Progressive myoclonic epilepsy type 1(EPM1) is a neurodegenerative disorder characterized by onset from age six to 15 years, stimulus-sensitive myoclonus, and tonic-clonic epileptic seizures. Some years after the onset, ataxia, incoordination, intentional tremor, and dysarthria develop. Individuals with EPM1 are cognitively mostly within the normal range, but show emotional lability and depression. The epileptic seizures are usually well controlled by anti-seizure medication, but the myoclonic jerks are progressive, action activated, and treatment resistant, and can be severely disabling.

Childhood absence epilepsy (CAE, ECA), a subtype of idiopathic generalized epilepsy (EIG; 600669), is characterized by a sudden and brief impairment of consciousness that is accompanied by a generalized, synchronous, bilateral, 2.5- to 4-Hz spike and slow-wave discharge (SWD) on EEG. Seizure onset occurs between 3 and 8 years of age and seizures generally occur multiple times per day. About 70% of patients experience spontaneous remission of seizures, often around adolescence. There are no structural neuropathologic findings in patients with ECA (Crunelli and Leresche, 2002). Genetic Heterogeneity of Susceptibility to Childhood Absence Epilepsy The ECA1 locus has been mapped to chromosome 8q24; see also EIG1 (see 600669), which also maps to 8q24. Susceptibility to the development of childhood absence epilepsy may be conferred by variation in several genes: ECA2 (see 607681), conferred by variation in the GABRG2 gene (137164) on chromosome 5q31.1; ECA4 (611136), conferred by variation in the GABRA1 gene (137160) on chromosome 5q34; ECA5 (612269), conferred by variation in the GABRB3 gene (137192) on chromosome 15q12; and ECA6 (see 611942), conferred by variation in the CACNA1H gene (607904) on chromosome 16p13. See EIG11 (607628) for discussion of a locus previously designated ECA3 on chromosome 3q26.

Mutations in the GABRG2 gene cause a spectrum of seizure disorders, ranging from early-onset isolated febrile seizures (FS) to childhood absence epilepsy (CAE) to generalized epilepsy with febrile seizures plus, type 3 (GEFS+3), which tends to represent a more severe phenotype. Patients with isolated febrile seizures usually have onset in the first year of life and show spontaneous remission by age 6 years. Many of these patients may later develop absence seizures, which may also spontaneously remit, whereas a few may continue to have various types of febrile and afebrile seizures that persist beyond childhood, consistent with GEFS+. There is phenotypic variability in the seizure type, even within a family carrying the same mutation, suggesting that other loci may be involved (summary by Singh et al., 1999 and Marini et al., 2003). For a phenotypic description and a discussion of genetic heterogeneity of familial febrile seizures, see 121210. For a general phenotypic description and a discussion of genetic heterogeneity of GEFS+, see 604233. For a phenotypic description and discussion of genetic heterogeneity of childhood absence epilepsy, see 600131.

For a general phenotypic description and a discussion of genetic heterogeneity of idiopathic generalized epilepsy, see 600669. Juvenile myoclonic epilepsy is a subtype of idiopathic generalized epilepsy; see 254770 for a general phenotypic description and a discussion of genetic heterogeneity of JME.

MRT74 is characterized by intellectual impairment, macrocephaly, and dysmorphic features. Epilepsy with eyelid myoclonus has also been reported (Almuriekhi et al., 2015; Mastrangelo et al., 2020).

Developmental and epileptic encephalopathy-56 (DEE56) is a neurodevelopmental disorder characterized by early-onset seizures in most patients, followed by impaired intellectual development, variable behavioral abnormalities, and sometimes additional neurologic features, such as ataxia (summary by Guella et al., 2017). For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

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

SETD1B-related neurodevelopmental disorder (SETD1B-NDD) is characterized by developmental delay (mainly affecting speech and language), intellectual disability, seizures, autism spectrum disorder or autism-like behaviors, and additional behavioral concerns. Speech delay and/or language disorder has been reported in most affected individuals. Delay in gross motor skills and mild-to-moderate intellectual disability are common. Most affected individuals have seizures with variable onset and seizure type. Behavioral issues including hyperactivity, aggression, anxiety, and sleep disorders have been reported in approximately half of individuals. Less common features include ophthalmologic manifestations and feeding issues.

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.