MERRF (myoclonic epilepsy with ragged red fibers) is a multisystem disorder characterized by myoclonus (often the first symptom) followed by generalized epilepsy, ataxia, weakness, exercise intolerance, and dementia. Onset can occur from childhood to adulthood, occurring after normal early development. Common findings are ptosis, hearing loss, short stature, optic atrophy, cardiomyopathy, cardiac dysrhythmias such as Wolff-Parkinson-White syndrome, and peripheral neuropathy. Pigmentary retinopathy, optic neuropathy, diabetes mellitus, and lipomatosis have been observed.

Gaucher disease (GD) encompasses a continuum of clinical findings from a perinatal lethal disorder to an asymptomatic type. The identification of three major clinical types (1, 2, and 3) and two other subtypes (perinatal-lethal and cardiovascular) is useful in determining prognosis and management. GD type 1 is characterized by the presence of clinical or radiographic evidence of bone disease (osteopenia, focal lytic or sclerotic lesions, and osteonecrosis), hepatosplenomegaly, anemia and thrombocytopenia, lung disease, and the absence of primary central nervous system disease. GD types 2 and 3 are characterized by the presence of primary neurologic disease; in the past, they were distinguished by age of onset and rate of disease progression, but these distinctions are not absolute. Disease with onset before age two years, limited psychomotor development, and a rapidly progressive course with death by age two to four years is classified as GD type 2. Individuals with GD type 3 may have onset before age two years, but often have a more slowly progressive course, with survival into the third or fourth decade. The perinatal-lethal form is associated with ichthyosiform or collodion skin abnormalities or with nonimmune hydrops fetalis. The cardiovascular form is characterized by calcification of the aortic and mitral valves, mild splenomegaly, corneal opacities, and supranuclear ophthalmoplegia. Cardiopulmonary complications have been described with all the clinical subtypes, although varying in frequency and severity.

GLB1-related disorders comprise two phenotypically distinct lysosomal storage disorders: GM1 gangliosidosis and mucopolysaccharidosis type IVB (MPS IVB). The phenotype of GM1 gangliosidosis constitutes a spectrum ranging from severe (infantile) to intermediate (late-infantile and juvenile) to mild (chronic/adult). Type I (infantile) GM1 gangliosidosis begins before age 12 months. Prenatal manifestations may include nonimmune hydrops fetalis, intrauterine growth restriction, and placental vacuolization; congenital dermal melanocytosis (Mongolian spots) may be observed. Macular cherry-red spot is detected on eye exam. Progressive central nervous system dysfunction leads to spasticity and rapid regression; blindness, deafness, decerebrate rigidity, seizures, feeding difficulties, and oral secretions are observed. Life expectancy is two to three years. Type II can be subdivided into the late-infantile (onset age 1-3 years) and juvenile (onset age 3-10 years) phenotypes. Central nervous system dysfunction manifests as progressive cognitive, motor, and speech decline as measured by psychometric testing. There may be mild corneal clouding, hepatosplenomegaly, and/or cardiomyopathy; the typical course is characterized by progressive neurologic decline, progressive skeletal disease in some individuals (including kyphosis and avascular necrosis of the femoral heads), and progressive feeding difficulties leading to aspiration risk. Type III begins in late childhood to the third decade with generalized dystonia leading to unsteady gait and speech disturbance followed by extrapyramidal signs including akinetic-rigid parkinsonism. Cardiomyopathy develops in some and skeletal involvement occurs in most. Intellectual impairment is common late in the disease with prognosis directly related to the degree of neurologic impairment. MPS IVB is characterized by skeletal dysplasia with specific findings of axial and appendicular dysostosis multiplex, short stature (below 15th centile in adults), kyphoscoliosis, coxa/genu valga, joint laxity, platyspondyly, and odontoid hypoplasia. First signs and symptoms may be apparent at birth. Bony involvement is progressive, with more than 84% of adults requiring ambulation aids; life span does not appear to be limited. Corneal clouding is detected in some individuals and cardiac valvular disease may develop.

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.

Succinic semialdehyde dehydrogenase (SSADH) deficiency is characterized by infantile-onset hypotonia, developmental delay, cognitive impairment, expressive language deficit, and mild ataxia. Epilepsy is present in about half of affected individuals and is more common in adults. Hyperkinetic behavior, aggression, self-injurious behaviors, hallucinations, and sleep disturbances have been reported in nearly half of all affected individuals, more commonly in those who are older. Basal ganglia signs including choreoathetosis, dystonia, and myoclonus have been reported in a few individuals with earlier-onset, more severe disease. Involvement beyond the central nervous system has not been described. Individuals with SSADH deficiency typically have 4-hydroxybutyric aciduria present on urine organic acid analysis. Head MRI reveals T2 hyperintensities in multiple regions, involving the globus pallidi, cerebellar dentate nuclei, subthalamic nuclei, subcortical white matter, and brain stem, as well as cerebral and sometimes cerebellar atrophy. EEG findings include background slowing and spike discharges that are usually generalized.

A rare disorder of histidine metabolism characterized by histidinuria without histidinemia due to impaired intestinal and renal tubular absorption of histidine. Developmental delay, intellectual disability, seizures, and mild dysmorphic features have been reported in association. There have been no further descriptions in the literature since 1992.

Idiopathic generalized epilepsy is a broad term that encompasses several common seizure phenotypes, classically including childhood absence epilepsy (CAE, ECA; see 600131), juvenile absence epilepsy (JAE, EJA; see 607631), juvenile myoclonic epilepsy (JME, EJM; see 254770), and epilepsy with grand mal seizures on awakening (Commission on Classification and Terminology of the International League Against Epilepsy, 1989). These recurrent seizures occur in the absence of detectable brain lesions and/or metabolic abnormalities. Seizures are initially generalized with a bilateral, synchronous, generalized, symmetrical EEG discharge (Zara et al., 1995; Lu and Wang, 2009). See also childhood absence epilepsy (ECA1; 600131), which has also been mapped to 8q24. Of note, benign neonatal epilepsy 2 (EBN2; 121201) is caused by mutation in the KCNQ3 gene (602232) on 8q24. Genetic Heterogeneity of Idiopathic Generalized Epilepsy EIG1 has been mapped to chromosome 8q24. Other loci or genes associated with EIG include EIG2 (606972) on 14q23; EIG3 (608762) on 9q32; EIG4 (609750) on 10q25; EIG5 (611934) on 10p11; EIG6 (611942), caused by mutation in the CACNA1H gene (607904) on 16p; EIG7 (604827) on 15q14; EIG8 (612899), caused by mutation in the CASR gene (601199) on 3q13.3-q21; EIG9 (607682), caused by mutation in the CACNB4 gene (601949) on 2q23; EIG10 (613060), caused by mutation in the GABRD gene (137163) on 1p36; EIG11 (607628), caused by variation in the CLCN2 gene (600570) on 3q36; EIG12 (614847), caused by mutation in the SLC2A1 gene (138140) on 1p34; EIG13 (611136), caused by mutation in the GABRA1 gene (137160) on 5q34; EIG14 (616685), caused by mutation in the SLC12A5 gene (606726) on 20q12; EIG15 (618357), caused by mutation in the RORB gene (601972) on 9q22; EIG16 (618596), caused by mutation in the KCNMA1 gene (600150) on chromosome 10q22; EIG17 (602477), caused by mutation in the HCN2 gene (602781) on chromosome 19p13.3; and EIG18 (619521) caused by mutation in the HCN4 gene (605206) on chromosome 15q24.

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.

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.

Mental retardation, microcephaly, spastic diplegia, seizures, and mild aminoaciduria are the principal features. Seemanova syndrome 1 and Paine syndrome are considered as variants of the same entity termed Paine-Seemanova syndrome.

Familial adult myoclonic epilepsy-1 (FAME1), also known as familial cortical myoclonic tremor associated with epilepsy-1 (FCMTE1), is characterized by autosomal dominant, adult-onset cortical myoclonus, with seizures in 40% of patients. Myoclonus is usually the first symptom and is characterized by tremulous finger movements and myoclonus of the extremities (summary by Depienne et al., 2010). FAME1 tends to occur in patients of southern Asian descent (summary by Bennett et al., 2020). Genetic Heterogeneity of Familial Adult Myoclonic Epilepsy See also FAME2 (607876), caused by mutation in the STARD7 gene (616712) on chromosome 2q11; FAME3 (613608), caused by mutation in the MARCHF6 gene (613297) on chromosome 5p15; FAME4 (615127), which maps to chromosome 3q26.32-q28; FAME5 (615400), caused by mutation in the CNTN2 gene (190197) on chromosome 1q32; FAME6 (618074), caused by mutation in the TNRC6A gene (610739) on chromosome 16p12; and FAME7 (618075), caused by mutation in the RAPGEF2 gene (609530) on chromosome 4. Progressive myoclonic epilepsy is a more severe disorder (see, e.g., EPM1, 254800).

The spectrum of ASAH1-related disorders ranges from Farber disease (FD) to spinal muscular atrophy with progressive myoclonic epilepsy (SMA-PME). Classic FD is characterized by onset in the first weeks of life of painful, progressive deformity of the major joints; palpable subcutaneous nodules of joints and mechanical pressure points; and a hoarse cry resulting from granulomas of the larynx and epiglottis. Life expectancy is usually less than two years. In the other less common types of FD, onset, severity, and primary manifestations vary. SMA-PME is characterized by early-childhood-onset progressive lower motor neuron disease manifest typically between ages three and seven years as proximal lower-extremity weakness, followed by progressive myoclonic and atonic seizures, tremulousness/tremor, and sensorineural hearing loss. Myoclonic epilepsy typically begins in late childhood after the onset of weakness and can include jerking of the upper limbs, action myoclonus, myoclonic status, and eyelid myoclonus. Other findings include generalized tremor, and cognitive decline. The time from disease onset to death from respiratory complications is usually five to 15 years.

The neuronal ceroid lipofuscinoses (NCL, or 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 (summary by Mole et al., 2005). For a general phenotypic description and a discussion of genetic heterogeneity of CLN, see CLN1 (256730).

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.

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.

Mosaic variegated aneuploidy (MVA) syndrome is a rare disorder in which some cells in the body have an abnormal number of chromosomes instead of the usual 46 chromosomes, a situation known as aneuploidy. Most commonly, cells have an extra chromosome, which is called trisomy, or are missing a chromosome, which is known as monosomy. In MVA syndrome, some cells are aneuploid and others have the normal number of chromosomes, which is a phenomenon known as mosaicism. Typically, at least one-quarter of cells in affected individuals have an abnormal number of chromosomes. Because the additional or missing chromosomes vary among the abnormal cells, the aneuploidy is described as variegated.\n\nThere are at least three types of MVA syndrome, each with a different genetic cause. Type 1 is the most common and displays the classic signs and symptoms described above. Type 2 appears to have slightly different signs and symptoms than type 1, although the small number of affected individuals makes it difficult to define its characteristic features. Individuals with MVA syndrome type 2 grow slowly before and after birth; however, their head size is typically normal. Some people with MVA syndrome type 2 have unusually short arms. Individuals with MVA syndrome type 2 do not seem to have an increased risk of cancer. Another form of MVA syndrome is characterized by a high risk of developing Wilms tumor. Individuals with this form may also have other signs and symptoms typical of MVA syndrome type 1.\n\nIn MVA syndrome, growth before birth is slow (intrauterine growth restriction). After birth, affected individuals continue to grow at a slow rate and are shorter than average. In addition, they typically have an unusually small head size (microcephaly). Another common feature of MVA syndrome is an increased risk of developing cancer in childhood. Cancers that occur most frequently in affected individuals include a cancer of muscle tissue called rhabdomyosarcoma, a form of kidney cancer known as Wilms tumor, and a cancer of the blood-forming tissue known as leukemia.\n\nLess commonly, people with MVA syndrome have eye abnormalities or distinctive facial features, such as a broad nasal bridge and low-set ears. Some affected individuals have brain abnormalities, the most common of which is called Dandy-Walker malformation. Intellectual disability, seizures, and other health problems can also occur in people with MVA syndrome.

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).

Molybdenum cofactor deficiency (MoCD) represents a spectrum, with some individuals experiencing significant signs and symptoms in the neonatal period and early infancy (termed early-onset or severe MoCD) and others developing signs and symptoms in childhood or adulthood (termed late-onset or mild MoCD). Individuals with early-onset MoCD typically present in the first days of life with severe encephalopathy, including refractory seizures, opisthotonos, axial and appendicular hypotonia, feeding difficulties, and apnea. Head imaging may demonstrate loss of gray and white matter differentiation, gyral swelling, sulci injury (typically assessed by evaluating the depth of focal lesional injury within the sulci), diffusely elevated T2-weighted signal, and panlobar diffusion restriction throughout the forebrain and midbrain with relative sparring of the brain stem. Prognosis for early-onset MoCD is poor, with about 75% succumbing in infancy to secondary complications of their neurologic disability (i.e., pneumonia). Late-onset MoCD is typically characterized by milder symptoms, such as acute neurologic decompensation in the setting of infection. Episodes vary in nature but commonly consist of altered mental status, dystonia, choreoathetosis, ataxia, nystagmus, and fluctuating hypotonia and hypertonia. These features may improve after resolution of the inciting infection or progress in a gradual or stochastic manner over the lifetime. Brain imaging may be normal or may demonstrate T2-weighted hyperintense or cystic lesions in the globus pallidus, thinning of the corpus callosum, and cerebellar atrophy.

The presence of an elevated amount of 5-hydroxylysine in the urine. This compound is a hydroxylated derivative of the amino acid lysine that is present in certain collagens.

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).

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.

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

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.

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.

Roifman-Chitayat syndrome (ROCHIS) is an autosomal recessive digenic disorder characterized by global developmental delay, variable neurologic features such as seizures, ataxia, and optic atrophy, dysmorphic facial features, distal skeletal anomalies, and combined immunodeficiency manifest as recurrent infections (summary by Sharfe et al., 2018).

Juvenile absence epilepsy is a subtype of idiopathic generalized epilepsy (IGE; see 600669). Manifestations occur around puberty, in contrast to childhood absence epilepsy (CAE; 600131), which begins at age 6 to 7 years. Absence seizures, generalized tonic-clonic seizures (GTCS), GTCS on awakening, and myoclonic seizures are the main features of JAE. (Commission on Classification and Terminology of the International League Against Epilepsy, 1989). Genetic Heterogeneity of Juvenile Absence Epilepsy See also susceptibility to juvenile absence epilepsy-2 (EJA2; see 607628), conferred by variation in the CLCN2 gene (600570) on chromosome 3q26.

Both juvenile myoclonic epilepsy and juvenile absence epilepsy are subtypes of idiopathic generalized epilepsy (EIG). For a general phenotypic description and a discussion of genetic heterogeneity of these disorders, see EIG1 (600669), EJM1 (254770), and EJA1 (607631).

Idiopathic generalized epilepsy (EIG) is a broad term that encompasses several common seizure phenotypes, classically including childhood absence epilepsy (CAE, ECA), juvenile absence epilepsy (JAE), and juvenile myoclonic epilepsy (JME, EJM) (Commission on Classification and Terminology of the International League Against Epilepsy, 1989). Generalized epilepsy with febrile seizures plus (GEFS+) shows phenotypic overlap with IGE, and includes patients with early-onset febrile seizures who later develop various types of febrile and afebrile seizures, such as those observed in EIG (summary by Singh et al., 1999). For a general phenotypic description and a discussion of genetic heterogeneity of EIG, see 600669. For a general phenotypic description and a discussion of genetic heterogeneity of GEFS+, see 604233. For a general phenotypic description and a discussion of genetic heterogeneity of EJM, see 254770.

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.

ISQMR is a severe autosomal recessive disorder characterized by ichthyosis apparent from birth, profound psychomotor retardation with essentially no development, spastic quadriplegia, and seizures (summary by Aldahmesh et al., 2011).

Spastic ataxia-5 (SPAX5) is an autosomal recessive neurodegenerative disorder characterized by early-onset spasticity resulting in significantly impaired ambulation, cerebellar ataxia, oculomotor apraxia, dystonia, and myoclonic epilepsy (summary by Pierson et al., 2011). For a discussion of genetic heterogeneity of spastic ataxia, see SPAX1 (108600).

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 (

A rare inborn error of metabolism characterized by low serum carnosinase activity, persistent carnosinuria, and carnosinemia. The clinical phenotype is highly variable, with some patients remaining asymptomatic, while others have been reported to show severe developmental delay, intellectual disability, hypotonia, seizures, and other neurological signs and symptoms.

Neuronal ceroid lipofuscinosis-11 is an autosomal recessive neurologic disorder characterized by rapidly progressive visual loss due to retinal dystrophy, seizures, cerebellar ataxia, and cerebellar atrophy. Cognitive decline may also occur (summary by Smith et al., 2012). For a general phenotypic description and a discussion of genetic heterogeneity of CLN, see CLN1 (256730).

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.

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

Progressive myoclonic epilepsy-10 (EPM10) is an autosomal recessive neurodegenerative disorder characterized by onset of progressive myoclonus, ataxia, spasticity, dysarthria, and cognitive decline in the first decade of life. The severity is variable, but some patients may become mute and bedridden with psychosis (summary by Turnbull et al., 2012). For a general phenotypic description and a discussion of genetic heterogeneity of progressive myoclonic epilepsy, see EPM1A (254800).

A rare genetic neurological disorder with characteristics of childhood-onset severe myoclonic and tonic-clonic seizures and early-onset ataxia leading to severe gait disturbances associated with normal to slightly diminished cognition. Scoliosis, diffuse muscle atrophy and subcutaneous fat loss, as well as developmental delay, may be associated. Brain MRI may reveal complete agenesis of the corpus callosum, ventriculomegaly, interhemispheric cysts and simplified gyration (frontally).

Developmental and epileptic encephalopathy-30 (DEE30) is a severe neurologic disorder characterized by onset of refractory seizures soon after birth or in the first months of life. Seizure types include early myoclonic encephalopathy (EME), Ohtahara syndrome, and infantile spasms; most are refractory to treatment. Patients with earlier seizure onset make essentially no developmental progress and may die in infancy. Those with later onset show profoundly impaired global development with absent speech, poor eye contact, inability to walk, behavioral abnormalities, and feeding difficulties that may require a feeding tube (summary by Hansen et al., 2015). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

Any autosomal recessive non-syndromic intellectual disability in which the cause of the disease is a mutation in the MBOAT7 gene.

Developmental and epileptic encephalopathy-52 (DEE52) is a severe autosomal recessive seizure disorder characterized by infantile onset of refractory seizures with resultant delayed global neurologic development. Affected individuals have impaired intellectual development and may have other persistent neurologic abnormalities, including axial hypotonia and spasticity; death in childhood may occur (summary by Patino et al., 2009 and Ramadan et al., 2017). Some patients with DEE52 may have a clinical diagnosis of Dravet syndrome (607208), which is characterized by the onset of seizures in the first year or 2 of life after normal early development. Developmental delay, impaired intellectual development, and behavioral abnormalities usually become apparent later between 1 and 4 years of age. Dravet syndrome may also include 'severe myoclonic epilepsy in infancy' (SMEI) (summary by Patino et al., 2009). For a discussion of genetic heterogeneity of DEE, see 308350.

Diencephalic-mesencephalic junction dysplasia syndrome-1 (DMJDS1) is an autosomal recessive neurodevelopmental disorder characterized by progressive microcephaly, severely delayed or even absent psychomotor development with profound intellectual disability, and spasticity or dystonia. Some patients may have seizures and/or visual impairment. Brain imaging shows a characteristic developmental malformation of the midbrain; subtle intracranial calcifications may also be present (summary by Aran et al., 2016 and Guemez-Gamboa et al., 2018). Genetic Heterogeneity of Diencephalic-Mesencephalic Junction Dysplasia Syndrome See also DMJDS2 (618646), caused by mutation in the GSX2 gene (616253) on chromosome 4q12.

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

DEDSM is a neurodevelopmental disorder characterized by global developmental delay, variable intellectual disability, and early-onset seizures with a myoclonic component. Most patients have delayed motor development and show abnormal movements, including ataxia, dystonia, and tremor (summary by Hamdan et al., 2017).

Combined oxidative phosphorylation deficiency-35 (COXPD35) is an autosomal recessive disorder characterized mainly by global developmental delay with intellectual disability, microcephaly, and early-onset myoclonic and other types of seizures. Affected individuals have variable deficiencies of mitochondrial respiratory enzyme complexes resulting from a defect in mitochondrial metabolism (summary by Kernohan et al., 2017). For a discussion of genetic heterogeneity of combined oxidative phosphorylation deficiency, see COXPD1 (609060).

Juvenile myoclonic epilepsy-10 is an autosomal dominant seizure disorder with variable manifestations, even within families. Affected individuals have febrile, myoclonic, tonic-clonic, or absence seizures, although several seizure types can occur in the same individual. The age of onset also shows great variability: some patients present in the first years of life, whereas other have onset of seizures in teenage years. EEG typically shows 3.5 to 5 Hz polyspike wave discharges. There is evidence of incomplete penetrance (summary by Bailey et al., 2018). For a general phenotypic description and a discussion of genetic heterogeneity of juvenile myoclonic epilepsy, see 254770.

Developmental and epileptic encephalopathy-63 (DEE63) is an autosomal recessive neurologic disorder characterized by early-onset refractory infantile spasms and myoclonic seizures in the first months to years of life. Affected individuals have severe to profound developmental delay, often with hypotonia and inability to sit or speak (summary by Redler et al., 2017). For a discussion of genetic heterogeneity of DEE, see 308350.

Glycosylphosphatidylinositol biosynthesis defect-17 (GPIBD17) is an autosomal recessive disorder characterized by variable neurologic deficits that become apparent in infancy or early childhood. Patients may present with early-onset febrile or afebrile seizures that tend to be mild or controllable. Other features may include learning disabilities, autism, behavioral abnormalities, hypotonia, and motor deficits. The phenotype is relatively mild compared to that of other GPIBDs (summary by Nguyen et al., 2018). 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.

Developmental and epileptic encephalopathy-2 (DEE2) is an X-linked dominant severe neurologic disorder characterized by onset of seizures in the first months of life and severe global developmental delay resulting in impaired intellectual development and poor motor control. Other features include lack of speech development, subtle dysmorphic facial features, sleep disturbances, gastrointestinal problems, and stereotypic hand movements. There is some phenotypic overlap with Rett syndrome (312750), but DEE2 is considered to be a distinct entity (summary by Fehr et al., 2013). For 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-102 (DEE102) is an autosomal recessive neurodevelopmental disorder characterized by global developmental delay and severe to profoundly impaired intellectual development with inability to walk or speak. Most patients have onset of variable types of seizures within the first year of life, and the seizures tend to be refractory. Additional features include progressive microcephaly, visual impairment, axial hypotonia, peripheral hypertonia, and nonspecific brain imaging abnormalities (Marafi et al., 2022). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.