Capsaicin evoked pain and allodynia in post-herpetic neuralgia.

The hypothesis that the pain and allodynia associated with post-herpetic neuralgia (PHN) is maintained by a combination of input from preserved primary afferent nociceptors and sensitization of central pain transmitting neurons was examined in 17 subjects with PHN. Pain, allodynia, thermal sensory function, cutaneous innervation, and response to controlled application of 0.075% capsaicin were measured. Compared to mirror-image skin, applying capsaicin on a 9 cm(2) area of PHN skin significantly increased overall PHN pain and allodynia in 11 of 17 subjects. These 'capsaicin responders' were characterized by higher average daily pain, higher allodynia ratings, and relatively preserved sensory function at baseline compared to the non-responders. In three of the 'capsaicin responders' the area of allodynia expanded into previously non-allodynic and non-painful skin that had normal sensory function and cutaneous innervation. These observations support the hypothesis that allodynia in some PHN patients is a form of chronic secondary hyperalgesia maintained by input from intact and possibly 'irritable' primary afferent nociceptors to a sensitized CNS.