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Polymyositis is a relatively uncommon inflammatory disease that leads to muscle weakness, swelling (inflammation), tenderness, and damage. It is part of a larger group of diseases called myositis.

Polymyositis is a skeletal muscle disease that is also known as idiopathic inflammatory myopathy. The cause is unknown. An autoimmune reaction or infection of the skeletal muscle may cause the disease.

Polymyositis can affect people at any age, but it most commonly occurs between ages 50 and 70, or in children ages 5 to 15. 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.

Other types of inflammatory muscle diseases exist, including inclusion body myositis.

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

• Difficulty swallowing
• Muscle pain
• Muscle weakness in the proximal muscles (shoulders, hips, etc.) that makes it difficult to raise the arms over the head, get up from a sitting position, or climb stairs
• Problems with the voice (caused by weak throat muscles)
• Shortness of breath

Patients may also have:

• Fatigue
• Fever
• Joint pain
• Loss of appetite
• Morning stiffness
• Weight loss

A similar condition called dermatomyositis causes a reddish-purple or red rash on the face or body.

Muscle weakness and tenderness can be signs of polymyositis. A rash is a sign of a similar condition, dermatomyositis.

Tests may include:

• Autoimmune antibodies and inflammation tests
• CPK
• Electromyography
• MRI of affected muscles
• Muscle biopsy
• Myoglobin in the urine
• Serum aldolase

Patients with this condition must be carefully monitored for cancer.

The main treatment is with corticosteroid medications. When muscle strength improves, 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, such as methotrexate and azathioprine, may be used.

Intravenous gamma globulin has been tried, with mixed results. It is too soon to know what role biologic drugs may play in treating this condition.

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

Response to treatment varies, and it depends on whether there are complications. The 5-year mortality can be as high as 20%.

Many patients, especially children, go into remission (a period when no symptoms are present) and recover. For most other people, immunosuppressant drugs can control the disease.

In adults, death may result from:

• Malnutrition
• Pneumonia
• Respiratory failure
• Severe, long-term muscle weakness

The major causes of death are cancer (malignancy) and lung disease.

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

Call your health care provider if you have symptoms of this disorder. Seek emergency treatment if you have shortness of breath and difficulty swallowing.

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.