Influence of thermode size for detecting heat pain dysfunction in a capsaicin model of epidermal nerve fiber loss

Quantitative sensory testing of heat pain sensation has become an important tool to evaluate small caliber afferent nerve function in peripheral neuro pathy. In earlier studies, we found that topical application of capsaicin i n humans results in the loss of epidermal nerve fibers (ENFs) with a corres ponding decrease in detection of heat pain sensation. Capsaicin may therefo re be a useful model for developing optimal psychophysical testing procedur es for detection of neuropathy in its early stages. Here we determined the influence of thermal probe (thermode) size in detecting the diminished heat pain sensation following capsaicin application. Twelve healthy volunteers applied 0.075% capsaicin topically to the volar forearm four times daily fo r 7 days. Psychophysical measures of heat pain, mechanical (sharp) pain, an d tactile threshold were obtained daily from untreated control skin and fro m capsaicin-treated skin during capsaicin application, and once weekly for 5 weeks following discontinuation of capsaicin. Heat pain sensation was ass essed using a large (30 x 30 mm) and small (3 x 3 mm) thermode and differen t algorithms to assess pain threshold and suprathreshold heat pain. Skin bi opsies were obtained and were processed for immunohistochemical localizatio n of (ENFs) using the pan neuronal marker protein gene product 9.5. Capsaic in produced a rapid decrease in the number of ENFs, with nearly complete di sappearance after 3 days of treatment. Heat pain evoked by the small, but n ot the large, thermode decreased dramatically after capsaicin treatment. Th e sensation of heat pain returned toward normal after 2-3 weeks following d iscontinuation of capsaicin treatment concordant with gradual reinnervation of the epidermis. Regression analysis indicated that the sensation of heat pain evoked by the small thermode correlated much better with the number o f ENFs than heat pain evoked by the large thermode. The detection of sharp pain decreased moderately after capsaicin treatment. Assessment of heat pai n sensation using small thermodes has potential for detecting sensory defic its in early stages of small fiber neuropathy. (C) 2001 International Assoc iation for the Study of Pain. Published by Elsevier Science B.V. All rights reserved.