TOPICAL CAPSAICIN SELECTIVELY ATTENUATES HEAT PAIN AND A-DELTA FIBER-MEDIATED LASER-EVOKED POTENTIALS

Cutaneous stimulation with CO2 laser pulses activates AS nociceptive a fferents and evokes late cerebral potentials (LEPs), the amplitude of which correlates parametrically with the perceived magnitude estimatio n of laser pulses. Capsaicin is known to desensitize the nociceptive t erminals of C fibers. In this double-blind, vehicle-controlled experim ent, we tested the hypothesis that topical capsaicin would inactivate A delta afferents and lead to an attenuation of the LEPs. Subjects app lied capsaicin cream to the dorsum of one hand and vehicle cream to th e other 3 times daily for a period of 5 weeks. At weekly intervals bef ore starting, during administration and after discontinuation of capsa icin, LEPs were recorded and psychophysical thresholds and magnitude e stimation for several sensory modalities were determined. The results of this study showed that topical capsaicin significantly and reversib ly decreased the magnitude estimation of suprathreshold heat pain, las er pulses and amplitude of the LEPs. There was no statistically signif icant difference in light touch, deep pain and mechanical pain detecti on thresholds between the capsaicin- and vehicle-treated hands. It ind icated that topical capsaicin caused a definite functional and reversi ble inactivation of A delta nociceptive afferent transmission. The dec line in the magnitude estimation of laser pulses concomitantly with th e attenuation of LEP amplitudes supports the hypothesis that some A de lta afferents mediate noxious heat in humans, These findings demonstra te the usefulness of LEP in the physiological evaluation of nociceptiv e pathways and its potential usefulness in objectively documenting the effect of pharmacological treatment on pain perception.