Could Reported Sex Differences in Hypertonic Saline-induced Muscle Pain be a dose Issue?

Higher levels of experimental muscle pain induced by injection of the same volume of noxious substances have been reported by women compared to men. This could hypothetically be related to the difference in muscle volume between men and women. The aim of this study was to investigate if the sex differences reported by intramuscular injection of hypertonic saline would disappear if a larger dose is given to men than women under similar conditions. Methods: Fifty-six healthy volunteers (25 men and 31 women) received hypertonic saline injection into the masseter muscle, 0.5 mL for men and 0.3 mL for women, to evoke pain. Pain intensity was assessed with 0-100 mm visual analogue scale (VAS) every 15 seconds until pain subsided or maximum 300s. VAS was also used to assess perceived unpleasantness and anxiety. Pain drawings were used to assess maximal pain distribution, and the McGill pain questionnaire to assess pain quality. Results: There was no sex difference in maximum pain intensity, unpleasantness, anxiety or pain drawing area, but the evoked pain had larger total pain area (p=0.005), and longer duration (p<0.001) in the men than women. The sexes also used some different pain descriptors. Conclusions: This study shows that the previously reported higher pain levels in women were abolished when a lower dose of hypertonic saline was injected into the masseter muscle of the women than men. This might indicate that the sex differences reported to hypertonic-induced muscle pain may be a dose issue. Further studies are required to validate these result