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A systemic connective tissue disease characterized by inflammatory and degenerative changes in the muscles (polymyositis) and frequently also in the skin (dermatomyositis), leading to symmetric weakness and some degree of muscle atrophy, principally of the limb girdles. Certain clinical findings are shared with progressive systemic sclerosis (PSS) or, less frequently, SLE or vasculitis. Classification of the types of myositis includes primary idiopathic polymyositis; childhood dermatomyositis or polymyositis; primary idiopathic dermatomyositis in adults; inclusion body myositis (IBM); dermatomyositis or polymyositis associated with malignant neoplasms; polymyositis or dermatomyositis associated with various connective tissue disease overlap syndromes, including mixed connective tissue disease and sclerodermatomyositis.

Etiology and Incidence

The etiology is unknown. The disease may be caused by an autoimmune reaction; deposits of IgM, IgG, and the 3rd component of complement have been found in the blood vessel walls of skeletal muscle (with particularly high frequency in childhood dermatomyositis). A cell-mediated immune reaction to muscle plays a role. Viruses may participate: Picornavirus-like structures have been found in muscle cells, and tubular inclusions resembling paramyxovirus nucleocapsid have been identified by electron microscopy in myocytes and endothelial cells of vessels in the skin and muscle. The association of malignancy with dermatomyositis suggests that a neoplasm may incite myositis as the result of an autoimmune reaction directed against a common antigen in muscle and tumor.

The disease is not rare; it is less common than SLE or PSS, but more frequent than polyarteritis nodosa. The female:male ratio is 2:1. The disease may appear at any age but occurs most commonly from age 40 to 60 or, in children, from age 5 to 15.


Microscopic examination of the skin may show epidermal atrophy, basal cell liquefaction and degeneration, vascular dilation, and lymphocytic infiltration of the dermis. Structural changes in affected muscle vary greatly. The most frequent abnormalities consist of necrosis; phagocytosis; regenerative activity reflected by basophilia, large vesicular nuclei, and prominent nucleoli; atrophy and degeneration of muscle fibers, especially in a perifascicular distribution in patients with dermatomyositis; internal migration of nuclei; vacuolation; fiber-size variation; and a lymphocytic infiltrate, often most prominent in a perivascular location. There is an increase in endomysial and later perimysial connective tissue. Recently identified as a subset of the inflammatory myopathies, inclusion body myositis (IBM) on muscle biopsy shows less fiber necrosis and perivascular inflammation but more frequently shows hypertrophied fibers containing vacuoles rimmed with basophilic granules. In childhood, there may be widespread ulceration and infarction in the GI tract related to necrotizing arteritis. Intimal proliferation and thrombosis of small arteries and veins follow.

Symptoms and Signs

Onset may be acute or insidious. Symptoms in children and adults are similar, the only distinction being that childhood onset is more likely to be very acute, and adult onset more insidious. An acute infection may precede or incite the initial symptoms, which consist of proximal muscle weakness (in patients with IBM, distal weakness is equal to or greater than proximal weakness), muscle tenderness and pain, rash, polyarthralgias, Raynaud's phenomenon, dysphagia, and constitutional complaints, most notably fever, fatigue, and weight loss.

The muscle weakness may appear suddenly and progress over weeks to months. Patients may have difficulty raising the arms above the shoulders, climbing steps, or arising from a sitting position, and be unable to raise the head from the pillow. Patients may become wheelchair- or bedridden because of weakness of pelvic and shoulder girdle muscle groups. The flexors of the neck may be severely affected. Weakness of the laryngeal musculature is responsible for dysphonia. Involvement of the striated muscle of the pharynx and upper portion of the esophagus leads to dysphagia and regurgitation. A diminution in peristaltic activity and dilation of the lower esophagus and small intestine may be indistinguishable from that found in PSS. (The diagnosis in patients with such GI changes, who are described as having minimal or mild scleroderma, may in fact be PSS with CREST syndrome--see above.) The muscles of the hands, feet, and face escape involvement. Contractures of limbs may develop late in the chronic stage.

The cutaneous eruption tends to be dusky and erythematous and to have an SLE-like butterfly distribution on the face. Periorbital edema with a heliotrope hue is pathognomonic. The skin rash may be slightly elevated and smooth or scaly, and may appear on the forehead, V of the neck and shoulders, chest and back, forearms and lower legs, elbows and knees, medial malleoli, and dorsum of the proximal interphalangeal and metacarpophalangeal joints. The base and sides of the fingernails may be hyperemic. The skin lesions frequently fade completely but may be followed by brownish pigmentation, atrophy, scarring, or vitiligo. Muscular pain, tenderness, and induration tend to be associated with the rash. The skin changes suggest scleroderma in a few patients. Subcutaneous calcification may occur, particularly in childhood: This is similar in distribution to that encountered in PSS but tends to be more extensive (calcinosis universalis), particularly in untreated or undertreated disease.

Polyarthralgia, accompanied at times by swelling, joint effusions, and other evidence of nondeforming arthritis, occurs in about 1/3 of patients. These rheumatic complaints tend to be mild and respond well to corticosteroids. Raynaud's phenomenon occurs with particularly high frequency in those patients in whom polymyositis coexists with other connective tissue disorders. Visceral involvement (with the exception of the pharynx and esophagus) is relatively uncommon in polymyositis compared to the high frequency of internal changes in other connective tissue diseases, such as SLE and PSS, but occasionally precedes weakness with presenting symptoms. Interstitial pneumonitis (manifested by dyspnea and cough) occurs and may dominate the clinical picture. Cardiac involvement, detected chiefly in the ECG (arrhythmias, conduction disturbances, abnormal systolic time intervals), has been reported with increasing frequency. Acute renal failure as a consequence of severe rhabdomyolysis with myoglobinuria (crush syndrome) has been reported. Sj?gren's syndrome occurs in some patients. Abdominal symptoms, more common in children, may be associated with hematemesis or melena from GI ulcerations that may progress to perforation and require surgical intervention.

An associated malignancy occurs in about 15% of men (and a smaller proportion of women) over age 50. There is no characteristic type or site.

Laboratory Findings

Laboratory studies are helpful but nonspecific. The ESR frequently is elevated. Antinuclear antibodies and/or LE cells are found in a few patients, most often those with another connective tissue disease. About 2/3 of patients have antibodies to a thymic nuclear antigen designated PM-1 or whole thymus and thymic nuclear extracts (Jo-1). The relationship between these autoantibodies and disease pathogenesis remains unclear. Serum muscle enzymes, especially the transaminases, creatine kinase (CK), and aldolase, are usually elevated; the most sensitive and useful is CK. Periodic enzyme determinations are helpful in monitoring treatment: Elevated levels decrease with effective therapy. However, these enzymes may be normal despite active disease in patients with chronic myositis and widespread muscle atrophy.


Five major criteria are useful in diagnosis: proximal muscle weakness; a characteristic skin rash; elevated muscle enzymes in the serum; muscle biopsy changes (often the definitive test); and a characteristic triad of electromyographic abnormalities: (1) spontaneous fibrillations and positive sharp potentials, with increased insertional irritability; (2) polyphasic short potentials during voluntary contraction; and (3) bizarre, repetitive, high-frequency discharges during mechanical stimulation. Preferred sites for biopsy are muscles that show electrical abnormalities, usually the deltoid and quadriceps femoris, but on the opposite extremities to avoid sites previously explored.

A malignancy should be considered in any adult with dermatomyositis, following up on clues in the basic assessment, but not pursuing extensive, invasive blind studies.


Relatively satisfactory and long remissions, even apparent recovery, have been reported, especially in children. Death in adults follows severe and progressive muscle weakness, dysphagia, malnutrition, aspiration pneumonia, or respiratory failure with superimposed pulmonary infection. Polymyositis tends to be more severe and resistant to treatment in those individuals with cardiac or pulmonary involvement. Death in children usually is a result of vasculitis of the bowel. The prognosis for patients with malignancy-associated myositis generally is determined by the malignancy prognosis.


The patient's activities should be curtailed until the inflammation subsides. Corticosteroids are the drugs of choice initially. For acute disease, prednisone is given 40 to 60 mg or more/day, together with antacids and potassium supplements. Serial measurements of muscle enzyme activity in serum (especially CK) provide the best guide of therapy effectiveness, falling toward or reaching normal values in most patients in 4 to 6 wk. This is followed by an improvement in muscle strength. Once the enzyme levels have returned to normal, the dose of prednisone is reduced slowly; if muscle enzymes rise, the dose is increased. In adults, maintenance therapy with prednisone (10 to 15 mg/day) usually is necessary indefinitely. Children require high initial doses of prednisone (30 to 60 mg/m/day). Occasional patients treated chronically with high doses of corticosteroids become increasingly weak because of a superimposed corticosteroid myopathy; the corticosteroids must then be discontinued and another agent (eg, an immunosuppressant) substituted. In childhood, it may be possible to discontinue prednisone after a year or more, with apparent remission. The myositis associated with nonresectable tumors, metastatic disease, or IBM usually is more refractory to corticosteroids.

Immunosuppressive agents, including methotrexate, cyclophosphamide, chlorambucil, and azathioprine have been beneficial in patients who fail to respond to corticosteroids alone. Some patients have received methotrexate for > 5 yr for the control of this disease. Effectiveness of IV immunoglobulins is being evaluated; preliminary data are encouraging. Malignancy-associated myositis often remits if the tumor is removed.

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