Background: Repeated sessions of transcranial direct current stimulation (tDCS) are increasingly used for therapeutic applications. However, adverse events (AEs) associated with repeated sessions have not been comprehensively evaluated.
Objective: The aim of this study was therefore to evaluate the safety of repeated sessions of tDCS, examining AE risk relative to tDCS exposure. Further, to identify whether certain participant populations are particularly at risk from tDCS.
Methods: A systematic review and meta-analysis included sham-controlled studies (up to June 2017) involving two or more tDCS sessions, spaced not more than a day apart. Data was extracted on AEs reported, total tDCS exposure (cumulative charge), and diagnostic groups (Healthy, Pain Disorder, Stroke, Neurocognitive Disorder, Neuropsychiatric Disorder, and Other). Univariate simple linear meta-regression analyses examined AE likelihood, comparing active and sham tDCS, with increasing exposure. Rates of AEs were compared for diagnostic groups.
Results: 158 studies (total 4130 participants) met inclusion criteria and were included for quantitative analyses. The incidence of AEs (examined per session, by proportion of participants, and by the number of studies reporting AEs) did not increase with higher levels of tDCS exposure. Furthermore, AE rates were not found to be greater for any diagnostic group.
Conclusions: Little evidence was found to suggest that repeated sessions of active tDCS pose increased risk to participants compared to sham tDCS within the limits of parameters used to date. Increased risks associated with greater levels of exposure to tDCS, or rare and under-reported AEs, however, cannot be ruled out.
Keywords: Adverse event; Repeated sessions; Safety; tDCS.
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