Sage Journals: Discover world-class research

Abstract

Background

Transauricular vagus nerve stimulation (taVNS) is being studied as a feasible intervention for stroke, but the mechanisms by which this non-invasive technique acts in the cortex are still broadly unknown.

Objectives

This study aimed to systematically review the current pre-clinical evidence in the auricular vagus nerve stimulation (aVNS) neuroplastic effects in stroke.

Methods

We searched, in December of 2022, in Medline, Cochrane, Embase, and Lilacs databases. The authors executed the extraction of the data on Excel. The risk of bias was evaluated by adapted Cochrane Collaboration’s tool for animal studies (SYRCLES’s RoB tool).

Results

A total of 8 studies published between 2015 and 2022 were included in this review, including 391 animal models. In general, aVNS demonstrated a reduction in neurological deficits (SMD = −1.97, 95% CI −2.57 to −1.36, I² = 44%), in time to perform the adhesive removal test (SMD = −2.26, 95% CI −4.45 to −0.08, I² = 81%), and infarct size (SMD = −1.51, 95% CI −2.42 to −0.60, I² = 58%). Regarding the neuroplasticity markers, aVNS showed to increase microcapillary density, CD31 proliferation, and BDNF protein levels and RNA expression.

Conclusions

The studies analyzed show a trend of results that demonstrate a significant effect of the auricular vagal nerve stimulation in stroke animal models. Although the aggregated results show high heterogeneity and high risk of bias. More studies are needed to create solid conclusions.

Keywords

stroke auricular vagal nerve stimulation animal models meta-analysis systematic review

Introduction

Stroke is the second leading cause of death¹ and the first leading cause of disability worldwide, causing a total economic burden of approximately of 100 billion dollars to the United States annually.² It can be caused either by an abrupt interruption of blood flow in the cerebral vascular territory, or blood leakage causing cerebral hemorrhage. In the ischemic stroke, the interruption of blood flow and thus shortage of glucose levels leads to the failure of energy-dependent channels, which in turn activates a series of intracellular events that culminate in increased oxidative stress.³ The activation of inflammatory pathways is a major secondary event to the progression of brain damage.

Currently, the only available therapeutic options at the acute stage rely on chemical or surgical reperfusion techniques. Still, only a minority of the patients are eligible to receive them.⁴ Also, the rapid blood reperfusion, even though crucial to restoring brain metabolic activity and reverse penumbra, may also lead to secondary lesions due to an abrupt increase in oxygen availability.⁵ Neuroprotectant agents targeting pathological events may help contain the detrimental cascade resultant from both ischemia and rapid reperfusion, extend the time window for treatment, and serve as an adjunct alternative to ineligible patients to reperfusion.⁶

Recently, animal studies have been investigating more accessible, less invasive interventions to deal with the consequences and complications of stroke. One of these techniques is noninvasive vagus nerve stimulation, which can be applied transcutaneously via the auricular branch of the vagus nerve or the cervical branch, with auricular vagus nerve stimulation (aVNS) being more thoroughly investigated for this condition.^7,8 Recent systematically reviewed results, on the aVNS’s electroencephalographic effects, suggests the capability of aVNS in modulate cortical activity.⁹ Furthermore, aVNS has been investigated in a variety of conditions, such as epilepsy, tinnitus, depression, Alzheimer’s disease, Schizophrenia, pain, migraine, and Parkinson’s disease, being considered safer, more accessible, and less invasive alternative than the surgical device.^9
-12 Currently, animal studies seem to be having positive results regarding the reestablishment of motor function in rats with post stroke sequelae after aVNS.^13
-22 However, there is a lack of full understanding and categorization of the current evidence, especially regarding the mechanisms behind aVNS’s effects that can be further analyzed.

Therefore, given the amplifying field of aVNS and its current potential relevancy for post stroke rehabilitation, we propose a systematic review and meta-analysis assessing the current pre-clinical evidence in aVNS in stroke aiming at understanding the mechanistic aspects of this intervention.

Methods

Data Sources and Study Selection

The team executed a systematic review of the literature and meta-analysis. The study followed the Preferred Reporting Items for Systematic Reviews and Meta-Analysis,²³ the Cochrane handbook for systematic reviews recommendations,²⁴ and a step-by-step guide to conduct systematic reviews of animal studies proposed by Lenaars et al.²⁵ This review was registered in PROSPERO, by the number 385026.

We used the search terms (“Transcutaneous vagal nerve stimulation” OR “Transcutaneous vagus nerve stimulation” OR “Auricular vagal nerve stimulation” OR “Auricular vagus nerve stimulation” OR “tVNS” OR “ taVNS” OR “aVNS” OR “VNS” OR “vagal nerve stimulation”) AND (“stroke” OR “cerebrovascular disease” OR “CVA”) OR (“poststroke” OR “post-stroke”) OR (“cerebrovasc*” OR “brain vasc*” OR “cerebral vasc*” OR “SAH”) OR (“ischemic transient attack”) OR (“ischemic stroke” OR “brain infarct*”) OR (“hemorrhagic stroke” OR “hemorrhagic stroke” OR “brain bleed”) AND ((animals) OR (rodents) OR (rats) OR (mice) OR (mouse) OR (mouse model*) OR (rodent model*) OR (rat model*) OR (mammal*) OR (animal)) in the Medline, Cochrane, Embase, and Lilacs databases. The search was performed in December 2022. The inclusion criteria were done as follows: (i) studies that used auricular vagus nerve stimulation in post cerebral vascular events in animal models and (ii) studies in English, Spanish, or Portuguese. Exclusion criteria were: (i) case reports, review articles, abstracts of conferences, letters, editorials, case control studies; (ii) ex vivo studies, in vitro studies, studies in humans or in sillico studies; and (iii) cervical vagal nerve stimulation.

The references gathered by the search were transferred to an excel for undergoing evaluation and screening. The titles and abstracts were analyzed by 2 separate authors (IR and AM). If the abstracts were not clear, a full text evaluation was retrieved. At the end, the authors reached a consensus regarding the inclusion and exclusion criteria, afterward the references underwent data extraction and analyses.

Acute ischemia/reperfusion models were considered acute with the outcomes assessed until 24 hours after the event, subacute between 24 hours and 6 months, and chronic if the outcomes were assessed after 6 months.

Data Extraction and Quality Assessment

Two authors (PSdM and JV) executed the extraction of the data on Excel, the process was supervised by a third senior investigator (FF). The authors extracted variables related to the animal models, study design, intervention, stimulation parameters, and outcomes related with neurological behavior, anatomopathological data, inflammatory markers, neuroplasticity markers, and other mechanistic outcomes disposed by the studies.

Quality Assessment

The risk of bias was evaluated by adapted Cochrane Collaboration’s tool for animal studies (SYRCLES’s RoB tool).²⁶ The tool allows the assessing of concerns of domains such as: sequence generation, baseline characteristics, allocation concealment, random housing, blinding (performance bias), random outcome assessment, blinding (detection bias), incomplete outcome data, selective outcome reporting, and other sources of bias. Two investigators (JV and ACG) rated the studies independently and came to a consensus regarding the evaluation, a third investigation was contacted in case of disagreement (PSdM).

Quantitative Analysis

Descriptive Data

We standardized the metrics of central tendency for mean and standard deviations (SD). When not reported, the means and SD or standard errors (SE) were extracted from the plots provided by the study using the WebPlotDigitizer software (https://automeris.io/WebPlotDigitizer/). SEs were transformed in SDs by the following formula: $S D = S E x \sqrt{S a m p l e s i z e}$ .²⁴

Outcomes

Primary outcomes were the behavioral assessments represented by the neurological deficits scores and adhesivel removal test. Secondary outcomes included lesion and neuroplastic anatomopathological results, such as infarct size, micro capillary density, markers of angiogenesis, and BDNF levels and expression. The mean and SDs post intervention were compared between aVNS and controls for each one of the outcomes.

Meta-Analysis

Based on the post intervention metrics, we performed a random-effects meta-analysis calculating the pooled standard mean differences between aVNS and controls and its 95% confidence intervals (CI) based on the inverse variance method. Heterogeneity were tested through I², we considered low heterogeneity when I² < 40%, moderate when 40% < I² < 60%, and high when I² > 60%.²⁴ A sensitivity analysis omitting each one of the studies was performed for those outcomes with more than 3 studies. The command “metacont,” for the pooled results, and “metainf,” for the sensitivity analysis, from the “meta” package in RStudio were used in this analysis. In order to test publication bias, we built a funnel plot for the main outcome using the command “funnel.meta” from the “meta” package in RStudio.²⁷

Results

Eligible Studies and Studies Characteristics

Primarily, we found 236 studies in all databases. Duplicates were excluded and the rest were filtered by title, abstract, and full text based on the eligibility criteria.

Finally, a total of 17 studies were selected and 9 of them were excluded because they were performed in humans. A total of 8 studies fulfilled the eligibility criteria and were included in this review (Supplemental 1).

The 8 studies were published between 2015 and 2022, they included 391 animal models. The average follow-up time of these models was 17 days ± 9.06, 7 studies analyzed subacute stroke phase and only 1 acute model. The most used model was the occlusion of the middle cerebral artery (n = 6), followed by the carotid artery occlusion (n = 2). All studies had a parallel design and analyzed the differences between active and sham aVNS after ischemia/reperfusion lesion. The majority of these studies used electroacupuncture aVNS (eaVNS; n = 5) and the rest used transcutaneous aVNS (taVNS; n = 3; Table 1).

Table 1.

Studies Characteristics.

Study	Study design	Model	Intervention	Sham	General sample size	Follow-up
Ay et al²²	Parallel	Acute MCAO rat models	eaVNS	Electrodes positioning with no stimulation	31	24 h
Choi et al²¹	Parallel	Subacute CAO rat models	taVNS	Stimulation in the helix	53	7 d
Jiang et al²⁰	Parallel	Subacute MCAO rat models	eaVNS	Electrodes positioning with no stimulation	32	21 d
Li et al¹⁹	Parallel	Subacute MCAO rat models	eaVNS	Electrodes positioning with no stimulation	64	28 d
Li et al¹⁸	Parallel	Subacute MCAO rat models	eaVNS	Electrodes positioning with no stimulation	72	28 d
Long et al¹⁷	Parallel	Subacute MCAO rat models	taVNS	Electrodes positioning with no stimulation	65	21 d
Ma et al¹⁶	Parallel	Subacute MCAO rat models	eaVNS	Electrodes positioning with no stimulation	26	7 d
Zhao et al¹⁵	Parallel	Subacute CAO rat models	taVNS	Stimulation in the helix	48	7 d

Abbreviations: MCAO: middle cerebral artery occlusion; CAO: carotid artery occlusion.

Regarding the stimulation parameters, all the studies stimulated the cavum concha. Two of them performed bilateral stimulation, while the rest stimulated the left ear. In general, the studies performed 22.38 ± 20.35 sessions, during 51.25 ± 30.18 minutes, with the frequency of 18.75 ± 3.31 Hz, the pulse width of 475.712 ± 59.49 ms, and the current intensity of 0.81 ± 0.50 mA (Table 2).

Table 2.

Stimulation Parameters.

Study	Device	Stimulation location	Number of sessions	Duration (min)	Frequency (Hz)	Pulse (ms)	Current intensity (mA)
Ay et al²²	Grass Model S48 stimulator and constant current unit, USA	Left cavum concha	1.00	60.00	20.00	500.00	0.50
Choi et al²¹	IX-RA-834 (iWorx Systems, Dover, NH, USA)	Left cymba concha	6.00	20.00	20.00	330.00	1.00
Jiang et al²⁰	Grass Model S48 stimulator, USA	Left cavum concha	21.00	120.00	20.00	500.00	0.50
Li et al¹⁹	Grass Model S48 stimulator, USA	Left cavum concha	56.00	60.00	20.00	500.00	0.50
Li et al¹⁸	Grass Model S48 stimulator, USA	Left cavum concha	56.00	30.00	20.00	500.00	0.50
Long et al¹⁷	HANS-100, Nanjing, China	Bilateral concha region	18.00	30.00	20.00	500.00	2.00
Ma et al¹⁶	Grass Model S48 stimulator	Left cavum concha	14.00	60.00	20.00	500.00	0.50
Zhao et al¹⁵	Nanjing Hanshi Electroacupuncture Instrument, China	Bilateral concha region	7.00	30.00	10.00	NI	1.00

Behavioral Outcomes

Most of the studies analyzed neurological deficits scores (n = 6), followed by the adhesivel removal test (n = 3). Choi 2022 analyzed the effects of taVNS in cognition using the NOR and Y-maze test and Long dysphagia analyzing body weight, food, and water intake. All studies were able to show an effect of auricular stimulation on those outcomes compared to controls (Table 3).

Table 3.

Behavioral Outcomes and Results.

Study	Outcome	Tool	Intervention	aVNS effects
Ay et al²²	Neurological deficit score	5-point scale	eaVNS	Better neurological scores 24 h after ischemia
Choi et al²¹	Cognitive Function	NOR test and Y-maze test	taVNS	Better discrimination index by the NOR test and rates of spontaneous alternations by the Y-maze test
Jiang et al²⁰	Neurological deficit score	Neurological deficit score, the beam-walking task and the staircase test	eaVNS	Improved level of neurological function.
Li et al¹⁹	Modified neurological severity score and sensorimotor deficit score	mNSS and adhesive removal SS test	eaVNS	Prevention of neurological impairment after injury in both tests and continuous improvement trend in neurofunctional recovery
Li et al¹⁸	Neurological deficit score and sensorimotor deficit score	mNSS and adhesive removal SS test	eaVNS	Prevention of neurological impairment leading to lower scores
Long et al¹⁷	Dysphagia	Body weight, food intake, and water intake	taVNS	Higher body weight gain, food intake, and water intake
Ma et al¹⁶	Neurological deficit score and sensorimotor deficit score	mNSS and adhesive removal SS test	eaVNS	Improvement in both neurological scores
Zhao et al¹⁵	Neurological deficit score	NI	taVNS	Improvements in neurological deficit scores

Abbreviations: NOR, Novel Objective recognition; mNSS, modified Neurological Severity Score; NI, non-informed.

Quantitative Analysis

Based on that, we could synthesize the data from the neurological deficits scores and the adhesive removal test.

For the neurological deficit score, 6 studies using I/R models were included in the analysis, showing an effect of aVNS in reducing neurological deficit in comparison to controls, the studies demonstrated a moderate heterogeneity. (SMD = −1.97, 95% CI −2.57 to −1.36, I² = 44%). A sensitivity analysis was performed omitting each one of the correspondent studies that did not impact the result (Supplemental 2A).

Regarding the adhesive removal test, 3 studies results were synthesized, the aVNS pooled effect reduced the time of the test, however detected a high heterogeneity between the studies (SMD = −2.26, 95% CI −4.45 to −0.08, I² = 81%; Figure 1).

Figure 1.

Forest plots of the aVNS effect on the neurological deficit score and adhesive removal test.

Anatomopathological/Inflammatory Outcomes

The authors included in this review analyzed anatomopathological outcomes to study the mechanistic effects of aVNS. Most of the studies analyze the infarct size (n = 4), the brain damage (n = 3), and inflammatory cytokines expression (n = 2). All the studies could detect a reduction in infarct size, brain damage, and inflammatory cytokines induced by aVNS compared to control after I/R (Table 4).

Table 4.

Anatomopathological Inflammatory Outcomes and Results.

Study	Outcome	Intervention	aVNS effects
Ay et al²²	Infarct size	eaVNS	Indirect and direct infarct volume reduction
Jiang et al²⁰	Infarct size	eaVNS	Infarct volume reduction
Jiang et al²⁰	Brain damage	eaVNS	Decreasing in brain pathology, denser neuropil regions, and higher number of surviving neurons.
Li et al¹⁸	Infarct size	eaVNS	Suppression of infarction volume enlargement after injury
Li et al¹⁸	Brain damage	eaVNS	Suppression of neuronal damage
Long et al¹⁷	Expression of inflammatory mediators	taVNS	Lower concentrations of IL-1β and TNF-α mediators in the white matter of the left hemisphere. Lower TLR4 expression intensity and MyD88 intensity.
Ma et al¹⁶	Infarct size	eaVNS	Infarct volume reduction
Zhao et al¹⁵	Brain damage	taVNS	Decreasing in the number of cells with nuclear contraction, lysis, and less damage
	Levels of Ach, IL-1β, IL-6, and TNF-α in the penumbra and motor cortex		Lower levels of TNF-α, IL-1β, and IL-6, and increased Ach levels in the ischemic penumbra and motor cortex.

Quantitative Analysis

Regarding infarct size, we could synthesize the data and calculate the pooled standardized mean difference between aVNS and control. Overall, aVNS decreased the infarct size after I/R compared to controls, however, the studies presented a modern high heterogeneity between them (SMD = −1.51, 95% CI −2.42 to −0.60. I² = 58%; Figure 2). A sensitivity analysis was performed omitting each one of the correspondent studies that did not impact the result (Supplemental 2B).

Figure 2.

Forest plot of the aVNS effect on infarct size.

Neuroplasticity Markers Outcomes

Different types of neuroplasticity markers were measured in the studies included in this review. Three studies described an increase in BDNF levels and expression in peri-infarct regions induced by aVNS. Other 3 have shown an increased microcapillary density and CD31/PCNA/KI67+/ALK5 levels suggesting an increase in angiogenesis promoted by aVNS after I/R. Other markers as phospho-endothelial nitric oxide synthase (p-eNOS), vascular endothelial growth factor (VEGF), α7nAch receptor, GAP-43, peroxisome proliferator activated receptor-γ (PPAR-γ), cAMP, PKA, p-CREB, growth differentiation factor 11 (GDF11), Cx43 expression, and length of myelin sheath were also measure. In general, aVNS was able to upregulate these metrics suggesting higher neuroplastic changes and increase in the microvasculature in the intervention group after injury. Li et al have tested the blockade of the α7nAch receptor and PPAR-γ, showing a reduction in the positive aVNS effects (Table 5).

Table 5.

Neuroplasticity Markers and Results.

Study	Outcome	Intervention	aVNS effects
Jiang et al²⁰	BDNF, p-eNOS, and VEGF expression in the ischemic penumbra	eaVNS	Upregulation of BDNF mRNA and protein expression around the ischemic boundary. Similar results were detected with p-eNOS and VEGF expression.
	Angiogenesis		Increased number of microvessels in the ischemic boundary
	PCNA and CD31		Increasing in the markers, showing an increased number of endothelial cells proliferation
Li et al¹⁹	α7nAchR expression	eaVNS	Reversion of the α7nAchR reduction and of mRNA expression levels of α7nAchR caused by injury
	Blockade of α7nAchR		Blockade of α7nAchR weakens the behavioral improvement, the increasing in the protein and mRNA expression levels of GAP-43, and the expression of BNDF, cAMP, PKA induced by eaVNS
	Axonal plasticity		Increasing in the protein and mRNA expression levels of GAP-43
	Expression of BDNF/cAMP/PKA/p-CREB		Increasing in the expression of BNDF, cAMP, PKA
Li et al¹⁸	PPAR-γ expression	eaVNS	Enhanced PPAR-γ expression in the peri-infarct cortex
	PPAR-γ inhibition		Inhibition of PPAR-γ weakens the improvements on neurological scores induced, the suppression of neuronal damage and infarction volume enlargement after injury, promotion of angiogenesis, and promotion BDNF, VEGF, and P-eNOS expression induced by eaVNS after injury
	BDNF, VEGF, P-eNOS expression		Promotion of BDNF, VEGF, and P-eNOS expression after injury
	Angiogenesis		Promotion of higher microvascular density
	Capillary endothelial cell proliferation in peri-infarct cerebral cortex		Promotion of endothelial cell proliferation showing increasing in Ki67+/CD31+ in peri-infarct cortex
Long et al¹⁷	Remyelination induction	taVNS	Thicker myelin sheaths
	Angiogenesis induction		Higher length density of the capillaries in the white matter of the left hemisphere and larger total volume of capillaries in the white matter of the left hemisphere
Ma et al¹⁶	Expression of GDF11	eaVNS	Upregulation of the expression of GDF11 protein in plasma
	Expression of GDF11 in peri-infarct cerebral cortex and spleen		Increased number of GDF11-positive cells in the peri-infarct cerebral cortex. Promotion of the expression of GDF11 mRNA. Reduced splenic GDF11 protein.
	Capillary endothelial cell proliferation in peri-infarct cerebral cortex		Vascular ECs labeled with Ki67+/CD31+ were detected in the peri-infarct cortex.
	Expression of CD31/ALK5 in capillary endothelial cell in peri-infarct cerebral cortex		Upregulation of the expression of CD31/ALK5
Zhao et al¹⁵	Levels of Cx43 phosphorylation		Decreasing in the levels of Cx43 phosphorylation in the motor cortex and ischemic penumbra

Quantitative Analysis

We were able to quantitatively synthesize the aVNS effects in microcapillary density, CD31 proliferation, and BDNF protein levels and RNA expression from the studies. aVNS showed to increase microcapillary density (SMD = 2.15, 95% CI 0.34 to 3.96, I² = 78%), increase CD31 proliferation (SMD = 3.16, 95% CI 0.59 to 5.73, I² = 87%), and BDNF protein levels (SMD = 4.11, 95% CI 1.01 to 7.22, I² = 89%) and RNA expression (SMD = 4.51, 95% CI 1.25 to 7.78, I² = 88%), showing a high heterogeneity between studies in all results (Figure 3).

Figure 3.

Forest plots of aVNS effects on microcapillary density, CD31 proliferation, BDNF protein levels and RNA expression.

Other Mechanistic Outcomes Related With aVNS Effects

Other ungrouped mechanistic outcomes included blood pressure and HR, cerebral blood flow, brain regions activated, and cerebrospinal fluid (CSF) circulation. Ay et al and Ma et al could not detect an eaVNS effect on blood pressure, HR, and cerebral blood flow. Choi et al have demonstrated an increase in CSF circulation induced by taVNS. Finally, Ay et al demonstrated the activation in nuclei tractus solitarii and locus coeruleus induced by eaVNS (Table 6).

Table 6.

Other Mechanistic Outcomes and Results.

Study	Outcome	Intervention	Results
Ay et al²²	Blood pressure and HR	eaVNS	No effect on blood pressure and heart rate
	Cerebral blood flow		No effect in the rCBF
	Brain regions activated		Different number of c-Fos positive cells in both nucleus tractus solitarii and locus coeruleus
Choi et al²¹	CSF circulation	taVNS	Increasing in CSF circulation and enhanced CSF flow in the perivascular space.
Ma et al¹⁶	Blood pressure	eaVNS	No significant overall effect on BP
	Cerebral blood flow		No overall effect on rCBF

Publication Bias

Regarding the main outcome, neurologic deficit scores, we aimed to assess the presence of publication bias. All the 6 studies that provided data on the score were included in the analysis. The funnel plot built shows a slight asymmetry in the button of the graph suggesting that a publication bias may be present overestimating the intervention effect. However, this analysis may be compromised by the low number of studies (Figure 4).

Figure 4.

Funnel plot based on Neurological Deficit Scores (main outcome) to assess publication bias.

Risk of Bias

The risk of bias was evaluated by the SYRCLE’s risk of bias tool, classifying 10 domains in low risk, unclear risk, or high risk of bias. As demonstrated in Table 7, the domain with lower risk of bias was the sequence generation. On the other hand, random housing was the domain with more studies classified as a high risk of bias. In general, all studies presented at least 3 domains classified as a high risk of bias and most of the domains presented an unclear risk (Table 7).

Table 7.

SYRCLE’s Risk of Bias Tool.

Study	Sequence generation	Baseline characteristics	Allocation concealment	Random housing	Blinding	Random outcome assessment	Blinding	Incomplete outcome data	Selective outcome reporting	Other sources of bias
Ay et al²²	Low risk	High risk	Unclear	High risk	Unclear	Unclear	High risk	High risk	Unclear	High risk
Choi et al²¹	Low risk	High risk	Unclear	High risk	Unclear	Unclear	High risk	Unclear	Unclear	Unclear
Jiang et al²⁰	Low risk	High risk	Unclear	High risk	Unclear	Unclear	High risk	High risk	Unclear	High risk
Li et al¹⁹	Low risk	High risk	Unclear	High risk	Unclear	Unclear	High risk	Unclear	Unclear	Unclear
Li et al¹⁸	Low risk	High risk	Unclear	High risk	Unclear	Unclear	High risk	Unclear	Unclear	Unclear
Long et al¹⁷	Low risk	High risk	Unclear	High risk	High risk	Unclear	High risk	High risk	Unclear	High risk
Ma et al¹⁶	Low risk	Unclear	High risk	High risk	Unclear	Unclear	Unclear	High risk	Unclear	High risk
Zhao et al¹⁵	Low risk	Unclear	High risk	High risk	High risk	Unclear	High risk	Unclear	Unclear	High risk

Discussion

In this review, we analyzed studies testing the effects of auricular vagus nerve stimulation in an induced ischemic-reperfusion animal models, the pooled results suggest that the intervention reduce motor and sensory deficits, infarct size, inflammatory outcomes, and induces changes in neuroplastic in the post-stroke recovery period. A total of 8 studies were included in the review, 6 of which investigated neurological deficit scores. A pooled analysis of these 6 demonstrated that aVNS stimulation reduced neurological deficits in I/R models. These studies had a moderate heterogeneity (44%) and a significant standardized mean difference with a relatively small CI (SMD = −1.97, 95% CI −2.57 to −1.36). Regarding the 3 studies analyzing the adhesive removal test, the animals that underwent active stimulation had significantly improved sensitivity and dexterity compared to the ones that had sham (SMD = −2.26, 95% CI −4.45 to −0.08), although the pooled results had a high heterogeneity (81%). There was also a decrease in infarct size (SMD = −1.51, 95% CI −2.42 to −0.60), with a moderate heterogeneity (58%). An increase in microcapillary density (SMD = 2.15, 95% CI 0.34), CD31 proliferation (SMD = 3.16, 95% CI 0.59 to 5.73), an increasing in BDNF peri-infarct enzyme concentration (SMD = 4.11, 95% CI 1.01 to 7.22) and RNA expression SMD = 4.51, 95% CI 1.25 to 7.78) was also present. All these results had high heterogeneity.

A sensitive analysis of the neurological deficit and infarct size was made omitting each study at time, the results did not change significantly in none of these situations, creating an understanding of congruence between the trials. Also, the risk of bias was analyzed by 2 separate authors that needed to agree with the evaluation. The SYRCLES’s RoB tool for animal studies was used analyzing 10 domains regarding the risk of bias in the trials. The domain with less concerns was the one testing the allocation sequence generation and the one with higher risk of bias was the domain testing the randomness of the animal’s selection. In general, all studies had an unclear to high risk of bias in most of the domains.

When compared to previous studies in invasive and non-invasive cervical stimulation, this meta-analysis conveyed similar clinical, neuroprotective, and anti-inflammatory results regarding aVNS.^28
-32 However, aVNS has been demonstrated to be a more feasible, accessible, and a safer therapy.¹²

AVNS Mechanisms of Behavioral Improvement

As demonstrated in our pooled results, behavioral outcomes were improved after auricular vagal nerve stimulation.

The direct comparison between infarct size of animals that underwent aVNS after the I/R model suggests that the intervention creates a morphological environment of neuroprotection. All the studies show an anatomically smaller lesion in the active groups when compared to the controls.

The improvement mechanisms can be explained by the secondary outcomes analyzed in this review. The 2 main hypotheses behind the aVNS effects on subacute stroke are the induced neuroprotection and neuroplasticity. The immune system plays an important role in the physiopathology of the acute and subacute ischemic stroke producing and releasing different cytokines and chemokines, such as interleukin 1b (IL-1b), interleukin 6 (IL-6), and tumor necrosis factor alpha (TNF-α).^33,34 In the subacute phase, local leukocytes release these factors, amplifying the brain-inflammatory response, more leukocytes activation, disruption of the BBB, brain edema, and neuronal death.^33,34 The vagal nerve stimulation has been demonstrated to modulate the immune system inducing anti-inflammatory states. A recent systematic review and meta-analysis conducted by our group detected the role of the vagal nerve stimulation in modulating IL-1b, IL-6, IL-10, and interferon gamma (IFN-g) in humans. We also could detect a role of the vagal stimulation in reducing C-reactive protein, a protein related with inflammatory states and widely used in clinical practice (A mechanistic analysis of the neural modulation of the inflammatory system via vagus nerve stimulation: A systematic review and meta-analysis—in submission).

The other mechanism behind the behavioral effects of the vagal nerve stimulation may be through the neuroplasticity induction. Plasticity is the ability to adapt in a changing environment. In pathological circumstances, the lack of neuroplasticity or a maladaptive reaction may prevent recovery. Regarding ischemic stroke, the reorganization of cortical representations on the surroundings or contralateral hemisphere occurs.^35,36 This reorganization can be directed to functional recovery enhancing the specificity of the readaptation targeting renormalize the circuitry that suffered maladaptation.³⁷ Different neuromodulatory techniques have shown to be able to induce neuroplasticity in different neurological conditions, such as traumatic brain injury, chronic pain, and stroke.^38
-40 One of the biomarkers widely related and used to measure neuroplasticity is the BDNF,^41
-43 responsible for neuron survival.⁴⁴ When compared to sham, our meta-analysis conveyed that, animals receiving active stimulation presented higher expression of BDNF enzyme in the periphery of the ischemic area. This neurotrophic factor is associated with the development of dendrites structures in the neurons, increasing their size and quantity.⁴⁵ It has also been demonstrated that BDNF is crucial for adult neurogenesis, with infusion of BDNF in the hippocampus of rats demonstrating an increased number of newborn neurons.⁴⁶ This BDNF induced neurogenesis has also been linked to increases of long-term potentiation, facilitating communication between the new cells and their functioning. Therefore, the fact that this substance is increased in the periphery of an ischemic area after vagal stimulation, is very likely one of the mechanisms by which the intervention increases neuroplasticity. Although the BDNF quantification was taken only from peri-infarct tissue, it is possible to rationalize that the increased transcription of this enzyme is present throughout the nervous system. This is possible because the intervention is systemic, therefore it is not likely that only the infarct tissue has an increase in this protein.

In addition, our analysis conveyed that, animals in the aVNS group showed higher microcapillary density and an increase in CD31 together with different markers after stimulation. CD31 has been demonstrated to be a sensitive and specific marker in vascular differentiation.^47,48 Therefore, the increase of this marker together with the microcapillary density may be associated with a greater endothelial proliferation and vascular support that could be protecting the neurons from the ischemic event.

Individual studies demonstrated other factors modulated by VNS not included in the meta-analysis that corroborate with our main hypothesis, such as increasing in VEGF, p-eNOS, PPAR-γ, and GDF11, important peptides related to cellular growth, angiogenesis, vascular relaxation, suppression platelet aggregation, proliferation of vascular smooth muscle, and neuroprotection.^49
-52 Furthermore, some studies have demonstrated aVNS as a modulator of transcription factors such as cAMP, PKA, p-CREB, and α7nAChR, involved neurogenesis, increased axonal regeneration and reorganization, and reinforcing the hypothesis behind the aVNS neuroplastic induction.^19,53
-57 However, in humans there are studies only testing the chronic effects of aVNS, and not the acute or subacute effects explored in this review. Therefore, this hypothesis and the demonstrated effects must be tested in future mechanistic randomized clinical trial in humans combined with the currently applied gold standard interventions.

Future Perspectives of aVNS

TaVNS is a promising intervention since it is non-invasive, affordable, usable, and it is effective to modulate vagal tone which would induce potential neural reorganization, and anti-inflammatory effects, allowing neuroplastic changes and reflecting the reduction of neurological deficits. According to several studies,^58
-60 taVNS is a safe alternative to the previous surgical device. Our recent meta-analysis of 167 studies assessing 5,827 subjects showed that this technique is a safe and feasible option for intervention, with a low incidence of adverse events and no severe adverse events reported associated with the stimulation.¹²

TaVNS can affect many of the bodily processes that the vagus nerve is engaged in, meaning it could be an intervention to a broad range of diseases, such as depression, diabetes, Alzheimer’s disease, arthritis, and stroke. For instance, a recent systematic review performed by our group has demonstrated the taVNS electroencephalographic effects current stage of evidence and the lack of homogenous studies and standardized metrics in the field.⁹ Thus, well designed, and multimodal mechanistic studies to explore the ways that taVNS works are required. A possible mechanism that has been studied recently is the modulatory effects of vagus stimulation in the cholinergic anti-inflammatory pathway, the microbiota-gut-brain axis, or the hypothalamic-pituitary-adrenal axis.^61,62 In one of our mechanistic meta-analyses on the field, we found that VNS significantly affected inflammatory cytokines and biomarkers including CRP, IL-10, and IFN-g. Additionally, taVNS had a modulatory influence on IL-1b, IL-6, and IL-10, according to subgroup analysis. (A mechanistic analysis of the neural modulation of the inflammatory system via vagus nerve stimulation: A systematic review and meta-analysis—peer review.) In the stroke field, to clarify taVNS’s therapeutic benefits on stroke, translational clinical trials in humans focusing on how the stimulation is able to reduce behavioral/neurological deficits acutely, subacutely, and chronically are necessary. To further examine the long-term efficacy and long-lasting mechanistic consequences, investigations with a larger number of sessions and assessments after longer periods are required.

Conclusion

This systematic review and meta-analysis gather the preclinical evidence of the effects of aVNS on stroke in rats. We demonstrated that aVNS reduced neurological deficits and improved sensorimotor performance after I/R induction in comparison to controls. These findings were corroborated by a reduction of infarct size, induction of angiogenesis, and increased neuroplastic markers promoted by aVNS, indicating the 2 main mechanisms of neuroprotection and neuroplasticity behind the behavioral neurological improvements. However, the results presented a high heterogeneity and risk of bias. Therefore, future well designed randomized clinical trials are needed in order to translate the pre-clinical effects of aVNS on stroke.

Supplemental Material

sj-docx-1-nnr-10.1177_15459683231177595 – Supplemental material for Understanding the Neuroplastic Effects of Auricular Vagus Nerve Stimulation in Animal Models of Stroke: A Systematic Review and Meta-Analysis

Supplemental material, sj-docx-1-nnr-10.1177_15459683231177595 for Understanding the Neuroplastic Effects of Auricular Vagus Nerve Stimulation in Animal Models of Stroke: A Systematic Review and Meta-Analysis by Paulo S. de Melo, MS, João Parente, MS, Ingrid Rebello-Sanchez, MS, Anna Marduy, MS, Anna Carolyna Gianlorenco, PhD, Chi Kyung Kim, PhD, Hyuk Choi, PhD, Jae-Jun Song, PhD and Felipe Fregni, PhD in Neurorehabilitation and Neural Repair

Footnotes

Acknowledgements

CKK, HC, and JS are directly associated with Neurive Co, a company developing neuromodulation technologies, such as taVNS, to treat common brain diseases.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: ACG is supported by the Federal University São Carlos and is a consultant for Neurive. PSdM JV, IR-S, and AM are consultants for Neurive. FF is supported by NIH grants, by a research grant and gift from Neurive to Spaulding Rehabilitation Hospital, and is a consultant for Neurive.

ORCID iDs

Jae-Jun Song

Felipe Fregni

Supplementary material for this article is available on the Neurorehabilitation & Neural Repair website along with the online version of this article.

References

Gorelick

PB.

The global burden of stroke: persistent and disabling. Lancet Neurol. 2019;18(5):417-418. doi:10.1016/s1474-4422(19)30030-4

Girotra

Lekoubou

Bishu

Ovbiagele

A contemporary and comprehensive analysis of the costs of stroke in the United States. J Neurol Sci. 2020;410:116643. doi:10.1016/j.jns.2019.116643

Astrup

Siesjö

Symon

Thresholds in cerebral ischemia - the ischemic penumbra. Stroke. 1981;12(6):723-725. doi:10.1161/01.str.12.6.723

Molina

Saver

JL.

Extending reperfusion therapy for acute ischemic stroke: emerging pharmacological, mechanical, and imaging strategies. Stroke. 2005;36(10):2311-2320. doi:10.1161/01.Str.0000182100.65262.46

Bai

Lyden

PD.

Revisiting cerebral postischemic reperfusion injury: new insights in understanding reperfusion failure, hemorrhage, and edema. Int J Stroke. 2015;10(2):143-152. doi:10.1111/ijs.12434

Chamorro

Dirnagl

Urra

Planas

. Neuroprotection in acute stroke: targeting excitotoxicity, oxidative and nitrosative stress, and inflammation. Lancet Neurol. 2016;15(8):869-881. doi:10.1016/s1474-4422(16)00114-9

Wang

Pan

, et al. Non-invasive vagus nerve stimulation in cerebral stroke: current status and future perspectives. Front Neurosci. 2022;16:820665. doi:10.3389/fnins.2022.820665

Baig

Kamarova

Ali

, et al. Transcutaneous vagus nerve stimulation (tVNS) in stroke: the evidence, challenges and future directions. Auton Neurosci. 2022;237:102909. doi:10.1016/j.autneu.2021.102909

Gianlorenco

ACL

de Melo

Marduy

, et al. Electroencephalographic patterns in taVNS: a systematic review. Biomedicines. 2022;10(9):2208.

10.

Wang

, et al. Transcutaneous auricular vagus nerve stimulators: a review of past, present, and future devices. Expert Rev Med Devices. 2022;19(1):43-61. doi:10.1080/17434440.2022.2020095

11.

Redgrave

Day

Leung

, et al. Safety and tolerability of transcutaneous vagus nerve stimulation in humans; a systematic review. Brain Stimul. 2018;11(6):1225-1238. doi:10.1016/j.brs.2018.08.010

12.

Kim

Marduy

de Melo

, et al. Safety of transcutaneous auricular vagus nerve stimulation (taVNS): a systematic review and meta-analysis. Sci Rep. 2022;12(1):22055. doi:10.1038/s41598-022-25864-1

13.

Zhang

Wang

Tan

Jia

Effect and safety of transcutaneous auricular vagus nerve stimulation on recovery of upper limb motor function in subacute ischemic stroke patients: a randomized pilot study. Neural Plast. 2020;2020:8841752. doi:10.1155/2020/8841752

14.

Tang

Zhou

, et al. Vagus nerve stimulation alleviated cerebral ischemia and reperfusion injury in rats by inhibiting pyroptosis via α7 nicotinic acetylcholine receptor. Cell Death Discovery. 2022;8(1):54. doi:10.1038/s41420-022-00852-6

15.

Zhao

J-J

Wang

Z-H

Zhang

Y-J

, et al. The mechanisms through which auricular vagus nerve stimulation protects against cerebral ischemia/reperfusion injury. Neural Regener Res. 2022;17(3):594-600. doi:10.4103/1673-5374.320992

16.

Zhang

Tan

Jin

Transcutaneous auricular vagus nerve stimulation regulates expression of growth differentiation factor 11 and activin-like kinase 5 in cerebral ischemia/reperfusion rats. J Neurol Sci. 2016;369:27-35. doi:10.1016/j.jns.2016.08.004

17.

Long

Zang

Jia

, et al. Transcutaneous auricular vagus nerve stimulation promotes white matter repair and improves dysphagia symptoms in cerebral ischemia model rats. Original research. Front Behav Neurosci. 2022;16:811419. doi:10.3389/fnbeh.2022.811419

18.

Zhang

, et al. PPAR-γ mediates Ta-VNS-induced angiogenesis and subsequent functional recovery after experimental stroke in rats. BioMed Res Int. 2020;2020:8163789. doi:10.1155/2020/8163789

19.

Zhang

, et al. α7nAchR mediates transcutaneous auricular vagus nerve stimulation-induced neuroprotection in a rat model of ischemic stroke by enhancing axonal plasticity. Neurosci Lett. 2020;730:135031. doi:10.1016/j.neulet.2020.135031

20.

Jiang

, et al. Auricular vagus nerve stimulation promotes functional recovery and enhances the post-ischemic angiogenic response in an ischemia/reperfusion rat model. Neurochem Int. 2016;97:73-82. doi:10.1016/j.neuint.2016.02.009

21.

Choi

Jang

Chung

Kim

SK.

Transcutaneous auricular vagus nerve stimulation enhances cerebrospinal fluid circulation and restores cognitive function in the rodent model of vascular cognitive impairment. Cells. 2022;11(19):3019.

22.

Napadow

Electrical stimulation of the vagus nerve dermatome in the external ear is protective in rat cerebral ischemia. Brain Stimul. 2015;8(1):7-12. doi:10.1016/j.brs.2014.09.009

23.

Page

McKenzie

Bossuyt

, et al. The PRISMA 2020 statement: an updated guideline for reporting systematic reviews. BMJ. 2021;372:n71. doi:10.1136/bmj.n71

24.

Julian

Higgins

Chandler

, et al. Cochrane Handbook for Systematic Reviews of Interventions. 2nd ed. John Wiley & Sons; 2019.

25.

Leenaars

Hooijmans

van Veggel

, et al. A step-by-step guide to systematically identify all relevant animal studies. Lab Anim. 2012;46(1):24-31. doi:10.1258/la.2011.011087

26.

Hooijmans

Rovers

de Vries

Leenaars

Ritskes-Hoitinga

Langendam

MW.

SYRCLE’s risk of bias tool for animal studies. BMC Med Res Methodol. 2014;14:43. doi:10.1186/1471-2288-14-43

27.

Balduzzi

Rücker

Schwarzer

How to perform a meta-analysis with R: a practical tutorial. Evid Based Ment Health. 2019;22(4):153-160. doi:10.1136/ebmental-2019-300117

28.

Gregory Sorensen

Vagus nerve stimulation reduces infarct size in rat focal cerebral ischemia. Neurosci Lett. 2009;459(3):147-51. doi:10.1016/j.neulet.2009.05.018

29.

Nasser

Simon

Transcutaneous cervical vagus nerve stimulation ameliorates acute ischemic injury in rats. Brain Stimul. 2016;9(2):166-173. doi:10.1016/j.brs.2015.11.008

30.

Sorensen

Vagus nerve stimulation reduces infarct size in rat focal cerebral ischemia: an unlikely role for cerebral blood flow. Brain Res. 2011;1392:110-115. doi:10.1016/j.brainres.2011.03.060

31.

Yamakawa

Matsumoto

Imamura

, et al. Electrical vagus nerve stimulation attenuates systemic inflammation and improves survival in a rat heatstroke model. PLoS One. 2013;8(2):e56728. doi:10.1371/journal.pone.0056728

32.

Yang

Orban

, et al. Non-invasive vagus nerve stimulation reduces blood-brain barrier disruption in a rat model of ischemic stroke. Brain Stimul. 2018;11(4):689-698. doi:10.1016/j.brs.2018.01.034

33.

Amantea

Nappi

Bernardi

Bagetta

Corasaniti

MT.

Post-ischemic brain damage: pathophysiology and role of inflammatory mediators. FEBS J. 2009;276(1):13-26. doi:10.1111/j.1742-4658.2008.06766.x

34.

Kriz

Inflammation in ischemic brain injury: timing is important. Crit Rev Neurobiol. 2006;18(1-2):145-157. doi:10.1615/critrevneurobiol.v18.i1-2.150

35.

Calautti

Baron

JC.

Functional neuroimaging studies of motor recovery after stroke in adults: a review. Stroke. 2003;34(6):1553-1566. doi:10.1161/01.Str.0000071761.36075.A6

36.

Nudo

RJ.

Recovery after damage to motor cortical areas. Curr Opin Neurobiol. 1999;9(6):740-747. doi:10.1016/s0959-4388(99)00027-6

37.

Kilgard

MP.

Harnessing plasticity to understand learning and treat disease. Trends Neurosci. 2012;35(12):715-722. doi:10.1016/j.tins.2012.09.002

38.

Villamar

Santos Portilla

Fregni

Zafonte

Noninvasive brain stimulation to modulate neuroplasticity in traumatic brain injury. Neuromodulation. 2012;15(4):326-338. doi:10.1111/j.1525-1403.2012.00474.x

39.

Ting

Fadul

Fecteau

Ethier

Neurostimulation for stroke rehabilitation. Front Neurosci. 2021;15:649459. doi:10.3389/fnins.2021.649459

40.

Xiong

H-Y

Zheng

J-J

Wang

X-Q

. Non-invasive brain stimulation for chronic pain: state of the art and future directions. Review. Front Mol Neurosci. 2022;15:888716. doi:10.3389/fnmol.2022.888716

41.

Yang

Nie

Shu

, et al. The role of BDNF on neural plasticity in depression. Front Cell Neurosci. 2020;14:82. doi:10.3389/fncel.2020.00082

42.

Grande

Fries

Kunz

Kapczinski

The role of BDNF as a mediator of neuroplasticity in bipolar disorder. Psychiatry Investig. 2010;7(4):243-250. doi:10.4306/pi.2010.7.4.243

43.

Koroleva

Tolmachev

Alifirova

, et al. Serum BDNF’s role as a biomarker for motor training in the context of AR-based rehabilitation after ischemic stroke. Brain Sci. 2020;10(9):623. doi:10.3390/brainsci10090623

44.

Barde

YA.

Trophic factors and neuronal survival. Neuron. 1989;2(6):1525-1534. doi:10.1016/0896-6273(89)90040-8

45.

Alonso

Medina

Pozzo-Miller

ERK1/2 activation is necessary for BDNF to increase dendritic spine density in hippocampal CA1 pyramidal neurons. Learn Mem. 2004;11(2):172-178. doi:10.1101/lm.67804

46.

Scharfman

Goodman

Macleod

Phani

Antonelli

Croll

Increased neurogenesis and the ectopic granule cells after intrahippocampal BDNF infusion in adult rats. Exp Neurol. 2005;192(2):348-356. doi:10.1016/j.expneurol.2004.11.016

47.

McKenney

Weiss

Folpe

AL.

CD31 expression in intratumoral macrophages: a potential diagnostic pitfall. Am J Surg Pathol. 2001;25(9):1167-1173. doi:10.1097/00000478-200109000-00007

48.

Folpe

Veikkola

Valtola

Weiss

SW.

Vascular endothelial growth factor receptor-3 (VEGFR-3): a marker of vascular tumors with presumed lymphatic differentiation, including Kaposi’s sarcoma, kaposiform and Dabska-type hemangioendotheliomas, and a subset of angiosarcomas. Mod Pathol. 2000;13(2):180-185. doi:10.1038/modpathol.3880033

49.

Lähteenvuo

Rosenzweig

Effects of aging on angiogenesis. Circ Res. 2012;110(9):1252-1264. doi:10.1161/circresaha.111.246116

50.

Kuo

Farnebo

, et al. Comparative evaluation of the antitumor activity of antiangiogenic proteins delivered by gene transfer. Proc Natl Acad Sci USA. 2001;98(8):4605-4610. doi:10.1073/pnas.081615298

51.

Vattulainen-Collanus

Akinrinade

, et al. Loss of PPARγ in endothelial cells leads to impaired angiogenesis. J Cell Sci. 2016;129(4):693-705. doi:10.1242/jcs.169011

52.

Cai

Yang

Liu

, et al. Peroxisome proliferator-activated receptor γ (PPARγ): a master gatekeeper in CNS injury and repair. Prog Neurobiol. 2018;163-164:27-58. doi:10.1016/j.pneurobio.2017.10.002

53.

Gao

Zhang

Cui

, et al. Ginsenoside Rb1 promotes motor functional recovery and axonal regeneration in post-stroke mice through cAMP/PKA/CREB signaling pathway. Brain Res Bull. 2020;154:51-60. doi:10.1016/j.brainresbull.2019.10.006

54.

Jiang

Liu

Zhang

Chen

Vagus nerve stimulation attenuates cerebral ischemia and reperfusion injury via endogenous cholinergic pathway in rat. PLoS One. 2014;9(7):e102342. doi:10.1371/journal.pone.0102342

55.

Liu

Tan

, et al. Gadd45b mediates axonal plasticity and subsequent functional recovery after experimental stroke in rats. Mol Neurobiol. 2015;52(3):1245-1256. doi:10.1007/s12035-014-8909-0

56.

Spencer

Mellado

Filbin

MT.

BDNF activates CaMKIV and PKA in parallel to block MAG-mediated inhibition of neurite outgrowth. Mol Cell Neurosci. 2008;38(1):110-116. doi:10.1016/j.mcn.2008.02.005

57.

Wang

Zhang

Zhao

Liu

Fastigial nucleus electrostimulation promotes axonal regeneration after experimental stroke via cAMP/PKA pathway. Neurosci Lett. 2019;699:177-183. doi:10.1016/j.neulet.2019.02.016

58.

Verma

Mudge

Kasole

, et al. Auricular vagus neuromodulation-a systematic review on quality of evidence and clinical effects. Front Neurosci. 2021;15:664740. doi:10.3389/fnins.2021.664740

59.

Stefan

Kreiselmeyer

Kerling

, et al. Transcutaneous vagus nerve stimulation (t-VNS) in pharmacoresistant epilepsies: a proof of concept trial. Epilepsia. 2012;53(7):e115-e118. doi:10.1111/j.1528-1167.2012.03492.x

60.

Ben-Menachem

Revesz

Simon

Silberstein

Surgically implanted and non-invasive vagus nerve stimulation: a review of efficacy, safety and tolerability. Eur J Neurol. 2015;22(9):1260-1268. doi:10.1111/ene.12629

61.

Goggins

Mitani

Tanaka

Clinical perspectives on vagus nerve stimulation: present and future. Clin Sci. 2022;136(9):695-709. doi:10.1042/cs20210507

62.

Kaniusas

Szeles

Kampusch

, et al. Non-invasive auricular vagus nerve stimulation as a potential treatment for Covid19-originated acute respiratory distress syndrome. Front Physiol. 2020;11:890. doi:10.3389/fphys.2020.00890

Supplementary Material

Please find the following supplemental material available below.

For Open Access articles published under a Creative Commons License, all supplemental material carries the same license as the article it is associated with.

For non-Open Access articles published, all supplemental material carries a non-exclusive license, and permission requests for re-use of supplemental material or any part of supplemental material shall be sent directly to the copyright owner as specified in the copyright notice associated with the article.

0.00 MB

0.43 MB