Besides the classical
inflammatory myopathies (IM),
dermatomyositis (DM),
polymyositis, and
inclusion body myositis, the much larger spectrum of IM includes focal and nodular
myositis, granulomatous
myositis,
macrophagic myofasciitis, graft vs. host
myositis, eosinophilic myositis, and other immune-associated conditions, some of them only recently described. In addition, paraneoplastic,
statin-induced and
critical illness myopathies have been considered immune-associated IM. Infectious, i.e., bacterial, viral, and parasitic IM are much less frequent in the northern hemisphere. In IM, muscle biopsy is an essential diagnostic procedure to initiate
therapy. The myopathological spectrum encompasses disease-specific histopathological features, such as perifascicular
atrophy in DM, non-necrotizing
granulomas in sarcoid
myopathy, autophagic vacuoles with tubulofilamentous inclusions in
inclusion body myositis, rarely electron microscopic criteria, such as undulating tubules in endothelial cells of DM specimens, and, foremost, immunohistochemical findings. These latter features concern inflammatory infiltrates, the muscle parenchyma, the interstitial compartment, and the vasculature with varying involvement of each component in the different IM. Differences in immunohistochemical parameters among the IM, such as major histocompatibility complexes I and II,
cytokines,
cell adhesion molecules, different types of inflammatory cells,
metalloproteinases, and
complement factors procure a large gamut of data, the individual patterns of which characterize the myopathology of individual IM.