Diagnostic uncertainty in the inflammatory myopathies.

A group of case histories with appropriate muscle biopsy findings is presented to demonstrate some atypical presentations of the inflammatory myopathies. Differential diagnostic possibilities are considered in presentations of idiopathic polymyositis, statin myotoxicity, the inflammatory component with the dysferlinopathies, treated dermatomyositis, a necrotizing myopathy with pipe-stem microvascular change, an inflammatory myopathy with abundant macrophages, inclusion-body myositis, and the differential diagnosis of problems with eosinophilic infiltration in the muscle biopsy. Attention is given to the role of membrane attack complex deposition in the microvasculature and the role of major histiocompatibility complex-1-expressing muscle fibers indicating activation of the endoplasmic reticulum stress response.