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Heart Drug Effective in Treating Symptom of Muscular Dystrophy

Heart Drug Effective in Treating Symptom of Muscular Dystrophy

Muscular dystrophy is a group of hereditary muscle diseases that weaken the muscles that move the human body. Muscular dystrophies are characterized by progressive skeletal muscle weakness, defects in muscle proteins, and the death of muscle cells and tissue.

Currently, there aren’t any drugs that are approved for treating myotonic dystrophy, which is an inherited disease characterized by the progressive weakening of muscles. Symptoms beside weakness may include muscle stiffness, problems walking, difficulty speaking and swallowing, and in some cases, cataracts and heart problems. However, the course of the disease can vary dramatically from one patient to another.


Mexiletine was found to be effective at treating the myotonia (muscle stiffness) that is at the center of the disease, according to some researchers at the University of Rochester Medical Center. Turning a key in a lock for patients with myotonia, or writing with a pen or pencil or picking up and setting down a pitcher of water are all formidable tasks. In some cases, the symptoms may first occur when the person is shaking someone’s hand, and then can’t relax his/her grip for several seconds.


This is the first placebo-controlled, double-blind study to show that mexiletene helps relieve myotonia, according to neurologist Richard Moxley III, M.D., an author of the study and an international expert on muscular dystrophy, despite the fact that several doctors have suspected this before.


Physicians who treat muscular dystrophy should know that mexiletine is an option, according to Moxley, the director of the University’s Neuromuscular Disease Center and professor of Neurology. This study was done to study this issue more closely even though several physicians who specialize in treating patients with myotonic dystrophy have found it to be effective for their patients. This medication addresses myotonia quite well without any additional risks.


These findings were discovered by one of the world’s premier groups who focus on research and new treatments for muscular dystrophy. Moxley began the world’s first muscular dystrophy registry ten years ago, which now includes more than 1,500 patients with either facioscapulohumeral dystrophy or myotonic dystrophy.


Moxley is also the head of the University’s Senator Paul D. Wellstone Muscular Dystrophy Cooperative Research Center, which is one of the three research centers that were originally created by the National Institutes of Health in 2003. Currently, the Rochester center is one of the six NIH Wellstone centers and has recently received $5 million in additional funding in September 2008 to continue its work for five more years.


In the trial of mexiletine, the amount of time it took patients to relax their grip after squeezing the handles of a computerized device that measures force was assessed. That relaxation takes one-third of a second or less in most healthy people. However, it may take several seconds for people with myotonic dystrophy.


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


It was found that myotonia was greatly alleviated in patients who took mexiletine at three daily doses of either 150 or 200 milligrams per dose. When the test of relaxation after grip was done, it was discovered that mexiletine reduces the abnormally long relaxation by 38% at the lower dose, and by 59% at the higher dose. On the other hand, the participants who received placebo didn’t show any benefits.


The participants of this study were admitted as inpatients and stayed several nights at the University’s Clinical Research Center, where the health of their heart could be monitored closely, because this drug could affect the heart. No adverse effects of mexiletine were found, and there weren’t any effects on normal cardiac rhythms either.


The core of the myotonia in this disease is the ion channel abnormality, and mexiletine acts to help the muscle compensate for. A sort of molecular stutter that causes electrical signaling in muscle cells to go awry is the cause of the myotonia, making the muscle stick in the “on” position. The number of functioning muscle chloride channels is markedly reduced due to this mutation, and the movement of the chloride ion across the muscle membrane is decreased, leading to excessive muscle irritability and repeated spontaneous activation of muscle fibers. This will result in muscle stiffness and delayed relaxation after contraction. To decrease muscle irritability, mexiletine works through the sodium channel that is functioning normally.


The new research findings on the effectiveness of mexiletine come amid several exciting research finds by Moxley’s colleagues. In one line of research led by Charles Thornton, M.D., the researchers discovered exactly how a faulty gene prevents normal proteins from doing their jobs, and so causes myotonic dystrophy. After this, experimental compounds were used to break up abnormal cellular deposits of toxic RNA in the nuclei of cells, eliminating myotonia in mice with myotonic dystrophy. Yet another team is part of an international study examining the genetic roots of facioscapulohumeral muscular dystrophy, which is the second most common form of muscular dystrophy in adults.


These findings about mexiletine, which is a chemical cousin of lidocaine, were published in the journal Neurology in May 4.


Prepared By: Dr. Mehyar Al-khashroum
Edited By: Miss Araz Kahvedjian

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