Effect of transcranial direct current stimulation combined with respiratory training on dysphagia in post-stroke patients

Abstract

BACKGROUND:

Transcranial direct current stimulation (tDCS) has a considerable advantage in the rehabilitation treatment of dysphagia.

OBJECTIVE:

The purpose of this study was to explore the effect of tDCS combined with respiratory training on dysphagia in post-stroke patients.

METHODS:

From December 2017 to January 2019, 64 post-stroke patients who were hospitalized in the Department of Neurology of the Second Hospital of Hebei Medical University were enrolled in this study. They were randomly divided into control and treatment groups ( $n=$ 32). Patients in the two groups received routine swallowing rehabilitation training and respiratory training. On this basis, the patients in the treatment group received tDCS. The anode was placed in the movement area of the pharyngeal cortex on the unaffected side of the patients’ bodies, and the cathode was placed in the upper orbital orbit on the opposite side. The current intensity was 1.5 mA, 20 min/time, 1 time/d, and 6 d/w. Before and after the treatment, the water swallow text, functional oral intake scale (FOIS), forced vital capacity (FVC) and peak expiratory flow (PEF) were assessed, and the correlation among them was evaluated.

RESULTS:

There were no differences in all indexes before and after treatment. After treatment, water swallow text, FOIS, FVC and PEF were all better than before treatment, and the clinical efficacy in the treatment group was significantly better than that in the control group. FVC and PEF were positively correlated with water swallow text and FOIS.

CONCLUSION:

tDCS combined with respiratory training may have a significant therapeutic effect on dysphagia in post-stroke patients.

Keywords

Transcranial direct current stimulation respiratory training dysphagia stroke

1. Introduction

Stroke is a group of diseases that cause brain tissue damage due to the sudden rupture of blood vessels in the brain or the inability of blood to flow into the brain because of clogged blood vessels. Dysphagia refers to the damage of structure and/or function of mandible, double palate, tongue, soft palate, pharynx and other organs in the process of food delivery from mouth to stomach, which is an independent risk factor of stroke [1]. Dysphagia can lead to drooling and aspiration pneumonia, salivation, malnutrition, dehydration, emaciation and other complications, and even death, which greatly affects the quality of life and brings economic burden to families and society [2]. There is no specific drug for treating dysphagia in stroke. Rehabilitation training has become the main treatment. However, the effect of traditional rehabilitation is limited, so it is necessary to find all feasible and effective methods of swallowing rehabilitation training.

Transcranial direct current stimulation (tDCS) is a non-invasive technique that uses constant, low-intensity direct current (1–2 mA) to regulate the activity of neurons in the cerebral cortex. It is safe, effective, simple to use, easy to carry, low cost [3]. tDCS has a considerable advantage in the rehabilitation treatment of dysphagia, especially in early rehabilitation, which makes it possible for patients to receive tDCS directly beside the bed.

In recent years, it has been reported that the pulmonary ventilation function of patients with dysphagia is weaker than that of healthy people, especially in the elderly [4]. Many experts and scholars have also found that the swallowing dysfunction of patients with stroke is closely related to the decline of respiratory function [5, 6, 7]. It is believed that respiratory training can improve the function of respiratory muscles, change the respiratory mode, improve the pulmonary ventilation function, reduce the pulmonary infection rate and the complications of dysphagia in post-stroke, and improve the quality of lif [8, 9, 10]. Furthermore, respiratory training can also regulate the immune system, and enhance physical fitness and immunity [11]. The purpose of this study was to explore the feasibility and efficacy of tDCS combined with respiratory training in the treatment of dysphagia in post-stroke patients.

2. Patients and methods

2.1 Patients

This study was approved by the Ethics Committee of the Second Hospital of Hebei Medical University. All patients signed an informed consent form before participating in the experiment. From December 2017 to January 2019, 64 post-stroke patients who were hospitalized in the Department of Neurology of the Second Hospital of Hebei Medical University were enrolled in this study.

Inclusion criteria: (1) Patients met the diagnostic criteria of China’s guidelines for the diagnosis and treatment of acute stroke in 2014 [12]; (2) The age of patients from the first onset is between 18 and 75 years old; (3) The course of disease is within 48 hours to 2 weeks after the disease is stable; (4) The total score of dysphagia screening (eating assessment tool, EAT-10) is more than 3 points; (5) The primary screening test of water swallowing is above grade 3; (6) Patients who were awake and able to cooperate with the trial.

Exclusion criteria: (1) Patients with a history of stroke and swallowing dysfunction; (2) Patients with serious liver and kidney disease, unstable myocardial infarction, and aortic aneurysm or chronic obstructive pulmonary disease; (3) Patients who had respiratory muscle training; (4) Patients who need drug treatment for epileptic seizures; (5) Elderly patients with multiple diseases; (6) Patients with tDCS contraindications.

The patients were randomly divided into control group and treatment group in accordance with the random number table ( $n=$ 32). The patients in both groups were treated with conventional medicine and swallowing rehabilitation. On this basis, the patients in control group received respiratory training, and those in the treatment group received tDCS and respiratory training.

2.2 Conventional swallowing rehabilitation

Conventional swallowing rehabilitation includes direct oral feeding training, ice stimulation, acid stimulation, oral exercise, shaker training, training method of swallowing on glottis, Mendelsohn maneuver and neuromuscular electrical stimulation, which were performed as previously described [5, 13].

2.3 Respiratory training

Respiratory training included lip retraction breathing and breathing trainer, 20 min/time, twice a day. Breath by shrinking lips [14]: Ask the patient to make the lips into a “woo” shape. Inhale with the mouth quickly, exhale slowly, inhale quickly, and then make a “woo” and “toot” sound slowly. Place a piece of toilet paper 10 cm in front of the patient’s mouth, and instruct the patient to blow up the toilet paper. Respiratory trainer: The therapist held the trainer in one hand and the ventilator in the other hand. Ask the patient to sit at the end and inhale or blow forcefully. The changes of blood pressure and heart rate were paid attention to.

2.4 tDCS

The IS200 transcranial direct current stimulator (Sichuan Intelligent Electronic Industry Co., Ltd., Chengdu, China) with 5 cm $\times$ 7 cm isotonic saline gelatin sponge electrode as stimulator electrode was used in this study. The anode was placed in the primary motor cortex of the contralateral side (15 cm from the center of the top, then 2 cm from the front of the forehead), and the cathode was placed in the contralateral upper orbit [15]. The current intensity was 1.5 mA, 20 min/time/d, 6 days/week.

2.5 Evaluation indexes

2.5.1 Water swallowing test

The water swallowing test was a screening method for drinking water situation and degree of dysphagia proposed by Japanese Jun Fu Wada in 1982 [2]. It has better sensitivity and specificity. Invalid: No improvement or even aggravation of clinical symptoms, no change in the rating of water swallowing test; improvement: Swallowing disorder is significantly improved, and the rating of water swallowing test is 1 grade higher than before; significant effect: The rating of water swallowing test is 2 grade higher than before; cure: The rating is 1 grade, and the drinking water problem is completely solved.

2.5.2 Functional Oral Intake Scale (FOIS)

FOIS is an evaluation method for food and water intake proposed by Crary et al. in 2005 [16]. It is divided into 7 levels from low to high. The higher the level, the better the swallowing function.

2.5.3 Forced vital capacity (FVC)

Spirolab III pulmonary function tester (Millways (Beijing) Medical Technology Co., Ltd., Beijing, China) was used to evaluate pulmonary function. FVC refers to the volume of air exhaled at the maximum effort and speed up to the residual volume (RV) position after the maximum inhalation to the total lung volume (TLC). Clinically, the lower limit of normal value (LLN) of 80% of normal value is taken as the reference index [17]. Peak expiratory flow (PEF) is an important index to evaluate this experiment. It has been reported that PEF in patients with acute and chronic stroke decreased by about 1/3 [18]. In clinic, the lower limit of normal value (LLN) of 80% of normal value is taken as the reference index [17].

2.6 Statistical analysis

Statistical analysis was made by software SPSS21.0 (IBM Corp., Armonk, NY, USA). The measurement data were expressed as means $\pm$ standard deviation (SD). The normality and homogeneity of variance of the data were tested. If it was consistent, $t$ -test was used to compare between the two groups and within the group. If not, nonparametric test was used. The results of pulmonary function evaluation and water swallowing test were analyzed by linear correlation. Differences were considered statistically significant when $P<$ 0.05.

3. Results

3.1 General clinical information

Approximately 85 people met the criteria of the study. Nineteen patients dropped out for various reasons. Because those who did not meet the inclusion criteria were not included, their data were not counted. A total of 64 patients with post-stoke were included in this study. There were no significant differences between the two groups in gender, age, pathological nature, location, pathological side, course of disease, and time of rehabilitation intervention before treatment ( $P>$ 0.05) (Table 1).

Table 1
Clinical characteristics of patients in the two groups

Groups	Treatment group	Control group
Cases	32	32
Age	60.22 $\pm$ 10.55	59.44 $\pm$ 10.16
Gender
Male	21	20
Female	11	12
EAT-10 scale	11.22 $\pm$ 2.45	11.28 $\pm$ 2.72
FVC	48.22 $\pm$ 21.33	46.81 $\pm$ 20.12
PEF	35.66 $\pm$ 19.27	39.69 $\pm$ 18.67
Course of disease (days)	8.22 $\pm$ 2.24	7.84 $\pm$ 2.33
Treatment of days	11.50 $\pm$ 2.76	10.75 $\pm$ 2.11
Pathologicalchanges
Medulla	21	22
Cortical	11	10
Lesion sides
Left	19	17
Right	13	15
Course of disease	7.84 $\pm$ 2.33	8.22 $\pm$ 2.24

3.2 Water swallowing test and FOIS

As shown in Tables 2 and 3, there were no significant differences between the two groups in water swallowing test and FOIS before treatment ( $P>$ 0.05). However, water swallowing test and FOIS after treatment were notably greater than those after treatment in both groups ( $P<$ 0.05). In addition, after treatment, water swallowing test and FOIS in treatment group were greater than those in control group ( $P<$ 0.05).

Table 2
Comparison of water swallowing test between the two groups (n)

Groups	Grade I	Grade II	Grade III	Grade IV	Grade V
Control group
Before	0	0	2	5	25
After	2	4	6	11	9
Treatment group
Before	0	0	1	10	21
After	4	8	11	6	3

Table 3

Comparison of FOIS between the two groups (n)

Groups	Grade I	Grade II	Grade III	Grade IV	Grade V	Grade VI	Grade VII
Control group
Before	9	10	6	4	3	0	0
After	6	3	6	8	3	5	1
Treatment group
Before	12	9	6	3	2	0	0
After	0	4	6	5	7	7	3

3.3 Clinical efficacy

According to the curative effect standard of water swallowing test, effective rate (%) $=$ (cure (n) $+$ significant effect (n) $+$ improvement (n))/total amount (n). The effective rate was 90.6% in the treatment group and 65.6% in the control group (Table 4) ( $P<$ 0.05). Along with that, two patients felt a decrease in power and a slight numbness. After a little posture adjustment, the adverse reactions disappeared. Other patients had no adverse reactions.

Table 4
Comparison of treatment effect between the two groups

Groups	Cure	Significant effect	Improvement	Invalid	Efficacy
Control group	4	15	10	3	65.6%
Treatment group	2	7	12	11	90.6%

3.4 FVC and PEF

There were no significant differences in FVC and PEF between the two groups before treatment ( $P>$ 0.05). After treatment, FVC and PEF in the two groups were higher than those before treatment ( $P<$ 0.05), and the improvement of FVC and PEF in the treatment group was higher than that in the control group ( $P<$ 0.05) (Table 5).

Table 5
Comparison of FVC and PEF between the two groups

Groups	FVC (%)		PEF (%)
	Before treatment	After treatment	Before treatment	After treatment
Control group	46.81 $\pm$ 20.12	56.81 $\pm$ 24.28	39.69 $\pm$ 18.07	44.41 $\pm$ 19.03
Treatment group	48.22 $\pm$ 21.33	68.28 $\pm$ 20.64	35.66 $\pm$ 19.27	54.31 $\pm$ 18.32

3.5 Correlation between water swallowing test and pulmonary function

After treatment, EAT-10, water swallowing test and FOIS were analyzed with FVC and PEF respectively, and there were statistical differences between them ( $P<$ 0.05) (Table 6). The results showed that EAT-10, water swallowing test and FOIS were significantly correlated with FVC and PEF.

Table 6
Correlation analysis between water swallowing teat and pulmonary function

Items	EAT-10	Water swallowing teat	FOIS
FVC	$r_{s}=-$ 0.628	$r_{s}=-$ 0.643	$r_{s}=$ 0.610
	$P<$ 0.000	$P<$ 0.000	$P<$ 0.000
PEF	$r_{s}=-$ 0.581	$r_{s}=-$ 0.549	$r_{s}=$ 0.569
	$P<$ 0.000	$P<$ 0.000	$P<$ 0.000

4. Discussion

In addition to the central pattern generator of medulla oblongata, swallowing function is also controlled by the central swallowing network of cortex and peripheral nerve, including the afferent fibers of trigeminal nerve, facial nerve, vagus nerve and hypoglossal nerve [19]. In addition, swallowing is also related to cerebellum and midbrain [20, 21].

Medulla oblongata is the center of respiratory function and interacts with swallowing function to provide corresponding airway protection function [6]. The central pattern generator (CPG) is composed of the dorsal swallow group (DSG) and ventral swallow group (VSG), which is responsible for transmitting sensory information to the medulla oblongata through the vagus nerve after food enters the pharyngeal period, and transmitting regulatory information to the pharynx, larynx and trachea, so as to seal the nasopharynx passage, thus completing the adjustment of respiratory mode and protecting the airway function. Therefore, this provides a theoretical basis for the treatment of dysphagia in post-stroke with respiratory training.

The hypoplasia of post-stroke patients is related to the decline of respiratory function. The medulla oblongata is the center of respiratory and swallowing function. When the lesions of dysphagia in post-stroke patients involve the medulla oblongata center, the control respiratory function of the cortical phrenic nerve pathway is damaged, resulting in the weakness of the chest or abdominal muscles, spasm, and the reduction of thoracic movement, so the corresponding pulmonary function is reduced, and ultimately leading to restrictive dyspnea [22]. Therefore, patients with dysphagia after stroke are prone to pneumonia. Pikus et al. compared the aspiration of dysphagia with the outcome of pneumonia by video fluoroscopic swallowing study (VFSS), and found that the incidence of pneumonia in this type of patients will increase correspondingly [23]. All respiratory training methods are to improve respiratory muscle function, change abnormal respiratory mode, enhance cough reflex, clear respiratory secretions, establish correct respiratory and swallowing coordination mode, so as to achieve the purpose of swallowing function recovery [11].

Our results proved that swallowing function and lung function are linearly related. Sangwon min et al. also showed that there was a significant correlation between lung function indexes and the severity of dysphagia [22]. After the treatment of respiratory training, FVC and PEF of lung function were improved [8, 18]. Our results are consistent with those research results. It showed that respiratory training can improve the FVC and PEF of stroke patients. tDCS treatment was added on the basis of respiratory training, and the results suggested that the FVC and PEF in the treatment group was higher than those in the control group, proving that tDCS combined with respiratory training was more effective in improving lung function.

The area related to swallowing is the pharyngeal cortex motor area. tDCS depolarizes the resting potential of cortical neurons and increases the cortical excitability of the motor area of the pharyngeal cortex. According to the study of Dipesh H. vasant, when tDCS acts on the motor area of the pharyngeal cortex, it can enhance the connection between the network of the motor area of the pharyngeal cortex and the synaptic function related to the task by enhancing and participating in the integration of functional information among the various swallowing cortex [24]. The central mode transmitters in the dorsolateral and ventrolateral areas of the medulla oblongata in the brain stem are regulated by the motor area of pharyngeal cortex [15]. The regulated information is fed back to the central mode generator of the medulla oblongata, and then the regulated signal is transmitted to the corresponding swallowing muscles, so as to achieve the effect of treating the swallowing dysfunction of lesions in the cortex [25].

Jefferson et al. First used tDCS to act on the left pharyngeal cortex motor area of healthy people in different doses and times in 2009, and found that the excitability of the pharyngeal cortex was increased [26]. After that, experts and scholars not only confirmed the effectiveness of TDCS in the treatment of swallowing, but also confirmed that tDCS stimulated the contralateral pharyngeal cortex motor area with better therapeutic effect [27, 28]. In this study, the therapeutic effect of tDCS combined with respiratory training was better than that of respiratory training group. This may be due to the role of TDCS anode in the treatment of pharyngeal cortex motor area through functional remodeling and synaptic connection, and the excitatory signals caused by tDCS are transmitted to the central pattern generator. The central pattern generator can also receive and control respiratory information [28]. Therefore, when tDCS acts on the motor area of the pharyngeal cortex, it can enhance the regulation of the central pattern generator in the brain stem on swallowing and breathing by enhancing the connection of brain functional areas, so as to enhance the respiratory function of patients while treating dysphagia. Therefore, the effect of respiratory training on dysphagia patients with stroke is better when combined with tDCS.

This study is not without limitations. Because the majority of the patients in the early stage of stroke have serious condition and short hospital stay, it is unable to use more objective criteria such as swallowing radiography for evaluation. At the same time, the treatment time of patients is shorter, which may lead to the deviation of research results. Therefore, in future research, we will continue to improve the plan design, do further in-depth research and follow-up observation of long-term efficacy.

5. Conclusion

Respiratory training has a positive effect on dysphagia in patients with post-stroke, while tDCS combined with respiratory training has a better effect on dysphagia in post-stroke.

Footnotes

Acknowledgments

None to report.

Conflict of interest

None of the authors have any conflict of interest to report.

Funding

None to report.

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Effect of transcranial direct current stimulation combined with respiratory training on dysphagia in post-stroke patients

Abstract

BACKGROUND:

OBJECTIVE:

METHODS:

RESULTS:

CONCLUSION:

Keywords

1. Introduction

2. Patients and methods

2.1 Patients

2.2 Conventional swallowing rehabilitation

2.3 Respiratory training

2.4 tDCS

2.5 Evaluation indexes

2.5.1 Water swallowing test

2.5.2 Functional Oral Intake Scale (FOIS)

2.5.3 Forced vital capacity (FVC)

2.6 Statistical analysis

3. Results

3.1 General clinical information

Table 1 Clinical characteristics of patients in the two groups

Table 2 Comparison of water swallowing test between the two groups (n)

Table 4 Comparison of treatment effect between the two groups

Table 5 Comparison of FVC and PEF between the two groups

Table 6 Correlation analysis between water swallowing teat and pulmonary function

5. Conclusion

Footnotes

Acknowledgments

Conflict of interest

Funding

References

Table 1
Clinical characteristics of patients in the two groups

Table 2
Comparison of water swallowing test between the two groups (n)

Table 4
Comparison of treatment effect between the two groups

Table 5
Comparison of FVC and PEF between the two groups

Table 6
Correlation analysis between water swallowing teat and pulmonary function