Researchers have found that mexiletine, an anti-arrhythmic medication now rarely prescribed, is effective at easing pain caused by the rare muscle disease nondystrophic myotonia.

Fifty-nine participants from four countries received either 200mg of mexiletine three times daily for four weeks, or a placebo. Then after one week, they received the other treatment for four weeks.

Nondystrophic myotonia sufferers reported that mexiletine improved their stiffness by at least 40% and halved their pain. Testing also showed that mexiletine reduced the abnormal electrical activity in muscles with patients able to relax their grip and open their eyes much faster.

Neurologists say that this study will inform research for treating many other patients who have one of the thousands of other diseases that are very rare.