Effects of intradermal foot and forearm capsaicin injections in normal and vulvodynia-afflicted women

Cutaneous response to capsaicin has been used to assess central sensitization in pain research. This study compared the response to intradermal capsaicin in the forearm and foot of vulvar vestibulitis (vestibulodynia)-afflicted cases and controls. We hypothesized that cases will experience greater spontaneous pain, larger cutaneous areas of punctate hyperalgesia and dynamic allodynia, and greater vascular flow than controls. We also hypothesized enhanced post-injection pain in the foot compared to the forearm based on dermatome proximity of the foot and vulva. Methods. Ten vulvar vestibulitis syndrome (VVS) cases and 10 age and ethnically matched controls underwent two randomized, cross-over trials with intra-dermal injections of capsaicin or a saline placebo in the forearm and foot. Outcome measures included spontaneous pain level, surface area of punctate hyperalgesia, surface area of dynamic allodynia, cutaneous blood flow, regional skin temperature and vital signs. Results. VVS cases experienced greater spontaneous pain, punctate hyperalgesia and dynamic allodynia than pain-free controls. Within the VVS group, post-capsaicin spontaneous pain, punctate hyperalgesia and dynamic allodynia were similar in the forearm and foot. Post-capsaicin blood flow did not differ between cases and controls by anatomic site. Measures of depression and anxiety correlated with spontaneous pain intensity but did not correlate with measures of hyperalgesia, allodynia, or blood flow. VVS cases had higher resting pulse rates and lower resting systolic blood pressures than in controls. Conclusion. VVS patients show enhancement of post-capsaicin pain response extending far beyond the anatomic location of the primary complaint.