Low incidence of SCN1A genetic mutation in patients with hemiconvulsion-hemiplegia-epilepsy syndrome.

Genetic mutations in SCN1A account for more than two-thirds of patients with classic Dravet syndrome. A role for SCN1A genetic mutations in the development of hemiconvulsion-hemiplegia-epilepsy (HHE) syndrome was recently suggested based on the observation that HHE syndrome and classic Dravet syndrome share many clinical features. We previously identified a 2 bp-deletion mutation in SCN1A in a Dravet patient, and we found out the patient also had HHE syndrome upon clinical re-evaluation. We subsequently screened 10 additional HHE patients for SCN1A. Among the 11 patients who were diagnosed with HHE syndrome, six patients had no other etiology with the exception of prolonged febrile illness, therefore classified as idiopathic HHE syndrome, whereas five patients were classified as symptomatic HHE syndrome. Direct sequencing of all coding exons and flanking intronic sequences of the SCN1A gene was performed, but we failed to identify additional mutations in 10 patients. The patient with SCN1A mutation had the earliest onset of febrile convulsion and hemiparesis. Our study suggests that SCN1A genetic mutation is only a rare predisposing cause of HHE syndrome.