New research supported by the National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS), the National Institute of Neurological Disorders and Stroke (NINDS), and other funding shows that the antiarrhythmic drug mexiletine appears to be safe and effective in reducing myotonia, a prominent symptom of myotonic dystrophy type 1 (DM1).

DM1 is one of the most common forms of adult muscular dystrophy and affects the voluntary muscles as well as many organs and organ systems, including the heart and blood vessels, digestive tract, and uterus. In addition to progressive muscle degeneration and weakness, people with the disease experience myotonia, or difficulty relaxing contracted voluntary muscles. This contributes to decreased dexterity, difficulty walking, problems with speech or swallowing, and muscle pain.

A few preliminary trials have suggested that mexiletine is useful in treating myotonic disorders, and neurologists have been prescribing it for that purpose for several years. However, two new studies reported in the journal Neurology, are the first randomized, double-blinded, placebo-controlled trials—the gold standard for studies of medical treatment—to show safety and efficacy of mexiletine as an antimyotonia treatment in DM1 at the dosages tested.

Led by Richard T. Moxley, M.D., at the University of Rochester Medical Center, each seven-week trial involved 20 patients with DM1. The first compared 150 milligrams (mg) of mexiletine three times a day to a placebo; the second compared 200 mg of mexiletine three times a day to a placebo. The results from both trials demonstrated that the drug at both doses was effective in quickly reducing myotonia without serious side effects. The drug blocks electrically charged sodium ions from passing through the cell membranes of skeletal muscle, which may reduce the abnormal electrical signaling associated with DM1.

Overall, the study suggests that this drug may be considered as an effective treatment for the muscle relaxation symptoms of patients with DM1. However, since some DM patients can have cardiac symptoms more severe than the patients in these trials, physicians are encouraged to carefully evaluate patients for cardiac symptoms before recommending mexiletine treatment. Further, long-term studies are needed to determine if the effects of mexiletine on myotonia is sustained and whether the drug improves the overall course of the disease and quality of life of the patients.

The mission of the National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS), a part of the U.S. Department of Health and Human Services' National Institutes of Health (NIH), is to support research into the causes, treatment, and prevention of arthritis and musculoskeletal and skin diseases; the training of basic and clinical scientists to carry out this research; and the dissemination of information on research progress in these diseases. For more information about NIAMS, call the information clearinghouse at (301) 495-4484 or (877) 22-NIAMS (free call) or visit the NIAMS Web site at


Logigian EL, Martens WB, Moxley RT 4th, McDermott MP, Dilek N, Wiegner AW, Pearson AT, Barbieri CA, Annis CL, Thornton CA, Moxley RT 3rd.. Mexiletine is an effective antimyotonia treatment in myotonic dystrophy type 1. Neurology. 2010 May 4: 74 (18):1441-8.

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