Time course of primary and secondary hyperalgesia after heat injury to the skin.

We have examined the time course of, and relationship between, primary and secondary hyperalgesia after thermal injury to the skin in humans. Burn injuries (15 x 25 mm rectangular thermode, 49 degrees C, 5 min) were produced in eight healthy, unmedicated male volunteers, on the medial side of the right calf, on two occasions at least 8 days apart. Heat pain detection thresholds (HPDT), heat pain tolerance (HPT), mechanical pain detection threshold (MPDT) and the intensity of burn-injury induced erythema (skin erythema index, SEI) were assessed inside the burn injury. HPT was assessed only in one study period. Areas of hyperalgesia to pinprick and brush were determined outside the injury. Assessments were made before and regularly for 72 h after the burn injury. There was a decrease in heat pain and mechanical thresholds, an increase in SEI inside the injury and development of mechanical hyperalgesia outside the injury. These alterations were consistent between the two examination days. The time course of the intensity of primary hyperalgesia was related closely to that of changes in area of secondary hyperalgesia, and hyperalgesia outside the injury did not outlast hyperalgesia inside the injury in any volunteer. These findings suggest post-injury development of secondary hyperalgesia to be a dynamic process, closely related in time to a peripheral nociceptive input, with reversal to normal when the peripheral lesion disappears. These observations may be relevant to the concept of "pre-emptive" analgesia.