Electrically-elicited surface mechanomyogram in myotonic dystrophy

The surface oscillation of the muscle during electrical stimulation is detectable by an accelerometer as a surface mechanomyogram (MMG). The aim of this study was to evaluate whether MMG properties reflect the mechanical muscle changes induced by myotonic dystrophy (MyD). To this end, the tibialis anterior of seven MyD patients and seven age- and sex-matched controls (C) was supramaximally stimulated at 1 (single twitches), 5, 10, 15, 20 Hz for 3 s at the most proximal motor point. RESULTS SINGLE TWITCHES: The MMG amplitude was 67% less, the duration 37% longer, the electromechanical delay 64% longer, and the spectrum mean frequency, 44% lower in the MyD patients than in controls. REPETITIVE STIMULATION: At each stimulation frequency, the average MMG peak-to-peak was less in the MyD patients than in C. CONCLUSION: The differences between the MMGs of MyD patients and C support the hypothesis that, together with the well-known changes in sarcolemmal excitability, an alteration in electromechanical coupling and a failure in contractile machinery may coexist in MyD.