Mexiletine (NaMuscla) for the treatment of myotonia in non-dystrophic myotonic disorders

Introduction: NaMuscla, (mexiletine), is the first licensed treatment for the Non-Dystrophic Myotonias (NDM). NDM are categorized by genetic ion channel dysfunction and cause significant morbidity. To date, off-license mexiletine, although less costly, has sometimes been subject to breaches in supply causing significant regional and national variation in availability. Areas covered: The evidence supporting mexiletine use in NDM, its mechanism of action, chemistry, and pharmacodynamics is reviewed. The evidence for other, unlicensed medications, used to treat myotonia as well as new antimyotonic compounds in development is also reviewed. Expert opinion: Mexiletine is an effective and safe treatment for NDM. However, while mexiletine is very effective in reducing muscle stiffness, it is less effective at treating the pain associated with NDM and some SCN4A genotypes may not respond to mexiletine treatment. In addition, gastrointestinal discomfort is frequent and may prevent adequate dose titration. Since the designation of mexiletine as an orphan drug for NDM, level 1 evidence for the antimyotonic effect of lamotrigine has emerged. However, no superiority trials have been completed. A head-to-head trial to compare the efficacy of mexiletine and lamotrigine in reducing both muscle stiffness and pain and to determine variation in genotype response would facilitate greater precision medicine in NDM.