The principal changes in muscle consist of infiltrates of inflammatory cells (lymphocytes, macrophages, plasma cells, and rare eosinophils and neutrophils) and destruction of muscle fibers with a phagocytic reaction. Perivascular (usually perivenular) inflammatory cell infiltration is the hallmark of polymyositis. Interstitial inflammatory cell infiltration is also a prominent feature of the disease, but lesser degrees of it may be seen in other conditions as a secondary reaction (e.g., in facioscapulohumeral and Becker’s muscular dystrophy).
Evidence of muscle fiber degeneration and regeneration is almost invariably present. Many of the residual muscle fibers are small, with increased numbers of sarcolemmal nuclei. Either the degeneration of muscle fibers or the infiltration of inflammatory cells may predominate in any given biopsy specimen. Blood vessel changes and perifascicular atrophy are more prominent in childhood dermatomyositis than in adult dermatomyositis and polymyositis. Capillary loss due to endothelial cell necrosis occurs particularly in the periphery of fascicles and may explain the perifascicular atrophy. Other features include reduplication of capillary basement membrane and the presence of tubular inclusions within endothelial cells. Type II muscle fiber atrophy and muscle infarcts also may be found. Vasculitis is also seen in polymyositis or dermatomyositis associated with connective tissue disorders.
Patients with dermatomyositis who have the characteristic skin rash, muscle weakness, EMG changes, and elevation of serum CK may not require a muscle biopsy to confirm the diagnosis. In the case of idiopathic polymyositis, however, a firm diagnosis must be based on the presence of a typical clinical picture, a typical EMG, elevation of serum CK, and a diagnostic muscle biopsy. All four of these criteria are required to be certain of the diagnosis, since inflammatory changes may occasionally occur in other myopathies (e.g., facioscapulohumeral muscular dystrophy) and in other connective tissue disorders without clear muscle weakness. However, in fewer than one-third of cases of polymyositis are ALL these criteria satisfied. It may be particularly difficult to obtain a diagnostic muscle biopsy because of the patchy nature of the disease. Thus, a therapeutic trial of glucocorticoids should be given when full investigation of a patient with significant disability leaves a diagnosis of “possible polymyositis,” usually because of a nondiagnostic muscle biopsy.
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