Dystrophia myotonica. Peripheral nerve involvement and pathogenetic implications

A comparative electrophysioloical study of patients with dystrophia myotonica and control subjects is presented. The study includes estimation of the number of motor axons innervating the extensor digitorum brevis muscle and measurements of the conduction of M- and F-waves along the deep peroneal nerve. There is unequivocal electrophysiological evidence of nerve involvement in the disease. This was indicated by (a) prolongation of the terminal latencies and slowing of the motor conduction of the deep peroneal nerve (P less than 0.001), (b) delayed conduction of the F-wave along the proximal segments of the nerves (P less than 0.02-0.01), (c) reduced number of motor axons innervating the extensor digitorum brevis muscle (P less than 0.001), (d) high amplitude motor unit potentials and discrete EMG activity on the maximal volitional contraction of minimally affected muscles in one patient. However, if the muscle changes were secondary to the nerve involvement occurring in the disease, one would expect that the electrophysiological findings of neuropathy would become more prominent in patients with severe muscle wasting and weakness. Our results showed that in some patients with marked muscle atrophy and weakness there was no evidence of nerve involvement while in other patients with slight to moderate degree of muscle weakness the electrophysiological studies indicated peripheral neuropathy. Therefore, it appears that both the nerves and the muscles are independently affected by the pleiotropic action of the responsible gene and the evidence does not suggest that the muscle atrophy of patients with dystrophia myotonica is entirely neural.