Sex differences and hormonal influences on response to cold pressor pain in humans.

UNLABELLED: Although most studies show that women have higher subjective pain ratings in response to painful stimuli, there is less consistency across studies with regard to the influence of gonadal hormones on pain responsivity. The present study evaluated sex differences in response to cold pressor pain in normally menstruating women (NMW), women maintained on oral contraceptives (OCW), and men. Testing occurred during 5 phases of the menstrual cycle. All participants completed 10 sessions (2 sessions per phase). During the cold pressor test, participants immersed the forearm into water maintained at 4 degrees C, and pain threshold and tolerance were measured. Subjective ratings of pain, physiologic indices, and plasma levels of estradiol and progesterone were also assessed. Both estradiol and progesterone levels varied as a function of menstrual cycle phase in NMW and were significantly higher in NMW compared with OCW and men. There were no significant differences in pain threshold or tolerance for any of the groups as a function of menstrual cycle phase. There were no significant differences in pain tolerance between groups. However, pain threshold was higher in NMW compared with OCW and men. When the data were reanalyzed across consecutive sessions, a significant sex-by-day interaction was observed for both threshold and tolerance. Specifically, pain threshold and tolerance were similar for NMW, OCW, and men, but these latencies changed at different rates across session days. Pain threshold remained relatively constant for both OCW and men, but it increased across days for NMW. Pain tolerance remained stable across sessions in OCW, a slow consistent increase was observed for men, whereas a sharper increase, followed by an asymptote, was observed for NMW. These results suggest that circulating gonadal hormones might mediate adaptation to cold pressor pain. PERSPECTIVE: The present study supports the notion that differences in pain perception between the sexes and among menstrual cycle phases are subtle. However, normally menstruating women exhibited an increase in pain tolerance and threshold over repeated stimulation, whereas men exhibited a shallow increase in pain threshold only, suggesting a sex difference in the adaptation to painful stimuli in men and women.