Phenytoin plasma concentrations in paroxysmal kinesigenic choreoathetosis.

We studied five patients with paroxysmal kinesigenic choreoathetosis (PKC) to evaluate the minimum effective plasma concentration of phenytoin. In two children, the minimum concentration necessary to control symptoms approximated the concentrations necessary to control epileptic seizures. In three adults, symptoms were controlled with concentrations of phenytoin well below the therapeutic range of phenytoin in epilepsy. These findings suggest an age-dependent change in the disease state, and support the concept that the clinical course of PKC may be explained by delayed maturation of extrapyramidal systems.