The Effect of Single-Session Transcranial Direct Current Stimulation on Cerebral Blood Flow: A Systematic Review and Meta-Analysis.

Transcranial direct current stimulation (tDCS) is a form of non-invasive brain stimulation that delivers a constant low electric current through electrodes placed on the scalp. A significant amount of research has explored the clinical potential of tDCS in psychiatric disorders. However, the mechanism of tDCS' behavioural effects remains largely unknown. Cerebral blood flow (CBF) is a surrogate marker of neural activity and has been used as an index of brain dysfunction in various psychiatric disorders. Studies have shown that single-session tDCS can modulate regional CBF in humans, however, these results as yet have not been systematically reviewed. The primary objective of this systematic review and meta-analysis was to evaluate the immediate effects of single-session tDCS on regional CBF. A literature search of English language publications in humans was conducted through Ovid database using Medline, Embase, and PsycINFO (last search in August 2023). The following search terms were used: (transcranial direct current stimulation or tDCS) and (cerebral blood flow or cerebrovascular circulation). Studies that reported sufficient data for calculating the pooled effect size of regional CBF change from baseline to immediately post-tDCS were included. The analysis was separated by anodal, cathodal, and sham stimulation. The search identified 188 English language publications. Four papers were deemed eligible for the analysis, while two of them were based on the same sample. Studies varied in terms of tDCS montage, stimulation protocol, and population studied. The meta-analysis found a decrease in regional CBF following single-session cathodal tDCS stimulation (n = 34) and no effect of single-session anodal (n = 58) or sham tDCS (n = 52). However, increased regional CBF following anodal tDCS was significantly associated with older age, sex (% male), and higher current intensity and density based on the results of exploratory moderator and subgroup analyses. The results of this meta-analysis contribute to the growing body of knowledge investigating the cerebral mechanisms of tDCS. Given the limited number and heterogeneity of available studies, further investigation into the effects of single- and multi-session tDCS on regional CBF in clinical or healthy participant samples is warranted to gain a better mechanistic understanding of tDCS' therapeutic effects.