The gold standard to characterize
idiopathic inflammatory myopathies is the morphological, immunohistochemical and immunopathological analysis of muscle biopsy. Mononuclear cell infiltrates and muscle fiber
necrosis are commonly shared histopathological features. Inflammatory cells that surround, invade and destroy healthy muscle fibers expressing MHC
class I antigen are the typical pathological finding of
polymyositis. Perifascicular
atrophy and microangiopathy strongly support a diagnosis of
dermatomyositis. Randomly distributed necrotic muscle fibers without mononuclear cell infiltrates represent the histopathological hallmark of immune-mediated necrotizing
myopathy; meanwhile, endomysial
inflammation and muscle fiber degeneration are the two main pathological features in
sporadic inclusion body myositis. A correct differential diagnosis requires immunopathological analysis of the muscle biopsy and has important clinical implications for therapeutic approach. In particular, unnecessary, potentially harmful, immune-suppressive
therapy should be avoided alike in dystrophic
myopathies with secondary
inflammation.