Assessing Offset Analgesia through electrical stimulations in healthy volunteers.

Background and aims Offset Analgesia (OA) is a disproportionally large decrease of pain perception evoked by a slight decrease of a painful cutaneous heat stimulus, resulting in a lower pain perception compared to a simple constant stimulus at the same temperature. This study aimed to investigate the possibility of evoking the same disproportional analgesic effect by applying electrical stimuli. Methods 24 subjects underwent two control-trials of 30 s constant intensity by applying either heat stimulation at 48 °C or an electrical stimulation at 150% of the individual electrical Pain Detection (ePD) on the volar forearm. OA-trials consisted of a 30 s stimulation, divided into three periods: T1 (5 s), T2 (5 s), and T3 (20 s), with stimuli intensities of 48 °C, 49°C and 48°C or 150% of ePD, 180% of ePD, and 150% of ePD. Subjects were asked to rate the pain intensity on an electronic Visual Analog Scale (VAS; 0: no pain; 10: worst imaginable pain), and were categorized as responders if they showed more than 30% lower VAS at heat OA-trials compared to heat control-trial. Repeated measures Analysis of Variance was applied to investigate the difference in pain intensity to the electrical OA-trials, compared with the electrical control-trials. Results Responders to the heat OA-trial also responded to the electrical OA-trial compared to the electrical control-trial, with an analgesic effect of 3.3±0.5 VAS points (P < 0.001). However, when analyzing all subjects, no difference was found comparing the electrical OA-trial (VAS 3.8±0.5) to the electrical control-trial (VAS 6.2±0.4; P > 0.5). Conclusions This study suggests that responders to the heat OA-paradigm also respond to the electrical OA-paradigm.