Sarcolemmal excitability in the myotonic dystrophies

Introduction: Chloride conductance disturbances contribute to sarcolemmal dysfunction in type 1 (DM1) and 2 (DM2) myotonic dystrophy. Studies using muscle velocity recovery cycles (MVRCs) suggest Na+/K+-ATPase activation becomes defective in advanced DM1. We used MVRCs to investigate muscle excitability in DM1 and DM2. Methods: MVRCs were measured for patients with mild (n = 8) and advanced DM1 (n = 11), DM2 (n = 4) and normal controls (n = 30). Results: Residual supernormality after multiple conditioning stimuli was increased in DM2 and advanced DM1. Advanced DM1 was distinguished by increases in muscle relative refractory period and reduced early supernormality, as well as peak amplitude decrements for the first and last responses in train during repetitive stimulation. Discussion: Prolongation of the muscle relative refractory period indicates that depolarization of the resting muscle membrane potential occurs in advanced DM1, with possible implications for future therapeutic approaches. This article is protected by copyright. All rights reserved.