GRIN2A-related speech disorders and epilepsy are characterized by speech disorders in all affected individuals and a range of epilepsy syndromes present in about 90%. Severe speech disorders observed can include dysarthria and speech dyspraxia, and both receptive and expressive language delay/regression; more mildly affected individuals may display subtly impaired intelligibility of conversational speech. Epilepsy features include seizure onset usually between ages three and six years, focal epilepsy with language and/or global developmental regression, and electroencephalogram (EEG) showing continuous spike-and-wave discharges in sleep or very active centrotemporal discharges. Seizure types include seizures associated with aura of perioral paresthesia, focal or focal motor seizures (often evolving to generalized tonic-clonic), and atypical absence seizures. Epilepsy syndromes can include: Landau-Kleffner syndrome (LKS), epileptic encephalopathy with continuous spike-and-wave during sleep (ECSWS), childhood epilepsy with centrotemporal spikes (CECTS), atypical childhood epilepsy with centrotemporal spikes (ACECTS), autosomal dominant rolandic epilepsy with speech dyspraxia (ADRESD), and infantile-onset epileptic encephalopathy.

A generalized myoclonic-atonic seizure is a type of generalized motor seizure characterized by a myoclonic jerk followed by an atonic motor component.

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.

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

The Lafora type of progressive myoclonic epilepsy is an autosomal recessive disorder characterized by insidious onset of progressive neurodegeneration between 8 and 18 years of age. Initial features can include headache, difficulties in school work, myoclonic jerks, generalized seizures, and often visual hallucination. The myoclonus, seizures, and hallucinations gradually worsen and become intractable. This is accompanied by progressive cognitive decline, resulting in dementia. About 10 years after onset, affected individuals are in near-continuous myoclonus with absence seizures, frequent generalized seizures, and profound dementia or a vegetative state. Histologic studies of multiple tissues, including brain, muscle, liver, and heart show intracellular Lafora bodies, which are dense accumulations of malformed and insoluble glycogen molecules, termed polyglucosans (review by Ramachandran et al., 2009). There is a slower progression of disease and later age at death in Lafora disease-2 than in Lafora disease-1 (MELF1, EPM2A; 254780); see Genotype/Phenotype Correlations. Myoclonic epilepsy of Lafora-1 is caused by mutation in the EPM2A gene (608072), which encodes laforin, on chromosome 6q24. For a discussion of genetic heterogeneity of progressive myoclonic epilepsy, see EPM1A (254800).

Childhood seizures associated with febrile episodes are relatively common and represent the majority of childhood seizures. A febrile convulsion is defined as a seizure event in infancy or childhood, usually occurring between 6 months and 6 years of age, associated with fever but without any evidence of intracranial infection or defined pathologic or traumatic cause (Nabbout et al., 2002). Although the majority of patients do not develop epilepsy, the risk of developing subsequent afebrile seizures is 5 to 7 times higher in those with a history of febrile seizures compared to the general population (Annegers et al., 1987; Hedera et al., 2006). The FEB1 locus maps to chromosome 8q13-q21. Genetic Heterogeneity of Familial Febrile Seizures See FEB2 (602477), caused by mutation in the HCN2 gene (602781) on chromosome 19p13; FEB3A (604403), caused by mutation in the SCN1A gene (182389) on chromosome 2q24; FEB4 (604352), caused by mutation in the ADGRV1 gene (602851) on chromosome 5q14; FEB8 (607681), caused by mutation in the GABRG2 gene (137164) on chromosome 5q31; and FEB11 (614418), caused by mutation in the CPA6 gene (609562) on chromosome 8q13. Several loci for familial febrile seizures have been identified: see FEB3B (613863) on chromosome 2q24; FEB5 (609255) on chromosome 6q22-q24; FEB6 (609253) on chromosome 18p11; FEB7 (611515) on chromosome 21q22; FEB9 (611634) on chromosome 3p24.2-p23; and FEB10 (612637) on chromosome 3q26. A phenotype termed 'generalized epilepsy with febrile seizures plus' (GEFS+; 604233) is a clinical subset of familial febrile convulsions in which affected individuals later develop afebrile seizures. GEFS+ is associated with mutations in several genes. Deprez et al. (2009) provided a review of the genetics of epilepsy syndromes starting in the first year of life, and included a diagnostic algorithm.

Any febrile seizures, familial in which the cause of the disease is a mutation in the ADGRV1 gene.

Generalized epilepsy with febrile seizures plus type 1 (GEFSP1) is an autosomal dominant neurologic disorder characterized by onset of seizures associated with fever in infancy or early childhood. There is wide phenotypic variability, even within families. In contrast to classic febrile seizures (see, e.g., FEB1, 121210), which affect approximately 3% of children under 6 years of age and typically spontaneously remit by age 6 years, patients with GEFSP1 either have febrile seizures extending beyond age 6 years or develop epilepsy with afebrile seizures. Other seizure types include absence seizures, partial seizures, myoclonic seizures, and atonic seizures. Some patients may have developmental delay after the onset of seizures (summary by Wallace et al., 1998 and Singh et al., 1999). Deprez et al. (2009) reviewed the genetics of epilepsy syndromes starting in the first year of life, and included a diagnostic algorithm. Genetic Heterogeneity of GEFS+ GEFS+ is a genetically heterogeneous disorder. See also GEFS+2 (604403), caused by mutation in the SCN1A gene (182389) on chromosome 2q24; GEFS+3 (see 607681), caused by mutation in the GABRG2 gene (137164) on chromosome 5q34; GEFS+5 (613060), associated with variation in the GABRD (137163) gene on chromosome 1p36; GEFS+9 (616172), caused by mutation in the STX1B gene (601485) on chromosome 16p11; GEFS+10 (618482), caused by mutation in the HCN1 gene (602780) on chromosome 5p12; GEFS+11 (602477), caused by mutation in the HCN2 gene (602781) on chromosome 19p13; and GEFS+12 (620755), caused by mutation in the SLC32A1 gene (616440) on chromosome 20q11. Several putative loci have also been identified; see GEFS+4 (609800), mapped to chromosome 2p24; GEFS+6 (612279), mapped to chromosome 8p23-p21; GEFS+7 (613863), mapped to chromosome 2q24; and GEFS+8 (613828), mapped to chromosome 6q16.3-q22.31.

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.

Any autosomal recessive non-syndromic intellectual disability in which the cause of the disease is a mutation in the GRIK2 gene.

Individuals with biallelic PRICKLE1-related disorders typically present with progressive myoclonus epilepsy (PME) with ataxia characterized by myoclonic seizures (lightning-like jerks), generalized convulsive seizures, varying degrees of neurologic regression mainly presenting with ataxia, and mild cognitive impairment or normal cognition. Onset of symptoms is between ages five and ten years. Action myoclonus may affect the limbs or bulbar muscles, while spontaneous myoclonus may occasionally involve facial muscles. Dysarthria may also be an early feature of this condition. The main seizure types are myoclonic or tonic-clonic with frequent nocturnal occurrence. Individuals with heterozygous PRICKLE1 pathogenic variants have presented with non-PME seizures (isolated myoclonic seizures, juvenile myoclonic epilepsy), myoclonic epilepsy, developmental delay, intellectual disability, autism spectrum disorder, and/or central nervous system malformations.

Patients with isolated febrile seizures (FEB3B) usually have onset between ages 5 months to 4 years and show spontaneous remission by age 6 years (summary by Singh et al., 2009), whereas patients with GEFS+ continue to have various types of febrile and afebrile seizures later in life (summary by Singh et al., 1999). For a general phenotypic description and a discussion of genetic heterogeneity of GEFS+, see 604233. For a phenotypic description and a discussion of genetic heterogeneity of familial febrile seizures, see 121210.

An X-linked dominant condition caused by mutation(s) in the IQSEC2 gene, encoding IQ motif and SEC7 domain-containing protein 2. It is characterized by substantially impaired intellectual functioning and behavioral abnormalities.

X-linked intellectual developmental disorder-98 (XLID98) is a neurodevelopmental disorder characterized by delayed psychomotor development, poor speech, behavioral abnormalities, poor overall growth, dysmorphic facial features, and often early-onset seizures. Some carrier females are unaffected, whereas other females with mutations are affected; males tend to be more severely affected than females. It is believed that the phenotypic variability and disease manifestations in female carriers results from skewed X-inactivation or cellular mosaicism (summary by de Lange et al., 2016).

CHD2-related neurodevelopmental disorders are characterized by early-onset epileptic encephalopathy (i.e., refractory seizures and cognitive slowing or regression associated with frequent ongoing epileptiform activity). Seizure onset is typically between ages six months and four years. Seizure types typically include drop attacks, myoclonus, and rapid onset of multiple seizure types associated with generalized spike-wave on EEG, atonic-myoclonic-absence seizures, and clinical photosensitivity. Intellectual disability and/or autism spectrum disorders are common.

Developmental and epileptic encephalopathy-19 (DEE19) is a neurologic disorder characterized by the onset of various types of seizures in the first year of life, usually between 8 and 12 months of age. Seizures are often triggered by fever, and status epilepticus may occur. Affected individuals subsequently show mildly to moderately impaired intellectual development. Brain imaging is typically normal. The clinical phenotype is similar to that of Dravet syndrome (DRVT; 607208) (summary by Carvill et al., 2014). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

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

Developmental and epileptic encephalopathy-26 (DEE26) is a neurologic disorder characterized by onset of variable types of seizures late in infancy or in the first years of life. Affected children show developmental delay with intellectual disability, poor speech, and behavioral abnormalities. EEG shows multifocal epileptic discharges, and may show hypsarrhythmia (summary by Torkamani et al., 2014). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

Generalized epilepsy with febrile seizures plus-9 is an autosomal dominant neurologic disorder characterized by onset of febrile and/or afebrile seizures in early childhood, usually before age 3 years. Seizure types are variable and include generalized tonic-clonic, atonic, myoclonic, complex partial, and absence. Most patients have remission of seizures later in childhood with no residual neurologic deficits, but rare patients may show mild developmental delay or mild intellectual disabilities (summary by Schubert et al., 2014). For a general phenotypic description and a discussion of genetic heterogeneity of GEFS+, see 604233.

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

Developmental and epileptic encephalopathy-31A (DEE31A) is an autosomal dominant neurologic disorder characterized by the global developmental delay apparent in early infancy. Most individuals have onset of various types of refractory seizures in the first months or years of life, which exacerbates the psychomotor deficits. Patients have hypotonia and profound intellectual disability with absent speech and inability to walk or ataxic gait. Some patients may have additional features, including dysmorphic features or cortical visual impairment (summary by the EuroEPINOMICS-RES Consortium et al., 2014 and Deciphering Developmental Disorders Study, 2015). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

A rare genetic, multiple congenital anomalies syndrome characterized by short stature, hand brachydactyly with hypoplastic distal phalanges, global development delay, intellectual disability, and more variably seizures, obesity, and craniofacial dysmorphism that includes microcephaly, high forehead, flat face, hypertelorism, deep set eyes, flat nasal bridge, averted nostrils, long philtrum, thin lip vermilion, and short neck.

Developmental and epileptic encephalopathy-43 (DEE43) is a neurologic disorder characterized by the onset of various types of seizures usually in the first year of life. The age at onset is highly variable, ranging from the neonatal period to about 12 months of age. Later onset may rarely occur. Seizure types include febrile, infantile spasms, focal, tonic-clonic, and myoclonic; they tend to be refractory to treatment. Affected individuals show global developmental delay with mild to moderate intellectual disability, although some may have normal early development before the onset of seizures. EEG shows focal, multifocal, or generalized sharp waves associated with seizures, sometimes with hypsarrhythmia. Additional more variable features include tube feeding, hypotonia, peripheral hypertonia, ataxia, dyskinesia, and behavioral difficulties, including aggression, ADHD, stereotypic, and impulsive behavior (summary by the Epi4K Consortium, 2016). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

Individuals with Okur-Chung neurodevelopmental syndrome (OCNDS) frequently have nonspecific clinical features, delayed language development, motor delay, intellectual disability (typically in the mild-to-moderate range), generalized hypotonia starting in infancy, difficulty feeding, and nonspecific dysmorphic facial features. Developmental delay affects all areas of development, but language is more impaired than gross motor skills in most individuals. Intellectual disability has been reported in about three quarters of individuals. Less common findings may include kyphoscoliosis, postnatal short stature, disrupted circadian rhythm leading to sleep disturbance, seizures, and poor coordination.

GNB1 encephalopathy (GNB1-E) is characterized by moderate-to-severe developmental delay / intellectual disability, structural brain abnormalities, and often infantile hypotonia and seizures. Other less common findings include dystonia, reduced vision, behavior issues, growth delay, gastrointestinal (GI) problems, genitourinary (GU) abnormalities in males, and cutaneous mastocytosis.

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).

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

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

GPIBD15 is an autosomal recessive disorder characterized by delayed psychomotor development, variable intellectual disability, hypotonia, early-onset seizures in most patients, and cerebellar atrophy, resulting in cerebellar signs including gait ataxia and dysarthria. The disorder is caused by a defect in glycosylphosphatidylinositol (GPI) biosynthesis (summary by Nguyen et al., 2017). For a discussion of genetic heterogeneity of GPI biosynthesis defects, see GPIBD1 (610293).

Developmental and epileptic encephalopathy-67 (DEE67) is characterized by the onset of various types of seizures in the first months of life, although later onset may occur in milder cases. The seizures tend to be resistant to treatment. Affected individuals have global developmental delay with impaired motor and intellectual development, poor or absent speech, movement disorders, and stereotypic or autistic behavior (summary by Chatron et al., 2018). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

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).

Progressive myoclonic epilepsy-6 (EPM6) is an autosomal recessive neurologic disorder characterized by onset of ataxia in the first years of life, followed by action myoclonus and seizures later in childhood, and loss of independent ambulation in the second decade. Cognition is not usually affected, although mild memory difficulties may occur in the third decade (summary by Corbett et al., 2011). For a discussion of genetic heterogeneity of progressive myoclonic epilepsy, see EPM1A (254800).

Neurodevelopmental disorder with microcephaly, epilepsy, and hypomyelination (NEDMEHM) is an autosomal recessive neurometabolic disorder characterized by these cardinal features. Patients also show an exaggerated startle reflex in early infancy (Rodan et al., 2018).

Developmental and epileptic encephalopathy-74 (DEE74) is neurologic disorder characterized by the onset of refractory seizures in the first months of life. Seizure types are variable and include infantile spasms, myoclonic, tonic, atonic, and absence, often with secondary generalization. Affected individuals have severe global developmental delay with hypotonia, severe motor impairment, roving eye movements, and absent language (summary by Shen et al., 2017). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

Glycosylphosphatidylinositol is a glycolipid that anchors more than 150 proteins to the cell surface, and these proteins, termed GPI-anchored proteins (GPI-APs), perform a variety of functions as enzymes, adhesion molecules, complement regulators, and coreceptors in signal transduction pathways. Reduced surface levels of GPI-APs or abnormal GPI-AP structure can therefore result in variable manifestations. Glycosylphosphatidylinositol biosynthesis defect-1 (GPIBD1) is characterized predominantly by portal hypertension due to portal vein thrombosis. Most patients have absence seizures, cerebral thrombosis, and macrocephaly. Some patients have mildly to moderately impaired intellectual development (summary by Makrythanasis et al., 2016; Pode-Shakked et al., 2019). Genetic Heterogeneity of Glycosylphosphatidylinositol Biosynthesis Defects Also see GPIBD2 (239300), caused by mutation in the PIGV gene (610274); GPIBD3 (614080), caused by mutation in the PIGN gene (606097); GPIBD4 (300868), caused by mutation in the PIGA gene (311770); GPIBD5 (280000), caused by mutation in the PIGL gene (605947); GPIBD6 (614749), caused by mutation in the PIGO gene (614730); GPIBD7 (615398), caused by mutation in the PIGT gene (610272); GPIBD8 (614207), caused by mutation in the PGAP2 gene (615187); GPIBD9 (615802), caused by mutation in the PGAP1 gene (611655); GPIBD10 (615716), caused by mutation in the PGAP3 gene (611801); GPIBD11 (616025), caused by mutation in the PIGW gene (610275); GPIBD12 (616809), caused by mutation in the PIGY gene (610662); GPIBD13 (616917), caused by mutation in the PIGG gene (616918); GPIBD14 (617599), caused by mutation in the PIGP gene (605938); GPIBD15 (617810), caused by mutation in the GPAA1 gene (603048); GPIBD16 (617816), caused by mutation in the PIGC gene (601730); GPIBD17 (618010), caused by mutation in the PIGH gene (600154); GPIBD18 (618143), caused by mutation in the PIGS gene (610271); GPIBD19 (618548), caused by mutation in the PIGQ gene (605754); GPIBD20 (618580), caused by mutation in the PIGB gene (604122); GPIBD21 (618590), caused by mutation in the PIGU gene (608528); GPIBD22 (618879), caused by mutation in the PIGK gene (605087); GPIBD23 (617020), caused by mutation in the ARV1 gene (611647); GPIBD24 (619356), caused by mutation in the PIGF gene (600153); and GPIBD25 (619985), caused by mutation in the C18ORF32 gene (619979).

Autosomal dominant intellectual developmental disorder-60 with seizures is characterized by global developmental delay apparent in infancy, followed by onset of seizures in the first years of life. Patients have delayed walking, an ataxic gait, and moderately to severely impaired intellectual development with poor speech (summary by Helbig et al., 2019).

Lissencephaly-10 (LIS10) is a neurologic disorder characterized by variably delayed development with mildly to moderately impaired intellectual development and language delay, as well as seizures, which are often intractable. There is a spectrum of severity, with some patients having normal early development and only borderline to mild cognitive impairment. Brain imaging shows features consistent with neuronal migration defects, including posterior-predominant lissencephaly, pachygyria, agyria, and subcortical band heterotopia (summary by Tsai et al., 2020). For a general description and a discussion of genetic heterogeneity of lissencephaly, see LIS1 (607432).

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

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

Developmental delay, impaired speech, and behavioral abnormalities, with or without seizures (DEDISB) is a neurodevelopmental disorder characterized by variably impaired skill acquisition apparent from infancy or early childhood. Affected individuals have predominant language delay with mild fine and gross motor deficits, although they usually are ambulatory by around 3 years of age. Most patients have mildly to moderately impaired intellectual development and behavioral abnormalities, including aggression, hyperactivity, and autism spectrum disorder. About half of individuals develop various types of seizures that may be refractory in some. More variable features include dysmorphic facial features, mild ocular anomalies, and nonspecific findings on brain imaging (Thomas et al., 2021).

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.

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

Developmental and epileptic encephalopathy-108 (DEE108) is characterized by the onset of multiple types of seizures in the first 2 years of life. Affected individuals often have normal early development before the onset of seizures, after which they show developmental regression with loss of skills, particularly language; most are nonverbal or speak only a few words. Other features included mildly delayed walking, unsteady gait, hypotonia, and behavioral abnormalities, such as ADHD or autism (Spinelli et al., 2021). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

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.

Early-onset epilepsy-3 with or without developmental delay (EPEO3) is an autosomal dominant neurologic disorder characterized by the onset of various types of seizures in the first months or years of life. Many patients present with febrile seizures and later develop afebrile seizures. The severity and disease course is highly variable: some affected individuals have global developmental delay or regression with impaired intellectual development, poor or absent speech, and motor delay, whereas others have normal psychomotor development. More severely affected individuals often show additional features, including hypotonia, gait ataxia, nonspecific dysmorphic features, behavioral abnormalities, and variable anomalies on brain imaging (Mattison et al., 2023, Zhao et al., 2023). For a discussion of genetic heterogeneity of EPEO, see 617290.

Developmental delay with or without epilepsy (DEVEP) is a clinically heterogeneous neurodevelopmental disorder characterized by motor delay, speech delay, and variably impaired intellectual development apparent from infancy or early childhood. Hypotonia and behavioral abnormalities are common. About half of affected individuals develop various types of seizures that are not as severe as observed in the allelic disorder DEE5. In general, the phenotype is similar to but milder than DEE5. Some individuals with DEVEP have ataxia or nystagmus associated with cerebellar atrophy on brain imaging, indicating phenotypic overlap with the allelic disorder SPG91 (Morsy et al., 2023). In a study of 31 individuals with SPTAN1 mutations, Morsy et al. (2023) delineated 3 distinct phenotypic subgroups: DEE5; a milder phenotype of developmental delay with or without seizures (DEVEP); and pure or complicated spastic paraplegia/ataxia (SPG91). Syrbe et al. (2017) similarly emphasized the remarkably broad phenotypic spectrum of neurologic disorders associated with heterozygous SPTAN1 mutations in their cohort study.