Capsaicin evoked pain and allodynia in post-herpetic neuralgia

The hypothesis that the pain and allodynia associated with post-herpetic ne uralgia (PHN) is maintained by a combination of input from preserved primar y afferent nociceptors and sensitization of central pain transmitting neuro ns was examined in 17 subjects with PHN. Pain, allodynia, thermal sensory f unction, cutaneous innervation, and response to controlled application of 0 .075% capsaicin were measured. Compared to mirror-image skin, applying caps aicin on a 9 cm(2) area of PHN skin significantly increased overall PHN pai n and allodynia in 11 of 17 subjects. These 'capsaicin responders' were cha racterized by higher average daily pain, higher allodynia ratings, and rela tively preserved sensory function at baseline compared to the non-responder s. In three of the 'capsaicin responders' the area of allodynia expanded in to previously non-allodynic and non-painful skin that had normal sensory fu nction and cutaneous innervation. These observations support the hypothesis that allodynia in some PHN patients is a form of chronic secondary hyperal gesia maintained by input from intact and possibly 'irritable' primary affe rent nociceptors to a sensitized CNS. (C) 2000 International Association fo r the Study of Pain. Published by Elsevier Science B.V. All rights reserved .