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Corresponding author Wei Lu, MM, Department of Rehabilitation, Jiangxi Provincial People's Hospital, No, 92 Aiguo Road, Donghu District, Nanchang, 330006, Jiangxi, People's Republic of China.
Department of Rehabilitation Medicine, Jiangxi Provincial People's Hospital, NanchangFirst Affiliated Hospital of Nanchang Medical College, NanchangDepartment of Rehabilitation Medicine, Nanchang Institute of Technology, Nanchang, China
This review aimed to systematically evaluate the effect of transcranial direct current stimulation (tDCS) on poststroke dysphagia.
Data Sources
PubMed, Cochrane Library (CENTRAL), Web of Science, VIP, CNKI, and Wanfang databases were systematically searched up to June 2021.
Study Selection
Randomized controlled trials (RCTs) on the effects of tDCS on poststroke dysphagia.
Data Extraction
The extracted data included the author, country of publication, time of publication, key elements of bias risk assessment (such as RCTs and blind methods), sample size and basic information (age, course of disease, stroke location), intervention measures, treatment methods of tDCS (stimulation location, intensity, duration), relevant outcome indicators, and relevant data (SDs).The Cochrane Risk of Bias Assessment Tool and Physiotherapy Evidence Database Scale were used to assess the risk of bias.
Data Synthesis
Sixteen RCTs were included in this meta-analysis. Overall, the results revealed a large and statistically significant pooled effect size (0.80; confidence interval [CI], 0.45-1.14; P<.001). The subgroup that explored the course of the disease yielded a large and significant effect size for the chronic phase group (0.80; CI, 0.43-1.16; P<.001). For the stimulation intensity, 1 mA and 1.6 mA showed a moderate and significant effect sizes (0.47; CI, 0.13-0.81; P=.006 vs 1.39; CI, 0.69-2.08; P<.001). In the subgroup analyses, the affected (0.87; CI, 0.26-1.48; P=.005) vs unaffected (0.61; CI, 0.23-0.99; P=.002) hemisphere showed a significant result, and stimulation of the affected hemisphere had a more obvious effect. Subgroup analysis of stroke location showed that tDCS was effective for dysphagia after unilateral hemispheric stroke, bulbar paralysis, and brainstem stroke but not for dysphagia after ataxic and basal ganglia stroke. However, the subgroup analysis of stroke location revealed a significant result (0.81; CI, 0.44-1.18; P<.001).
Conclusions
This meta-analysis demonstrated the height and significant beneficial effect of tDCS on improving poststroke dysphagia.
and the risk of aspiration pneumonia is 3 times higher and the mortality rate is 5.4 times higher in patients with dysphagia after stroke than that in patients without.
Poststroke dysphagia not only seriously affects the recovery process and quality of life in patients but also imposes a huge economic burden on families. At present, there is no evidence-based basis for rehabilitation treatment methods used in clinical practice. It is necessary to conduct randomized controlled trials (RCTs) to confirm their effects.
Transcranial direct current stimulation (tDCS), a noninvasive brain stimulation technology, is used as a nondrug intervention treatment for some neurologic or mental diseases because of its noninvasiveness, safety, and portability.
Transcranial direct current stimulation potentiates improvements in functional ability in patients with chronic stroke receiving constraint-induced movement therapy.
The tDCS regulates the transmembrane potential of neurons to produce hyperpolarization or depolarization by transmitting weak currents through the skull, thus changing the plasticity of the stimulated cerebral cortex.
Improving working memory: the effect of combining cognitive activity and anodal transcranial direct current stimulation to the left dorsolateral prefrontal cortex.
Improving working memory: the effect of combining cognitive activity and anodal transcranial direct current stimulation to the left dorsolateral prefrontal cortex.
When the anodic electrode is placed in the pharyngeal motor cortex or the functional brain area related to swallowing, the resting membrane potential of neurons can be depolarized, and the spontaneous discharge of neurons can be increased, thus increasing cortical excitability.
In addition, tDCS regulates the synaptic microenvironment by inducing long-term neurochemical changes in the N-methyl-D-aspartate receptor or gamma-aminobutyric acid activity and thus plays a role in regulating synaptic plasticity.
tDCS-induced alterations in GABA concentration within primary motor cortex predict motor learning and motor memory: a 7 T magnetic resonance spectroscopy study.
It has also been suggested that tDCS may involve transient changes in the protein channel density located at the stimulating electrode. Cortical excitability can be modulated by altering the local oxygenated hemoglobin concentration and blood flow.
A series of neurophysiological and behavioral results suggests that tDCS has a positive effect on the plasticity of cortical neurons and swallowing function.
Transcranial direct current stimulation reverses neurophysiological and behavioural effects of focal inhibition of human pharyngeal motor cortex on swallowing.
This review aimed to evaluate the clinical effects of tDCS on poststroke dysphagia using a meta-analysis.
Methods
Search strategy
We searched the following 6 electronic databases from their inception to June 2021: PubMed, Cochrane Library (CENTRAL), Web of Science, VIP, CNKI, and Wanfang. Additionally, literature was identified by citation tracking in reference lists from the identified articles. We restricted the search to studies published in English and Chinese.
The following English search terms were used: (transcranial direct current stimulation OR tDCS OR brain stimulation OR non-invasive brain stimulation) AND (dysphagia OR post-stroke dysphagia OR swallowing disorders OR swallowing difficulty) AND (stroke OR ischemic stroke OR hemorrhagic stroke OR cerebral apoplexy OR acute cerebral accident OR cerebrovascular accident OR brainstem stroke).
Selection criteria
Inclusion criteria
According to the principle of Population, Intervention, Comparator, Outcomes, and Study Designs, the following inclusion criteria were determined: (1) the participants were all patients with stroke that was confirmed by computed tomography or magnetic resonance imaging; (2) tDCS was used as the intervention; (3) the article included at least 1 of the following standardized, validated dysphagia scales: Modified Mann Assessment of Swallowing Ability, Standardized Swallowing Assessment, Functional Oral Intake Scale, Dysphagia Outcome Severity Scale, Functional Dysphagia Scale, and Video Fluoroscopic Swallowing Study; (4) the study was clinical RCT of tDCS for the treatment of dysphagia after stroke.
Exclusion criteria
(1) The article was not an RCT; (2) the article was a repetitive literature, review, and nonpublic literature, such as a conference paper; (3) swallowing dysfunction in the article was caused by other diseases, such as craniocerebral trauma and Parkinson disease; (4) poor rating on the Physiotherapy Evidence Database (PEDro) Scale (0-3 of 10); (5) incomplete data.
Process of identification
Two researchers individually screened the literature, extracted the data, and carried out cross-checks to select the literature that was consistent with the content of this study. Ambiguous titles and abstracts were sent for full-text review to avoid erroneous exclusion of potential studies; notably, if the participants received a therapeutic intervention (that is, a combination of neuromuscular electrical stimulation and another secondary intervention), the study was not excluded. There is an ethical obligation to provide patients with alternative treatments; however, noninvasive brain stimulation is rarely used alone and often includes paired stimuli that increase the sample size and allow for greater generalization.
Data extraction
Data were extracted from each article for the meta-analysis. The extracted data included the author, country of publication, time of publication, key elements of bias risk assessment (such as RCTs and blind methods), sample size and basic information (age, course of disease, stroke location), intervention measures, treatment methods of tDCS (stimulation location, intensity, duration), relevant outcome indicators, and relevant data (SDs). We defined current intensity >1mA as “high-intensity” and ≤1 mA as “low-intensity.” Table 1 presents basic information on the included studies.
Table 1Characteristics of included trials investigating tDCS in patients with poststroke dysphagia
Transcranial direct current stimulation improves the swallowing function in patients with cricopharyngeal muscle dysfunction following a brainstem stroke.
Cold stimulation, soft palate lifting training, feeding training
Anodal to affected projection area of oropharyngeal cortex
1.2 mA
20 min/10 d
VFSS
NOTE. n1 was the control group and n2 was the intervention group; a represents the contralateral stimulation group and the pseudobulbar paralysis group; b represents the affected side stimulation group and the true bulbar paralysis group; c is for control group.
Abbreviations: DOSS, Dysphagia Outcome Severity Scale; FDS, Functional Dysphagia Scale; FOIS, Functional Oral Intake Scale; MMASA, Modified Mann Assessment of Swallowing Ability; VFSS, Video Fluoroscopic Swallowing Study.
Any disagreements were resolved in a consensus meeting. The PEDro Scale is intended to identify the internal validity of clinical studies and sufficient statistical data to guide clinical decisions, with scores ranging from 0-10, with ≤3 considered poor, 4-5 fair, 6-8 good, and 9-10 excellent.
This review only included studies with scores of ≥4. We also used the Cochrane bias risk assessment tool to assess the risk bias of the RCTs included in the literature and carried out bias risk assessment from 6 aspects: selection bias, implementation bias, measurement bias, follow-up bias, reporting bias, and other bias.
Statistical analyses
All statistical analyses were performed using Reviewer Manager Software 5.4.
,a A meta-analysis was performed for standardized mean difference (SMD) between the stimulation and control groups across the whole and predefined subgroups (stimulation site, current intensity, course of stroke, site of stroke). The data we extracted from the included studies included group sizes, group mean differences, and pooled SDs. For studies that reported median and IQR, the mean and SDs were estimated using methods previously described.
For dichotomous outcomes, the odds ratios were first calculated and then converted to SMD using the equation below, as previously described48:
For each study, the SMD was calculated and synthesized into an effect size using Hedges’ g. SMD eliminates not only the effect of weights and measures on the results but also the effect of the absolute value size. Hedges’ g is a variation of Cohen's d and corrects for biases due to the small sample sizes.
Higgins JPT, Thomas J, Chandler J, et al, editors. Cochrane handbook for systematic reviews of interventions version 6.1 (updated September 2020). Available at: www.training.cochrane.org/handbook. Accessed April 15, 2021.
In the interpretation of the combined effect size, a P value ≤.05 was considered statistically significant.
This meta-analysis used the random-effects model, which allows within- and between-study variances (unlike fixed-effects that ignore between-study variance) and is, hence, more conservative when dealing with heterogeneity.
The I2 statistic, which is useful in suggesting the effect of heterogeneity by describing the percentage of variation across studies, was used to assess statistical heterogeneity. Here, I2<25%, 25%-75%, and >75% were regarded as low, moderate, and high, respectively.
,b we removed 100 duplicate titles. Of the remaining 173 articles, 66 were excluded by reading titles and abstracts, including reviews, conference papers, and more, and 62 articles unrelated to the research content were excluded. The remaining 45 studies were evaluated. Among them, 3 studies were excluded because of poor quality of literature, 9 studies were excluded because of repeated publication and inconformity of outcome indicators, 3 studies were excluded because of lack of available data, and 14 studies were excluded because of inconformity of inclusion criteria. The remaining 16 RCTs were included in the present study. The literature retrieval and screening process is shown in figure 1.
Criterion 2 (hidden allocation) from the PEDro scale was disputed among the 2 raters who independently evaluated all studies. However, all differences were resolved through consultation, and consensus was reached. All the trials ranged from 5-9, with a mean internal validity score of 6.12. This indicated that the overall quality of the included RCTs was high. No study was excluded because of poor PEDro scores. We also used the Cochrane bias risk assessment tool to assess the risk bias of RCTs included in the literature (Fig 2, Fig 3). Additionally, we tested for publication bias using a simple funnel plot. Funnel plots should appear symmetrical because asymmetrical plots indicate publication bias, which is usually positive.
In our study, dots were symmetrically scattered around the size of the aggregation effect (vertical line), which indicates that there was no publication bias in this review (fig 4).
Of the 16 included RCTs, 10 were conducted in China, 2 in South Korea, and 1 each in the United States, Japan, Italy, and Germany. The trials were reported between the beginning of 2011 and beginning of 2021. The distribution of dysphagia types was as follows: 8 included studies were unilateral hemisphere poststroke dysphagia, 3 were brainstem dysphagia, 1 was stroke with bulbar palsy dysphagia, 1 was ataxia dysphagia, and 1 was basal ganglia dysphagia. The other 2 studies did not specify the location of the stroke, but both had dysphagia after stroke. The course of disease in the 3 included cases was less than 1 month, and that in the 13 was more than 1 month.
Outcomes
The outcome measures used differed across the trials. Five of the included RCTs in this meta-analysis used the Dysphagia Outcome and Severity Scale
More than 1 outcome measure was used in 11 of the included 16 studies. Wherever the directionality of the scales differed, that is, greater number means improvement vs greater number means decline, the effect sizes were multiplied by −1 to make the scales uniform in direction across all trials. Excluding the effect of treatment prior to baseline assessment (prior to tDCS treatment), we found the duration of treatment was between 4 days and 8 weeks. In this study, our analysis was limited between baseline and post-treatment results, and we did not assess long-term efficacy owing to the variability of the study and lack of information.
Stimulation protocols
All included RCTs were performed using anode tDCS, and 1 study was a 3-arm trial. Five of them were on the unaffected hemisphere, 7 on the affected hemisphere, and 3 included bihemispheric stimulation (anodal tDCS to both affected and unaffected hemispheres). One trial used dual stimulation (anodal tDCS to the affected and cathodal tDCS to the unaffected); details of the stimulation parameters are shown in table 1.
Synthesized data analyses
Overall summary effect
Overall, results from all included trials, including two 3-arm trials, a total of 18 RCTs revealed a height and statistically significant pooled effect size in favor of tDCS on poststroke dysphagia (g=0.8; 95% confidence interval [CI], 0.45-1.14; P<.001). The I2 value, which is a measure of statistical heterogeneity, was 77%, indicating high heterogeneity between trials. Five trials had a small negative effect. Thirteen trials had moderate to large positive effect sizes, ranging from 0.40-4.09, but only 7 trials were considered statistically significant (fig 5A and table 2).
Fig 5(A) Meta-analysis forest plot of the overall effects of tDCS on poststroke dysphagia. Data derived from a random-effects model. (B) Affected vs unaffected hemisphere. Subgroup analysis that shows the effect sizes for affected vs unaffected hemisphere stimulation. (C) Acute vs chronic. Subgroup analysis that shows the effect sizes for stimulation during the acute vs chronic stroke phase. (D) Stimulation intensity. Subgroup analysis that shows the effect sizes for stimulation intensity. (E) Stroke location. Subgroup analysis that shows the effect sizes for stroke location.
Fig 5(A) Meta-analysis forest plot of the overall effects of tDCS on poststroke dysphagia. Data derived from a random-effects model. (B) Affected vs unaffected hemisphere. Subgroup analysis that shows the effect sizes for affected vs unaffected hemisphere stimulation. (C) Acute vs chronic. Subgroup analysis that shows the effect sizes for stimulation during the acute vs chronic stroke phase. (D) Stimulation intensity. Subgroup analysis that shows the effect sizes for stimulation intensity. (E) Stroke location. Subgroup analysis that shows the effect sizes for stroke location.
Fig 5(A) Meta-analysis forest plot of the overall effects of tDCS on poststroke dysphagia. Data derived from a random-effects model. (B) Affected vs unaffected hemisphere. Subgroup analysis that shows the effect sizes for affected vs unaffected hemisphere stimulation. (C) Acute vs chronic. Subgroup analysis that shows the effect sizes for stimulation during the acute vs chronic stroke phase. (D) Stimulation intensity. Subgroup analysis that shows the effect sizes for stimulation intensity. (E) Stroke location. Subgroup analysis that shows the effect sizes for stroke location.
−0.25 to 1.97 −1.31 to 0.68 −0.26 to 1.55 −0.32 to 1.12 −0.62 to 0.92 −0.42 to 0.82 −0.94 to 1.09 −0.47 to 1.62 0.04 to 1.73 −0.06 to 1.2 −0.21 to 0.81 0.44 to 2.09 0.52 to 1.42 0.07 to 1.25 −0.06 to 0.96 0.45 to 1.53 3.09 to 5.10 0.69 to 2.08 0.45 to 1.14
Large Small Moderate Moderate Small Small Small Moderate Large Moderate Small Large Large Moderate Moderate Large Large Large
The 95% confidence interval of effect size doesn't include 0.
The tDCS on the affected vs unaffected hemisphere revealed a moderate and significant pooled effect size for both (g=0.87; CI, 0.26-1.48; P=.005 vs g=0.61; CI, 0.23-0.99; P=.002). The measure of heterogeneity was much lower for the unaffected group than for the affected group (I2=35% vs 85%). Three studies were excluded from this analysis because they used bihemispheric stimulation (see fig 5B).
Patients with acute vs chronic stroke
The chronicity of stroke was classified as acute (0-14 days) and chronic (beyond 15 days). Similarly, the application of tDCS in the acute vs chronic stroke phase yielded a moderate and significant effect size for both groups (g=0.40; CI, −0.07 to 0.86; P=.09 vs g=0.80; CI, 0.43-1.16; P<.001). The I2 values for the 2 groups were 0% and 76%, respectively (see fig 5C).
Stimulation intensity
The 2 high-intensity stimulation studies that used 2 mA showed a small, nonsignificant effect size of 0.36 (CI, −0.19 to 0.91; P=.20). The statistical heterogeneity of this group was small, with an I2 value of 2%. Application of 1 mA current strength for 20 min/d, as in the 7 RCTs, revealed a moderate, significant effect size of 0.47 (CI, 0.13-0.81; P=.006). This group was homogenous (27%). The 2 studies that used 1.4 mA and 1 study that used 1.6 mA showed a moderate, significant effect size of 0.53 (CI, 0.07-0.99; P=.02) and 1.39 (CI, 0.69-2.08; P<.001). Two studies that used 1.2 mA showed a large but nonsignificant effect size of 2.50 (CI, −0.56 to 5.56; P=.11). One study that used 1.5 mA showed a moderate but nonsignificant effect size of 0.57 (CI, −0.06 to 1.20; P=.08). The last study was excluded from this analysis because its current intensity was unclear (see fig 5D).
Stroke location
Nine trials using tDCS to the unilateral hemisphere demonstrated a large and significant pooled effect size of 0.82 (CI, 0.11-1.53; P=.02) (see fig 5E). Three studies on the brainstem demonstrated a large and significant pooled effect size (1.06, CI 0.58-1.53; P<.001), as shown in fig 5B. Studies using tDCS to the bulbar paralysis demonstrated a large and significant pooled effect size of 0.71 (CI, 0.18-1.25; P=.008). Two studies on the cerebellum and basal ganglia showed a small, nonsignificant effect size of 0.40 (CI, −0.32 to 1.12; P=.28) and 0.57 (CI, −0.06 to 1.20; P=.08).
Higgins JPT, Thomas J, Chandler J, et al, editors. Cochrane handbook for systematic reviews of interventions version 6.1 (updated September 2020). Available at: www.training.cochrane.org/handbook. Accessed April 15, 2021.
reported slight tingling or itching at the beginning of tDCS treatment, but no serious adverse events occurred. No cases of shedding due to tDCS treatment were reported in any of the studies (table 3).
Table 3Safety and adverse events included in the study
Transcranial direct current stimulation improves the swallowing function in patients with cricopharyngeal muscle dysfunction following a brainstem stroke.
Transcranial direct current stimulation improves the swallowing function in patients with cricopharyngeal muscle dysfunction following a brainstem stroke.
The treatment of poststroke dysphagia is expensive and potentially fatal. Studies have shown that at least 1 in every 2 patients with stroke has dysphagia. Many studies have explored whether tDCS can be used to treat dysphagia. At present, there are few RCTs investigating transcranial direct current in the treatment of stroke-related dysphagia, so there is no reliable evaluation of the therapeutic effect. In this context, a systematic review would be helpful for synthesizing the results of these trials. This meta-analysis aimed to evaluate the effect of tDCS on the improvement of poststroke dysphagia.
Sixteen RCTs met the inclusion criteria for this review. Two trials contained 2 eligible treatment arms performed on independent participants. Therefore, 18 trials were included in this study. The synthesized findings demonstrate that the use of transcranial direct facilitated recovery in poststroke dysphagia. When the 18 trials were combined, a large and significant pooled effect size emerged (0.80; 95% CI, 0.45-1.14; P<.001).
This meta-analysis builds on the findings of an earlier meta-analysis.
a moderate effect size of 0.55 (CI, 0.17-0.93; P=.004) of noninvasive brain stimulation on poststroke dysphagia was demonstrated. Both of these meta-analyses included tDCS as well as transcranial magnetic stimulation studies. The tDCS subgroup analysis did not reach statistical significance. A meta-analysis of noninvasive brain stimulation compared with sham stimulation in patients with dysphagia after stroke was also performed previously.
and colleagues conducted a meta-analysis of the effects of neurostimulation on poststroke dysphagia, including repetitive transcranial magnetic stimulation, tDCS, surface neuromuscular electrical stimulation, and pharyngeal electrical stimulation; however, only 5-7 studies on tDCS have been included. Our study, based on a large sample size from all RCTs, showed that tDCS improves swallowing function in patients with poststroke dysphagia.
The effectiveness of tDCS stimulation on the affected hemisphere in improving dysphagia after stroke may be related to the bilateral cortical representation of swallowing. The study also analyzed other factors that appeared in the trial results, which are discussed below.
Affected vs unaffected
The issue of which hemisphere to stimulate is an important factor. Our study found that the excitatory stimulation of tDCS on both the unaffected and affected sides was statistically significant in the improvement of poststroke dysphagia.
From the perspective of anatomic structure, the swallowing cortex and neural network structure of the affected cerebral hemisphere were destroyed after stroke, while stimulation of the unaffected cerebral hemisphere was minimally affected by nerve loss or tissue injury. Remodeling of the unaffected cerebral hemisphere is the basis for the spontaneous recovery of swallowing disorders after stroke.
However, it is probable that stimulation of the affected hemispheres would increase the chance of stimulating the infarct volume. We applied tDCS to the affected hemisphere in 1 case. Yang et al
found increased glucose metabolism in the postcentral gyrus of the unaffected hemisphere and believed that tDCS might activate a large area of the cortical network engaged in swallowing recovery rather than activate the only stimulated area under the electrode. In addition, some functional neuroimaging studies suggested that affected motor regions could be targeted because the reactivation of intact portions of the affected motor cortex is associated with better outcomes after stroke.
Our results also showed that stimulation of the affected hemisphere had a larger and more significant pooled effect size than stimulation of the unaffected hemisphere (see fig 4). The effect size of the affected hemisphere (0.87, P=.005) was much higher than that of the unaffected hemisphere (0.61, P=.002), but the heterogeneity of the affected side stimulus was higher (I2=85%, 0.87). A sensitivity analysis was conducted. Because of the differences between the outcome indicators adopted by Liu et al and other included studies, after eliminating the study by Liu
a random-effect model was used for meta-analysis, and I2=37% (0.58). The research of Liu has a great influence on the heterogeneity of the articles. This may be because of an advantage in the age of the participants included in the study compared with other studies and a large gap in the course of disease among the participants included.
Some scholars believe that the ipsilateral cortex is more important in swallowing function because the lesion site of patients with dysphagia may be more concentrated on the dominant side of swallowing.
who demonstrated improvement in dysphagia after stroke by stimulating the swallowing cortex of the affected hemisphere using repetitive transcranial magnetic stimulation. The results of this review suggest that stimulation of both the affected and unaffected hemispheres with tDCS may improve dysphagia after a stroke. More RCTs will help further understand the mechanism of tDCS in the treatment of dysphagia after stroke.
Patients with acute vs chronic stroke
While numerous studies have investigated the use of tDCS in the chronic phase of stroke and have shown that it is safe and effective in improving function, data on safety and, more importantly, efficacy in acute stroke remains sparse. The spontaneous recovery process during the acute phase is presumed to be related to cortical reorganization and an increase in pharyngeal motor representation in the contralesional motor cortex.
Hamdy S, Rothwell JC, Aziz Q, Thompson DG. Organization and reorganization of human swallowing motor cortex: implications for recovery after stroke. Clin Sci (Lond) 200;99:151-157.
Hence, it may be advantageous to facilitate this course using neuromodulatory techniques such as tDCS.
In the subgroup analysis, we found that tDCS improved swallowing function in patients with stroke in the convalescent stage but not in the acute phase, which may be related to the small number of studies included in the acute phase. This review did not retrieve clinical trials within 72 hours of the onset of stroke and 1.5 years after the onset of stroke, and at present, there is no conclusion on when patients with dysphagia after stroke should receive tDCS after the attack of stroke.
Stimulation intensity
In the present review, the 9 RCTs in the high stimulation group (>1mA) used a larger current, higher current density, and longer stimulation duration than the trials in the low-stimulation group (=1mA) (see table 1); however, in the high-stimulation group, only the current intensity of 1.6 mA showed a moderate and significant effect. Despite the overall high stimulation intensity, the effect size was very small. In contrast, the low-intensity stimulation group showed moderate and significant results. Notably, variables that might have confused outcomes were not accounted for: stroke period, age, severity of stroke, dysphagia at baseline, or when swallowing treatment was performed.
Stroke location
This meta-analysis showed that tDCS has a good therapeutic effect on dysphagia after unilateral hemisphere stroke and brainstem stroke, but it is not obvious for dysphagia after stroke in the basal ganglia and cerebellum. The recovery of swallowing function depends on the development of plasticity in the swallowing motor cortex. Most studies suggest that the recovery of deglutition after unilateral cerebral apoplexy depends on the functional remodeling of the deglutition motor cortex in the healthy hemisphere.
However, some studies suggest that enhancing the plasticity of the undamaged motor cortex around the affected hemisphere is also beneficial to the recovery of swallowing function. The basal ganglia include a group of cell bodies located under the cortex of the brain, whose function is to regulate muscle tone and maintain stability of movement. Basal ganglia function injury leads to high or low muscle tone and aimless movement tDCS is performed by placing anodic electrodes in the pharyngeal motor cortex or in functional areas of the brain related to swallowing to depolarize the resting membrane potential of neurons and increase the spontaneous discharge of neurons, thereby increasing the excitability of the cortex. It may not target the basal ganglia or cerebellum.
Treatment of dysphagia is closely related to neural injury mechanisms. Swallowing function involves the primary motor sensory cortex, insular lobe, cingulate gyrus, prefrontal cortex, temporal lobe, parietooccipital region, and other brain regions, which constitute the central network of cortical swallowing. These brain regions participate in the swallowing process together.
Study limitations
There are several limitations that must be addressed. First, the included studies were heterogeneous in treatment protocols and outcome measurements. One study, in particular, Liu et al,
might have shown such a large effect size owing to several factors influencing the precision of their results. When excluding this study from the analysis, the I2 dropped from 77% to 44% and the pooled effect size dropped from 0.80 to 0.64, but the results were still considered moderate and remained significant (P<.001).
Second, patients in different studies had different characteristics. Stroke location (unilateral hemisphere and brainstem), stroke type (ischemic and hemorrhagic), and time post onset (acute and chronic stroke) are just a few of the diverse variables that could have confounded the results. However, regardless of the lesion type, it is presumed that the cases of dysphagia were neurogenic and therefore should have benefitted from interventions that promoted neuroplasticity.
Third, some studies combined noninvasive brain stimulation with other methods (such as, neuromuscular electrical stimulation and Mendelsohn maneuver). This review did not exclude any RCTs in which paired stimulation was provided. Certainly, by combining stimulation with another form of intervention, a confounding variable comes into play: the type and strength of secondary action. Although some studies have shown that multiple stimulation work better than 1 stimulus alone, this does not negate the importance of neurostimulation.
assessed patients' nutrition-related indicators and infection indicators and found that there were statistically significant differences in nutrition-related indexes and infection indexes between the 2 groups before and after tDCS treatment (P<.05). More clinical studies are needed to confirm the effects of tDCS on mortality, pneumonia, quality of life, and length of hospital stay in patients with poststroke dysphagia.
Finally, under normal circumstances, meta-analysis should be based on individual data; however, because of the lack of individual data, this method is not feasible. In addition, although all recent studies on this topic were retrieved, the number of trials and participants included in the trials were relatively small.
Conclusions
The systematic evaluation results showed that anode tDCS had a better effect on poststroke dysphagia. Although these methods have been tested in a number of studies and in different patient populations of late, larger multicenter RCTs with clinically relevant endpoints are needed for a final assessment of their respective effectiveness.
Transcranial direct current stimulation potentiates improvements in functional ability in patients with chronic stroke receiving constraint-induced movement therapy.
Improving working memory: the effect of combining cognitive activity and anodal transcranial direct current stimulation to the left dorsolateral prefrontal cortex.
tDCS-induced alterations in GABA concentration within primary motor cortex predict motor learning and motor memory: a 7 T magnetic resonance spectroscopy study.
Transcranial direct current stimulation reverses neurophysiological and behavioural effects of focal inhibition of human pharyngeal motor cortex on swallowing.
Transcranial direct current stimulation improves the swallowing function in patients with cricopharyngeal muscle dysfunction following a brainstem stroke.
Higgins JPT, Thomas J, Chandler J, et al, editors. Cochrane handbook for systematic reviews of interventions version 6.1 (updated September 2020). Available at: www.training.cochrane.org/handbook. Accessed April 15, 2021.
Hamdy S, Rothwell JC, Aziz Q, Thompson DG. Organization and reorganization of human swallowing motor cortex: implications for recovery after stroke. Clin Sci (Lond) 200;99:151-157.
Supported by the Science and Technology Project of Health Commission of Jiangxi Province (202130250), and the Key Research and Developement Plan of Jiangxi Province (20203BBGL73125; 20192BBG70016).
Ethical Standard Statement: All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki Declaration and its later amendments or comparable ethical standards.
The 95% confidence interval of effect size doesn't include 0.
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