Effects of contact-heat stimulation intensity on the reliability of subjective pain rating of individuals with different heat tolerance

Abstract

Background: Studies revealed that contact-heat stimulations mediate pain perception due to temporal summation of second pain (TSSP). How heat intensity affects the reliability of the pain rating of individuals with different heat tolerance is not well examined. This study investigated (1) the influence of the preceding contact-heat stimuli with different levels of intensity on the reliability of subjective pain rating and (2) the differences in reliability of subjective pain rating of participants with high and low sensory sensitivity or heat tolerance. Method: Participants with intact sensory function were divided into (1) high (n = 17) and low (n = 13) sensitivity groups based on the cutoff temperature of 42°C or (2) high (n = 18) and low (n = 12) pain-tolerance groups based on the cutoff temperature of 47°C equivalent to numerical rating scale (NRS) of 7. In each trial, participants were given a pair of 2-s contact-heat stimuli (an interstimulus interval of 2.5 s) at the left thenar eminence and were asked to report an NRS rating. Four blocks of intensity combinations were given: Low–Low, High–High, Low–High, and High–Low conditions, with 72 trials in each block. Results: Findings revealed that high heat-tolerance group results had lower intraclass correlation coefficients (ICCs) when contact-heat stimuli were preceded by another with higher intensity (ICC = 0.551–0.747) compared to those preceded by lower intensity (ICC = 0.724–0.818). In contrast, the ICCs of the low heat-tolerance group were found to be relatively higher regardless of heat intensity (ICC = 0.595–0.806). Conclusions: The TSSP effect reflected by lower pain rating reliability appears to be induced in the high heat-tolerance group when a contact-heat stimulation is preceded by another stimulation with higher intensity but with the same duration. This is possibly due to the longer offset time of contact-heat stimulations with higher intensity, and also the top-down modulatory effects in this high heat-tolerance group. Further electrophysiological studies would be needed to investigate the underlying neural processes of TSSP in individuals with different heat tolerance.