Secondary hyperalgesia to mechanical but not heat stimuli following a capsaicin injection in hairy skin

A psychophysical investigation was carried out to examine whether heat hyperalgesia exists within the secondary mechanical hyperalgesia zone surrounding a capsaicin injection site on hairy skin. A non-contact laser stimulator was used to deliver temperature controlled stimuli to sites within and outside the zone of mechanical hyperalgesia. Heat testing was carried out before and after the intradermal injection of 50 micrograms of capsaicin into the volar forearm. The zones of mechanical hyperalgesia to punctate and stroking stimuli and the region of flare were also mapped after the capsaicin injection. Heat pain thresholds inside the secondary mechanical hyperalgesic zone were not significantly different from thresholds outside the secondary mechanical hyperalgesia zone. In addition, pain ratings to an ascending series of heat stimuli delivered inside the zone of secondary hyperalgesia were not significantly different from pain ratings outside the zone of secondary hyperalgesia. Thus, there was no evidence for heat hyperalgesia within the zone of secondary hyperalgesia to punctate mechanical stimuli. Though the areas of punctate and stroking hyperalgesia were correlated, no correlation existed between the magnitude of capsaicin evoked pain and the areas mechanical hyperalgesia to punctuate and stroking stimuli or the area of flare. This suggests that independent mechanisms may mediate evoked pain, central sensitization that leads to mechanical hyperalgesia, and axon reflexive flare.