Evoked isometric muscle contractions in myopathies: analysis of pathophysiological properties by different stimulus patterns

Isometric twitches of the adductor pollicis muscle following ulnar nerve stimulation were investigated in healthy subjects (n=35) and patients with different types of myopathies (myotonic dystrophy, n=19; limb girdle muscular dystrophy, n=10; metabolic myopathy, n=6). The changes within the rising part (i.e. within the contraction time, CT) of the isometric twitches after single stimuli were similar in myotonic and limb girdle dystrophies: the first part of CT, which lasts until the maximal contraction rate is achieved, was shortened, whereas the following second part of CT, which lasts until the maximal twitch force is achieved, was normal. In metabolic myopathies the first part was normal, whereas the second part was prolonged. The relaxation was prolonged in all types of myopathies, particularly in metabolic myopathies. Using double stimuli with short interstimulus intervals (ISI), the absolute refractory period of the muscle contraction (healthy subjects: 1.35±0.16 ms) was shortened in patients with myotonic dystrophy (1.02±0.11 ms). In the other types of myopathies, the absolute refractory period was only shortened provided that the single twitch force was clearly reduced. A similar dependence on a reduced single twitch force was also found with regard to the maximal force development with two stimuli and the corresponding ISI: the force contributed by a second stimulus was pathologically enhanced if the single twitch force was clearly reduced. The ISI related to the maximal force with two stimuli was shifted towards very short values (healthy subjects: 10.5 ms, myotonic dystrophy: 4.6 ms, limb girdle dystrophies: 5.0 ms). Our results can be attributed to altered kinetics of calcium release and uptake by the sarcoplasmic reticulum in myopathies.