The syndrome of 'continuous muscle fiber activity.'.

A 7-year-old boy who suffered from increasing stiffness and contractures of the extremities had distally pronounced atrophy and absent tendon reflexes. Electromyography showed continuous electrical activity during rest, sleep, after intravenous injection of diazepam, and after peripheral nerve block. The H reflex was elicitable; the silent period after the reflex was absent. Histopathological examination of the peroneus muscle disclosed a marked preponderance of type I fibers and slight atrophy of the type II fibers. Electron microscopic examination of the endplates demonstrated a marked atrophy of the postsynaptic regions and widened synaptic clefts. After one year's treatment with phenytoin, 200 mg daily, the patient showed an almost normal muscle tone. As not all of these electrophysiological phenomena can be fully explained by disturbances of the nerve terminals or the endplates, a further anomaly proximal from the peripheral nerve block seems to have been present.