Clubfoot is a congenital limb deformity defined as fixation of the foot in cavus, adductus, varus, and equinus (i.e., inclined inwards, axially rotated outwards, and pointing downwards) with concomitant soft tissue abnormalities (Cardy et al., 2007). Clubfoot may occur in isolation or as part of a syndrome (e.g., diastrophic dysplasia, 222600). Clubfoot has been reported with deficiency of long bones and mirror-image polydactyly (Gurnett et al., 2008; Klopocki et al., 2012).

Smith-Lemli-Opitz syndrome (SLOS) is a congenital multiple-anomaly / cognitive impairment syndrome caused by an abnormality in cholesterol metabolism resulting from deficiency of the enzyme 7-dehydrocholesterol (7-DHC) reductase. It is characterized by prenatal and postnatal growth restriction, microcephaly, moderate-to-severe intellectual disability, and multiple major and minor malformations. The malformations include distinctive facial features, cleft palate, cardiac defects, underdeveloped external genitalia in males, postaxial polydactyly, and 2-3 syndactyly of the toes. The clinical spectrum is wide; individuals with normal development and only minor malformations have been described.

Craniofrontonasal syndrome is an X-linked developmental disorder that shows paradoxically greater severity in heterozygous females than in hemizygous males. Females have frontonasal dysplasia, craniofacial asymmetry, craniosynostosis, bifid nasal tip, grooved nails, wiry hair, and abnormalities of the thoracic skeleton, whereas males typically show only hypertelorism (Twigg et al., 2004; Wieland et al., 2004).

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).

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.

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).

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).

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.

PAX6-related aniridia occurs either as an isolated ocular abnormality or as part of the Wilms tumor-aniridia-genital anomalies-retardation (WAGR) syndrome. Aniridia is a pan ocular disorder affecting the cornea, iris, intraocular pressure (resulting in glaucoma), lens (cataract and lens subluxation), fovea (foveal hypoplasia), and optic nerve (optic nerve coloboma and hypoplasia). Individuals with aniridia characteristically show nystagmus and impaired visual acuity (usually 20/100 - 20/200); however, milder forms of aniridia with subtle iris architecture changes, good vision, and normal foveal structure do occur. Other ocular involvement may include strabismus and occasionally microphthalmia. Although the severity of aniridia can vary between and within families, little variability is usually observed in the two eyes of an affected individual. WAGR syndrome. The risk for Wilms tumor is 42.5%-77%; of those who develop Wilms tumor, 90% do so by age four years and 98% by age seven years. Genital anomalies in males can include cryptorchidism and hypospadias (sometimes resulting in ambiguous genitalia), urethral strictures, ureteric abnormalities, and gonadoblastoma. While females typically have normal external genitalia, they may have uterine abnormalities and streak ovaries. Intellectual disability (defined as IQ <74) is observed in 70%; behavioral abnormalities include attention-deficit/hyperactivity disorder (ADHD), autism spectrum disorder, anxiety, depression, and obsessive-compulsive disorder. Other individuals with WAGR syndrome can have normal intellect without behavioral problems.

Encephalocraniocutaneous lipomatosis (ECCL) comprises a spectrum of predominantly congenital anomalies. In its typical form, ECCL is characterized by congenital anomalies of the skin (nevus psiloliparus, patchy or streaky non-scarring alopecia, subcutaneous lipomas in the frontotemporal region, focal skin aplasia or hypoplasia on the scalp, and/or small nodular skin tags on the eyelids or between the outer canthus and tragus), eye (choristoma), and brain (in particular intracranial and spinal lipomas). To a much lesser degree, the bones and the heart can be affected. About 40% of affected individuals have bilateral abnormalities of the skin or the eyes. About one third of affected individuals have normal cognitive development, another one third have mild developmental delay (DD) or intellectual disability (ID), and the final one third have severe or unspecified DD/ID. Half of individuals have seizures. Affected individuals are at an increased (i.e., above the general population) risk of developing brain tumors, particularly low-grade gliomas such as pilocytic astrocytomas. There is evidence that oculoectodermal syndrome (OES) may constitute a clinical spectrum with ECCL, with OES on the mild end and ECCL on the more severe end of the spectrum.

Syndrome with characteristics of psychomotor delay, brachycephaly with flat face, small nose, microstomia, cleft palate, cataract, hearing loss, hypoplastic scrotum and digital anomalies. Less than 10 patients have been described in the literature so far. Although the majority of reported cases were sporadic, the syndrome has been reported in one pair of siblings (a brother and sister) with an apparently autosomal recessive inheritance pattern.

A rare neurologic disease typically characterized by the triad of eye, central nervous system and skin malformations, and often associated with an intellectual disability.

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.

CLCN4-related neurodevelopmental disorder (CLCN4-NDD), an X-linked disorder, is characterized in the 36 males reported to date by developmental delay or intellectual disability, behavioral/mental health issues (e.g., autism spectrum disorder, anxiety, hyperactivity, and bipolar disorder), epilepsy, and gastrointestinal dysfunction. The five heterozygous females with a de novo CLCN4 variant reported to date had findings very similar to those of affected males. Twenty-two of 25 heterozygous females identified in family studies following identification of an affected male were unaffected or had only mild specific learning difficulties and/or mental health concerns, whereas three were more severely affected.

Bohring-Opitz syndrome (BOS) is characterized by distinctive facial features and posture, growth failure, variable but usually severe intellectual disability, and variable anomalies. The facial features may include microcephaly or trigonocephaly / prominent (but not fused) metopic ridge, hypotonic facies with full cheeks, synophrys, glabellar and eyelid nevus flammeus (simplex), prominent globes, widely set eyes, palate anomalies, and micrognathia. The BOS posture, which is most striking in early childhood and often becomes less apparent with age, is characterized by flexion at the elbows with ulnar deviation and flexion of the wrists and metacarpophalangeal joints. Feeding difficulties in early childhood, including cyclic vomiting, have a significant impact on overall health; feeding tends to improve with age. Seizures are common and typically responsive to standard epileptic medications. Minor cardiac anomalies and transient bradycardia and apnea may be present. Affected individuals may experience recurrent infections, which also tend to improve with age. Isolated case reports suggest that individuals with BOS are at greater risk for Wilms tumor than the general population, but large-scale epidemiologic studies have not been conducted.

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.

Goldberg-Shprintzen syndrome (GOSHS) is an autosomal recessive multiple congenital anomaly syndrome characterized by impaired intellectual development, microcephaly, and dysmorphic facial features. Most patients also have Hirschsprung disease and/or gyral abnormalities of the brain, consistent with defects in migration of neural crest cells and neurons. Other features, such as megalocornea or urogenital anomalies, may also be present. Goldberg-Shprintzen syndrome has some resemblance to Mowat-Wilson syndrome (MOWS; 235730) but is genetically distinct (summary by Drevillon et al., 2013).

Combined oxidative phosphorylation deficiency is an autosomal recessive multisystem disorder with variable manifestations resulting from a defect in the mitochondrial oxidative phosphorylation (OXPHOS) system. Onset occurs at or soon after birth, and features can include growth retardation, microcephaly, hypertonicity, axial hypotonia, encephalopathy, cardiomyopathy, and liver dysfunction. Death usually occurs in the first weeks or years of life (summary by Smits et al., 2011). Genetic Heterogeneity of Combined Oxidative Phosphorylation Deficiency See also COXPD2 (610498), caused by mutation in the MRPS16 gene (609204) on 10q22; COXPD3 (610505), caused by mutation in the TSFM gene (604723) on 12q14; COXPD4 (610678), caused by mutation in the TUFM gene (602389) on 16p11; COXPD5 (611719), caused by mutation in the MRPS22 gene (605810) on 3q23; COXPD6 (300816), caused by mutation in the AIFM1 gene (300169) on Xq26; COXPD7 (613559), caused by mutation in the MTRFR gene (613541) on 12q24; COXPD8 (614096), caused by mutation in the AARS2 gene (612035) on 6p21; COXPD9 (614582), caused by mutation in the MRPL3 gene (607118) on 3q22; COXPD10 (614702), caused by mutation in the MTO1 gene (614667) on 6q13; COXPD11 (614922), caused by mutation in the RMND1 gene (614917) on 6q25; COXPD12 (614924), caused by mutation in the EARS2 gene (612799) on 16p13; COXPD13 (614932), caused by mutation in the PNPT1 gene (610316) on 2p16; COXPD14 (614946), caused by mutation in the FARS2 gene (611592) on 6p25; COXPD15 (614947), caused by mutation in the MTFMT gene (611766) on 15q; COXPD16 (615395), caused by mutation in the MRPL44 gene (611849) on 2q36; COXPD17 (615440), caused by mutation in the ELAC2 gene (605367) on 17p11; COXPD18 (615578), caused by mutation in the SFXN4 gene (615564) on 10q26; COXPD19 (615595), caused by mutation in the LYRM4 gene (613311) on 6p25; COXPD20 (615917), caused by mutation in the VARS2 gene (612802) on 6p21; COXPD21 (615918), caused by mutation in the TARS2 gene (612805) on 1q21; COXPD22 (616045), caused by mutation in the ATP5A1 gene (164360) on 18q12; COXPD23 (616198), caused by mutation in the GTPBP3 (608536) gene on 19p13; COXPD24 (616239), caused by mutation in the NARS2 gene (612803) on 11q14; COXPD25 (616430), caused by mutation in the MARS2 gene (609728) on 2q33; COXPD26 (616539), caused by mutation in the TRMT5 gene (611023) on 14q23; COXPD27 (616672), caused by mutation in the CARS2 gene (612800) on 13q34; COXPD28 (616794), caused by mutation in the SLC25A26 gene (611037) on 3p14; COXPD29 (616811), caused by mutation in the TXN2 gene (609063) on 22q12; COXPD30 (616974), caused by mutation in the TRMT10C gene (615423) on 3q12; and COXPD31 (617228), caused by mutation in the MIPEP gene (602241) on 13q12; COXPD32 (617664), caused by mutation in the MRPS34 gene (611994) on 16q13; COXPD33 (617713), caused by mutation in the C1QBP gene (601269) on 17p13; and COXPD34 (617872), caused by mutation in the MRPS7 gene (611974) on 17q25; COXPD35 (617873), caused by mutation in the TRIT1 gene (617840) on 1p34; COXPD36 (617950), caused by mutation in the MRPS2 gene (611971) on 9q34; COXPD37 (618329), caused by mutation in the MICOS13 gene (616658) on 19p13; COXPD38 (618378), caused by mutation in the MRPS14 gene (611978) on 1q23; COXPD39 (618397), caused by mutation in the GFM2 gene (606544) on 5q13; COXPD40 (618835), caused by mutation in the QRSL1 gene (617209) on 6q21; COXPD41 (618838), caused by mutation in the GATB gene (603645) on 4q31; COXPD42 (618839), caused by mutation in the GATC gene (617210) on 12q24; COXPD43 (618851), caused by mutation in the TIMM22 gene (607251) on 17p13; COXPD44 (618855), caused by mutation in the FASTKD2 gene (612322) on 2q33; COXPD45 (618951), caused by mutation in the MRPL12 gene (602375) on 17q25; COXPD46 (618952), caused by mutation in the MRPS23 gene (611985) on 17q22; COXPD47 (618958), caused by mutation in the MRPS28 gene (611990) on 8q21; COXPD48 (619012), caused by mutation in the NSUN3 gene (617491) on 3q11; COXPD49 (619024), caused by mutation in the MIEF2 gene (615498) on 17p11; COXPD50 (619025), caused by mutation in the MRPS25 gene (611987) on 3p25; COXPD51 (619057), caused by mutation in the PTCD3 gene (614918) on 2p11; COXPD52 (619386), caused by mutation in the NFS1 gene (603485) on 20q11; COXPD53 (619423), caused by mutation in the C2ORF69 gene (619219) on 2q33; and COXPD54 (619737), caused by mutation in the PRORP gene (609947) on 14q13.; COXPD55 (619743), caused by mutation in the POLRMT gene (601778) on 19p13; COXPD56 (620139), caused by mutation in the TAMM41 gene (614948) on 3p25; COXPD57 (620167), caused by mutation in the CRLS1 gene (608188) on 20p12; COXPD58 (620451), caused by mutation in the TEFM gene (616422) on 17q11; and COXPD59 (620646), caused by mutation in the MRPL39 gene (611845) on 21q21.

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).

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.

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.

Pontocerebellar hypoplasia (PCH) refers to a heterogeneous group of disorders characterized by an abnormally small cerebellum and brainstem. Clinical features vary, but usually include severe developmental delay, dysmorphic features, seizures, and early death (summary by Durmaz et al., 2009). For a phenotypic description and a discussion of genetic heterogeneity of PCH, see PCH1 (607596).

The constitutional deletion of chromosome 1p36 results in a syndrome with multiple congenital anomalies and mental retardation (Shapira et al., 1997). Monosomy 1p36 is the most common terminal deletion syndrome in humans, occurring in 1 in 5,000 births (Shaffer and Lupski, 2000; Heilstedt et al., 2003). See also neurodevelopmental disorder with or without anomalies of the brain, eye, or heart (NEDBEH; 616975), which shows overlapping features and is caused by heterozygous mutation in the RERE gene (605226) on proximal chromosome 1p36. See also Radio-Tartaglia syndrome (RATARS; 619312), caused by mutation in the SPEN gene (613484) on chromosome 1p36, which shows overlapping features.

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.

The Hedera type of X-linked syndromic intellectual developmental disorder (MRXSH) is characterized by global developmental delay apparent from infancy and progressive neurologic decline with abnormal movements, spasticity, and seizures. Brain imaging shows volume loss of cortical white and gray matter, thin corpus callosum, and myelination defects, consistent with a neurodegenerative process. Only males are affected (summary by Hirose et al., 2019).

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).

The creatine deficiency disorders (CDDs), inborn errors of creatine metabolism and transport, comprise three disorders: the creatine biosynthesis disorders guanidinoacetate methyltransferase (GAMT) deficiency and L-arginine:glycine amidinotransferase (AGAT) deficiency; and creatine transporter (CRTR) deficiency. Developmental delay and cognitive dysfunction or intellectual disability and speech-language disorder are common to all three CDDs. Onset of clinical manifestations of GAMT deficiency (reported in ~130 individuals) is between ages three months and two years; in addition to developmental delays, the majority of individuals have epilepsy and develop a behavior disorder (e.g., hyperactivity, autism, or self-injurious behavior), and about 30% have movement disorder. AGAT deficiency has been reported in 16 individuals; none have had epilepsy or movement disorders. Clinical findings of CRTR deficiency in affected males (reported in ~130 individuals) in addition to developmental delays include epilepsy (variable seizure types and may be intractable) and behavior disorders (e.g., attention deficit and/or hyperactivity, autistic features, impulsivity, social anxiety), hypotonia, and (less commonly) a movement disorder. Poor weight gain with constipation and prolonged QTc on EKG have been reported. While mild-to-moderate intellectual disability is commonly observed up to age four years, the majority of adult males with CRTR deficiency have been reported to have severe intellectual disability. Females heterozygous for CRTR deficiency are typically either asymptomatic or have mild intellectual disability, although a more severe phenotype resembling the male phenotype has been reported.

X-linked spondyloepimetaphyseal dysplasia with hypomyelinating leukodystrophy (SEMDHL) is an X-linked recessive developmental disorder characterized by slowly progressive skeletal and neurologic abnormalities, including short stature, large and deformed joints, significant motor impairment, visual defects, and sometimes cognitive deficits. Affected individuals typically have normal early development in the first year or so of life, followed by development regression and the development of symptoms. Brain imaging shows white matter abnormalities consistent with hypomyelinating leukodystrophy (summary by Miyake et al., 2017).

Oculofaciocardiodental (OFCD) syndrome is a condition that affects the development of the eyes (oculo-), facial features (facio-), heart (cardio-) and teeth (dental). This condition occurs only in females.\n\nThe eye abnormalities associated with OFCD syndrome can affect one or both eyes. Many people with this condition are born with eyeballs that are abnormally small (microphthalmia). Other eye problems can include clouding of the lens (cataract) and a higher risk of glaucoma, an eye disease that increases the pressure in the eye. These abnormalities can lead to vision loss or blindness.\n\nPeople with OFCD syndrome often have a long, narrow face with distinctive facial features, including deep-set eyes and a broad nasal tip that is divided by a cleft. Some affected people have an opening in the roof of the mouth called a cleft palate.\n\nHeart defects are another common feature of OFCD syndrome. Babies with this condition may be born with a hole between two chambers of the heart (an atrial or ventricular septal defect) or a leak in one of the valves that controls blood flow through the heart (mitral valve prolapse).\n\nTeeth with very large roots (radiculomegaly) are characteristic of OFCD syndrome. Additional dental abnormalities can include delayed loss of primary (baby) teeth, missing or abnormally small teeth, misaligned teeth, and defective tooth enamel.

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).

Al-Gazali-Bakalinova syndrome (AGBK) is characterized by multiple epiphyseal dysplasia, macrocephaly, and distinctive facial features including frontal bossing, hypertelorism, flat malar regions, low-set ears, and short neck. Other features include pectus excavatum, spindle-shaped fingers, clinodactyly, prominent joints, and genu valgum (summary by Ali et al., 2012).

FLNA deficiency is associated with a phenotypic spectrum that includes FLNA-related periventricular nodular heterotopia (Huttenlocher syndrome), congenital heart disease (patent ductus arteriosus, atrial and ventricular septal defects), valvular dystrophy, dilation and rupture of the thoracic aortic, pulmonary disease (pulmonary hypertension, alveolar hypoplasia, emphysema, asthma, chronic bronchitis), gastrointestinal dysmotility and obstruction, joint hypermobility, and macrothrombocytopenia.

Spastic paraplegia 15 (SPG15), typically an early-onset complex hereditary spastic paraplegia, is characterized by progressive spasticity that begins in the lower extremities and is associated with several manifestations resulting from central and peripheral nervous system dysfunction. While onset of spasticity is typically in mid- to late childhood or adolescence (i.e., between ages 5 and 18 years), other manifestations, such as developmental delay or learning disability, may be present earlier, often preceding motor involvement. Individuals with adult onset have also been reported.

Patients with mutations in the receptor for insulin-like growth factor I show intrauterine growth retardation and postnatal growth failure, resulting in short stature and microcephaly. Other features may include delayed bone age, developmental delay, and dysmorphic features.

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).

PEHO is a severe autosomal recessive neurodevelopmental disorder characterized by extreme cerebellar atrophy due to almost total loss of granule neurons. Affected individuals present in early infancy with hypotonia, profoundly delayed psychomotor development, optic atrophy, progressive atrophy of the cerebellum and brainstem, and dysmyelination. Most patients also develop infantile seizures that are often associated with hypsarrhythmia on EEG, and many have peripheral edema (summary by Anttonen et al., 2017).

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.

A form of congenital disorders of N-linked glycosylation characterized by generalized hypotonia, craniofacial dysmorphism (prominent occiput, short palpebral fissures, long eyelashes, broad nose, high arched palate, retrognathia), hypoplastic genitalia, seizures, feeding difficulties, hypoventilation, severe hypogammaglobulinemia with generalized edema and increased resistance to particular viral infections (particularly to enveloped viruses). The disease is caused by loss-of-function mutations in the gene MOGS (2p13.1).

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.

The primary characteristics of the Frank-ter Haar syndrome (FTHS) are brachycephaly, wide fontanels, prominent forehead, hypertelorism, prominent eyes, macrocornea with or without glaucoma, full cheeks, small chin, bowing of the long bones, and flexion deformity of the fingers. Protruding, simple ears and prominent coccyx are also regarded as important diagnostic signs (summary by Maas et al., 2004). Borrone syndrome was described as a severe progressive multisystem disorder with features overlapping those of FTHS, including thick skin, acne conglobata, osteolysis, gingival hypertrophy, brachydactyly, camptodactyly, and mitral valve prolapse. Although it was initially thought to be a distinct phenotype, mutations in the FTHS-associated gene SH3PXD2B have been identified in patients diagnosed with Borrone syndrome. The earlier differential description was attributed to phenotypic variability as well as to differences in the ages at which patients were examined (Wilson et al., 2014).

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.

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.

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).

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.

Yunis-Varon syndrome (YVS) is a severe autosomal recessive disorder characterized by skeletal defects, including cleidocranial dysplasia and digital anomalies, and severe neurologic involvement with neuronal loss. Enlarged cytoplasmic vacuoles are found in neurons, muscle, and cartilage. The disorder is usually lethal in infancy (summary by Campeau 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.

Spastic paraplegia 11 (SPG11) is characterized by progressive spasticity and weakness of the lower limbs frequently associated with the following: mild intellectual disability with learning difficulties in childhood and/or progressive cognitive decline; peripheral neuropathy; pseudobulbar involvement; and increased reflexes in the upper limbs. Less frequent findings include: cerebellar signs (ataxia, nystagmus, saccadic pursuit); retinal degeneration; pes cavus; scoliosis; and parkinsonism with characteristic brain MRI features that include thinning of the corpus callosum. Onset occurs mainly during infancy or adolescence (range: age 1-31 years) and in rare cases as late as age 60 years. Most affected individuals become wheelchair bound one or two decades after disease onset.

In WDR62 primary microcephaly (WDR62-MCPH), microcephaly (occipitofrontal circumference [OFC] = -2 SD) is usually present at birth, but in some instances becomes evident later in the first year of life. Growth is otherwise normal. Except for brain malformations in most affected individuals, no other congenital malformations are observed. Central nervous system involvement can include delayed motor development, mild-to-severe intellectual disability (ID), behavior problems, epilepsy, spasticity, and ataxia.

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).

Microcephalic osteodysplastic primordial dwarfism type I is a severe autosomal recessive skeletal dysplasia characterized by dwarfism, microcephaly, and neurologic abnormalities, including mental retardation, brain malformations, and ocular/auditory sensory deficits. Patients often die in early childhood (summary by Pierce and Morse, 2012).

The phenotypic spectrum of SOX2 disorder includes anophthalmia and/or microphthalmia, brain malformations, developmental delay / intellectual disability, esophageal atresia, hypogonadotropic hypogonadism (manifest as cryptorchidism and micropenis in males, gonadal dysgenesis infrequently in females, and delayed puberty in both sexes), pituitary hypoplasia, postnatal growth delay, hypotonia, seizures, and spastic or dystonic movements.

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.

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).

Congenital glutamine deficiency is a severe autosomal recessive disorder characterized by onset at birth of encephalopathy, lack of normal development, seizures, and hypotonia associated with variable brain abnormalities (summary by Haberle et al., 2011).

Pterygia, impaired intellectual development, and distinctive craniofacial features is a chromosomal disorder characterized by these cardinal features. Craniofacial features include trigonocephaly and retrognathia. Intellectual development may be severely impaired (summary by Devriendt et al., 2000).

A rare complex type of hereditary spastic paraplegia with characteristics of slowly progressive spastic paraplegia (with walking difficulties appearing at onset at 6-7 years of age) associated with mild intellectual disability. Brain imaging reveals thin corpus callosum, cortical and cerebellar atrophy, and pontine dysraphia. The SPG32 phenotype has been mapped to a locus on chromosome 14q12-q21.

Meckel syndrome is an autosomal recessive pre- or perinatal lethal disorder characterized by a combination of renal cysts and variably associated features including developmental anomalies of the central nervous system (typically occipital encephalocele), hepatic ductal dysplasia and cysts, and postaxial polydactyly (summary by Baala et al., 2007). For a more complete phenotypic description and information on genetic heterogeneity of Meckel syndrome, see MKS1 (249000).

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.

Mutations in the KCTD7 gene cause a severe neurodegenerative phenotype characterized by onset of intractable myoclonic seizures before age 2 years and accompanied by developmental regression. The initial description was consistent with a form of progressive myoclonic epilepsy (designated here as EPM3), whereas a later report identified intracellular accumulation of autofluorescent lipopigment storage material, consistent with neuronal ceroid lipofuscinosis (designated CLN14). Ultrastructural findings on skin biopsies thus appear to be variable. However, clinical features are generally consistent between reports (summary by Staropoli et al., 2012). For a general phenotypic description and a discussion of genetic heterogeneity of progressive myoclonic epilepsy, see EPM1A (254800). For a general phenotypic description and a discussion of genetic heterogeneity of neuronal ceroid lipofuscinosis, see CLN1 (256730).

Combined saposin deficiency (PSAPD), a deficiency of prosaposin and saposins A, B, C, and D, is a fatal infantile storage disorder with hepatosplenomegaly and severe neurologic disease (summary by Hulkova et al., 2001).

This syndrome is characterized by severe hypotonia, lactic acidemia and congenital hyperammonemia. It has been described in three newborns born to consanguineous parents. Ultrasound examination during the 36th week of pregnancy revealed generalized edema. Hypertrophic cardiomyopathy and tubulopathy developed within the first week of life and the infants died within the first month. The activities of enzymes in the mitochondrial respiratory chain were reduced in the muscles of the patients. Mutations were identified in the MRPS22 gene on chromosome 3q23, encoding a mitochondrial ribosomal protein

SADDAN dysplasia (severe achondroplasia with developmental delay and acanthosis nigricans) is a very rare skeletal dysplasia characterized by the constellation of these features. Radiology reveals 'ram's horn' shaped clavicles and reverse bowing of lower limbs. Approximately half of patients die before the fourth week of life secondary to respiratory failure (summary by Zankl et al., 2008).

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).

1q21.1 microduplication is a chromosomal change in which a small amount of genetic material on chromosome 1 is abnormally copied (duplicated). The duplication occurs on the long (q) arm of the chromosome at a location designated q21.1.\n\nSome people with a 1q21.1 microduplication have developmental delay and intellectual disability that is typically mild to moderate. Individuals with this condition can also have features of autism spectrum disorder. These disorders are characterized by impaired communication and socialization skills, as well as delayed development of speech and language. Expressive language skills (vocabulary and the production of speech) tend to be more impaired than receptive language skills (the ability to understand speech) in affected individuals. In childhood, 1q21.1 microduplications may also be associated with an increased risk of attention-deficit/hyperactivity disorder (ADHD) and other behavioral problems. Psychiatric disorders such as schizophrenia or mood disorders such as anxiety or depression occur in some affected individuals, usually during adulthood. Rarely, recurrent seizures (epilepsy) occur in people with a 1q21.1 microduplication.\n\nSome individuals with a 1q21.1 microduplication are born with malformations of the heart, including a particular combination of heart defects known as tetralogy of Fallot. Less commonly, other physical malformations such as the urethra opening on the underside of the penis (hypospadias) in males, inward- and upward-turning feet (clubfeet), or misalignment of the hip joint (hip dysplasia) are present at birth. Individuals with a 1q21.1 microduplication may also have a larger than average head size or taller than average adult stature. Some have slightly unusual facial features such as wide-set eyes or low-set ears. As adults, individuals with a 1q21.1 microduplication may be prone to develop cysts, swollen and knotted (varicose) veins, or carpal tunnel syndrome, which is characterized by numbness, tingling, and weakness in the hands and fingers. However, there is no particular pattern of physical abnormalities that characterizes 1q21.1 microduplications. Signs and symptoms related to the chromosomal change vary even among affected members of the same family. Some people with the duplication have no identified physical, intellectual, or behavioral abnormalities.

POLR3-related leukodystrophy, a hypomyelinating leukodystrophy with specific features on brain MRI, is characterized by varying combinations of four major clinical findings: Neurologic dysfunction, typically predominated by motor dysfunction (progressive cerebellar dysfunction, and to a lesser extent extrapyramidal [i.e., dystonia], pyramidal [i.e., spasticity] and cognitive dysfunctions). Abnormal dentition (delayed dentition, hypodontia, oligodontia, and abnormally placed or shaped teeth). Endocrine abnormalities such as short stature (in ~50% of individuals) with or without growth hormone deficiency, and more commonly, hypogonadotropic hypogonadism manifesting as delayed, arrested, or absent puberty. Ocular abnormality in the form of myopia, typically progressing over several years and becoming severe. POLR3-related leukodystrophy and 4H leukodystrophy are the two recognized terms for five previously described overlapping clinical phenotypes (initially described as distinct entities before their molecular basis was known). These include: Hypomyelination, hypodontia, hypogonadotropic hypogonadism (4H syndrome); Ataxia, delayed dentition, and hypomyelination (ADDH); Tremor-ataxia with central hypomyelination (TACH); Leukodystrophy with oligodontia (LO); Hypomyelination with cerebellar atrophy and hypoplasia of the corpus callosum (HCAHC). Age of onset is typically in early childhood but later-onset cases have also been reported. An infant with Wiedemann-Rautenstrauch syndrome (neonatal progeroid syndrome) was recently reported to have pathogenic variants in POLR3A on exome sequencing. Confirmation of this as a very severe form of POLR3-related leukodystrophy awaits replication in other individuals with a clinical diagnosis of Wiedemann-Rautenstrauch syndrome.

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.

SLC25A24 Fontaine progeroid syndrome is a multisystem connective tissue disorder characterized by poor growth, abnormal skeletal features, and distinctive craniofacial features with sagging, thin skin, and decreased subcutaneous fat suggesting an aged appearance that is most pronounced in infancy and improves with time. Characteristic radiographic features include turribrachycephaly with widely open anterior fontanelle, craniosynostosis, and anomalies of the terminal phalanges. Cardiovascular, genitourinary, ocular, and gastrointestinal abnormalities may also occur. To date, 13 individuals with a molecularly confirmed diagnosis of SLC25A24 Fontaine progeroid syndrome have been described.

STXBP1 encephalopathy with epilepsy is characterized by early-onset encephalopathy with epilepsy (i.e., moderate-to-severe intellectual disability, refractory seizures, and ongoing epileptiform activity). The median age of onset of seizures is six weeks (range 1 day to 13 years). Seizure types can include infantile spasms; generalized tonic-clonic, clonic, or tonic seizures; and myoclonic, focal, atonic, and absence seizures. Epilepsy syndromes can include Ohtahara syndrome, West syndrome, Lennox-Gaustaut syndrome, and Dravet syndrome (not SCN1A-related), classic Rett syndrome (not MECP2-related), and atypical Rett syndrome (not CDKL5-related). The EEG is characterized by focal epileptic activity, burst suppression, hypsarrhythmia, or generalized spike-and-slow waves. Other findings can include abnormal tone, movement disorders (especially ataxia and dystonia), and behavior disorders (including autism spectrum disorder). Feeding difficulties are common.

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).

Congenital fibrosis of the extraocular muscles (CFEOM) encompasses several different inherited strabismus syndromes characterized by congenital restrictive ophthalmoplegia affecting extraocular muscles innervated by the oculomotor and/or trochlear nerves. If all affected members of a family have classic CFEOM with bilateral involvement and inability to raise the eyes above midline, the phenotype is classified as CFEOM1 (135700). CFEOM2 (602078) shows autosomal recessive inheritance. CFEOM3 is characterized by autosomal dominant inheritance of a more variable phenotype than classic CFEOM1. Individuals with CFEOM3 may not have bilateral involvement, may be able to raise the eyes above midline, or may not have blepharoptosis (reviews by Yamada et al., 2004 and Heidary et al., 2008). Yamada et al. (2003) concluded that CFEOM3 is a relatively rare form of CFEOM. Genetic Heterogeneity of CFEOM3 The CFEOM3 phenotype is genetically heterogeneous; see also CFEOM3B (135700), caused by mutation in the KIF21A gene on chromosome 12q12, and CFEOM3C (609384), which maps to chromosome 13q.

Spastic paraplegia-18B (SPG18B) is a severe autosomal recessive neurologic disorder characterized by onset in early childhood of progressive spastic paraplegia resulting in motor disability. Most affected individuals have severe psychomotor retardation. Some may develop significant joint contractures (summary by Alazami et al., 2011 and Yildirim et al., 2011).

Cerebellar ataxia, impaired intellectual development, and dysequilibrium syndrome (CAMRQ) is a genetically heterogeneous disorder characterized by congenital cerebellar ataxia and intellectual disability (summary by Gulsuner et al., 2011). For a discussion of genetic heterogeneity of CAMRQ, see CAMRQ1 (224050).

A very rare, complex form of hereditary spastic paraplegia characterised by a late-onset, slowly progressive spastic paraplegia associated with mild ataxia and dysarthria, upper extremity involvement (i.e. loss of finger dexterity, dysmetria), and mild cognitive impairment, without the presence of nystagmus. A hypomyelinating leucodystrophy and thin corpus callosum is observed in all cases and psychomotor development is normal or near normal. Caused by mutations in the GJC2 gene (1q41-q42) encoding the gap junction gamma-2 protein.

Any autosomal recessive non-syndromic intellectual disability in which the cause of the disease is a mutation in the TRAPPC9 gene.

A rare syndrome with characteristics of pre-natal onset growth retardation (low birth weight and short stature), hypotonia, developmental delay and intellectual disability associated with microcephaly and craniofacial (low anterior hairline, hypotelorism, thick lips with carp-shaped mouth, high-arched palate, low-set ears), cardiac (conotruncal heart malformations such as tetralogy of Fallot) and skeletal (hypoplastic thumbs and first metacarpals) abnormalities.

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.

Autosomal recessive spastic paraplegia-46 (SPG46) is a neurodegenerative disorder characterized by onset in childhood of slowly progressive spastic paraplegia and cerebellar signs. Some patients have cognitive impairment, cataracts, and cerebral, cerebellar, and corpus callosum atrophy on brain imaging (summary by Boukhris et al., 2010 and Martin et al., 2013). For a discussion of genetic heterogeneity of autosomal recessive spastic paraplegia, see SPG5A (270800).

CDG IIe is caused by a mutation that impairs the integrity of the conserved oligomeric Golgi (COG) complex and alters Golgi trafficking, resulting in the disruption of multiple glycosylation pathways. For a general discussion of CDGs, see CDG1A (212065).

Potocki-Lupski syndrome (PTLS) is characterized by cognitive, behavioral, and medical manifestations. Cognitively, most individuals present with developmental delay, later meeting criteria for moderate intellectual disability. Behaviorally, issues with attention, hyperactivity, withdrawal, and anxiety may be seen. Some individuals meet criteria for autism spectrum disorder. Medically, hypotonia, oropharyngeal dysphagia leading to failure to thrive, congenital heart disease, hypoglycemia associated with growth hormone deficiency, and mildly dysmorphic facial features are observed. Medical manifestations typically lead to identification of PTLS in infancy; however, those with only behavioral and cognitive manifestations may be identified in later childhood.

Spastic paraplegia 3A (SPG3A; also known as ATL1-HSP) is characterized by progressive bilateral and mostly symmetric spasticity and weakness of the legs. Compared to other forms of autosomal dominant hereditary spastic paraplegia (HSP), in which diminished vibration sense (caused by degeneration of the corticospinal tracts and dorsal columns) and urinary bladder hyperactivity are present in all affected individuals, these findings occur in a minority of individuals with SPG3A. The average age of onset is four years. More than 80% of reported individuals manifest spastic gait before the end of the first decade of life. Most persons with early-onset ATL1-HSP have a "pure" ("uncomplicated") HSP; however, complicated HSP with axonal motor neuropathy and/or distal amyotrophy with lower motor neuron involvement (Silver syndrome phenotype) has been observed. The rate of progression in ATL1-HSP is slow, and wheelchair dependency or need for a walking aid (cane, walker, or wheelchair) is relatively rare.

The Catania type of acrofacial dysostosis is characterized by intrauterine growth retardation, short stature, microcephaly, intellectual disability, widow's peak, mandibulofacial dysostosis without cleft palate, ear anomalies, mild pre- and postaxial limb hypoplasia with brachydactyly, mild interdigital webbing, dental anomalies, and cryptorchidism and hypospadias in males (Opitz et al., 1993; Wulfsberg et al., 1996).

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.

Frontonasal dysplasia-2 (FND2) is an autosomal recessive disorder characterized by variable degrees of alopecia, skull defects, hypertelorism, depressed nasal bridge and ridge with notched alae nasi, and abnormal central nervous system findings (summary by Kariminejad et al., 2014).

The congenital variant of Rett syndrome is a severe neurodevelopmental disorder with features of classic Rett syndrome (RTT; 312750), but earlier onset in the first months of life. Classic Rett syndrome shows later onset and is caused by mutation in the MECP2 gene (300005).

14q11.2 microdeletion syndrome is a recently described syndrome characterized by developmental delay, hypotonia and facial dysmorphism.

Developmental and epileptic encephalopathy-5 (DEE5) is a neurologic disorder characterized by global developmental delay and the onset of tonic seizures or infantile spasms in the first months of life. The seizures tend to be refractory to treatment, and EEG shows hypsarrhythmia, consistent with a clinical diagnosis of West syndrome. Affected individuals have severely impaired psychomotor development with lack of visual attention, poor head control, feeding difficulties, microcephaly, and spastic quadriplegia. Brain imaging may show cerebral atrophy and hypomyelination (summary by Saitsu et al., 2010). For a general phenotypic description and a discussion of genetic heterogeneity of developmental and epileptic encephalopathy, see 308350.

Spastic paraplegia-48 (SPG48) is an autosomal recessive neurologic disorder characterized by spasticity of the lower limbs resulting in gait difficulties. Most patients have onset in mid- or late-adulthood, although childhood onset has been reported in 1 patient. Additional features may include parkinsonism, urinary incontinence, neuropathy, and mild cognitive impairment (summary by Hirst et al., 2015). For a discussion of genetic heterogeneity of autosomal recessive SPG, see SPG5A (270800).

KCNQ2-related disorders represent a continuum of overlapping neonatal epileptic phenotypes ranging from self-limited familial neonatal epilepsy (SLFNE) at the mild end to neonatal-onset developmental and epileptic encephalopathy (NEO-DEE) at the severe end. Additional, less common phenotypes consisting of neonatal encephalopathy with non-epileptic myoclonus, infantile or childhood-onset developmental and epileptic encephalopathy (DEE), and isolated intellectual disability (ID) without epilepsy have also been described. KCNQ2-SLFNE is characterized by seizures that start in otherwise healthy infants between two and eight days after term birth and spontaneously disappear between the first and the sixth to 12th month of life. There is always a seizure-free interval between birth and the onset of seizures. Seizures are characterized by sudden onset with prominent motor involvement, often accompanied by apnea and cyanosis; video EEG identifies seizures as focal onset with tonic stiffening of limb(s) and some migration during each seizure's evolution. About 30% of individuals with KCNQ2-SLFNE develop epileptic seizures later in life. KCNQ2-NEO-DEE is characterized by multiple daily seizures beginning in the first week of life that are mostly tonic, with associated focal motor and autonomic features. Seizures generally cease between ages nine months and four years. At onset, EEG shows a burst-suppression pattern or multifocal epileptiform activity; early brain MRI can show basal ganglia hyperdensities and later MRIs may show white matter or general volume loss. Moderate-to-profound developmental impairment is present.

PCH2D is an autosomal recessive disorder characterized by progressive microcephaly, postnatal onset of progressive atrophy of the cerebrum and cerebellum, profound mental retardation, spasticity, and variable seizures (summary by Ben-Zeev et al., 2003). For a general phenotypic description and a discussion of genetic heterogeneity of pontocerebellar hypoplasia type 2, see PCH2A (277470).

The chromosome 13q14 deletion syndrome is characterized by retinoblastoma (180200), variable degrees of mental impairment, and characteristic facial features, including high forehead, prominent philtrum, and anteverted earlobes (summary by Caselli et al., 2007).

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.

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.

Methylmalonate semialdehyde dehydrogenase deficiency is a rare autosomal recessive inborn error of metabolism with a highly variable phenotype. Some patients may be asymptomatic, whereas others show global developmental delay, nonspecific dysmorphic features, and delayed myelination on brain imaging. Laboratory studies typically show increased urinary 3-hydroxyisobutyric acid, although additional metabolic abnormalities may also be observed (summary by Marcadier et al., 2013).

Hyperphosphatasia with impaired intellectual development syndrome-3 (HPMRS3) is an autosomal recessive disorder characterized by severe intellectual disability, hypotonia with poor motor development, poor speech, and increased serum alkaline phosphatase (summary by Hansen et al., 2013). However, the severity of the disorder can also vary to include more mild intellectual impairment (Krawitz et al., 2013). The disorder is caused by a defect in glycosylphosphatidylinositol (GPI) biosynthesis. For a discussion of genetic heterogeneity of HPMRS, see HPMRS1 (239300). For a discussion of genetic heterogeneity of GPI biosynthesis defects, see GPIBD1 (610293).

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.

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.

The chromosome 8q21.11 deletion syndrome is characterized by impaired intellectual development and common facial dysmorphic features (summary by Palomares et al., 2011).

Primary microcephaly-epilepsy-permanent neonatal diabetes syndrome is a rare, genetic, neurologic disease characterized by congenital microcephaly, severe, early-onset epileptic encephalopathy (manifesting as intractable, myoclonic and/or tonic-clonic seizures), permanent, neonatal, insulin-dependent diabetes mellitus, and severe global developmental delay. Muscular hypotonia, skeletal abnormalities, feeding difficulties, and dysmorphic facial features (including narrow forehead, anteverted nares, small mouth with deep philtrum, tented upper lip vermilion) are frequently associated. Brain MRI reveals cerebral atrophy with cortical gyral simplification and aplasia/hypoplasia of the corpus callosum.

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.

The defining clinical characteristics of the microcephaly-capillary malformation (MIC-CAP) syndrome are typically present at birth: microcephaly and generalized cutaneous capillary malformations (a few to hundreds of oval/circular macules or patches varying in size from 1-2 mm to several cm), hypoplastic distal phalanges of the hands and/or feet, early-onset intractable epilepsy, and profound developmental delay. Seizures, which can be focal, tonic, and complex partial and can include infantile spasms, appear to stabilize after age two years. Myoclonus of the limbs and eyelids is common; other abnormal movements (dyskinetic, choreiform) may be seen. To date, the diagnosis has been confirmed in 18 individuals from 15 families.

POLR3-related leukodystrophy, a hypomyelinating leukodystrophy with specific features on brain MRI, is characterized by varying combinations of four major clinical findings: Neurologic dysfunction, typically predominated by motor dysfunction (progressive cerebellar dysfunction, and to a lesser extent extrapyramidal [i.e., dystonia], pyramidal [i.e., spasticity] and cognitive dysfunctions). Abnormal dentition (delayed dentition, hypodontia, oligodontia, and abnormally placed or shaped teeth). Endocrine abnormalities such as short stature (in ~50% of individuals) with or without growth hormone deficiency, and more commonly, hypogonadotropic hypogonadism manifesting as delayed, arrested, or absent puberty. Ocular abnormality in the form of myopia, typically progressing over several years and becoming severe. POLR3-related leukodystrophy and 4H leukodystrophy are the two recognized terms for five previously described overlapping clinical phenotypes (initially described as distinct entities before their molecular basis was known). These include: Hypomyelination, hypodontia, hypogonadotropic hypogonadism (4H syndrome); Ataxia, delayed dentition, and hypomyelination (ADDH); Tremor-ataxia with central hypomyelination (TACH); Leukodystrophy with oligodontia (LO); Hypomyelination with cerebellar atrophy and hypoplasia of the corpus callosum (HCAHC). Age of onset is typically in early childhood but later-onset cases have also been reported. An infant with Wiedemann-Rautenstrauch syndrome (neonatal progeroid syndrome) was recently reported to have pathogenic variants in POLR3A on exome sequencing. Confirmation of this as a very severe form of POLR3-related leukodystrophy awaits replication in other individuals with a clinical diagnosis of Wiedemann-Rautenstrauch syndrome.

The Zaki-Gleeson syndrome is an autosomal recessive neurodevelopmental disorder characterized by profound mental retardation, severe microcephaly, poor growth, cerebellar hypoplasia, and second-degree cardiac conduction defects (Zaki et al., 2011).

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.

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).

Infantile cerebellar-retinal degeneration (ICRD) is a severe autosomal recessive neurodegenerative disorder characterized by onset between ages 2 and 6 months of truncal hypotonia, athetosis, seizures, and ophthalmologic abnormalities, particularly optic atrophy and retinal degeneration. Affected individuals show profound psychomotor retardation, with only some achieving rolling, sitting, or recognition of family. Brain MRI shows progressive cerebral and cerebellar degeneration (summary by Spiegel et al., 2012). A subset of patients may have a milder phenotype with variable features, including ataxia, developmental delay, and behavioral abnormalities (Blackburn et al., 2020). Mutation in the ACO2 gene also causes isolated optic atrophy (OPA9; 616289).

Coffin-Siris syndrome (CSS) is classically characterized by aplasia or hypoplasia of the distal phalanx or nail of the fifth and additional digits, developmental or cognitive delay of varying degree, distinctive facial features, hypotonia, hirsutism/hypertrichosis, and sparse scalp hair. Congenital anomalies can include malformations of the cardiac, gastrointestinal, genitourinary, and/or central nervous systems. Other findings commonly include feeding difficulties, slow growth, ophthalmologic abnormalities, and hearing impairment.

Complex cortical dysplasia with other brain malformations-13 (CDCBM13) is an autosomal dominant neurodevelopmental disorder characterized by global developmental delay with impaired intellectual development. Brain imaging shows variable neuronal migration defects resulting in cortical malformations, including pachygyria. More variable features include early-onset seizures and dysmorphic features. Some patients may also show signs of peripheral neuropathy, such as abnormal gait, hyporeflexia, and foot deformities (summary by Willemsen et al., 2012 and Poirier et al., 2013). For a discussion of genetic heterogeneity of CDCBM, see CDCBM1 (614039).

Fatty acid hydroxylase-associated neurodegeneration (FAHN) is characterized early in the disease course by central nervous system involvement including corticospinal tract involvement (spasticity), mixed movement disorder (ataxia/dystonia), and eye findings (optic atrophy, oculomotor abnormalities), and later in the disease course by progressive intellectual impairment and seizures. With disease progression, dystonia and spasticity compromise the ability to ambulate, leading to wheelchair dependence. Life expectancy is variable. FAHN is considered to be a subtype of neurodegeneration with brain iron accumulation (NBIA).

Spastic paraplegia-54 (SPG54) is a complicated form of spastic paraplegia, a neurodegenerative disorder affecting fibers of the corticospinal tract. Affected individuals have delayed psychomotor development, intellectual disability, and early-onset spasticity of the lower limbs. Brain MRI shows a thin corpus callosum and periventricular white matter lesions. Brain magnetic resonance spectroscopy shows an abnormal lipid peak (summary by Schuurs-Hoeijmakers et al., 2012). For a general phenotypic description and a discussion of genetic heterogeneity of autosomal recessive spastic paraplegia, see 270800.

A rare complex type of hereditary spastic paraplegia with characteristics of childhood onset of progressive spastic paraplegia associated with optic atrophy (with reduced visual acuity and central scotoma), ophthalmoplegia, reduced upper-extremity strength and dexterity, muscular atrophy in the lower extremities and sensorimotor neuropathy. Caused by mutations in the C12ORF65 gene (12q24.31) encoding probable peptide chain release factor C12ORF65, mitochondrial.

Spastic paraplegia-56 with or without pseudoxanthoma elasticum (SPG56) is an autosomal recessive neurodegenerative disorder characterized by early-onset progressive lower-limb spasticity resulting in walking difficulties. Upper limbs are often also affected, and some patients may have a subclinical axonal neuropathy (summary by Tesson et al., 2012). Some patients also have pseudoxanthoma elasticum (Legrand et al., 2021). For a general phenotypic description and a discussion of genetic heterogeneity of autosomal recessive spastic paraplegia, see 270800.

Pontine tegmental cap dysplasia (PTCD) refers to a neurologic condition characterized by a distinct pattern of hindbrain malformations apparent on brain imaging. The abnormalities affect the pons, medulla, and cerebellum. In neuroradiologic studies, the ventral side of the pons is flattened, whereas there is vaulting ('capping') of the dorsal pontine border into the fourth ventricle. Affected individuals show a variety of neurologic deficits, most commonly sensorineural deafness, impaired cranial nerve function, and variable psychomotor retardation (summary by Barth et al., 2007).

TECPR2-related hereditary sensory and autonomic neuropathy with intellectual disability (TECPR2-HSAN with ID) is characterized by developmental delay and subsequent intellectual disability, behavioral abnormalities, neurologic manifestations (muscular hypotonia, sensory neuropathy with lower-limb hypo- or areflexia and ataxic gait), and autonomic dysfunction (including central hypoventilation and apnea, gastrointestinal dysmotility, dysphagia, and gastroesophageal reflux disease with recurrent aspiration). To date, more than 30 individuals with TECPR2-HSAN with ID have been identified.

Microphthalmia with linear skin defects (MLS) syndrome is characterized by unilateral or bilateral microphthalmia and/or anophthalmia and linear skin defects, usually involving the face and neck, which are present at birth and heal with age, leaving minimal residual scarring. Other findings can include a wide variety of other ocular abnormalities (e.g., corneal anomalies, orbital cysts, cataracts), central nervous system involvement (e.g., structural anomalies, developmental delay, infantile seizures), cardiac concerns (e.g., hypertrophic or oncocytic cardiomyopathy, atrial or ventricular septal defects, arrhythmias), short stature, diaphragmatic hernia, nail dystrophy, hearing impairment, and genitourinary malformations. Inter- and intrafamilial variability is described.

Complex cortical dysplasia with other brain malformations-7 is an autosomal dominant, clinically heterogeneous disorder showing a wide spectrum of abnormalities of cortical brain development. The most severely affected patients are fetuses with microlissencephaly, absence of the cortical plate, agenesis of the corpus callosum, and severely hypoplastic brainstem and cerebellum. Other patients have lissencephaly, polymicrogyria, cortical dysplasia, or neuronal heterotopia. Those with less severe malformations can survive, but usually have some degree of neurologic impairment, such as mental retardation, seizures, and movement abnormalities (summary by Chang et al., 2006; Fallet-Bianco et al., 2014). For a discussion of genetic heterogeneity of CDCBM, see CDCBM1 (614039).

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).

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).

KCNT1-related epilepsy is most often associated with two phenotypes: epilepsy of infancy with migrating focal seizures (EIMFS) and autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE). EIMFS is characterized by seizures, typically focal and asynchronous, beginning in the first six months of life with associated developmental plateau or regression. Autonomic manifestations (e.g., perioral cyanosis, flushing, apnea) are common. Seizures are intractable to multiple anticonvulsants and progress to become nearly continuous by age six to nine months. ADNFLE is characterized by clusters of nocturnal motor seizures that vary from simple arousals to hyperkinetic events with tonic or dystonic features. Individuals with KCNT1-related ADNFLE are more likely to develop seizures at a younger age, have cognitive comorbidity, and display psychiatric and behavioral problems than individuals with ADNFLE resulting from other causes. Less common seizure phenotypes in individuals with KCNT1-related epilepsy include West syndrome, Ohtahara syndrome, early myoclonic encephalopathy, leukodystrophy and/or leukoencephalopathy, focal epilepsy, and multifocal epilepsy. Additional neurologic features include hypotonia, microcephaly developing by age 12 months, strabismus, profound developmental delay, and additional movement disorders. Other systemic manifestations including pulmonary hemorrhage caused by prominent systemic-to-pulmonary collateral arteries or cardiac arrhythmia have been reported.

Pontocerebellar hypoplasia type 8 is an autosomal recessive neurodevelopmental disorder characterized by severe psychomotor retardation, abnormal movements, hypotonia, spasticity, and variable visual defects. Brain MRI shows pontocerebellar hypoplasia, decreased cerebral white matter, and a thin corpus callosum (summary by Mochida et al., 2012). For a general phenotypic description and a discussion of genetic heterogeneity of PCH, see PCH1 (607596).

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).

CTNNB1 neurodevelopmental disorder (CTNNB1-NDD) is characterized in all individuals by mild-to-profound cognitive impairment and in up to 39% of reported individuals by exudative vitreoretinopathy, an ophthalmologic finding consistent with familial exudative vitreoretinopathy (FEVR). Other common findings include truncal hypotonia, peripheral spasticity, dystonia, behavior problems, microcephaly, and refractive errors and strabismus. Less common features include intrauterine growth restriction, feeding difficulties, and scoliosis.

Lissencephaly-5 (LIS5) is an autosomal recessive brain malformation characterized by cobblestone changes in the cortex, more severe in the posterior region, and subcortical band heterotopia. Affected individuals have hydrocephalus, seizures, and severely delayed psychomotor development (Radmanesh et al., 2013). For a general phenotypic description and a discussion of genetic heterogeneity of lissencephaly, see LIS1 (607432).

A rare multiple congenital anomalies syndrome with characteristics of distinctive facial appearance (low frontal hairline, bilateral ptosis, prominent eyes, flat midface, broad, ?at nares, Cupid''s bow upper lip vermilion and small, low-set, posteriorly rotated ears), cleft palate, conductive hearing loss, heart defects (atrial or ventricular septal defect) and mild developmental delay/intellectual disability.

Congenital disorder of glycosylation type IIm, or developmental and epileptic encephalopathy-22 (DEE22), is an X-linked dominant severe neurologic disorder characterized by infantile-onset seizures, hypsarrhythmia on EEG, hypotonia, and developmental delay associated with severe intellectual disability and lack of speech. These features are consistent with developmental and epileptic encephalopathy (DEE). Brain malformations usually include cerebral and cerebellar atrophy. Additionally, some patients may have dysmorphic features or coarse facies (Ng et al., 2013; Kodera et al., 2013). For a general discussion of CDGs, see CDG1A (212065) and CDG2A (212066). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

Complex cortical dysplasia with other brain malformations (CDCBM) is a disorder of aberrant neuronal migration and disturbed axonal guidance. Affected individuals have mild to severe mental retardation, strabismus, axial hypotonia, and spasticity. Brain imaging shows variable malformations of cortical development, including polymicrogyria, gyral disorganization, and fusion of the basal ganglia, as well as thin corpus callosum, hypoplastic brainstem, and dysplastic cerebellar vermis. Extraocular muscles are not involved (summary by Poirier et al., 2010). Genetic Heterogeneity of Complex Cortical Dysplasia with Other Brain Malformations See also CDCBM2 (615282), caused by mutation in the KIF5C gene (604593) on chromosome 2q23; CDCBM3 (615411), caused by mutation in the KIF2A gene (602591) on chromosome 5q12; CDCBM4 (615412), caused by mutation in the TUBG1 gene (191135) on chromosome 17q21; CDCBM5 (615763), caused by mutation in the TUBB2A gene (615101) on chromosome 6p25; CDCBM6 (615771), caused by mutation in the TUBB gene (191130) on chromosome 6p21; CDCBM7 (610031), caused by mutation in the TUBB2B gene (612850) on chromosome 6p25; CDCBM9 (618174), caused by mutation in the CTNNA2 gene (114025) on chromosome 2p12; CDCBM10 (618677), caused by mutation in the APC2 gene (612034) on chromosome 19p13; CDCBM11 (620156), caused by mutation in the KIF26A gene (613231) on chromosome 14q32; CDCBM12 (620316), caused by mutation in the CAMSAP1 gene (613774) on chromosome 9q34; CDCBM13 (614563), caused by mutation in the DYNC1H1 gene (600112) on chromosome 14q32; CDCBM14A (606854) and CDCBM14B (615752), caused by mutation in the ADGRG1 gene (604110) on chromosome 16q21; and CDCBM15 (618737), caused by mutation in the TUBGCP2 gene (617817) on chromosome 10q26. The designation CDCBM8 was previously used to represent a phenotype caused by mutation in the TUBA8 gene (see 605742.0001) on chromosome 22q11; the patients with this phenotype were subsequently found to have a homozygous mutation in the SNAP29 gene (604202.0002), also on chromosome 22q11, that may have been responsible for the disorder. The same mutation in SNAP29 causes a similar disorder, CEDNIK syndrome (609528). See also lissencephaly (e.g., LIS1, 607432), which shows overlapping features and may result from mutation in tubulin genes.

Any complex cortical dysplasia with other brain malformations in which the cause of the disease is a mutation in the KIF5C gene.

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).

Any complex cortical dysplasia with other brain malformations in which the cause of the disease is a mutation in the KIF2A gene.

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.

Developmental and epileptic encephalopathy-17 (DEE17) is a severe neurologic disorder characterized by onset of intractable seizures in the first weeks or months of life. EEG often shows a burst-suppression pattern consistent with a clinical diagnosis of Ohtahara syndrome. Affected infants have very poor psychomotor development and may have brain abnormalities, such as cerebral atrophy or thin corpus callosum. Some patients may show involuntary movements (summary by Nakamura et al., 2013). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

Van Maldergem syndrome is an autosomal recessive disorder characterized by intellectual disability, typical craniofacial features, auditory malformations resulting in hearing loss, and skeletal and limb malformations. Some patients have renal hypoplasia. Brain MRI typically shows periventricular nodular heterotopia (summary by Cappello et al., 2013). For a discussion of genetic heterogeneity of Van Maldergem syndrome, see 601390.

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.

A rare, genetic, syndromic intellectual disability disease characterized by progressive postnatal microcephaly and global developmental delay, as well as moderate to profound intellectual disability, difficulty or inability to walk, pyramidal signs (including spasticity, hyperreflexia and extensor plantar response) and thin corpus callosum revealed by brain imaging. Ophthalmologic signs (including nystagmus, strabismus and abnormal retinal pigmentation), foot deformity and genital anomalies may also be associated.

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.

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.

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.

An extremely rare and complex form of hereditary spastic paraplegia with characteristics of onset in infancy of spastic paraplegia (presenting with delayed walking and a scissors gait) associated with short stature and normal cognition. Periventricular deep white matter changes in the corpus callosum are noted on brain imaging. SPG63 is caused by a homozygous mutation in the AMPD2 gene (1p13.3) encoding AMP deaminase 2.

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.

A rare pure or complex form of hereditary spastic paraplegia with characteristics of onset in infancy of progressive lower limb spasticity, abnormal gait, increased deep tendon reflexes and extensor plantar responses that may be associated with intellectual disability. Additional signs such as contractures in the lower limbs, amyotrophy, clubfoot and optic atrophy, have also been reported. Caused by homozygous mutation in the NT5C2 gene on chromosome 10q24.

Bardet-Biedl syndrome-19 (BBS19) is an autosomal recessive ciliopathy characterized by obesity, impaired intellectual development, polydactyly, renal failure, retinitis pigmentosa, and hypogonadism (Aldahmesh et al., 2014). For a general phenotypic description and a discussion of genetic heterogeneity of Bardet-Biedl syndrome, see BBS1 (209900).

A form of congenital disorders of N-linked glycosylation with characteristics of neurologic abnormalities (global developmental delay in language, social skills and fine and gross motor development, intellectual disability, hypotonia, microcephaly, seizures/epilepsy), facial dysmorphism (deep set eyes, large ears, hypoplastic vermillion of upper lip, large mouth with widely spaced teeth), feeding problems often due to chewing difficulties and aversion to food with certain textures, failure to thrive, gastrointestinal abnormalities (reflux or vomiting) and strabismus. The disease is caused by mutations in the gene SSR4 (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.

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).

Short stature with microcephaly and distinctive facies (SSMCF) is characterized by pre- or postnatal growth retardation, frontal bossing, high forehead, sparse hair and eyebrows, and telecanthus. Patients also show skin dyspigmentation, with hyper- and/or hypopigmented areas (Leduc et al., 2016).

Neurodevelopmental disorder with dysmorphic features, spasticity, and brain abnormalities (NEDDSBA) is an autosomal recessive neurodevelopmental disorder characterized by severely delayed global development, with hypotonia, impaired intellectual development, and poor or absent speech. Most patients have spasticity with limb hypertonia and brisk tendon reflexes. Additional features include nonspecific dysmorphic facial features, structural brain abnormalities, and cortical visual impairment (summary by Bosch et al., 2015). Novarino et al. (2014) labeled the disorder 'spastic paraplegia-67' (SPG67). The disorder is caused by a defect in glycosylphosphatidylinositol (GPI) biosynthesis. For a discussion of genetic heterogeneity of GPI biosynthesis defects, see GPIBD1 (610293).

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).

The main features of Xia-Gibbs syndrome (XGS), present in a majority of affected individuals, include delayed motor milestones, speech delay with severely limited or absent speech, moderate-to-severe cognitive impairment, hypotonia, structural brain anomalies, and nonspecific dysmorphic features. Other features may include sleep apnea, movement disorders (ataxia, tremors, and bradykinesias) that often become apparent in childhood or adolescence, short stature, seizures, eye anomalies, behavioral concerns, autism spectrum disorder, scoliosis, and laryngomalacia.

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).

Developmental and epileptic encephalopathy-23 (DEE23) is an autosomal recessive neurologic disorder characterized by the onset of intractable seizures in the first months of life (range, 2-6 months). Affected individuals have severely impaired psychomotor development with poor or absent speech, cortical blindness, and dysmorphic facial features (summary by Perrault et al., 2014).

Combined oxidative phosphorylation defect type 20 is a rare mitochondrial oxidative phosphorylation disorder characterized by variable combination of psychomotor delay, hypotonia, muscle weakness, seizures, microcephaly, cardiomyopathy and mild dysmorphic facial features. Variable types of structural brain anomalies have also been reported. Biochemical studies typically show decreased activity of mitochondrial complexes (mainly complex I).

Webb-Dattani syndrome is an autosomal recessive disorder characterized by frontotemporal hypoplasia, globally delayed development, and pituitary and hypothalamic insufficiency due to hypoplastic development of these brain regions. Patients present soon after birth with multiple pituitary hormonal deficiencies and subsequently develop microcephaly, seizures, and spasticity. Other features include postretinal blindness and renal abnormalities (summary by Webb et al., 2013).

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.

Severe combined immunodeficiency (SCID) due to DNA-PKcs deficiency is an extremely rare type of SCID (see this term) characterized by the classical signs of SCID (severe and recurrent infections, diarrhea, failure to thrive), absence of T and B lymphocytes, and cell sensitivity to ionizing radiation.

Pontocerebellar hypoplasia type 1C is a severe autosomal recessive neurodegenerative disorder characterized by severe muscle weakness and failure to thrive apparent in the first months of life. Affected infants showed delayed psychomotor development, often with visual and hearing impairment, and may die of respiratory failure. Brain imaging typically shows cerebellar hypoplasia, hypoplasia of the corpus callosum, and immature myelination (summary by Boczonadi et al., 2014). For a phenotypic description and a discussion of genetic heterogeneity of PCH, see PCH1A (607596).

Hypomyelinating leukodystrophy-9 is an autosomal recessive neurologic disorder characterized by onset of delayed psychomotor development, spasticity, and nystagmus in the first year of life. Additional neurologic features such as ataxia and abnormal movements may also occur. Brain imaging shows diffuse hypomyelination affecting all regions of the brain (summary by Wolf et al., 2014). For a general phenotypic description and a discussion of genetic heterogeneity of HLD, see 312080.

Developmental and epileptic encephalopathy-28 (DEE28) is an autosomal recessive severe neurologic disorder characterized by the onset of refractory seizures in the first months of life. Affected individuals have severe axial hypotonia and profoundly impaired psychomotor development. More severely affected patients have acquired microcephaly, poor or absent visual contact, and retinal degeneration; early death may occur (summary by Mignot et al., 2015). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

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).

Combined oxidative phosphorylation deficiency-24 (COXPD24) is an autosomal recessive mitochondrial disorder with wide phenotypic variability. Most patients present in infancy with delayed neurodevelopment, refractory seizures, hypotonia, and hearing impairment due to auditory neuropathy. Less common features may include cortical blindness, renal dysfunction, and/or liver involvement, suggestive of Alpers syndrome (MTDPS4A; 203700). Patients with the severe phenotype tend to have brain abnormalities on imaging, including cerebral atrophy and hyperintensities in the basal ganglia and brainstem, consistent with Leigh syndrome. Laboratory values may be normal or show increased lactate and evidence of mitochondrial respiratory chain defects, particularly in muscle. Some patients achieve little developmental milestones and may die in infancy or early childhood. However, some patients have a less severe phenotype manifest only by myopathy (summary by Sofou et al., 2015, Vanlander et al., 2015, and Mizuguchi et al., 2017). For a discussion of genetic heterogeneity of combined oxidative phosphorylation deficiency, see COXPD1 (609060).

Childhood-onset spasticity with hyperglycinemia is an autosomal recessive disorder characterized by onset of slowly progressive spasticity that results in impaired gait in the first decade of life. Imaging of the central nervous system shows leukodystrophy and/or lesions in the upper spinal cord. More variable features include visual defects and mild learning disabilities. Serum glycine is increased, but CSF glycine is only mildly increased or normal; serum lactate is normal. The disorder represents a form of 'variant' nonketotic hyperglycinemia and is distinct from classic nonketotic hyperglycinemia (NKH, or GCE; 605899), which is characterized by significantly increased CSF glycine. Several forms of 'variant' NKH, including SPAHGC, appear to result from defects of mitochondrial lipoate biosynthesis (summary by Baker et al., 2014).

Microcephaly, short stature, and impaired glucose metabolism-2 (MSSGM2) is an autosomal recessive syndrome characterized by microcephaly associated with impaired intellectual development, and short stature. Patients develop diabetes in the second or third decade of life, and hypothyroidism and delayed puberty have also been reported (Abdulkarim et al., 2015; Kernohan et al., 2015). For a discussion of genetic heterogeneity of microcephaly, short stature, and impaired glucose metabolism, see MSSGM1 (616033).

Severe hypotonia-psychomotor developmental delay-strabismus-cardiac septal defect syndrome is a rare, genetic, non-dystrophic congenital myopathy disorder characterized by a neonatal-onset of severe generalized hypotonia associated with mild psychomotor delay, congenital strabismus with abducens nerve palsy, and atrial and/or ventricular septal defects. Cryptorchidism is commonly reported in male patients and muscle biopsy typically reveals increased variability in muscle fiber size.

UNC80 deficiency is characterized by hypotonia, strabismus, oral motor dysfunction, postnatal growth deficiency, and developmental delay. The majority of individuals do not learn to walk. All individuals lack expressive language; however, many have expressive body language, and a few have used signs to communicate. Seizures may develop during infancy or childhood. Additional features can include nystagmus, extremity hypertonia, a high-pitched cry, repetitive and self-stimulatory behaviors, constipation, clubfeet, joint contractures, and scoliosis. For most individuals the UNC80 deficiency syndrome is not progressive. Individuals have slow acquisition of skills and do not have a loss of skills suggestive of neurodegeneration.

Spastic paraplegia and psychomotor retardation with or without seizures is an autosomal recessive complex neurodevelopmental disorder with onset in infancy. Affected children show hypotonia followed by severely impaired global development and significant motor disability. Most develop seizures in childhood and have speech delay. Other features, such as ocular abnormalities, foot deformities, hypoplasia of the corpus callosum, and decreased white matter, are more variable (summary by Hollstein et al., 2015).

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).

Palatal anomalies-widely spaced teeth-facial dysmorphism-developmental delay syndrome is a rare, genetic multiple congenital anomalies/dysmorphic syndrome characterized by global developmental delay, axial hypotonia, palate abnormalities (including cleft palate and/or high and narrow palate), dysmorphic facial features (including prominent forehead, hypertelorism, downslanting palpebral fissures, wide nasal bridge, thin lips and widely spaced teeth), and short stature. Additional manifestations may include digital anomalies (such as brachydactyly, clinodactyly, and hypoplastic toenails), a single palmar crease, lower limb hypertonia, joint hypermobility, as well as ocular and urogenital anomalies.

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 tetraplegia, thin corpus callosum, and progressive microcephaly is an autosomal recessive neurodevelopmental disorder characterized by onset of those features and severely impaired global development in early infancy. Most patients are unable to achieve independent walking or speech; some patients have seizures (summary by Srour et al., 2015 and Heimer et al., 2015).

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).

Neurodevelopmental disorder with hearing loss, seizures, and brain abnormalities (NEDHSB) is an autosomal recessive disorder characterized by severe neurologic impairment including impaired intellectual development, epilepsy, microcephaly, abnormal muscle tone, and sensorineural hearing loss. Most affected individuals are nonambulatory, cannot sit unassisted, and have no speech development. More variable features include feeding difficulties, poor growth, cortical visual impairment, spasticity, scoliosis, immunodeficiency, and thrombocytopenia (Tanaka et al., 2015).

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).

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).

POLR3-related leukodystrophy, a hypomyelinating leukodystrophy with specific features on brain MRI, is characterized by varying combinations of four major clinical findings: Neurologic dysfunction, typically predominated by motor dysfunction (progressive cerebellar dysfunction, and to a lesser extent extrapyramidal [i.e., dystonia], pyramidal [i.e., spasticity] and cognitive dysfunctions). Abnormal dentition (delayed dentition, hypodontia, oligodontia, and abnormally placed or shaped teeth). Endocrine abnormalities such as short stature (in ~50% of individuals) with or without growth hormone deficiency, and more commonly, hypogonadotropic hypogonadism manifesting as delayed, arrested, or absent puberty. Ocular abnormality in the form of myopia, typically progressing over several years and becoming severe. POLR3-related leukodystrophy and 4H leukodystrophy are the two recognized terms for five previously described overlapping clinical phenotypes (initially described as distinct entities before their molecular basis was known). These include: Hypomyelination, hypodontia, hypogonadotropic hypogonadism (4H syndrome); Ataxia, delayed dentition, and hypomyelination (ADDH); Tremor-ataxia with central hypomyelination (TACH); Leukodystrophy with oligodontia (LO); Hypomyelination with cerebellar atrophy and hypoplasia of the corpus callosum (HCAHC). Age of onset is typically in early childhood but later-onset cases have also been reported. An infant with Wiedemann-Rautenstrauch syndrome (neonatal progeroid syndrome) was recently reported to have pathogenic variants in POLR3A on exome sequencing. Confirmation of this as a very severe form of POLR3-related leukodystrophy awaits replication in other individuals with a clinical diagnosis of Wiedemann-Rautenstrauch syndrome.

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).

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).

Hypomyelinating leukodystrophy-10 is an autosomal recessive neurologic disorder characterized by postnatal progressive microcephaly, severely delayed psychomotor development, and hypomyelination on brain imaging (summary by Nakayama et al., 2015). For a general phenotypic description and a discussion of genetic heterogeneity of HLD, see 312080.

White-Sutton syndrome is a neurodevelopmental disorder characterized by a wide spectrum of cognitive dysfunction, developmental delays (particularly in speech and language acquisition), hypotonia, autism spectrum disorder, and other behavioral problems. Additional features commonly reported include seizures, refractive errors and strabismus, hearing loss, sleep disturbance (particularly sleep apnea), feeding and gastrointestinal problems, mild genital abnormalities in males, and urinary tract involvement in both males and females.

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).

Lethal congenital contracture syndrome-7, an axoglial form of arthrogryposis multiplex congenita (AMC), is characterized by congenital distal joint contractures, polyhydramnios, reduced fetal movements, and severe motor paralysis leading to death early in the neonatal period (Laquerriere et al., 2014). For a general phenotypic description and a discussion of genetic heterogeneity of lethal congenital contracture syndrome, see LCCS1 (253310).

Neurodevelopmental disorder with spastic paraplegia and microcephaly (NEDSPM) is an autosomal recessive neurologic syndrome characterized by delayed psychomotor development with delayed walking, moderately to severely impaired intellectual development, and poor or absent speech. More severely affected individuals show poor overall growth with progressive microcephaly, axial hypotonia, oromotor dysfunction with drooling, joint contractures, and spastic paraplegia resulting in walking difficulties. Some patients may develop seizures; nonspecific dysmorphic features have also been reported (summary by Hengel et al., 2018 and Ouyang et al., 2019).

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).

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).

An extremely rare form of carbohydrate deficient glycoprotein syndrome with, in the single reported case to date, seizures, some dysmorphic features, axial hypotonia, slight peripheral hypertonia and hyperreflexia.

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.

Developmental and epileptic encephalopathy-48 (DEE48) is a severe autosomal recessive neurologic disorder characterized by global developmental delay with intellectual disability and absent speech; poor, if any, motor development; and onset of seizures usually in the first year of life, although later onset has been reported. Affected individuals have poor eye contact and may develop microcephaly and abnormal movements (summary by Assoum 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).

Neonatal intractable myoclonus (NEIMY) is a severe neurologic disorder characterized by the onset of intractable myoclonic seizures soon after birth. Affected infants have intermittent apnea, abnormal eye movements, pallor of the optic nerve, and lack of developmental progress. Brain imaging shows a progressive leukoencephalopathy. Some patients may die in infancy. There is phenotypic and biochemical evidence of mitochondrial dysfunction (summary by Duis et al., 2016).

Progressive encephalopathy with amyotrophy and optic atrophy (PEAMO) is a severe autosomal recessive neurodegenerative disorder characterized by delayed development with hypotonia apparent in infancy and subsequent motor regression. Most affected individuals are unable to or lose the ability to sit and show distal amyotrophy and weakness of all 4 limbs. The patients are cognitively impaired and unable to speak or have severe dysarthria. Additional features include optic atrophy, thin corpus callosum, and cerebellar atrophy (Sferra et al., 2016).

Periventricular nodular heterotopia-7 (PVNH7) is a neurologic disorder characterized by abnormal neuronal migration during brain development resulting in delayed psychomotor development and intellectual disability; some patients develop seizures. Other features include cleft palate and 2-3 toe syndactyly (summary by Broix et al., 2016). For a phenotypic description and a discussion of genetic heterogeneity of periventricular heterotopia, see 300049.

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).

Developmental and epileptic encephalopathy-45 (DEE45) is a neurologic disorder characterized by global developmental delay apparent in infancy or early childhood and onset of seizures within the first 12 months of life. Affected individuals have severely impaired intellectual development, hypotonia, and other persistent neurologic deficits (summary by Burgess et al., 2019). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

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).

Developmental and epileptic encephalopathy-44 (DEE44) is an autosomal recessive neurologic disorder characterized by the onset of refractory infantile spasms or myoclonus usually in the first weeks or months of life, up to about 12 months of age. Affected infants may have normal or mildly delayed development before the onset of seizures, but thereafter show developmental stagnation and severe neurologic impairment. EEG in some patients shows hypsarrhythmia, consistent with a clinical diagnosis of West syndrome. Additional features include poor feeding and poor overall growth with microcephaly, axial hypotonia with peripheral hypertonia or spasticity, abnormal movements, limited eye contact, and profoundly impaired intellectual development with absent language. Many patients require tube feeding, and some die in childhood (summary by Muona et al., 2016; Colin et al., 2016). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

Developmental and epileptic encephalopathy-41 (DEE41) is a neurologic disorder characterized by the onset of seizures in the first days or weeks of life. Affected infants show severely impaired psychomotor development with hypotonia, spasticity, lack of speech, poor visual fixation, feeding difficulties sometimes necessitating tube feeding, poor overall growth and microcephaly, and contractures. Brain imaging may show delayed myelination, thin corpus callosum, and cerebral atrophy (summary by the EPI4K Consortium, 2016). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, 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 with or without polydactyly, see SRTD1 (208500).

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).

HIVEP2-related intellectual disability is a neurological disorder characterized by moderate to severe developmental delay and intellectual disability and mild physical abnormalities (dysmorphic features). Early symptoms of the condition include weak muscle tone (hypotonia) and delayed development of motor skills, such as sitting, standing, and walking. After learning to walk, many affected individuals continue to have difficulty with this activity; their walking style (gait) is often unbalanced and wide-based. Speech is also delayed, and some people with this condition never learn to talk. Most people with HIVEP2-related intellectual disability also have unusual physical features, such as widely spaced eyes (hypertelorism), a broad nasal bridge, or fingers with tapered ends, although there is no characteristic pattern of such features among affected individuals. Many people with the condition exhibit neurodevelopmental disorders, such as hyperactivity, attention deficit disorder, aggression, anxiety, and autism spectrum disorder, which is a group of developmental disorders characterized by impaired communication and social interaction.\n\nOther features of HIVEP2-related intellectual disability include mild abnormalities in the structure of the brain and an abnormally small brain and head size (microcephaly). Less common health problems include seizures; recurrent ear infections; and eye disorders, such as eyes that do not look in the same direction (strabismus), "lazy eye" (amblyopia), and farsightedness (hyperopia). Some people with HIVEP2-related intellectual disability have gastrointestinal problems, which can include backflow of acidic stomach contents into the esophagus (gastroesophageal reflux) and constipation.

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.

Any autosomal recessive cerebellar ataxia in which the cause of the disease is a mutation in the VWA3B gene.

Coffin-Siris syndrome is a rare congenital disorder characterized by delayed psychomotor development, intellectual disability, coarse facial features, and hypoplasia of the distal phalanges, particularly the fifth digit. Other features may also be observed, including congenital heart defects, hypoplasia of the corpus callosum, and poor overall growth with short stature and microcephaly (summary by Wieczorek et al., 2013). Patients with SMARCE1 mutations have a wide spectrum of manifestations, including severe to moderate intellectual disability and heart defects (summary by Kosho et al., 2014). For a general phenotypic description and a discussion of genetic heterogeneity of Coffin-Siris syndrome, see CSS1 (135900).

Heart and brain malformation syndrome (HBMS) is a severe autosomal recessive multiple congenital anomaly syndrome characterized by profoundly delayed psychomotor development, dysmorphic facial features, microphthalmia, cardiac malformations, mainly septal defects, and brain malformations, including Dandy-Walker malformation (summary by Shaheen et al., 2016). Homozygous mutation in the SMG9 gene can also cause neurodevelopmental disorder with intention tremor, pyramidal signs, dyspraxia, and ocular anomalies (NEDITPDO; 619995), a less severe neurodevelopmental disorder.

Neurodevelopmental disorder with or without hypotonia, seizures, and cerebellar atrophy (NEDHSCA) is an autosomal recessive disorder characterized by severely delayed psychomotor development, hypotonia apparent since infancy, and early-onset seizures in most patients. Some patients may have additional features, such as cerebellar atrophy, ataxia, and nonspecific dysmorphic features. NEDHSCA is one of a group of similar neurologic disorders resulting from biochemical defects in the glycosylphosphatidylinositol (GPI) biosynthetic pathway. Some patients with NEDHSCA may have the Emm-null blood group phenotype (see 619812) (summary by Makrythanasis et al., 2016; Duval et al., 2021). For a discussion of genetic heterogeneity of GPI biosynthesis defects, see GPIBD1 (610293).

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).

Any non-syndromic X-linked intellectual disability in which the cause of the disease is a mutation in the FRMPD4 gene.

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.

Xq25 duplication syndrome is an X-linked neurodevelopmental disorder characterized by delayed development and intellectual disability associated with abnormal behavior and dysmorphic facial features. Additional variable features may include thin corpus callosum on brain imaging and sleep disturbances. Carrier females may be mildly affected (summary by Leroy et al., 2016).

Noonan syndrome-like disorder with loose anagen hair is characterized by facial features similar to those observed in Noonan syndrome (163950), including hypertelorism, ptosis, downslanting palpebral fissures, low-set posteriorly angulated ears, and overfolded pinnae. In addition, patients display short stature, frequently with growth hormone (GH; see 139250) deficiency; cognitive deficits; relative macrocephaly; small posterior fossa resulting in Chiari I malformation; hypernasal voice; cardiac defects, especially dysplasia of the mitral valve and septal defects; and ectodermal abnormalities, in which the most characteristic feature is the hair anomaly, including easily pluckable, sparse, thin, slow-growing hair (summary by Bertola et al., 2017). Reviews Komatsuzaki et al. (2010) reviewed the clinical manifestations of patients with Noonan syndrome, Costello syndrome (218040), and cardiofaciocutaneous syndrome (CFC; see 115150) compared to patients with mutations in the SHOC2 gene. They noted that although there is phenotypic overlap among the disorders, loose anagen/easily pluckable hair had not been reported in mutation-positive patients with Noonan, CFC, or Costello syndrome, and appeared to be a distinctive feature of SHOC2 mutation-positive patients. Genetic Heterogeneity of Noonan Syndrome-Like Disorder with Loose Anagen Hair NSLH2 (617506) is caused by mutation in the PPP1CB gene (600590) on chromosome 2p23.

CDK13-related disorder, reported in 43 individuals to date, is characterized in all individuals by developmental delay / intellectual disability (DD/ID); nearly all individuals older than age one year display impaired verbal language skills (either absent or restricted speech). Other common findings are recognizable facial features in some individuals, behavioral problems (autism spectrum disorder or autistic traits/stereotypies, attention-deficit/hyperactivity disorder), feeding difficulties in infancy, structural cardiac defects, and seizures.

A rare congenital disorder of glycosylation caused by mutations in the COG2 gene and with characteristics of normal presentation at birth, followed by progressive deterioration with postnatal microcephaly, developmental delay, intellectual disability, seizures, spastic quadriplegia, liver dysfunction, hypocupremia and hypoceruloplasminemia in the first year of life. Diffuse cerebral atrophy and thin corpus callosum may be observed on brain MRI.

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).

NMIHBA is a severe autosomal recessive neurodevelopmental and neurodegenerative disorder characterized by global developmental delay apparent from infancy and profoundly impaired intellectual development. Affected individuals have microcephaly with accompanying dysmorphic features, truncal hypotonia, peripheral spasticity, and lack of independent ambulation or speech acquisition. Brain imaging shows variable abnormalities, including cortical atrophy, thin corpus callosum, cerebellar hypoplasia, and delayed myelination (summary by Zollo 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).

Neurodegeneration with brain iron accumulation refers to a group of neurodegenerative disorders characterized by progressive motor and cognitive dysfunction beginning in childhood or young adulthood. Patients show extrapyramidal motor signs, such as spasticity, dystonia, and parkinsonism. Brain imaging shows iron accumulation in the basal ganglia (summary by Dusi et al., 2014). For a general phenotypic description and a discussion of genetic heterogeneity of NBIA, see NBIA1 (234200).

Developmental and epileptic encephalopathy-55 (DEE55) is an autosomal recessive neurologic disorder characterized by onset of refractory seizures in the first weeks or months of life. Affected individuals have an extremely poor outcome, with profoundly impaired intellectual development, absent speech, spastic quadriplegia, and dyskinetic movements. Most have cortical visual impairment and require a feeding tube. Brain imaging shows nonspecific abnormalities, including cerebral atrophy, thin corpus callosum, and abnormal signals in the white matter. Death in childhood may occur. Biochemically, the disorder is associated with impaired synthesis of GPI-anchored proteins (summary by Vetro et al., 2020). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350. For a discussion of genetic heterogeneity of GPI biosynthesis defects, see GPIBD1 (610293).

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).

Childhood-onset neurodegeneration with brain atrophy (CONDBA) is a severe progressive neurodegenerative disorder characterized by loss of motor and cognitive skills between ages 2 and 7 years. Affected individuals may have normal development or mild developmental delay, but all eventually lose all motor skills, resulting in inability to walk, absence of language, and profound intellectual disability. Brain imaging shows progressive cerebral and cerebellar atrophy (summary by Edvardson 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).

Pontocerebellar hypoplasia type 11 (PCH11) is an autosomal recessive neurodevelopmental disorder characterized by severely delayed psychomotor development with impaired intellectual development and poor speech, microcephaly, dysmorphic features, and pontocerebellar hypoplasia on brain imaging. Additional features are more variable (summary by Marin-Valencia et al., 2017). For a general phenotypic description and a discussion of genetic heterogeneity of PCH, see PCH1 (607596).

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).

A rare genetic multiple congenital anomalies/dysmorphic syndrome characterized by global developmental delay and moderate to severe intellectual disability, as well as variable other manifestations, such as macro- or microcephaly, epilepsy, hypotonia, behavioral problems, stereotypic movements, and facial dysmorphism (including arched eyebrows, long palpebral fissures, prominent nasal bridge, upturned nose, dysplastic ears, and broad mouth), among others. Brain imaging may show cerebellar anomalies, hypoplastic corpus callosum, enlarged ventricles, polymicrogyria, or white matter abnormalities.

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.

A rare genetic disorder with characteristics of infantile onset of stagnation and regression of motor and language development resulting in complete lack of communication and purposeful movement. Further neurological manifestations include truncal hypotonia, appendicular spasticity, dystonia, optic disc pallor, peripheral neuropathy and neurogenic bladder. Patients also present multiple contractures, late-onset relative macrocephaly, short stature and facial dysmorphism (including coarse facial features, sloping forehead, thick eyebrows, low-set ears, prominent nose, flat philtrum, and prominent lower lip). Brain imaging at advanced stages shows diffuse abnormal white matter signal and severe atrophy. Sural nerve biopsy reveals decreased myelination.

Developmental and epileptic encephalopathy-57 (DEE57) is a neurologic disorder characterized by global developmental delay with hypotonia, variably impaired intellectual development, and poor or absent language. Affected individuals have onset of refractory multifocal seizures in the first days or months of life, and may show developmental regression. EEG patterns include hypsarrhythmia, suggesting a clinical diagnosis of West syndrome, background slowing, and epilepsy of infancy with migrating focal seizures (EIMFS). Some patients may have mild dysmorphic features (summary by Ambrosino et al., 2018 and Mao et al., 2020). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

MRT61 is an autosomal recessive neurodevelopmental disorder characterized by delayed psychomotor development, moderate to severe intellectual disability, and variable dysmorphic facial features. More severely affected patients may develop refractory seizures and have brain abnormalities, including hypoplasia of the corpus callosum (summary by Alwadei et al., 2016).

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).

Neurodevelopmental disorder with microcephaly, seizures, and cortical atrophy (NDMSCA) is an autosomal recessive disorder characterized by severe global developmental delay with poor motor and intellectual function apparent soon after birth, as well as postnatal progressive microcephaly. Most patients develop early-onset, frequent, and often intractable seizures, compatible with an epileptic encephalopathy. Other features include poor feeding, poor overall growth, absent speech, poor or absent eye contact, inability to achieve walking, hypotonia, and peripheral spasticity. Brain imaging usually shows progressive cerebral atrophy, thin corpus callosum, and abnormalities in myelination. Death in childhood may occur (summary by Siekierska et al., 2019).

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.

Van Maldergem syndrome is an autosomal recessive disorder characterized by intellectual disability, typical craniofacial features, auditory malformations resulting in hearing loss, and skeletal and limb malformations. Some patients have renal hypoplasia. Brain MRI typically shows periventricular nodular heterotopia (summary by Cappello et al., 2013). Genetic Heterogeneity of Van Maldergem Syndrome See also VMLDS2 (615546), caused by mutation in the FAT4 gene (612411) on chromosome 4q28.

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.

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).

Neurodevelopmental disorder with or without seizures and gait abnormalities (NEDSGA) is an autosomal dominant disorder characterized by global developmental delay apparent from infancy or early childhood, resulting in variably impaired intellectual development that can range from profound with absent speech to mild with an ability to attend special schools. Most affected individuals show irritability, stiffness, and hypertonia early in life, which progresses to spasticity and impaired gait later. Some patients may develop seizures of variable severity early in life (summary by Martin et al., 2017).

Neurodegeneration with brain iron accumulation-7 (NBIA7) is characterized by iron accumulation in the basal ganglia and manifests as a progressive extrapyramidal syndrome with dystonia, rigidity, and choreoathetosis. Severity and rate of progression are variable (Drecourt et al., 2018). For a general phenotypic description and a discussion of genetic heterogeneity of NBIA, see NBIA1 (234200).

SCN3A-related neurodevelopmental disorder (SCN3A-ND) encompasses a spectrum of clinical severity associated with epilepsy and/or brain malformation. Affected individuals may have (a) developmental and epileptic encephalopathy (DEE) (i.e., intractable seizures with developmental delays associated with ongoing epileptiform EEG activity) with or without malformations of cortical development; or (b) malformations of cortical development with or without mild focal epilepsy. Some degree of early childhood developmental delay is seen in all affected individuals; the severity varies widely, ranging from isolated speech delay to severe developmental delay. Infantile hypotonia is common but may be mild or absent in those without DEE. In those with DEE, seizure onset is typically in the first six to 12 months of life. A variety of seizure types have been described. Seizures remain intractable to multiple anti-seizure medications in approximately 50% of individuals with DEE without malformations of cortical development (MCD) and in 90% of individuals with DEE and MCD. Seizures may be absent or infrequent in those without DEE. Brain MRI findings range from normal to showing thinning or hypoplasia of the corpus callosum, to various malformations of cortical development. Autonomic dysregulation, oromotor dysfunction leading to the need for gastrostomy tube placement, progressive microcephaly, hyperkinetic movement disorder, and cortical visual impairment can also be seen in those with DEE.

Hypomyelinating leukodystrophy-15 is an autosomal recessive neurodegenerative disorder characterized by onset of motor and cognitive impairment in the first or second decade of life. Features include dystonia, ataxia, spasticity, and dysphagia. Most patients develop severe optic atrophy, and some have hearing loss. Brain imaging shows hypomyelinating leukodystrophy with thin corpus callosum. The severity of the disorder is variable (summary by Mendes et al., 2018) For a discussion of genetic heterogeneity of HLD, see 312080.

Hypomyelinating leukodystrophy-16 is an autosomal dominant neurologic disorder characterized by onset of hypotonia, nystagmus, and mildly delayed motor development in infancy. Affected individuals have motor disabilities, including ataxic or broad-based gait, hyperreflexia, intention tremor, dysmetria, and a mild pyramidal syndrome. Some patients have cognitive impairment, whereas others may have normal cognition or mild intellectual disability with speech difficulties. Brain imaging typically shows hypomyelination, leukodystrophy, and thin corpus callosum (summary by Simons et al., 2017). For a general phenotypic description and a discussion of genetic heterogeneity of hypomyelinating leukodystrophy, see 312080.

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.

JABELS is an autosomal recessive neurodevelopmental disorder characterized by developmental delay and intellectual disability with additional variable features. Patients have onset of symptoms in infancy, but the severity is highly variable. Some patients have social interaction and learn to walk but have an ataxic gait and abnormal movements, such as tremor or dystonia, whereas others do not achieve any motor control and are unable to speak. Additional features may include retinal anomalies, visual impairment, microcephaly, abnormal foot or hand posturing, and kyphoscoliosis; some patients have dysmorphic facial features or seizures. Brain imaging typically shows cerebellar atrophy and hypoplasia of the corpus callosum (summary by et al., 2016 and Bertoli-Avella et al., 2018).

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.

Hypomyelinating leukodystrophy-17 is an autosomal recessive neurodevelopmental disorder characterized by poor, if any, development apparent from infancy. Affected individuals never learn to walk or speak, and have early-onset multifocal seizures, spasticity, poor overall growth, and microcephaly (up to -10 SD). Brain imaging shows multiple abnormalities, including cerebral and cerebellar atrophy, thin corpus callosum, abnormal signals in the basal ganglia, and features suggesting hypo- or demyelination. Some patients may die in childhood (summary by Shukla et al., 2018). For a general phenotypic description and a discussion of genetic heterogeneity of hypomyelinating leukodystrophy, see 312080.

Developmental and epileptic encephalopathy (DEE93) is an autosomal dominant neurologic disorder characterized by delayed psychomotor development, early-onset refractory seizures, and impaired intellectual development. The severity of the phenotype is highly variable: some patients may be nonverbal and nonambulatory with spastic quadriparesis and poor eye contact, whereas others have moderate intellectual disability (summary by Fassio et al., 2018). For a discussion of genetic heterogeneity of DEE, see 308350.

Combined oxidative phosphorylation deficiency-21 (COXPD21) is an autosomal recessive disorder characterized either by onset within the first months of life of severe hypotonia, failure to thrive, epilepsy and early death or by onset after 6 months of life with a milder course and longer survival (summary by Zheng et al., 2022). For a discussion of genetic heterogeneity of combined oxidative phosphorylation deficiency, see COXPD1 (609060).

Combined oxidative phosphorylation deficiency-12 (COXPD12) is an autosomal recessive mitochondrial neurologic disorder characterized by onset in infancy of hypotonia and delayed psychomotor development, or early developmental regression, associated with T2-weighted hyperintensities in the deep cerebral white matter, brainstem, and cerebellar white matter. Serum lactate is increased due to a defect in mitochondrial respiration. There are 2 main phenotypic groups: those with a milder disease course and some recovery of skills after age 2 years, and those with a severe disease course resulting in marked disability (summary by Steenweg et al., 2012). For a discussion of genetic heterogeneity of combined oxidative phosphorylation deficiency, see COXPD1 (609060).

A rare subtype of orofaciodigital syndrome, with autosomal recessive inheritance and C2CD3 mutations. The disease has characteristics of severe microcephaly, trigonocephaly, severe intellectual disability and micropenis, in addition to oral, facial and digital malformations (gingival frenulum, lingual hamartomas, cleft/lobulated tongue, cleft palate, telecanthus, up-slanting palpebral fissures, microretrognathia, postaxial polydactyly of hands and duplication of hallux). Corpus callosum agenesis and vermis hypoplasia with molar tooth sign on brain imaging are also associated.

Polycystic lipomembranous osteodysplasia with sclerosing leukoencephalopathy (PLOSL) is characterized by fractures (resulting from radiologically demonstrable polycystic osseous lesions), frontal lobe syndrome, and progressive presenile dementia beginning in the fourth decade. The clinical course of PLOSL can be divided into four stages: 1. The latent stage is characterized by normal early development. 2. The osseous stage (3rd decade of life) is characterized by pain and tenderness, mostly in ankles and feet, usually following strain or injury. Fractures are typically diagnosed several years later, most commonly in the bones of the extremities. 3. In the early neurologic stage (4th decade of life), a change of personality begins to develop insidiously. Affected individuals show a frontal lobe syndrome (loss of judgment, euphoria, loss of social inhibitions, disturbance of concentration, and lack of insight, libido, and motor persistence) leading to serious social problems. 4. The late neurologic stage is characterized by progressive dementia and loss of mobility. Death usually occurs before age 50 years.

Neurodevelopmental disorder with cerebellar atrophy and with or without seizures (NEDCAS) is an autosomal recessive disorder characterized by intellectual disability associated with ataxia (summary by Engel et al., 2023).

MFRG is an autosomal recessive syndrome in which microcephaly, unilateral renal agenesis, ambiguous genitalia, and facial dysmorphisms, including severe micrognathia, are observed in most patients. Variable brain, cardiac, and skeletal anomalies are present, including corpus callosum agenesis or dysgenesis, lissencephaly, atrial and ventricular septal defects, patent ductus arteriosus, hypoplastic right ventricle, and joint contractures (Shaheen et al., 2016).

Squalene synthase deficiency (SQSD) is a rare inborn error of cholesterol biosynthesis with multisystem clinical manifestations similar to Smith-Lemli-Optiz syndrome. Key clinical features include facial dysmorphism, a generalized seizure disorder presenting in the neonatal period, nonspecific structural brain malformations, cortical visual impairment, optic nerve hypoplasia, profound developmental delay / intellectual disability, dry skin with photosensitivity, and genital malformations in males.

Complex cortical dysplasia with other brain malformations-9 is a severe autosomal recessive disorder characterized by profoundly impaired motor and cognitive development apparent from early infancy. Affected individuals develop intractable seizures and are unable to speak or ambulate. Brain imaging shows pachygyria as well as hypogenesis of the corpus callosum and other variable brain abnormalities. The phenotype results from impaired cortical neuronal migration (summary by Schaffer et al., 2018). For a discussion of genetic heterogeneity of CDCBM, see CDCBM1 (614039).

Congenital hypomyelinating neuropathy-3 is an autosomal recessive neurologic disorder characterized by onset of neurogenic muscle impairment in utero. Affected individuals present at birth with severe hypotonia, often causing respiratory insufficiency or failure and inability to swallow or feed properly. They have profoundly impaired psychomotor development and may die in infancy or early childhood. Those that survive are unable to sit or walk. Sural nerve biopsy shows hypomyelination of the nerve fibers, and brain imaging often shows impaired myelination and cerebral and cerebellar atrophy. Nerve conduction velocities are severely decreased (about 10 m/s) or absent due to improper myelination (summary by Vallat et al., 2016 and Low et al., 2018). For a discussion of genetic heterogeneity of CHN, see CHN1 (605253).

Polycystic lipomembranous osteodysplasia with sclerosing leukoencephalopathy-2 (PLOSL2), or Nasu-Hakola disease, is a recessively inherited presenile frontal dementia with leukoencephalopathy and basal ganglia calcification. In most cases the disorder first manifests in early adulthood as pain and swelling in ankles and feet, followed by bone fractures. Neurologic symptoms manifest in the fourth decade of life as a frontal lobe syndrome with loss of judgment, euphoria, and disinhibition. Progressive decline in other cognitive domains begins to develop at about the same time. The disorder culminates in a profound dementia and death by age 50 years (summary by Klunemann et al., 2005). For a discussion of genetic heterogeneity of polycystic lipomembranous osteodysplasia with sclerosing leukoencephalopathy, see 221770.

A rare genetic disease characterized by infantile onset of severe inflammatory bowel disease manifesting with bloody diarrhea and failure to thrive, and central nervous system disease with global developmental delay and regression, impaired speech, hypotonia, hyperreflexia, and epilepsy. Brain imaging shows global cerebral atrophy, thin corpus callosum, delayed myelination, and posterior leukoencephalopathy. Cases with recurrent infections and impaired T-cell responses to stimulation, as well as decreased T-cell subsets, have been reported.

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).

ASXL3-related disorder is characterized by developmental delay or intellectual disability, typically in the moderate to severe range, with speech and language delay and/or absent speech. Affected individuals may also display autistic features. There may be issues with feeding. While dysmorphic facial features have been described, they are typically nonspecific. Affected individuals may also have hypotonia that can transition to spasticity resulting in unusual posture with flexion contractions of the elbows, wrists, and fingers. Other findings may include poor postnatal growth, strabismus, seizures, sleep disturbance, and dental anomalies.

Hypomyelination with brainstem and spinal cord involvement and leg spasticity is an autosomal recessive leukoencephalopathy characterized by onset in the first year of life of severe spasticity, mainly affecting the lower limbs and resulting in an inability to achieve independent ambulation. Affected individuals show delayed motor development and nystagmus; some may have mild mental retardation. Brain MRI shows hypomyelination and white matter lesions in the cerebrum, brainstem, cerebellum, and spinal cord (summary by Taft et al., 2013).

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).

Combined oxidative phosphorylation deficiency-21 (COXPD11) is a severe multisystemic autosomal recessive disorder characterized by neonatal hypotonia and lactic acidosis. Affected individuals may have respiratory insufficiency, foot deformities, or seizures, and all reported patients have died in infancy. Biochemical studies show deficiencies of multiple mitochondrial respiratory enzymes (summary by Garcia-Diaz et al., 2012). For a discussion of genetic heterogeneity of combined oxidative phosphorylation deficiency, see COXPD1 (609060).

Lissencephaly-9 with complex brainstem malformation (LIS9) is an autosomal dominant neurologic disorder characterized by global developmental delay apparent since infancy, impaired intellectual development with poor or absent speech, and sometimes abnormal or involuntary movements associated with abnormal brain imaging that typically shows pachygyria, lissencephaly, and malformation of the brainstem consistent with a neuronal migration defect (summary by Dobyns et al., 2018). For a general description and a discussion of genetic heterogeneity of lissencephaly, see LIS1 (607432).

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, neurometabolic disease characterized by microcephaly, short stature, epilepsy, cerebral hypomyelination, severe global developmental delay, and progressive spasticity. Macrocytic anemia and hyperthermia have also been reported in association. Brain imaging reveals delayed myelination with minimal progression over time, mild cerebellar atrophy and/or thin corpus callosum.

Turnpenny-Fry syndrome (TPFS) is characterized by developmental delay, impaired intellectual development, impaired growth, and recognizable facial features that include frontal bossing, sparse hair, malar hypoplasia, small palpebral fissures and oral stoma, and dysplastic 'satyr' ears. Other common findings include feeding problems, constipation, and a range of brain, cardiac, vascular, and skeletal malformations (Turnpenny et al., 2018).

Developmental and epileptic encephalopathy-72 (DEE72) is neurologic disorder characterized by the onset of infantile spasms around 5 months of age. The seizures tend to be refractory to treatment. 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. Additional more variable features include hyperkinetic movements and cortical visual impairment (summary by Sega et al., 2019). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

Developmental and epileptic encephalopathy-73 (DEE73) is a neurologic disorder characterized by the onset of refractory seizures in the first months of life. Affected individuals meet almost no developmental milestones: they have hypotonia and are unable to walk, speak, or feed properly. They have poor overall growth with small head circumference and dysmorphic facial features. Additional manifestations include cortical visual impairment with roving eye movements and variable hearing loss (summary by Edvardson et al., 2019). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

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.

Developmental and epileptic encephalopathy-75 (DEE75) is an autosomal recessive neurodevelopmental and neurodegenerative disorder characterized by onset of severe refractory seizures in the first months of life. Patients often have global developmental delay before the onset of seizures, and thereafter achieve few milestones. EEG usually shows multifocal spikes and hypsarrhythmia, consistent with a clinical diagnosis of West syndrome. They have severely impaired intellectual development with inability to walk, absent speech, and hypotonia with axial hyperreflexia. Brain imaging shows progressive cerebral atrophy, frontal lobe atrophy, white matter abnormalities, and delayed myelination. Since the disorder is due to mitochondrial dysfunction, some patients may develop other organ involvement, including cardiomyopathy or liver and renal dysfunction. Death may occur in childhood (summary by Yin et al., 2018). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

Neurodevelopmental disorder with or without variable brain abnormalities (NEDBA) is characterized by global developmental delay apparent from infancy or early childhood, resulting in mildly delayed walking, variably impaired intellectual development, and poor or absent speech. Additional features may include hypotonia, spasticity, or ataxia. About half of patients have abnormal findings on brain imaging, including cerebral or cerebellar atrophy, loss of white matter volume, thin corpus callosum, and perisylvian polymicrogyria. Seizures are not a prominent finding, and although some patients may have nonspecific dysmorphic facial features, there is no common or consistent gestalt (summary by Platzer et al., 2019).

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).

Developmental and epileptic encephalopathy-76 (DEE76) is an autosomal recessive neurodevelopmental disorder characterized by early-onset, usually refractory, seizures, severely delayed global development, hypotonia, peripheral spasticity, and abnormalities on brain imaging, mainly cerebral atrophy and delayed myelination. Some patients may have additional features, such as scoliosis or microcephaly. The disorder may result in death in childhood (summary by Bell et al., 2019). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

Intellectual developmental disorder with severe speech and ambulation defects (IDDSSAD) is an autosomal dominant neurodevelopmental disorder with onset of features in infancy or early childhood. Affected individuals have global developmental delay with impaired intellectual development and absent speech, and most cannot walk independently. Common dysmorphic features include prominent forehead and wide mouth (summary by Bell et al., 2019).

ATN1-related neurodevelopmental disorder (ATN1-NDD) is characterized by developmental delay / intellectual disability. Other neurologic findings can include infantile hypotonia, brain malformations, epilepsy, cortical visual impairment, and hearing loss. Feeding difficulties, present in some individuals, may require gastrostomy support when severe; similarly, respiratory issues, present in some, may require respiratory support after the neonatal period. Distinctive facial features and hand and foot differences are common. Other variable findings can include cardiac malformations and congenital anomalies of the kidney and urinary tract (CAKUT). To date, 18 individuals with ATN1-NDD have been identified.

Holoprosencephaly-12 with or without pancreatic agenesis (HPE12) is a developmental disorder characterized by abnormal separation of the embryonic forebrain (HPE) resulting in dysmorphic facial features and often, but not always, impaired neurologic development. Most patients with this form of HPE also have congenital absence of the pancreas, resulting in early-onset type 1 diabetes mellitus and requiring pancreatic enzyme replacement. Other features may include hearing loss and absence of the gallbladder (summary by De Franco et al., 2019 and Kruszka et al., 2019). For a phenotypic description and a discussion of genetic heterogeneity of holoprosencephaly, see HPE1 (236100).

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).

Developmental and epileptic encephalopathy-79 (DEE79) is a severe neurologic disorder characterized by onset of refractory seizures in the first months of life. Affected individuals have severely impaired psychomotor development and may show hypotonia or spasticity. Brain imaging may show hypomyelination, cerebral atrophy, and thinning of the corpus callosum (summary by Butler et al., 2018 and Hernandez et al., 2019). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

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 brain anomalies, seizures, and scoliosis (NEDBSS) is an autosomal recessive disorder characterized by severely impaired psychomotor development, hypotonia, seizures, and structural brain anomalies, including thin corpus callosum and cerebellar atrophy. Other features include scoliosis, dysmorphic facies, and visual impairment. Affected individuals are usually unable to walk or speak and may require tube feeding in severe cases. The disorder is caused by a defect in glycosylphosphatidylinositol (GPI) biosynthesis (summary by Knaus et al., 2019). 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).

Snijders Blok-Fisher syndrome (SNIBFIS) is a neurodevelopmental disorder characterized by global developmental delay, hypotonia, variable impaired intellectual development, and specifically impaired speech and language acquisition. Patients achieve independent ambulation and most have mildly to moderately impaired cognition with autistic features, although a few may develop seizures and have a more severe phenotype. Dysmorphic features include abnormal, cupped, or prominent ears and ocular anomalies. Mutations usually occur de novo, although 1 family with autosomal dominant inheritance has been reported (summary by Snijders Blok et al., 2019).

Pontocerebellar hypoplasia type 13 (PCH13) is an autosomal recessive disorder characterized by global developmental delay, impaired intellectual development with absent speech, microcephaly, and progressive atrophy of the cerebellar vermis and brainstem. Additional features, including seizures and visual impairment, are variable (summary by Uwineza et al., 2019). For a general phenotypic description and a discussion of genetic heterogeneity of PCH, see PCH1A (607596).

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.

Intellectual developmental disorder with impaired language and dysmorphic facies (IDDILF) is an autosomal dominant disorder characterized by global developmental delay apparent from infancy, impaired language development, and dysmorphic facial features, including hypertelorism, epicanthal folds, and abnormal palpebral fissures. Some patients may have additional findings, including feeding difficulties, mild cardiac or genitourinary defects, and distal skeletal anomalies (summary by Balak 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).

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).

Developmental and epileptic encephalopathy-82 (DEE82) is an autosomal recessive mitochondriopathy manifest as early-onset metabolic epileptic encephalopathy. Soon after birth, affected individuals exhibit hypotonia, feeding difficulties, and global developmental delay even before the onset of seizures in the first year of life. The severity is variable, but all patients have severely impaired intellectual development with absent speech and spastic tetraplegia. Other features include poor overall growth with microcephaly and recurrent infections. Brain imaging shows cerebral atrophy, thin corpus callosum, cerebellar hypoplasia, and white matter abnormalities. Laboratory studies show increased serum lactate and ammonia. Importantly, treatment with combined pyridoxine and serine can result in significant improvement in seizures as well as some mild developmental progress (summary by van Karnebeek et al., 2019). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

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).

Complex cortical dysplasia with other brain malformations-15 (CDCBM15) is an autosomal recessive neurologic disorder characterized by progressive microcephaly associated with abnormal facial features, hypotonia, and variable global developmental delay with impaired intellectual development. Brain imaging shows variable malformation of cortical development on the lissencephaly spectrum, mainly pachygyria and thin corpus callosum (summary by Mitani et al., 2019). For a discussion of genetic heterogeneity of CDCBM, see CDCBM1 (614039).

Developmental and epileptic encephalopathy-83 (DEE83) is a severe autosomal recessive neurodevelopmental disorder characterized by onset of frequent seizures in the first days to months of life that are usually refractory to medical treatment and are associated with significant EEG abnormalities. Affected individuals have profoundly impaired development, with no motor or language skill acquisition, poor or absent visual tracking, and poor oromotor function necessitating tube feeding. Many patients die in the first years of life (summary by Perenthaler et al., 2020). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

Neurodevelopmental disorder with hypotonia and autistic features with or without hyperkinetic movements (NEDHAHM) is characterized by axial hypotonia apparent from birth, global developmental delay with impaired intellectual development and poor or absent language acquisition, and behavioral abnormalities, including autistic features, poor social interaction, and hang-wringing. Most patients have childhood-onset seizures that are usually responsive to medication, and a subset of patients develop cortical visual impairment and involuntary hyperkinetic movements, including chorea and dystonia. Some of the features are reminiscent of Rett syndrome (RTT; 312750) (summary by Salpietro 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.

Developmental and epileptic encephalopathy-85 with or without midline brain defects (DEE85) is an X-linked neurologic disorder characterized by onset of severe refractory seizures in the first year of life, global developmental delay with impaired intellectual development and poor or absent speech, and dysmorphic facial features. The seizures tend to show a cyclic pattern with clustering. Many patients have midline brain defects on brain imaging, including thin corpus callosum and/or variable forms of holoprosencephaly (HPE). The severity and clinical manifestations are variable. Almost all reported patients are females with de novo mutations predicted to result in a loss of function (LOF). However, some patients may show skewed X inactivation, and the pathogenic mechanism may be due to a dominant-negative effect. The SMC1A protein is part of the multiprotein cohesin complex involved in chromatid cohesion during DNA replication and transcriptional regulation; DEE85 can thus be classified as a 'cohesinopathy' (summary by Symonds et al., 2017 and Kruszka et al., 2019). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

Neurodevelopmental disorder with hypotonia, neonatal respiratory insufficiency, and thermodysregulation (NEDHRIT) is a severe autosomal recessive disorder characterized by neonatal respiratory distress, poor feeding, and impaired global development. Affected individuals are unable to walk or speak and have poor or absent eye contact. Some patients may develop seizures (summary by Wagner 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).

Alopecia-intellectual disability syndrome-4 (APMR4) is characterized by alopecia universalis, scaly skin, and psychomotor retardation of varying degrees (Besnard et al., 2019). For a discussion of genetic heterogeneity of alopecia-intellectual disability syndrome, see APMR1 (203650).

Neurodevelopmental disorder with or without autistic features and/or structural brain abnormalities (NEDASB) is an early-onset neurologic disorder characterized by global developmental delay, poor or absent speech and language development, and behavioral abnormalities reminiscent of autism spectrum disorder (ASD; 209850) or Angelman syndrome (AS; 105830). Additional features may include poor overall growth with small head circumference, axial hypotonia, spasticity, and seizures. Some patients have abnormal findings on brain imaging, including cerebral atrophy, cerebellar atrophy, and/or thin corpus callosum (summary by Mattioli et al., 2020).

Neurodevelopmental disorder with hypotonia, microcephaly, and seizures (NEDHYMS) is an autosomal recessive disorder characterized by global developmental delay with axial hypotonia, inability to sit or walk, and severely impaired intellectual development with absent language. Most patients develop early-onset intractable seizures that prevent normal development. Additional features include feeding difficulties with poor overall growth and microcephaly. Some patients may have spastic quadriplegia, poor eye contact due to cortical blindness, variable dysmorphic features, and nonspecific abnormalities on brain imaging (summary by Tan et al., 2020).

Nizon-Isidor syndrome (NIZIDS) is a neurodevelopmental disorder characterized by global developmental delay, mildly delayed walking, poor speech and language, variably impaired intellectual development, and behavioral abnormalities, such as autistic features or attention deficit-hyperactivity disorder (ADHD). Some patients may have additional features, including nonspecific facial dysmorphism, gastrointestinal difficulties, distal hand anomalies, and thin corpus callosum on brain imaging (summary by Nizon et al., 2019).

Early-onset seizures with neurodegeneration and brain calcifications (SENEBAC) is an autosomal recessive encephalopathy characterized by onset of refractory seizures in the first year of life. Affected individuals tend to have normal or mildly delayed development early in life, but show significant and progressive developmental regression associated with seizure onset. Features include hypotonia, peripheral spasticity, poor eye contact, and absent speech. Most require tube feeding; death in childhood may occur. Brain imaging shows cerebral atrophy, loss of white matter, and punctate calcifications, suggestive of abnormal neuroinflammation (summary by Smith et al., 2020 and Dong et al., 2020).

Leukoencephalopathy, developmental delay, and episodic neurologic regression syndrome (LEUDEN) is characterized by global developmental delay apparent in early childhood, followed by episodic neurologic regression or decompensation associated with systemic stress, such as febrile infection. Affected individuals have hypotonia, gait difficulties or ataxia, poor or absent speech with dysarthria, and variable motor abnormalities, including spasticity, dystonia, extrapyramidal signs, and tremor. Many patients have seizures. Brain imaging shows diffuse white matter abnormalities, poor myelination, thin corpus callosum, and generalized cerebral atrophy with enlarged ventricles. The clinical features of the disorder and the abnormal brain imaging findings are progressive (summary by Mao et al., 2020).

Neurodevelopmental disorder and structural brain anomalies with or without seizures and spasticity (NEDBASS) is an autosomal recessive neurologic disorder characterized by global developmental delay apparent from early infancy, poor overall growth often with microcephaly, impaired intellectual development with delayed or absent speech, axial hypotonia, and peripheral spasticity. Additional common but variable features include early-onset seizures, optic atrophy with poor visual fixation, and dysmorphic facial features. Brain imaging shows cerebral atrophy, poor or absent myelination with loss of white matter volume, and often hypoplasia of the corpus callosum and/or cerebellum. Early death may occur (summary by Bend et al., 2020).

Microcephaly, developmental delay, and brittle hair syndrome (MDBH) is a multisystem disorder with clinical variability. Affected individuals show cognitive and motor disabilities, as well as some degree of fine, brittle hair with microscopic shaft abnormalities. Other shared features include failure to thrive in early childhood and short stature, with some patients exhibiting feeding difficulties and hepatic steatosis (Kuo et al., 2019).

Neurodevelopmental, jaw, eye, and digital syndrome (NEDJED) is characterized by phenotypic diversity, with patients exhibiting a range of overlapping phenotypes. Most patients show developmental delay ranging from mild to severe, and often have behavioral disorders as well. Brain imaging shows hypoplasia of the corpus callosum, prominence of lateral ventricles, and/or white matter abnormalities. Many patients have retro- or micrognathia, but mild prognathism has also been observed. Ocular anomalies are variably present, and may be severe and complex; however, some patients show only mild myopia. Abnormalities of fingers and toes include brachydactyly, clinodactyly, syndactyly, and contractures; polydactyly is rarely seen (Holt et al., 2019).

Periventricular nodular heterotopia-9 (PVNH9) is an autosomal dominant neurologic disorder characterized as a malformation of cortical development. Anterior predominant PVNH, thin corpus callosum, and decreased white matter volume are found on brain imaging, but the clinical effects are variable. Most patients have impaired intellectual development and cognitive defects associated with low IQ (range 50 to 80), learning disabilities, and behavior abnormalities. Some patients develop seizures that tend to have a focal origin. However, some mutation carriers may be less severely affected with borderline or even normal IQ, suggesting incomplete penetrance of the phenotype (summary by Heinzen et al., 2018, Walters et al., 2018). For a discussion of genetic heterogeneity of periventricular nodular heterotopia, see 300049.

Neurodevelopmental disorder with seizures, hypotonia, and brain imaging abnormalities (NEDSHBA) is an autosomal recessive neurodevelopmental disorder characterized by global developmental delay, severe to profound intellectual impairment, early-onset refractory seizures, hypotonia, failure to thrive, and progressive microcephaly. Brain imaging shows cerebral atrophy, thin corpus callosum, and myelination defects. Death in childhood may occur (summary by Marafi 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.

Sodium-dependent multivitamin transporter deficiency (SMVTD) is an autosomal recessive multisystemic metabolic disorder with highly variable manifestations. Affected individuals usually present at birth or in infancy with severe feeding problems, gastrointestinal reflux, cyclic vomiting, and diarrhea associated with failure to thrive. Gastrointestinal hemorrhage may occur; tube-feeding is often required for a short time. The course and severity of the disease varies: some patients have episodes of acute metabolic decompensation during infection that respond well to treatment, whereas others show more permanent neurologic regression with loss of early motor and cognitive milestones in the first year or so of life. Less severely affected patients have normal development or mild growth and motor delays, whereas more severely affected individuals may have seizures, ataxia, spasticity, peripheral neuropathy, immune defects, and osteopenia. In severely affected patients, brain imaging shows cerebral, cerebellar, and brainstem atrophy and thin corpus callosum. Treatment with biotin, pantothenic acid, and alpha-lipoic acid has been shown to result in significant clinical improvement (Byrne et al., 2019; Hauth et al., 2022).

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).

Neurodevelopmental disorder with progressive spasticity and brain white matter abnormalities (NEDSWMA) is an autosomal recessive disorder characterized by impaired psychomotor development apparent in infancy. Affected individuals show poor overall growth, progressive microcephaly, and axial hypotonia, with later onset of spasticity. The disorder is progressive. Some patients show normal early development, but later have regression of motor, cognitive, and language skills. More variable features include seizures, joint contractures, ocular disturbances, episodic respiratory failure, and nonspecific dysmorphic facial features. The intellectual impairment is variable, ranging from poor visual contact with inability to walk or speak to milder intellectual disability with the ability to say some words. Brain imaging shows variable white matter abnormalities, including thin corpus callosum and poor myelination (summary by Husain et al., 2020).

Hypomyelinating leukodystrophy-20 (HLD20) is an autosomal recessive neurodegenerative disorder characterized by the loss of developmental milestones at about 12 to 16 months of age after normal early development. Patients lose motor, language, and cognitive skills and show poor overall growth with microcephaly. The disorder is progressive, resulting in feeding difficulties and spastic quadriplegia. Some patients may have seizures. Brain imaging shows subcortical white matter abnormalities and a thin corpus callosum, suggesting a myelination defect. Death usually occurs in childhood (Al-Abdi et al., 2020). For a discussion of genetic heterogeneity of HLD, see 312080.

Neurodevelopmental disorder with seizures and brain atrophy (NEDSEBA) is an autosomal recessive disorder with highly variable manifestations and severity of these core features. The most severely affected individuals develop symptoms in utero, which may lead to spontaneous abortion or planned termination. Those that survive may present with severe seizures at birth, have poor overall growth with small head circumference, achieve no developmental progress, and show significant brain atrophy and other brain abnormalities. Patients at the mildest end of the phenotypic spectrum have onset of seizures later in childhood and show developmental delay with mildly impaired intellectual development and minimal brain atrophy (summary by Coulter et al., 2020).

Neurodevelopmental disorder with microcephaly, seizures, and brain atrophy (NEDMISB) is an autosomal recessive disorder characterized by severe global developmental delay, developmental regression with loss of milestones, severe microcephaly, and brain abnormalities, primarily cerebral atrophy and hypoplasia of the corpus callosum. Affected individuals develop seizures in the first year of life; eventually they are unable to sit, feed, or communicate, and may be unresponsive to stimuli. Other features include muscle weakness, spasticity with hyperreflexia, irritability, and contractures (Coulter et al., 2020).

Neurodevelopmental disorder with microcephaly, language delay, and gait abnormalities (NEDMILG) is an autosomal recessive disorder characterized by global developmental delay apparent in infancy. Affected individuals have delayed walking with variable gait abnormalities, impaired intellectual development with poor or absent speech and language, and progressive microcephaly. More variable features include hypotonia, early-onset seizures, and a peripheral demyelinating or axonal peripheral sensorimotor neuropathy. The disease follows a neurodegenerative course in many patients; clinical features suggest involvement of both the central and peripheral nervous systems (Manole et al., 2020).

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).

Developmental and epileptic encephalopathy-89 (DEE89) is a severe autosomal recessive disorder characterized by profound global developmental delay with impaired intellectual development, absent speech, inability to sit or walk due to axial hypotonia and spastic quadriparesis, and onset of seizures in the first days or months of life. EEG shows suppression-burst pattern or hypsarrhythmia, consistent with DEE or a clinical diagnosis of West syndrome. More variable features include joint contractures with foot deformities, dysmorphic facial features with cleft palate, and omphalocele. Affected individuals have poor motor skills, poor eye contact, and lack of language development; some die in infancy or early childhood. Brain imaging may be normal or show nonspecific abnormalities (summary by Chatron et al., 2020).

Kaya-Barakat-Masson syndrome (KABAMAS) is a severe autosomal recessive neurodevelopmental disorder characterized by profoundly impaired global development with variable motor abnormalities, such as axial hypotonia, peripheral spasticity, dystonia, and poor coordination, resulting in the inability to sit or walk. Affected individuals have impaired intellectual development with absent speech, poor eye contact, and feeding difficulties, resulting in poor overall growth, sometimes with microcephaly. Dysmorphic features are generally not present. Additional more variable features include early-onset seizures, ocular anomalies, foot deformities, and nonspecific brain imaging findings, such as thin corpus callosum and cerebral, cerebellar, or pontine atrophy. Some patients may die in infancy or early childhood (summary by AlMuhaizea et al., 2020 and Diaz et al., 2020).

Congenital muscular dystrophies resulting from defective glycosylation of alpha-dystroglycan (DAG1; 128239) are characterized by early onset of muscle weakness, usually before ambulation is achieved; intellectual disability mild brain anomalies are variable (Balci et al., 2005; Godfrey et al., 2007). Congenital muscular dystrophy-dystroglycanopathies with or without impaired intellectual development (type B) represent the intermediate range of the spectrum of dystroglycanopathies. They are less severe than muscular dystrophy-dystroglycanopathy with brain and eye anomalies (type A; see MDDGA1, 236670), previously designated Walker-Warburg syndrome (WWS) or muscle-eye-brain disease (MEB), and more severe than limb-girdle muscular dystrophy-dystroglycanopathy (type C; see MDDGC1, 609308). Genetic Heterogeneity of Congenital Muscular Dystrophy-Dystroglycanopathy with or without Impaired Intellectual Development (Type B) Congenital muscular dystrophy with impaired intellectual development due to defective glycosylation of DAG1 is genetically heterogeneous. See also MDDGB2 (613156), caused by mutation in the POMT2 gene (607439); MDDGB3 (613151), caused by mutation in the POMGNT1 gene (606822); MDDGB4 (613152), caused by mutation in the FKTN gene (607440); MDDGB5 (616612), caused by mutation in the FKRP gene (606596); MDDGB6 (608840), caused by mutation in the LARGE gene (603590); MDDGB14 (615351), caused by mutation in the GMPPB gene (615320); and MDDGB15 (618992), caused by mutation in the DPM3 gene (605951).

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).

Joubert syndrome-37 (JBTS37) is an autosomal recessive neurodevelopmental ciliopathy characterized classically by a distinctive hindbrain malformation affecting the midbrain and cerebellum, recognizable as the 'molar tooth sign' on brain imaging. Affected individuals have hypotonia, ataxia, and variably impaired intellectual development. Additional variable features, such as postaxial polydactyly, liver or kidney anomalies, retinal dystrophy, and coloboma, may also occur. In severe cases, affected fetuses with these malformations may be terminated (summary by Latour et al., 2020). For a phenotypic description and a discussion of genetic heterogeneity of Joubert syndrome, see JBTS1 (213300).

Neurofacioskeletal syndrome with or without renal agenesis (NFSRA) is characterized by developmental delay and/or intellectual disability; corpus callosum hypoplasia or agenesis; facial dysmorphism, including upslanting palpebral fissures, broad nasal tip, and wide mouth; and skeletal abnormalities, including short stature, scoliosis, and flexion contractures, with broad fingertips and/or toes. Renal agenesis, unilateral or bilateral, has also been observed in some patients (Schneeberger et al., 2020).

SOFM is characterized by marked short stature, oligodontia, mild facial dysmorphism, and motor delay. Endosteal hyperostosis has also been observed, and patients may exhibit some features of progeria (Terhal et al., 2020; Beauregard-Lacroix et al., 2020).

SHILCA is characterized by early-onset retinal degeneration in association with sensorineural hearing loss, short stature, vertebral anomalies, and epiphyseal dysplasia, as well as motor and intellectual delay. Delayed myelination, leukoencephalopathy, and hypoplasia of the corpus callosum and cerebellum have been observed on brain MRI (Bedoni et al., 2020).

Pontocerebellar hypoplasia type 1F (PCH1F) is an autosomal recessive neurologic disorder characterized by hypotonia, global developmental delay, poor overall growth, and dysmorphic facial features. Brain imaging shows pontocerebellar hypoplasia, thin corpus callosum, cerebral atrophy, and delayed myelination (summary by Somashekar et al., 2021). For a phenotypic description and a discussion of genetic heterogeneity of PCH, see PCH1A (607596).

Neurodevelopmental disorder with dysmorphic facies and cerebellar hypoplasia (NEDFACH) is an autosomal recessive disorder characterized by global developmental delay and intellectual disability. The phenotype is variable: more severely affected individuals have poor overall growth with microcephaly, delayed walking, spasticity, and poor or absent speech, whereas others may achieve more significant developmental milestones and even attend special schooling. Brain imaging shows abnormalities of the cerebellum, most commonly cerebellar hypoplasia, although other features, such as thin corpus callosum and delayed myelination, may also be present. Dysmorphic facial features include sloping forehead, upslanting palpebral fissures, and hypertelorism. Additional more variable manifestations may include cardiac ventricular septal defect, spasticity, cataracts, optic nerve hypoplasia, seizures, and joint contractures (summary by Van Bergen et al., 2020).

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).

Martsolf syndrome-2 (MARTS2) is an autosomal recessive disorder with the main features of congenital cataracts, mildly to severely impaired intellectual development, and facial dysmorphism. Other features include brain malformations, microcephaly, and hypogonadism-hypogenitalism (summary by Koparir et al., 2019).

Primary ciliary dyskinesia-47 and lissencephaly (CILD47) is an autosomal recessive disorder characterized by onset of recurrent respiratory infections and respiratory dysfunction caused by defective mucociliary clearance in early childhood. Affected individuals also have neurologic features, such as impaired intellectual development and central hypotonia, associated with structural brain abnormalities, most notably lissencephaly and thin or absent corpus callosum. The disorder results from impaired function of motile ciliopathy and can be classified as 'reduced generation of multiple motile cilia' (RGMC). Situs inversus is not observed (summary by Wallmeier et al., 2021). For a phenotypic description and a discussion of genetic heterogeneity of primary ciliary dyskinesia, see CILD1 (244400).

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.

Neurodevelopmental disorder with hearing loss and spasticity (NEDHLS) is characterized by hearing loss, global developmental delay/impaired intellectual development, spastic-dystonic cerebral palsy, focal or generalized epilepsy, and microcephaly. Some children present with hypotonia (Richard et al., 2021).

Zaki syndrome (ZKS) is characterized by developmental delay, progressive microcephaly, and short stature, as well as dysmorphic features including sparse scalp hair, cupped ears, wide nose and mouth, short philtrum, and high-arched palate. Other variable features have been observed, including ocular, skeletal, cardiac, and renal anomalies (Chai et al., 2021).

Neurodevelopmental disorder with microcephaly, seizures, and neonatal cholestasis (NEDMSC) is an autosomal recessive disorder characterized by severely impaired global development apparent from infancy, progressive microcephaly, and neonatal cholestasis manifest as jaundice and elevated liver enzymes. The liver disease resolves, but affected individuals show feeding difficulties, failure to thrive, hypotonia, seizures, hyperkinetic movements, irritability, and poor eye contact or vision, and achieve almost no motor or cognitive developmental milestones. Brain imaging demonstrates agenesis or hypoplasia of the corpus callosum. Death in early childhood may occur (summary by Schneeberger et al., 2021).

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).

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).

Combined oxidative phosphorylation deficiency-27 (COXPD27) is an autosomal recessive multisystem disorder characterized mainly by neurologic features, including delayed development, seizures, abnormal movements, and neurologic regression. Age at onset, ranging from infancy to late childhood, and severity are variable. Other features include hypotonia, myoclonus, brain imaging abnormalities, and evidence of mitochondrial dysfunction in skeletal muscle. Liver dysfunction has also been reported (summary by Samanta et al., 2018). For a discussion of genetic heterogeneity of combined oxidative phosphorylation deficiency, see COXPD1 (609060).

Seizures, cortical blindness, and microcephaly syndrome (SCBMS) is an autosomal recessive neurodevelopmental disorder characterized by microcephaly, early-onset seizures, severely delayed psychomotor development, and cortical blindness. Affected individuals also tend to show poor overall growth with short stature (summary by Ercan-Sencicek et al., 2015).

Charcot-Marie-Tooth disease type 2X (CMT2X) is an autosomal recessive, slowly progressive, axonal peripheral sensorimotor neuropathy characterized by lower limb muscle weakness and atrophy associated with distal sensory impairment and gait difficulties. Some patients also have involvement of the upper limbs. Onset usually occurs in the first 2 decades of life, although later onset can also occur (summary by Montecchiani et al., 2016) For a phenotypic description and a discussion of genetic heterogeneity of axonal CMT, see CMT2A1 (118210).

Neurocardiofaciodigital syndrome (NCFD) is characterized by severe developmental delay, variable brain anomalies, congenital heart defects, dysmorphic facial features, and a distinctive type of synpolydactyly with a supernumerary hypoplastic digit between the fourth and fifth digits of the hands and/or feet. Other features include eye abnormalities, hearing impairment, and electroencephalogram anomalies (summary by Horn et al., 2021).

Craniofacial dysmorphism, skeletal anomalies, and impaired intellectual development syndrome-1 (CFSMR1) is characterized by cranial involvement with macrocrania at birth, brachycephaly, anomalies of middle fossa structures including hypoplasia of corpus callosum, enlargement of septum pellucidum, and dilated lateral ventricles, as well as cortical atrophy and hypodensity of the gray matter. Facial dysmorphisms include flat face, hypertelorism, epicanthal folds, synophrys, broad nasal bridge, cleft lip and cleft palate, and low-set posteriorly rotated ears. Patients also exhibit short neck and multiple costal and vertebral anomalies. The face is rather characteristic, and various authors have consistently reported affable/friendly personality, despite intellectual delay (summary by Alanay et al., 2014). Genetic Heterogeneity of Craniofacial Dysmorphism, Skeletal Anomalies, and Impaired Intellectual Development Syndrome CFSMR2 (616994) is caused by mutation in the RAB5IF gene (619960) on chromosome 20q11.

WAC-related intellectual disability (ID) is typically characterized by variable degrees of developmental delay and/or intellectual disability. Behavioral abnormalities including anxiety, attention-deficit/hyperactivity disorder, and/or autism spectrum disorder are observed in the majority of older children and adults. Most affected infants have significant but nonspecific features at birth such as neonatal hypotonia and feeding problems. Some affected individuals come to medical attention with respiratory or vision problems. Facial features may be mildly dysmorphic, but are nonspecific. To date, 18 individuals have been identified with WAC-related ID.

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 dysmorphic facies and skeletal and brain abnormalities (NEDDFSB) is a multisystemic developmental disorder characterized by feeding difficulties, poor overall growth, and global developmental delay with moderate to severely impaired intellectual development and poor or absent speech. Affected individuals have dysmorphic facial features and skeletal defects, mainly affecting the distal extremities. More variable additional findings include hypotonia, seizures, and ocular defects. Brain imaging tends to show structural defects of the corpus callosum and cerebellar hypoplasia (Duijkers et al., 2019).

Neurodevelopmental disorder with growth retardation, dysmorphic facies, and corpus callosum abnormalities (NEDGFC) is an autosomal recessive disorder characterized by these cardinal features apparent from infancy. There is phenotypic variability both in disease manifestations and severity. More severely affected individuals are unable to walk independently, are nonverbal, and may have other anomalies, including congenital heart defects, feeding difficulties, or skeletal defects, whereas others show mildly delayed motor and speech acquisition with mild or borderline intellectual disability (summary by von Elsner et al., 2022).

Neurodevelopmental disorder with speech impairment and with or without seizures (NEDSIS) is a phenotypically heterogeneous neurologic disorder whose severity appears to depend on the functional effect of the CACNA1I mutation. Severely affected individuals present in infancy with profound global developmental delay, hypotonia, delayed or absent walking, absent speech, feeding difficulties, cortical visual impairment, and onset of hyperexcitability and seizures in the first months or years of life. They achieve little or no developmental progress and may be tube-fed. Mutations in these individuals occurred de novo. In contrast, a milder phenotype associated with an inherited mutation has been found in a family with mild to moderate cognitive impairment and mild speech delay, usually without seizures (El Ghaleb et al., 2021).

Developmental and epileptic encephalopathy-110 (DEE110) is an autosomal recessive disorder characterized by profound global developmental delay and hypotonia apparent in infancy followed by onset of seizures in the first months or years of life. Affected individuals achieve almost no developmental milestones and show impaired intellectual development, poor or absent speech, inability to walk or grasp objects, peripheral spasticity, and poor eye contact. Brain imaging shows hypoplastic corpus callosum and cortical atrophy (Dahimene et al., 2022). For a discussion of genetic heterogeneity of DEE, see 308350.

Rabin-Pappas syndrome (RAPAS) is a multisystemic disorder characterized by severely impaired global development apparent from infancy, feeding difficulties with failure to thrive, small head circumference, and dysmorphic facial features. Affected individuals have impaired intellectual development and hypotonia; they do not achieve walking or meaningful speech. Other neurologic findings may include seizures, hearing loss, ophthalmologic defects, and brain imaging abnormalities. There is variable involvement of other organ systems, including skeletal, genitourinary, cardiac, and possibly endocrine (Rabin et al., 2020).

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).

Autosomal dominant intellectual developmental disorder-70 (MRD70) is characterized by mild global developmental delay, moderately impaired intellectual disability with speech difficulties, and behavioral abnormalities. More variable findings may include hypotonia and dysmorphic features (Rabin et al., 2020)

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).

Neurodevelopmental disorder with poor growth and behavioral abnormalities (NEDGBA) is an autosomal recessive disorder characterized by global developmental delay, moderately to severely impaired intellectual development, often with absent speech, and behavioral abnormalities, including hyperactivity, short attention span, and ADHD. Affected individuals show failure to thrive with poor overall growth; some have microcephaly. Additional features may include nonspecific facial dysmorphism, hypotonia, and feeding difficulties (Vogt et al., 2022; Meng et al., 2023).

Hypomyelinating leukodystrophy-26 with chondrodysplasia (HLD26) is characterized by severe psychomotor delay, predominantly involving motor and expressive language development, with cerebral and cerebellar atrophy and corpus callosum hypoplasia. In addition, patients show pre- and postnatal growth retardation, early-onset scoliosis, and dislocations of large joints (Guasto et al., 2022). For a general phenotypic description and a discussion of genetic heterogeneity of HLD, see HLD1 (312080).