A Systematic Review of Reporting Adverse Effects Associated With Transcranial Direct Current Stimulation in Chronic Pain

Background: Transcranial direct current stimulation (tDCS) has expanded its use for the treatment of chronic pain. Trials have proven the efficacy of tDCS, but there are still uncertainties over adverse effects (AEs). Areas of Uncertainty: There are no systematic data assessing tDCS safety profile in chronic pain conditions. Data Sources: A comprehensive systematic review was performed using PubMed and Web of Science databases. It included randomized sham-controlled clinical trials conducted in patients with different chronic pain conditions treated with tDCS. Based on the level of AEs reported, studies were classified as follows: type I, studies that provided numerical data on AEs; type II, studies that mentioned the occurrence of AEs; type III, studies that reported the absence of AEs; and type IV, studies that made no mention of AEs. Therapeutic Advances: The review included 141 studies: 60 type I, 21 type II, 34 type III, and 26 type IV. There were no significant differences in study design between studies that reported AEs quantitatively and those that reported the absence of AEs. Considering the number of studies that reported AEs quantitatively, the most documented for active versus sham groups were headache (18.86% vs. 19.23%) and tingling (15.43% vs. 19.23%). However, when considering frequency, the most common AEs observed for active versus sham groups were tingling (26.3% vs. 23.65%) and skin redness (17.6% vs. 9.56%). In comparing headache and tingling, no statistically significant difference between active and sham groups could be identified. Conclusions: In the present form of reporting AEs of tDCS in clinical studies involving patients with chronic pain, this procedure seems to be safe. Nevertheless, we identified diverse modalities of reporting and assessing AEs, which should raise the need for a standardized procedure in this domain.