Effects of Transcranial Direct Current Stimulation on Cognitive Function in Older Adults with and without Mild Cognitive Impairment: A Systematic Review and Meta-Analysis of Randomized Controlled Trials

Abstract

<b><i>Introduction:</i></b> Noninvasive brain stimulation (NIBS) has shown benefits for cognitive function in older adults. However, the effects of transcranial direct current stimulation (tDCS) on cognitive function in older adults are inconsistent across studies, and the evidence for tDCS has limitations. We aim to explore whether tDCS can improve cognitive function and different cognitive domains (i.e., learning and memory and executive function) in adults aged 65 years and older with and without mild cognitive impairment and to further analyze the influencing factors of tDCS. <b><i>Methods:</i></b> Five English databases (PubMed, Cochrane Library, EMBASE, Web of Science, the cumulative Index to Nursing and Allied Health Literature [CINAHL]) and four Chinese databases were searched from inception to October 14, 2023. Literature screening, data extraction, and quality assessment were completed independently by two reviewers. All statistical analyses were conducted using RevMan software (version 5.3). Standardized mean difference (SMD) along with a 95% confidence interval (CI) was used to express the effect size of the outcomes, and a random-effect model was also used. <b><i>Results:</i></b> A total of 10 RCTs and 1,761 participants were included in the meta-analysis, and the risk of bias in those studies was relatively low. A significant effect favoring tDCS on immediate postintervention cognitive function (SMD = 0.16, <i>Z</i> = 2.36, <i>p</i> = 0.02) was found. However, the effects on immediate postintervention learning and memory (SMD = 0.20, <i>Z</i> = 2.00, <i>p</i> = 0.05) and executive function (SMD = 0.10, <i>Z</i> = 1.22, <i>p</i> = 0.22), and 1-month postintervention cognitive function (SMD = 0.12, <i>Z</i> = 1.50, <i>p</i> = 0.13), learning and memory (SMD = 0.17, <i>Z</i> = 1.39, <i>p</i> = 0.16), and executive function (SMD = 0.08, <i>Z</i> = 0.67, <i>p</i> = 0.51) were not statistically significant. <b><i>Conclusion:</i></b> tDCS can significantly improve the immediate postintervention cognitive function of healthy older adults and MCI elderly individuals. Additional longitudinal extensive sample studies are required to clarify the specific effects of tDCS on different cognitive domains, and the optimal tDCS parameters need to be explored to guide clinical practice.