Isometric muscle contractions after double pulse stimulation. comparison of healthy subjects and patients with myotonic dystrophy

Isometric contractions of the adductor pollicis 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 contributed 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 amplitude) increased (mean + 204%) with a maximum at 4.8-ms ISI. An abnormal increase of force was only recorded if the single twitch force was clearly reduced. The absolute refractory period of muscle contraction (normal range 1.2–1.6 ms, mean 1.35 ms) was shortened in all patients (mean 1.01 ms) except one (1.2 ms). The ISI showing the maximal force were related to those showing the maximal prolongation of the contraction time in healthy subjects (r = 0.71) but not in patients. The rate of force development contributed by a second stimulus was slower than expected from the summation of two single twiches with short stimulus intervals (3–40 ms) a phenomenon called early depression. In patients, 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 other disturbances it is suggested that activation of diseased muscle by motoneurons may be less effective, being an additional factor leading to weakness in myotonic dystrophy.