Polymyositis is a relatively uncommon inflammatory disease that leads to significant muscle weakness.

Polymyositis is a skeletal muscle disease also known as idiopathic inflammatory myopathy. The cause of this disorder is unknown. It is thought 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 those between 50 and 70 years old, or in children between 5 and 15 years old. It affects women twice as often as men. It is more common in African-Americans than Caucasians. Overall, about 1 in 100,000 people are diagnosed with some form of inflammatory myopathy each year.

Muscle weakness may appear suddenly or occur slowly over weeks or months. The weakness is due to inflammation and the breakdown of the muscles. Patients with this disease may have difficulty raising their arms over the head, rising from a sitting position, or climbing stairs. The voice may be affected through weakness of the throat muscles.

A similar condition, called dermatomyositis, is marked by a dusky, red rash that appears over the face, knuckles, neck, shoulders, upper chest, and back.