THE BACK PAGE RESEARCH REVIEW Probing TENS effect on brain through imaging REVIEWED BY SHAWN THISTLE STUDY TITLE: Brain mechanisms of pain relief by transcutaneous electrical nerve stimula-tion: A functional magnetic resonance imaging study Authors: Choi JC, Kim J, Kang E, et al. Publication Information: European Journal of Pain 2016; 20: 92–105. T welve men and 12 women were selected for this study. All participants had two channel TENS applied to their lower left leg skin with one cathode electrode over the fibular neck and the anode electrode from the same channel applied to the skin di-rectly above the lateral malleolus. The other cathode electrode applied to the skin on the posterior left leg at the level of the fibular head, with the anode electrode from that second channel applied at the posterior left ankle. This allowed one channel to stimulate the common fibular (peroneal) nerve and the other to stimulate the tibial nerve. Noxious thermal stimuli at 45C were applied to the left lower leg skin through a thermode applied midway between the cathodes (fibular head level) and anodes (ankle level) with a Velcro strap. A duration of 15 seconds was selected for the study intervention because pain perception has been shown to increase over the first 10 seconds and then remain roughly con-stant. Subjects were placed in the MRI, and individual TENS levels that were perceived as strong, but not pain-ful, were identified for each participant (all under 80 Hz and 60 ms pulse duration). Each participant received the same painful stimuli in both the pain and pain+TENS conditions. In the pain+TENS condition, the partici-pants also received TENS for 15 sec-onds, which was increased in a step-wise manner to overcome any habituation effect. Participants in the pain+TENS condition began with a comfortable TENS intensity (CTI) for 5 seconds, then received CTI plus 1 mA for the second 5 second period, and then CTI plus 2 mA for the third 5 second period. Six men and six women received the pain-only condition followed by the pain+TENS condition. The other six men and six women received the pain+TENS condition first followed by the pain-only condition. All par-ticipants remained in the MRI scan-ner for the duration of the study. In the pain+TENS condition, PAG functional connectivity with the lat-eral prefrontal cortex increased, likely activating the descending pain-inhib-itory pathway. As well, the superior parietal lobe, right postcentral cortex (S1), bilateral parietal operculum (S2), supramarginal cortex, and right ventrolateral prefrontal cor tex (VLPFC) were all significantly acti-vated in the pain+TENS condition when compared to the pain-only condition. This may indicate the stimulation of large-diameter A-beta fibers, leading to reduced pain. Results In the pain-only condition, the left anteroventral thalamus, bilateral rACC (rostral anterior cingulate cor-tex), and right hippocampal cortex were more highly activated. The ante-rior thalamic nuclei have many projec-tions to the ACC, which has been shown to process the affective-motiva-tional dimension of pain. The hip-pocampal cortex activates in response to noxious stimulation and may con-tribute to negative effect associated with pain. Left PAG functional con-nectivity with the left, but not the right, lateral prefrontal cortex (PFC) significantly increased as the TENS effect increased in the pain+TENS condition. Both the lateral PFC and PAG are implicated in endogenous pain inhibition. Thus, the increase in TENS effect may activate the de-scending pain-inhibitory pathway. The participants reported lower per-ceived pain and pain-related unpleas-antness in the pain+TENS condition compared to the pain-only condition. S1, S2 and parietal cortices were acti-vated by non-painful TENS. TENS also increased PAG functional con-nectivity with the lateral prefrontal cortex. This would likely result in ac-tivation of the descending pain-inhib-itory pathway to produce pain relief. Increasing TENS intensity in a step-wise fashion effectively prevented temporal summation. This study suggests that TENS application is ef-fective for pain reduction. This mir-rors what many see in clinical practice. Whether this effect is physiological, or represents some degree of a placebo effect, helping our patients reduce pain levels remains the most impor-tant outcome. www.canadianchiropractor.ca Application, conclusions DR. SHAWN THISTLE, DC, is the founder and CEO of RRS Education (rrseducation.com), which helps busy clinicians integrate current research evidence rationally into practice. He also maintains a practice in Toronto, lectures at CMCC and provides medicolegal consulting. Email him at [email protected]. 30 Canadian Chiropractor February 2017
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