Myositis as a cause of muscle pain

dIn inflammatory myopathies muscle pain is caused by irritation of the noci ceptors. The phenomenon is mediated by substances such as histamine or brad ykinin produced by histiocytes and lymphocytes, Furthermore, neuropeptides like substance P or calcitonin-gene related peptide may be involved in this process. The earliest stage of an inflammatory myopathy is interstitial my ositis, In acute forms, muscle pain is very prominent, In focal nodular myo sitis, prominent pain and slight muscle weakness is followed by muscle atro phy. Dermatomyositis is caused by a vasculitis of small blood vessels, asso ciated with deposits of C5b9-complement and tubulovesicular inclusions. Mus cle fiber damage is pronounced in the perifascicular region, Muscle pain is common in acute cases. Idiopathic polymyositis produces the pattern of dif fuse polymyositis with infiltration of cytotoxic CD8-positive lymphocytes t hat invade non-necrotic muscle fibers. Muscle pain is less prominent, and m ay be absent in chronic forms, A special form of the chronic painless diffu se polymyositis is inclusion body myositis, which occurs especially in the elderly. Filamentous inclusions in nucleus and cytoplasm are pathognomic. T he absence of muscle pain in this subtype of myositis is not yet understood .