Modulation of pain-descending inhibitory pathway using transcranial direct current stimulation priming protocols in healthy subjects: systematic review

BACKGROUND AND OBJECTIVES: Priming is a phenomenon in which brain activity can shift in an inhibitory or excitatory direction, potentially increasing synaptic efficiency in response to a previous input. Transcranial direct current stimulation (tDCS) is a neuromodulation technique that has been extensively investigated as an alternative treatment in pain processing changes. Priming techniques can improve pain relief mechanisms in healthy subjects. However, no systematic reviews have been published that summarize these findings. The objective of this review was to identify and evaluate studies that used tDCS as priming or testing protocols and investigate its effects on the descending inhibitory pathway of pain in healthy people. CONTENTS: Two independent reviewers searched Medline, Embase, CINAHL, Web of Science, PsycINFO, PEDro, Scopus, and Cochrane databases until January 2024 for studies using tDCS as a priming or testing protocol in healthy subjects to assess changes in the pain descending pathway. Four studies were eligible. Two studies showed that cathodic tDCS increases pain threshold when applied before 1Hz rTMS (repetitive transcranial magnetic stimulation), and this may be mediated by homeostatic metaplasticity mechanisms. Two studies have shown that anodal tDCS combined with exercise can activate central pain control mechanisms; the use of both at the same time may have resulted in a synergistic effect and greater analgesia. CONCLUSION: The priming approach of cathodal or anodal tDCS appears to change the pain threshold in healthy people, however, the effect is reliant on the test stimulus used and may increase or reverse the intended effect.