Effect of acupuncture on temporal summation of pain: a randomised, double-blind, sham-controlled study
Creators: name: Feng, Jian
Publisher: name: RMIT University
Year: 2024

description: BACKGROUND: The analgesic effects of manual acupuncture (MA) and electro-acupuncture (EA) have been studied in healthy humans and patients with pain. The advantage of studying pain in healthy humans is that the intensity of stimulation can be accurately controlled and thereby the analgesic effect can be quantitatively assessed. However, an important difference between experimentally induced pain and clinical pain is central sensitisation, that is, an enhanced activity of the central nervous system (CNS). Temporal summation (TS) of pain refers to pain induced by repeated stimulations at sub-threshold level. It is a central phenomenon that reflects the sensitivity of CNS. The electrical TS pain model has been validated and applied to quantitatively determine the levels of analgesia and the central inhibition effects of analgesic medications.<br><br>OBJECTIVES: The present study aimed to: 1. systematically review available randomised, controlled trials (RCTs) of acupuncture on experimentally induced pain in healthy humans; 2. conduct a RCT to assess the effect of MA and EA on TS of pain and the spatial characteristics of this effect (i.e. the same and different dermatome segments to the acupuncture point), and the temporal factors (i.e. immediately after and 24-hours after intervention).<br><br>METHODS: The systematic review was carried out in accordance with the requirements of a Cochrane Systematic Review. The methodological quality and quality of the acupuncture techniques of the included RCTs were assessed. The Review Management software (RevMan version 4.2, The Cochrane Library) was used for data extraction and data analysis.<br><br>For the present experiment, 27 healthy volunteers were recruited and randomly assigned to either EA, MA or sham-acupuncture (SA) group, with nine volunteers in each group. The acupuncture sites were ST36 and ST40 on the dominant leg. Both the volunteers and the assessor were blinded to the treatment allocation. A second researcher, who was blinded to the outcome assessment processes, delivered all the interventions. Each treatment lasted for 25 minutes. To test pain thresholds, transcutaneous electrical stimulation was delivered to three sites: 1. the tibia anterior muscle along the sural nerve path of the treatment leg and parallel to the mid-point between ST36 and ST40; 2. the same area on the other leg; and 3. the dorsum of the non-dominant forearm along the median nerve path and 3 to 4 cm above the wrist crease. Pain thresholds to single electrical stimulation (SPT) and to TS stimulation (TST) were assessed before, 30-minutes after and 24-hours after the intervention. Ratings to supra-threshold stimulation at intensities of 1.2 and 1.4 times the TST were assessed with a visual analogue scale. The level of anxiety was assessed before and after acupuncture. <br><br>Data on pain thresholds and anxiety scores were analysed with one-way analysis of variance (ANOVA) using Statistical Package for the Social Sciences (SPSS, Version 13.0) to detect between treatment group differences. Significance for each of the ANOVAs was assessed at α = 0.05 / 3 = 0.0167 (Bonferroni Correction). When a significant ANOVA was obtained, Bonferroni corrected post-hoc analyses were applied for multiple-comparisons. Equivalence of the groups on demographic variables was assessed by ANOVA and chi-square tests. Power analysis and sample size calculations were performed using MINITAB (Version 15.0).<br><br>RESULTS: The literature search identified 605 papers, however, only nine papers met the inclusion criteria and thus included in this review. The methodological quality and quality of the acupuncture procedures were satisfactory. In these studies, the pain models and interventions varied substantially, therefore, meta-analysis was not practicable. Four studies employed both invasive and non-invasive controls with three of them reported that the invasive controls induced significantly stronger analgesia than the non-invasive controls. One study reported that there was no difference between EA and MA tested using a heat pain model.<br><br>In the present RCT, baseline values were comparable among the three groups with respect of SPT and TST. Overall, the baseline values of TST were lower than those of SPT. Within group comparison, the level of anxiety did not change significantly after the inventions. Between group comparisons, when sufficient statistical power was demonstrated, indicated that EA significantly increased SPT and TST on the treatment leg 24-hour after the treatment when compared with SA. In addition, the EA effect was not found on the non-treatment leg or the forearm. For the delivery of the supra-threshold stimulation, 1.2 and 1.4 times of the TST of each time point, instead of baseline TST, was mistakenly applied. Data from this component of the study were not analysed, however presented in the thesis for information.<br><br>As the first study in this field, the current findings provide the base for sample size calculation. For example, the sample sizes for EA and MA comparisons with 80% statistical power at a significance level of 0.05 will be 21 subjects in each group to detect the immediate effect of acupuncture on TST, ; and 11 to detect the effect of acupuncture after 24-hour.<br><br>CONCLUSIONS: The systematic review showed that there has been only a small number of experimental RCTs. Comparing acupuncture with non-invasive control, significant acupuncture analgesia was reported. These studies also demonstrated that invasive controls produced analgesia. Thus, future studies should consider using non-invasive intervention as control. No conclusion could be drawn regarding the relative analgesic effect of EA versus MA.<br><br>Consistent with previous studies, TS of pain can be successfully elicited in healthy humans with electrical stimulation. This study on TS demonstrated that the effect of EA was stronger than SA. The fact that such an effect increased within 24 hours after acupuncture might indicate the potential role of neurohumoral mechanisms in acupuncture analgesia. The spatial effect of acupuncture tended to be localised at the needling site. It is important to note that acupuncture increased both SPT and TST, which may suggest that both peripheral and central nervous systems mechanisms are involved in acupuncture analgesia.<br>