13. Which is the best evoked potential technique for assessing the nociceptive system? Preliminary results of a neurophysiological study in healthy humans

Laser evoked potentials (LEPs) and contact heat evoked potentials (CHEPs) are widely agreed method for investigating the nociceptive system. Concentric electrodes (CE) have also recently been introduced to measure pain-related evoked potentials (PREPs) and, thereby, to assess the nociceptive system in patients. Although some authors have reported that low intensity CE stimulation, evoking pinprick sensation, selectively activates nociceptive fibres, the CE reliability in the assessment of nociceptive system is still unclear. In this study we aimed to verify whether low and high intensity CE stimulations selectively activates nociceptive fibres. To do so we recorded LEPs, CHEPs and PREPs before and after capsaicin-induced skin denervation. To date, we have enrolled 6 healthy subjects. All subjects underwent a baseline recording of LEPs, CHEPs and PREPs after stimulation of the right forearm. For PREP recording we used both low intensity stimulation, evoking a distinct pinprick sensation and high intensity stimulation, evoking an electrical painless sensation. After the baseline evoked potential recordings, we applied an 8% capsaicin plaster on the right forearm. After one-two weeks, we recorded LEPs, CHEPs and PREPs after stimulation of the capsaicin-induced denervated forearm skin. In a small skin area spared by evoked potential stimulations we have also collected a skin biopsy for assessing the skin denervation. While LEPs and CHEPs were suppressed after topical application of capsaicin, low and high intensity PREPs did not differ before and after capsaicin-induced skin denervation. The skin biopsy documented the skin denervation induced by the capsaicin plaster. Our data indicate that both low and high intensity CE stimulation elicit PREPs after skin denervation, thus suggesting that the CE stimulation probably coactivates non-nociceptive fibres.