Experimenter- and Infrared Thermography–Derived Measures of Capsaicin-Induced Neurogenic Flare Among Non-Hispanic White and Black Adults

Abstract Objective Capsaicin is a widely utilized experimental pain stimulus; however, few studies have reported on ethnic differences in pain responses to capsaicin. The present study used infrared thermography to 1) measure differences in capsaicin-induced neurogenic flare between non-Hispanic black (NHB) and non-Hispanic white (NHW) adults and 2) determine the association between neurogenic flare and secondary hyperalgesia. Methods Fifty-four participants (NHB N = 28) underwent heat/capsaicin sensitization model procedures. Neurogenic flare was examined using experimenter (i.e., subjective) and thermography (i.e., objective) measurements. A typically nonpainful mechanical punctate probe was used to measure secondary hyperalgesia. Results Ethnic groups did not significantly differ in age, sex, marital status, or personal income. Although experimenters rated a significantly wider area of capsaicin-related neurogenic flare among NHW compared with NHB participants (F1, 52 = 8.33, P = 0.006), thermography results showed no differences between groups in neurogenic flares (F1, 52 = 0.01, P = 0.93). Further, although NHB individuals reported greater average pain during the capsaicin procedures compared with NHW individuals (NHB = 58.57 [3.67], NHW = 46.46 [3.81]; F2, 51 = 5.19, P = 0.03), the groups did not differ in secondary hyperalgesia (F2, 51 = 0.03, P = 0.86), and ethnicity did not moderate the association between neurogenic flare and secondary hyperalgesia (F3, 50 = 0.24, P = 0.87). Conclusions Findings cautiously support the use of infrared thermography over subjective experimenter report when measuring neurogenic inflammation in diverse samples. However, infrared thermography should not be used as a diagnostic tool for pain, given the lack of association between these factors. Future research is warranted to replicate these findings in a larger and more diverse sample to determine accurate neurogenic inflammation measures across other ethnic minority populations.