Effect of electroacupuncture on capsaicin induced hyperalgesia in healthy volunteers: a randomised and sham acupuncture controlled study

Background and Aim<br><br>One of the key features of clinical pain is hyperalgesia, which is enhanced sensitivity to painful stimulation. Sensitisation in the central and peripheral nervous systems underlies this phenomenon. Previous studies mainly focused on testing the analgesic effects of acupuncture in healthy humans using transient painful stimulation that had imited clinical implication.<br><br>The current project aimed to bridge the gap between experimental and clinical pain by employing a validated human hyperalgesia model to assess the anti-sensitisation effect of acupuncture.<br><br>Methods and Results<br><br>This project consists of three stages, including 1) literature review; 2) assessment of the reproducibility of a commonly used hyperalgesia model; and 3) evaluation of the anti-sensitisation effect of electroacupuncture (EA).<br><br>For the literature review, the terms "acupuncture", "electroacupuncture", "hyperalgesia" and "allodynia" and their combinations were searched in the major databases. Thirty-two (32) papers were selected. All of them were animal studies. No human study was identified. All studies used EA as the intervention. Overall, EA significantly reduced hyperalgesia to mechanical and heat stimulation when compared with sham procedures.<br><br>In the first experiment, the reproducibility of the topical capsaicin model was assessed in 12 healthy human subjects. The reproducibility of this model was low and not appropriate for the purpose of the current study.<br><br>In the second experiment, a previously validated heat/capsaicin model was used. Hyperalgesia was induced by heating (45˚C) followed with topical application of capsaicin cream (0.075%) to an area of skin (3×3 cm²) in the middle of the non-dominant forearm. The sensitised area was then rekindled with heat stimulation (40˚C for 5 minutes) for four times. Twenty (20) subjects were recruited and randomly allocated to receive real or sham EA (REA alternating at 5-15 Hz or SEA) on eight acupuncture points located on the four extremities. The treatment lasted for 25 minutes and was delivered by two acupuncturists blinded to the outcome measures. Pain rating to long thermal stimulation (40˚C for 1 minute) and heat pain threshold were measured before and after the establishment of hyperalgesia, immediately and 50-60 minutes after the intervention. The area of mechanical hyperalgesia was measured once after hyperalgesia being established and twice after the intervention.<br><br>There was no group difference in the magnitude of hyperalgesia before the intervention. Area of mechanical hyperalgesia decreased to 30% of the baseline in the REA group and 24% of the baseline in the SEA group. Two-way ANOVA showed a statistically significant reduction in the area in both groups, but without difference between the groups. Similar analgesic effects were found in the heat pain threshold and pain rating to long thermal stimulation without any group difference.<br><br>Conclusion<br><br>The current experiment was the first to assess the anti-hyperalgesia effect of acupuncture using a human model. REA was not better than SEA in reducing hyperalgesia in healthy humans. The results contradictory to the results of animal studies. Further studies need to examine the effect of EA with a high frequency and to use a different hyperalgesia model to verify the current findings. <br>