Charcot Marie Tooth Disease is a family of inherited peripheral neuropathies, with over 70 causative genes identified to date.1-4 Muscle cramps are frequent in CMT, affecting up to 85% of patients with some subtypes of CMT. These cramps impact quality of life and have been identified as an important therapeutic target for clinical trials in CMT.1-4 There is no FDA approved treatment for muscle cramps.5 Mexiletine is a sodium channel blocker approved for treatment of arrhythmias. As a sodium channel blocker, mexiletine offers the promise of effective therapy for muscle cramps.

This study will provide data on the short term efficacy of oral mexiletine in helping to prevent muscle cramps in adults with CMT. The study will also assess the short-term safety and tolerability of low dose mexiletine in adults with CMT.

Inclusion Criteria:

• The subject has known CMT, previously diagnosed by a neurologist with subspeciality expertise in neuromuscular disorders/peripheral neuropathy. The diagnosis of CMT relies on a combination of clinical history (including family history), neurological examination electrophysiological features, and prior genetic testing results.

  • The subject is at least 18 years old, and has signed the Informed Consent Form.
  • The subject is ambulatory (cane, walker, orthoses allowed).
  • The subject has experienced muscle cramps and has known provokable hamstring or calf muscle cramps on MVC.
  • The subject has a reliable method of birth control (if a female subject of child bearing potential). Reliable birth control is defined as Hormonal contraception Intrauterine contraception/device Any two barrier methods (combination of male or female condom with diaphragm sponge or cervical cap) together with spermicidal foam/gel/film/cream/suppository True abstinence

Exclusion Criteria:

• The subject has a known neuropathy from another source (e.g., diabetes, drug induced, alcohol, etc.).

  • The subject has an untreated medical disorder known to predispose to muscle cramps
  • The subject is pregnant or nursing, has a known mexiletine allergy or has taken mexiletine in the past 12 weeks.
  • The subject is participating in another therapeutic trial.
  • The subject has second or 3rd degree heart block, atrial flutter/fibrillation, ventricular arrhythmias, or is receiving treatment of a cardiac arrhythmia.
  • The subject is currently taking another agent for muscle cramps or a muscle relaxant (e.g., benzodiazepine, baclofen, tizanidine, soma, meprobromate).
  • The subject is on another sodium channel blocker or medication that precludes administration of mexiletine.
  • The subject has known diabetes mellitus, liver or kidney disease requiring ongoing treatment, untreated thyroid dysfunction, symptomatic cardiomyopathy, or symptomatic coronary artery disease.
  • The subject, in the opinion of the investigator, is unsuitable for enrollment for any other reason