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.

An extremely rare form of hereditary episodic ataxia with characteristics of recurrent episodes of vertigo and ataxia lasting several hours.

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.

A temporal lobe epilepsy that has material basis in heterozygous mutation in the CPA6 gene on chromosome 8q13.

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.

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

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

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).