In Reply:-Dr. Vandam correctly points out that the medium in which capsaicin is dissolved may itself cause pain, perhaps by destroying peripheral nerve fibers as has been shown to occur with 5–10% benzyl alcohol. He mentions that Zostrix, an OTC preparation of capsaicin, contains benzyl alcohol and acetyl alcohol. We did not use this preparation but prepared our injectate from purified capsaicin powder (Fluka, Ronkonkoma, NY) and dissolved it in 15% Tween-80 in water at a concentration of 10 mg/ml. We are unaware of reports that examine the direct effects of Tween-80 on the peripheral nerve fibers. In previous studies (Simone et al., 1989) injection of this vehicle alone does not cause secondary hyperalgesia. However, even if the vehicle had caused peripheral nerve injury or sensitization, our findings would still support the conclusion that in normal human volunteers, mechanical allodynia and hyperalgesia caused by a focal noxious stimulus spread beyond the distribution of the nerve supplying the injection site. This indicates that strong peripheral nociceptor input can produce mechanical allodynia and hyperalgesia by sensitizing central nervous system neurons.
Christine N. Sang, M.D., M.P.H.
Richard H. Gracely, Ph.D.
Mitchell B. Max, M.D.
Neurobiology and Anesthesiology Branch; National Institute of Dental Research; National Institutes of Health; 9000 Rockville Pike; Bethesda, Maryland 20892
Gary J. Bennett, Ph.D.
Department of Neurology; Allegheny University of the Health Sciences; Mail Stop 423; Broad and Vine Streets; Philadelphia, Pennsylvania 19102
(Accepted for publication April 10, 1997.)