Offset analgesia evoked by non-contact thermal stimulator

Abstract

Abstract Objective The objective of this study was to test if offset analgesia could be evoked using a noncontact thermal stimulator. Offset analgesia [J. Neurophysiol. 87:2205–2208, 2002] is defined as an unproportionally large decrease in pain intensity following a slight decrease in stimulation intensity. The importance of differences in thermal properties between human hairy and glabrous skin was investigated. Methods A 20W diode laser (970 nm) was used for the thermal stimulation. A fast (50 images/s) infrared camera measured the skin temperature and a temperature controlled feedback control loop adjusted the laser power. 8 subjects participated in this study. Stimulations were applied on the dorsum side and in the palm of the hand. Subjects were instructed to continuously rate the pain intensity. First the subject was stimulated using both a rising 35–45 °C staircase and a decreasing 45–35 °C staircase in both skin types; each staircase step was 1 °C and lasted for 15 s. Offset analgesia was tested by stimulating the hairy skin on the dorsum of the hand using two sequential temperature plateaus (48–48 °C, 48–49 °C, 49–48 °C and 49–49 °C). Each plateau was held for 5 s. Results For the staircase stimulations identical surface temperatures were perceived significantly higher in glabrous than in hairy skin (p < 0.001). The offset analgesia test showed that a decrease in temperature from 49 to 48 °C evoked a drop in the pain rating which was significantly lower than observed during a 48–48 °C stimulation (p < 0.001) indicating offset analgesia. Conclusion A non-contact thermal stimulator is able to evoke offset analgesia. Furthermore, it was noted that a high penetration laser causes higher pain ratings in glabrous skin than in hairy skin—a relationship which is opposite to low penetration lasers (CO2 laser) and contact heat stimulation.