Transcranial non-invasive brain stimulation in swallowing rehabilitation following stroke - A review of the literature

Background: This descriptive review of the literature outlines the current evidence-base underpinning the potential of transcranial brain stimulation techniques to modulate swallowing function in healthy individuals and in treating post-stroke dysphagia. Methods: Published research was identified by review of scientific databases (Scopus, Medline Ovid, Science Direct, AMED and Google Scholar) using relevant keywords. In addition, the reference lists of identified articles were scrutinized to identify further potentially relevant papers. Studies employing variants of transcranial magnetic or direct current stimulation for the purpose of modulating swallowing motor cortical excitability in healthy participants or dysphagia following stroke were included. Due to a significant heterogeneity in stimulation paradigms, all included studies were summarised and descriptively analysed in relation to the participants tested, cortical representations targeted by brain stimulation and outcome measures used. Results: Seventeen studies met inclusion criteria (seven evaluating healthy participants, 10 evaluating participants presenting with post-stroke dysphagia). Cortical stimulation most commonly targeted pharyngeal motor representations (13/17 studies). In the 10 clinical studies, stimulation was applied contralesionally (5/10 studies), ipsilesionally (3/10 studies) or bilaterally (2/10 studies). A range of behavioural and neurophysiological outcome measures demonstrated positive effects on swallowing function across studies. Conclusion: There is promising proof of concept that non-invasive brain stimulation may provide a useful adjunct to post-stroke swallowing rehabilitation practice. Eventual transition of optimal paradigms into routine clinical practice will be accompanied by practical considerations in relation to local and national frameworks, e.g. the prescription and provision of treatment.