Continuous motor unit activity confined to the upper extremities.

We report a patient with clinical vermiform motor activity, muscle cramps, delayed relaxation of grip, and continuous motor unit discharges who developed mild symmetric symptoms in the upper extremities following a viral infection. Treatment with carbamazepine produced considerable symptomatic improvement. Needle electromyography (EMG) in several muscles showed irregular grouped spontaneous discharges composed of potentials resembling normal motor units. The discharges occurred asynchronously in distal muscles with identical and different peripheral nerve innervation but were not seen in more proximal muscles innervated by the same nerves. Our EMG findings indicate an origin of this spontaneous activity in the terminal branched motor nerves.