The role of SCN1A gene mutations in the etiology of severe myoclonic epilepsy of infancy (SMEI) was investigated in 93 patients followed at the Hopital Saint Vincent de Paul, Paris, and other centers in France and Italy. SCN1A mutations occurred in 33 patients (35%). Parents of three patients (10%) who carried the inherited mutations were asymptomatic or had a milder form of epilepsy. Patients with the mutations had a greater frequency of unilateral motor seizures than those without and a more frequent family history of epilepsy. 
COMMENT. SMEI caused by SCN1A mutations is characterized by unilateral motor seizures, and 10% of cases are inherited from an asymptomatic or mildly affected parent. SMEI has a mean age of onset of 5 months, more than half the seizures are febrile, seizures are of all types, absence seizures with a myoclonic component are often triggered by photic stimulation, status epilepticus occurs in 75% and is often precipitated by fever, ataxia develops in >80%, psychomotor delay is common to all, and seizures are refractory to medication.