Single fiber EMG and repetitive nerve stimulation of the same extensor digitorum communis muscle in myasthenia gravis

Objective: To compare voluntary single fiber electromyography (v-SFEMG) and repetitive nerve stimulation (RNS) at the same extensor digitorum communis (EDC) muscle in myasthenia gravis (MG). Methods: We examined v-SFEMG and RNS successively on the same day in the same EDC muscle. We studied 45 examinations of both v-SFEMG and RNS in 29 patients suffering from MG, together with examinations of RNS in 30 control subjects. Results: Forty-one of 45 (91%) v-SFEMGs showed abnormal results, whereas only 18/45 (40%) RNSs showed an abnormal decrement. The percentage of decrement showed similar correlations with 3 v-SFEMG parameters: percentage of abnormal pairs, percentage of blocking pairs, and the mean MCD value. Examinations showing a significant decrement in RNS had at least 60%, and usually no less than 90%, abnormal pairs, and 10–80% blocking pairs. Some muscles without a decrement had up to 50% blocking pairs. Conclusions: These results suggest that the blocking phenomenon observed in v-SFEMG is not a direct counterpart of the decrement in RNS. This must be partly because fibers contributing to the decrement are continuously blocked during voluntary contraction, and partly, because smaller motor units explored by v-SFEMG are probably more abnormal in MG than larger motor units mainly contributing to a decrement. Both factors make v-SFEMG much more sensitive than RNS.