Investigating mechanisms behind offset analgesia: Effect on spinal responses during thermal stimulation.

Introduction Offset analgesia (OA) is a temporal perceptual mechanism in which subjective pain ratings decrease disproportionally when a noxious heat stimulus is decreased by 1-3 ◦C. Whether OA is a peripheral, spinal or supraspinal mechanism remains unknown. The stimulation of afferent nociceptors in the foot, leads to a spinal nociceptive withdrawal reflex (NWR) which is mediated through the wide dynamic range (WDR) neurons and therefore under descending control. We hypothesized that OA affects the spinal nociceptive neurons resulting in an attenuation of the NWR during OA. Methods Four heat stimulations profiles were applied to the lower legs divided into four segments of 5 s, 5 s, 5 s, and 15 s, respectively: Offset Analgesia Trial (OAT) (48, 49, 48, 48 ◦C), Offset Baseline Trial (OBT) (48, 49, 32, 32 ◦C), Constant Heat Trial (CHT) (4 × 48 ◦C), and Baseline Trial (BT) (4 × 32 ◦C). Subjects rated the pain intensity continuously using a visual analog scale (VAS). NWR were evoked by electrical stimulation of the plantar foot and assessed once during each segment by recording EMG from the tibialis anterior muscle. Results VAS-ratings were lower during the third period of OAT compared to CHT (p < 0.001). However, there was no difference (p > 0.05) comparing the NWR size between OAT, OBT, CHT, and BT throughout the time periods. Conclusions The NWR was not affected by OA suggesting that spinal WDR plays a limited role in the OA mechanism. Whether peripheral- or supraspinal mechanisms are responsible the OA phenomenon remains unknown.