A systematic review and meta-analysis of the impact of transcranial direct current stimulation on cognitive function in older adults with cognitive impairments: the influence of dosage parameters

Introduction Numerous studies have demonstrated the effects of transcranial direct current stimulation (tDCS) on cognitive function in older people. This study further explores the impact of tDCS and its dosage parameters on cognitive enhancement in older people with cognitive impairments. Methods Randomized controlled trials (RCTs) published through November 2023 were retrieved from PubMed, Scopus, EMBASE, EBSCO, and the Cochrane Library. Participants were older adults with cognitive impairments, including Alzheimer's disease (AD), mild cognitive impairment (MCI), and dementia, diagnosed using standard criteria (DSM-IV/V, NINCDS-ADRDA, Petersen, MoCA, CDR). Standardized mean difference (SMD) values were analyzed. Results Nineteen RCTs were included. tDCS significantly improved Mini-Mental State Examination (MMSE) scores immediately post-intervention (SMD=0.51, p=0.005) and at follow-up (SMD=2.29, p=0.0003). Significant effects were observed when tDCS was used alone (SMD=0.39, p=0.04), at current densities ≤ 0.06 mA/cm² (SMD=0.25, p=0.04), with session durations > 20 min (SMD=0.89, p=0.01), up to 15 sessions (SMD=0.28, p=0.009), and with the active electrode placed over the temporal area (SMD=0.33, p=0.02). People with AD showed greater improvements than those with MCI or dementia (SMD=0.91, p=0.02). However, tDCS did not significantly improve memory or executive function. Conclusion tDCS demonstrated efficacy in enhancing global cognition in older people with cognitive impairments, providing insight into optimal parameters for clinical application. However, no improvements were observed in memory or executive function.