Effects of lidocaine patch on intradermal capsaicin-induced pain: a double-blind, controlled trial

This study evaluated the effects of topical lidocaine on skin sensation and on intradermal capsaicin-induced pain and hyperalgesia. A randomized, double-blinded, placebo controlled methodology was used. After baseline sensory testing, a placebo patch and a lidocaine patch were randomized to the volar aspect of the left or right forearm for 4 hours. The right forearm patch was removed, the sensory testing was repeated, and capsaicin was injected intradermally at the site. Pain scores were measured at the time of injection and every 2.5 minutes for 10 minutes followed by measurement of the hyperalgesic area to von Frey hair and stroking, flare response, and repeat sensory testing. At the completion of the testing on the right forearm, the left forearm patch was removed and the procedures described for the right forearm were repeated for the left forearm. There was a significant reduction in cool sensation, warm sensation, and touch thresholds in the lidocaine but not placebo patch arm. The lidocaine patch had no significant effect on hot pain or mechanical pain thresholds. Intradermal capsaicin resulted in a significant decrease in hot pain and mechanical pain thresholds; however, lidocaine was unable to significantly reverse the thermal or mechanical hyperalgesia induced by capsaicin. The lidocaine patch did not reduce flare area, nor areas of hyperalgesia or allodynia. This study suggests that the sodium channels and the capsaicin receptors function independently to control peripheral terminal depolarization.

The sodium channel and the transient receptor potential vanilloid 1 (TRPV1) receptor coexist on peripheral terminals of unmyelinated fibers. This study showed that activation of the TRPV1 receptor can depolarize the fibers in the presence of sodium channel blockade. This suggests that the sodium channel and TRPV1 receptor function independently in depolarizing the fibers.