Late sensory function after intraoperative capsaicin wound instillation

Background: Intense capsaicin‐induced C‐fiber stimulation results in reversible lysis of the nerve soma, thereby making capsaicin wound instillation of potential interest for the treatment of post‐operative pain. Clinical histological and short‐term sensory studies suggest that the C‐fiber function is partly re‐established after skin injection of capsaicin. However, no study has evaluated the long‐term effects of wound instillation of purified capsaicin on sensory functions. Methods: Patients included in a double‐blind placebo‐controlled randomized study of the analgesic effect of capsaicin after groin hernia repair were examined by quantitative sensory testing before, 1 week and 2 years post‐operatively. The primary endpoint was occurrence of hyperalgesia/allodynia. The secondary endpoints were acute and late sensory changes between the two patient groups. Patients were blinded to the allocated treatment. Results: Twenty (100%) capsaicin and 16 (76%) placebo‐treated patients were seen at the year follow‐up. Hyperalgesia was seen in five capsaicin‐ vs. one placebo‐treated patient (P=0.2). The mechanical detection threshold was significantly increased on the operated side in the capsaicin vs. placebo group at the 1‐week follow‐up (P<0.05), but was not different at the year follow‐up (P=0.3). There were no other significant differences in sensory function on the operated side between groups at the pre‐operative, 1‐week or year post‐operative follow‐up (P>0.05). The sensory function on the contralateral side was comparable between groups throughout the study (P>0.1). Conclusion: This small‐volume study calls for further long‐term safety studies of wound capsaicin instillation.