GABA-transaminase deficiency (GABATD) is characterized by neonatal or early infantile-onset encephalopathy, hypotonia, hypersomnolence, epilepsy, choreoathetosis, and accelerated linear growth. Electroencephalograms show burst-suppression, modified hypsarrhythmia, multifocal spikes, and generalized spike-wave. Severity varies, but most patients have profound developmental impairment and some patients die in infancy (summary by Koenig et al., 2017).

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.

A rare glycogen storage disease with fetal or neonatal onset of severe cardiomyopathy with non-lysosomal glycogen accumulation and fatal outcome in infancy. Patients present with massive cardiomegaly, severe cardiac and respiratory complications and failure to thrive. Non-specific facial dysmorphism, bilateral cataracts, macroglossia, hydrocephalus, enlarged kidneys and skeletal muscle involvement have been reported in some cases.

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

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.

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

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

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

Lethal neonatal rigidity and multifocal seizure syndrome (RMFSL) is a severe autosomal recessive epileptic encephalopathy characterized by onset of rigidity and intractable seizures at or soon after birth. Affected infants achieve no developmental milestones and die within the first months or years of life (summary by Saitsu et al., 2014).

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

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

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.

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

Lissencephaly-7 with cerebellar hypoplasia (LIS7) is a severe neurodevelopmental disorder characterized by lack of psychomotor development, facial dysmorphism, arthrogryposis, and early-onset intractable seizures resulting in death in infancy (Magen et al., 2015). For a general description and a discussion of genetic heterogeneity of lissencephaly, see LIS1 (607432).

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.

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

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.

COXPD33 is an autosomal recessive multisystem disorder resulting from a defect in mitochondrial energy metabolism. The phenotype is highly variable, ranging from death in infancy to adult-onset progressive external ophthalmoplegia (PEO) and myopathy. A common finding is cardiomyopathy and increased serum lactate (summary by Feichtinger et al., 2017). For a discussion of genetic heterogeneity of combined oxidative phosphorylation deficiency, see COXPD1 (609060).

Developmental and epileptic encephalopathy-60 (DEE60) is an autosomal recessive neurologic disorder characterized by the onset of infantile spasms, seizures, or myoclonus in the first months of life. EEG typically shows hypsarrhythmia, consistent with a clinical diagnosis of West syndrome. Affected individuals have severe global developmental delay with inability to sit, walk, or speak. Brain imaging may show brain atrophy and hippocampal malrotation (summary by Mutoh et al., 2018). For a discussion of genetic heterogeneity of DEE, see 308350.

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

Lethal congenital contracture syndrome-5 (LCCS5) is an autosomal recessive disorder characterized by decreased fetal movements, joint contractures, hypotonia, skeletal abnormalities with thin bones, and brain and retinal hemorrhages (Koutsopoulos et al., 2013). For a general phenotypic description and a discussion of genetic heterogeneity of LCCS, see LCCS1 (253310).

Developmental and epileptic encephalopathy-95 (DEE95) is a severe autosomal recessive disorder characterized by severely impaired global development, hypotonia, weakness, ataxia, coarse facial features, and intractable seizures. More variable features may include abnormalities of the hands and feet, inguinal hernia, and feeding difficulties. The disorder is part of a group of similar neurologic disorders resulting from biochemical defects in the glycosylphosphatidylinositol (GPI) biosynthetic pathway (summary by Nguyen et al., 2018). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350. For a discussion of genetic heterogeneity of GPI biosynthesis defects, see GPIBD1 (610293).

Developmental and epileptic encephalopathy-71 (DEE71) is characterized by early neonatal refractory seizures, respiratory failure, structural brain abnormalities and cerebral edema, with death within weeks after birth. Glutamine levels are significantly increased (z score 3.2-11.7). Three patients have been described (summary by Rumping et al., 2019).

Developmental and epileptic encephalopathy-81 (DEE81) is an autosomal recessive neurodevelopmental disorder typically characterized by onset of severe refractory seizures soon after birth or in the first months of life. Affected individuals show little developmental progress with no eye contact and no motor or cognitive development. Other features may include facial dysmorphism, such as hypotonic facies and epicanthal folds, as well as sensorineural hearing loss and peripheral neuropathy. Brain imaging shows cerebral atrophy, impaired myelination, thin corpus callosum, and progressive leukoencephalopathy (summary by Esposito et al., 2019; Maddirevula et al., 2019). For a discussion of genetic heterogeneity of DEE, see 308350.

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

Myopathy, epilepsy, and progressive cerebral atrophy (MEPCA) is a severe autosomal recessive disorder with onset in utero or at birth. Affected individuals have hypotonia with respiratory or feeding difficulties apparent from birth and often associated with contractures of the large joints. There is little spontaneous movement: skeletal muscle biopsy and electrophysiologic studies are consistent with a myopathy or myasthenic disorder. Patients also develop refractory seizures with burst-suppression pattern or hypsarrhythmia on EEG. Brain imaging shows progressive cerebral atrophy and myelination defects. All patients reported to date died within the first year of life (summary by Schorling et al., 2017).

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

Developmental and epileptic encephalopathy-90 (DEE90) is an X-linked neurologic disorder characterized by onset of refractory seizures in the first days or months of life. Although most patients have focal seizures associated with oromotor automatisms and apnea, various seizure types may occur, including epileptic spasms, generalized tonic-clonic, and absence. EEG shows multifocal discharges; hypsarrhythmia, intermittent burst suppression, and slow spike-wave background resembling Lennox-Gastaut syndrome may also be observed. Affected individuals have global developmental delay with variable severity, but it is usually profound or severe. Some are unable to walk or speak, whereas others may achieve some milestones and show autistic features (summary by Fry et al., 2021). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

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. 

Pontocerebellar hypoplasia type 1E (PCH1E) is an autosomal recessive neurologic disorder characterized by severe hypotonia and respiratory insufficiency apparent soon after birth. Virtually all patients die in the first days or weeks of life. Postmortem examination and brain imaging show pontocerebellar atrophy and loss of anterior motor neurons in the spinal cord. Additional more variable features may include optic atrophy, peripheral neuropathy, dysmorphic features, congenital contracture or foot deformities, and seizures (summary by Braunisch et al., 2018). For a phenotypic description and a discussion of genetic heterogeneity of PCH, see PCH1A (607596).

Developmental and epileptic encephalopathy-96 (DEE96) is characterized by onset of seizures in the first days or weeks of life. Affected infants have tonic or myoclonic seizures associated with burst-suppression pattern on EEG. They also have hypotonia with respiratory insufficiency that may result in premature death. Those that survive have profound developmental delay and persistent seizures (summary by Suzuki et al., 2019). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

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-3 (DEE3) is an autosomal recessive neurologic disorder characterized by onset of refractory seizures in the first weeks to months of life. The prognosis is poor, and affected children either may die within 1 to 2 years after birth or survive in a persistent vegetative state. The EEG pattern often shows a suppression-burst pattern with high-voltage bursts of slow waves mixed with multifocal spikes alternating with isoelectric suppression phases; these features are reminiscent of a clinical diagnosis of Ohtahara syndrome. Some patients may have hypsarrhythmia on EEG, consistent with a clinical diagnosis of West syndrome (summary by Molinari et al., 2005, Molinari et al., 2009). For a discussion of genetic heterogeneity of DEE, see 308350.

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 epilepsy and brain atrophy (NEDEBA) is an autosomal recessive disorder characterized by early-onset progressive myoclonus epilepsy with ataxia (summary by Bott et al., 2021).

Complex cortical dysplasia with other brain malformations-12 (CDCBM12) is an autosomal recessive disorder of developmental neuronal migration characterized by severe to profound neurodevelopmental delay with absent speech, central hypotonia, peripheral spasticity, cortical visual impairment, and dysmorphic craniofacial features. Affected individuals usually have feeding difficulties and show minimal developmental progress of motor or cognitive skills. Most have microcephaly and develop early-onset refractory seizures. Brain imaging shows cortical abnormalities, such as lissencephaly and pachygyria, as well as other brain malformations, including thin or absent corpus callosum, dysplastic basal ganglia, and mild cerebellar hypoplasia. Due to the function of CAMSAP1 in microtubule stability and maintenance, this disorder can be classified as a 'tubulinopathy' (Khalaf-Nazzal et al., 2022). For a discussion of genetic heterogeneity of CDCBM, see CDCBM1 (614039).

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).

Mitochondrial dysfunctions syndrome-7 (MMDS7) is an autosomal recessive disorder characterized by a clinical spectrum ranging from neonatal fatal glycine encephalopathy to an attenuated phenotype of developmental delay, behavioral problems, limited epilepsy, and variable movement problems (Arribas-Carreira et al., 2023). For a general description and a discussion of genetic heterogeneity of multiple mitochondrial dysfunctions syndrome, see MMDS1 (605711).