Muscle biopsy findings in inflammatory myopathies

The inflammatory myopathies encompass a heterogeneous group of acquired muscle diseases characterized clinically, by muscle weakness, and histologically, by inflammatory infiltrates within the skeletal muscles. The group of these myopathies comprise three major and discrete subsets: polymyositis (PM), dermatomyositis (DM), and inclusion body myositis (IBM). Each subset retains its characteristic clinical, immunopathologic, and morphologic features regardless of whether it occurs separately or in connection with other systemic diseases. Although the diagnosis of these disorders is based on the combination of clinical examination, electromyographic data, serum muscle enzyme levels, various autoantibodies, and the muscle biopsy findings, the muscle biopsy offers the most definitive diagnostic information in the majority of the cases. This article summarizes the main histologic features that characterize PM, DM, or IBM and emphasizes the main pitfalls associated with interpretation of the biopsies.