A Systematic Review of Transcranial Direct Current Stimulation in Primary Progressive Aphasia: Methodological Considerations

Abstract

A variety of tDCS approaches has been used to investigate the potential of tDCS to improve language outcomes, or slow down the decay of language competences caused by Primary Progressive Aphasia (PPA). The employed stimulation protocols and study designs in PPA are generally speaking similar to those deployed in post-stroke aphasic populations. These two etiologies of aphasia however differ substantially in their pathophysiology, and for both conditions the optimal stimulation paradigm still needs to be established. A systematic review was done and after applying inclusion and exclusion criteria, 15 articles were analyzed focusing on differences and similarities across studies especially focusing on PPA patient characteristics (age, PPA variant, language background), tDCS stimulation protocols (intensity, frequency, combined therapy, electrode configuration) and study design as recent reviews and group outcomes for individual studies suggest tDCS is an effective tool to improve language outcomes, while methodological approach and patient characteristics are mentioned as moderators that may influence treatment effects. We found that studies of tDCS in PPA have clinical and methodological and heterogeneity regarding patient populations, stimulation protocols and study design. While positive group results are usually found irrespective of these differences, the magnitude, duration and generalization of these outcomes differ when comparing stimulation locations, and when results are stratified according to the clinical variant of PPA. We interpret the results of included studies in light of patient characteristics and methodological decisions. Further, we highlight the role neuroimaging can play in study protocols and interpreting results and make recommendations for future work.

Association of Long-Term Speech Therapy and Neuromodulation in Primary Progressive Aphasia: Lessons from a Case Report

“…A standard 1.5 mA stimulation was first planned, but during the pilot trial, the patient asked to decrease the intensity. Thus, 1 mA intensity was chosen in order to compromise between the literature and the patient's feelings [17]. The current was ramped up for 30 s at the beginning and ramped down for 15 s at the end of the stimulation.…”

“…However, most of the studies are of rather short therapy [13]. In their systematic review, Coemans et al (2021) [17] reported that 13 of the 17 studies focused stimulation sites on the main locus of atrophy in nfvPPA patients either with an anode placed in the left inferior frontal, corresponding to the F7 electrode in the EEG 10-20 electrode position system or in the left temporoparietal areas, and the cathode placed on the right fronto-orbital area, forehead, or right shoulder. Interestingly, a longitudinal study showed improved sentence processing and neurocognitive process changes after 18 months of speech therapy in a patient with nfvPPA [18].…”

Baseline functional connectivity predicts who will benefit from neuromodulation: evidence from primary progressive aphasia

“…Recent studies from different groups, including our own, have identified several baseline factors that predict efficacy of tDCS for a given patient or patient group [see [6][7][8] for reviews]. Five predictive factors have been identified: (1) baseline language and cognitive performance [9,10], (2) baseline cortical volume and thickness [11][12][13], (3) baseline white matter integrity [14], (4) clinical variant PPA [15], and (5) baseline sleep efficiency [16].…”

“…There is feasibility and efficacy of tDCS stimulation to trained items, as well as transfer of benefits to untrained items/words [4–6]. Investigation of factors that predict treatment response in PPA is at a nascent stage (see [7,8] for recent reviews) and is challenging, because PPA is a heterogenous clinical syndrome that includes at least three different clinical variants with varying patterns of decline [2]. Early prediction of treatment outcomes (before any treatment starts) is important in a neurodegenerative disorder because losing time means losing brain tissue.…”

Positive changes to written language following phonological treatment in logopenic variant primary progressive aphasia: Case report

“…Transcranial direct current stimulation (tDCS) has emerged as a strong candidate approach in the effort to maximize the effects of behavioral rehabilitation in PPA (for recent reviews see Nissim et al, 2020 ; Coemans et al, 2021 ). In relation to other neuromodulatory approaches, tDCS advantages include good tolerance and portability for clinical use ( Nissim et al, 2020 ).…”