Polymyositis is a systemic connective tissue disease characterized by inflammatory and degenerative changes in proximal muscles sometimes accompanied by characteristic skin rash. If skin manifestations are associated, it is designated as dermatomyositis. The classic lilac-colored (heliotrope) rash is on the eyelids, bridge of the nose, cheeks (butterfly distribution), forehead, chest, elbows, knees and knuckles, and around the nail beds.
Diagnosis usually relies on 4 findings - weakness, CPK elevation, abnormal EMG, and findings on muscle biopsy. Presence of skin rash of dermatomyositis also helpful.
The exact cause of polymyositis is unknown. Dermatomyositis and polymyositis are conditions of presumed autoimmune etiology in which the skeletal muscle is damaged by a nonsuppurative inflammatory process dominated by lymphocytic infiltration. Potential factors include: Inciting viral infection, T cell activation, Cytokine release, and Immune-mediate muscle destruction.
Risk factors include a family history of autoimmune disease or vasculitis.
Infectious myositis can result from viral, bacterial, or parasitic infections. Experimental viral myositis can be induced in animals by coxsackievirus. A mild inflammatory myopathy can occur with influenza and coxsackieviruses in humans. A focal infective myositis due to streptococcal or staphylococcal infection is mostly seen in the tropics. Rare cases of polymyositis have been found to result from infection with toxoplasma, viruses, and spirochetes (Lyme disease). Trichinosis may be confused with idiopathic polymyositis, particularly with a history of raw or undercooked pork ingestion.
Drug-induced myopathies can result from: Cholesterol lowering agents, Colchicine, Corticosteroids, Ethanol (alcohol), Chloroquine, and Zidovudine (AZT). Electrolyte disorders (magnesium, calcium, potassium) may also lead to a myopathy. Rhabdomyolysis and myoglobinuria have been associated with intake of amphotericin B, e-aminocaproic acid, fenfluramine, heroin, and phencyclidine. A predominantly hypokalemic myopathy may result from prolonged use of diuretics, carbenoxolone, and azathioprine. Penicillamine has been reported to produce a myositis. The use of clofibrate, cimetidine, chloroquine, colchicine, carbimazole, cyclosporine, emetine, gemfibrozil, growth hormone, ketoconazole, leuprolide, lovastatin, phenytoin, provastatin, tretinoin, and, recently, AZT has been associated with a myopathy.
Eosinophilia-Myalgia Syndrome is more common in women has been associated with ingestion of contaminated batches of L-tryptophan that was removed from the marketplace by the FDA.
Lab tests will show an increased creatine kinase (CK), aldolase, SGOT, LDH, ESR. Positive rheumatoid factor in less than 50% of patients. Positive ANA is found in more than 50% of patients. Leukocytosis is found in less than 50% of patients. Anemia is present in less than 50% of patients. Hyperglobulinemia in less than 50% of patients. Increased creatinine in less than 50% of patients. Urinalysis will show myoglobinuria.
Myositis specific antibodies have been described in a minority of patients - most are anti-synthetase antibodies, anti-Jo-1 is the most common and has been found in about 20% of patients; associated with an increased incidence of interstitial lung disease.
Microscopic analysis of a muscle biopsy shows: muscle fiber degeneration, phagocytosis of muscle debris, perifascicular muscle fiber atrophy, inflammatory cell infiltrates in adult form, inclusion bodies (electron microscopy), and Sarcoplasmic basophilia. Muscle fibers are increased in size.
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