Angelman syndrome (AS) is characterized by severe developmental delay or intellectual disability, severe speech impairment, gait ataxia and/or tremulousness of the limbs, and unique behavior with an apparent happy demeanor that includes frequent laughing, smiling, and excitability. Microcephaly and seizures are also common. Developmental delays are first noted at around age six months; however, the unique clinical features of AS do not become manifest until after age one year.

Borjeson-Forssman-Lehmann syndrome (BFLS) is an uncommon X-linked intellectual developmental disorder that evolves with age. Clinical manifestations in males are quite variable, with the most consistent features being initial hypotonia, mild to moderate impaired intellectual development, large fleshy ears, underdeveloped genitalia, gynecomastia, truncal obesity, tapering fingers, and shortening of the fourth and fifth toes. Heterozygous females may have a milder similar clinical phenotype, which can include hypothyroidism; however, many carrier females appear unaffected (summary by Crawford et al., 2006).

Hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome is a disorder of the urea cycle and ornithine degradation pathway. Clinical manifestations and age of onset vary among individuals even in the same family. Neonatal onset (~8% of affected individuals). Manifestations of hyperammonemia usually begin 24-48 hours after feeding begins and can include lethargy, somnolence, refusal to feed, vomiting, tachypnea with respiratory alkalosis, and/or seizures. Infantile, childhood, and adult onset (~92%). Affected individuals may present with: Chronic neurocognitive deficits (including developmental delay, ataxia, spasticity, learning disabilities, cognitive deficits, and/or unexplained seizures); Acute encephalopathy secondary to hyperammonemic crisis precipitated by a variety of factors; and Chronic liver dysfunction (unexplained elevation of liver transaminases with or without mild coagulopathy, with or without mild hyperammonemia and protein intolerance). Neurologic findings and cognitive abilities can continue to deteriorate despite early metabolic control that prevents hyperammonemia.

Tyrosinemia type III (TYRSN3), an autosomal recessive disorder caused by a deficiency in the activity of 4-hydroxyphenylpyruvate dioxygenase (HPD), is characterized by elevated levels of blood tyrosine and massive excretion of its derivatives into urine. Patients with this disorder have mildly impaired intellectual development and/or convulsions, with the absence of liver damage (summary by Tomoeda et al., 2000).

The spectrum of isolated sulfite oxidase deficiency ranges from classic early-onset (severe) disease to late-onset (mild) disease. Classic ISOD is characterized in the first few hours to days of life by intractable seizures, feeding difficulties, and rapidly progressive encephalopathy manifest as abnormal tone (especially opisthotonus, spastic quadriplegia, and pyramidal signs) followed by progressive microcephaly and profound intellectual disability. Lens subluxation or dislocation, another characteristic finding, may be evident after the newborn period. Children usually die during the first few months of life. Late-onset ISOD manifests between ages six and 18 months and is characterized by ectopia lentis (variably present), developmental delay/regression, movement disorder characterized by dystonia and choreoathetosis, ataxia, and (rarely) acute hemiplegia as a result of metabolic stroke. The clinical course may be progressive or episodic. In the episodic form encephalopathy, dystonia, choreoathetosis, and/or ataxia are intermittent.

A type of central congenital hypothyroidism, a permanent thyroid deficiency that is present from birth, characterized by low levels of thyroid hormones due to a deficiency in TSH synthesis.

Congenital disorder of glycosylation type IIc (CDG2C) is an autosomal recessive disorder characterized by moderate to severe psychomotor retardation, mild dysmorphism, and impaired neutrophil motility. It is a member of a group of disorders with a defect in the processing of protein-bound glycans. For a general overview of congenital disorders of glycosylation (CDGs), see CDG1A (212065) and CDG2A (212066). Frydman (1996) contended that the neutrophil defect in CDG2C, which has been referred to as 'leukocyte adhesion deficiency type II' (LAD2), is a manifestation of the disorder and that there are no cases of 'primary' LAD II. Etzioni and Harlan (1999) provided a comprehensive review of both leukocyte adhesion deficiency-1 (LAD1; 116920) and LAD2. While the functional neutrophil studies are similar in the 2 LADs, the clinical course is milder in LAD2. Furthermore, patients with LAD2 present other abnormal features, such as growth and mental retardation, which are related to the primary defect in fucose metabolism. Delayed separation of the umbilical cord occurs in LAD1. For a discussion of genetic heterogeneity of LAD, see 116920.

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

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.

SCN1A seizure disorders encompass a spectrum that ranges from simple febrile seizures and generalized epilepsy with febrile seizures plus (GEFS+) at the mild end to Dravet syndrome and intractable childhood epilepsy with generalized tonic-clonic seizures (ICE-GTC) at the severe end. Phenotypes with intractable seizures including Dravet syndrome are often associated with cognitive decline. Less commonly observed phenotypes include myoclonic astatic epilepsy (MAE), Lennox-Gastaut syndrome, infantile spasms, epilepsy with focal seizures, and vaccine-related encephalopathy and seizures. The phenotype of SCN1A seizure disorders can vary even within the same family.

Kleefstra syndrome is characterized by intellectual disability, autistic-like features, childhood hypotonia, and distinctive facial features. The majority of individuals function in the moderate-to-severe spectrum of intellectual disability although a few individuals have mild delay and total IQ within low-normal range. While most have severe expressive speech delay with little speech development, general language development is usually at a higher level, making nonverbal communication possible. A complex pattern of other findings can also be observed; these include heart defects, renal/urologic defects, genital defects in males, severe respiratory infections, epilepsy / febrile seizures, psychiatric disorders, and extreme apathy or catatonic-like features after puberty.

Allan-Herndon-Dudley syndrome (AHDS), an X-linked disorder, is characterized in males by neurologic findings (hypotonia and feeding difficulties in infancy, developmental delay / intellectual disability ranging from mild to profound) and later-onset pyramidal signs, extrapyramidal findings (dystonia, choreoathetosis, paroxysmal movement disorder, hypokinesia, masked facies), and seizures, often with drug resistance. Additional findings can include dysthyroidism (manifest as poor weight gain, reduced muscle mass, and variable cold intolerance, sweating, elevated heart rate, and irritability) and pathognomonic thyroid test results. Most heterozygous females are not clinically affected but may have minor thyroid test abnormalities.

A form of ectodermal dysplasia syndrome characterized by a short stature of prenatal onset, alopecia, ichthyosis, photophobia, ectrodactyly, seizures, scoliosis, multiple contractures, fusions of various bones (particularly elbows, carpals, metacarpals, and spine), intellectual disability, and facial dysmorphism (microdolichocephaly, madarosis, large ears and long nose). ACD syndrome overlaps with ichthyosis follicularis-alopecia-photophobia syndrome.

An extremely rare syndrome with characteristics of facial dysmorphism, severe intellectual deficiency, cardiac and intestinal anomalies, and growth retardation. Only four cases have been reported in the literature, in three unrelated families. Dysmorphic features include bilateral cleft lip and palate, bulbous nasal tip and eye anomalies. The condition seems to be inherited as an autosomal recessive trait.

Proud syndrome is an X-linked developmental disorder characterized by agenesis of the corpus callosum, severe mental retardation, seizures, and spasticity. Males are severely affected, whereas females may be unaffected or have a milder phenotype (Proud et al., 1992). Proud syndrome is part of a phenotypic spectrum of disorders caused by mutation in the ARX gene comprising a nearly continuous series of developmental disorders ranging from lissencephaly (LISX2; 300215) to Proud syndrome to infantile spasms without brain malformations (DEE1; 308350) to syndromic (309510) and nonsyndromic (300419) mental retardation (Kato et al., 2004; Wallerstein et al., 2008).

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.

The spectrum of MECP2-related phenotypes in females ranges from classic Rett syndrome to variant Rett syndrome with a broader clinical phenotype (either milder or more severe than classic Rett syndrome) to mild learning disabilities; the spectrum in males ranges from severe neonatal encephalopathy to pyramidal signs, parkinsonism, and macroorchidism (PPM-X) syndrome to severe syndromic/nonsyndromic intellectual disability. Females: Classic Rett syndrome, a progressive neurodevelopmental disorder primarily affecting girls, is characterized by apparently normal psychomotor development during the first six to 18 months of life, followed by a short period of developmental stagnation, then rapid regression in language and motor skills, followed by long-term stability. During the phase of rapid regression, repetitive, stereotypic hand movements replace purposeful hand use. Additional findings include fits of screaming and inconsolable crying, autistic features, panic-like attacks, bruxism, episodic apnea and/or hyperpnea, gait ataxia and apraxia, tremors, seizures, and acquired microcephaly. Males: Severe neonatal-onset encephalopathy, the most common phenotype in affected males, is characterized by a relentless clinical course that follows a metabolic-degenerative type of pattern, abnormal tone, involuntary movements, severe seizures, and breathing abnormalities. Death often occurs before age two years.

Any non-syndromic X-linked intellectual disability in which the cause of the disease is a mutation in the PAK3 gene.

Intellectual developmental disorder-29 (XLID29) is a nonspecific form of XLID. It is part of a phenotypic spectrum of disorders caused by mutation in the ARX gene comprising a nearly continuous series of developmental disorders ranging from lissencephaly (LISX2; 300215) to Proud syndrome (300004) to infantile spasms without brain malformations (DEE1; 308350) to Partington syndrome (309510) (Kato et al., 2004; Wallerstein et al., 2008).

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

Nicolaides-Baraitser syndrome (NCBRS) is characterized by sparse scalp hair, prominence of the inter-phalangeal joints and distal phalanges due to decreased subcutaneous fat, characteristic coarse facial features, microcephaly, seizures, and developmental delay / intellectual disability. Seizures are of various types and can be difficult to manage. Developmental delay / intellectual disability (ID) is severe in nearly a half, moderate in a third, and mild in the remainder. Nearly a third never develop speech or language skills.

Holoprosencephaly-9 refers to a disorder characterized by a wide phenotypic spectrum of brain developmental defects, with or without overt forebrain cleavage abnormalities. It usually includes midline craniofacial anomalies involving the first branchial arch and/or orbits, pituitary hypoplasia with panhypopituitarism, and postaxial polydactyly. The disorder shows incomplete penetrance and variable expressivity (summary by Roessler et al., 2003 and Bertolacini et al., 2012). For general phenotypic information and a discussion of genetic heterogeneity of holoprosencephaly, see HPE1 (236100).

Cerebral dysgenesis, neuropathy, ichthyosis, and keratoderma syndrome (CEDNIK) refers to a unique constellation of clinical manifestations including global developmental delay with hypotonia, roving eye movements or nystagmus, poor motor skills, and impaired intellectual development with speech delay. More variable features include microcephaly, feeding difficulties, seizures, ocular anomalies, hearing loss, and nonspecific dysmorphic facial features. Palmoplantar keratoderma and ichthyosis or neuropathy develop in some patients. Brain magnetic resonance imaging (MRI) shows varying degrees of cerebral dysgenesis, including absence of the corpus callosum and cortical dysplasia, as well as hypomyelination, white matter loss, and white matter signal anomalies suggestive of a leukodystrophy. Some patients may show developmental regression; many die in childhood (Fuchs-Telem et al., 2011; Mah-Som et al., 2021). With more patients being reported, several authors (Diggle et al., 2017; Llaci et al., 2019; Mah-Som et al., 2021) have observed that the dermatologic features and peripheral neuropathy show reduced penetrance and are more variable manifestations of this disorder, as they are not observed in all patients with biallelic SNAP29 mutations.

Alpha-N-acetylgalactosaminidase (NAGA) deficiency is a very rare lysosomal storage disorder. It is clinically heterogeneous with 3 main phenotypes: type I is an infantile-onset neuroaxonal dystrophy; type II, also known as Kanzaki disease (609242), is an adult-onset disorder characterized by angiokeratoma corporis diffusum and mild intellectual impairment; and type III is an intermediate disorder with mild to moderate neurologic manifestations (Desnick and Schindler, 2001).

Distal arthrogryposis type 4 (DA4) is distinguished by the presence of scoliosis (summary by Bamshad et al., 2009). For a phenotypic description and a discussion of genetic heterogeneity of distal arthrogryposis, see DA1 (108120).

Any autosomal recessive non-syndromic intellectual disability in which the cause of the disease is a mutation in the CC2D1A gene.

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

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

Developmental and epileptic encephalopathy-8 (DEE8) is an X-linked disorder characterized by seizure onset before 2 years of age and severe developmental delay. Some patients have hyperekplexia (summary by Shimojima et al., 2011). For general phenotypic descriptions and discussions of genetic heterogeneity of developmental and epileptic encephalopathy and hyperekplexia, see DEE1 (308350) and HKPX1 (149400), respectively.

Claes-Jensen type of X-linked syndromic intellectual developmental disorder (MRXSCJ) is characterized by impaired intellectual development with substantial clinical heterogeneity in affected males. However, males are usually reported to have short stature, microcephaly, hyperreflexia, and aggressive behavior. In rare cases, female carriers exhibit mildly impaired intellectual development or learning difficulties (summary by Guerra et al., 2020).

MECP2 duplication syndrome is a severe neurodevelopmental disorder characterized by early-onset hypotonia, feeding difficulty, gastrointestinal manifestations including gastroesophageal reflux and constipation, delayed psychomotor development leading to severe intellectual disability, poor speech development, progressive spasticity, recurrent respiratory infections (in ~75% of affected individuals), and seizures (in ~50%). MECP2 duplication syndrome is 100% penetrant in males. Occasionally females have been described with a MECP2 duplication and a range of findings from mild intellectual disability to a phenotype similar to that seen in males. In addition to the core features, autistic behaviors, nonspecific neuroradiologic findings on brain MRI, mottled skin, and urogenital anomalies have been observed in several affected boys.

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

A rare, genetic disorder of amino acid absorption and transport, characterized by generalized hypotonia at birth, neonatal/infantile failure to thrive (followed by hyperphagia and rapid weight gain in late childhood), cystinuria type 1, nephrolithiasis, growth retardation due to growth hormone deficiency, and minor facial dysmorphism. Dysmorphic features mainly include dolichocephaly and ptosis. Nephrolithiasis occurs at variable ages.

Peroxisomal acyl-CoA oxidase deficiency is a disorder of peroxisomal fatty acid beta-oxidation. See also D-bifunctional protein deficiency (261515), caused by mutation in the HSD17B4 gene (601860) on chromosome 5q2. The clinical manifestations of these 2 deficiencies are similar to those of disorders of peroxisomal assembly, including Zellweger cerebrohepatorenal syndrome (see 214100) and neonatal adrenoleukodystrophy (see 601539) (Watkins et al., 1995).

IDDAS is a neurodevelopmental disorder characterized by varying degrees of intellectual disability, autism spectrum disorder, and language deficits (Deriziotis et al., 2014; den Hoed et al., 2018).

Any autosomal recessive non-syndromic intellectual disability in which the cause of the disease is a mutation in the PRSS12 gene.

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.

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.

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.

Rhizomelic chondrodysplasia punctata (RCDP) is a peroxisomal disorder characterized by disproportionately short stature primarily affecting the proximal parts of the extremities, a typical facial appearance including a broad nasal bridge, epicanthus, high-arched palate, dysplastic external ears, and micrognathia, congenital contractures, characteristic ocular involvement, dwarfism, and severe mental retardation with spasticity. Biochemically, plasmalogen synthesis and phytanic acid alpha-oxidation are defective. Most patients die in the first decade of life. RCDP1 (215100) is the most frequent form of RCDP (summary by Wanders and Waterham, 2005). Whereas RCDP1 is a peroxisomal biogenesis disorder (PBD), RCDP2 is classified as a single peroxisome enzyme deficiency (Waterham and Ebberink, 2012). For a discussion of genetic heterogeneity of rhizomelic chondrodysplasia punctata, see 215100.

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

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.

This syndrome has characteristics of intellectual deficit, calcification of the choroid plexus and elevated levels of cerebrospinal fluid protein. It has been described in two sibships from two unrelated families. The seven children of one of the sibships were born to consanguineous parents. Some patients also had strabismus, hyperactive deep tendon reflexes and foot deformities.

Alopecia-intellectual disability syndrome (APMR) is a rare autosomal recessive disorder in which affected individuals show loss of hair on the scalp, absence of eyebrows, eyelashes, and axillary and pubic hair, and mildly to severely impaired intellectual development (summary by Wali et al., 2007). Genetic Heterogeneity of Alopecia-Intellectual Disability Syndrome Loci for alopecia-intellectual disability syndrome have been mapped to chromosome 3q26.2-q26.31 (APMR2; 610422) and chromosome 18q11.2-q12.2 (APMR3; 613930). APMR4 (618840) is caused by mutation in the LSS gene (600909) on chromosome 21q22.

KAT6B disorders include genitopatellar syndrome (GPS) and Say-Barber-Biesecker-Young-Simpson variant of Ohdo syndrome (SBBYSS) which are part of a broad phenotypic spectrum with variable expressivity; individuals presenting with a phenotype intermediate between GPS and SBBYSS have been reported. Both phenotypes are characterized by some degree of global developmental delay / intellectual disability; hypotonia; genital abnormalities; and skeletal abnormalities including patellar hypoplasia/agenesis, flexion contractures of the knees and/or hips, and anomalies of the digits, spine, and/or ribs. Congenital heart defects, small bowel malrotation, feeding difficulties, slow growth, cleft palate, hearing loss, and dental anomalies have been observed in individuals with either phenotype.

The neuronal ceroid lipofuscinoses (NCL; CLN) are a clinically and genetically heterogeneous group of neurodegenerative disorders characterized by the intracellular accumulation of autofluorescent lipopigment storage material in different patterns ultrastructurally. The clinical course includes progressive dementia, seizures, and progressive visual failure (Mole et al., 2005). For a discussion of genetic heterogeneity of neuronal ceroid lipofuscinosis, see CLN1 (256730).

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).

This syndrome has characteristics of severe intellectual deficit, patella luxations, acromicria, hypogonadism, facial dysmorphism (including midface hypoplasia and premature frontotemporal balding). It has been described in three unrelated males.

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

The spectrum of MECP2-related phenotypes in females ranges from classic Rett syndrome to variant Rett syndrome with a broader clinical phenotype (either milder or more severe than classic Rett syndrome) to mild learning disabilities; the spectrum in males ranges from severe neonatal encephalopathy to pyramidal signs, parkinsonism, and macroorchidism (PPM-X) syndrome to severe syndromic/nonsyndromic intellectual disability. Females: Classic Rett syndrome, a progressive neurodevelopmental disorder primarily affecting girls, is characterized by apparently normal psychomotor development during the first six to 18 months of life, followed by a short period of developmental stagnation, then rapid regression in language and motor skills, followed by long-term stability. During the phase of rapid regression, repetitive, stereotypic hand movements replace purposeful hand use. Additional findings include fits of screaming and inconsolable crying, autistic features, panic-like attacks, bruxism, episodic apnea and/or hyperpnea, gait ataxia and apraxia, tremors, seizures, and acquired microcephaly. Males: Severe neonatal-onset encephalopathy, the most common phenotype in affected males, is characterized by a relentless clinical course that follows a metabolic-degenerative type of pattern, abnormal tone, involuntary movements, severe seizures, and breathing abnormalities. Death often occurs before age two years.

MBD5 haploinsufficiency is a neurodevelopmental disorder characterized by developmental delay, intellectual disability, severe speech impairment, seizures, sleep disturbances, and abnormal behaviors. Most children lack speech entirely or have single words, short phrases, or short sentences. Seizures are present in more than 80% of children; onset is usually around age two years. Sleep disturbances, present in about 90%, can result in excessive daytime drowsiness. Abnormal behaviors can include autistic-like behaviors (80%) and self-injury and aggression (>60%).

Any autosomal recessive non-syndromic intellectual disability in which the cause of the disease is a mutation in the TUSC3 gene.

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.

Any hypothyroidism, congenital, nongoitrous in which the cause of the disease is a mutation in the NKX2-5 gene.

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).

Kahrizi syndrome is an autosomal recessive neurodevelopmental disorder characterized by mental retardation, cataracts, coloboma, kyphosis, and coarse facial features (summary by Kahrizi et al., 2009). See also congenital disorder of glycosylation type Iq (CDG1Q; 612379), an allelic disorder with overlapping features.

Autosomal recessive primary microcephaly (often shortened to MCPH, which stands for "microcephaly primary hereditary") is a condition in which infants are born with a very small head and a small brain. The term "microcephaly" comes from the Greek words for "small head."\n\nInfants with MCPH have an unusually small head circumference compared to other infants of the same sex and age. Head circumference is the distance around the widest part of the head, measured by placing a measuring tape above the eyebrows and ears and around the back of the head. Affected infants' brain volume is also smaller than usual, although they usually do not have any major abnormalities in the structure of the brain. The head and brain grow throughout childhood and adolescence, but they continue to be much smaller than normal.\n\nPeople with MCPH usually have few or no other features associated with the condition. Some have a narrow, sloping forehead; mild seizures; problems with attention or behavior; or short stature compared to others in their family. The condition typically does not affect any other major organ systems or cause other health problems.\n\nMCPH causes intellectual disability, which is typically mild to moderate and does not become more severe with age. Most affected individuals have delayed speech and language skills. Motor skills, such as sitting, standing, and walking, may also be mildly delayed.

Maternal 15q duplication syndrome (maternal dup15q) is characterized by hypotonia and motor delays, intellectual disability, autism spectrum disorder (ASD), and epilepsy including infantile spasms. Rarely, maternal dup15q may also be associated with psychosis or sudden unexplained death. Those with a maternal isodicentric 15q11.2-q13.1 supernumerary chromosome are typically more severely affected than those with an interstitial duplication.

Any autosomal dominant non-syndromic intellectual disability in which the cause of the disease is a mutation in the KIRREL3 gene.

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.

Congenital disorders of glycosylation (CDGs) are a genetically heterogeneous group of autosomal recessive disorders caused by enzymatic defects in the synthesis and processing of asparagine (N)-linked glycans or oligosaccharides on glycoproteins. Type I CDGs comprise defects in the assembly of the dolichol lipid-linked oligosaccharide (LLO) chain and its transfer to the nascent protein. These disorders can be identified by a characteristic abnormal isoelectric focusing profile of plasma transferrin (Leroy, 2006). For a discussion of the classification of CDGs, see CDG1A (212065).

Individuals with the 15q13.3 recurrent deletion may have a wide range of clinical manifestations. The deletion itself may not lead to a clinically recognizable syndrome and a subset of persons with the recurrent deletion have no obvious clinical findings, implying that penetrance for the deletion is incomplete. A little over half of individuals diagnosed with this recurrent deletion have intellectual disability or developmental delay, mainly in the areas of speech acquisition and cognitive function. In the majority of individuals, cognitive impairment is mild. Other features reported in diagnosed individuals include epilepsy (in ~30%), mild hypotonia, and neuropsychiatric disorders (including autism spectrum disorder, attention-deficit/hyperactivity disorder, mood disorder, schizophrenia, and aggressive or self-injurious behavior). Congenital malformations are uncommon.

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

Christianson syndrome (referred to as CS in this GeneReview), an X-linked disorder, is characterized in males by cognitive dysfunction, behavioral disorder, and neurologic findings (e.g., seizures, ataxia, postnatal microcephaly, and eye movement abnormalities). Males with CS typically present with developmental delay, later meeting criteria for severe intellectual disability (ID). Behaviorally, autism spectrum disorder and hyperactivity are common, and may resemble the behaviors observed in Angelman syndrome. Hypotonia and oropharyngeal dysphagia in infancy may result in failure to thrive. Seizures, typically beginning before age three years, can include infantile spasms and tonic, tonic-clonic, myoclonic, and atonic seizures. Subsequently, regression (e.g., loss of ambulation and ability to feed independently) may occur. Manifestations in heterozygous females range from asymptomatic to mild ID and/or behavioral issues.

Temple-Baraitser syndrome is a rare developmental disorder characterized by severe mental retardation and anomalies of the first ray of the upper and lower limbs with absence/hypoplasia of the nails. Most patients also have seizures; various dysmorphic facial features have been reported (summary by Jacquinet et al., 2010).

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).

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.

Congenital disorders of glycosylation (CDGs) are a genetically heterogeneous group of autosomal recessive disorders caused by enzymatic defects in the synthesis and processing of asparagine (N)-linked glycans or oligosaccharides on glycoproteins. These glycoconjugates play critical roles in metabolism, cell recognition and adhesion, cell migration, protease resistance, host defense, and antigenicity, among others. CDGs are divided into 2 main groups: type I CDGs (see, e.g., CDG1A, 212065) comprise defects in the assembly of the dolichol lipid-linked oligosaccharide (LLO) chain and its transfer to the nascent protein, whereas type II CDGs refer to defects in the trimming and processing of the protein-bound glycans either late in the endoplasmic reticulum or the Golgi compartments. The biochemical changes of CDGs are most readily observed in serum transferrin (TF; 190000), and the diagnosis is usually made by isoelectric focusing of this glycoprotein (reviews by Marquardt and Denecke, 2003; Grunewald et al., 2002). Genetic Heterogeneity of Congenital Disorder of Glycosylation Type II Multiple forms of CDG type II have been identified; see CDG2B (606056) through CDG2Z (620201), and CDG2AA (620454) to CDG2BB (620546).

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

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

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

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

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.

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

A rare genetic multiple congenital anomalies/dysmorphic syndrome with the association of severe intellectual disability, strabismus and anterior maxillary protrusion with vertical maxillary excess, open bite and prominent crowded teeth. Mild cochlear hearing loss has been reported in addition.

Developmental and epileptic encephalopathy-11 (DEE11) is a neurologic disorder characterized by onset of seizures in the first days, weeks, or months of life. Some patients may have later onset. Seizures comprise multiple types, including tonic, generalized, and myoclonic, and tend to be refractory to medication. However, some patients with onset of seizures before 3 months of age may respond to sodium channel blockers, particularly phenytoin. About half of patients become seizure-free in childhood. Affected individuals have global developmental delay, usually with severely impaired intellectual development, although some may be less severely affected and show autism spectrum disorder. Additional common features include microcephaly, hypotonia, and abnormal movements, such as dystonia, dyskinesias, and choreoathetotic movements. Brain imaging may show white matter defects. The phenotype is highly variable, even in patients with the same mutation (summary by Ogiwara et al., 2009; Howell et al., 2015; Wolff et al., 2017). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

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.

SUCLG1-related mitochondrial DNA (mtDNA) depletion syndrome, encephalomyopathic form with methylmalonic aciduria is characterized in the majority of affected newborns by hypotonia, muscle atrophy, feeding difficulties, and lactic acidosis. Affected infants commonly manifest developmental delay / cognitive impairment, growth retardation / failure to thrive, hepatopathy, sensorineural hearing impairment, dystonia, and hypertonia. Notable findings in some affected individuals include hypertrophic cardiomyopathy, epilepsy, myoclonus, microcephaly, sleep disturbance, rhabdomyolysis, contractures, hypothermia, and/or hypoglycemia. Life span is shortened, with median survival of 20 months.

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

Myotonic dystrophy type 1 (DM1) is a multisystem disorder that affects skeletal and smooth muscle as well as the eye, heart, endocrine system, and central nervous system. The clinical findings, which span a continuum from mild to severe, have been categorized into three somewhat overlapping phenotypes: mild, classic, and congenital. Mild DM1 is characterized by cataract and mild myotonia (sustained muscle contraction); life span is normal. Classic DM1 is characterized by muscle weakness and wasting, myotonia, cataract, and often cardiac conduction abnormalities; adults may become physically disabled and may have a shortened life span. Congenital DM1 is characterized by hypotonia and severe generalized weakness at birth, often with respiratory insufficiency and early death; intellectual disability is common.

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.

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.

Keppen-Lubinsky syndrome (KPLBS) is a rare disorder characterized by severely delayed psychomotor development, hypertonia, hyperreflexia, generalized lipodystrophy giving an aged appearance, and distinctive dysmorphic features, including microcephaly, prominent eyes, narrow nasal bridge, and open mouth (summary by Masotti et al., 2015).

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.

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.

Chromosome 20q11-q12 deletion syndrome is characterized by global developmental delay, poor overall growth, sometimes with severe feeding difficulties, facial dysmorphism, and distal skeletal anomalies. Some patients may have hearing impairment, retinopathy, or cardiac defects. It is a multisystemic disorder with variable features (summary by Loddo et al., 2018).

Any Pitt-Hopkins-like syndrome in which the cause of the disease is a mutation in the NRXN1 gene.

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.

SCN8A-related epilepsy with encephalopathy is characterized by developmental delay, seizure onset in the first 18 months of life (mean 4 months), and intractable epilepsy characterized by multiple seizure types (generalized tonic-clonic seizures, infantile spasms, and absence and focal seizures). Epilepsy syndromes can include Lennox-Gastaut syndrome, West syndrome, and epileptic encephalopathies (e.g., Dravet syndrome). Hypotonia and movement disorders including dystonia, ataxia, and choreoathetosis are common. Psychomotor development varies from normal prior to seizure onset (with subsequent slowing or regression after seizure onset) to abnormal from birth. Intellectual disability, present in all, ranges from mild to severe (in ~50% of affected individuals). Autistic features are noted in some. Sudden unexpected death in epilepsy (SUDEP) of unknown cause has been reported in approximately 10% of published cases. To date SCN8A-related epilepsy with encephalopathy has been reported in the literature in about 50 individuals.

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).

Any autosomal recessive primary microcephaly in which the cause of the disease is a mutation in the CEP135 gene.

Microcephalic primordial dwarfism, Dauber type is a rare, genetic developmental defect during embryogenesis characterized by severe pre- and postnatal growth retardation, severe microcephaly, severe developmental delay and intelletual disability, severe adult short stature and facial dysmorphism (incl. hypotelorism, small ears, prominent nose). Other reported features include skeletal anomalies (Madelung deformity, clinodactyly, mild lumbar scoliosis, bilateral hip dysplasia) and seizures. Absence of thelarche and menarche is also associated.

Alazami syndrome is an autosomal recessive disorder characterized by severe growth restriction present at birth, severely impaired intellectual development, and distinctive facial features. Some patients have been reported with skeletal and behavioral features (summary by Imbert-Bouteille et al., 2019).

GAND syndrome is a neurodevelopmental syndrome characterized by global developmental delay apparent from infancy, with motor delay and moderate to severely impaired intellectual development. Most patients have poor speech acquisition, especially expressive language development, and may manifest signs of speech apraxia. Affected individuals have hypotonia and feeding difficulties in infancy, as well as common dysmorphic features, such as macrocephaly, frontal bossing, hypertelorism, deep-set eyes, posteriorly rotated ears, and elongated wide nose with prominent nasal tip. More variable features may include seizures, cardiac abnormalities, and nonspecific findings on brain imaging (summary by Shieh et al., 2020).

Mitochondrial complex III deficiency can be fatal in childhood, although individuals with mild signs and symptoms can survive into adolescence or adulthood.\n\nMost people with mitochondrial complex III deficiency have a buildup of a chemical called lactic acid in the body (lactic acidosis). Some affected individuals also have buildup of molecules called ketones (ketoacidosis) or high blood glucose levels (hyperglycemia). Abnormally high levels of these chemicals in the body can be life-threatening.\n\nThe severity of mitochondrial complex III deficiency varies widely among affected individuals. People who are mildly affected tend to have muscle weakness (myopathy) and extreme tiredness (fatigue), particularly during exercise (exercise intolerance). More severely affected individuals have problems with multiple body systems, such as liver disease that can lead to liver failure, kidney abnormalities (tubulopathy), and brain dysfunction (encephalopathy). Encephalopathy can cause delayed development of mental and motor skills (psychomotor delay), movement problems, weak muscle tone (hypotonia), and difficulty with communication. Some affected individuals have a form of heart disease called cardiomyopathy, which can lead to heart failure. \n\nMitochondrial complex III deficiency is a genetic condition that can affect several parts of the body, including the brain, kidneys, liver, heart, and the muscles used for movement (skeletal muscles). Signs and symptoms of mitochondrial complex III deficiency usually begin in infancy but can appear later.

Deafness, dystonia, and cerebral hypomyelination is an X-linked recessive mental retardation syndrome characterized by almost no psychomotor development, dysmorphic facial features, sensorineural deafness, dystonia, pyramidal signs, and hypomyelination on brain imaging (summary by Cacciagli et al., 2013).

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

Severe intellectual disability-short stature-behavioral abnormalities-facial dysmorphism syndrome is a rare, genetic, syndromic intellectual disability disorder characterized by severe intellectual disability with limited or absent speech and language, short stature, acquired microcephaly, kyphoscoliosis or scoliosis, and behavioral disturbances that include hyperactivity, stereotypy and aggressiveness. Facial dysmorphism, that typically includes sloping forehead, mild synophrys, deep-set eyes, strabismus, anteverted large ears, prominent nose and dental malposition, is also characteristic.

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.

Any non-syndromic X-linked intellectual disability in which the cause of the disease is a mutation in the GDI1 gene.

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).

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.

Any autosomal recessive non-syndromic intellectual disability in which the cause of the disease is a mutation in the LMAN2L gene.

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).

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

Rhizomelic chondrodysplasia punctata (RCDP) is a peroxisomal disorder characterized by disproportionately short stature primarily affecting the proximal parts of the extremities, a typical facial appearance including a broad nasal bridge, epicanthus, high-arched palate, dysplastic external ears, and micrognathia, congenital contractures, characteristic ocular involvement, dwarfism, and severe mental retardation with spasticity. Biochemically, plasmalogen synthesis and phytanic acid alpha-oxidation are defective. Most patients die in the first decade of life (summary by Wanders and Waterham, 2005). For a discussion of genetic heterogeneity of rhizomelic chondrodysplasia punctata, see 215100.

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).

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).

Any autosomal recessive primary microcephaly in which the cause of the disease is a mutation in the SASS6 gene.

Any autosomal dominant non-syndromic intellectual disability in which the cause of the disease is a mutation in the EEF1A2 gene.

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).

A rare genetic syndromic intellectual disability characterized by global developmental delay, moderate to severe intellectual disability, motor and language impairment, behavioral abnormalities (with mood instability, aggression and self-mutilation) and progressive hand tremor. Facial dysmorphism includes narrow palpebral fissures, large ears, long philtrum and prominent chin.

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

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

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

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.

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

Developmental and epileptic encephalopathy-37 (DEE37) is an autosomal recessive epileptic-dyskinetic neurologic disorder characterized by the onset of intractable seizures or abnormal movements in the first months or years of life. Patients typically have normal or only mildly delayed development in early infancy, but then show developmental regression and stagnation after the onset of seizures, which can occur between about 6 months to 2 years of age. In addition to epileptic encephalopathy, affected individuals also manifest a hyperkinetic movement disorder with choreoathetosis, spasticity, and rigidity. There is severely impaired intellectual development and function, loss of verbal skills with absent speech, and impaired volitional movements (summary by Madeo et al., 2016). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

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).

Developmental and epileptic encephalopathy-36 (DEE36) is an X-linked neurodevelopmental disorder characterized by the onset of seizures at a mean age of 6.5 months. Most patients present with infantile spasms associated with hypsarrhythmia on EEG, consistent with a clinical diagnosis of West syndrome. The seizures tend to be refractory to treatment, although some patients may respond to benzodiazepines or a ketogenic diet. Affected individuals have severely delayed psychomotor development with poor motor function, severe intellectual disability, poor or absent speech, and limited eye contact. More variable features include feeding difficulties sometimes requiring tube feeding, ocular defects including cortical visual impairment, dysmorphic facial features, and scoliosis or osteopenia. The vast majority of patients reported have been females, although rare affected males with a similar phenotype have been described. Most patients show normal N-glycosylation on transferrin isoelectric focusing, but some show abnormal N-glycosylation consistent with CDG type I (summary by Ng et al., 2020). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350. For a discussion of the classification of CDGs, see CDG1A (212065).

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

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

SPTBN4 disorder is typically characterized by severe-to-profound developmental delay and/or intellectual disability, although two individuals in one family had a milder phenotype, including one individual with normal cognitive development. Speech and language skills are often severely limited. Affected individuals rarely achieve head control. Most are unable to sit, stand, or walk. Affected individuals typically have congenital hypotonia that may transition to hypertonia. Axonal motor neuropathy leads to hyporeflexia/areflexia and weakness, which can result in respiratory difficulties requiring ventilatory support. Most affected individuals require tube feeding for nutrition. Half of affected individuals develop seizures. Cortical visual impairment and auditory neuropathy have also been reported.

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).

3-Methylglutaconic aciduria type IX (MGCA9) is an autosomal recessive disorder characterized by early-onset seizures, severely delayed psychomotor development and intellectual disability. Patients have hypotonia or spasticity, and laboratory investigations show increased serum lactate and 3-methylglutaconic aciduria, suggestive of a mitochondrial defect (summary by Shahrour et al., 2017). For a phenotypic description and a discussion of genetic heterogeneity of 3-methylglutaconic aciduria, see MGCA type I (250950).

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.

Hyperphosphatasia with impaired intellectual development syndrome-1 (HPMRS1) is an autosomal recessive disorder characterized by impaired intellectual development, various neurologic abnormalities such as seizures and hypotonia, and hyperphosphatasia. Other features include facial dysmorphism and variable degrees of brachytelephalangy (summary by Krawitz et al., 2010). The disorder is caused by a defect in glycosylphosphatidylinositol biosynthesis; see GPIBD1 (610293). Genetic Heterogeneity of Hyperphosphatasia with Impaired Intellectual Development Syndrome See also HPMRS2 (614749), caused by mutation in the PIGO gene (614730) on chromosome 9p13; HPMRS3 (614207), caused by mutation in the PGAP2 gene (615187) on chromosome 11p15; HPMRS4 (615716), caused by mutation in the PGAP3 gene (611801) on chromosome 17q12; HPMRS5 (616025), caused by mutation in the PIGW gene (610275) on chromosome 17q12; and HPMRS6 (616809), caused by mutation in the PIGY gene (610662) on chromosome 4q22. Knaus et al. (2018) provided a review of the main clinical features of the different types of HPMRS, noting that some patients have a distinct pattern of facial anomalies that can be detected by computer-assisted comparison, particularly those with mutations in the PIGV and PGAP3 genes. Individuals with HPMRS have variable increased in alkaline phosphatase (AP) as well as variable decreases in GPI-linked proteins that can be detected by flow cytometry. However, there was no clear correlation between AP levels or GPI-linked protein abnormalities and degree of neurologic involvement, mutation class, or gene involved. Knaus et al. (2018) concluded that a distinction between HPMRS and MCAHS (see, e.g., 614080), which is also caused by mutation in genes involved in GPI biosynthesis, may be artificial and even inaccurate, and that all these disorders should be considered and classified under the more encompassing term of 'GPI biosynthesis defects' (GPIBD).

Zimmermann-Laband syndrome is a rare disorder characterized by gingival fibromatosis, dysplastic or absent nails, hypoplasia of the distal phalanges, scoliosis, hepatosplenomegaly, hirsutism, and abnormalities of the cartilage of the nose and/or ears (summary by Balasubramanian and Parker, 2010). Genetic Heterogeneity of Zimmermann-Laband Syndrome ZLS2 (616455) is caused by mutation in the ATP6V1B2 gene (606939) on chromosome 8p21. ZLS3 (618658) is caused by mutation in the KCNN3 gene (602983) on chromosome 1q21.

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.

Developmental and epileptic encephalopathy-58 (DEE58) is a severe neurodevelopmental disorder characterized by the onset of infantile spasms and refractory seizures in the first days or months of life. Affected individuals have global developmental delay with impaired intellectual development, usually with absent speech and inability to walk. Additional features include optic atrophy with poor or absent visual fixation, hypotonia, feeding difficulties, and spasticity (summary by Hamdan et al., 2017). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

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).

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

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.

Zellweger spectrum disorder (ZSD) is a phenotypic continuum ranging from severe to mild. While individual phenotypes (e.g., Zellweger syndrome [ZS], neonatal adrenoleukodystrophy [NALD], and infantile Refsum disease [IRD]) were described in the past before the biochemical and molecular bases of this spectrum were fully determined, the term "ZSD" is now used to refer to all individuals with a defect in one of the ZSD-PEX genes regardless of phenotype. Individuals with ZSD usually come to clinical attention in the newborn period or later in childhood. Affected newborns are hypotonic and feed poorly. They have distinctive facies, congenital malformations (neuronal migration defects associated with neonatal-onset seizures, renal cysts, and bony stippling [chondrodysplasia punctata] of the patella[e] and the long bones), and liver disease that can be severe. Infants with severe ZSD are significantly impaired and typically die during the first year of life, usually having made no developmental progress. Individuals with intermediate/milder ZSD do not have congenital malformations, but rather progressive peroxisome dysfunction variably manifest as sensory loss (secondary to retinal dystrophy and sensorineural hearing loss), neurologic involvement (ataxia, polyneuropathy, and leukodystrophy), liver dysfunction, adrenal insufficiency, and renal oxalate stones. While hypotonia and developmental delays are typical, intellect can be normal. Some have osteopenia; almost all have ameleogenesis imperfecta in the secondary teeth.

IDDMSSD is a neurodevelopmental disorder characterized by impaired intellectual development, poor speech, postnatal macrocephaly, and seizures (Harms et al., 2018).

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).

Alpha-thalassemia X-linked intellectual disability (ATR-X) syndrome is characterized by distinctive craniofacial features, genital anomalies, hypotonia, and mild-to-profound developmental delay / intellectual disability (DD/ID). Craniofacial abnormalities include small head circumference, telecanthus or widely spaced eyes, short triangular nose, tented upper lip, and thick or everted lower lip with coarsening of the facial features over time. While all affected individuals have a normal 46,XY karyotype, genital anomalies comprise a range from hypospadias and undescended testicles, to severe hypospadias and ambiguous genitalia, to normal-appearing female external genitalia. Alpha-thalassemia, observed in about 75% of affected individuals, is mild and typically does not require treatment. Osteosarcoma has been reported in a few males with germline pathogenic variants.

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).

Brain small vessel disease-3 (BSVD3) is an autosomal recessive disorder resulting from fragility of cerebral vessels causing an increased risk of intracranial bleeding. The resultant phenotype is highly variable depending on timing and location of the intracranial bleed. Some patients may have onset in utero or early infancy, with subsequent global developmental delay, spasticity, and porencephaly on brain imaging. Other patients may have normal or mildly delayed development with sudden onset of intracranial hemorrhage causing acute neurologic deterioration (summary by Miyatake et al., 2018). For a discussion of genetic heterogeneity of brain small vessel disease, see BSVD1 (175780).

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).

Shukla-Vernon syndrome (SHUVER) is an X-linked recessive neurodevelopmental disorder characterized by global developmental delay, variably impaired intellectual development, and behavioral abnormalities, including autism spectrum disorder and ADHD. Dysmorphic features are common and may include tall forehead, downslanting palpebral fissures, and tapering fingers. Some patients may have seizures and/or cerebellar atrophy on brain imaging. Carrier mothers may have mild manifestations, including learning disabilities (summary by Shukla 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.

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).

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).

Progressive myoclonic epilepsy-11 (EPM11) is a neurodegenerative disorder characterized by onset of developmental regression and various types of seizures around 2 years of age after relatively normal early development. The seizures are usually refractory to treatment and are associated with multiple abnormalities on EEG. During the first and second decades, affected individuals develop additional neurologic signs and symptoms, including pyramidal, extrapyramidal, and cerebellar signs such as spasticity, loss of independent ambulation, myoclonus, tremor, and ataxia. Cognitive impairment is severe, and patients can speak only a few words or are non-verbal (summary by Hamanaka et al., 2020). For discussion of genetic heterogeneity of progressive myoclonic epilepsy, see EPM1A (254800).

Congenital disorder of glycosylation type IIt (CDG2t) is an autosomal recessive multisystemic metabolic disorder characterized by global developmental delay, poor overall growth, severely impaired intellectual development with absent language, and behavioral abnormalities. Most patients develop early-onset seizures; brain imaging tends to show white matter abnormalities. Variable dysmorphic features, including long face, almond-shaped eyes, protruding maxilla, and short philtrum, are also present. The disorder, which is associated with low levels of HDL cholesterol, results from defective posttranslational O-linked glycosylation of certain plasma lipids and proteins (summary by Zilmer et al., 2020). For an overview of congenital disorders of glycosylation, see CDG1A (212065) and CDG2A (212066).

Arthrogryposis multiplex congenita-5 (AMC5) is an autosomal recessive disorder characterized by severe joint contractures apparent at birth. Affected individuals usually have hypertonia and abnormal movements suggestive of dystonia, as well as feeding and/or breathing difficulties. More variable features may include poor overall growth, strabismus, dysmorphic facies, and global developmental delay with impaired speech (summary by Kariminejad et al., 2017).

IDDEBF is a severe disorder characterized by impaired intellectual development, epilepsy, behavioral abnormalities, and coarse facies. Brain MRI findings may include delayed myelination in the deep parietal lobes (Kvarnung et al., 2018).

Kilquist syndrome (KILQS) is an autosomal recessive multisystem disorder characterized by neurologic, gastrointestinal, and secretory dysfunction. Affected individuals present at birth with hypotonia, feeding difficulties, mild dysmorphic features, and sensorineural hearing loss. They show poor overall growth associated with gastrointestinal anomalies such as gastroesophageal reflux or midgut malrotation, as well as profound global developmental delay with inability to sit or speak. Tear, sweat, and saliva production is also impaired, causing dry mouth and recurrent bronchial mucus plugging. Some of the clinical features are reminiscent of cystic fibrosis (CF; 219700) (summary by Stodberg et al., 2020).

Neurodevelopmental disorder with dysmorphic facies, sleep disturbance, and brain abnormalities (NEDFASB) is a syndromic disorder with multisystemic involvement. Affected individuals have severe global developmental delay with severely impaired intellectual development, poor or absent language, behavioral abnormalities, seizures, and sleep disturbances. Craniofacial dysmorphisms, while variable, include round face, prognathism, depressed nasal bridge, and cleft or high-arched palate. Brain imaging shows dysgenesis of the corpus callosum and progressive cerebellar atrophy. Additional features may include genitourinary tract anomalies, hearing loss, and mild distal skeletal defects (summary by Humbert 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).

Chromosome 13q33-q34 deletion syndrome is associated with developmental delay and/or impaired intellectual development, facial dysmorphism, and an increased risk for epilepsy, cardiac defects and additional anatomic anomalies (summary by Sagi-Dain et al., 2019).

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).

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.

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

CUL3-related neurodevelopmental disorder is a condition that affects neurological and physical development. Children with CUL3-related neurodevelopmental disorder may have intellectual disability or specific learning disorders. They may also experience delayed development of speech and motor skills, such as sitting and walking. Some individuals with this condition may have autism spectrum disorder, a developmental condition that affects communication and social skills. \n\nMovement abnormalities can also occur in people with CUL3-related neurodevelopmental disorder. Affected individuals may have weak muscle tone (hypotonia) in childhood. In adulthood, they may develop involuntary muscle tensing (dystonia), rhythmic shaking (tremor), or other uncontrolled movements (spasms). \n\nPeople with CUL3-related neurodevelopmental disorder can have distinctive facial features, including a long, triangular-shaped face; a large forehead; a large, rounded nose; small ears; deep-set eyes; or a pointed chin. Some affected individuals have a larger than normal head (macrocephaly). \n\nMany people with CUL3-related neurodevelopmental disorder have hand and foot abnormalities. Hand abnormalities can include small pinky (fifth) fingers that curve inward (clinodactyly), narrow thumbs, underdevelopment of the muscle at the base of the thumb (thenar hypoplasia), or a single crease across the palm of the hand. Foot abnormalities can include high arches of the feet (pes cavus); bunions; fusion of the skin between some toes (cutaneous syndactyly); or joint deformities (contractures) in the ankles, feet, or toes. A few individuals with CUL3-related neurodevelopmental disorder have an abnormally curved lower back (lordosis) or a spine that curves to the side (scoliosis). \n\nSome affected infants have a backflow of stomach acids into the esophagus (gastroesophageal reflux disease or GERD), which tends to go away after childhood. Rarely, recurrent seizures (epilepsy), congenital heart abnormalities, or genitourinary abnormalities occur in people with CUL3-related neurodevelopmental disorder. 

Baralle-Macken syndrome (BARMACS) is an autosomal recessive disorder characterized by global developmental delay apparent from infancy, difficulty walking or inability to walk, and impaired intellectual development with poor or absent speech. Affected individuals develop early-onset cataracts; some may have microcephaly. Additional more variable features may include dysmorphic facial features, metabolic abnormalities, spasticity, and lymphopenia (summary by Macken et al., 2021).

CIMDAG syndrome (CIMDAG) is a multisystemic disorder characterized by severely impaired psychomotor development and hematologic abnormalities apparent from early infancy. Affected individuals show poor overall growth with microcephaly, impaired intellectual development, poor or absent speech, poor eye contact, and motor problems, such as inability to walk, hypotonia, and spasticity. Brain imaging typically shows cerebral and cerebellar atrophy, thin corpus callosum, and delayed myelination. The associated hematologic abnormalities are variable, but are mostly consistent with congenital dyserythropoietic anemia (CDA) (summary by Rodger et al., 2020 and Seu et al., 2020).

Pontocerebellar hypoplasia type 14 (PCH14) is a severe autosomal recessive neurodevelopmental disorder characterized by congenital onset of progressive microcephaly and poor or absent psychomotor development with severely impaired intellectual development apparent from birth. Other features may include hypotonia, spastic quadriplegia, and early-onset seizures. Brain imaging shows pontocerebellar hypoplasia, agenesis or partial agenesis of the corpus callosum, and sometimes a simplified gyral pattern. Early death may occur (summary by et al., 2021). For a phenotypic description and a discussion of genetic heterogeneity of PCH, see PCH1A (607596).

Pontocerebellar hypoplasia type 15 (PCH15) is a severe autosomal recessive neurodevelopmental disorder characterized by congenital onset of progressive microcephaly and poor or absent psychomotor development with severely impaired intellectual development apparent from birth. Other features may include spastic quadriplegia, early-onset seizures, and chronic anemia and thrombocytopenia. Brain imaging shows pontocerebellar hypoplasia and partial agenesis of the corpus callosum (summary by et al., 2021). For a phenotypic description and a discussion of genetic heterogeneity of PCH, see PCH1A (607596).

Onychodystrophy, osteodystrophy, impaired intellectual development, and seizures syndrome (OORS) is an autosomal recessive syndromic developmental disorder characterized by global developmental delay with impaired intellectual development, dysmorphic facial features, and hypoplastic terminal phalanges and nails. Patients have seizures or tonic posturing. The disorder is associated with a defect in GPI anchoring of membrane-bound proteins (summary by Salian et al., 2021). For a discussion of genetic heterogeneity of GPI biosynthesis defects, see GPIBD1 (610293). See also DOORS syndrome (220500), which shows some overlapping clinical features.

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).

X-linked syndromic intellectual developmental disorder with pigmentary mosaicism and coarse facies (MRXSPF) is characterized by a phenotypic triad of severe developmental delay, coarse facial dysmorphisms, and Blaschkoid pigmentary mosaicism. Additional clinical features may include epilepsy, orthopedic abnormalities, hypotonia, and growth abnormalities. The disorder affects both males and females (Villegas et al., 2019; Diaz et al., 2020).

Focal segmental glomerulosclerosis and neurodevelopmental syndrome (FSGSNEDS) is characterized by global developmental delay and renal dysfunction manifest as proteinuria and nephrotic syndrome apparent from infancy or early childhood. Some patients present with renal disease, whereas others present with developmental delay and develop renal disease later in childhood. Renal biopsy shows focal segmental glomerulosclerosis (FSGS), but the course of the disease is variable: some patients have transient proteinuria and others require renal transplant. Neurodevelopmental features are also variable, with some patients having only mildly impaired intellectual development, and others having a severe developmental disorder associated with early-onset refractory seizures or epileptic encephalopathy. Additional features, including feeding difficulties, poor overall growth, and nonspecific dysmorphic facial features, are commonly observed (summary by Assoum et al., 2018 and Weng et al., 2021).

Ritscher-Schinzel syndrome-4 (RTSC4) is characterized by a constellation of congenital anomalies, including dysmorphic craniofacial features and structural brain anomalies, such as Dandy-Walker malformation (220200), hindbrain malformations, or agenesis of the corpus callosum, associated with global developmental delay and impaired intellectual development. Congenital cardiac defects have been reported in 1 family (summary by Ritscher et al., 1987 and Jeanne et al., 2021). For a discussion of genetic heterogeneity of Ritscher-Schinzel syndrome, see RTSC1 (220210).

Luo-Schoch-Yamamoto syndrome (LUSYAM) is a neurodevelopmental disorder characterized by global developmental delay and impaired intellectual development apparent from infancy. Affected individuals have delayed walking, early-onset seizures, hypotonia, dysmorphic facial features, and white matter abnormalities on brain imaging (Luo et al., 2021).

SIMHA syndrome is characterized by short stature, impaired intellectual development, microcephaly, hypotonia, and ocular anomalies. Inter- and intrafamilial phenotypic variability has been observed (Kambouris et al., 2014; Zahra et al., 2020).

Neurodevelopmental disorder with hypotonia and gross motor and speech delay (NEDHMS) is an autosomal recessive disorder characterized by severe global developmental delay apparent from infancy. Affected individuals have axial hypotonia and limited ability to walk, including some who are nonambulatory with lower limb spasticity, impaired intellectual development, and poor or absent speech and language. Additional more variable features may include seizures, behavioral problems, distal skeletal anomalies, and dysmorphic facial features (Melo et al., 2021).

Hengel-Maroofian-Schols syndrome (HEMARS) is an autosomal recessive neurodevelopmental disorder characterized by severe global developmental delay apparent from infancy or early childhood. Affected individuals have delayed walking or inability to walk, impaired intellectual development with poor or absent speech, pyramidal signs manifest as lower limb spasticity, poor overall growth often with short stature and microcephaly, and dysmorphic facial features. Some patients develop seizures. Brain imaging shows thinning of the posterior part of the corpus callosum, delayed myelination, and cerebral and cerebellar atrophy (Hengel et al., 2021).

Neurodevelopmental disorder with hyperkinetic movements and dyskinesia (NEDHYD) is an autosomal recessive complex neurologic disorder characterized by severe global developmental delay with axial hypotonia, impaired intellectual development, poor overall growth, and abnormal involuntary hyperkinetic movements, including dystonia, myoclonus, spasticity, and orofacial dyskinesia. It is the most severe manifestation of ADCY5-related dyskinetic disorders (summary by Okamoto et al., 2021 and Kaiyrzhanov et al., 2021).

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

Craniofacial dysmorphism, skeletal anomalies, and impaired intellectual development syndrome-2 (CFSMR2) is characterized by flat face, low-set ears, and cleft lip and palate, as well as costovertebral anomalies including bifid and fused ribs, vertebral segmentation defects, and scoliosis. Intellectual delay can be severe, with absent speech (Alanay et al., 2014). For a general phenotypic description and discussion of genetic heterogeneity of CFSMR, see CFSMR1 (213980).

Developmental and epileptic encephalopathy-103 (DEE103) is characterized by onset of various types of seizures in the first year of life, most of which are refractory to treatment. Affected individuals show global developmental delay with impaired intellectual development ranging from mild to severe. Additional features may include hypotonia, ataxia, and behavioral abnormalities, including autism and hyperactivity (Schwarz et al., 2022). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

Autosomal recessive intellectual developmental disorder-76 (MRT76) is characterized by impaired intellectual development, absent speech, poor sleep, abnormal EEG with seizures, normal brain imaging, and precocious puberty (Ismail et al., 2022).

Autosomal dominant intellectual developmental disorder-68 (MRD68) is characterized by developmental delay/intellectual disability, microcephaly, poor growth, feeding difficulties, and dysmorphic features. Some patients may have autism spectrum disorder or attention deficit-hyperactivity disorder (ADHD) (Cif et al., 2020).

X-linked intellectual developmental disorder-110 (XLID110) is characterized by moderately to severely impaired intellectual development.

Neurodevelopmental disorder with microcephaly, short stature, and speech delay (NEDMISS) is an autosomal recessive disorder characterized by global developmental delay with severely impaired intellectual development usually accompanied by behavioral abnormalities. Other features may include hypotonia, abnormal gait, mild dysmorphism, and seizures (Rawlins et al., 2022).

Neurodevelopmental disorder with microcephaly, hypotonia, and absent language (NEDMHAL) is a severe autosomal recessive disorder characterized by the constellation of these features. Behavioral problems and hearing loss are also present (Ansar et al., 2020).

Birk-Aharoni syndrome (BKAH) is a severe neurodevelopmental disorder characterized developmental delay, impaired intellectual development, absent speech, spastic tetraplegia with central hypotonia, chorea, inability to walk, hearing loss, micropenis, undescended testes, and mildly elevated liver enzymes (Aharoni et al., 2022).

Neurodevelopmental disorder with craniofacial dysmorphism and skeletal defects (NEDCDS) is characterized by global developmental delay, severely impaired intellectual development with poor or absent speech, characteristic facial features, and variable skeletal abnormalities. Additional features include feeding difficulties, inability to walk or walking with an abnormal gait, and cerebellar or other abnormalities on brain imaging (Reichert et al., 2020).

Developmental and epileptic encephalopathy-109 (DEE109) is characterized by the onset of various types of seizures in the first months or years of life. Affected individuals show developmental delay before and concurrent with the onset of seizures. Features include impaired intellectual development with poor speech, ataxic gait, coordination problems, and behavioral abnormalities (Manivannan et al., 2022). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

Autosomal recessive intellectual developmental disorder-78 (MRT78) is a neurodevelopmental disorder characterized by impaired intellectual development that is usually mild, but shows variable severity. Affected individuals have microcephaly and mild short stature. Additional features may include ocular abnormalities and mild skeletal defects (Haag et al., 2021).

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-25 (HLD25) is an autosomal recessive disorder characterized by horizontal nystagmus, hypotonia, and global developmental delay apparent soon after birth or in infancy. Most patients show gradual clinical improvement over time with resolution of the nystagmus in early childhood. Many achieve developmental milestones and may have normal cognition, although the severity of the disorder varies and some patients may have persistent neurologic deficits, such as ataxia or intellectual disability. Brain imaging shows hypomyelination that may also improve with time (Yan et al., 2022; do Rosario et al., 2022). For a general phenotypic description and a discussion of genetic heterogeneity of HLD, see 312080.

Neurodevelopmental disorder with absent speech and movement and behavioral abnormalities (NEDSMB) is an autosomal recessive disorder characterized by global developmental delay and severely impaired intellectual development with aggressive behavior. Mild dysmorphic features and hypodontia are also present (Faqeih et al., 2023).

Autosomal recessive intellectual developmental disorder-79 (MRT79) is characterized by global developmental delay apparent from infancy. Affected individuals have mildly delayed walking with an ataxic gait and severely impaired intellectual development with poor or absent speech. Additional features may include postnatal microcephaly and dysmorphic features (Van Bergen et al., 2022).

Glycine encephalopathy (GCE), also called nonketotic hyperglycinemia (NKH), is an inborn error of metabolism characterized by accumulation of a large amount of glycine in body fluids. Typical cases have severe neurologic features, including seizures, lethargy, and muscular hypotonia soon after birth, and most die with the neonatal period; atypical cases have later onset and less severe psychomotor development (summary by Nanao et al., 1994). For a general description and a discussion of genetic heterogeneity of glycine encephalopathy, see GCE1 (605899).