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Some patients also have shortness of breath, an irregular heartbeat, or other heart problems. Many times, polymyositis is idiopathic. That means the cause of the disease is not known. But, there are some theories about the causes of polymyositis, including:
1 Polymyositis and dermatomyositis (PM/DM) are chronic inflammatory diseases of muscle. 2  Muscle weakness is the most common symptom of PM/DM. 3  The cause of PM/DM is unknown.his results in weakness that can be severe. Polymyositis is a chronic illness featuring progressive muscle weakness with periods of increased symptoms, called flares or relapses, and minimal or no symptoms, known as remissions.