Transcutaneous auricular vagal nerve stimulation for consciousness recovery in patients with prolonged disorders of consciousness (TAVREC): study protocol for a multicenter, triple-blind, randomized controlled trial in China


Prolonged disorders of consciousness (pDoC) are among the severest sequelae following brain damage, primarily including vegetative state/unresponsive wakefulness syndrome (vs/UWS) and minimally conscious state (MCS).1 Patients in vs/UWS condition exhibit spontaneous eye opening and only show reflexive movements without any behavioural evidence of consciousness, while people with MCS show clear, discernible but fluctuant signs of awareness, thus encompassing a variety of behavioural presentations.1 2 In recent decades, advances in emergency and intensive care have contributed to a higher survival rate of patients with severe brain injury, leading to a significant increase in prevalence and incidence of pDoC. Patients with pDoC are vulnerable and at high risk of medical complications (approximately 0.40 medical event per week per patient) posing great challenges to healthcare staff engaged in this field.3 4 Moreover, long-term care for patients with pDoC often entails significant emotional and financial strain for the patients’ families and involves a series of ethical and legal issues.5 Hence, developing effective strategies for pDoC is pressing.

Several pharmacological interventions have been suggested to improve consciousness or functional recovery in DoC patients. Among them, neurostimulants such as levodopa, bromocriptine and methylphenidate appeared to have no influence on recovery of full consciousness or meaningful neurobehavioral improvement in a retrospective cohort study.6 Zolpidem, an agonist on subtype 1 of the inhibitory gamma-aminobutyric acid receptor, seems to induce a paradoxical transient awakening effect in only about five percent of patients.7 8 One large randomised controlled trial (RCT) provided class II evidence that amantadine can result in a faster consciousness recovery in patients with pDoC, but differences to placebo disappeared at the end of a 2-week washout period.9 Given those results, amantadine is the only respective medication recommended in the 2018 DoC guideline of the American Academy of Neurology.10 As regards non-pharmacological interventions, a recent RCT suggested that trigeminal nerve stimulation may increase local brain metabolism and promote functional recovery in patients with pDoC; effectiveness of this therapy remains, however, to be confirmed by larger trials.11 While a number of trials reported improved signs of consciousness in patients with MCS after transcranial direct current stimulation (tDCS)12 13 ; however, effectiveness of tDCS for pDoC patients was challenged in a double-blind cross-over study, in which repeated tDCS did not exert remarkable short-term clinical and EEG effects.14 Repeated transcranial magnetic stimulation has been explored in treatment of vs and MCS, but its clinical efficacy is still controversial and awaits confirmation through high-quality intervention studies.15 16 In addition, it is still unclear whether benefits of invasive brain stimulation outweigh its risks in the treatment of DoC.17–19 Therefore, novel and more effective therapeutic interventions for pDoC are needed.

Transcutaneous auricular vagal nerve stimulation (taVNS) is a safe, non-invasive therapeutic option that has been applied in treatment of epilepsy, depression, obesity and stroke.20 21 Application of taVNS is supposed to activate nuclei located in the brainstem areas via stimulating the auricular branch of the vagus nerve, which then regulates other brain region activities through ascending projections.22 So far as taVNS in pDoC is concerned, mostly case reports or uncontrolled studies reported positive outcomes including increases in scores of the Coma Recovery Scale-Revised (CRS-R), gradual evolution of electroencephalography signals or cerebral metabolic improvements.23–25 The only small RCT in the field indicated that CRS-R total scores improved markedly in patients with MCS after active taVNS stimulation compared with sham stimulation. Adverse effects described did not have any obvious relation with taVNS.26 However, the above RCT and cohort studies used the CRS-R total score as primary outcome without clear indication of what constitutes clinical meaningful effects.27 Additional better-powered trials focusing on clinically meaningful improvement and safety of taVNS in pDoC population are thus required.

Given the above rationale, we propose a multicentre triple-blind RCT to investigate the effectiveness of taVNS on consciousness recovery in patients with pDoC (TAVREC) based on multimodal assessments and to indicate the safety of this therapeutic strategy. The results of this trial will provide high-level evidence to evaluate clinical prospect of taVNS in pDoC and possibly identify a new clinical therapy for this disease.

Methods and analysis

Study design

The TAVREC trial is a multicentre, triple-blind, randomised controlled trial with 4 weeks intervention and 4 weeks follow-up period. The trial protocol was written in accordance with the Standard Protocol Items: Recommendations for Interventional Trials 2013 Statement. Ethics approval was granted by the Research Ethics Committee of the First Affiliated Hospital of Nanjing Medical University (Reference number: 2023-SR-392, 11 July 2023). The TAVREC trial has been prospectively registered at the Chinese Clinical Trials Registry ( ChiCTR2300073950, 26 July 2023.

Table 1 provides an overview of trial registration information.

Table 1

WHO trial registration data set for TAVREC trial

Study setting

The TAVREC Trial Collaboration Group consists of three participating centres from China. The First Affiliated Hospital of Nanjing Medical University is a 4500-bed metropolitan, primary referral hospital, which provides standard therapy for pDoC with aetiology including cardiac arrest, brain injury and spinal cord injury, among others. Nanjing Zijin Hospital is a 300-bed medium-sized, secondary referral hospital focusing on treatment of and research on pDoC with capacity to provide comprehensive therapy for 100~200 patients with pDoC annually. Taizhou Rehabilitation Hospital is a 480-bed medium-sized, primary referral hospital, providing standard therapy for patients within its high-dependency unit including patients with pDoC. The First Affiliated Hospital of Nanjing Medical University will implement the TAVREC programme guidelines and standard operation procedures in all participating centres starting with a kick-off meeting and carry out regular visits to the participating hospitals.

Eligibility and withdrawal criteria

Eligible participants are those who meet the consolidated criteria as listed in table 2.

Table 2

Inclusion, exclusion and withdrawal criteria of TAVREC trial

Enrollment, randomisation and allocation

Candidates will be identified based on medical records and invited to participate in the study through proxy. A trained accredited examiner will assess patients’ eligibility for the study at the time of admission with the prespecified criteria and determine the current state of awareness by means of at least two CRS-R evaluations. Family members of patients who meet the eligibility criteria will receive a detailed explanation of the trial purpose and procedures, and consent (see online supplemental material for the informed consent form) will be obtained from the legal representative. They also will be informed that they have the right to withdraw from the trial at any time during the trial period.

The randomisation procedure will be performed centrally by the Institute of Clinical Medicine from the First Affiliated Hospital of Nanjing Medical University (allocation centre) with SAS (V.9.4). Individuals will be randomised in a 1:1 ratio to either taVNS or control arms using block randomisation, with alternating random block sizes (4 to 8). Following acquisition of informed consent from legal guardians of eligible participate, an independent trial assistant at each participating centre will register patients, then receive the respective unlabeled group allocation letter (A vs B) revealed by computer.28 An overview of recruitment, randomisation and allocation is presented in figure 1.

Figure 1
Figure 1

Trial flow diagram.


Patients in the taVNS group will be treated with taVNS plus conventional therapy. Conventional therapy mainly comprises multimodal sensory and auditory stimulations, bedside physical therapy and pharmacological therapy (online supplemental table S1).1 2 An electrical stimulator (tVNS501, Changzhou Rishena Medical Device Co., Ltd., Jiangsu, China) will be applied at the surface above the left cymba conchae to provide the VNS using the following parameters: 30 s monophasic square current of pulse width 300 μs, frequency of 25 Hz and intensity of 1 mA followed by 30 s rest.24–26 taVNS will be carried out for 60 min two times per day for 4 weeks (5 days per week).24 26 To ensure adequate contact between the electrodes and the skin of the ear, a module will be installed in the device that can monitor anode and cathode impedance with the skin. This module will produce an alarm sound when the fit is suboptimal. Patients in the control groups will receive a taVNS placebo procedure in addition to conventional therapy. For the taVNS placebo, the stimulator will offer a built-in placebo mode: on activation, VNS with identical parameters as above specified will solely be provided for 5 s at the beginning and the end of each 60 min stimulation period to mimic the somatosensory artefact of the actual taVNS. The same parameters as for actual taVNS will be displayed on the device interface.

Supplemental material

Outcome measures

Primary outcomes

The primary outcome of the TAVREC trial is RIC at week 4. Improvement of consciousness level is defined as moving from a lower consciousness level to a superior consciousness level, such as moving from vs to MCS. The diagnosis of consciousness level is based on a comprehensive assessment using subscale scores of the CRS-R, the most widely used behavioural assessment tool for diagnosis of patients with DoC recommended by US and European guidelines.10 29 30 Table 3 shows the CRS-R diagnostic standard for the levels of vs/UWS, MCS and emergence from minimally conscious state.31 Two examiners will independently administer CRS-R subscales for consciousness level evaluation, with any discrepancy resolved by consensus or the involvement of a third researcher if consensus cannot be reached.

Table 3

Diagnostic criteria for vs, the MCS and the EMCS according to CRS-R scale

Secondary outcomes

  1. CRS-R scores including subscale scores and total scores will be evaluated. Higher CRS-R scores are associated with better conscious levels.

  2. In addition to CRS-R, GCS and FOUR total and subscale scores will be documented as supplementary scales evaluating the consciousness level. GCS scores are arrived at by investigating eye, motor and verbal response, providing a total score ranging from 3 to 15. The total score is intended to reflect severity of injury, with scores of 3–8, 9–12 and 13–15 indicating a severe, moderate and mild injury, respectively.32 FOUR is a reproducible rapid assessment tool for consciousness state, evaluating four components (eye, motor, brainstem and respiration) on a scale of 0–4 in each domain.33 Higher FOUR scores are associated with better conscious levels.

  3. EEG signals will be recorded through a 32-channel EEG device. Data will be processed and analysed with MATLAB (The Mathworks, Natick, MA) using a combination of EEGLAB and Chronux toolbox. Power spectrum density within fixed bands will be generated using spectral and cross-spectral decomposition of the resting-state EEG data, and the relative percentage contribution of delta (0–4 Hz), theta (4–8 Hz), alpha (8–13 Hz), beta (13–25 Hz) and gamma (25–40 Hz) bands to the total power will be calculated.34–36 Moreover, functional connectivity between regions of interest will be estimated using the weighted Phase Lag Index and the Coherence Index.34 37

  4. BAEP assessment will take place based on the international standard 10/20 system for electrode placement. The differentiation and latency of I to V waves will be evaluated, and Hall grading criteria will serve as the grading standard of BAEP (online supplemental table S2).38

  5. USEP will be elicited by bilateral stimulation of the median nerves at wrists. The differentiation and latency of N9, N13 and N20 waves will be evaluated, and Judson’s grading criteria will serve as grading standard (online supplemental table S3).39

  6. Neuroimaging parameters associated with DoC will include cerebral blood flow in mL/100 g/min, standardised uptake value ratio, amplitude of low-frequency fluctuation, functional connectivity, fractional anisotropy and mean diffusivity using PET/fMRI.29 40 41

  7. Biomarkers associated with consciousness levels will include neuron-specific enolase, neurofilament light chain and S100B in serum.42–44 Fasting blood will be collected into standard serum tubes through vacuette system. Following centrifugation at 1000×g for 10 min at 4°C, serum will be isolated and stored at −80°C. Concentrations of those biomarkers are planned to be assayed using ELISA kits.

  8. Adverse events. For adverse event evaluation, the attending health professionals and patients’ relatives or caregivers are required to pay attention to any abnormal responses of the patient on a daily basis. All reported abnormal signs will be rated by two blinded assessors who have access to event and medical history, and identified adverse events will be recorded with the adverse event report form. Severity will be rated on a 3-point Likert scale ranging from mild to severe. Death, life-threatening events, extension of hospitalisation time and permanent or severe disability will be defined as serious adverse events. Relationship with the intervention will be assessed following the World Health Organization-The Uppsala Monitoring Centre system.45

Data collection

Data in terms of consciousness levels, CRS-R total scores and subscale scores, GCS and FOUR will be collected at baseline (T0), week 1 (T1), week 2 (T2), week 4 (T3, the end of intervention period) and week 8 (T4, the end of follow-up period). RIC will be calculated at the same time points except at the baseline. Outcomes involving spectral power estimation and functional connectivity from EEG, grading of BEAP and USEP, and serum biomarkers will be evaluated at T0, T3 and T4. The PET/fMRI will be performed at T0 and T3 for neuroimaging parameter evaluation. Adverse event will be evaluated on a daily basis throughout the trial process.

Apart from the above outcomes, demographics and clinical characteristics will also be collected directly from medical files at randomisation, including but not limited to the following information: centre of recruitment, phone number, age, gender, height in centimetre, weight in kilogram, nationality, occupation, education level, handedness, aetiology, course of illness, GCS score at emergency department, medical history for current disease (eg, history of head trauma, history of craniotomy, history of brain oedema, history of tracheal intubation, history of tracheotomy and history of mechanical ventilation), duration of sedation, length of stay (LOS) in hospital, LOS in intensive care unit and comorbidities (diabetes mellitus, hyperlipidaemia, hypertension, atrial fibrillation, valvular heart disease, respiratory infection or others). Scheduled data collection at each time point is presented in table 4.

Table 4

Scheduled events and timeline of the TAVREC trial

Data management and monitoring

All data will be managed with an electronic data capture (EDC) system developed specifically for the TAVREC programme. Two independent trial assistants at each participating centre will perform data entry and sign for this process. A specifically designed computer programme will detect typographic errors and missing data. Any discrepancy will be solved by consensus achieved by checking raw-data, repeated patient assessment or discussion. For ensuring the data security and confidentiality, only authorised investigators will have access to the EDC system. Programme meetings will be periodically hold in primary investigator, coordinators in each participating centre, members of the data safety monitoring board, data analysts, statisticians and trial assistants to (1) monitor and review the participant safety; (2) review patient recruitment, accrual and withdrawal; (3) request the conduct of interim data analyses; (4) discuss continuing or modifying the trial; and (5) terminate the trial on any severe adverse events considered to have resulted from the TAVREC programme.


TAVREC trial employs a triple-blind design. Independent trial assistants registering patients will receive group allocation displayed as A or B without labels and distribute taVNS devices accordingly. A placebo mode of taVNS with the same parameter displayed in the device interface will be employed in the control group, effectively blinding patients, families and care providers. Allocation will, furthermore, be kept concealed up until final data analysis. This is achieved through keeping intervention labels on a secure password-protected server accessible only to the trial monitoring committee, ensuring that physicians responsible for conventional therapy, assessors, data analysts, and statisticians are continuously blinded to group allocation until conclusion of the study.

Sample size calculation

Based on a previous study, RIC in pDoC patients receiving usual treatment and taVNS at the fourth week were 6.90% and 28.57%, respectively.26 For a conservative purpose, we considered a 20% increase of RIC after taVNS intervention from a 5% baseline. Using a chi-square test for two groups, we performed sample size calculation with a desired power of 80% and an alpha error of 5%, leading to a minimum number of 49 participants per group. Accounting for 15% attrition, the estimated sample size is 58 per group.

Statistical analysis

All analyses will be performed using the Stata version 16 (StataCorp, College Station, Texas, USA) or other software, as appropriate. Main analyses will be conducted on intention-to-treat basis. Two-sided p values of less than 0.05 were considered statistically significant. JDR, the senior statistician from the Institute for Disaster Management and Reconstruction of Sichuan University, is responsible for the statistical analyses.

The demographic and clinical characteristics of participants and outcomes for both groups during the trial period will be represented as means with SD or medians with interquartile ranges for continuous variables and as percentages for categorical variables, as appropriate. The primary outcome (RIC at week 4) will be analysed with logistic regression. Secondary outcomes, including longitudinal RIC, CRS-R scores, GCS scores, FOUR scores, indexes from EEG, BAEP, USEP, neuroimaging parameters by PET/MR and serum biomarkers linked to consciousness level will be estimated for group differences with mixed effects ordered logistic regression or mixed effects linear regression. Adverse events will be descriptively analysed.

Data will be assumed to be missing at random (MAR); missing values will be handled with multiple imputation using chined equations based on observed model parameters (ie, centre, time point, and the interaction of group and time point).46 The reliability and robustness of the MAR assumption will be tested with sensitivity analysis.

Prespecified sensitivity analysis includes estimation of all primary and secondary outcomes with above models on the per-protocol sample and for complete cases, as well as assessing those outcomes on two types of multiply imputed datasets. First, multiple imputation with chained equations will be performed under an extended MAR assumption, that is, that missing values will also be dependent on observed values of auxiliary variables not included in aforementioned models. Second, controlled multiple imputation will be employed for simulating a non-MAR scenario where patients with missing assessments in the taVNS group followed the pattern of change in controls.

Exploratory subgroup analysis with respect to age, gender, BMI, aetiology, course of illness, GCS score at emergency department, medical history for current disease, duration of sedation, LOS in hospital, LOS in intensive care unit and comorbidities will be performed with exploratory post hoc sensitivity adjusted analyses to determine the effect of specific covariates.

In addition, interim analysis will be scheduled halfway through enrollment to check for potential issues as regards sample handling or data collection.

Quality assurance

For enhancing normalisation and uniformity in this multicentre RCT, a standard operation procedure (SOP) throughout enrollment, allocation, intervention, data collection and sample manage procedure has been drafted. A standard operation video regarding intervention and data collection is provided as well. Physicians, physiotherapists, assessors and trial assistants from all participating centres will receive training in SOP and are required to comply with instructions as specified in SOP booklet and digital video recordings. Notably, all participating centres of the TAVREC Trial Collaboration Group will provide sufficient LOS in hospital to fulfil the therapeutic and assessment requirements for patient included in the trial. If a patient is discharged early, assessors must do their best to follow-up for all assessment time points. This may include phone calls with patients and/or relatives, inviting patients back to hospitals or visits to homes or nursing homes if patients live in reasonable range of the responsible hospital (200 km).

Patient and public involvement

Patients and/or the public were not involved in the design, or conduct, or reporting, or dissemination plans of this research.

Strengths and limitations

To our knowledge, TAVREC will be the first multicentre RCT assessing efficacy of taVNS in patients with pDoC. In this programme, three medical centres providing comprehensive therapy for a sizeable number of pDoC patients will collaborate. In particular, TAVREC benefits from bias reduction through a triple-blind design that includes a specifically designed taVNS placebo mode. Importantly and different from previous studies, the TAVREC programme also uses a multimodal assessment system consisting of clinical scales, family-reported data, neuroelectrophysiological parameters, neuroimaging indicators and serum biomarkers.1,17,18 Based on those multimodal outcomes, we expect to not only generate new evidence as regards the efficacy of taVNS in facilitating consciousness recovery but also provide an initial database for enhancing the understanding of mechanisms responsible for taVNS effects in pDoC patients

There are several possible limitations of our study. First, the primary outcome CRS-R depends to some degree on the subjective perception of assessors. However, assessor blinding and independent administration of the CRS-R subscales by two assessors should reduce risk of bias. Second, subgroup analysis will only be exploratory as the trial has not been powered accordingly, but may, nonetheless, serve the design of future studies that aim to look into differential effects across subgroups. Third, although the TAVREC trial includes neuroelectrophysiological parameters and neuroimaging indicators that may contribute to explore the mechanism of taVNS, the trial is not focused on the in-depth investigation of underlying biomechanical mechanisms.

Trial status

The trial is ongoing and is actively enrolling. Enrollment of patients was initiated by the three participating centres on 26 July 2023. With an expected inclusion rate of 10 to 20 patients per month, an estimated recruitment period of 6–12 months is required.

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