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.

Polymyositis is a systemic disease, which means it affects the whole body. Symptoms include:

• Muscle weakness in the proximal muscles (shoulders, hips, etc.)
• Muscle pain
• Difficulty swallowing
• Shortness of breath

Patients may also have morning stiffness, joint pain, fatigue, anorexia, weight loss, and fever.

• Electromyography
• MRI of affected muscles
• Muscle biopsy
• Serum aldolase , elevated
• CPK , elevated

The disease is treated with corticosteroid medications. When an improvement in muscle strength is achieved, usually in 4 to 6 weeks, the medication is slowly tapered off. Maintenance therapy with prednisone may be continued indefinitely. In people who fail to respond to corticosteroids, medications to suppress the immune system may be used.

If the condition is associated with a tumor , the condition may improve if the tumor is removed.

Remission (a period when no symptoms are present) and recovery occur in many patients, especially children. For most others, immunosuppressant drugs can control the disease. In adults, death may result from severe and prolonged muscle weakness, malnutrition, pneumonia , or respiratory failure. The major causes of death are cancer (malignancy) and lung disease.

• Associated malignancy
• Heart disease , lung disease , or abdominal complications
• Calcium deposits in the affected muscles, especially in children with the disease

Call your health care provider if symptoms of this disorder occur; shortness of breath and difficulty swallowing can indicate an urgent need for treatment.

Harris ED, Budd RC, Genovese MC, Firestein GS, Sargent JS, Sledge CB. Kelley's Textbook of Rheumatology. 7th ed. St. Louis, Mo: WB Saunders; 2005:1312-1316.

Goldman L, Ausiello D. Goldman: Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders; 2007.