Effectiveness of tDCS to enhance Aphasia Treatment outcomes Following Stroke: Evidence from Systematic Reviews and Meta-Analyses

Objective(s): Transcranial direct current stimulation (tDCS) is an adjuvant to speech therapy for aphasia following stroke, using anodal (excitatory), cathodal (inhibitory), and bihemispheric stimulation. Questions remain whether tDCS effects surpass behavioral treatment alone. In this project we appraised the quality of systematic reviews (SRs) of the burgeoning tDCS literature and examined the weight of the evidence for aphasia outcomes
Data Sources: PsychInfo, PubMed, SpeechBite, ASHA Compedium, Cochrane;
Study Selection: 16 SRs were identified by two reviewers, nine that included meta-analysis (MA) to quantify results;
Data Extraction: Two reviewers reliably coded SRs for methodological rigor using AMSTAR2 (Shea et al., 2017) and summarized study characteristics;
Data Synthesis: The quality of the SRs ranged from low (Monti et al., 2013 = 2) to high (Elsner et al., 2019 = 14.5), M=7.8/16. The number of original studies included in the SRs ranged from 3 to 48, as some reviews included RCTs/randomized crossover trials, and others summarized all trials. Most studies examined chronic nonfluent aphasia. SRs reported positive tDCS effects for general aphasia tests, and measures of repetition and speech fluency following left anodal stimulation. Outcomes for noun and verb naming improved following left anodal and cathodal, right anodal and cathodal, and bihemispheric stimulation. Only Ding et al. (2022) reported positive MA findings of tDCS for general aphasia batteries, repetition, speech fluency, and auditory comprehension. While earlier MAs reported no significant effects for naming, seven others since 2016 reported significant effects. Naming effects of tDCS were greater in chronic than subacute aphasia, and in nouns than verbs.

Conclusions: The conclusions of MAs summarizing randomized trials merged with earlier SRs reporting positive benefits of tDCS over speech therapy alone. Consistent results are reported for naming measures, leaving some questions about tDCS effects for functional aphasia recovery. Important questions remain before the technique can be recommended for clinical practice.
Author(s) Disclosures: The authors have no conflict of interest to declare.