I.I.1 Myopathologic classification of acquired immune and inflammatory myopathies

Abstract

It is now difficult to classify acquired immune and inflammatory myopathies (IIMs) into traditional groups: dermatomyositis, polymyositis, and inclusion body myositis (IBM). Myopathologic classification of IIM provides more specific and inclusive diagnostic information. Pathologic features used to define IIM subtypes include: Muscle fiber changes; Immune abnormalities (cellular, humoral); and Tissue types involved (connective tissue, vessels, muscle fibers). Some features, like B-cell foci and alkaline phosphatase positive vessels or perimysium, occur more generally with treatable IIM. Myopathologic subtypes of IIM include: (1) Immune myopathies with perimysial pathology (IMPP) have perimysial connective tissue with fragmentation and histiocytic cellularity. Jo-1 antibody associated IIM often have IMPP pathology; (2) Myovasculopathies have damaged perimysial or endomysial vessels. Muscle damage may be due to ischemia rather than direct immune attack. Childhood dermatomyositis syndromes are often myovasculopathies. (3). Immune polymyopathies are active myopathies with necrosis but little inflammation. Serum CK is very high. Myopathies with signal recognition particle antibodies are an example. (4) Immune myopathies with endomysial pathology (IM-EP) often have C5b-9 complement deposition on endomysial connective tissue, and mononuclear cell foci. Brachio-cervical inflammatory myopathies illustrate IM-EP. (5) Histiocytic inflammatory myopathies have granulomas and focal invasion of muscle fibers by mononuclear cells. They include sarcoid myopathy. (6) Inflammatory myopathies with vacuoles, aggregates and mitochondrial pathology (IM-VAMP) in myofibers can also have diffusely upregulated myofiber MHC I, CD4 and CD8 cell foci and NT5C1A antibodies. IBM is a subgroup. IM-VAMP are poorly treatable. Myopathologic classification of IIM adds diagnostic and prognostic accuracy, and focuses testing, treatment, and pathogenic investigations.