Dravet syndrome (previously known as severe myoclonic epilepsy of infancy; SMEI) is a genetic early-onset epileptic encephalopathy most commonly (approximately 80%) caused by mutations in the SCN1A gene, which encodes a voltage-gated sodium channel. Dravet syndrome is being increasingly recognized but remains rare with an incidence of about 1:15 700 in the United States.

Seizure onset occurs within the first year of life in an otherwise normally developing infant. All seizure types can occur and are often resistant to antiepileptic drugs. The course of epilepsy tends to follow a pattern of febrile seizures at initial diagnosis, followed by a "catastrophic" phase of multiple seizure types that are poorly responsive to medication, followed by relative stabilization after about 5 years. Developmental delay becomes apparent within the second year of life and behavioral problems can be prominent. Other neurologic signs include hypotonia, ataxia, and myoclonus. A later-onset gait deterioration and a characteristic "crouch gait" stance have been described. Patients may have a family history of epilepsy or febrile seizures.