Dravet syndrome (previously known as severe myoclonic epilepsy of infancy, SMEI), typically presents in the first year of life in a normal child with prolonged, febrile and afebrile, focal (usually hemiclonic) or generalized convulsive seizures. Other seizure types including myoclonic (hyperlink) and atypical absence seizures (hyperlink) appear between the age of 1 and 4 years. Seizures are usually intractable and from the second year of life children demonstrate cognitive and behaviour impairments. The clinical diagnosis is supported by the presence of abnormalities in the sodium channel gene SCN1A (found in 75% of cases).
NOTE the term Dravet syndrome is now also used to encompass atypical or borderline cases, previously known as severe myoclonic epilepsy of infancy - borderland (SMEB).
NOTE Dravet syndrome may be considered an 'epileptic encephalopathy'. This term denotes the concept that the epileptic activity itself might directly contribute additional cognitive and behavioral impairments over those expected from the underlying etiology alone, and that suppression of epileptic activity might minimize this additional impairment.
This syndrome is characterized by onset of seizures typically around 6 months of age. Most have had seizure onset less than 15 months of age, however a small minority of cases have onset in the second year of life. Both sexes are affected. Antecedent, birth and neonatal history is normal. The first seizure is associated with a fever in about 60% of cases. Not all patients start with febrile convulsions. Immunization may be a non-specific trigger to the first seizure leading to an earlier age of seizure onset, but cases with onset with a vaccine proximate seizure have the same outcome as other children with Dravet syndrome. Sensitivity of seizures to fever may persist throughout life. Head size and neurological examination are usually normal initially, over time ataxia and pyramidal signs may develop. Development is typically normal in the first year of life, with plateauing or regression in later years.
CAUTION Anti-seizure medications that have sodium channel blocking properties may aggravate seizures in this syndrome.