Exercise-induced membrane failure in paramyotonia congenita.

A marked decrease in the amplitude of the evoked thenar compound muscle action potential followed brief exercise at room temperature in two patients with paramyotonia congenita. This decline was more rapid and pronounced in our patient with coexistent thyrotoxicosis. The prolonged recovery period in paramyotonia congenita differs from the brief exercise-induced decreased compound muscle action potential seen in myotonia congenita and myotonic dystrophy. These findings suggest that exercise, like cold exposure, decreases membrane excitability in paramyotonia congenita.