Efficacy of Transcranial Direct Current Stimulation in Patients with Primary Progressive Aphasia: A Systematic Review and Updated Meta-analysis.

Background: Primary progressive aphasia (PPA) is a group of dementias characterized by progressive loss of specific language functions with relative sparing of other cognitive domains. Transcranial direct current stimulation (tDCS) has been introduced as a potential therapeutic option for PPA. We aim to evaluate the efficacy of tDCS in improving cognitive and language functions in patients with PPA.

Methods: PubMed, Embase, Cochrane Library, and Web of Science were searched up to June 2024 to identify randomized controlled studies (RCTs) comparing tDCS vs sham procedure in patients with PPA.

Results: Our analysis encompassed ten RCTs comparing the efficacy of tDCS against sham treatment, with a total of 167 participants. Due to the crossover design implemented in several studies, 94.5% of participants received active tDCS, while 91.1% received the sham intervention. The findings demonstrated significant improvements immediately post-tDCS (SMD 0.29, 95% CI 0.003-0.57, p = 0.05, I2 = 19%) and sustained benefits after two months or more (SMD 0.51, 95% CI 0.10-0.92, p = 0.01, I2 = 11%) in the untrained naming task. Similarly, for the trained naming tasks, significant improvement was observed (SMD 0.96; 95% CI 0.27 to 1.65; p < 0.01; I2 = 0%), even after the end of the tDCS treatment, with effects persisting for up to two months. (SMD 0.64; 95% CI 0.15 to 1.13; p = 0.01; I2 = 8%). However, no significant improvements were observed in cognitive tasks, untrained comprehension, language, and trained semantic tasks.

Conclusion: Our findings suggest that tDCS may be efficacious in augmenting naming tasks both acutely and over a prolonged period of up to two months. However, its effects on overall cognitive performance remain inconclusive. Further robust trials are warranted to elucidate its impact on cognitive function.