Electrical parameters of transcranial direct current stimulation that effectively alter cerebral blood flow in experimental animals: a systematic review

Abstract

Objective: To identify the electrical parameters of transcranial direct current stimulation (tDCS) that effectively alter cerebral blood flow in rats. Methodology: Six eletronic databases were searched with no time or language restrictions to identify experimental studies with rats using tDCS with anodal and/or cathodal stimulation with or without a comparison group. Internal validity was assessed via the following criteria: housing, lighting, temperature, water/food, groups randomization and ethical aspects. The ‘Laboratory Systematic Review Center for Laboratory animal Experimentation’ (SYRCLE) tool was used to assess risk of bias. The tDCS electrical parameters and cerebral blood flow were considered as primary outcomes and cerebral histological alterations as the secondary outcome. Results: Four articles were included. All four studies were considered to present a high level of scientific bias. The electrical tDCS parameters implemented were heterogeneous but overall, tDCS with anodal stimulation promoted an increase in cerebral blood flow while the cathodal stimulation decreased it. Cerebral histological alterations were assessed in two studies and tissue necrosis was reported in only one animal per study. Conclusion: The identification of tDCS electrical parameters that effectively alter cerebral blood flow in rats was not possible due to the heterogeneity of tDCS protocols being implemented in the literature. Considering the high risk of scientific bias in the included studies, the current available evidence regarding tDCS efficacy is insufficient and inconclusive.