Is excitation–contraction coupling impaired in myasthenia gravis?

OBJECTIVE: To investigate whether excitation-contraction (E-C) coupling of muscle is impaired in patients with myasthenia gravis (MG). METHODS: In 51 patients with generalized MG and 35 normal subjects, compound muscle action potentials (CMAPs) of the abductor pollicis brevis, and movement-related potentials using an accelerometer placed at the thumb tip were simultaneously recorded after median nerve stimulation at the wrist. The E-C coupling time (ECCT) was estimated by a latency difference between CMAP and movement-related potential. Antibodies against acetylcholine receptor (AChR), ryanodine receptor (RyR), and muscle specific receptor tyrosine kinase (MuSK) were measured by immunoassays. RESULTS: The mean ECCT was significantly longer in patients with MG (mean+/-SEM; 2.79+/-0.1 ms; p=0.002) than in normal controls (2.52+/-0.1 ms). Among MG patients, the mean ECCT was longer for patients with thymoma than for those without it (P=0.04), and was shorter for patients treated with FK506 (an immunosuppressant and also an enhancer of RyR related Ca(2+) release) than for those not receiving this treatment (p=0.04). ECCT had no significant correlation with anti-AChR, anti-RyR, or anti-MuSK antibodies. CONCLUSIONS: In MG, E-C coupling appears to be impaired, particularly in patients with thymoma, and FK506 possibly facilitates E-C coupling. SIGNIFICANCE: The functional implication of impaired E-C coupling is not established, but it may contribute to muscle weakness in patients with MG.