Referred Muscle Pain and Hyperalgesia from Viscera: Clinical and Pathophysiological Aspects

Referred muscle pain, i.e., pain perceived in a musculoskeletal area other than the site of the noxious stimulation, is seen frequently in the clinical setting. Referred pain from viscera is the most paradigmatic form of this phenomenon and is frequently accompanied locally by secondary hyperalgesia. In patients with different diseases of internal organs (gastrointestinal and urinary tracts, female reproductive organs) this muscle hyperalgesia, testified by a pain threshold decrease, proves to be an early process, proportional in extent to the number and intensity of visceral pain episodes and prolonged in duration. Referred muscle pain with hyperalgesia is normally attributed to processes of central sensitization which, as shown by electrophysiological studies on animal models of this condition, take place in the Central Nervous System, triggered by the massive afferent visceral barrage upon convergent viscero-somatic neurons. The visceral barrage, however, is also likely to activate a reflex arc towards the periphery (afferent branch: visceral afferent fibers, efferent branch: somatic efferences towards the muscle) resulting in sustained muscle contraction and subsequent local sensitization of nociceptors. This hypothesis is supported by the results of recent studies in the area of referred muscle hyperalgesia caused by an artificial ureteric stone in rats, where positivity was found for a number of morphofunctional indices of skeletal muscle contraction (decrease in I band length/sarcomere length ratio and in 3Hryanodine binding, increase in Ca 2+ ATPase activity and muscle cell membrane fluidity). These changes were proportional to the degree of the hyperalgesia behaviorally monitored in the animals.