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 sa me EDC muscle. We studied 45 examinations of both v-SFEMG and RNS in 29 pat ients suffering from MG, together with examinations of RNS in 30 control su bjects. Results: Forty-one of 45 (91%) v-SFEMGs showed abnormal results, whereas on ly 18/45 (40%) RNSs showed an abnormal decrement. The percentage of decreme nt showed similar correlations with 3 v-SFEMG parameters: percentage of abn ormal pairs, percentage of blocking pairs, and the mean MCD value. Examinat ions showing a significant decrement in RNS had at least 60%. and usually n o less than 90%, abnormal pairs, and 10-80% blocking pairs. Some muscles wi thout 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 blocke d during voluntary contraction, and partly, because smaller motor units exp lored by v-SFEMG are probably more abnormal in MG than larger motor units m ainly contributing to a decrement. Both factors make v-SFEMG much more sens itive than RNS. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.