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.

Muscle weakness may appear suddenly or occur slowly over weeks or months. There may be difficulty raising the arms over the head, rising from a sitting position, or climbing stairs. The voice may be affected through weakness of the throat muscles. Joint pain, inflammation of the heart, and lung disease may occur.

A similar condition, called dermatomyositis, is evident when a dusky, red rash appears over the face, neck, shoulders, upper chest, and back. A malignancy may be associated with this disorder. Each year, between 2 and 10 people per million develop the disease.

• muscle weakness, proximal muscles (shoulders, hips, etc.)
• muscle pain
• difficulty swallowing
• shortness of breath
• hand tremor

• electromyography
• MRI of affected muscles
• muscle biopsy
• serum creatinine, elevated
• serum aldolase, elevated
• CPK, elevated

• associated malignancy
• heart disease, lung disease, or abdominal complications
• calcium deposits in the affected muscles, especially in children with the disease