Electrophysiological abnormalities in cases of dystrophia myotonica

EMG, nerve conductions, repetitive nerve stimulation and SSEPs were studied in 14 patients with dystrophia myotonica. Fibrillation potentials were seen in two of these patients. Motor nerve conduction velocities of the medial popliteal nerve were decreased in 4 of the 13 patients studied. Three of these patients also showed decrease in the antidromic sensory conduction velocities. Motor conduction velocities of the lateral popliteal nerve were similarly reduced in four patients. Taking both the nerves into consideration, 6 of the 14 patients showed a decrease in the motor nerve conduction velocities of either, or both, of these nerves. Repetitive nerve stimulation showed abnormal responses in 13 of these 14 cases. SSEPs from stimulation of the lateral popliteal nerve showed increased latencies in three of the ten cases studied. Sensory conduction velocities as measured from SSEPs were decreased in three of the four cases. SSEPs for the medial popliteal nerve showed similar values. The significance of these abnormalities has been discussed.