Late components of motor unit potentials in a patient with myoglobinuria.

A 45-year-old woman had severe weakness, myoglobinuria, and elevated muscle enzyme levels in the serum after a flulike illness. Electromyography some four weeks later showed motor unit potentials with late components; these remained present on subsequent electromyographic examinations performed at approximately monthly intervals for the following year. The persistence of motor unit potentials with late components is taken to imply that the late components were caused either by sprouts that failed to undergo myelination or, more likely, by ectopically innervated muscle fibers.