Inclusion body myositis is a rare myopathy that, like polymyositis, is thought to be mediated by an immune reaction to muscle fibers via cytotoxic T-lymphocytes. The typical presentation is insidious onset of slowly progressive, non-painful, muscular weakness over years in a person over age 50. Males are affected twice as often as females. Unlike polymyositis, no response to corticosteroid therapy occurs. There is chronic inflammation, and within some muscle fibers can be seen vacuoles.

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