Long‐term Safety and Efficacy of Mexiletine for Patients With Myotonic dystrophy type 1 and 2

Introduction: Longterm safety data in the non‐dystrophic myotonias is reassuring but there is no equivalent data in DM 1 and 2. Objective: To examine efficacy and safety of mexiletine in DM1 and DM 2 patients. Methods: Retrospective cohort study in two UK specialist neuromuscular centres. Results: 20 patients(17 DM1 and 3 DM2) were identified. Age at treatment:37.5(12.2)years, treatment duration:1.9(1.8)years, cumulative duration:44.1years. 19/20 reported some response. Mean(S.D.) effective and tolerated daily dose was 365.6(159.9)mg, median=300mg, range=100‐600mg,2‐3 divided doses. There were no serious adverse events. 5/20 reported dyspepsia, 3 stopped mexiletine despite routine dyspeptic treatment. Paired pre‐treatment and peak dose assessment revealed no change in PR interval:165.6vs170.0ms;p=0.40, QTc:439.7vs432ms;p=0.76. 8 were assessed by cardiology prior to treatment: 5/8x dysrhythmia; 2x DCCV, 1x ablation, 1x ICD and 1x PPM. Mexiletine did not precipitate symptomatic arrhythmia in any patient. Conclusion: The duration of mexiletine exposure provides reassuring safety data despite cardiac comorbidity in DM1.