Myositis means inflammation of the muscles that you use to move your body. An injury, infection, or autoimmune disease can cause it. Two specific kinds are polymyositis and dermatomyositis. Polymyositis causes muscle weakness, usually in the muscles closest to the trunk of your body. Dermatomyositis causes muscle weakness, plus a skin rash. Other symptoms of myositis may include: fatigue after walking or standing, tripping or falling, trouble swallowing or breathing. Doctors may use a physical exam, lab tests, imaging tests and a muscle biopsy to diagnose myositis. There is no cure for these diseases, but you can treat the symptoms.