The cause of this disorder is unknown. It is theorized that an autoimmune reaction or a viral infection of the skeletal muscle may cause the disease. It can affect people at any age, but most commonly occurs in people 40 to 60 years old, or in children from 5 to 15 years old.

It affects women much more often than men. Muscle weakness may appear suddenly or occur slowly over weeks or months. There may be difficulty with raising the arms over the head, rising from a sitting position, and climbing stairs.

A dusky, purplish red rash may appear over the face, neck, shoulders, upper chest, and back. Joint pain, inflammation of the heart, and lung (pulmonary) disease may occur.

A malignancy may sometimes be associated with this disorder. A similar condition is called polymyositis when the symptoms occur without any skin manifestations.