Pathophysiology of Inflammatory Muscle Pain

Objectives: Immunogenic inflammatory myopathies represent the most important group of acquired muscular disorders. Based on clinical as well as histopathological criteria they can be divided into interstitial myositis, focal nodular myositis, and polymyositis. Muscle pain is prominent especially in acute myositis, but is missed in chronic cases and especially inclusion body myositis. Findings: Muscle pain is caused by excitation of intramuscular free nerve endings, so called nociceptors. In skeletal muscle, nociceptors seem to be preferentially localized next to small vessels. The process of nociception is mediated by endogenous substances like bradykinin, serotonin, and histamine, which are released from inflammatory cells and bind to specific receptors in the nociceptor membrane. Furthermore, neuropeptides like substance P, calcitonin-gene related peptide, or nerve growth factor, which are altered in inflammatory myopathies, may be involved in this process. Conclusion: The concept of nociceptor sensitization by chemical stimuli, which are released during inflammation, is supported by numerous studies, and may explain the origin of inflammatory muscle pain especially in dermatomyositis with an local effect on the intramuscular vessels. Nevertheless, the absence of muscle pain in chronic myositis and inclusion body myositis is not yet fully understood. (C) 2002 by The Haworth Press, Inc. All rights reserved.