Patients with different myotonic syndromes were evaluated for the presence of transient decrease of muscle fiber excitability. The evoked compound muscle action potential (CMAP) was recorded from the hypothenar eminence; single, supramaximal stimuli were applied to the ulnar nerve at the wrist before and after various periods of voluntary, isometric exercise of the hand muscles. A variable decline of the CMAP after exercise was present in all subjects with myotonia. The results were similar to those obtained with rapid repetitive nerve stimulation.
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