Effect of ambient temperature on human pain and temperature perception

Background Animal studies show reduced nociceptive responses to noxious hea t stimuli and increases in endogenous beta-endorphin levels in cold environ ments, suggesting that human pain perception may be dependent on ambient te mperature. However, studies of changes in local skin temperature on human p ain perception have yielded variable results, This study examines the effec t of both warm and cool ambient temperature on the perception of noxious an d innocuous mechanical and thermal stimuli. Methods Ten subjects (7 men and 3 women, aged 20-23 yr) used visual analog scales to rate the stimulus intensity, pain intensity, and unpleasantness o f thermal (0-50 degrees C) and mechanical (1.2-28.9 g) stimuli applied on t he volar forearm with a 1-cm(2) contact thermode and von Frey filaments, re spectively. Mean skin temperatures were measured throughout the experiment by infrared pyrometer, Each subject was tested in ambient temperatures of 1 5 degrees C (cool), 25 degrees C (neutral), and 35 degrees C (warm) on sepa rate days, after a 30-min acclimation to the environment. Studies began in the morning after an 8-h fast. Results Mean skin temperature was altered by ambient temperature (cool room : 30.1 degrees C; neutral room: 33.4 degrees C; warm room: 34.5 degrees C; P < 0.0001). Ambient temperature affected both hear (44-50 degrees C) and c old (25-0 degrees C) perception (P < 0.01), Stimulus intensity ratings tend ed to be lower in the cool than in the neutral environment (P < 0.07) but w ere not different between the neutral and warm environments. Unpleasantness ratings revealed that cold stimuli were more unpleasant than hot stimuli i n the cool room and that noxious heat stimuli were more unpleasant in a war m environment, Environmental temperature did not alter ratings of warm (37 and 40 degrees C) or mechanical stimuli. Conclusions: These results indicate that, in humans, a decrease in skin tem perature following exposure to cool environments reduces thermal pain. Supp ression of A delta primary afferent cold fiber activity has been shown to i ncrease cold pain produced by skin cooling. Our current findings may repres ent the reverse phenomenon, i.e., a reduction in thermal nociceptive transm ission by the activation of A delta cutaneous cold fibers.