The efficacy of transcranial direct current stimulation and transcranial magnetic stimulation for chronic orofacial pain: A systematic review

Background: Transcranial Direct Current Stimulation (tDCS) and Transcranial Magnetic Stimulation (TMS) have been described as promising alternatives to treat different pain syndromes. This study evaluated the effects of TMS and tDCS in the treatment of chronic orofacial pain, through a systematic review. Methods: An electronic search was performed in major databases: MEDLINE, Scopus, Web of Science, Cochrane, Embase, LILACS, BBO, Open Gray and CINAHL. The eligibility criteria comprised randomized clinical trials (RCTs) that applied TMS or tDCS to treat chronic orofacial pain. The variables analyzed were pain, functional limitation, quality of life, tolerance to treatment, somatosensory changes, and adverse effects. The risk of bias was assessed through the Cochrane Collaboration tool, and the certainty of evidence was evaluated through GRADE. The protocol was registered in the PROSPERO database (CRD42018090774). Results: The electronic search resulted in 636 studies. Thereafter, the eligibility criteria were applied and the duplicates removed, resulting in eight RCTs (four TMS and four tDCS). The findings of these studies suggest that rTMS applied to the Motor cortex (M1), the dorsolateral prefrontal cortex (DLPFC) and the secondary somatosensory cortex (S2) provide adequate orofacial pain relief. Two studies reported significant pain improvement with tDCS applied over M1 while the other two failed to demonstrate significant effects compared to placebo. Conclusions: rTMS, applied to M1, DLPFC or S2, is a promising approach for the treatment of chronic orofacial pain. Moreover, tDCS targeting M1 seems to be also effective in chronic orofacial pain treatment. The included studies used a wide variety of therapeutic protocols. In addition, most of them used small sample sizes, with a high risk of biases in their methodologies, thus producing a low quality of evidence. The results indicate that further research should be carried out with caution and with better-standardized therapeutic protocols.