Introduction

With the development of medical technology, the mortality rate of patients with severe traumatic brain injury (TBI), stroke, and hypoxic-ischemic encephalopathy (HIE) has decreased significantly. At the same time, most survivors suffer from temporary or prolonged disorders of consciousness (DOC), and thus the number of patients with DOC is increasing year by year.1 Studies have shown that patients with impaired consciousness lasting up to 1 month have only about a 50% or lower chance of spontaneous awakening, and most awakened patients remain severely disabled for life.2 Prolonged disorders of consciousness (pDOC) not only impose a huge financial burden on families but also predispose them to ethical and legal issues.3 However, there is a lack of effective wake-up promotion programs, and the clinical treatment of pDOC remains tricky.

Non-invasive neuromodulation techniques, such as repetitive transcranial magnetic stimulation (rTMS) and transcranial direct current stimulation (tDCS), have been used to treat DOC, but with unsatisfactory results.2 Consciousness is determined by both correct perceptions of the surroundings and oneself and normal arousal. Perception relies on the functional and structural integrity of the functional cortical network and cortical and subcortical neural network connections, whereas arousal is maintained by the brainstem’s superior reticular activation system.4,5 However, the currently used transcranial magnetic stimulation (TMS) modalities mainly excite the thalamus through cortical stimulation and do not achieve activation of the superior brainstem reticular system.6

It has been shown that the vagus nerve can join the superior reticular activation system through the solitary nucleus of the brainstem, thus participating in the modulation of the consciousness loop.7 Vagus nerve modulation (VNM) includes vagus nerve stimulation (VNS) and Vagus nerve magnetic modulation (VNMM).8,9 VNS is left vagus nerve modulation using implanted electrodes. VNMM is extracranial vagus nerve stimulation using repetitive transcranial magnetic stimulation. Studies have shown that VNS can improve motor function after stroke.8,10–12 VNM also promotes the release of plasticity-enhancing neurotransmitters, such as acetylcholine, norepinephrine, etc.8,13 However, VNS is less generalizable and acceptable due to its invasive nature in humans.14 So then, the researchers chose to stimulate the vagus nerve ear branch using TMS to observe its effect on post-stroke dysphagia. The results showed that the extracranial vagus nerve can promote esophageal motor evoked potential (MEP) responses and effectively improve post-stroke swallowing disorders.9 VNMM can provide similar effects as conventional VNS without surgery, avoiding the potential risks associated with surgical complications.

Based on these findings, this study used noninvasive VNMM to treat pDOC patients, observe its effect and safety in pDOC, and determine whether this innovative approach could be an effective complement to wake-promoting protocols.

Methods

All included patients were assessed for Coma Recovery Scale-Revised (CRS-R), somatosensory evoked potentials (SEP), and brainstem auditory evoked potentials (BAEP) prior to treatment. The same components were then assessed immediately after 20 VNMM treatments (Figure 1A). The data supporting the results of this study can be obtained from the corresponding author upon reasonable request.

Figure 1 Study protocol (A) Timeline of the study protocol: pre-treatment assessment of CRS-R, SEP and BAEP, followed by the implementation of 20 VNM sessions and immediate post-treatment assessment of CRS-R, SEP and BAEP. (B) Stimulation target: the center of the figure-of-eight coil was placed on the left posterior auricular mastoid.

Abbreviations: CRS-R, The Coma Recovery Scale-Revised; SEP, Somatosensory evoked potentials; BAEP, Brainstem auditory evoked potentials; VNMM, vagus nerve magnetic modulation.

Patients Selection

Inclusion criteria for this study:1 met the definition of pDOC, diagnosed as coma, vegetative state/non-responsive awakening syndrome, minimally conscious state according to the Revised Coma Recovery Scale;2,15 no history of epilepsy and no other central nervous system disorders unrelated to disorders of consciousness;3 no use of central excitatory drugs during treatment;4 consent of the patient and family was obtained and informed consent for treatment was signed before enrollment. Patients were excluded from the study in the following cases:1 patients with contraindications to magnetic stimulation, such as those wearing pacemakers or containing metal in the skull;2 patients with unstable vital signs (respiratory or hemodynamic instability), combined with severe cardiac, hepatic, renal, or pulmonary impairment or other serious medical conditions;3 patients with a history of seizures; and4 patients who were unable to cooperate with the assessment or therapist treatment methods.

The study was approved by the medical ethics committee of the affiliated hospital of north Sichuan medical college, and informed consent was obtained from all enrolled patients. This study complies with the Declaration of Helsinki. It was registered with the Chinese Clinical Trials Registry (ChiCTR2100053959). The authors expressly state that there is no conflict of interest with this article.

Vagus Nerve Modulation

The patient lies on the right side of the bed and stimulation was performed on the left mastoid using a figure-of-8 focusing coil as a guide, with the coil attachment stalk position noted to not interfere with the therapeutic manipulation (Figure 1B). Parameters: output intensity at 100% of resting motor threshold at a frequency of 10 Hz for 20 min/session,16 once daily for 5 days per week for 4 weeks of treatment. This study used a TMS System (magneuro60 stimulator, Nanjing Vishee Medical Technology Co., Ltd., Nanjing, China).

Clinical Assessment

All assessments were performed by a physician who was not responsible for the treatment (Master Yang).

Clinical presentation remains the gold standard for the diagnosis of disorders of consciousness. CRS-R score is 23.17 Patients are assessed in 5 areas: motor function, visual function, auditory function, communication function, oromotor function, and arousal. The CRS-R assessment is performed by a physician who is not in charge of VNMM treatment. Patients were diagnosed based on the highest score obtained in 3 CRS-R assessments performed within a week. Depending on the score, they are classified as coma, vegetative state/unresponsive wakefulness syndrome (VS/UWS), minimally conscious state ‘minus’ (MCS-), minimally conscious state ‘plus’ (MCS+), and exit minimally conscious state (EMCS).

The GCS score, which refers to the Glasgow Coma Index assessment, includes three aspects: eye-opening response, verbal response, and body movement.18 It is a common indicator used in medicine to assess the degree of coma in patients. The total score of the GCS scale ranges from 3 to 15, with 15 points indicating conscious consciousness and 13–14 points indicating a mild disturbance of consciousness. The score of 9–12 was in moderate disturbance of consciousness. 3–8 was classified as a severe disturbance of consciousness. When the score is less than 8 as coma, less than 3 as deep coma or brain death.

Evoked Potentials

Evoked potentials can be used to detect the integrity of cortical and brainstem pathways in patients with impaired consciousness. Before and after treatment, patients’ SEP and BAEP were examined by electromyography. The SEP and BAEP were clinically graded according to the results of the examinations, which were performed by a physician who was not in charge of VNMM treatment. SEP grading was performed according to Judson’s grading criteria:19 Grade I: normal CCT bilaterally; Grade II: prolonged or asymmetric CCT unilaterally or bilaterally; Grade III: the disappearance of cortical potential waves (N20 waves) unilaterally or bilaterally. BAEP grading was performed using Hall grading judgment criteria:20 generally, the side with the worse BAEP examination result is used for grading, Grade I: normal; Grade II. Mild abnormality, good differentiation of I to V waves, but with one of the following conditions, such as I, III, or (and) V wave PL prolongation. I~III, III~V or (and) I~V wave IPL prolongation, III~V/I~III inter-peak delay ratio >1, V/I amplitude ratio <0. 5; Grade III: moderate abnormality: poor differentiation of III and/or V wave, poor repeatability or no V wave; Grade IV: severe abnormality: only I wave or no wave at all.

Safety Monitoring

Throughout the experiment, changes in the patients’ vital signs were closely monitored. And record the adverse reactions we observed or reported by the patient’s family.

Statistical Analysis

IBM SPSS Statistics for Windows, Version 26.0 (IBM Corp, Armonk, NY, United States) was used for statistical analysis. 2-tailed P<0.05 was considered a statistically significant difference. Continuous variables were expressed as mean with standard deviation or median with interquartile range, whereas categorical data were expressed as counts and percentages. The Shapiro–Wilk test was used to assess if the scores confirmed a normal distribution. The rank sum test was used for categorical variables and the Wilcoxon for continuous variables. Percentage bar graphs were plotted using Excel 2020 software (Microsoft).

Results

The clinical characteristics of the patients are shown in Table 1. No patients withdrew from the study and did not experience any significant adverse events.

Table 1 Baseline Characteristics of Patients with Prolonged Disorders of Consciousness

Clinical Assessment

The CRS-R showed a significant improvement after the VNMM intervention, compared to the pre-treatment period. (Figure 2 and Table 2). CRS-R improved from 7.88±2.93 to 11.53±4.94. GCS score also improved from 7.65±1.9 to 9.18±2.65. Compared with pre-treatment, both CRS-R and GCS scores were statistically significant (P<0.05). Significant improvements were seen in all six subscales of the CRS-R, except oromotor (Table 3).

Table 2 Outcomes

Table 3 Outcomes of the CRS-R Subscales

Figure 2 Behavioral responses to VNMM. (A) CRS-R scores of 17 patients before and after treatment. (B) The proportion of each state of consciousness before and after VNMM shows that higher states of consciousness (EMCS and (MCS+) were increased, while lower states of consciousness (VS/UWS and MCS-) were affected.

Abbreviations: VNMM, vagus nerve magnetic modulation; CRS-R, The Coma Recovery Scale-Revised; EMCS, exit minimally conscious state; MCS+, minimally conscious state “plus”; MCS-, minimally conscious state “minus”; VS/UWS, vegetative state/unresponsive wakefulness syndrome.

Evoked Potential Analysis

There was no significant change in SEP after treatment compared with that before treatment, and only one patient improved from grade II to grade I. In contrast, the BAEP improved more significantly after treatment than before treatment, and the number of BAEP class I increased from 3 to 5, class II increased by 1, and class III decreased from 4 to 1 after treatment, with statistically significant differences (P=0.025) (Figure 3 and Table 2).

Figure 3 Distribution of evoked potential grading before and after treatment. (A) The proportion of each SEP grading before and after treatment is shown. This distribution shows that one patient changed from SEP grade II to grade I after treatment. (B) The proportion of each BAEP grading before and after treatment is shown. The results show a more significant improvement in BAEP in treated patients.

Abbreviations: SEP, somatosensory evoked potentials; BAEP, brainstem auditory evoked potentials.

Discussion

To our knowledge, this is the first protocol to study the effect and safety of VNMM in patients with pDOC. The results suggest that VNMM has the potential to promote awakening in pDOC patients and is expected to improve the conscious function of pDOC patients. After VNMM treatment, patients’ behavioral responses were significantly better than before treatment. In addition, patients also showed more significant improvement in BAEP outcomes.

Loss of consciousness is associated with a loss of connectivity in the default mode network, frontoparietal network, thalamocortical network, and cerebellar cortical circulation.21,22 Further studies have revealed that the central thalamus is a specific node that supports arousal of the brain or central thalamic connections to various cortical areas.23 Consciousness in a normal physiological state is determined by the correct perception of the surroundings and oneself and by normal arousal. Perception depends on the functional and structural integrity of the cortical functional network and the cortical and subcortical neural network connections, and arousal is maintained by the superior brainstem reticular agonist system.4,5 TMS is a neuromodulation technique that generates an induced electric field by stimulating tissues using a pulsed magnetic field of a certain intensity, which activates cortical effects and excites neuromuscular to achieve neural remodeling and promote consciousness recovery. Currently, the targets of magnetic stimulation for pDOC are mostly focused on the prefrontal dorsolateral cortex and primary motor cortical areas.24,25 The excitatory effect is mainly produced by indirect stimulation of the extensive cortical and subcortical tissues (especially the thalamus) connected to them. The currently used transcranial magnetic stimulation modalities mainly produce excitatory effects in the thalamus through cortical stimulation and do not achieve activation of the superior brainstem reticular system.

The superior vagal fibers are relayed to other nuclei in the brain through the solitary nucleus, which receives most of the afferent information from the vagus nerve. This sensory information is then integrated and projected to higher centers such as the medullary reticular formation and the nucleus accumbens, which is closely related to the brainstem superior reticular activation system. Studies have confirmed that vagal electrical stimulation has a pro-wake function, directly regulates brainstem activity, and reaches the dorsal nucleus of the middle suture and the thalamus via the nucleus accumbens, with positive effects on the reticular formation and thalamic metabolism.7 In contrast, transcutaneous vagal electrical stimulation enters the isolated nucleus of the brainstem through the auricular branch of the vagus nerve and joins the superior reticular activating system, which is also involved in the modulation of the consciousness circuit.26 This led us to ponder whether the vagus nerve could be a new target for magnetic stimulation in the treatment of patients with pDOC.

Previous studies have shown that left-sided VNM combined with hemiplegic limb training can improve motor function in rat stroke models or in patients with hemiplegia.8,10–12 In addition, VNM also promotes the release of neurotransmitters such as acetylcholine and norepinephrine and cytokines such as brain-derived neurotrophic factors, thus mediating the process of neuroplasticity.8,13 A clinical study showed that VNMM can promote esophageal MEP response and effectively improve post-stroke swallowing disorder.9 However, no studies have reported the use of VNMM in patients with pDOC. Our findings suggest that VNMM can promote recovery in patients with PDOC (especially MCS patients). In the present study, we observed a mean increase of about 4 points in CRS-R scores after 20 VNMM treatments, while BAEP outcomes showed improvement. Our findings support that VNMM can improve patients with pDOC.

In previous studies, the mean increase in CRS-R in pDOC patients after intervention with high-frequency rTMS ranged from 1.25 to 1.57.16,27 The increase in CRS-R scores in pDOC patients after intervention with tDCS, another commonly used neuromodulation technique, ranged from approximately 1.7 to 5.92.28,29 In the present study after VNMM intervention, patients had a mean increase in CRS-R of approximately 4 points, which was significantly better than previous rTMS stimulation protocols and did not differ significantly from the effect of tDCS. In other words, compared with other noninvasive neuromodulation techniques, there was no significant difference in the effect of VNMM for pDOC. Moreover, VNMM compensates for some shortcomings of rTMS: first, pDOC patients with cranial defects can also be treated through the vagus nerve ear branch. Secondly, VNMM can modulate consciousness through the brainstem’s superior reticular activating system as an effective complement to TMS for pDOC.26 Finally, VNMM can be both antiepileptic and may also help patients to recover motor and swallowing functions.9,12,30 It has the advantage of a single target with multiple benefits.

Safety Analysis

The safety of TMS has been widely reported, and no significant abnormal reactions have been observed during its use. Some studies have pointed out that TMS may have the risk of inducing adverse effects such as seizures, tinnitus, and headache. In a clinical study of magnetic modulation of the vagus nerve, there were no adverse events.9 In the present study, all patients tolerated VNMM without any adverse reactions during treatment. However, there is still insufficient experience with the implementation of VNM, and clinicians should exercise caution when performing VNMM treatment.

Limitations

First, the small sample size of our study did not allow for a stratified comparison of patients’ states of consciousness (coma, VS/UWS, MCS), and etiology (TBI, stroke, HIE, etc.). Second, no follow-up was performed to observe the long-term effects of VNMM on patients with prolonged disorders of consciousness. In addition, the study was self-controlled and not randomized. Therefore, future studies need to conduct randomized controlled trials to analyze the effect of VNMM in patients with prolonged disorders of consciousness and to focus on whether there are differences in the effect of VNMM in patients with prolonged disorders of consciousness of different etiologies and states of consciousness.

Conclusion

In the present study, we indicate that vagus nerve magnetic modulation holds promise as a novel therapeutic approach in patients with chronic disturbances of consciousness. In the available research data, MCS patients had the best effect, followed by UWS patients. And it has good security. This study provides a preliminary indication of the potential of VNMM in the rehabilitation of pDOC patients. It provides the basis for a phase 2 or phase 3 study of VNMM in patients with pDOC.

Acknowledgments

This work is supported by the Open Project of the Key Laboratory of Rehabilitation Medicine in Sichuan Province. Lingling Wang and Qing Wu are co-first authors for this study.

Disclosure

The authors report there are no competing interests to declare.

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