Digenic mutations in severe myoclonic epilepsy of infancy.

The clinical features of severe myoclonic epilepsy of infancy (SMEI) resemble those of mitochondrial diseases, although most patients have the sodium channel (SCN1A) mutation. We describe a patient with SMEI and enlarged muscle mitochondria associated with mutations in mitochondrial polymerase gamma 1 (POLG1) and SCN1A. Due to increased risk of valproate-induced liver failure in patients with POLG1 mutations, we recommend POLG1 gene analysis for SMEI patients before valproate administration.