Polymyositis And Dermatomyositis

Polymyositis and dermatomyositis are diffuse, inflammatory myopathies that produce symmetrical weakness of striated muscles, especially proximal muscles of the shoulder, pelvic girdle, neck, and pharynx. In dermatomyositis, such weakness is accompanied by cutaneous involvement. These disorders usually progress slowly, with frequent exacerbations and remissions.

Polymyositis and dermatomyositis are twice as common in women as in men (with the exception of dermatomyositis with cancer, which is most common in men over age 40). Generally, the prognosis worsens with age. Although 80% to 90% of affected children regain normal function if properly treated, untreated childhood dermatomyositis may rapidly progress to disabling contractures and muscle atrophy.

Causes

Although the cause of polymyositis and dermatomyositis is unknown, the disorders may result from autoimmunity, perhaps combined with defective T-cell function. Presumably, the patient's T cells inappropriately recognize muscle fiber antigens as foreign and release lymphotoxins that cause diffuse or focal muscle fiber degeneration. Regeneration of new muscle cells follows, producing remission.

These disorders may be associated with allergic reactions, systemic lupus erythematosus (SLE), scleroderma, rheumatoid arthritis, Sjogren's syndrome, penicillamine administration, systemic viral infection, and carcinomas of the lung, breast, and other organs.

Signs and Symptoms

• The main sign is muscle weakness. Usually the first muscles affected are in the hips and thighs, though any other muscle in the body can be affected.  It may be hard to climb stairs, get dressed or get out of bed.
• Pain in the joints between bones. However, the joints are usually not warm or swollen, as with some other forms of arthritis.
• Deep red (almost purple) rash, usually on the face, scalp, neck and chest. However, the rash can appear on any part of the body.
• Coughing and shortness of breath.

Diagnostic tests

Muscle biopsy that shows necrosis, degeneration, regeneration, and interstitial chronic lymphocytic infiltration allows diagnosis.

Appropriate laboratory tests differentiate polymyositis from disorders that cause similar muscular or cutaneous symptoms, such as muscular dystrophy, advanced trichinosis, psoriasis, seborrheic dermatitis, and SLE.

Treatment

High-dose corticosteroid therapy (40 to 60 mg/day) relieves inflammation and lowers muscle enzyme levels. Within 2 to 6 weeks after treatment begins, serum muscle enzyme levels usually return to normal and muscle strength improves, permitting a gradual titration of corticosteroid dosage. If the patient responds poorly to corticosteroids, treatment may include cytotoxic or immunosuppressant drugs, such as cyclophosphamide, azathioprine, and methotrexate, given intermittently I.V or daily by mouth.

Supportive therapy includes bed rest during the acute phase, range-of-motion exercises to prevent contractures, analgesics and application of heat to relieve painful muscle spasms, and diphenhydramine to relieve itching. Patients over age 40 need thorough assessment for coexisting cancer.

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