A medication most often used to treat heart arrhythmias also reduces a central symptom of myotonic dystrophy, the most common type of muscular dystrophy in adults. The findings about the medication Mexiletine ( Mexitil ), a Lidocaine congener, are published in the journal Neurology.

Currently there is no drug approved to treat myotonic dystrophy, an inherited disease that is marked by progressive muscle weakness. While the course of the disease can vary dramatically from patient to patient, symptoms besides weakness can include muscle stiffness, difficulty speaking and swallowing, problems walking, and in some patients, heart problems and cataracts. Physicians estimate that approximately 40,000 Americans have the condition.

The researchers at the University of Rochester Medical Center found that Mexiletine is effective at treating the myotonia.

In the trial of Mexiletine, researchers measured the amount of time it took patients to relax their grip after squeezing the handles of a computerized device that measures force. For most healthy people, that relaxation takes one-third of a second or less. But for people with myotonic dystrophy, that relaxation can take many seconds.

Researchers studied two groups of 20 patients who had myotonic dystrophy, all confirmed through genetic analysis. Each participant received either placebo, or 150 or 200 milligrams of Mexiletine three times a day, for seven weeks. Then, after a period of several weeks where they received no drug, participants were switched to the other treatment for another seven weeks.

The team found that Mexiletine at three daily doses of either 150 or 200 milligrams per dose does a great deal to alleviate myotonia. In their test of relaxation after grip, the team found that Mexiletine reduces the abnormally long relaxation by 38% at the lower dose and 59% at the higher dose. No benefit at all was seen for participants on placebo.

Because the drug can affect the heart, participants in the study were admitted as inpatients and stayed several nights at the University's Clinical Research Center, where their heart health could be monitored closely. The team found no adverse effects of Mexiletine, including no effects on normal cardiac rhythms.

Mexiletine acts to help the muscle compensate for the ion channel abnormality that is at the core of the myotonia in this disease. The myotonia is caused by a sort of molecular stutter that causes electrical signaling in muscle cells to go awry, in effect making muscle stick in the on position. The mutation markedly reduces the number of functioning muscle chloride channels and causes decreased movement of the chloride ion across the muscle membrane, leading to excessive muscle irritability and repeated spontaneous activation of muscle fibers. This results in muscle stiffness and delayed relaxation after contraction. Mexiletine works through the sodium channel, which is functioning normally, to decrease muscle irritability.

Source: University of Rochester Medical Center, 2010

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