A comprehensive electrophysiological study has been made of the extensor digitorum brevis muscle and its motor innervation in 17 patients with dystrophia myotonica. The mean contraction and half-relaxation times were prolonged in the isometric twitches of dystrophic muscles. Decremental responses to repetitive motor nerve stimulation were found in two patients. All the terminal latency measurements were normal and only one patient had a reduced nerve conduction velocity. As the patients aged their muscles became weaker, due to a progressive loss of motor units. This finding, and the normal sizes of many surviving motor units, suggested that the muscle changes resulted from a primary defect of motor innervation.
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