondary overgrowth of specific brain structures resulting in ventriculomegaly, a markedly thick corpus callosum, and cerebellar tonsillar ectopia with crowding of the posterior fossa. Vascular malformations may include capillary, venous, and less frequently, arterial or mixed (capillary-lymphatic-venous or arteriovenous) malformations. Lymphatic malformations may be in various locations (internal and/or external) and can cause various clinical issues, including swelling, pain, and occasionally localized bleeding secondary to trauma. Lipomatous overgrowth may occur ipsilateral or contralateral to a vascular malformation, if present. The degree of intellectual disability appears to be mostly related to the presence and severity of seizures, cortical dysplasia (e.g., polymicrogyria), and hydrocephalus. Many children have feeding difficulties that are often multifactorial in nature. Endocrine issues affect a small number of individuals and most commonly include hypoglycemia (largely hypoinsulinemic hypoketotic hypoglycemia), hypothyroidism, and growth hormone deficiency.

Desmosterolosis is a rare autosomal recessive disorder characterized by multiple congenital anomalies and elevated levels of the cholesterol precursor desmosterol in plasma, tissue, and cultured cells (summary by Waterham et al., 2001).

Bleeding problems are common in the vascular type of Ehlers-Danlos syndrome and are caused by unpredictable tearing (rupture) of blood vessels and organs. These complications can lead to easy bruising, internal bleeding, a hole in the wall of the intestine (intestinal perforation), or stroke. During pregnancy, women with vascular Ehlers-Danlos syndrome may experience rupture of the uterus. Additional forms of Ehlers-Danlos syndrome that involve rupture of the blood vessels include the kyphoscoliotic, classical, and classical-like types.\n\nMany people with the Ehlers-Danlos syndromes have soft, velvety skin that is highly stretchy (elastic) and fragile. Affected individuals tend to bruise easily, and some types of the condition also cause abnormal scarring. People with the classical form of Ehlers-Danlos syndrome experience wounds that split open with little bleeding and leave scars that widen over time to create characteristic "cigarette paper" scars. The dermatosparaxis type of the disorder is characterized by loose skin that sags and wrinkles, and extra (redundant) folds of skin may be present.\n\nAn unusually large range of joint movement (hypermobility) occurs in most forms of Ehlers-Danlos syndrome, and it is a hallmark feature of the hypermobile type. Infants and children with hypermobility often have weak muscle tone (hypotonia), which can delay the development of motor skills such as sitting, standing, and walking. The loose joints are unstable and prone to dislocation and chronic pain. In the arthrochalasia type of Ehlers-Danlos syndrome, infants have hypermobility and dislocations of both hips at birth.\n\nThe various forms of Ehlers-Danlos syndrome have been classified in several different ways. Originally, 11 forms of Ehlers-Danlos syndrome were named using Roman numerals to indicate the types (type I, type II, and so on). In 1997, researchers proposed a simpler classification (the Villefranche nomenclature) that reduced the number of types to six and gave them descriptive names based on their major features. In 2017, the classification was updated to include rare forms of Ehlers-Danlos syndrome that were identified more recently. The 2017 classification describes 13 types of Ehlers-Danlos syndrome.\n\nOther types of Ehlers-Danlos syndrome have additional signs and symptoms. The cardiac-valvular type causes severe problems with the valves that control the movement of blood through the heart. People with the kyphoscoliotic type experience severe curvature of the spine that worsens over time and can interfere with breathing by restricting lung expansion. A type of Ehlers-Danlos syndrome called brittle cornea syndrome is characterized by thinness of the clear covering of the eye (the cornea) and other eye abnormalities. The spondylodysplastic type features short stature and skeletal abnormalities such as abnormally curved (bowed) limbs. Abnormalities of muscles, including hypotonia and permanently bent joints (contractures), are among the characteristic signs of the musculocontractural and myopathic forms of Ehlers-Danlos syndrome. The periodontal type causes abnormalities of the teeth and gums.\n\nEhlers-Danlos syndrome is a group of disorders that affect connective tissues supporting the skin, bones, blood vessels, and many other organs and tissues. Defects in connective tissues cause the signs and symptoms of these conditions, which range from mildly loose joints to life-threatening complications.

Wolf-Hirschhorn syndrome is a congenital malformation syndrome characterized by pre- and postnatal growth deficiency, developmental disability of variable degree, characteristic craniofacial features ('Greek warrior helmet' appearance of the nose, high forehead, prominent glabella, hypertelorism, high-arched eyebrows, protruding eyes, epicanthal folds, short philtrum, distinct mouth with downturned corners, and micrognathia), and a seizure disorder (Battaglia et al., 2008).

Autosomal recessive osteopetrosis-5 (OPTB5) is a form of infantile malignant osteopetrosis, characterized by defective osteoclast function resulting in decreased bone resorption and generalized osteosclerosis. Defective resorption causes development of densely sclerotic fragile bones and progressive obliteration of the marrow spaces and cranial foramina. Marrow obliteration is associated with extramedullary hematopoiesis and hepatosplenomegaly, and results in anemia and thrombocytopenia, whereas nerve entrapment accounts for progressive blindness and hearing loss. Other major manifestations include failure to thrive, pathologic fractures, and increased infection rate. Most affected children succumb to severe bone marrow failure and overwhelming infection in the first few years of life (summary by Quarello et al., 2004).

Congenital plasminogen deficiency is a rare autosomal recessive disorder characterized clinically by chronic mucosal pseudomembranous lesions consisting of subepithelial fibrin deposition and inflammation. The most common clinical manifestation is ligneous ('wood-like') conjunctivitis, a redness and subsequent formation of pseudomembranes mostly on the palpebral surfaces of the eye that progress to white, yellow-white, or red thick masses with a wood-like consistency that replace the normal mucosa. The lesions may be triggered by local injury and/or infection and often recur after local excision. Pseudomembranous lesions of other mucous membranes often occur in the mouth, nasopharynx, trachea, and female genital tract. Some affected children also have congenital occlusive hydrocephalus. A slightly increased female:male ratio has been observed (1.4:1 to 2:1) (Schuster and Seregard, 2003; Tefs et al., 2006). Type I plasminogen deficiency is characterized by decreased serum plasminogen activity, decreased plasminogen antigen levels, and clinical symptoms, whereas type II plasminogen deficiency, also known as 'dysplasminogenemia,' is characterized by decreased plasminogen activity with normal or slightly reduced antigen levels. Patients with type II deficiency are usually asymptomatic. Ligneous conjunctivitis and pseudomembranous formation has only been associated with type I plasminogen deficiency. Presumably, normal amounts of plasminogen antigen with decreased activity, as seen in type II, is sufficient for normal wound healing (Schuster and Seregard, 2003).

Syndrome with characteristics of severe psychomotor retardation, failure to thrive and intolerance to wheat and dairy products. So far, only two cases have been described. The disease is caused by mutations in the COG8 gene, which encodes a subunit of the COG complex. This complex is involved vesicle transport in the Golgi apparatus.

A rare genetic neurological disorder with characteristics of pregnancy complicated by polyhydramnios, severe intractable epilepsy presenting in infancy, severe hypotonia, decreased muscle mass, global developmental delay, craniofacial dysmorphism (long face, large forehead, peaked eyebrows, broad nasal bridge, hypertelorism, large mouth with thick lips), and macrocephaly due to megalencephaly and hydrocephalus in most patients. Additional features that have been reported include cardiac anomalies like atrial septal defects, diabetes insipidus and nephrocalcinosis among others.

An autosomal recessive condition caused by mutation(s) in the ERCC4 gene, encoding DNA repair endonuclease XPF. it is characterized by characterized by cutaneous photosensitivity and progeroid features in multiple organ systems.

Pitt-Hopkins syndrome (PTHS) is characterized by significant developmental delays with moderate-to-severe intellectual disability and behavioral differences, characteristic facial features, and episodic hyperventilation and/or breath-holding while awake. Speech is significantly delayed and most individuals are nonverbal with receptive language often stronger than expressive language. Other common findings are autism spectrum disorder symptoms, sleep disturbance, stereotypic hand movements, seizures, constipation, and severe myopia.

Disease caused by homozygous mutation in the prosaposin gene (PSAP) on chromosome 10q22. The disease is genetically distinct from Krabbe disease. Clinical features include onset in infancy with respiratory and neurologic involvement.

Endocrine-cerebro-osteodysplasia (ECO) syndrome is characterized by various anomalies of the endocrine, cerebral, and skeletal systems resulting in neonatal mortality.

1q41q42 microdeletion syndrome is a chromosomal anomaly characterized by a severe developmental delay and/or intellectual disability, typical facial dysmorphic features, brain anomalies, seizures, cleft palate, clubfeet, nail hypoplasia and congenital heart disease.

Chromosome 2p16.1-p15 deletion syndrome is a neurodevelopmental disorder characterized by delayed psychomotor development, intellectual disability, and variable but distinctive dysmorphic features, including microcephaly, bitemporal narrowing, smooth and long philtrum, hypertelorism, downslanting palpebral fissures, broad nasal root, thin upper lip, and high palate. Many patients have behavioral disorders, including autistic features, as well as structural brain abnormalities, such as pachygyria or hypoplastic corpus callosum. Those with deletions including the BCL11A gene (606557) also have persistence of fetal hemoglobin (HbF), which is asymptomatic and does not affected hematologic parameters or susceptibility to infection (summary by Funnell et al., 2015). Point mutation in the BCL11A gene causes intellectual developmental disorder with persistence of fetal hemoglobin (617101), which shows overlapping features. See also fetal hemoglobin quantitative trait locus-5 (HBFQTL5; 142335).

BCARD syndrome is an autosomal recessive connective tissue disorder characterized by bone abnormalities, including low bone mineral density, scoliosis, contractures of the fingers and other joints, prominent knees, and rare pathologic fractures; cataract and other ocular abnormalities, including high myopia, optically empty vitreous, and risk for retinal detachment; risk of arterial rupture due to vascular aneurysm or dissection; and sensorineural deafness. Affected individuals also exhibit recognizable craniofacial dysmorphisms, and variable skin features have been observed, including reduced palmar creases, soft skin with easy bruising, and blistering. Developmental delay, which is present in most patients, may be attributable to sensory deficits or medical complications (Ewans et al., 2019).

TSEN54 pontocerebellar hypoplasia (TSEN54-PCH) comprises three PCH phenotypes (PCH2, 4, and 5) that share characteristic neuroradiologic and neurologic findings. The three PCH phenotypes (which differ mainly in life expectancy) were considered to be distinct entities before their molecular basis was known. PCH2. Children usually succumb before age ten years (those with PCH4 and 5 usually succumb as neonates). Children with PCH2 have generalized clonus, uncoordinated sucking and swallowing, impaired cognitive development, lack of voluntary motor development, cortical blindness, and an increased risk for rhabdomyolysis during severe infections. Epilepsy is present in approximately 50%. PCH4. Neonates often have seizures, multiple joint contractures ("arthrogryposis"), generalized clonus, and central respiratory impairment. PCH5 resembles PCH4 and has been described in one family.

Classic Joubert syndrome (JS) is characterized by three primary findings: A distinctive cerebellar and brain stem malformation called the molar tooth sign (MTS). Hypotonia. Developmental delays. Often these findings are accompanied by episodic tachypnea or apnea and/or atypical eye movements. In general, the breathing abnormalities improve with age, truncal ataxia develops over time, and acquisition of gross motor milestones is delayed. Cognitive abilities are variable, ranging from severe intellectual disability to normal. Additional findings can include retinal dystrophy, renal disease, ocular colobomas, occipital encephalocele, hepatic fibrosis, polydactyly, oral hamartomas, and endocrine abnormalities. Both intra- and interfamilial variation are seen.

Turner-type X-linked syndromic intellectual developmental disorder (MRXST) is a neurodevelopmental disorder with a highly variable phenotype. Some affected families show X-linked recessive inheritance, with only males being affected and carrier females having no abnormal findings. In other affected families, males are severely affected, and female mutation carriers show milder cognitive abnormalities or dysmorphic features. In addition, there are female patients with de novo mutations who show the full phenotype, despite skewed X-chromosome inactivation. Affected individuals show global developmental delay from infancy, with variably impaired intellectual development and poor or absent speech, often with delayed walking. Dysmorphic features are common and can include macrocephaly, microcephaly, deep-set eyes, hypotelorism, small palpebral fissures, dysplastic, large, or low-set ears, long face, bitemporal narrowing, high-arched palate, thin upper lip, and scoliosis or mild distal skeletal anomalies, such as brachydactyly or tapered fingers. Males tend to have cryptorchidism. Other features, such as hypotonia, seizures, and delayed bone age, are more variable (summary by Moortgat et al., 2018).

A syndromic X-linked intellectual disability characterized by moderate intellectual disability with variable occurrence of asthenic body habitus, dysmorphic features, autistic features, macrocephaly, seizures, myoclonic jerks, and hyporeflexia that has material basis in mutation in the GRIA3 gene on chromosome Xq25.

Roifman-Chitayat syndrome (ROCHIS) is an autosomal recessive digenic disorder characterized by global developmental delay, variable neurologic features such as seizures, ataxia, and optic atrophy, dysmorphic facial features, distal skeletal anomalies, and combined immunodeficiency manifest as recurrent infections (summary by Sharfe et al., 2018).

RNAse T2-deficient leukoencephalopathy is a disorder that affects the brain. People with RNAse T2-deficient leukoencephalopathy have neurological problems that become apparent during infancy; the problems generally do not worsen over time (progress). Most affected individuals have severe intellectual disability; muscle stiffness (spasticity); and a delay in developing motor skills such as sitting, crawling, and walking. Some do not learn to walk, and most do not develop the ability to speak. Other neurological features that can occur in RNAse T2-deficient leukoencephalopathy include hearing loss caused by abnormalities in the inner ear (sensorineural deafness), seizures, involuntary writhing movements of the hands (athetosis), uncontrolled muscle tensing (dystonia), and involuntary eye movements (nystagmus). In addition to the neurological problems associated with this disorder, some affected individuals have unusual facial features sometimes described as a "doll-like face."\n\nThe neurological problems in this disorder are caused by abnormalities in the brain. People with this condition have leukoencephalopathy, an abnormality of the brain's white matter that can be detected with medical imaging. White matter consists of nerve fibers covered by a fatty substance called myelin. Myelin insulates nerve fibers and promotes the rapid transmission of nerve impulses. In people with RNAse T2-deficient leukoencephalopathy, myelin is not made in sufficient amounts during development, leading to patchy white matter abnormalities (lesions) in the brain. In addition, individuals with RNAse T2-deficient leukoencephalopathy may have cysts in regions of the brain called the temporal lobes and enlargement of the fluid-filled cavities (ventricles) near the center of the brain. The white matter lesions are primarily concentrated around the cysts and the ventricles. An abnormally small head and brain size (microcephaly) often occurs in this disorder.

AP-4-associated hereditary spastic paraplegia (HSP), also known as AP-4 deficiency syndrome, is a group of neurodegenerative disorders characterized by a progressive, complex spastic paraplegia with onset typically in infancy or early childhood. Early-onset hypotonia evolves into progressive lower-extremity spasticity. The majority of children become nonambulatory and usually wheelchair bound. Over time spasticity progresses to involve the upper extremities, resulting in a spastic tetraplegia. Associated complications include dysphagia, contractures, foot deformities, dysregulation of bladder and bowel function, and a pseudobulbar affect. About 50% of affected individuals have seizures. Postnatal microcephaly (usually in the -2SD to -3SD range) is common. All have developmental delay. Speech development is significantly impaired and many affected individuals remain nonverbal. Intellectual disability in older children is usually moderate to severe.

Cerebral palsy (CP) is defined as a nonprogressive but not unchanging disorder of posture or movement, caused by an abnormality of the brain and first evident at the stage of rapid brain development (Hughes and Newton, 1992). Cerebral palsy can be classified according to the type of movement disorder: spastic cerebral palsy accounts for approximately 60% of cases and can be subdivided into hemiplegic, diplegic, quadriplegic, and monoplegic types, whereas other forms include athetoid/dyskinetic, ataxic (605388), and mixed (Gustavson et al., 1969). Genetic Heterogeneity of Spastic Quadriplegic Cerebral Palsy See also CPSQ3 (617008), caused by mutation in the ADD3 gene (601568) on 10q24. Related phenotypes that were formerly classified in the CPSQ series include spastic paraplegia-47 (SPG47; 614066), spastic paraplegia-50 (SPG50; 612936), spastic paraplegia-51 (SPG51; 613744), spastic paraplegia-52 (SPG52; 614067), and neurodevelopmental disorder with progressive spasticity and brain white matter abnormalities (NEDSWMA; 619026).

Oxoglutarate dehydrogenase deficiency (OGDHD) is an autosomal recessive disorder associated with features of infantile- and pediatric-onset basal ganglia-associated movement disorders, hypotonia, developmental delays, ataxia, and seizures (summary by Yap et al., 2021).

Congenital alveolar capillary dysplasia with misalignment of pulmonary veins (ACDMPV) is characterized histologically by failure of formation and ingrowth of alveolar capillaries that then do not make contact with alveolar epithelium, medial muscular thickening of small pulmonary arterioles with muscularization of the intraacinar arterioles, thickened alveolar walls, and anomalously situated pulmonary veins running alongside pulmonary arterioles and sharing the same adventitial sheath. Less common features include a reduced number of alveoli and a patchy distribution of the histopathologic changes. The disorder is associated with persistent pulmonary hypertension of the neonate and shows varying degrees of lability and severity (Boggs et al., 1994). Affected infants present with respiratory distress resulting from pulmonary hypertension in the early postnatal period, and the disease is uniformly fatal within the newborn period (Vassal et al., 1998). Additional features of ACDMPV include multiple congenital anomalies affecting the cardiovascular, gastrointestinal, genitourinary, and musculoskeletal systems, as well as disruption of the normal right-left asymmetry of intrathoracic or intraabdominal organs (Sen et al., 2004).

Congenital muscular dystrophy-dystroglycanopathy with brain and eye anomalies (type A), which includes both the more severe Walker-Warburg syndrome (WWS) and the slightly less severe muscle-eye-brain disease (MEB), is an autosomal recessive disorder with characteristic brain and eye malformations, profound mental retardation, congenital muscular dystrophy, and death usually in the first years of life. It represents the most severe end of a phenotypic spectrum of similar disorders resulting from defective glycosylation of DAG1 (128239), collectively known as 'dystroglycanopathies' (van Reeuwijk et al., 2005). For a general phenotypic description and a discussion of genetic heterogeneity of muscular dystrophy-dystroglycanopathy type A, see MDDGA1 (236670).

MDDGB3 is an autosomal recessive congenital muscular dystrophy with impaired intellectual development and mild brain abnormalities (Clement et al., 2008). It is part of a group of similar disorders resulting from defective glycosylation of alpha-dystroglycan (DAG1; 128239), collectively known as 'dystroglycanopathies' (Mercuri et al., 2009). For a discussion of genetic heterogeneity of congenital muscular dystrophy-dystroglycanopathy type B, see MDDGB1 (613155).

Congenital muscular dystrophy-dystroglycanopathy with brain and eye anomalies (type A), which includes both the more severe Walker-Warburg syndrome (WWS) and the slightly less severe muscle-eye-brain disease (MEB), is an autosomal recessive disorder with characteristic brain and eye malformations, profound mental retardation, congenital muscular dystrophy, and death usually in the first years of life. It represents the most severe end of a phenotypic spectrum of similar disorders resulting from defective glycosylation of DAG1 (128239), collectively known as 'dystroglycanopathies' (Beltran-Valero de Bernabe et al., 2004). For a general phenotypic description and a discussion of genetic heterogeneity of muscular dystrophy-dystroglycanopathy type A, see MDDGA1 (236670).

Congenital muscular dystrophy-dystroglycanopathy with brain and eye anomalies (type A), which includes both the more severe Walker-Warburg syndrome (WWS) and the slightly less severe muscle-eye-brain disease (MEB), is an autosomal recessive disorder with characteristic brain and eye malformations, profound mental retardation, congenital muscular dystrophy, and death usually in the first years of life. It represents the most severe end of a phenotypic spectrum of similar disorders resulting from defective glycosylation of DAG1 (128239), collectively known as 'dystroglycanopathies' (Godfrey et al., 2007). For a general phenotypic description and a discussion of genetic heterogeneity of muscular dystrophy-dystroglycanopathy type A, see MDDGA1 (236670).

MDDGB2 is an autosomal recessive congenital muscular dystrophy associated with impaired intellectual development and mild structural brain abnormalities (Yanagisawa et al., 2007). It is part of a group of similar disorders, collectively known as 'dystroglycanopathies,' resulting from defective glycosylation of alpha-dystroglycan (DAG1; 128239) (Godfrey et al., 2007). For a discussion of genetic heterogeneity of congenital muscular dystrophy-dystroglycanopathy type B, see MDDGB1 (613155).

Microcephaly, seizures, and developmental delay (MCSZ) is an autosomal recessive neurodevelopmental disorder with onset in infancy. There is a range of phenotypic severity: some patients develop refractory seizures in infancy, consistent with a developmental and epileptic encephalopathy (DEE), whereas others have more well-controlled seizures and a more protracted course associated with cerebellar atrophy and peripheral neuropathy (Shen et al., 2010 and Poulton et al., 2013). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

Neurodevelopmental disorder with hypotonia, stereotypic hand movements, and impaired language (NEDHSIL) is characterized by global developmental delay with hypotonia, poor motor development with limited walking, impaired intellectual development with poor or absent speech, and behavioral abnormalities. Almost all affected individuals demonstrate repetitive stereotypic hand movements that can be categorized as hyperkinetic and resembling those of Rett syndrome (RTT; 312750). About 80% of patients develop various types of seizures that may be refractory to treatment. Additional features may include dysmorphic facial features, particularly dysplastic ears, poor eye contact, episodic hyperventilation, tendency to infection, and abnormalities on brain imaging, such as enlarged ventricles, thin corpus callosum, and delayed myelination (summary by Vrecar et al., 2017, Paciorkowski et al., 2013).

19p13.13 deletion syndrome is a condition that results from a chromosomal change in which a small piece of chromosome 19 is deleted in each cell. The deletion occurs on the short (p) arm of the chromosome at a position designated p13.13.\n\nFeatures commonly associated with this chromosomal change include an unusually large head size (macrocephaly), tall stature, and intellectual disability that is usually moderate in severity. Many affected individuals have significantly delayed development, including speech, and children may speak few or no words. Weak muscle tone (hypotonia) and problems with coordinating muscle movement (ataxia) contribute to delays in gross motor skills (such as sitting and walking) and fine motor skills (such as holding a pencil).\n\nOther signs and symptoms that can occur with 19p13.13 deletion syndrome include seizures, abnormalities of brain structure, and mild differences in facial features (such as a prominent forehead). Many affected individuals have problems with feeding and digestion, including constipation, diarrhea, vomiting, and abdominal pain. Eye problems that can impair vision are also common. These include eyes that do not point in the same direction (strabismus) and underdevelopment of the optic nerves, which carry visual information from the eyes to the brain.\n\nThe signs and symptoms of 19p13.13 deletion syndrome vary among affected individuals. In part, this variation occurs because the size of the deletion, and the number of genes it affects, varies from person to person.

HDBSCC is an autosomal recessive disorder with a distinctive phenotype comprising hemorrhagic destruction of the brain, subependymal calcification, and congenital cataracts. Affected individuals have a catastrophic neurologic clinical course resulting in death in infancy (summary by Akawi et al., 2013).

AP-4-associated hereditary spastic paraplegia (HSP), also known as AP-4 deficiency syndrome, is a group of neurodegenerative disorders characterized by a progressive, complex spastic paraplegia with onset typically in infancy or early childhood. Early-onset hypotonia evolves into progressive lower-extremity spasticity. The majority of children become nonambulatory and usually wheelchair bound. Over time spasticity progresses to involve the upper extremities, resulting in a spastic tetraplegia. Associated complications include dysphagia, contractures, foot deformities, dysregulation of bladder and bowel function, and a pseudobulbar affect. About 50% of affected individuals have seizures. Postnatal microcephaly (usually in the -2SD to -3SD range) is common. All have developmental delay. Speech development is significantly impaired and many affected individuals remain nonverbal. Intellectual disability in older children is usually moderate to severe.

The classic phenotype of megalencephalic leukoencephalopathy with subcortical cysts (MLC) is characterized by early-onset macrocephaly, often in combination with mild gross motor developmental delay and seizures; gradual onset of ataxia, spasticity, and sometimes extrapyramidal findings; and usually late onset of mild mental deterioration. Macrocephaly, observed in virtually all individuals, may be present at birth but more frequently develops during the first year of life. The degree of macrocephaly is variable and can be as great as 4 to 6 SD above the mean in some individuals. After the first year of life, head growth rate normalizes and growth follows a line parallel to and usually several centimeters above the 98th centile. Initial mental and motor development is normal in most individuals. Walking is often unstable, followed by ataxia of the trunk and extremities, then minor signs of pyramidal dysfunction and brisk deep-tendon stretch reflexes. Almost all individuals have epilepsy from an early age. The epilepsy is typically well controlled with anti-seizure medication, but status epilepticus occurs relatively frequently. Mental deterioration is late and mild. Disease severity ranges from independent walking for a few years only to independent walking in the fifth decade. Some individuals have died in their teens or twenties; others are alive in their fifties. An improving phenotype has a similar initial presentation with delayed mental or motor development, followed by an improving clinical course: macrocephaly usually persists, but some children become normocephalic; motor function improves or normalizes; hypotonia and clumsiness may persist in some or neurologic examination may become normal. Some have intellectual disability that is stable, with or without autism. Epilepsy and status epilepticus may occur.

An autosomal recessive muscular dystrophy caused by mutations in the POMGNT1 gene. It is associated with characteristic brain and eye malformations, profound mental retardation, and death usually in the first years of life.

Ogden syndrome (OGDNS) is an X-linked neurodevelopmental disorder characterized by postnatal growth failure, severely delayed psychomotor development, variable dysmorphic features, and hypotonia. Many patients also have cardiac malformations or arrhythmias (summary by Popp et al., 2015).

Intellectual disability-alacrima-achalasia syndrome is a rare, genetic intellectual disability syndrome characterized by delayed motor and cognitive development, absence or severe delay in speech development, intellectual disability, and alacrima. Achalasia/dysphagia and mild autonomic dysfunction (i.e. anisocoria) have also been reported in some patients. The phenotype is similar to the one observed in autosomal recessive Triple A syndrome, but differs by the presence of intellectual disability in all affected individuals.

Multiple congenital anomalies-hypotonia-seizures syndrome-2 (MCAHS2) is an X-linked recessive neurodevelopmental disorder characterized by dysmorphic features, neonatal hypotonia, early-onset myoclonic seizures, and variable congenital anomalies involving the central nervous, cardiac, and urinary systems. Some affected individuals die in infancy (summary by Johnston et al., 2012). The phenotype shows clinical variability with regard to severity and extraneurologic features. However, most patients present in infancy with early-onset epileptic encephalopathy associated with developmental arrest and subsequent severe neurologic disability; these features are consistent with a form of developmental and epileptic encephalopathy (DEE) (summary by Belet et al., 2014, Kato et al., 2014). The disorder is caused by a defect in glycosylphosphatidylinositol (GPI) biosynthesis. For a discussion of genetic heterogeneity of MCAHS, see MCAHS1 (614080). For a discussion of nomenclature and genetic heterogeneity of DEE, see 308350. For a discussion of genetic heterogeneity of GPI biosynthesis defects, see GPIBD1 (610293).

AP-4-associated hereditary spastic paraplegia (HSP), also known as AP-4 deficiency syndrome, is a group of neurodegenerative disorders characterized by a progressive, complex spastic paraplegia with onset typically in infancy or early childhood. Early-onset hypotonia evolves into progressive lower-extremity spasticity. The majority of children become nonambulatory and usually wheelchair bound. Over time spasticity progresses to involve the upper extremities, resulting in a spastic tetraplegia. Associated complications include dysphagia, contractures, foot deformities, dysregulation of bladder and bowel function, and a pseudobulbar affect. About 50% of affected individuals have seizures. Postnatal microcephaly (usually in the -2SD to -3SD range) is common. All have developmental delay. Speech development is significantly impaired and many affected individuals remain nonverbal. Intellectual disability in older children is usually moderate to severe.

Hydrolethalus syndrome is an autosomal recessive embryonic lethal disorder characterized by hydrocephaly or anencephaly, postaxial polydactyly of the upper limbs, and pre- or postaxial polydactyly of the lower limbs. Duplication of the hallux is a common finding. HLS2 is considered a ciliopathy (summary by Putoux et al., 2011). Acrocallosal syndrome (ACLS; 200990) is an allelic disorder with a less severe phenotype. For a discussion of genetic heterogeneity of hydrolethalus syndrome, see 236680.

RAB18 deficiency is the molecular deficit underlying both Warburg micro syndrome (characterized by eye, nervous system, and endocrine abnormalities) and Martsolf syndrome (characterized by similar – but milder – findings). To date Warburg micro syndrome comprises >96% of reported individuals with genetically defined RAB18 deficiency. The hallmark ophthalmologic findings are bilateral congenital cataracts, usually accompanied by microphthalmia, microcornea (diameter <10), and small atonic pupils. Poor vision despite early cataract surgery likely results from progressive optic atrophy and cortical visual impairment. Individuals with Warburg micro syndrome have severe to profound intellectual disability (ID); those with Martsolf syndrome have mild to moderate ID. Some individuals with RAB18 deficiency also have epilepsy. In Warburg micro syndrome, a progressive ascending spastic paraplegia typically begins with spastic diplegia and contractures during the first year, followed by upper-limb involvement leading to spastic quadriplegia after about age five years, often eventually causing breathing difficulties. In Martsolf syndrome infantile hypotonia is followed primarily by slowly progressive lower-limb spasticity. Hypogonadism – when present – manifests in both syndromes, in males as micropenis and/or cryptorchidism and in females as hypoplastic labia minora, clitoral hypoplasia, and small introitus.

GRIN1-related neurodevelopmental disorder (GRIN1-NDD) is characterized by mild-to-profound developmental delay / intellectual disability (DD/ID) in all affected individuals. Other common manifestations are epilepsy, muscular hypotonia, movement disorders, spasticity, feeding difficulties, and behavior problems. A subset of individuals show a malformation of cortical development consisting of extensive and diffuse bilateral polymicrogyria. To date, 72 individuals with GRIN1-NDD have been reported.

Brain small vessel disease-2 is an autosomal dominant disorder characterized by variable neurologic impairment resulting from disturbed vascular supply that leads to cerebral degeneration. The disorder is often associated with 'porencephaly' on brain imaging. Affected individuals typically have hemiplegia, seizures, and intellectual disability, although the severity is variable (summary by Yoneda et al., 2012). For a discussion of genetic heterogeneity of brain small vessel disease, see BSVD1 (175780).

Neurodevelopmental disorder with hypotonia, craniofacial abnormalities, and seizures (NEDHCS) is an autosomal recessive syndrome characterized primarily by hypotonia and poor feeding apparent in early infancy. Affected individuals have severe global developmental delay, early-onset intractable seizures, and recognizable craniofacial dysmorphism with skull abnormalities. The disorder is believed to be unique to the Amish population, where it exhibits a founder effect (summary by Ammous et al., 2021).

Baraitser-Winter cerebrofrontofacial (BWCFF) syndrome is a multiple congenital anomaly syndrome characterized by typical craniofacial features and intellectual disability. Many (but not all) affected individuals have pachygyria that is predominantly frontal, wasting of the shoulder girdle muscles, and sensory impairment due to iris or retinal coloboma and/or sensorineural deafness. Intellectual disability, which is common but variable, is related to the severity of the brain malformations. Seizures, congenital heart defects, renal malformations, and gastrointestinal dysfunction are also common.

Developmental and epileptic encephalopathy-1 (DEE1) is a severe form of epilepsy characterized by frequent tonic seizures or spasms beginning in infancy with a specific EEG finding of suppression-burst patterns, characterized by high-voltage bursts alternating with almost flat suppression phases. Approximately 75% of DEE1 patients progress to tonic spasms with clustering, arrest of psychomotor development, and hypsarrhythmia on EEG (Kato et al., 2007). DEE1 is part of a phenotypic spectrum of disorders caused by mutation in the ARX gene comprising a nearly continuous series of developmental disorders ranging from lissencephaly (LISX2; 300215) to Proud syndrome (300004) to infantile spasms without brain malformations (DEE) to syndromic (309510) and nonsyndromic (300419) mental retardation. Although males with ARX mutations are often more severely affected, female mutation carriers may also be affected (Kato et al., 2004; Wallerstein et al., 2008). Reviews Deprez et al. (2009) reviewed the genetics of epilepsy syndromes starting in the first year of life and included a diagnostic algorithm. Genetic Heterogeneity of Developmental and Epileptic Encephalopathy Also see DEE2 (300672), caused by mutation in the CDKL5 gene (300203); DEE3 (609304), caused by mutation in the SLC25A22 gene (609302); DEE4 (612164), caused by mutation in the STXBP1 gene (602926); DEE5 (613477), caused by mutation in the SPTAN1 gene (182810); DEE6A (607208), also known as Dravet syndrome, caused by mutation in the SCN1A gene (182389); DEE6B (619317), also caused by mutation in the SCN1A gene; DEE7 (613720), caused by mutation in the KCNQ2 gene (602235); DEE8 (300607), caused by mutation in the ARHGEF9 gene (300429); DEE9 (300088), caused by mutation in the PCDH19 gene (300460); DEE10 (613402), caused by mutation in the PNKP gene (605610); DEE11 (613721), caused by mutation in the SCN2A gene (182390); DEE12 (613722), caused by mutation in the PLCB1 gene (607120); DEE13 (614558), caused by mutation in the SCN8A gene (600702); DEE14 (614959), caused by mutation in the KCNT1 gene (608167); DEE15 (615006), caused by mutation in the ST3GAL3 gene (606494); DEE16 (615338), caused by mutation in the TBC1D24 gene (613577); DEE17 (615473), caused by mutation in the GNAO1 gene (139311); DEE18 (615476), caused by mutation in the SZT2 gene (615463); DEE19 (615744), caused by mutation in the GABRA1 gene (137160); DEE20 (300868), caused by mutation in the PIGA gene (311770); DEE21 (615833), caused by mutation in the NECAP1 gene (611623); DEE22 (300896), caused by mutation in the SLC35A2 gene (314375); DEE23 (615859), caused by mutation in the DOCK7 gene (615730); DEE24 (615871), caused by mutation in the HCN1 gene (602780); DEE25 (615905), caused by mutation in the SLC13A5 gene (608305); DEE26 (616056), caused by mutation in the KCNB1 gene (600397); DEE27 (616139), caused by mutation in the GRIN2B gene (138252); DEE28 (616211), caused by mutation in the WWOX gene (605131); DEE29 (616339), caused by mutation in the AARS gene (601065); DEE30 (616341), caused by mutation in the SIK1 gene (605705); DEE31A (616346) and DEE31B (620352), caused by mutation in the DNM1 gene (602377); DEE32 (616366), caused by mutation in the KCNA2 gene (176262); DEE33 (616409), caused by mutation in the EEF1A2 gene (602959); DEE34 (616645), caused by mutation in the SLC12A5 gene (606726); DEE35 (616647), caused by mutation in the ITPA gene (147520); DEE36 (300884), caused by mutation in the ALG13 gene (300776); DEE37 (616981), caused by mutation in the FRRS1L gene (604574); DEE38 (617020), caused by mutation in the ARV1 gene (611647); DEE39 (612949), caused by mutation in the SLC25A12 gene (603667); DEE40 (617065), caused by mutation in the GUF1 gene (617064); DEE41 (617105), caused by mutation in the SLC1A2 gene (600300); DEE42 (617106), caused by mutation in the CACNA1A gene (601011); DEE43 (617113), caused by mutation in the GABRB3 gene (137192); DEE44 (617132), caused by mutation in the UBA5 gene (610552); DEE45 (617153), caused by mutation in the GABRB1 gene (137190); DEE46 (617162), caused by mutation in the GRIN2D gene (602717); DEE47 (617166), caused by mutation in the FGF12 gene (601513); DEE48 (617276), caused by mutation in the AP3B2 gene (602166); DEE49 (617281), caused by mutation in the DENND5A gene (617278); DEE50 (616457) caused by mutation in the CAD gene (114010); DEE51 (617339), caused by mutation in the MDH2 gene (154100); DEE52 (617350), caused by mutation in the SCN1B gene (600235); DEE53 (617389), caused by mutation in the SYNJ1 gene (604297); DEE54 (617391), caused by mutation in the HNRNPU gene (602869); DEE55 (617599), caused by mutation in the PIGP gene (605938); DEE56 (617665), caused by mutation in the YWHAG gene (605356); DEE57 (617771), caused by mutation in the KCNT2 gene (610044); DEE58 (617830), caused by mutation in the NTRK2 gene (600456); DEE59 (617904), caused by mutation in the GABBR2 gene (607340); DEE60 (617929), caused by mutation in the CNPY3 gene (610774); DEE61 (617933), caused by mutation in the ADAM22 gene (603709); DEE62 (617938), caused by mutation in the SCN3A gene (182391); DEE63 (617976), caused by mutation in the CPLX1 gene (605032); DEE64 (618004), caused by mutation in the RHOBTB2 gene (607352); DEE65 (618008), caused by mutation in the CYFIP2 gene (606323); DEE66 (618067), caused by mutation in the PACS2 gene (610423); DEE67 (618141), caused by mutation in the CUX2 gene (610648); DEE68 (618201), caused by mutation in the TRAK1 gene (608112); DEE69 (618285), caused by mutation in the CACNA1E gene (601013); DEE70 (618298) caused by mutation in the PHACTR1 gene (608723); DEE71 (618328), caused by mutation in the GLS gene (138280); DEE72 (618374), caused by mutation in the NEUROD2 gene (601725); DEE73 (618379), caused by mutation in the RNF13 gene (609247); DEE74 (618396), caused by mutation in the GABRG2 gene (137164); DEE75 (618437), caused by mutation in the PARS2 gene (612036); DEE76 (618468), caused by mutation in the ACTL6B gene (612458); DEE77 (618548), caused by mutation in the PIGQ gene (605754); DEE78 (618557), caused by mutation in the GABRA2 gene (137140); DEE79 (618559), caused by mutation in the GABRA5 gene (137142); DEE80 (618580), caused by mutation in the PIGB gene (604122); DEE81 (618663), caused by mutation in the DMXL2 gene (612186); DEE82 (618721), caused by mutation in the GOT2 gene (138150); DEE83 (618744), caused by mutation in the UGP2 gene (191760); DEE84 (618792), caused by mutation in the UGDH gene (603370); DEE85 (301044), caused by mutation in the SMC1A gene (300040); DEE86 (618910), caused by mutation in the DALRD3 gene (618904); DEE87 (618916), caused by mutation in the CDK19 gene (614720); DEE88 (618959), caused by mutation in the MDH1 gene (152400); DEE89 (619124), caused by mutation in the GAD1 gene (605363); DEE90 (301058), caused by mutation in the FGF13 gene (300070); DEE91 (617711), caused by mutation in the PPP3CA gene (114105); DEE92 (617829), caused by mutation in the GABRB2 gene (600232); DEE93 (618012), caused by mutation in the ATP6V1A gene (607027); DEE94 (615369), caused by mutation in the CHD2 gene (602119); DEE95 (618143), caused by mutation in the PIGS gene (610271); DEE96 (619340), caused by mutation in the NSF gene (601633); DEE97 (619561), caused by mutation in the iCELF2 gene (602538); DEE98 (619605), caused by mutation in the ATP1A2 gene (182340); DEE99 (619606), caused by mutation in the ATP1A3 gene (182350); DEE100 (619777), caused by mutation in the FBXO28 gene (609100); DEE101 (619814), caused by mutation in the GRIN1 gene (138249); DEE102 (619881), caused by mutation in the SLC38A3 gene (604437); DEE103 (619913), caused by mutation in the KCNC2 gene (176256); DEE104 (619970), caused by mutation in the ATP6V0A1 gene (192130); DEE105 (619983), caused by mutation in the HID1 gene (605752); DEE106 (620028), caused by mutation in the UFSP2 gene (611482); DEE107 (620033), caused by mutation in the NAPB gene (

Cornelia de Lange syndrome (CdLS) encompasses a spectrum of findings from mild to severe. Severe (classic) CdLS is characterized by distinctive facial features, growth restriction (prenatal onset; <5th centile throughout life), hypertrichosis, and upper-limb reduction defects that range from subtle phalangeal abnormalities to oligodactyly (missing digits). Craniofacial features include synophrys, highly arched and/or thick eyebrows, long eyelashes, short nasal bridge with anteverted nares, small widely spaced teeth, and microcephaly. Individuals with a milder phenotype have less severe growth, cognitive, and limb involvement, but often have facial features consistent with CdLS. Across the CdLS spectrum IQ ranges from below 30 to 102 (mean: 53). Many individuals demonstrate autistic and self-destructive tendencies. Other frequent findings include cardiac septal defects, gastrointestinal dysfunction, hearing loss, myopia, and cryptorchidism or hypoplastic genitalia.

CDG2L is an autosomal recessive multisystem disorder apparent from birth or early infancy. It is characterized by poor growth, gastrointestinal and liver abnormalities, delayed psychomotor development, hypotonia, recurrent infections, hematologic abnormalities, increased bleeding tendency, and hyperhidrosis or hyperkeratosis. More variable features include nonspecific dysmorphic facial features and cardiac septal defects. The disorder often results in death in infancy or the first years of life (summary by Rymen et al., 2015). For a general discussion of CDGs, see CDG1A (212065) and CDG2A (212066).

Congenital muscular dystrophy-dystroglycanopathy with brain and eye anomalies (type A), which includes both the more severe Walker-Warburg syndrome (WWS) and the slightly less severe muscle-eye-brain disease (MEB), is an autosomal recessive disorder with characteristic brain and eye malformations, profound mental retardation, congenital muscular dystrophy, and death usually in the first years of life. It represents the most severe end of a phenotypic spectrum of similar disorders resulting from defective glycosylation of alpha-dystroglycan (DAG1; 128239), collectively known as 'dystroglycanopathies' (summary by Roscioli et al., 2012). For a general phenotypic description and a discussion of genetic heterogeneity of muscular dystrophy-dystroglycanopathy type A, see MDDGA1 (236670).

Cornelia de Lange syndrome (CdLS) encompasses a spectrum of findings from mild to severe. Severe (classic) CdLS is characterized by distinctive facial features, growth restriction (prenatal onset; <5th centile throughout life), hypertrichosis, and upper-limb reduction defects that range from subtle phalangeal abnormalities to oligodactyly (missing digits). Craniofacial features include synophrys, highly arched and/or thick eyebrows, long eyelashes, short nasal bridge with anteverted nares, small widely spaced teeth, and microcephaly. Individuals with a milder phenotype have less severe growth, cognitive, and limb involvement, but often have facial features consistent with CdLS. Across the CdLS spectrum IQ ranges from below 30 to 102 (mean: 53). Many individuals demonstrate autistic and self-destructive tendencies. Other frequent findings include cardiac septal defects, gastrointestinal dysfunction, hearing loss, myopia, and cryptorchidism or hypoplastic genitalia.

Hyperphosphatasia with impaired intellectual development syndrome-2 (HPMRS2) is an autosomal recessive disorder characterized by moderately to severely delayed psychomotor development, facial dysmorphism, brachytelephalangy, and increased serum alkaline phosphatase (hyperphosphatasia). Some patients may have additional features, such as cardiac septal defects or seizures (summary by Krawitz et al., 2012). The disorder is caused by a defect in glycosylphosphatidylinositol (GPI) biosynthesis. For a discussion of genetic heterogeneity of hyperphosphatasia with impaired intellectual development syndrome, see HPMRS1 (239300). For a discussion of genetic heterogeneity of GPI biosynthesis defects, see GPIBD1 (610293).

Malan syndrome (MALNS) is clinically characterized by overgrowth, advanced bone age, macrocephaly, and dysmorphic facial features. Patients develop marfanoid habitus, with long and slender body, very low body mass, long narrow face, and arachnodactyly, with age. Impaired intellectual development and behavior anomalies are present (summary by Martinez et al., 2015).

Congenital muscular dystrophy-dystroglycanopathy with brain and eye anomalies (type A) is an autosomal recessive disorder with characteristic brain and eye malformations, profound mental retardation, congenital muscular dystrophy, and death usually in the first years of life. The phenotype includes the alternative clinical designation Walker-Warburg syndrome (WWS). The disorder represents the most severe end of a phenotypic spectrum of similar disorders resulting from defective glycosylation of alpha-dystroglycan (DAG1; 128239), collectively known as 'dystroglycanopathies' (summary by Manzini et al., 2012). For a general phenotypic description and a discussion of genetic heterogeneity of muscular dystrophy-dystroglycanopathy type A, see MDDGA1 (236670).

Pontocerebellar hypoplasia type 7 (PCH7) is a severe neurologic condition characterized by delayed psychomotor development, hypotonia, breathing abnormalities, and gonadal abnormalities (summary by Anderson et al., 2011). For a general phenotypic description and a discussion of genetic heterogeneity of PCH, see PCH1 (607596).

Congenital hydrocephalus-2 (HYC2) is a congenital disorder with onset in utero. Affected individuals have hydrocephalus with variably dilated ventricles and variable neurologic sequelae. Some individuals have other brain abnormalities, including lissencephaly, thinning of the corpus callosum, and neuronal heterotopia. Most patients have delayed motor development and some have delayed intellectual development and/or seizures. Additional congenital features, including cardiac septal defects, iris coloboma, and nonspecific dysmorphic features, may be observed. Some patients die in utero, in infancy, or in early childhood, whereas others have long-term survival (summary by Shaheen et al., 2017). For a discussion of genetic heterogeneity of congenital hydrocephalus, see 233600.

Meckel syndrome, also known as Meckel-Gruber syndrome, is a severe pleiotropic autosomal recessive developmental disorder caused by dysfunction of primary cilia during early embryogenesis. There is extensive clinical variability and controversy as to the minimum diagnostic criteria. Early reports, including that of Opitz and Howe (1969) and Wright et al. (1994), stated that the classic triad of Meckel syndrome comprises (1) cystic renal disease; (2) a central nervous system malformation, most commonly occipital encephalocele; and (3) polydactyly, most often postaxial. However, based on a study of 67 patients, Salonen (1984) concluded that the minimum diagnostic criteria are (1) cystic renal disease; (2) CNS malformation, and (3) hepatic abnormalities, including portal fibrosis or ductal proliferation. In a review of Meckel syndrome, Logan et al. (2011) stated that the classic triad first described by Meckel (1822) included occipital encephalocele, cystic kidneys, and fibrotic changes to the liver. Genetic Heterogeneity of Meckel Syndrome See also MKS2 (603194), caused by mutation in the TMEM216 gene (613277) on chromosome 11q12; MKS3 (607361), caused by mutation in the TMEM67 gene (609884) on chromosome 8q; MKS4 (611134), caused by mutation in the CEP290 gene (610142) on chromosome 12q; MKS5 (611561), caused by mutation in the RPGRIP1L gene (610937) on chromosome 16q12; MKS6 (612284), caused by mutation in the CC2D2A gene (612013) on chromosome 4p15; MKS7 (267010), caused by mutation in the NPHP3 (608002) gene on chromosome 3q22; MKS8 (613885), caused by mutation in the TCTN2 gene (613846) on chromosome 12q24; MKS9 (614209), caused by mutation in the B9D1 gene (614144) on chromosome 17p11; MKS10 (614175), caused by mutation in the B9D2 gene (611951) on chromosome 19q13; MKS11 (615397), caused by mutation in the TMEM231 gene (614949) on chromosome 16q23; MKS12 (616258), caused by mutation in the KIF14 gene (611279) on chromosome 1q32; MKS13 (617562), caused by mutation in the TMEM107 gene (616183) on chromosome 17p13; and MKS14 (619879), caused by mutation in the TXNDC15 gene (617778) on chromosome 5q31.

Neuronal ceroid lipofuscinosis-13 (CLN13) is an autosomal recessive neurodegenerative disorder characterized by adult onset of progressive cognitive decline and motor dysfunction leading to dementia and often early death. Some patients develop seizures. Neurons show abnormal accumulation of autofluorescent material (summary by Smith et al., 2013). Adult-onset neuronal ceroid lipofuscinosis is sometimes referred to as Kufs disease (see 204300). In a review of the classification of CLN disease, Gardner and Mole (2021) noted that the CLN13 phenotype corresponds to 'Kufs type B', which is characterized by dementia and a variety of motor signs (Smith et al., 2013). For a discussion of genetic heterogeneity of neuronal ceroid lipofuscinosis (CLN), see CLN1 (256730).

Congenital muscular dystrophy-dystroglycanopathy with brain and eye anomalies (type A) is a autosomal recessive disorder associated with severe neurologic defects and resulting in early infantile death. The phenotype includes the alternative clinical designations Walker-Warburg syndrome (WWS) and muscle-eye-brain disease (MEB). The disorder represents the most severe end of a phenotypic spectrum of similar disorders resulting from defective glycosylation of alpha-dystroglycan (DAG1; 128239), collectively known as dystroglycanopathies (summary by Buysse et al., 2013). For a general phenotypic description and a discussion of genetic heterogeneity of muscular dystrophy-dystroglycanopathy type A, see MDDGA1 (236670).

Multiple mitochondrial dysfunctions syndrome-3 (MMDS3) is an autosomal recessive severe neurodegenerative disorder characterized by loss of previously acquired developmental milestones in the first months or years of life. Some affected patients have normal development in early infancy before the onset of symptoms, whereas others show delays from birth. Features included loss of motor function, spasticity, pyramidal signs, loss of speech, and cognitive impairment. The disease course is highly variable: some patients die of respiratory failure early in childhood, whereas some survive but may be bedridden with a feeding tube. Less commonly, some patients may survive and have a stable course with motor deficits and mild or even absent cognitive impairment, although there may be fluctuating symptoms, often in response to infection. Other variable features include visual problems and seizures. Brain imaging shows diffuse leukodystrophy in the subcortical region, brainstem, cerebellum, and spinal cord. Laboratory studies tend to show increased lactate and CSF glycine, and decreased activity of mitochondrial complexes I and II, although these findings are also variable. There may be additional biochemical evidence of mitochondrial dysfunction (summary by Liu et al., 2018). For a general description and a discussion of genetic heterogeneity of multiple mitochondrial dysfunctions syndrome, see MMDS1 (605711).

The chromosome 3q13.31 deletion syndrome is characterized by marked developmental delay, characteristic facies with a short philtrum and protruding lips, and abnormal male genitalia (Molin et al., 2012). Patients with Primrose syndrome (PRIMS; 259050) exhibit features overlapping those of the chromosome 3q13.31 deletion syndrome but also have ossified ear cartilage, severe muscle wasting, and abnormalities of glucose metabolism resulting in insulin-resistant diabetes mellitus in adulthood. Primrose syndrome is caused by mutation in the ZBTB20 gene (606025) on chromosome 3q13.

FBXL4-related encephalomyopathic mitochondrial DNA (mtDNA) depletion syndrome is a multi-system disorder characterized primarily by congenital or early-onset lactic acidosis and growth failure, feeding difficulty, hypotonia, and developmental delay. Other neurologic manifestations can include seizures, movement disorders, ataxia, autonomic dysfunction, and stroke-like episodes. All affected individuals alive at the time they were reported (median age: 3.5 years) demonstrated significant developmental delay. Other findings can involve the heart (hypertrophic cardiomyopathy, congenital heart malformations, arrhythmias), liver (mildly elevated transaminases), eyes (cataract, strabismus, nystagmus, optic atrophy), hearing (sensorineural hearing loss), and bone marrow (neutropenia, lymphopenia). Survival varies; the median age of reported deaths was two years (range 2 days – 75 months), although surviving individuals as old as 36 years have been reported. To date FBXL4-related mtDNA depletion syndrome has been reported in 50 individuals.

A rare, genetic, neurodevelopmental disorder characterized by global developmental delay, borderline to severe intellectual disability, feeding difficulties, behavioral anomalies, vision anomalies and mild facial dysmorphism. Other associated features may include microcephaly, short stature, urogenital or palatal anomalies (e.g. cleft palate), minor cardiac defects, recurrent infections or hearing loss.

Asparagine synthetase deficiency (ASD) mainly presents as a triad of congenital microcephaly, severe developmental delay, and axial hypotonia followed by spastic quadriplegia. Low cerebrospinal fluid (CSF) asparagine level can help the clinician in differentiating this disorder from others. In most cases age of onset of apnea, excessive irritability, and seizures is soon after birth. Affected individuals typically do not acquire any developmental milestones. Spastic quadriplegia can lead to severe contractures of the limbs and neurogenic scoliosis. Feeding difficulties (gastroesophageal reflux disease, frequent vomiting, swallowing dysfunction, and gastroesophageal incoordination) are a significant problem in most affected individuals. A majority have cortical blindness. MRI findings are nonspecific but may include generalized atrophy and simplified gyral pattern.

Short-rib thoracic dysplasia (SRTD) with or without polydactyly refers to a group of autosomal recessive skeletal ciliopathies that are characterized by a constricted thoracic cage, short ribs, shortened tubular bones, and a 'trident' appearance of the acetabular roof. SRTD encompasses Ellis-van Creveld syndrome (EVC) and the disorders previously designated as Jeune syndrome or asphyxiating thoracic dystrophy (ATD), short rib-polydactyly syndrome (SRPS), and Mainzer-Saldino syndrome (MZSDS). Polydactyly is variably present, and there is phenotypic overlap in the various forms of SRTDs, which differ by visceral malformation and metaphyseal appearance. Nonskeletal involvement can include cleft lip/palate as well as anomalies of major organs such as the brain, eye, heart, kidneys, liver, pancreas, intestines, and genitalia. Some forms of SRTD are lethal in the neonatal period due to respiratory insufficiency secondary to a severely restricted thoracic cage, whereas others are compatible with life (summary by Huber and Cormier-Daire, 2012 and Schmidts et al., 2013). There is phenotypic overlap with the cranioectodermal dysplasias (Sensenbrenner syndrome; see CED1, 218330). For a discussion of genetic heterogeneity of short-rib thoracic dysplasia, see SRTD1 (208500).

Autosomal recessive intellectual developmental disorder-41 (MRT41) is characterized by macrocephaly and global developmental delay. Some patients have seizures (Baple et al., 2014).

An autosomal dominant condition caused by mutation(s) in the TUBB2A gene, encoding tubulin beta-2A chain. It is characterized by cortical dysplasia and is associated with impaired intellectual development, hypotonia, global developmental delay, cortical dysplasia, and dysmorphic corpus callosum.

Congenital hydrocephalus-1 (HYC1) is characterized by onset in utero of enlarged ventricles due to a disturbance of cerebrospinal fluid accumulation. Affected individuals may have neurologic impairment (summary by Drielsma et al., 2012). Hydrocephalus can also be caused by Arnold-Chiari malformation, atresia of foramen of Magendie, stenosis of aqueduct of Sylvius (307000), toxoplasmosis, hydranencephaly, etc. Furthermore, it develops in infancy or childhood in achondroplasia (100800) and in Hurler disease (607014). Genetic Heterogeneity of Congenital Hydrocephalus See also HYC2 (615219), caused by mutation in the MPDZ gene (603785) on chromosome 9p23; HYC3 (617967), caused by mutation in the WDR81 gene (614218) on chromosome 17p13; HYC4 (618667), caused by mutation in the TRIM71 gene (618570) on chromosome 3p22; and HYC5 (620241), caused by mutation in the SMARCC1 gene (601732) on chromosome 3p23. An X-linked form of congenital hydrocephalus (HSAS, HYCX; 307000) is caused by mutation in the L1CAM gene on (308840) on chromosome Xq28.

MPPH (megalencephaly-postaxial polydactyly-polymicrogyria-hydrocephalus) syndrome is a developmental brain disorder characterized by megalencephaly (brain overgrowth) with the cortical malformation bilateral perisylvian polymicrogyria (BPP). At birth the occipital frontal circumference (OFC) ranges from normal to 6 standard deviations (SD) above the mean for age, sex, and gestational age; in older individuals the range is from 3 to 10 SD above the mean. A variable degree of ventriculomegaly is seen in almost all children with MPPH syndrome; nearly 50% of individuals have frank hydrocephalus. Neurologic problems associated with BPP include oromotor dysfunction (100%), epilepsy (50%), and mild-to-severe intellectual disability (100%). Postaxial hexadactyly occurs in 50% of individuals with MPPH syndrome.

Polymicrogyria is a condition characterized by abnormal development of the brain before birth. The surface of the brain normally has many ridges or folds, called gyri. In people with polymicrogyria, the brain develops too many folds, and the folds are unusually small. The name of this condition literally means too many (poly-) small (micro-) folds (-gyria) in the surface of the brain.\n\nPolymicrogyria can affect part of the brain or the whole brain. When the condition affects one side of the brain, researchers describe it as unilateral. When it affects both sides of the brain, it is described as bilateral. The signs and symptoms associated with polymicrogyria depend on how much of the brain, and which particular brain regions, are affected.\n\nResearchers have identified multiple forms of polymicrogyria. The mildest form is known as unilateral focal polymicrogyria. This form of the condition affects a relatively small area on one side of the brain. It may cause minor neurological problems, such as mild seizures that can be easily controlled with medication. Some people with unilateral focal polymicrogyria do not have any problems associated with the condition.\n\nBilateral forms of polymicrogyria tend to cause more severe neurological problems. Signs and symptoms of these conditions can include recurrent seizures (epilepsy), delayed development, crossed eyes, problems with speech and swallowing, and muscle weakness or paralysis. The most severe form of the disorder, bilateral generalized polymicrogyria, affects the entire brain. This condition causes severe intellectual disability, problems with movement, and seizures that are difficult or impossible to control with medication.\n\nPolymicrogyria most often occurs as an isolated feature, although it can occur with other brain abnormalities. It is also a feature of several genetic syndromes characterized by intellectual disability and multiple birth defects. These include 22q11.2 deletion syndrome, Adams-Oliver syndrome, Aicardi syndrome, Galloway-Mowat syndrome, Joubert syndrome, and Zellweger spectrum disorder.

Progressive microcephaly with seizures and cerebral and cerebellar atrophy is a severe autosomal recessive neurodevelopmental and neurodegenerative disorder with onset in the first days or months of life. Patients are born with microcephaly and soon develop intractable seizures, resulting in profoundly delayed development and hypotonia (summary by Zhang et al., 2014).

Pontocerebellar hypoplasia type 9 (PCH9) is an autosomal recessive neurodevelopmental and neurodegenerative disorder characterized by severely delayed psychomotor development, progressive microcephaly, spasticity, seizures, and brain abnormalities, including brain atrophy, thin corpus callosum, and delayed myelination (summary by Akizu et al., 2013). For a general phenotypic description and a discussion of genetic heterogeneity of PCH, see PCH1 (607596).

Pontocerebellar hypoplasia type 2E is an autosomal recessive neurodegenerative disorder characterized by profound mental retardation, progressive microcephaly, spasticity, and early-onset epilepsy (summary by Feinstein et al., 2014). For a general phenotypic description and a discussion of genetic heterogeneity of pontocerebellar hypoplasia type 2, see PCH2A (277470).

ADNP-related disorder is characterized by hypotonia, severe speech and motor delay, mild-to-severe intellectual disability, and characteristic facial features (prominent forehead, high anterior hairline, wide and depressed nasal bridge, and short nose with full, upturned nasal tip) based on a cohort of 78 individuals. Features of autism spectrum disorder are common (stereotypic behavior, impaired social interaction). Other common findings include additional behavioral problems, sleep disturbance, brain abnormalities, seizures, feeding issues, gastrointestinal problems, visual dysfunction (hypermetropia, strabismus, cortical visual impairment), musculoskeletal anomalies, endocrine issues including short stature and hormonal deficiencies, cardiac and urinary tract anomalies, and hearing loss.

Tatton-Brown-Rahman syndrome (TBRS) is an overgrowth / intellectual disability syndrome characterized by length/height and/or head circumference =2 SD above the mean for age and sex, obesity / increased weight, intellectual disability that ranges from mild to severe, joint hypermobility, hypotonia, behavioral/psychiatric issues, kyphoscoliosis, and seizures. Individuals with TBRS have subtle dysmorphic features, including a round face with coarse features, thick horizontal low-set eyebrows, narrow (as measured vertically) palpebral fissures, and prominent upper central incisors. The facial gestalt is most easily recognizable in the teenage years. TBRS may be associated with an increased risk of developing acute myeloid leukemia. There are less clear associations with aortic root dilatation and increased risk of other hematologic and solid tumors.

MPPH (megalencephaly-postaxial polydactyly-polymicrogyria-hydrocephalus) syndrome is a developmental brain disorder characterized by megalencephaly (brain overgrowth) with the cortical malformation bilateral perisylvian polymicrogyria (BPP). At birth the occipital frontal circumference (OFC) ranges from normal to 6 standard deviations (SD) above the mean for age, sex, and gestational age; in older individuals the range is from 3 to 10 SD above the mean. A variable degree of ventriculomegaly is seen in almost all children with MPPH syndrome; nearly 50% of individuals have frank hydrocephalus. Neurologic problems associated with BPP include oromotor dysfunction (100%), epilepsy (50%), and mild-to-severe intellectual disability (100%). Postaxial hexadactyly occurs in 50% of individuals with MPPH syndrome.

MPPH (megalencephaly-postaxial polydactyly-polymicrogyria-hydrocephalus) syndrome is a developmental brain disorder characterized by megalencephaly (brain overgrowth) with the cortical malformation bilateral perisylvian polymicrogyria (BPP). At birth the occipital frontal circumference (OFC) ranges from normal to 6 standard deviations (SD) above the mean for age, sex, and gestational age; in older individuals the range is from 3 to 10 SD above the mean. A variable degree of ventriculomegaly is seen in almost all children with MPPH syndrome; nearly 50% of individuals have frank hydrocephalus. Neurologic problems associated with BPP include oromotor dysfunction (100%), epilepsy (50%), and mild-to-severe intellectual disability (100%). Postaxial hexadactyly occurs in 50% of individuals with MPPH syndrome.

Neu-Laxova syndrome-2 (NLS2) is a rare autosomal recessive disorder characterized by a recognizable pattern of severe congenital malformations leading to prenatal or early postnatal lethality. Affected individuals have abnormal craniofacial features, microcephaly, intrauterine growth retardation, ichthyosis, flexion deformities, limb malformations, and edema of the hands and feet. Some patients have malformations of the central nervous system, such as abnormal gyration (summary by Acuna-Hidalgo et al., 2014). For a discussion of genetic heterogeneity of Neu-Laxova syndrome, see NLS1 (256520).

Any autosomal recessive non-syndromic intellectual disability in which the cause of the disease is a mutation in the NDST1 gene.

Cerebellofaciodental syndrome is an autosomal recessive neurodevelopmental disorder characterized by delayed development, intellectual disability, abnormal facial and dental findings, and cerebellar hypoplasia (summary by Borck et al., 2015).

Lissencephaly-6 (LIS6) is an autosomal recessive neurodevelopmental disorder characterized by severe microcephaly and developmental delay. Brain imaging shows variable malformations of cortical development, including lissencephaly, pachygyria, and hypoplasia of the corpus callosum (summary by Mishra-Gorur et al., 2014). For a general description and a discussion of genetic heterogeneity of lissencephaly, see LIS1 (607432).

Tenorio syndrome (TNORS) is characterized by overgrowth, macrocephaly, and impaired intellectual development. Some patients may have mild hydrocephaly, hypoglycemia, and inflammatory diseases resembling Sjogren syndrome (270150) (summary by Tenorio et al., 2014).

Luscan-Lumish syndrome (LLS) is characterized by macrocephaly, intellectual disability, speech delay, low sociability, and behavioral problems. More variable features include postnatal overgrowth, obesity, advanced carpal ossification, developmental delay, and seizures (Luscan et al., 2014; Lumish et al., 2015)

Complex lethal osteochondrodysplasia of the Symoens-Barnes-Gistelinck type is characterized by severe skeletal osteopenia, microcephaly, multiple fractures, and congenital anomalies including ascites, pleural effusion, and intracranial ventriculomegaly (Symoens et al., 2015).

Any Seckel syndrome in which the cause of the disease is a mutation in the TRAIP gene.

Takenouchi-Kosaki syndrome is a highly heterogeneous autosomal dominant complex congenital developmental disorder affecting multiple organ systems. The core phenotype includes delayed psychomotor development with variable intellectual disability, dysmorphic facial features, and cardiac, genitourinary, and hematologic or lymphatic defects, including thrombocytopenia and lymphedema. Additional features may include abnormalities on brain imaging, skeletal anomalies, and recurrent infections. Some patients have a milder disease course reminiscent of Noonan syndrome (see, e.g., NS1, 163950) (summary by Martinelli et al., 2018).

Congenital symmetric circumferential skin creases is characterized by the folding of excess skin, which leads to ringed creases, primarily of the limbs. Affected individuals also exhibit intellectual disability, cleft palate, and dysmorphic features (summary by Isrie et al., 2015). For a discussion of genetic heterogeneity of congenital symmetric circumferential skin creases, see CSCSC1 (156610).

Hypomyelinating leukodystrophy-12 (HLD12) is an autosomal recessive neurologic disorder characterized by severely delayed or even lack of psychomotor development that becomes apparent in the first months of life. Patients are markedly disabled, with acquired microcephaly, lack of speech, and often lack of spontaneous movement due to hypotonia and spasticity. Brain imaging shows delayed myelination (summary by Edvardson et al., 2015). For a general phenotypic description and a discussion of genetic heterogeneity of HLD, see 312080. In a review of the pathogenesis of disorders with prominent dystonia or opisthotonic posturing as a feature, Monfrini et al. (2021) classified HLD12 as belonging to a group of neurologic disorders termed 'HOPS-associated neurologic disorders (HOPSANDs), which are caused by mutations in genes encoding various components of the autophagic/endolysosomal system, including VPS11.

Spastic paraplegia-75 (SPG75) is an autosomal recessive, slowly progressive neurodegenerative disorder characterized by onset of spastic paraplegia and cognitive impairment in childhood (summary by Lossos et al., 2015). For a discussion of genetic heterogeneity of autosomal recessive SPG, see SPG5A (270800).

Smith-Kingsmore syndrome (SKS) is a rare autosomal dominant syndromic intellectual disability syndrome characterized by macrocephaly, seizures, umbilical hernia, and facial dysmorphic features including frontal bossing, midface hypoplasia, small chin, hypertelorism with downslanting palpebral fissures, depressed nasal bridge, smooth philtrum, and thin upper lip (Smith et al., 2013; Baynam et al., 2015).

Au-Kline syndrome is characterized by developmental delay and hypotonia with moderate-to-severe intellectual disability, and typical facial features that include long palpebral fissures, ptosis, shallow orbits, large and deeply grooved tongue, broad nose with a wide nasal bridge, and downturned mouth. There is frequently variable autonomic dysfunction (gastrointestinal dysmotility, high pain threshold, heat intolerance, recurrent fevers, abnormal sweating). Congenital heart disease, hydronephrosis, palate abnormalities, and oligodontia are also reported in the majority of affected individuals. Additional complications can include craniosynostosis, feeding difficulty, vision issues, osteopenia, and other skeletal anomalies.

Short-rib thoracic dysplasia (SRTD) with or without polydactyly refers to a group of autosomal recessive skeletal ciliopathies that are characterized by a constricted thoracic cage, short ribs, shortened tubular bones, and a 'trident' appearance of the acetabular roof. SRTD encompasses Ellis-van Creveld syndrome (EVC) and the disorders previously designated as Jeune syndrome or asphyxiating thoracic dystrophy (ATD), short rib-polydactyly syndrome (SRPS), and Mainzer-Saldino syndrome (MZSDS). Polydactyly is variably present, and there is phenotypic overlap in the various forms of SRTDs, which differ by visceral malformation and metaphyseal appearance. Nonskeletal involvement can include cleft lip/palate as well as anomalies of major organs such as the brain, eye, heart, kidneys, liver, pancreas, intestines, and genitalia. Some forms of SRTD are lethal in the neonatal period due to respiratory insufficiency secondary to a severely restricted thoracic cage, whereas others are compatible with life (summary by Huber and Cormier-Daire, 2012 and Schmidts et al., 2013). There is phenotypic overlap with the cranioectodermal dysplasias (Sensenbrenner syndrome; see CED1, 218330). For a discussion of genetic heterogeneity of short-rib thoracic dysplasia with or without polydactyly, see SRTD1 (208500).

In patients with SSMED, short stature and microcephaly are apparent at birth, and there is progressive postnatal growth failure. Endocrine dysfunction, including hypergonadotropic hypogonadism, multinodular goiter, and diabetes mellitus, is present in affected adults. Progressive ataxia has been reported in some patients, with onset ranging from the second to fifth decade of life. In addition, a few patients have developed tumors, suggesting that there may be a predisposition to tumorigenesis. In contrast to syndromes involving defects in other components of the nonhomologous end-joining (NHEJ) complex (see, e.g., 606593), no clinically overt immunodeficiency has been observed in SSMED, although laboratory analysis has revealed lymphopenia or borderline leukopenia in some patients (Murray et al., 2015; Bee et al., 2015; de Bruin et al., 2015; Guo et al., 2015).

A rare genetic neurological disorder with characteristics of childhood-onset severe myoclonic and tonic-clonic seizures and early-onset ataxia leading to severe gait disturbances associated with normal to slightly diminished cognition. Scoliosis, diffuse muscle atrophy and subcutaneous fat loss, as well as developmental delay, may be associated. Brain MRI may reveal complete agenesis of the corpus callosum, ventriculomegaly, interhemispheric cysts and simplified gyration (frontally).

Congenital muscular dystrophy-dystroglycanopathy with brain and eye anomalies (type A) is an autosomal recessive disorder with characteristic brain and eye malformations, profound mental retardation, and congenital muscular dystrophy. The phenotype includes the alternative clinical designation Walker-Warburg syndrome (WWS), which is associated with death in infancy. The disorder represents the most severe end of a phenotypic spectrum of similar disorders resulting from defective glycosylation of alpha-dystroglycan (DAG1), collectively known as 'dystroglycanopathies' (summary by Geis et al., 2013 and Riemersma et al., 2015). For a general phenotypic description and a discussion of genetic heterogeneity of muscular dystrophy-dystroglycanopathy type A, see MDDGA1 (236670).

PI4KA-related disorder is a clinically variable disorder characterized primarily by neurologic dysfunction (limb spasticity, developmental delay, intellectual disability, seizures, ataxia, nystagmus), gastrointestinal manifestations (multiple intestinal atresia, inflammatory bowel disease), and combined immunodeficiency (leukopenia, variable immunoglobulin defects). Age of onset is typically antenatal or in early childhood; individuals can present with any combination of these features. Rare individuals present with later-onset hereditary spastic paraplegia. Brain MRI findings can include hypomyelinating leukodystrophy, cerebellar hypoplasia/atrophy, thin or dysplastic corpus callosum, and/or perisylvian polymicrogyria.

Neurodevelopmental disorder with progressive microcephaly, spasticity, and brain imaging abnormalities (NEDMISBA) is an autosomal recessive disorder characterized by a spectrum of neurologic abnormalities apparent from early infancy. Affected individuals have impaired intellectual development with poor speech, progressive microcephaly, and appendicular spasticity. Brain imaging usually shows abnormalities, including enlarged ventricles, white matter defects, and atrophy or hypoplasia of brain tissue. Some patients have a more severe phenotype with seizures, lack of developmental milestones, and early death (summary by Harel et al., 2018).

The Cincinnati type of acrofacial dysostosis is a ribosomopathy characterized by a spectrum of mandibulofacial dysostosis phenotypes, with or without extrafacial skeletal defects (Weaver et al., 2015). In addition, a significant number of neurologic abnormalities have been reported, ranging from mild delays to refractory epilepsy, as well as an increased incidence of congenital heart defects, primarily septal in nature (Smallwood et al., 2023).

Basel-Vanagaite-Smirin-Yosef syndrome is an autosomal recessive multiple congenital anomaly disorder characterized by severely delayed psychomotor development resulting in mental retardation, as well as variable eye, brain, cardiac, and palatal abnormalities (summary by Basel-Vanagaite et al., 2015).

PPP2R1A-related neurodevelopmental disorder (NDD) is characterized by: severe, persistent hypotonia; developmental delay with variable intellectual outcomes, typically in the moderate-to-severe intellectual disability range; seizures (more commonly seen in individuals with microcephaly and/or severe intellectual disability); attention-deficit/hyperactivity disorder and other behavioral problems (anxiousness, repetitive movements, self-injurious or destructive behavior, and autism spectrum disorder); feeding and swallowing issues; and dysmorphic features of the head and face. A minority of affected individuals have ear anomalies, hearing loss, ptosis, generalized joint hypermobility, and patent ductus arteriosus. Brain MRI findings are nonspecific but typically include complete or partial agenesis of the corpus callosum. Nonprogressive ventriculomegaly may be seen in a subset of affected individuals and is often associated with specific pathogenic variants in PPP2R1A: c.544C>T (p.Arg182Trp) and c.547C>T (p.Arg183Trp).

Lipoyl transferase 1 deficiency is a very rare inborn error of metabolism disorder, with a highly variable phenotype, typically characterized by neonatal to infancy-onset of seizures, psychomotor delay, and abnormal muscle tone that may include hypo- and/or hypertonia, resulting in generalized weakness, dystonic movements, and/or progressive respiratory distress, associated with severe lactic acidosis and elevated lactate, ketoglutarate and 2-oxoacids in urine. Additional manifestations may include dehydration, vomiting, signs of liver dysfunction, extrapyramidal signs, spastic tetraparesis, brisk deep tendon reflexes, speech impairment, swallowing difficulties, and pulmonary hypertension.

Female-restricted X-linked syndromic intellectual developmental disorder-99 (MRXS99F) is an X-linked dominant neurodevelopmental disorder characterized by delayed psychomotor development and mild to moderate intellectual disability. Affected females can have a wide range of additional congenital anomalies, including scoliosis, postaxial polydactyly, mild cardiac or urogenital anomalies, dysmorphic facial features, and mild structural brain abnormalities (summary by Reijnders et al., 2016).

X-linked syndromic intellectual developmental disorder-33 (MRXS33) is an X-linked recessive neurodevelopmental disorder characterized by delayed psychomotor development, intellectual disability, and characteristic facial features (summary by O'Rawe et al., 2015).

Band heterotopia (BH) is a neuronal migration disorder in which aberrantly located neurons, in the form of a band in the brain white matter, are present below a cortex that appears relatively normal by magnetic resonance imaging (MRI). Clinically, patients show severe developmental delay with intellectual disability, seizures, hypotonia, and hydrocephalus (Kielar et al., 2014, Shaheen et al., 2017).

Congenital muscular dystrophy-dystroglycanopathy with brain and eye anomalies (type A), which includes both the more severe Walker-Warburg syndrome (WWS) and the slightly less severe muscle-eye-brain disease (MEB), is a genetically heterogeneous autosomal recessive disorder with characteristic brain and eye malformations, profound mental retardation, congenital muscular dystrophy, and early death. The phenotype commonly includes cobblestone (type II) lissencephaly, cerebellar malformations, and retinal malformations. More variable features include macrocephaly or microcephaly, hypoplasia of midline brain structures, ventricular dilatation, microphthalmia, cleft lip/palate, and congenital contractures (Dobyns et al., 1989). Those with a more severe phenotype characterized as Walker-Warburg syndrome often die within the first year of life, whereas those characterized as having muscle-eye-brain disease may rarely acquire the ability to walk and to speak a few words. These are part of a group of disorders resulting from defective glycosylation of DAG1 (128239), collectively known as 'dystroglycanopathies' (Godfrey et al., 2007). Genetic Heterogeneity of Congenital Muscular Dystrophy-Dystroglycanopathy with Brain and Eye Anomalies (Type A) Muscular dystrophy-dystroglycanopathy with brain and eye anomalies (type A) is genetically heterogeneous and can be caused by mutation in other genes involved in DAG1 glycosylation: see MDDGA2 (613150), caused by mutation in the POMT2 gene (607439); MDDGA3 (253280), caused by mutation in the POMGNT1 gene (606822); MDDGA4 (253800), caused by mutation in the FKTN gene (607440); MDDGA5 (613153), caused by mutation in the FKRP gene (606596); MDDGA6 (613154), caused by mutation in the LARGE gene (603590); MDDGA7 (614643), caused by mutation in the ISPD gene (CRPPA; 614631); MDDGA8 (614830) caused by mutation in the GTDC2 gene (POMGNT2; 614828); MDDGA9 (616538), caused by mutation in the DAG1 gene (128239); MDDGA10 (615041), caused by mutation in the TMEM5 gene (RXYLT1; 605862); MDDGA11 (615181), caused by mutation in the B3GALNT2 gene (610194); MDDGA12 (615249), caused by mutation in the SGK196 gene (POMK; 615247); MDDGA13 (615287), caused by mutation in the B3GNT1 gene (B4GAT1; 605517); and MDDGA14 (615350), caused by mutation in the GMPPB gene (615320).

A congenital cortical development anomaly due to abnormal neuronal migration involving neocortical and hippocampal lamination, corpus callosum, cerebellum and brainstem. A large clinical spectrum can be observed, from children with severe epilepsy and intellectual and motor deficit to cases with severe cerebral dysgenesis in the antenatal period leading to pregnancy termination due to the severity of the prognosis.

Optic atrophy-11 (OPA11) is an autosomal recessive disorder characterized by delayed psychomotor development, intellectual disability, ataxia, optic atrophy, and leukoencephalopathy on brain imaging. Laboratory studies are consistent with mitochondrial dysfunction (summary by Hartmann et al., 2016). For a discussion of genetic heterogeneity of optic atrophy, see OPA1 (165500).

Early-onset vitamin B6-dependent epilepsy-1 (EPEO1) is an autosomal recessive neurologic disorder characterized by onset of seizures in the neonatal period or first months of life. The seizures show favorable response to treatment with activated vitamin B6 (pyridoxal 5-prime-phosphate; PLP) and/or pyridoxine. However, most patients show delayed psychomotor development (Darin et al., 2016). Genetic Heterogeneity of Early-Onset Epilepsy EPEO2 (618832) is caused by mutation in the SETD1A gene (611052) on chromosome 16p11. EPEO3 (620465) is caused by mutation in the ATP6V0C gene (108745) on chromosome 16p13. EPEO4 (266100) is caused by mutation in the ALDH7A1 gene (107323) on chromosome 5q23. EPEO5 (615400) is caused by mutation in the CNTN2 gene (190197) on chromosome 1q32.

Developmental and epileptic encephalopathy-49 (DEE49) is a severe autosomal recessive neurologic disorder characterized by onset of seizures in the neonatal period, global developmental delay with intellectual disability and lack of speech, hypotonia, spasticity, and coarse facial features. Some patients may have brain calcifications on imaging (summary by Han et al., 2016). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

GDACCF is an intellectual disability syndrome apparent soon after birth with neonatal hypotonia, poor feeding, and respiratory insufficiency followed by delayed psychomotor development and intellectual disability with poor speech. Brain imaging shows aplasia or hypoplasia of the corpus callosum. Affected individuals have variable dysmorphic facial features, and some may have dysplastic, cystic kidneys or mild cardiac defects (summary by Stevens et al., 2016).

Lissencephaly-8 (LIS8) is an autosomal recessive neurologic disorder characterized by delayed psychomotor development, intellectual disability with poor or absent speech, early-onset refractory seizures, and hypotonia. Brain imaging shows variable features, including cortical gyral abnormalities and hypoplasia of the corpus callosum, brainstem, and cerebellum (Jerber et al., 2016). For a general description and a discussion of genetic heterogeneity lissencephaly, see LIS1 (607432).

MGCA8 is an autosomal recessive metabolic disorder resulting in death in infancy. Features include hypotonia, abnormal movements, respiratory insufficiency with apneic episodes, and lack of developmental progress, often with seizures. Brain imaging is variable, but may show progressive cerebral atrophy. Laboratory studies show increased serum lactate and 3-methylglutaconic aciduria, suggesting a mitochondrial defect (summary by Mandel et al., 2016). For a phenotypic description and a discussion of genetic heterogeneity of 3-methylglutaconic aciduria, see MGCA type I (250950).

PEBAT is an autosomal recessive neurodevelopmental disorder characterized by severely delayed psychomotor development apparent soon after birth or in infancy, profound intellectual disability, poor or absent speech, and seizures. Most patients are never able to walk due to hypotonia or spasticity. Brain imaging shows cerebral and cerebellar atrophy, thin corpus callosum, and secondary hypomyelination. The disorder shows progressive features, including microcephaly, consistent with a neurodegenerative process (summary by Miyake et al., 2016; Flex et al., 2016).

Shashi-Pena syndrome is a neurodevelopmental syndrome characterized by delayed psychomotor development, variable intellectual disability, hypotonia, facial dysmorphism, and some unusual features, including enlarged head circumference, glabellar nevus flammeus, and deep palmar creases. Some patients may also have atrial septal defect, episodic hypoglycemia, changes in bone mineral density, and/or seizures (summary by Shashi et al., 2016).

Early-onset progressive encephalopathy with brain edema and/or leukoencephalopathy-1 (PEBEL1) is an autosomal recessive severe neurometabolic disorder characterized by rapidly progressive neurologic deterioration that is usually associated with a febrile illness. Affected infants tend to show normal early development followed by acute psychomotor regression with ataxia, hypotonia, respiratory insufficiency, and seizures, resulting in coma and death in the first years of life. Brain imaging shows multiple abnormalities, including brain edema and signal abnormalities in the cortical and subcortical regions (summary by Kremer et al., 2016). Genetic Heterogeneity of PEBEL See also PEBEL2 (618321), caused by mutation in the NAXD gene (615910) on chromosome 13q34.

CHD4 neurodevelopmental disorder (CHD4-NDD) is associated with developmental delay, speech delay, and usually mild-to-moderate intellectual disability. Variability between individuals with CHD4-NDD is significant, and a few have normal intelligence. Other manifestations can include brain anomalies, heart defects, and skeletal abnormalities; less common features are hypogonadism in males, hearing impairment, and ophthalmic abnormalities. Most affected individuals have mild nonspecific dysmorphic facial features with or without macrocephaly.

ZTTK syndrome (ZTTKS) is a severe multisystem developmental disorder characterized by delayed psychomotor development and intellectual disability. Affected individuals have characteristic dysmorphic facial features, hypotonia, poor feeding, poor overall growth, and eye or visual abnormalities. Most patients also have musculoskeletal abnormalities, and some have congenital defects of the heart and urogenital system. Brain imaging usually shows developmental abnormalities such as gyral changes, cortical and/or cerebellar atrophy, and thin corpus callosum (summary by Kim et al., 2016).

Autosomal recessive primary microcephaly-17 (MCPH17) is a severe neurologic disorder characterized by very small head circumference that is apparent at birth and worsens over time (up to -12 SD). Affected individuals have delayed psychomotor development, intellectual disability, spasticity, axial hypotonia, and dysmorphic features. Brain imaging shows a simplified gyral pattern; more severe cases have lissencephaly with hypoplasia of the brainstem and cerebellum (summary by Harding et al., 2016). For a phenotypic description and a discussion of genetic heterogeneity of primary microcephaly, see MCPH1 (251200).

Neurodevelopmental disorder with microcephaly and gray sclerae (NEDMIGS) is a severe autosomal recessive disorder characterized by impaired global development with hypotonia often precluding independent ambulation, profoundly impaired intellectual development with poor or absent language, mild microcephaly, and abnormal visual fixation. Patients also have gray sclerae and may have coarse facial features. Most affected individuals have seizures; some may have brain imaging abnormalities (summary by Shaheen et al., 2016 and Froukh et al., 2020).

Macrocephaly, dysmorphic facies, and psychomotor retardation (MDFPMR) is an autosomal recessive neurodevelopmental disorder characterized by large head and somatic overgrowth apparent at birth followed by global developmental delay. Affected individuals have characteristic dysmorphic facial features and persistently large head, but increased birth weight normalizes with age. Additional neurologic features, including seizures, hypotonia, and gait ataxia, may also occur. Patients show severe intellectual impairment (summary by Ortega-Recalde et al., 2015).

RERE-related disorders are characterized by neurodevelopmental problems with or without structural anomalies of the eyes, heart, kidneys, and genitourinary tract and mild sensorineural hearing loss. Hypotonia and feeding problems are common among affected individuals. Developmental delay and intellectual disability range from mild to profound. Behavior problems may include attention-deficit/hyperactivity disorder, self-injurious behavior, and autism spectrum disorder. A variety of eye anomalies (coloboma, optic nerve anomalies, microphthalmia, and/or Peter's anomaly) and vision issues (myopia, anisometropia, astigmatism, exotropia, esotropia) have been reported. Congenital heart defects, most commonly septal defects, have also been described. Genitourinary abnormalities include vesicoureteral reflux, and cryptorchidism and hypospadias in males. Sensorineural hearing loss can be unilateral or bilateral.

Witteveen-Kolk syndrome (WITKOS) is an autosomal dominant disorder with characteristic distinctive facial features, microcephaly, short stature, and mildly impaired intellectual development with delayed cognitive and motor development and subtle anomalies on MRI-brain imaging (summary by Balasubramanian et al., 2021).

Meester-Loeys syndrome (MRLS) is an X-linked disorder characterized by early-onset aortic aneurysm and dissection. Other recurrent findings include hypertelorism, pectus deformity, joint hypermobility, contractures, and mild skeletal dysplasia (Meester et al., 2017).

GLYT1 encephalopathy is characterized in neonates by severe hypotonia, respiratory failure requiring mechanical ventilation, and absent neonatal reflexes; encephalopathy, including impaired consciousness and unresponsiveness, may be present. Arthrogryposis or joint laxity can be observed. Generalized hypotonia develops later into axial hypotonia with limb hypertonicity and a startle-like response to vocal and visual stimuli which should not be confused with seizures. To date, three of the six affected children reported from three families died between ages two days and seven months; the oldest reported living child is severely globally impaired at age three years. Because of the limited number of affected individuals reported to date, the phenotype has not yet been completely described.

A brain disorder caused by pathogenic variants in NFIA that is characterized by developmental delay, corpus callosum agenesis/hypoplasia and craniofacial dysmorphism, such as macrocephaly (caused by hydrocephalus or ventriculomegaly), low-set ears, anteverted nostrils and micrognathia. Urinary tract defects (e.g. vesicoureteral reflux, urinary incontinence) are also frequently associated. Other reported variable manifestations include hypotonia, tethered spinal cord, Chiari type I malformation and seizures.

HNRNPU-related neurodevelopmental disorder (HNRNPU-NDD) is characterized by developmental delay and intellectual disability – typically moderate to severe – with speech and language delay and/or absent speech. Affected individuals may also display autistic features. There may be feeding difficulties during the neonatal period as well as hypotonia, which often remains lifelong. Dysmorphic features have been described but they are nonspecific. Affected individuals are likely to experience seizures (most commonly tonic-clonic or absence) that may be refractory to treatment. Nonspecific brain MRI findings include ventriculomegaly and thinning of the corpus callosum. Less common findings include cardiac abnormalities, strabismus, undescended testes in males, renal anomalies, and skeletal features, including joint laxity, polydactyly, and scoliosis. Rarely, abnormal breathing patterns, including hyperventilation and apnea, may be present and can lead to sleep disturbance.

Pseudo-TORCH syndrome-2 (PTORCH2) is an autosomal recessive multisystem disorder characterized by antenatal onset of intracranial hemorrhage, calcification, brain malformations, liver dysfunction, and often thrombocytopenia. Affected individuals tend to have respiratory insufficiency and seizures, and die in infancy. The phenotype resembles the sequelae of intrauterine infection, but there is no evidence of an infectious agent. The disorder results from inappropriate activation of the interferon (IFN) immunologic pathway (summary by Meuwissen et al., 2016). For a discussion of genetic heterogeneity of PTORCH, see PTORCH1 (251290).

Autosomal recessive cutis laxa type IID (ARCL2D) is characterized by generalized skin wrinkling with sparse subcutaneous fat and dysmorphic progeroid facial features. Most patients also exhibit severe hypotonia as well as cardiovascular and neurologic involvement (summary by Van Damme et al., 2017). For a general phenotypic description and a discussion of genetic heterogeneity of autosomal recessive cutis laxa, see ARCL1A (219100).

IDDFSDA is an autosomal recessive severe multisystem disorder characterized by poor overall growth, developmental delay, early-onset seizures, intellectual disability, and dysmorphic features. There is phenotypic variability. The most severely affected patients have a neurodevelopmental disorder with microcephaly, absent speech, and inability to walk, and they require feeding tubes. Some patients have congenital heart defects or nonspecific abnormalities on brain imaging. Less severely affected individuals have mild to moderate intellectual disability with normal speech and motor development (summary by Santiago-Sim et al., 2017).

NEDIM is a neurodevelopmental and neurodegenerative disorder characterized by delayed psychomotor development and infantile or childhood onset of hyperkinetic involuntary movements, including chorea and athetosis. The abnormal movements can be severe, sometimes resulting in inability to sit, walk, speak, or eat. Hyperkinetic movements can be exacerbated by specific triggers, such as stress, illness, or high temperature. Some patients have brain abnormalities, such as cerebral atrophy or thin corpus callosum, and some patients may develop seizures (summary by Ananth et al., 2016 and Danti et al., 2017).

Neurodevelopmental disorder with progressive microcephaly, spasticity, and brain anomalies (NDMSBA) is an autosomal recessive neurodevelopmental disorder characterized by infantile onset of progressive microcephaly and spasticity and severe global developmental delay resulting in profoundly impaired intellectual development and severely impaired or absent motor function. More variable features include seizures and optic atrophy. Brain imaging may show myelinating abnormalities and white matter lesions consistent with a leukoencephalopathy, as well as structural anomalies, including thin corpus callosum, gyral abnormalities, and cerebral or cerebellar atrophy. Some patients die in early childhood (summary by Falik Zaccai et al., 2017 and Hall et al., 2017).

The name HIST1H1E syndrome has been proposed as a mnemonic for the characteristic features of this emerging, recognizable phenotype: hypotonia; intellectual disability with behavioral issues; skeletal; testes (undescended) and thyroid; heart anomalies (most commonly atrial septal defect); and ectodermal issues (including sparse hair, thin nails, and abnormal dentition). In the 47 affected individuals reported to date, predominant findings were intellectual disability (ranging from mild to profound) and behavioral problems (combinations of anxiety/phobias, obsessive behaviors, attention-deficit/hyperactivity disorder, and autistic spectrum disorder/traits among others). Skeletal involvement can include scoliosis and decreased bone mineral density. Other findings in some include seizures, craniosynostosis, and hearing loss. Life expectancy does not appear to be reduced in HIST1H1E syndrome.

Fanconi anemia (FA) is characterized by physical abnormalities, bone marrow failure, and increased risk for malignancy. Physical abnormalities, present in approximately 75% of affected individuals, include one or more of the following: short stature, abnormal skin pigmentation, skeletal malformations of the upper and/or lower limbs, microcephaly, and ophthalmic and genitourinary tract anomalies. Progressive bone marrow failure with pancytopenia typically presents in the first decade, often initially with thrombocytopenia or leukopenia. The incidence of acute myeloid leukemia is 13% by age 50 years. Solid tumors – particularly of the head and neck, skin, and genitourinary tract – are more common in individuals with FA.

ISCA1-related multiple mitochondrial dysfunctions syndrome (ISCA1-MMDS) is a severe neurodegenerative condition typically characterized by either no attainment of developmental milestones or very early loss of achieved milestones, seizures in early infancy, development of spasticity with exaggerated deep tendon reflexes, nystagmus, and risk for sensorineural hearing loss. Affected individuals may also demonstrate elevated blood lactate levels with an elevated lipid-lactate peak on brain MR spectroscopy. Further brain MRI findings may include extensive cerebral and cerebellar deep white matter hyperintensities, marked dilatation of the cerebral ventricles, and pachygyria. Prognosis is poor and most individuals succumb to an intercurrent illness in early childhood.

WDR26-related intellectual disability (ID) is characterized by developmental delay / intellectual disability, characteristic facial features, hypotonia, epilepsy, and infant feeding difficulties. To date 15 individuals, ages 24 months to 34 years, have been reported. Developmental delay is present in all individuals and ranges from mild to severe. All individuals have delayed speech. Although some begin to develop speech in the second year, others have remained nonverbal. Seizures, present in all affected individuals reported to date, can be febrile or non-febrile (tonic-clonic, absence, rolandic seizures); most seizures are self limited or respond well to standard treatment. Affected individuals are generally described as happy and socially engaging; several have stereotypies / autistic features (repetitive or rocking behavior, abnormal hand movements or posturing, and at times self-stimulation).

NELABA is a severe autosomal recessive metabolic disorder characterized by onset at birth of progressive encephalopathy associated with increased serum lactate. Affected individuals have little or no psychomotor development and show brain abnormalities, including cerebral atrophy, cysts, and white matter abnormalities. Some patients die in infancy (summary by Habarou et al., 2017).

Al Kaissi syndrome (ALKAS) is an autosomal recessive developmental disorder characterized by growth retardation, spine malformation, particularly of the cervical spine, dysmorphic facial features, and delayed psychomotor development with moderate to severe intellectual disability (summary by Windpassinger et al., 2017).

NEMMLAS is an autosomal recessive multisystemic disorder characterized by delayed psychomotor development, intellectual disability, and abnormal motor function, including hypotonia, dystonia, ataxia, and spasticity. Patient tissues may show deficiencies in one or more of the mitochondrial oxidative phosphorylation (OXPHOS) enzymes, but this is not a constant finding (summary by Wortmann et al., 2017).

Galloway-Mowat syndrome is a renal-neurologic disease characterized by early-onset nephrotic syndrome associated with microcephaly, gyral abnormalities of the brain, and delayed psychomotor development. Most patients have dysmorphic facial features, often including hypertelorism, ear abnormalities, and micrognathia. Other features, such as arachnodactyly and visual impairment, are more variable. Most patients die in the first years of life (summary by Braun et al., 2017). For a general phenotypic description and a discussion of genetic heterogeneity of GAMOS, see GAMOS1 (251300).

Galloway-Mowat syndrome is a renal-neurologic disease characterized by early-onset nephrotic syndrome associated with microcephaly, gyral abnormalities, and delayed psychomotor development. Most patients have dysmorphic facial features, often including hypertelorism and ear abnormalities. Other features, such as arachnodactyly and visual or hearing impairment, are more variable. Most patients die in the first years of life (summary by Braun et al., 2017). For a general phenotypic description and a discussion of genetic heterogeneity of GAMOS, see GAMOS1 (251300).

Classic Joubert syndrome (JS) is characterized by three primary findings: A distinctive cerebellar and brain stem malformation called the molar tooth sign (MTS). Hypotonia. Developmental delays. Often these findings are accompanied by episodic tachypnea or apnea and/or atypical eye movements. In general, the breathing abnormalities improve with age, truncal ataxia develops over time, and acquisition of gross motor milestones is delayed. Cognitive abilities are variable, ranging from severe intellectual disability to normal. Additional findings can include retinal dystrophy, renal disease, ocular colobomas, occipital encephalocele, hepatic fibrosis, polydactyly, oral hamartomas, and endocrine abnormalities. Both intra- and interfamilial variation are seen.

Autosomal recessive primary microcephaly-19 (MCPH19) is a rare congenital brain defect resulting in a reduction of occipitofrontal head circumference by at least 3 standard deviations, severe developmental delay, failure to thrive, cortical blindness, and spasticity (DiStasio et al., 2017). For a general phenotypic description and a discussion of genetic heterogeneity of primary microcephaly, see MCPH1 (251200).

NEDMIAL is a neurodevelopmental disorder characterized by delayed psychomotor development and hypotonia apparent from early infancy, resulting in feeding difficulties, ataxic gait or inability to walk, delayed or absent speech development, and impaired intellectual development, sometimes with behavioral abnormalities, such as hand-flapping. Additional common features may include sleep disorder, nonspecific dysmorphic facial features, and joint hyperlaxity (summary by Lessel et al., 2017 and Mannucci et al., 2021).

NDAGSCW is a neurodevelopmental disorder characterized by severely delayed psychomotor development apparent from infancy. Affected individuals have delayed and difficulty walking, intellectual disability, absent speech, and variable additional features, including hip dysplasia, tapering fingers, and seizures. Brain imaging shows decreased cortical white matter, often with decreased cerebellar white matter, thin corpus callosum, and thin brainstem (summary by Lamers et al., 2017).

Any Neu-Laxova syndrome in which the cause of the disease is a mutation in the PHGDH gene.

Adams-Oliver syndrome (AOS) is characterized by aplasia cutis congenita (ACC) of the scalp and terminal transverse limb defects (TTLD). ACC lesions usually occur in the midline of the parietal or occipital regions, but can also occur on the abdomen or limbs. At birth, an ACC lesion may already have the appearance of a healed scar. ACC lesions less than 5 cm often involve only the skin and almost always heal over a period of months; larger lesions are more likely to involve the skull and possibly the dura, and are at greater risk for complications, which can include infection, hemorrhage, or thrombosis, and can result in death. The limb defects range from mild (unilateral or bilateral short distal phalanges) to severe (complete absence of all toes or fingers, feet or hands, or more, often resembling an amputation). The lower extremities are almost always more severely affected than the upper extremities. Additional major features frequently include cardiovascular malformations/dysfunction (23%), brain anomalies, and less frequently renal, liver, and eye anomalies.

Knobloch syndrome-1 (KNO1) is an autosomal recessive developmental disorder primarily characterized by typical eye abnormalities, including high myopia, cataracts, dislocated lens, vitreoretinal degeneration, and retinal detachment, with occipital skull defects, which can range from occipital encephalocele to occult cutis aplasia (summary by Aldahmesh et al., 2011). Genetic Heterogeneity of Knobloch Syndrome KNO2 (618458) is caused by mutation in the PAK2 gene (605022) on chromosome 3q29.

ALKKUCS is an autosomal recessive severe neurodevelopmental disorder characterized by arthrogryposis, brain abnormalities associated with cerebral parenchymal underdevelopment, and global developmental delay. Most affected individuals die in utero or soon after birth. Additional abnormalities may include hypotonia, dysmorphic facial features, and involvement of other organ systems, such as cardiac or renal. The few patients who survive have variable intellectual disability and may have seizures (summary by Gueneau et al., 2018).

NEDMEBA is an autosomal recessive neurodegenerative disorder characterized by global developmental delay, severe intellectual disability with poor or absent speech and autistic stereotypic behaviors, microcephaly, early-onset generalized seizures, and hypotonia (summary by Marin-Valencia et al., 2018).

Short-rib thoracic dysplasia (SRTD) with or without polydactyly refers to a group of autosomal recessive skeletal ciliopathies that are characterized by a constricted thoracic cage, short ribs, shortened tubular bones, and a 'trident' appearance of the acetabular roof. SRTD encompasses Ellis-van Creveld syndrome (EVC) and the disorders previously designated as Jeune syndrome or asphyxiating thoracic dystrophy (ATD), short rib-polydactyly syndrome (SRPS), and Mainzer-Saldino syndrome (MZSDS). Polydactyly is variably present, and there is phenotypic overlap in the various forms of SRTDs, which differ by visceral malformation and metaphyseal appearance. Nonskeletal involvement can include cleft lip/palate as well as anomalies of major organs such as the brain, eye, heart, kidneys, liver, pancreas, intestines, and genitalia. Some forms of SRTD are lethal in the neonatal period due to respiratory insufficiency secondary to a severely restricted thoracic cage, whereas others are compatible with life (summary by Huber and Cormier-Daire, 2012 and Schmidts et al., 2013). There is phenotypic overlap with the cranioectodermal dysplasias (Sensenbrenner syndrome; see CED1, 218330). For a discussion of genetic heterogeneity of short-rib thoracic dysplasia with or without polydactyly, see SRTD1 (208500).

NDPLHS is an autosomal dominant disorder characterized by developmental stagnation or regression apparent in the first years of life and manifest as loss of purposeful hand movements, loss of language, and intellectual disability. Additional features may include stereotypic movements, dystonia, gait abnormalities, sleep disturbances, and small hands and feet. The phenotype is reminiscent of Rett syndrome (RTT; 312750) (summary by Yoo et al., 2017).

Developmental and epileptic encephalopathy-59 (DEE59) is characterized by severe global developmental delay apparent in infancy with onset of various types of seizures in the first months of life (range 3 to 11 months). The seizures are usually refractory and are often associated with hypsarrhythmia on EEG, although brain imaging is usually normal. More severely affected individuals may be unable to speak or walk, have poor interaction, and require a feeding tube (summary by the EuroEPINOMICS-RES Consortium et al., 2014). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

El-Hattab-Alkuraya syndrome is characterized by microcephaly (often early onset and progressive); severe-to-profound developmental delay; refractory and early-onset seizures; spastic quadriplegia with axial hypotonia; and growth deficiency with poor weight gain and short stature. Characteristic findings on brain imaging include cerebral atrophy that is disproportionately most prominent in the frontal lobes; ex vacuo ventricular dilatation with notable posterior horn predominance; brain stem volume loss with flattening of the belly of the pons; and symmetric under-opercularization. Neurologic involvement is progressive, with significant morbidity and mortality.

Developmental and epileptic encephalopathy-64 (DEE64) is a neurodevelopmental disorder characterized by onset of seizures usually in the first year of life and associated with intellectual disability, poor motor development, and poor or absent speech. Additional features include hypotonia, abnormal movements, and nonspecific dysmorphic features. The severity is variable: some patients are unable to speak, walk, or interact with others as late as the teenage years, whereas others may have some comprehension (summary by Straub et al., 2018). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

Developmental and epileptic encephalopathy-65 (DEE65) is characterized by onset of intractable seizures of various types usually within the first months or years of life, severe to profound psychomotor developmental delay, and mild facial dysmorphism (summary by Nakashima et al., 2018). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

CMH27 is a severe, early-onset cardiomyopathy with morphologic features of both dilated and hypertrophic disease, characterized by biventricular involvement and atypical distribution of hypertrophy. Heterozygotes are at increased risk of developing cardiomyopathy (Almomani et al., 2016). For a general phenotypic description and a discussion of genetic heterogeneity of hypertrophic cardiomyopathy, see CMH1 (192600). An oligogenic form of hypertrophic cardiomyopathy, involving heterozygous mutations in the ALPK3, TTN (188840), and MYL3 (160790) genes has also been reported in 1 family.

CAFDADD is a multisystemic developmental disorder with variable cardiac and digital anomalies and facial dysmorphism. Some patients may have seizures and ocular/aural abnormalities (Tokita et al., 2018).

Snijders Blok-Campeau syndrome (SNIBCPS) is an autosomal dominant neurodevelopmental disorder characterized by global developmental delay with impaired intellectual development and delayed speech acquisition. Affected individuals tend to have expressive language deficits, with speech apraxia and dysarthria. Other features include macrocephaly and characteristic facial features, such as prominent forehead and hypertelorism, hypotonia, and joint laxity. The severity of the neurologic deficits and presence of nonneurologic features is variable (summary by Snijders Blok et al., 2018).

Mega-corpus-callosum syndrome with cerebellar hypoplasia and cortical malformations (MCCCHCM) is an autosomal dominant neurodevelopmental disorder characterized by global developmental delay, impaired intellectual development, and characteristic brain abnormalities on brain imaging. Affected individuals have enlargement of the corpus callosum, enlarged ventricles, and cerebellar and brainstem hypoplasia. Other features may include lack of speech development, gait instability, and seizures. Some patients with MAST1 mutations may have impaired intellectual development and/or autism spectrum disorder without significant findings on brain imaging (summary by Tripathy et al., 2018).

Fibrosis, neurodegeneration, and cerebral angiomatosis (FINCA) is characterized by severe progressive cerebropulmonary symptoms, resulting in death in infancy from respiratory failure. Features include malabsorption, progressive growth failure, recurrent infections, chronic hemolytic anemia, and transient liver dysfunction. Neuropathology shows increased angiomatosis-like leptomeningeal, cortical, and superficial white matter vascularization and congestion, vacuolar degeneration and myelin loss in white matter, as well as neuronal degeneration. Interstitial fibrosis and granuloma-like lesions are seen in the lungs, and there is hepatomegaly with steatosis and collagen accumulation (Uusimaa et al., 2018).

SMALED2B is a severe neuromuscular disorder with onset in utero. Affected individuals show decreased fetal movements and are usually born with congenital contractures consistent with arthrogryposis multiplex congenita (AMC). After birth, they have severe hypotonia and muscle atrophy as well as respiratory insufficiency due to muscle weakness. Some patients may have dysmorphic facial features and/or abnormalities on brain imaging. Many patients die in early childhood (summary by Storbeck et al., 2017) For discussion of genetic heterogeneity of lower extremity-predominant spinal muscular atrophy, see SMALED1 (158600).

Developmental and epileptic encephalopathy-70 (DEE70) is neurologic disorder characterized by the onset of epileptic spasms or seizures in the first months of life. EEG may show hypsarrhythmia, consistent with a clinical diagnosis of West syndrome. Affected individuals show severely delayed psychomotor development with impaired or absent walking and language skills; intellectual impairment ranges from moderate to severe (summary by Hamada et al., 2018). For a discussion of genetic heterogeneity of DEE, see 308350.

PAFAH1B1-related lissencephaly/subcortical band heterotopia (SBH) comprises a spectrum of severity. Affected newborns typically have mild-to-moderate hypotonia, feeding difficulties, and poor head control. During the first years, neurologic examination typically demonstrates poor visual tracking and response to sounds, axial hypotonia, and mild distal spasticity that can transition over time to more severe spasticity. Seizures occur in more than 90% of individuals with lissencephaly and often include infantile spasms. Seizures are often drug resistant, but even with good seizure control, the best developmental level achieved (excluding the few individuals with partial lissencephaly) is the equivalent of about age three to five months. In individuals with PAFAH1B1-related lissencephaly/SBH, developmental delay ranges from mild to severe. Other findings in PAFAH1B1-related lissencephaly/SBH include feeding issues and aspiration (which may result in need for gastrostomy tube placement), progressive microcephaly, and occasional developmental regression.

Neurodevelopmental disorder with brain abnormalities, poor growth, and dysmorphic facies (NEDBGF) is an autosomal recessive disorder characterized by global developmental delay with delayed walking, impaired intellectual development, and speech delay apparent from infancy or early childhood. Most patients have dysmorphic facial features, often with microcephaly and strabismus, and white matter abnormalities on brain imaging. More variable features may include teeth anomalies, distal joint contractures, spasticity, peripheral neuropathy, and behavioral problems (summary by Sharkia et al., 2019).

The spectrum of FARS2 deficiency ranges from the infantile-onset phenotype, characterized by epileptic encephalopathy with lactic acidosis and poor prognosis (70% of affected individuals), to the later-onset phenotype, characterized by spastic paraplegia, less severe neurologic manifestations, and longer survival (30% of affected individuals). To date FARS2 deficiency has been reported in 37 individuals from 25 families. Infantile-onset phenotype. Seizures are difficult to control and may progress quickly at an early age to intractable seizures with frequent status epilepticus; some children have hypsarrhythmia on EEG. All have developmental delay; most are nonverbal and unable to walk. Feeding difficulties are common. More than half of affected children die in early childhood. Later-onset phenotype. All affected individuals have spastic paraplegia manifested by weakness, spasticity, and exaggerated reflexes of the lower extremities associated with walking difficulties; some have developmental delay/intellectual disability; some have brief seizures that resolve over time.

Pontocerebellar hypoplasia type 10 is an autosomal recessive neurodevelopmental and neurodegenerative disorder characterized by severely delayed psychomotor development, progressive microcephaly, spasticity, seizures, and brain abnormalities, including brain atrophy and delayed myelination. Some patients have dysmorphic features and an axonal sensorimotor neuropathy (summary by Karaca et al., 2014 and Schaffer et al., 2014). For a general phenotypic description and a discussion of genetic heterogeneity of PCH, see PCH1 (607596).

Hypomagnesemia, seizures, and impaired intellectual development-2 (HOMGSMR2) is characterized by generalized seizures in infancy, severe hypomagnesemia, and renal magnesium wasting. Seizures persist despite magnesium supplementation and are associated with significantly impaired intellectual development (Schlingmann et al., 2018). For a discussion of genetic heterogeneity of hypomagnesemia, seizures, and impaired intellectual development, see HOMGSMR1 (616418).

Early-onset progressive encephalopathy with brain edema and/or leukoencephalopathy-2 (PEBEL2) is an autosomal recessive severe neurometabolic disorder characterized by rapidly progressive neurologic deterioration that is usually associated with a febrile illness. Affected infants tend to show normal early development followed by acute psychomotor regression with ataxia, hypotonia, and sometimes seizures, resulting in death in the first years of life. Brain imaging shows multiple abnormalities, including brain edema and signal abnormalities in the cortical and subcortical regions (summary by Van Bergen et al., 2019). For a discussion of genetic heterogeneity of PEBEL, see PEBEL1 (617186).

Polymicrogyria with or without vascular-type Ehlers-Danlos syndrome is an autosomal recessive disorder with a highly variable phenotype. Although all patients have polymicrogyria and other variable structural brain anomalies on imaging, only some show developmental delay and/or seizures. Similarly, only some patients have connective tissue defects that particularly affect the vascular system and can result in early death (summary by Vandervore et al., 2017).

Houge-Janssens syndrome-3 (HJS3) is characterized by global developmental delay apparent from infancy. The phenotype is highly variable: patients may have hypotonia, behavioral abnormalities, and abnormalities on brain imaging, including enlarged ventricles, thin corpus callosum, and sometimes small brainstem. Many develop seizures, sometimes refractory, and some may have nonspecific dysmorphic features. Intellectual impairment can vary from mild to profound, and some patients may benefit from special education and respond well to speech therapy (summary by Reynhout et al., 2019). For a discussion of genetic heterogeneity of HJS, see HJS1 (616355).

A rare, genetic, multiple congenital anomalies/dysmorphic syndrome characterized by variable intellectual disability and/or developmental delay, epilepsy, generalized hypertrichosis, severe gingival overgrowth and visual impairment in some patients. Common craniofacial features include bitemporal narrowing, bushy and straight eyebrows, long eyelashes, low-set ears, deep/short philtrum, everted upper lip, prominent upper and lower vermilion, wide mouth, micrognathia, and retrognathia.

Neurodevelopmental disorder with progressive movement abnormalities, cognitive decline, and brain abnormalities (NEDMCB) is an autosomal recessive disorder characterized by global developmental delay and developmental regression resulting in variably impaired intellectual development with poor or absent speech, difficulty walking or inability to walk, and various movement abnormalities, including spasticity, hypertonia, dystonia, tremor, and myoclonus. Affected individuals usually show poor overall growth, often with microcephaly, hypotonia, limb contractures, and cataracts. Most have progressive brain imaging abnormalities, including enlarged ventricles, white matter loss, and cerebellar atrophy. A subset of patients have combined malonic and methylmalonic aciduria (CMAMMA), although this is not a reliable biomarker (Ortigoza-Escobar et al., 2024).

Combined oxidative phosphorylation deficiency-39 (COXPD39) is an autosomal recessive multisystem disorder resulting from a defect in mitochondrial energy metabolism. Affected individuals show global developmental delay, sometimes with regression after normal early development, axial hypotonia with limb spasticity or abnormal involuntary movements, and impaired intellectual development with poor speech. More variable features may include hypotonia, seizures, and features of Leigh syndrome (256000) on brain imaging. There are variable deficiencies of the mitochondrial respiratory chain enzyme complexes in patient tissues (summary by Glasgow et al., 2017). For a discussion of genetic heterogeneity of combined oxidative phosphorylation deficiency, see COXPD1 (609060).

Developmental delay with or without dysmorphic facies and autism (DEDDFA) is a complex neurodevelopmental disorder apparent from infancy or early childhood and associated with variably impaired intellectual development. Some patients may be severely affected with no speech and inability to walk, whereas others may be able to attend special schools or have normal intellectual function associated with autism spectrum disorder and mild speech delay. Genetic analysis has suggested that the phenotype can be broadly categorized into 2 main groups. Patients with TRRAP mutations affecting residues 1031-1159 have a more severe disorder, often with multisystem involvement, including renal, cardiac, and genitourinary systems, as well as structural brain abnormalities. Patients with mutations outside of that region tend to have a less severe phenotype with a higher incidence of autism and usually no systemic involvement. Patients in both groups usually have somewhat similar dysmorphic facial features, such as upslanting palpebral fissures, hypertelorism, low-set ears, and broad or depressed nasal bridge, although these features are highly variable (summary by Cogne et al., 2019).

Khan-Khan-Katsanis syndrome (3KS) is an autosomal recessive neurodevelopmental disorder with variable involvement of the ocular, renal, skeletal, and sometimes cardiac systems. Affected individuals present at birth with multiple congenital anomalies, defects in urogenital and limb morphogenesis, poor overall growth with microcephaly, and global developmental delay (summary by Khan et al., 2019).

Brain abnormalities, neurodegeneration, and dysosteosclerosis (BANDDOS) is an autosomal recessive disorder characterized by brain abnormalities, progressive neurologic deterioration, and sclerotic bone dysplasia similar to dysosteosclerosis (DOS). The age at onset is highly variable: some patients may present in infancy with hydrocephalus, global developmental delay, and hypotonia, whereas others may have onset of symptoms in the late teens or early twenties after normal development. Neurologic features include loss of previous motor and language skills, cognitive impairment, spasticity, and focal seizures. Brain imaging shows periventricular white matter abnormalities and calcifications, large cisterna magna or Dandy-Walker malformation, and sometimes agenesis of the corpus callosum (summary by Guo et al., 2019).

Basilicata-Akhtar syndrome (MRXSBA) is characterized by global developmental delay apparent from infancy, feeding difficulties, hypotonia, and poor or absent speech. Most patients are able to walk, although they may have an unsteady gait or spasticity. Additional findings include dysmorphic facial features and mild distal skeletal anomalies. Males and females are similarly affected (summary by Basilicata et al., 2018).

Neurodevelopmental disorder with visual defects and brain anomalies (NEDVIBA) is characterized by global developmental delay with impaired intellectual development and speech delay, variable visual defects, including retinitis pigmentosa and optic atrophy, hypotonia or hypertonia, and variable structural brain abnormalities. Other nonspecific features may be found (summary by Okur et al., 2019).

Multiple congenital anomalies-hypotonia-seizures syndrome-4 (MCAHS4) is an autosomal recessive neurologic disorder characterized by onset of refractory seizures in the first months of life. Patients have severe global developmental delay, and may have additional variable features, including dysmorphic or coarse facial features, visual defects, and mild skeletal or renal anomalies. At the cellular level, the disorder is caused by a defect in the synthesis of glycosylphosphatidylinositol (GPI), and thus affects the expression of GPI-anchored proteins at the cell surface (summary by Starr et al., 2019). For a discussion of genetic heterogeneity of MCAHS, see MCAHS1 (614080). For a discussion of genetic heterogeneity of DEE, see 308350. For a discussion of genetic heterogeneity of GPI biosynthesis defects, see GPIBD1 (610293).

Neurodevelopmental disorder with structural brain anomalies and dysmorphic facies (NEDBAF) is a complex syndromic disorder including features of moderate to severe psychomotor delay leading to impaired intellectual development, dysplastic corpus callosum, cortical malformations, hypotonia, dyspraxia, musculoskeletal abnormalities, and feeding difficulties. Seizures occur in about half of patients. Dysmorphic features include wide forehead with frontal bossing and high anterior hairline, prominent eyes with upslanted palpebral fissures, arched eyebrows, long eyelashes, midface hypoplasia, broad nasal bridge, and anteverted nares (summary by Scala et al., 2022).

Developmental and epileptic encephalopathy-80 (DEE80) is an autosomal recessive neurologic disorder characterized by the onset of refractory seizures in the first year of life. Patients have severe global developmental delay and may have additional variable features, including dysmorphic or coarse facial features, distal skeletal abnormalities, and impaired hearing or vision. At the cellular level, the disorder is caused by a defect in the synthesis of glycosylphosphatidylinositol (GPI), and thus affects the expression of GPI-anchored proteins at the cell surface (summary by Murakami et al., 2019). For a discussion of genetic heterogeneity of DEE, see 308350. For a discussion of genetic heterogeneity of GPI biosynthesis defects, see GPIBD1 (610293).

Neurodevelopmental disorder with hypotonia and variable intellectual and behavioral abnormalities (NEDHIB) is characterized by early-onset hypotonia, delayed walking, poor speech, and impaired intellectual development. Additional features may include feeding difficulties, dysmorphic features, and visual defects. Brain imaging tends to show delayed myelination, thin corpus callosum, and/or enlarged ventricles. The severity of the disorder is highly variable; initial evidence suggests that the severity may depend on the type of mutation (summary by Haijes et al., 2019).

Weiss-Kruszka syndrome is characterized by metopic ridging or synostosis, ptosis, nonspecific dysmorphic features, developmental delay, and autistic features. Brain imaging may identify abnormalities of the corpus callosum. Developmental delay can present as global delay, motor delay, or speech delay. Affected individuals may also have ear anomalies, feeding difficulties (sometimes requiring placement of a gastrostomy tube), and congenital heart defects. There is significant variability in the clinical features, even between affected members of the same family.

Noonan syndrome (NS) is characterized by characteristic facies, short stature, congenital heart defect, and developmental delay of variable degree. Other findings can include broad or webbed neck, unusual chest shape with superior pectus carinatum and inferior pectus excavatum, cryptorchidism, varied coagulation defects, lymphatic dysplasias, and ocular abnormalities. Although birth length is usually normal, final adult height approaches the lower limit of normal. Congenital heart disease occurs in 50%-80% of individuals. Pulmonary valve stenosis, often with dysplasia, is the most common heart defect and is found in 20%-50% of individuals. Hypertrophic cardiomyopathy, found in 20%-30% of individuals, may be present at birth or develop in infancy or childhood. Other structural defects include atrial and ventricular septal defects, branch pulmonary artery stenosis, and tetralogy of Fallot. Up to one fourth of affected individuals have mild intellectual disability, and language impairments in general are more common in NS than in the general population.

Halperin-Birk syndrome (HLBKS) is an autosomal recessive neurodevelopmental disorder characterized by structural brain defects, spastic quadriplegia with multiple contractures, profound developmental delay, seizures, dysmorphism, cataract, and optic nerve atrophy. Death occurs in early childhood (Halperin et al., 2019).

Neurodevelopmental disorder with dysmorphic facies and distal skeletal anomalies (NEDDFSA) is a global neurodevelopmental disorder with highly variable features. Patients often show poor feeding, poor overall growth, and hypotonia from early infancy, followed by mildly delayed motor development, poor language acquisition, and behavioral abnormalities. Intellectual development varies from severe with absent speech to mild with the ability to attend special schools. Common features include dysmorphic facial features with notable eye anomalies, joint hypermobility, and mild skeletal anomalies of the hands and feet (summary by Carapito et al., 2019).

Congenital hydrocephalus-4 (HYC4) is characterized by inadequate passage of cerebrospinal fluid (CSF) from its point of production to its point of absorption, in the absence of obstruction (communicating hydrocephalus). Patients undergo surgical shunting at birth because of extreme ventriculomegaly. Patients have neurodevelopmental delay and epilepsy (Furey et al., 2018).

Complex cortical dysplasia with other brain malformations-10 (CDCBM10) is an autosomal recessive neurodevelopmental disorder characterized by severely impaired global development associated with abnormalities on brain imaging, including lissencephaly, cortical dysplasia, subcortical heterotopia, and paucity of white matter. The disorder results from defective neuronal migration during brain development. Affected individuals often develop seizures, are unable to walk, and do not acquire language (summary by Lee et al., 2019). For a discussion of genetic heterogeneity of CDCBM, see CDCBM1 (614039).

Neurodevelopmental disorder with nonspecific brain abnormalities is a highly variable syndrome characterized by impaired intellectual development and behavioral abnormalities associated with structural changes on brain imaging. Some patients have seizures, hypotonia, and scoliosis/kyphosis. Cognitive function ranges from severely impaired to the ability to attend schools with special assistance (summary by Fischer-Zirnsak et al., 2019).

Neurodevelopmental disorder with microcephaly, cortical malformations, and spasticity (NEDMCMS) is an autosomal recessive disorder characterized by severe to profound global developmental delay, early-onset seizures, microcephaly, and polymicrogyria and/or cerebral atrophy on brain imaging. Most affected individuals are unable to walk or speak and have profoundly impaired intellectual development, as well as axial hypotonia and peripheral spasticity. Rare individuals may be less severely affected (summary by Vandervore et al., 2019).

DDX3X-related neurodevelopmental disorder (DDX3X-NDD) typically occurs in females and very rarely in males. All affected individuals reported to date have developmental delay / intellectual disability (ID) ranging from mild to severe; about 50% of affected girls remain nonverbal after age five years. Hypotonia, a common finding, can be associated with feeding difficulty in infancy. Behavioral issues can include autism spectrum disorder, attention-deficit/hyperactivity disorder and hyperactivity, self-injurious behavior, poor impulse control, and aggression. Other findings can include seizures, movement disorders (dyskinesia, spasticity, abnormal gait), vision and hearing impairment, congenital heart defects, respiratory difficulties, joint laxity, and scoliosis. Neuroblastoma has been observed in three individuals.

Female-restricted Wieacker-Wolff syndrome (WRWFFR) is an X-linked dominant syndromic form of neurogenic arthrogryposis multiplex congenita (AMC) with central and peripheral nervous system involvement. Affected individuals have decreased fetal movements causing the development of contractures in utero and resulting in AMC and diffuse contractures involving the large and small joints apparent at birth. There is global developmental delay with difficulty walking or inability to walk, hypotonia that often evolves to spasticity, and variably impaired intellectual development with poor or absent speech and language. Dysmorphic facial features, including hypotonic facies, ptosis, microretrognathia, and small mouth, are seen in most patients. Seizures are uncommon; some patients have evidence of a peripheral motor neuropathy with distal muscle weakness. The level of X inactivation in lymphocytes and fibroblasts is often skewed, but may not predict the severity of the phenotype. Most cases occur sporadically; rare X-linked dominant inheritance has been reported in families (summary by Frints et al., 2019).

Developmental and epileptic encephalopathy-84 (DEE84) is an autosomal recessive neurologic disorder characterized by onset of refractory seizures in the first months or years of life. Affected individuals have severely impaired global development with impaired intellectual development, absent speech, and inability to walk. Other features include axial hypotonia, peripheral spasticity, feeding difficulties that sometimes necessitate tube feeding, and mild dysmorphic facial features. Brain imaging may show nonspecific findings such as cerebral/cerebellar atrophy and/or hypomyelination. The severity of the disorder is variable (summary by Hengel et al., 2020). For a discussion of genetic heterogeneity of DEE, see 308350.

Beck-Fahrner syndrome (BEFAHRS) is a developmental disorder characterized by global developmental delay with variably impaired intellectual development. Affected individuals often have behavioral abnormalities, such as autistic features or attention deficit-hyperactivity disorder (ADHD), as well as learning disabilities. Most patients have hypotonia and dysmorphic facies. Some may have growth abnormalities, including overgrowth or poor growth, poor feeding, and rarely, seizures. Although both monoallelic and biallelic mutations have been reported, some heterozygous carriers in autosomal recessive families may have milder symptoms; thus, both groups are included in this entry (summary by Beck et al., 2020).

Sandestig-Stefanova syndrome (SANDSTEF) is an autosomal recessive developmental syndrome characterized by pre- and postnatal microcephaly, trigonocephaly, congenital cataract, microphthalmia, facial gestalt, camptodactyly, loss of periventricular white matter, thin corpus callosum, delayed myelinization, and poor prognosis (Sandestig et al., 2019).

CSGALNACT1 deficiency is characterized by mild skeletal dysplasia, joint hypermobility, and advanced bone age. Shortness of long bones is evident prenatally, and patients exhibit short stature and relative macrocephaly. Advanced carpotarsal bone age and monkey-wrench appearance of the femur observed in infancy may disappear with age (Mizumoto et al., 2020).

The IFAP/BRESHECK syndrome is an X-linked multiple congenital anomaly disorder with variable severity. The classic triad, which defines IFAP, is ichthyosis follicularis, atrichia, and photophobia. Some patients have additional features, including mental retardation, brain anomalies, Hirschsprung disease, corneal opacifications, kidney dysplasia, cryptorchidism, cleft palate, and skeletal malformations, particularly of the vertebrae, which constitutes BRESHECK syndrome (summary by Naiki et al., 2012). Genetic Heterogeneity of IFAP Syndrome IFAP syndrome-2 (IFAP2; 619016) is caused by heterozygous mutation in the SREBF1 gene (184756) on chromosome 17p11.

Mitochondrial complex IV deficiency nuclear type 2 (MC4DN2) is an autosomal recessive multisystem metabolic disorder characterized by the onset of symptoms at birth or in the first weeks or months of life. Affected individuals have severe hypotonia, often associated with feeding difficulties and respiratory insufficiency necessitating tube feeding and mechanical ventilation. The vast majority of patients develop hypertrophic cardiomyopathy in the first days or weeks of life, which usually leads to death in infancy or early childhood. Patients also show neurologic abnormalities, including developmental delay, nystagmus, fasciculations, dystonia, EEG changes, and brain imaging abnormalities compatible with a diagnosis of Leigh syndrome (see 256000). There may also be evidence of systemic involvement with hepatomegaly and myopathy, although neurogenic muscle atrophy is more common and may resemble spinal muscular atrophy type I (SMA1; 253300). Serum lactate is increased, and laboratory studies show decreased mitochondrial complex IV protein and activity levels in various tissues, including heart and skeletal muscle. Most patients die in infancy of cardiorespiratory failure (summary by Papadopoulou et al., 1999). For a discussion of genetic heterogeneity of mitochondrial complex IV (cytochrome c oxidase) deficiency, see 220110.

Li-Ghorbani-Weisz-Hubshman syndrome (LIGOWS) is a neurodevelopmental disorder characterized by global developmental delay, mild to moderately impaired intellectual development with language delay, and mild dysmorphic features. Affected individuals may have behavioral abnormalities and difficulties with numbers and understanding certain concepts, such as money. Some patients have seizures. Brain imaging often shows enlarged ventricles, thin corpus callosum, and gray matter nodular heterotopia, suggesting abnormal cortical brain development. More variable additional features may be present (summary by Li et al., 2020).

Mitochondrial complex IV deficiency nuclear type 7 (MC4DN7) is an autosomal recessive metabolic encephalomyopathic disorder with highly variable manifestations. Only a few patients have been reported. Some patients have normal early development then show rapid neurodegeneration with progressive muscle weakness, gait disturbances, and cognitive decline in mid to late childhood. Other features may include seizures and visual impairment. Brain imaging shows progressive leukodystrophy with cystic lesions. In contrast, at least 1 patient has been reported who presented in the neonatal period with metabolic acidosis, hydrocephalus, hypotonia, and cortical blindness. This patient developed hypertrophic cardiomyopathy resulting in early death. All patients had increased serum lactate and decreased levels and activity of mitochondrial respiratory complex IV (summary by Massa et al., 2008 and Abdulhag et al., 2015). For a discussion of genetic heterogeneity of mitochondrial complex IV (cytochrome c oxidase) deficiency, see 220110.

Mitochondrial complex IV deficiency nuclear type 15 (MC4DN15) is an autosomal recessive multisystem metabolic disorder characterized by the onset of symptoms in infancy. Affected individuals show global developmental delay, poor feeding, short stature with microcephaly, proximal muscle weakness, and distal spasticity. Other manifestations include scoliosis, primary pulmonary hypertension, childhood-onset refractory seizures, and inability to walk. Brain imaging shows features consistent with Leigh syndrome (see 256000) and enlarged ventricles. Laboratory studies show increased serum and CSF lactate, as well as decreased levels and activity of mitochondrial respiratory complex IV (summary by Hallmann et al., 2016). For a discussion of genetic heterogeneity of mitochondrial complex IV (cytochrome c oxidase) deficiency, see 220110.

Cleft palate, proliferative retinopathy, and developmental delay (CPPRDD) is characterized by motor and speech delay, with intellectual disability ranging from mild to severe. Brain imaging shows ventriculomegaly as well as other malformations (Harel et al., 2019).

Neurodevelopmental disorder with cardiomyopathy, spasticity, and brain abnormalities (NEDCASB) is an autosomal recessive multisystemic disorder characterized by global neurodevelopmental delay, severely impaired intellectual development, poor overall growth, and spasticity of the lower limbs resulting in gait difficulties. Most affected individuals also develop progressive hypertrophic cardiomyopathy in childhood or have cardiac developmental anomalies. Additional more variable features include dysmorphic facies and axonal sensory peripheral neuropathy. Brain imaging tends to show thin corpus callosum and polymicrogyria (summary by Garcia-Cazorla et al., 2020).

Intellectual developmental disorder with paroxysmal dyskinesia or seizures (IDDPADS) is an autosomal recessive complex neurologic disorder characterized by global developmental delay with impaired intellectual development and language delay. In addition, most patients develop a paroxysmal hyperkinetic movement disorder in the first months or years of life manifest as sudden falls or backward propulsion, eye or head deviation, and dystonic limb posturing followed by chorea and dyskinetic movements. The episodes are pharmacoresistant to anticonvulsant medication. EEG may show interictal abnormalities, but are usually not consistent with epilepsy. However, some patients may also develop epileptic seizures or only have seizures without a movement disorder (summary by Doummar et al., 2020).

RAB18 deficiency is the molecular deficit underlying both Warburg micro syndrome (characterized by eye, nervous system, and endocrine abnormalities) and Martsolf syndrome (characterized by similar – but milder – findings). To date Warburg micro syndrome comprises >96% of reported individuals with genetically defined RAB18 deficiency. The hallmark ophthalmologic findings are bilateral congenital cataracts, usually accompanied by microphthalmia, microcornea (diameter <10), and small atonic pupils. Poor vision despite early cataract surgery likely results from progressive optic atrophy and cortical visual impairment. Individuals with Warburg micro syndrome have severe to profound intellectual disability (ID); those with Martsolf syndrome have mild to moderate ID. Some individuals with RAB18 deficiency also have epilepsy. In Warburg micro syndrome, a progressive ascending spastic paraplegia typically begins with spastic diplegia and contractures during the first year, followed by upper-limb involvement leading to spastic quadriplegia after about age five years, often eventually causing breathing difficulties. In Martsolf syndrome infantile hypotonia is followed primarily by slowly progressive lower-limb spasticity. Hypogonadism – when present – manifests in both syndromes, in males as micropenis and/or cryptorchidism and in females as hypoplastic labia minora, clitoral hypoplasia, and small introitus.

X-linked multiple congenital anomalies-neurodevelopmental syndrome (MCAND) is an X-linked recessive congenital multisystemic disorder characterized by poor growth, global developmental delay with impaired intellectual development, and variable abnormalities of the cardiac, skeletal, and genitourinary systems. Most affected individuals also have hypotonia and dysmorphic craniofacial features. Brain imaging typically shows enlarged ventricles and thin corpus callosum; some have microcephaly, whereas others have hydrocephalus. The severity of the disorder is highly variable, ranging from death in early infancy to survival into the second or third decade. Pathogenetically, the disorder results from disrupted gene expression and signaling during embryogenesis, thus affecting multiple systems (summary by Tripolszki et al., 2021 and Beck et al., 2021). Beck et al. (2021) referred to the disorder as LINKED syndrome (LINKage-specific deubiquitylation deficiency-induced Embryonic Defects).

Autosomal dominant primary microcephaly-26 (MCPH26) is characterized by progressive microcephaly beginning at birth and associated with global developmental delay with variably impaired intellectual development. Some patients may have only mild learning difficulties or speech delay, whereas other are more severely affected with the inability to walk or speak. Additional features may include short stature, spasticity, feeding difficulties requiring tube feeding, and nonspecific dysmorphic facial features. Brain imaging in some patients shows a simplified gyral pattern or dysgenesis of the corpus callosum, suggesting abnormal neuronal migration (summary by Cristofoli et al., 2020). For a general phenotypic description and a discussion of genetic heterogeneity of primary microcephaly, see MCPH1 (251200).

Autosomal dominant primary microcephaly-27 (MCPH27) is characterized by small head circumference apparent in early childhood and associated with global developmental delay manifest as delayed walking, inability to walk, impaired intellectual development, and poor or absent speech. Most patients have variable and nonspecific additional features, including facial dysmorphism, hypotonia, limb hypertonia, poor feeding, and distal skeletal anomalies. Brain imaging may show enlarged ventricles or gyral abnormalities, but most have normal imaging (Parry et al., 2021). For a general phenotypic description and a discussion of genetic heterogeneity of primary microcephaly, see MCPH1 (251200).

Autosomal dominant intellectual developmental disorder-64 (MRD64) is characterized by mildly to severely impaired intellectual development (ID) with speech delays. Most patients also have autism spectrum disorder (ASD). Additional features are highly variable but may include motor delay, attention deficit-hyperactivity disorder (ADHD), and nonspecific dysmorphic features (summary by Mirzaa et al., 2020).

Den Hoed-de Boer-Voisin syndrome (DHDBV) is characterized by global developmental delay with moderately to severely impaired intellectual development, poor or absent speech, and delayed motor skills. Although the severity of the disorder varies, many patients are nonverbal and have hypotonia with inability to sit or walk. Early-onset epilepsy is common and may be refractory to treatment, leading to epileptic encephalopathy and further interruption of developmental progress. Most patients have feeding difficulties with poor overall growth and dysmorphic facial features, as well as significant dental anomalies resembling amelogenesis imperfecta. The phenotype is reminiscent of Kohlschutter-Tonz syndrome (KTZS; 226750). More variable features of DHDBV include visual defects, behavioral abnormalities, and nonspecific involvement of other organ systems (summary by den Hoed et al., 2021).

KINSSHIP syndrome (KINS) is an autosomal dominant disorder characterized by a recognizable pattern of anomalies including developmental delay, impaired intellectual development, seizures, mesomelic dysplasia, dysmorphic facial features, horseshoe or hypoplastic kidney, and failure to thrive (summary by Voisin et al., 2021).

Neurodevelopmental disorder with seizures and gingival overgrowth (NEDSGO) is an autosomal recessive disorder with a highly variable phenotype. Some patients have early normal development with developmental regression apparent in the first years of life, whereas others present with hypotonia or delayed development. Most patients develop significant gingival hypertrophy associated with a prominent mandible or cherubism in the first years of life. Other more variable features may include coarse facial features, optic atrophy, sensorineural hearing loss, ataxia, and seizures. Brain imaging may show cerebellar or cerebral atrophy and enlarged ventricles. There is a wide phenotypic spectrum with features that may develop with age; the disorder appears to comprise a continuum of evolving neurologic manifestations (Harms et al., 2020).

Mitochondrial complex IV deficiency nuclear type 22 (MC4DN22) is an autosomal recessive metabolic disorder characterized by neonatal hypertrophic cardiomyopathy, encephalopathy, and severe lactic acidosis with fatal outcome (Wintjes et al., 2021). For a discussion of genetic heterogeneity of mitochondrial complex IV (cytochrome c oxidase) deficiency, see 220110.

Neurodevelopmental disorder with hypotonia, facial dysmorphism, and brain abnormalities (NEDHFBA) is an autosomal recessive neurologic syndrome characterized by global developmental delay with severely impaired intellectual development, hypotonia and muscle weakness, often resulting in the inability to walk or sit, and characteristic coarse facial features. Additional features include feeding difficulties, respiratory distress, scoliosis, poor visual function, and rotary nystagmus. Brain imaging shows variable abnormalities, including enlarged ventricles, decreased white matter volume, white matter changes, thin corpus callosum, and cerebellar hypoplasia (summary by Loddo et al., 2020).

Infantile-onset multisystem neurologic, endocrine, and pancreatic disease-2 (IMNEPD2) is an autosomal recessive multisystemic disorder characterized by cholestatic hepatitis, poor feeding associated with poor overall growth, and hypoglycemia apparent from infancy. Most, but not all, patients have variable global developmental delay. Additional common features include sensorineural deafness, retinal abnormalities with visual defects, and hypotonia. Some patients have endocrine abnormalities, including hyperinsulinemic hypoglycemia, pancreatic dysfunction, hypothyroidism, and primary amenorrhea. Additional features may include hypertriglyceridemia, anemia, proteinuria, increased lactate, and recurrent infections. Brain imaging often shows dysmyelination, thin corpus callosum, cerebral atrophy, and white matter abnormalities. Although the clinical manifestations and severity of the disorder are highly variable, death in early childhood may occur (summary by Williams et al., 2019 and Zeiad et al., 2021). For a discussion of genetic heterogeneity of IMNEPD, see IMNEPD1 (616263).

Developmental delay, impaired speech, and behavioral abnormalities (DDISBA) is characterized by global developmental delay apparent from early childhood. Intellectual disability can range from mild to severe. Additional variable features may include dysmorphic facial features, seizures, hypotonia, motor abnormalities such as Tourette syndrome or dystonia, and hearing loss (summary by Cousin et al., 2021).

Short-rib thoracic dysplasia-21 (SRTD21) is characterized by rhizomelic limb shortening with bowing of long bones and metaphyseal abnormalities, narrow chest with short broad ribs, and trident pelvis. Other features include hypotonia and global developmental delay, with corpus callosum hypoplasia and cerebellar vermis abnormalities on brain imaging, which may show the 'molar tooth' sign (Hammarsjo et al., 2017). For a general phenotypic description and discussion of genetic heterogeneity of SRTD, see SRTD1 (208500). Mutation in the KIAA0753 gene also causes orofaciodigital syndrome (OFD15; 617127) and Joubert syndrome (JBTS28; 619476), phenotypes with features overlapping those of SRTD21.

DEGCAGS syndrome is an autosomal recessive syndromic neurodevelopmental disorder characterized by global developmental delay, coarse and dysmorphic facial features, and poor growth and feeding apparent from infancy. Affected individuals have variable systemic manifestations often with significant structural defects of the cardiovascular, genitourinary, gastrointestinal, and/or skeletal systems. Additional features may include sensorineural hearing loss, hypotonia, anemia or pancytopenia, and immunodeficiency with recurrent infections. Death in childhood may occur (summary by Bertoli-Avella et al., 2021).

Ventriculomegaly and arthrogryposis (VENARG) is a severe autosomal recessive congenital disorder characterized by the onset of features in utero that are not compatible with life. Affected pregnancies are terminated spontaneously or by plan due to the severity of the defects. Prenatal ultrasound and autopsy show limb contractures consistent with arthrogryposis and enlarged brain ventricles that may be associated with hydrocephalus, abnormalities of the corpus callosum, and cerebellar hypoplasia. Some affected fetuses may also have congenital heart disease and hydrops fetalis (summary by Mero et al., 2017 and El-Dessouky et al., 2020).

Neurodevelopmental disorder with hypotonia and dysmorphic facies (NEDHYDF) is characterized by global developmental delay and hypotonia apparent from birth. Affected individuals have variably impaired intellectual development, often with speech delay and delayed walking. Seizures are generally not observed, although some patients may have single seizures or late-onset epilepsy. Most patients have prominent dysmorphic facial features. Additional features may include congenital cardiac defects (without arrhythmia), nonspecific renal anomalies, joint contractures or joint hyperextensibility, dry skin, and cryptorchidism. There is significant phenotypic variability in both the neurologic and extraneurologic manifestations (summary by Tan et al., 2022).

Pontocerebellar hypoplasia type 16 (PCH16) is an autosomal recessive severe neurodevelopmental disorder characterized by hypotonia and severe global developmental delay apparent from early infancy. Although the severity of the disorder is variable, most affected individuals achieve only a few, if any, developmental milestones. Most are unable to walk or speak, have eye abnormalities with poor visual contact, and develop early-onset epilepsy. Other features may include stereotypic movements, spasticity, and progressive microcephaly. Brain imaging shows pontocerebellar hypoplasia, often with thin corpus callosum, atrophy of the thalamus and basal ganglia, enlarged ventricles, and white matter abnormalities (summary by Ucuncu et al., 2020). For a phenotypic description and a discussion of genetic heterogeneity of PCH, see PCH1A (607596).

Intellectual developmental disorder with hypotonia, impaired speech, and dysmorphic facies (IDDHISD) is characterized by global developmental delay with impaired intellectual development and poor or absent speech, hypotonia, ophthalmologic abnormalities, and nonspecific dysmorphic features. Some affected individuals have seizures, and a few have involvement of other organ systems (Goodman et al., 2021).

Developmental and epileptic encephalopathy-97 (DEE97) is characterized by developmental delay, epileptic encephalopathy, and impaired intellectual development. Other clinical features may include autistic features and hypotonia. For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

Neurodevelopmental disorder with impaired language and ataxia and with or without seizures (NEDLAS) is characterized by axial hypotonia and global developmental delay apparent in early infancy. Affected individuals have delayed walking with gait ataxia and poor language development. Behavioral abnormalities also commonly occur. The severity is highly variable: a subset of patients have a more severe phenotype with early-onset seizures resembling epileptic encephalopathy, inability to walk or speak, and hypomyelination on brain imaging (summary by Stolz et al., 2021).

Galloway-Mowat syndrome-9 (GAMOS9) is an autosomal recessive disorder characterized by onset of nephrotic syndrome with proteinuria in infancy or early childhood. The renal disease is slowly progressive, but some affected individuals may develop end-stage renal disease in the first decade. Renal biopsy shows focal segmental glomerulosclerosis (FSGS) or diffuse mesangial sclerosis (DMS). Affected individuals also have developmental delay and secondary microcephaly. Additional features may include facial dysmorphism and gastroesophageal reflux. Early death may occur (Arrondel et al., 2019). For a discussion of genetic heterogeneity of GAMOS, see GAMOS1 (251300).

Developmental and epileptic encephalopathy-99 (DEE99) is characterized by onset of seizures in early childhood associated with global developmental delay and severely impaired intellectual development. Other features may include hypotonia, quadriparesis, nystagmus, and apnea. Brain imaging may be normal or show nonspecific and variable abnormalities, including cerebral atrophy and polymicrogyria. The severity is variable; some patients die of refractory status epilepticus (summary by Vetro et al., 2021). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

Yoon-Bellen neurodevelopmental syndrome (YOBELN) is an autosomal recessive disorder characterized mainly by global developmental delay with variably impaired intellectual development. The manifestations and severity of the phenotype are highly variable. Additional neurologic features may include hypotonia, spasticity, ataxia, hearing loss, visual problems, seizures, and nonspecific anomalies on brain imaging (summary by Yap et al., 2021).

A rare genetic neurological disorder with characteristics of early-onset severe global developmental delay with regression, congenital or acquired microcephaly, hearing loss, truncal hypotonia, appendicular spasticity, and dystonia and/or myoclonus. Additional reported manifestations include seizures, optic atrophy, cortical visual impairment, scoliosis, and dysphagia. Brain imaging shows pontine hypoplasia, partial agenesis of the corpus callosum, and diffuse cerebral atrophy with relative sparing of the cerebellum.

Infantile hypotonia with psychomotor retardation and characteristic facies-3 is a severe autosomal recessive neurodevelopmental disorder with onset at birth or in early infancy. Most affected individuals show very poor, if any, normal psychomotor development, poor speech, and inability to walk independently (summary by Bhoj et al., 2016). For a general phenotypic description and a discussion of genetic heterogeneity of infantile hypotonia with psychomotor retardation and characteristic facies, see IHPRF1 (615419).

A rare mitochondrial oxidative phosphorylation disorder with decreased respiratory complex I and IV enzyme activity. Characteristics of this disease hypotonia, global developmental delay, neonatal onset of progressive pectus carinatum without other skeletal abnormalities, poor growth, sensorineural hearing loss, dysmorphic features and brain abnormalities such as cerebral atrophy, quadriventricular dilatation and thin corpus callosum posteriorly.

DYRK1A syndrome is characterized by intellectual disability including impaired speech development, autism spectrum disorder including anxious and/or stereotypic behavior problems, and microcephaly. Affected individuals often have a clinically recognizable phenotype including a typical facial gestalt, feeding problems, seizures, hypertonia, gait disturbances, and foot anomalies. The majority of affected individuals function in the moderate-to-severe range of intellectual disability; however, individuals with mild intellectual disability have also been reported. Other medical concerns relate to febrile seizures in infancy; the development of epilepsy with seizures of the atonic, absence, and generalized myoclonic types; short stature; and gastrointestinal problems. Ophthalmologic, urogenital, cardiac, and/or dental anomalies have been reported.

Combined D-2- and L-2-hydroxyglutaric aciduria (D2L2AD) is an autosomal recessive neurometabolic disorder characterized by neonatal-onset encephalopathy with severe muscular weakness, intractable seizures, respiratory distress, and lack of psychomotor development resulting in early death. Brain imaging shows abnormalities including enlarged ventricles, delayed myelination, and germinal layer cysts (Muntau et al., 2000). See also isolated L-2-hydroxyglutaric aciduria (236792) and isolated D-2-hydroxyglutaric aciduria (see 600721).

Carey-Fineman-Ziter syndrome-1 (CFZS1) is a multisystem congenital disorder characterized by hypotonia, Moebius sequence (bilateral congenital facial palsy with impairment of ocular abduction), Pierre Robin complex (micrognathia, glossoptosis, and high-arched or cleft palate), delayed motor milestones, and failure to thrive. More variable features include dysmorphic facial features, brain abnormalities, and intellectual disability. It has been postulated that many clinical features in CFZS1 may be secondary effects of muscle weakness during development or brainstem anomalies (summary by Pasetti et al., 2016). Di Gioia et al. (2017) determined that CFZS1 represents a slowly progressive congenital myopathy resulting from a defect in myoblast fusion. Genetic Heterogeneity of Carey-Fineman-Ziter Syndrome Carey-Fineman-Ziter syndrome-2 (CFZS2) is caused by mutation in the MYMX gene (619912) on chromosome 6p21.

Bryant-Li-Bhoj neurodevelopmental syndrome-1 (BRYLIB1) is a highly variable phenotype characterized predominantly by moderate to severe global developmental delay with impaired intellectual development, poor or absent speech, and delayed motor milestones. Most patients have hypotonia, although some have peripheral hypertonia. Common features include abnormal head shape, variable dysmorphic facial features, oculomotor abnormalities, feeding problems, and nonspecific brain imaging abnormalities. Additional features may include hearing loss, seizures, short stature, and mild skeletal defects (summary by Bryant et al., 2020). Genetic Heterogeneity of Bryant-Li-Bhoj Neurodevelopmental Syndrome See also BRYLIB2 (619721), caused by heterozygous mutation in the H3F3B gene (601058).

Combined oxidative phosphorylation deficiency-55 (COXPD55) is characterized by global developmental delay, hypotonia, short stature, and impaired intellectual development with speech disabilities in childhood. Indolent progressive external ophthalmoplegia phenotype has been described in 1 patient (summary by Olahova et al., 2021). For a discussion of genetic heterogeneity of combined oxidative phosphorylation deficiency, see COXPD1 (609060).

Congenital disorder of deglycosylation-2 (CDDG2) is an autosomal recessive disorder with variable associated features such as dysmorphic facies, impaired intellectual development, and brain anomalies, including polymicrogyria, interhemispheric cysts, hypothalamic hamartoma, callosal anomalies, and hypoplasia of brainstem and cerebellar vermis (Maia et al., 2022). For a discussion of genetic heterogeneity of congenital disorder of deglycosylation, see CDGG1 (615273).

Developmental and epileptic encephalopathy-100 (DEE100) is a severe neurologic disorder characterized by global developmental delay and onset of variable types of seizures in the first months or years of life. Most patients have refractory seizures and show developmental regression after seizure onset. Affected individuals have ataxic gait or inability to walk and severe to profoundly impaired intellectual development, often with absent speech. Additional more variable features may include axial hypotonia, hyperkinetic movements, dysmorphic facial features, and brain imaging abnormalities (summary by Schneider et al., 2021). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

Neurodevelopmental disorder with central hypotonia and dysmorphic facies (NEDCHF) is an autosomal dominant disorder characterized by global developmental delay, impaired intellectual development, seizures, distinctive facial features, scoliosis, delayed closure of the anterior fontanel, and nonspecific brain abnormalities (Wakeling et al., 2021).

Neurodevelopmental disorder with neuromuscular and skeletal abnormalities (NEDNMS) is an autosomal recessive disorder characterized by global developmental delay apparent from infancy or early childhood. The severity of the disorder is highly variable. Affected individuals show impaired intellectual development and motor delay associated with either severe hypotonia or hypertonia and spasticity. Most affected individuals have skeletal defects and dysmorphic facial features. Some may have ocular or auditory problems, peripheral neuropathy, behavioral abnormalities, and nonspecific findings on brain imaging (Kurolap et al., 2022).

Hypomyelinating leukodystrophy-24 (HLD24) is an autosomal dominant disorder characterized by global developmental delay and neurologic deterioration (Segawa et al., 2021). For a general phenotypic description and a discussion of genetic heterogeneity of HLD, see 312080.

Holoprosencephaly-14 (HPE14) is an autosomal recessive condition characterized by severe developmental delay secondary to brain malformations within the holoprosencephaly spectrum (Drissi et al., 2022). For general phenotypic information and a discussion of genetic heterogeneity of holoprosencephaly, see HPE1 (236100).

Neurodevelopmental disorder with dystonia and seizures (NEDDS) is a severe autosomal recessive disorder characterized by hypotonia and dystonic posturing apparent from early infancy. Affected individuals show global developmental delay with inability to walk or speak and have profoundly impaired intellectual development, often with behavioral abnormalities. Additional features may include other extrapyramidal movements, seizures or seizure-like activity, and cerebellar hypoplasia on brain imaging (Sleiman et al., 2022).

Neurodevelopmental disorder with seizures, microcephaly, and brain abnormalities (NEDSMBA) is an autosomal recessive disorder characterized by a core phenotype of moderate to profound developmental delay, progressive microcephaly, epilepsy, and periventricular calcifications (summary by Rosenhahn et al., 2022).

Neurodevelopmental disorder with microcephaly, cerebral atrophy, and visual impairment (NEDMVIC) is an autosomal recessive disorder characterized by global developmental delay, impaired intellectual development, facial dysmorphism, and microcephaly (Ziegler et al., 2022).

Autosomal dominant spastic paraplegia-88 (SPG88) is characterized by onset of symptoms in the first year of life. Affected individuals show delayed motor development with walking difficulties due to spasticity of the lower limbs. The disorder is slowly progressive, but variable in severity; some patients are unable to ambulate independently. Most patients have a pure form of the disorder, although rare patients have been reported to have additional features, including peripheral neuropathy, speech delay, ADHD, and nonspecific brain imaging abnormalities (Schob et al., 2021, Estiar et al., 2022, De Winter et al., 2022). For a general phenotypic description and a discussion of genetic heterogeneity of autosomal dominant spastic paraplegia, see SPG3A (182600).

Complex cortical dysplasia with other brain malformations-11 (CDCBM11) is an autosomal recessive disorder characterized by dilated ventricles and reduced white matter and associated with axonal developmental defects (Qian et al., 2022). For a discussion of genetic heterogeneity of CDCBM, see CDCBM1 (614039).

Congenital muscular dystrophy with or without seizures (MYOS) is an autosomal recessive disorder characterized by severe muscle hypotonia apparent from birth, as well as developmental delay. Laboratory studies show increased serum creatine kinase and muscle biopsy shows nonspecific dystrophic features. Most patients develop seizures or have abnormal epileptiform findings on EEG studies; other variable findings may include feeding difficulties, nystagmus, myopathic facies, areflexia, and brain atrophy on MRI (summary by Larson et al., 2018 and Henige et al., 2021).

Autosomal recessive primary microcephaly-30 (MCPH30) is characterized by small head circumference, poor overall growth, and global developmental delay with variably impaired intellectual development. Affected individuals have been reported to have variable additional congenital anomalies, including atrial septal defect, dysmorphic facial features, tracheal stenosis, and anomalies of the skin and teeth (Carvalhal et al., 2022). For a general phenotypic description and a discussion of genetic heterogeneity of primary microcephaly, see MCPH1 (251200).

Congenital disorder of glycosylation type IIy (CDG2Y) is an autosomal recessive multisystemic congenital disorder characterized by poor overall growth and global developmental delay with impaired intellectual development. Other features may include hypotonia, seizures, brain imaging abnormalities, dysmorphic features, and various skeletal defects. Laboratory studies show a subtle type II glycosylation defect of serum transferrin (Tambe et al., 2020). For a general discussion of CDGs, see CDG1A (212065).

Neurodevelopmental disorder with dysmorphic facies and ischiopubic hypoplasia (NEDFIH) is an autosomal recessive disorder characterized by these features and moderate to severe global developmental delay. Affected individuals show episodic regression during periods of stress, including seizures or infection, the latter of which may be associated with lymphopenia. Brain imaging shows diminished white matter volume, enlarged ventricles, and thin corpus callosum (Muffels et al., 2023).

PPP2R5D-related neurodevelopmental disorder is characterized by mild to severe neurodevelopmental delay. Pronounced hypotonia with delay in gross motor skills is common. Onset of independent walking varies widely and ataxia is reported. All reported individuals have speech impairment, with a wide range of abilities. Autism spectrum disorder is reported in six individuals. Macrocephaly is common. Seizures and ophthalmologic abnormalities are reported in fewer than half of individuals. Additional anomalies include skeletal, endocrine, and cardiac malformations, each reported in a few individuals. To date, 23 individuals with PPP2R5D-related neurodevelopmental disorder have been reported.

X-linked intellectual developmental disorder-111 (XLID111) is a neurodevelopmental disorder characterized by different degrees of impaired intellectual development associated with motor, speech, and behavioral impairments (El Chehadeh et al., 2022).

Neurodevelopmental disorder with hypotonia, dysmorphic facies, and skeletal anomalies, with or without seizures (NEDFSS), is characterized by these features and global developmental delay with delayed or absent walking, moderate to severely impaired intellectual development, and poor or absent speech acquisition. Affected individuals may also have behavioral abnormalities. About half of patients develop various types of seizures that are usually well-controlled with medication. Rare patients are noted to have heat intolerance or insensitivity to pain (Lines et al., 2022).

Neurooculorenal syndrome (NORS) is an autosomal recessive developmental disorder with highly variable clinical manifestations involving several organ systems. Some affected individuals present in utero with renal agenesis and structural brain abnormalities incompatible with life, whereas others present in infancy with a neurodevelopmental disorder characterized by global developmental delay and dysmorphic facial features that may be associated with congenital anomalies of the kidney and urinary tract (CAKUT). Additional more variable features may include ocular anomalies, most commonly strabismus, congenital heart defects, and pituitary hormone deficiency. Brain imaging usually shows structural midline defects, including dysgenesis of the corpus callosum and hindbrain. There is variation in the severity, manifestations, and expressivity of the phenotype, even within families (Rasmussen et al., 2018; Munch et al., 2022).

Neurodegeneration and seizures due to copper transport defect (NSCT) is an autosomal recessive disorder of copper transport characterized by hypotonia, global developmental delay, seizures, and rapid brain atrophy (summary by Dame et al., 2023).

Leukoencephalopathy with vanishing white matter-4 (VWM4) is a chronic and progressive autosomal recessive leukoencephalopathy characterized by neurologic deterioration with cerebellar ataxia, spasticity, and relatively mild mental decline. Onset is usually in childhood; early development may be normal. Female patients may experience ovarian failure. Magnetic resonance imaging (MRI) and magnetic resonance spectroscopy are diagnostic and show a diffuse abnormality of the cerebral white matter beginning in the presymptomatic stage, with increasing amounts of the abnormal white matter vanishing and being replaced by cerebrospinal fluid; autopsy confirms these findings (summary by van der Knaap et al., 2002 and Fogli et al., 2003). For a discussion of genetic heterogeneity of VWM, see 603896.

Developmental and epileptic encephalopathy-31B (DEE31B) is an autosomal recessive neurologic disorder with early-onset epilepsy, generalized muscular hypotonia, visual impairment, and severe neurodevelopmental delay (Yigit et al., 2022).

Neurodevelopmental disorder with motor and language delay, ocular defects, and brain abnormalities (NEDMLOB) is an autosomal recessive neurologic disorder characterized by the onset of features in infancy or early childhood. Affected individuals show hypotonia, severe motor delay with ataxic gait or sometimes an inability to achieve walking, and impaired intellectual development with speech and language delay. Ocular defects can include optic atrophy, nystagmus, strabismus, and retinal dystrophy. Additional features may include seizures (in some), dysmorphic facial features, poor overall growth, and variable brain imaging abnormalities (Tepe et al., 2023).

Neurodevelopmental disorder with hypotonia and speech delay, with or without seizures (NEDHSS) is characterized by global developmental delay, impaired intellectual development with poor or absent speech, and fine and gross motor delay. Most affected individuals are severely affected and may be unable to walk, have feeding difficulties requiring tube-feeding, and develop early-onset seizures. Additional features may include cortical blindness and nonspecific structural brain abnormalities. Rare individuals present only with hypotonia and mild developmental delay (Paul et al., 2023).

Thrombocytopenia-11 with multiple congenital anomalies and dysmorphic facies (THC11) is a syndromic disorder characterized by dysmorphic facial features, multiple congenital anomalies that may involve the heart, brain, genitourinary, endocrine, and/or skeletal systems, chronic and persistent thrombocytopenia, sometimes with leukopenia or anemia, poor growth with microcephaly, hypotonia, and mildly impaired intellectual development or learning disabilities. The disorder results from constitutive activation of the RAS signaling pathway and can be considered a RASopathy (Niemann et al., 2020; Miller et al., 2022). For a discussion of genetic heterogeneity of thrombocytopenia, see 313900.

Hypomyelinating leukodystrophy-27 (HLD27) is an autosomal recessive neurologic disorder characterized by global developmental delay with impaired motor and intellectual development apparent from infancy. Affected individuals have poor or absent speech, ataxic gait or inability to sit or walk, spasticity, and abnormal eye movements (nystagmus, gaze palsy). Some patients have seizures. Disease progression and developmental regression consistent with neurodegeneration is often observed. Brain imaging shows progressive hypomyelinating leukodystrophy, cerebral and cerebellar atrophy, and thin corpus callosum (Misceo et al., 2023). For a general phenotypic description and a discussion of genetic heterogeneity of HLD, see 312080.

Polydactyly-macrocephaly syndrome (PDMCS) is characterized by postaxial polydactyly and progressive macrocephaly. Variable ocular anomalies have been observed, including microphthalmia and coloboma as well as delayed visual maturation. Neurodevelopmental anomalies are also present, including global developmental delay and autism or autistic traits, with prominent perivascular spaces on brain imaging (Harris et al., 2024).