N-acetylglutamate synthase deficiency (NAGSD) is an autosomal recessive disorder of the urea cycle. The clinical and biochemical features of the disorder are indistinguishable from carbamoyl phosphate synthase I deficiency (237300), since the CPS1 enzyme (608307) has an absolute requirement for NAGS (Caldovic et al., 2007).

Pyridoxine-dependent epilepsy – ALDH7A1 (PDE-ALDH7A1) is characterized by seizures not well controlled with anti-seizure medication that are responsive clinically and electrographically to large daily supplements of pyridoxine (vitamin B6). This is true across a phenotypic spectrum that ranges from classic to atypical PDE-ALDH7A1. Intellectual disability is common, particularly in classic PDE-ALDH7A1. Classic PDE-ALDH7A1. Untreated seizures begin within the first weeks to months of life. Dramatic presentations of prolonged seizures and recurrent episodes of status epilepticus are typical; recurrent self-limited events including partial seizures, generalized seizures, atonic seizures, myoclonic events, and infantile spasms also occur. Electrographic seizures can occur without clinical correlates. Atypical PDE-ALDH7A1. Findings in untreated individuals can include late-onset seizures beginning between late infancy and age three years, seizures that initially respond to anti-seizure medication and then become intractable, seizures during early life that do not respond to pyridoxine but are subsequently controlled with pyridoxine several months later, and prolonged seizure-free intervals (=5 months) that occur after discontinuation of pyridoxine.

Any Gaucher disease in which the cause of the disease is a mutation in the PSAP gene.

Thiamine metabolism dysfunction syndrome-5 (THMD5) is an autosomal recessive metabolic disorder due to an inborn error of thiamine metabolism. The phenotype is highly variable, but in general, affected individuals have onset in early childhood of acute encephalopathic episodes associated with increased serum and CSF lactate. These episodes result in progressive neurologic dysfunction manifest as gait disturbances, ataxia, dystonia, and spasticity, which in some cases may result in loss of ability to walk. Cognitive function is usually preserved, although mildly delayed development has been reported. These episodes are usually associated with infection and metabolic decompensation. Some patients may have recovery of some neurologic deficits (Mayr et al., 2011). For a discussion of genetic heterogeneity of disorders due to thiamine metabolism dysfunction, see THMD1 (249270).

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.

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

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

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

Developmental and epileptic encephalopathy-24 (DEE24) is a neurologic disorder characterized by onset of refractory seizures in infancy, severely impaired global development, intellectual disability, and behavioral abnormalities. Most patients have onset of variable types of seizures between 4 and 13 months of age, but earlier onset in the first days of life has also been reported. Seizures are often triggered by fever, at least initially; status epilepticus may occur (summary by Nava et al., 2014 and Marini et al., 2018). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

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

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

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

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

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

Developmental and epileptic encephalopathy-66 (DEE66) is a neurologic disorder characterized by the onset of various types of seizures in the first days or weeks of life. Most seizures have focal origins; secondary generalization is common. Seizure control is difficult at first, but may become easier with time. Affected individuals show global developmental delay with hypotonia, behavioral abnormalities, and dysmorphic features or ophthalmologic defects. Brain imaging often shows cerebellar dysgenesis. A subset of patients have extraneurologic manifestations, including hematologic and distal limb abnormalities (summary by Olson et al., 2018). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

Neurodevelopmental disorder with speech impairment and behavioral abnormalities (NEDLIB) is characterized by impaired intellectual development or developmental delay, behavioral abnormalities including autistic features, and language impairment. Other features include seizures and developmental regression (Salpietro et al., 2019).

Episodic ataxia type 9 (EA9) is a neurologic disorder characterized by onset of ataxic episodes in the first years of life. Features may include difficulty walking, dizziness, slurred speech, headache, vomiting, and pain. The ataxic episodes vary in frequency and duration; most tend to occur every few weeks or months and last minutes to hours. Prior to the EA, most patients have neonatal- or infantile-onset tonic or generalized tonic-clonic (GTC) seizures that may be severe and refractory to medication, but remit later in infancy or early childhood, either spontaneously or concurrently with medication. Some patients have mildly delayed development with speech delay and/or autistic features or mildly impaired intellectual development. However, others show normal psychomotor development. Treatment of the ataxic episodes with acetazolamide is effective in about 50% of patients (summary by Schwarz et al., 2019). For a phenotypic description and discussion of genetic heterogeneity of episodic ataxia, see EA1 (160120).

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

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

Developmental and epileptic encephalopathy-98 (DEE98) is characterized by onset of seizures in the first decade (range infancy to late childhood) associated with variable global developmental delay. Other features may include hypotonia, spasticity, and quadriparesis. Brain imaging may be normal or show nonspecific and variable abnormalities, including polymicrogyria. The severity is variable; some patients may die of refractory status epilepticus (summary by Vetro et al., 2021). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

Developmental and epileptic encephalopathy-104 (DEE104) is an autosomal dominant disorder characterized by developmental delay in the first few months of life and drug-resistant focal and generalized tonic-clonic seizures (summary by Bott et al., 2021). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

Developmental and epileptic encephalopathy-107 (DEE107) is a severe autosomal recessive neurologic disorder characterized by onset of seizures in the first months of life and severe global developmental delay, profound intellectual disability, progressive microcephaly, and hypotonia (Conroy et al., 2016). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

Mitochondrial complex IV deficiency nuclear type 23 (MC4DN23) is an autosomal recessive disorder characterized by infantile-onset encephalopathy (Rius et al., 2022). For a discussion of genetic heterogeneity of mitochondrial complex IV (cytochrome c oxidase) deficiency, see 220110.

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