Stroke is the leading cause of death and disability worldwide1, and dysphagia is one of the common complications of stroke, and studies have shown that the incidence of dysphagia in patients with acute stroke ranges from 37–78%2. Post stroke dysphagia(PSD) can lead to serious complications, such as aspiration, pneumonia, and malnutrition3, which greatly reduces the quality of life of patients. Currently, treatment modalities for PSD include swallowing training for muscle strength, Mendelssohn training, peripheral sensory stimulation, Chinese acupuncture training, peripheral electrical stimulation, and central nervous system stimulation4. However, these training methods often require a high degree of cooperation from the patients, so a treatment method to overcome this problem is needed.
Transcranial magnetic stimulation(TMS), a non-invasive central nervous system stimulation that is safe, non-invasive, and does not require active patient participation, has been gradually applied to stroke rehabilitation in recent years5.It is based on the principle of electromagnetic conversion, which generates an electric current that affects the brain's metabolism and neuroelectric activity. The repetitive, continuous, and regular stimulation of repetitive transcranial magnetic stimulation(rTMS) can produce a cumulative effect that excites more neurons. rTMS has been shown to alter the excitability of the cerebral cortex depending on the frequency of the stimulation used, with high frequencies (usually 5hz and above) increasing excitability and low frequencies (usually 1hz or below) decreasing excitability6. It not only alters the excitability of the cerebral cortex at the site of stimulation, but also in remote areas of functional anatomical connectivity7, activating more neuronal reorganization with sustained biological effects8. Previous studies have also indicated that rTMS may also improve patients with dysphagia in brainstem stroke9.
Currently, there are different theoretical bases for the application of rTMS in PSD, mainly including the "interhemispheric competition model" and the "healthy-side vicariation model", etc. The "interhemispheric competition model" suggests that, in general, the relationship between the two hemispheres is inhibitory and in dynamic balance. "interhemispheric competition model suggests that, in general, there is a relationship of mutual inhibition between the two hemispheres of the brain and that they are in dynamic equilibrium10. When one cerebral hemisphere is damaged, there is an imbalance in the function of both cerebral hemispheres. The inhibition of the affected hemisphere on the healthy hemisphere is weakened, and the inhibition of the healthy hemisphere on the affected hemisphere is relatively enhanced11. Transcranial magnetic stimulation based on this theory often uses both high-frequency excitation of the affected side and low-frequency inhibition of the healthy side.
The "healthy-side vicariation model" suggests that the recovery of swallowing function in patients with PSD is associated with compensatory cortical reorganization in the healthy-side hemisphere12. Fraser et al.13 found that improved swallowing in PSD patients was associated with increased excitability in the healthy hemisphere.Several studies14–16 based on the "healthy-side vicariation model" have confirmed the validity of this theory by using high-frequency TMS to stimulate the healthy side of PSD patients. This contradicts the low-frequency inhibition of lateralized therapy in the interhemispheric competition model. Since transcranial magnetism can also alter the excitability of deep distal regions7, low-frequency stimulation may inhibit the excitability of the brainstem in PSD patients, which is detrimental to the recovery of swallowing function. Multiple meta-analyses17,18 have found that using rTMS to stimulate the healthy side of PSD patients has better results than stimulating the affected side. Therefore, we chose to use high-frequency stimulation on the healthy side. High-frequency stimulation includes stimulation frequencies such as 5hz, 10hz, etc. A study by Gow et al.14 found that rTMS at 5hz had a more pronounced excitatory effect on the brain than at 10hz, and that high-frequency stimulation may not be as good as higher frequencies, and that higher frequencies may be more likely to bring about an adverse reaction. Lee et al. 19showed that in terms of selecting the stimulation site for rTMS for PSD, stimulation of the mandibuloglossus muscle cortical representative area group was more effective than the bunion abductor muscle cortical representative area group. This is consistent with previous studies in which the selected stimulation site was the pharyngeal motor cortex representative area15,20.
Not only the treatment, but also the assessment of PSD is quite important. At present, the assessment of swallowing function of PSD patients mainly uses some scales and so on, but the scale assessment lacks certain objectivity. Functional near-infrared spectroscopic(fNIRS), as an objective and non-invasive assessment tool, has the advantages of better temporal resolution, low signal-to-noise ratio (SNR)21, portability, immunity to interference22, inexpensive, easy to operate, safe and non-invasive23. It can detect changes in the concentration of oxyhemoglobin and deoxyhemoglobin (total oxyhemoglobin) in regional brain tissue, as well as temporal changes in cerebral blood flow24. Neural remodeling is one of the important mechanisms for functional recovery after stroke. fNIRS signaling has good responsiveness to changes in oxygenation in stroke patients and can monitor activation and connectivity of brain regions related to swallowing function in PSD patients in real time25, which in turn can be used to evaluate brain functional remodeling by monitoring brain activity26.
Although there have been studies on the improvement of PSD by healthy-side high-frequency rTMS, there have been no studies assessing the activation of relevant brain regions before and after its treatment by fNIRS and exploring the possible mechanisms involved. The aim of this study was to observe the effect of high-frequency rTMS stimulation of the healthy mandibular hyoid cortical area at 5 hz on PSD, and to explore the possible mechanisms by assessing it with fNIRS, so as to provide a theoretical basis for the clinical use of rTMS in the treatment of patients with PSD.