ISOMETRIC MUSCLE CONTRACTIONS AFTER DOUBLE-PULSE STIMULATION - COMPARISON OF HEALTHY-SUBJECTS AND PATIENTS WITH MYOTONIC-DYSTROPHY

Isometric contractions of the adductor policis muscle were studied in healthy subjects and patients with myotonic dystrophy after single and double stimuli of the ulnar nerve using a wide range of interstimulus intervals (ISI, 0.4-180 ms). In healthy subjects, the force contribut ed by a second stimulus was greater than the single twitch force being maximal (mean + 140%) at 12-ms ISI. In myotonic dystrophy, the force contributed by the second stimulus was (relative to a reduced twitch a mplitude) increased (mean + 204%) with a maximum at 4.8-ms ISI. An abn ormal increase of force was only recorded if the single twitch force w as clearly reduced. The absolute refractory period of muscle contracti on (normal range 1.2-1.6 ms, mean 1.35 ms) was shortened in all patien ts (mean 1.01 ms) except one (1.2 ms). The ISI showing the maximal for ce were related to those showing the maximal prolongation of the contr action time in healthy subjects (r = 0.71) but not in patients. The ra te of force development contributed by a second stimulus was slower th an expected from the summation of two single twiches with short stimul us intervals (3-40 ms) a phenomenon called early depression. In patien ts, the early depression was reduced or abolished within this range of ISI as has been found in dystrophic mouse muscles. The optimal ISI in patients was shifted towards very short times and together with the o ther disturbances it is suggested that activation of diseased muscle b y motoneurons may be less effective, being an additional factor leadin g to weakness in myotonic dystrophy.