Treatment of chronic post-stroke dysphagia using 5Hz repetitive transcranial magnetic stimulation (rTMS)

Abstract

Purpose Dysphagia (swallowing disorders) is common after stroke. Stroke recovery relies on the reorganization of neurological systems (neural plasticity). Studies have found that repetitive transcranial magnetic stimulation (rTMS) can promote neural plasticity and improve swallowing functions in acute stroke survivors. rTMS is a non-invasive neuromodulation technique that can excite or inhibit cortical excitability using electromagnetic induction. Cortical excitability refers to how readily motor responses can be elicited by external stimulation of the cortex. It is quantified by the minimum stimulus intensity required to trigger motor responses (motor threshold). The tongue is important for swallowing. However, little is known about the changes in cortical excitability of the tongue with aging and after stroke. For the clinical application of rTMS, its therapeutic potential for chronic post-stroke dysphagia remains uncertain. Therefore, the present thesis aimed to investigate (1) the changes in cortical excitability of the tongue with aging and stroke, and (2) the effectiveness of rTMS as a treatment for chronic post-stroke dysphagia. Method Four studies were carried out. Study 1 compared the motor thresholds of the tongue among 35 healthy adults and six chronic stroke survivors (Aim 1). Study 2 to 4 investigated the effectiveness of rTMS as a dysphagia treatment for chronic stroke survivors (Aim 2). Study 2 investigated its effects on swallowing functions, tongue strength and swallowing-related quality of life in 15 individuals with chronic post-stroke dysphagia. Among these participants, eleven received rTMS and four received sham rTMS for 10 days. The outcomes were assessed at 2, 6 and 12 months post-rTMS. Following Study 2, some modifications on the electrode arrangement for motor response measurement were made. Study 3 investigated the effects of such modifications on the rTMS outcomes on swallowing functions in six individuals with chronic post-stroke dysphagia. Past studies have reported high inter-individual variability in the responses to rTMS. Therefore, Study 4 analyzed the individual responses to rTMS for a comprehensive understanding of its therapeutic potential. Results Study 1 showed that the cortical excitability of the tongue was significantly reduced in adults aged above 65 and in stroke survivors when compared to age-matched healthy adults. Study 2 found no significant group treatment effects of rTMS on swallowing functions, tongue strength, or swallowing-related quality of life. Study 3 revealed that using a fixed inter-electrode distance for motor response measurement improved the rTMS outcomes on swallowing functions when compared to the original design in Study 2. Upon individual analysis, Study 4 revealed that 10 of the 17 participants who received rTMS showed greater improvements in swallowing functions than the sham rTMS group. They had higher cortical excitability than those who did not benefit from rTMS. The treatment effects were maintained up to 12 months post-rTMS in two participants. Conclusion The present thesis showed that aging and stroke reduce the cortical excitability of the tongue. Substantial inter-individual variability in the responses to rTMS was observed. Group-level analysis revealed insignificant treatment effects. However, individual analysis suggested that individuals with mildly- or moderately-impaired cortical excitability may respond better to rTMS than those with severely-impaired cortical excitability.