Facial action myoclonus in patients with olivopontocerebellar atrophy.

We studied four patients with familial olivopontocerebellar atrophy (OPCA) who had abnormal twitching of the cheeks and perioral muscles induced by facial movements. With the muscles at rest, electromyographic (EMG) recordings of the orbicularis oris and risorius muscles revealed myokymic discharges in the absence of visible movements. With voluntary contraction, the EMG showed synchronous discharges in the orbicularis oris and risorius muscles ipsilaterally associated with visible twitching. The duration of the EMG bursts was 10 to 75 ms with a frequency of 8 to 25 Hz, which suggested that the abnormal twitching was most consistent with a myoclonic disorder. Because it was induced by activation of the facial muscles, this movement disorder represents a form of action myoclonus.