Dysphagia is one of the most common problems following a stroke, with a high incidence of 80% [1]. Although the prevalence of dysphagia gradually decreases over time, 50% of patients still have symptoms of dysphagia at six months after stroke onset [2]. Dysphagia increases the incidence of undernutrition, dehydration, aspiration pneumonia, prolonged hospital stay, and high medical expenses or even death ^[1][3][4][1,3,4]. Conventional rehabilitation methods include dietary changes, pronunciation training, posture training, oral ice stimulation, swallowing training, and pulse electrotherapy, which are widely used to enhance the recovery of deglutition function. However, conventional rehabilitation may need frequent treatment over many weeks before obtaining a good clinical response, leading to poor patient compliance and, eventually, poor clinical outcomes [5]. In some clinical studies, botulinum toxin injections are also recommended for treating dysphagia [6]. However, this is a treatment option for only few patients suffering from post-stroke dysphagia. Therefore, novel, effective treatment methods are desperately needed to improve swallowing dysfunction after stroke.

To date, there have been several important non-invasive neuromodulations developed to manage neuropsychiatric disease. To be specific, the repetitive transcranial magnetic stimulation (rTMS), theta burst stimulation (TBS, a variant of rTMS), non-invasive vagal nerve stimulation, pharyngeal electrical stimulation (PES), neuromuscular electrical stimulation (NMES), transcranial direct current stimulation (tDCS), and transcranial random noise stimulation (tRNS, a variant of tDCS). Further, there have been several important studies addressing the efficacy and safety of these non-invasive neuromodulations in neuropsychiatric disease, such as dementia and minimal cognitive impairment [7], cognition in brain disorder [8], Alzheimer’s disease [9], and Parkinson’s disease with mild cognitive impairment [10]. Moreover, these methods are also recommended for the treatment of post-stroke dysphagia ^{[11][12][13][14]}[11,12,13,14]. Among them, tDCS is a promising adjunct therapy to improve deglutitive function.

2. Effect of Stimulation Site of tDCS on Post-Stroke Dysphagia

Subgroup analyses of the modified Mann assessment of swallowing ability and the functional oral intake scale demonstrated that anodal tDCS of the damaged hemisphere and bilateral hemispheres could significantly affect deglutition function in stroke patients. When tDCS is used in different brain areas, it can give rise to various manifestations, for instance, changes in brain networks, cognitive performance, and brain metabolite and neurotransmitter levels ^[15][16][17][42,43,44]. Since swallowing has bilateral hemispheric representation, the reorganization of the damaged cerebral hemisphere may also play an essential role in recovering deglutition function after stroke ^[18][19][45,46]. For the meta-analysis results of the modified Mann assessment of swallowing ability, the weighted effect size for the bilateral hemisphere was large at 6.19 compared to a medium effect size of 5.87 for the undamaged hemisphere. For the meta-analysis results of the functional oral intake scale, the weighted effect size for the bilateral hemisphere was large at 0.86 compared to the medium effect size of 0.48 for the undamaged hemisphere. These results suggests that anodal tDCS of the bilateral hemisphere is superior to the undamaged hemispheres for improving deglutition function after stroke. The Our results are consistent with previous studies showing that the application of tDCS to the bilateral hemisphere may have some inherent advantages over applying it to the undamaged hemisphere. Bilateral tDCS can affect neuronal activity and connectivity within and across the sensorimotor cortical network in the brain ^[20][47]. Of course, this result requires further RCT confirmation.

3. Effect of Intense Stimulation of tDCS on Post-Stroke Dysphagia

The subgroup analysis of the dysphagia outcome and severity scale demonstrated that both low and high-intensity stimulation with anodal tDCS can significantly affect deglutition function in stroke patients. Notably, high-intensity stimulation with anodal tDCS has more advantages than low-intensity stimulation for improving deglutition function after stroke. It is generally known that tDCS is a non-invasive technique that uses a constant, low-intensity direct current (1~2 mA) to regulate neuronal activity in the cerebral cortex. Previous studies had identified that high-intensity (2 mA) stimulation with tDCS had a greater effect on neural plasticity than low-intensity (1 mA) stimulation ^[21][22][48,49]. Here, reswearchers divided stimulation with tDCS into high intensity (1.6–2 mA) or low intensity (1–1.5 mA) according to the included studies’ characteristics. The results showed that high-intensity stimulation has a better effect size than low-intensity stimulation. As described in previous studies ^[23][50], high-intensity stimulation resulted in a significant increase of motor-evoked potentials amplitudes, whereas low-intensity stimulation is also associated with less variability in corticospinal excitability. Moreover, higher cortical excitability is associated with better swallowing function recovery ^[24][51].

4. Duration Stimulation of tDCS

Apart from the intensity of stimulation by tDCS, the duration of stimulation is also an element that can have an impact on the efficacy of tDCS ^[25][52]. It has been shown in human studies that tDCS duration varied from 3 to 40 min ^[26][53]. In Rour researcher, we found that the longest duration of stimulation was 40 min. However, too few durations exist for tDCS for post-stroke dysphagia and it was hard to apply subgroup analyses. Therefore, researcherswe were not able to investigate the optimal duration of stimulation.

5. Treatment Period of tDCS

TIn this study, the treatment period for tDCS differed between studies. MIn a previous study, multiple stimulation with tDCS per week may produce a cumulative effect on brain activity and increase its impact on behavioral outcomes ^[27][54]. It is generally thought that the short-term and long-term effects of tDCS are different, one of which is resting membrane potential depolarization through non-synaptic mechanisms ^[28][55], and the other is N-methyl-D-aspartate-dependent mechanisms ^[29][56].

6. Adverse Effects of tDCS on Post-Stroke Dysphagia

RWesearchers should recognize that for any stimulation protocol there exists a certain degree of risk that could cause problems in particular cases. Many questions remain open until extensive research or clinical experience is gained. In general, low intensity (1–2 mA) tDCS is considered safe ^[30][18]. However, this evidence was collected mainly from healthy subjects and neurological and psychiatric patients. In theour research, some studies expressly affirmed that there were no adverse effects reported for post-stroke dysphagia. It should be noted explicitly that one study that used the highest intensity (2 mA) for tDCS declared no adverse events occurred ^[31][29]. However, a large sample study of tDCS of healthy subjects and other diseases has reported some negative effects, such as pain, fatigue, itching, etc. ^[32][57]. Thus, small sample sizes may explain why the studies mincludentioned d in this review did not report adverse effects.