Dermatomyositis is a muscle disease characterized by inflammation and a skin rash. It is a type of inflammatory myopathy. The cause of dermatomyositis is unknown. Experts think it may be due to a viral infection of the muscles or a problem with the body's immune system. It can also sometimes occur in patients who have cancer of the abdomen, lung or other body area. Anyone can develop dermatomyositis, but it most commonly occurs in children age 5 - 15 and adults age 40 - 60. Women develop this condition more often than men do. Polymyositis is a similar condition, but the symptoms occur without a skin rash. Symptoms: Difficulty swallowing; Muscle weakness, stiffness, or soreness; Purple or violet colored upper eyelids; Purple-red skin rash; Shortness of breath; The muscle weakness may appear suddenly or develop slowly over weeks or months. You may have difficulty raising your arms over your head, rising from a sitting position, and climbing stairs. The rash may appear over the face, knuckles, neck, shoulders, upper chest, and back.