The Impact of Transcranial Direct Current Stimulation (tDCS) on Memory Function in Older Adults with Mild Cognitive Impairment: A Systematic Review

Abstract Background: Mild Cognitive Impairment (MCI) represents a transitional phase between normal aging and dementia, predominantly affecting memory (Sanz‐Blasco et al., 2022). MCI affects approximately 19.7% (95% CI: 18.3–21.1%) of adults aged ≥50 years globally (Song et al., 2023), underscoring its clinical significance amid rapid population aging. Pharmacological treatments (e.g., cholinesterase inhibitors) show limited efficacy in slowing MCI progression, with meta-analyses reporting minimal cognitive gains (e.g., ΔMMSE <1.5 points; (Petersen et al., 2018)). Transcranial direct current stimulation (tDCS) emerges as a non-invasive alternative, modulating cortical excitability via weak direct currents (Sudbrack-Oliveira et al., 2021). With the growing elderly population worldwide and the limited efficacy of pharmacological treatments, non-invasive methods like transcranial direct current stimulation (tDCS) have gained attention for potential cognitive enhancement (Siegert et al., 2021). This systematic review addresses the gap in evidence synthesis for tDCS efficacy on memory in MCI, particularly regarding optimal stimulation parameters. Objective: To evaluate tDCS effects on memory performance in older adults with MCI and identify protocol-specific predictors of success. Methods: A systematic review was conducted using PubMed, Scopus, and Web of Science (up to April 2025) following PRISMA guidelines. Inclusion criteria: RCTs/non-RCTs assessing tDCS monotherapy or combined with cognitive training in adults ≥60 years with MCI. No meta-analysis was performed due to heterogeneity; narrative synthesis prioritized. Ten studies (N=428) met criteria. Risk of bias was assessed via JBI RCT checklist. Results: In this systematic review, significant improvements were observed in verbal memory, including RAVLT test results (3/5 studies; p<0.05), and in recognition memory (p<0.001; (Cotelli et al., 2022)). Spatial memory was also enhanced by increasing WMS-RC test scores, especially in episodic memory latency (p<0.05; (Gu et al., 2022)). In the field of neurophysiological indicators, a significant decrease in ERP wavelength of 27 ms, a significant increase in signal amplitude of 1.8 microvolts, and an enhancement of fronto-parietal connectivity (ρ=0.59, p=0.02) were reported, indicating improved neural activity and connectivity. Subgroup analyses showed that left dorsolateral prefrontal cortex(L-DLPFC) stimulation had a more significant effect (8 of 10 studies, effect size g=0.42), sessions ≥10 had a greater effect (6 of 10 studies, g=0.61), and stimulation intensity of 2 mA (7 of 10 studies, g=0.55) were more effective than the other conditions. Mild side effects such as redness and burning were reported in 20 to 30% of cases, assessed by structured questionnaires, and no serious events were reported. The quality assessment of the studies indicated a low risk of bias with a mean JBI score of 11 out of 13. These findings suggest that the method under review is generally effective and safe. Conclusion: Preliminary evidence suggests tDCS—particularly anodal left DLPFC stimulation at 2 mA for ≥10 sessions—may enhance memory in MCI, supported by neurophysiological changes. However, heterogeneity in protocols and small sample sizes necessitate cautious interpretation. Future trials should prioritize standardized protocols, biomarker-guided personalization, and longitudinal designs.

کلیدواژه ها:

Mild Cognitive Impairment ، transcranial direct current stimulation ، memory ، aging ، systematic review