A Surface EMG analysis of sarcolemma excitability alteration and myofibre degeneration in Steinert disease

Objectives: To apply surface electromyography (EMG) using low and high stimulation frequencies, to establish the contribution of myotonia and/or dystrophy to muscle impairment in subjects affected by myotonic dystrophy (MyD). Methods: A motor point stimulation protocol, at 15 and 35 Hz, was carried out on the tibialis anterior (TA) of 25 MyD patients. These were subdivided into 3 subgroups, MyD3 (9), MyD4 (10) and MyD5 (6), on the basis of their TA MRC score. The surface myoelectric signal was recorded and the average rectified value of amplitude (ARV) was evaluated. In 4 MyD5 patients we simultaneously recorded EMG and force. Results: Each subgroup presented a characteristic ARV trend both at 15 and 35 Hz - increasing in MyD3 (like the controls), slightly decreasing in MyD4 and clearly decreasing in MyD5. The low frequency permitted a greater resolution of the statistical analysis. Two out of 4 patients showed an opposite trend of ARV with respect to the force, whereas the others presented a parallel decreasing trend. Conclusions: The analysis of the ARV during a stimulated contraction permits the identification and quantification of the sarcolemma excitability alteration and/or the myofibre degeneration contributing to muscle impairment in MyD. The lower frequency is more sensitive and offers better inter-experiment repeatability. © 2001 Published by Elsevier Science Ireland Ltd.