The role of muscle biopsy in the diagnosis of inflammatory myopathy

Muscle biopsies of patients with immune-mediated inflammatory myopathies are characterized by a combination of chronic inflammatory infiltrates and muscle fiber necrosis. In addition, there are typical histochemical, immunohistochemical, and ultrastructural findings of each type of inflammatory myopathy. Accurate classification of inflammatory myopathies is important because inclusion body myositis does not respond to immunosuppressive treatment, polymyositis is frequently part of an overlap syndrome, and dermatomyositis is associated with cancer in adults. Necrotizing myopathy without inflammation may be due to a partially treated inflammatory myopathy, a toxic myopathy, or a paraneoplastic myopathy. Metabolic myopathies and muscular dystrophies may clinically and rarely pathologically mimic inflammatory myopathy. Correct identification of these entities depends on better awareness of clinical similarities between metabolic myopathy and inflammatory myopathy; appropriate utilization of immunohistochemical, ultrastructural, biochemical, and genetic techniques on muscle samples in difficult cases; and maximizing communication between the clinician and the pathologist.