Phenytoin-induced choreoathetosis in patients with severe myoclonic epilepsy in infancy.

We describe three patients with severe myoclonic epilepsy in infancy (SME) who suffer from choreoathetosis due to the adverse effect of phenytoin. Choreoathetosis appeared when these patients were 8, 19, and 21 years old, 2 days to 6 months after increasing the phenytoin dosage. Choreoathetosis disappeared when the phenytoin dosage was decreased. The two elder patients experienced episodic and rather paroxysmal onset of long-lasting choreoathetosis, requiring the differential diagnosis from degenerative disease. In one of the patients, an ictal SPECT revealed decreased perfusion in the basal ganglia contralateral to the unilateral choreoathetosis. Polypharmacy, including carbamazepine and zonisamide, may have facilitated the onset of choreoathetosis. Phenytoin-induced choreoathetosis in the patients with SME is an important differential diagnosis among degenerative disorders involving involuntary movements. The episodic and paroxysmal nature of this movement disorder can delay its diagnosis and effective treatment. Patients with SME appear to be particularly vulnerable to this side effect of phenytoin, indicating the possible involvement of basal ganglia in the pathophysiology of this type of epilepsy.