Effect of Sympathetic nerve block on acute inflammatory pain and hyperalgesia

Sympathetic nerve blocks relieve pain in certain chronic pain states, but the role of the sympathetic pathways in acute pain is unclear. Thus the authors wanted to determine whether a sympathetic block could reduce acute pain and hyperalgesia after a heat injury in healthy volunteers. The study was made as a randomized, single blinded investigation, in which the volunteers served as their own controls. A lumbar sympathetic nerve block and a contralateral placebo block were performed in 24 persons by injecting 10 ml bupivacaine (0.5%) and 10 ml saline, respectively. The duration and quality of blocks were evaluated by the sympatogalvanic skin response and skin temperature. 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 injuries. Of the 24 volunteers, eight were excluded because of somatic block or incomplete sympathetic block. The study revealed no significant differences between sympathetic block and placebo for pain or mechanical allodynia during injury, or pain thresholds, pain responses to heat, or areas of secondary hyperalgesia after the injury. The comparisons were done for the period when the block was effective. Sympathetic nerve block did not change acute inflammatory pain or hyperalgesia after a heat injury in human skin.