EFFECTS OF SYMPATHETIC-NERVE BLOCK ON ACUTE INFLAMMATORY PAIN AND HYPERALGESIA

Background: Sympathetic nerve blocks relieve pain in certain chronic p ain states, but the role of the sympathetic pathways in acute pain is unclear. Thus the authors wanted to determine whether a sympathetic bl ock could reduce acute pain and hyperalgesia after a heat injury in he althy volunteers. Methods: The study was made as a randomized, single- blinded investigation, in which the volunteers served as their own con trols, A lumbar sympathetic nerve block and a contralateral placebo bl ock were performed in 24 persons by injecting 10 ml bupivacaine (0.5%) and 10 ml saline, respectively, The duration and quality of blocks we re evaluated by the sympathogalvanic skin response and skin temperatur e, Bilateral heat injuries were produced on the medial surfaces of the calves with a 50 x 25 mm thermode (47 degrees C, 7 min) 45 min after the blocks. Pain intensity induced by heat, pain thresholds to thermal and mechanical stimulation, and secondary hyperalgesia were assessed before block, after block, and 1, 2, 4, and 6 h after the heat injurie s. Results: Of the 24 volunteers, eight were excluded because of somat ic block or incomplete sympathetic block. The study revealed no signif icant differences between sympathetic block and placebo for pain or me chanical allodynia during injury, or pain thresholds, pain responses t o heat, or areas of secondary hyperalgesia after the injury. The compa risons were done for the period when the block was effective. Conclusi on Sympathetic nerve block did not change acute inflammatory pain or h yperalgesia after a heat injury in human skin.