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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:
If dermatomyositis occurs along with polymyositis, symptoms may also include: 1  Skin rash. 2  This can be either raised and smooth or scaly. 3  It may appear on the forehead, the neck, shoulders, chest and back, forearms and lower legs, elbows and knees or the joints of the fingers, toes, wrists and ankles.oth Polymyositis and dermatomyositis have symptoms in common with sclerosis or sometimes lupus. Signs of these conditions may appear suddenly and severely or gradually and more mildly. Symptoms are more likely to be gradual in older people.