A methodological description of the technique, potential targets, and proper administration of transcutaneous auricular vagus nerve stimulation (taVNS) on the human ear is described.
Non-invasive vagus nerve stimulation (VNS) may be administered via a novel, emerging neuromodulatory technique known as transcutaneous auricular vagus nerve stimulation (taVNS). Unlike cervically-implanted VNS, taVNS is an inexpensive and non-surgical method used to modulate the vagus system. taVNS is appealing as it allows for rapid translation of basic VNS research and serves as a safe, inexpensive, and portable neurostimulation system for the future treatment of central and peripheral disease. The background and rationale for taVNS is described, along with electrical and parametric considerations, proper ear targeting and attachment of stimulation electrodes, individual dosing via determination of perception threshold (PT), and safe administration of taVNS.
Cranial nerve X, better known as the vagus nerve, is a large nerve tract that originates in the brainstem of the central nervous system and travels throughout the periphery, targeting every major organ in the thorax and abdomen (Figure 1)1. Vagus nerve stimulation (VNS) involves surgical implantation of bipolar electrodes around the left cervical branch of the vagus nerve. Electrical pulses are delivered to the vagus nerve via an implanted pulse generator (IPG) surgically implanted in the chest2. Although VNS is currently FDA-approved for epilepsy, refractory depression, and chronic obesity, it is a costly procedure requiring a hospital visit and surgery. Long-term safety of VNS is well established, and the majority of safety considerations regard current intensity related side effects (hoarse voice, throat pain) without serious stimulation-related adverse effects over the past 25 years of its clinical use3.
Recently, a noninvasive form of VNS known as transcutaneous auricular vagus nerve stimulation (taVNS) has emerged4. taVNS delivers electrical stimulation to the auricular branch of the vagus nerve (ABVN), an easily accessible target that innervates the human ear5. Over the last decade, several groups have demonstrated the safety and tolerability of this method6,7,8, including central and peripheral nervous system effects9,10, and behavioral effects7,11,12,13 in neuropsychiatric populations. taVNS is also being explored in individuals as a promising enhancer of cognitive14,15 and social functioning16,17,18. As taVNS is becoming established, it offers the ability for researchers and clinicians to rapidly translate the promising VNS research that has been described in various disorders ranging from neurological and psychological trauma19,20,21, addiction22, inflammation23, and tinnitus24,25.
In principle, taVNS is methodologically similar to conventionally administered transcutaneous electrical nerve stimulation (TENS) used to treat musculoskeletal pain disorders26. The difference is that taVNS is delivered to specific anatomical ear targets that are believed to be innervated by the ABVN5. The field is still determining optimal stimulation targets27, although the two most common placements are the anterior wall of the outer ear canal (tragus) and the cymba conchae. Sham stimulation may be conducted by stimulating the earlobe of the ear, an area believed to have minimal ABVN innervation (Figure 2). Alternatively, sham may be delivered via a passive control method in which electrodes are attached to active sites, but no stimulation is delivered. Stimulation parameters may vary between groups, however according to the literature, stimulation is delivered in a pulsatile fashion (pulse width: 250–500 μs, frequency: 10–25 Hz) and delivered at an individualized constant current (<5 mA). Stimulation current varies by individual and experimental protocol, with many groups exploring various intensities as a function of an individual perceptual threshold (PT). The PT is defined as the minimum amount of current eliciting a perceived sensation at the target site and is usually determined via parametric estimation by customized sequential testing (PEST) software described in this report.
taVNS is a safe technique that may be administered in the laboratory or clinical setting. Side-effects of taVNS are minimal, with skin irritation or redness being the most common side-effect. Most taVNS studies explore stimulation of the left ear, as it is believed to be safer, although data in a large trial (Badran et al. 2018) reveal that right-sided stimulation has no increases in the risk of adverse events. Due to the wealth of literature in unilateral-left side stimulation, we will illustrate the typical taVNS set-up for laboratory studies investigating the use of left-sided taVNS as an intervention.
This experimental protocol illustrates a typical taVNS set-up for use in a laboratory or clinical setting in which we target stimulating the anterior wall of the auditory canal (tragus) in a supine posture with an 8mm diameter round metal electrode. These methods can be mimicked for alternative active treatment sites by simply changing electrode position to the cymba concha. All methods and procedures have been IRB approved by the Human Research Protection Program (HRPP) at City College New York.
1. Materials
2. Ear Targeting and Skin Preparation
3. Electrode Preparation and Placement
4. Determination of Perceptual Threshold (PT)
NOTE: Perceptual threshold is a critical value used to determine the power of taVNS stimulation. This value is defined as the minimum amount of electricity required to perceive electrical stimulation on the skin described as a pricking or tingling sensation.
5. Delivering Stimulation
6. After taVNS
When proper skin preparation is conducted, perceptual thresholds are inversely correlated with stimulation pulse width. As pulse width increases, the perceptual threshold decreases (Figure 7). Initial studies by this group exploring the effect of pulse width on PT in healthy individuals (meeting inclusion/exclusion criteria listed above), determined that the combined overall (n=15, 7 female, mean age 26.5 ± 4.99) PT at 100 μs = 3.92 ± 1.1 mA; 200 μs = 2.24 ± 0.74 mA; 500 μs = 1.24 ± 0.41 mA. These thresholds suggest that a constant current stimulator with capacity of delivering up to 5 mA of current is required for stimulation of 500 μs pulse width parameters, and a minimum of a 10mA stimulator is required for lower pulse widths (Table 2). Fine tuning of the current is required, with increments of 0.1 mA are necessary for precise stimulation.
Delivering stimulation at 200% PT is tolerable and relatively pain free as demonstrated by pain numerical rating scales (NRS) scales9,30. The NRS scale is a rating system for pain from 0-10 in which individuals report pain or discomfort29. Both Active and Sham stimulation rate similarly low pain levels (NRS <3 for all stimulation pulse widths. More specifically, the biologically-active pulse width of 500 μs delivered at 25 Hz is reported on average to rate as Active = 1.98 ± 0.83, Sham = 2.17 ± 1.27 (n=25, 9 female, mean age 25.16 ± 4.16 years) (Table 3). Pain ratings for other parameters are no more painful than the 25 Hz parameter and the details can be found in the groups' prior work30.
Safety and tolerability of 30 min to 1 hour sessions at a 20–50% duty cycle has been widely reported in the literature with some studies delivering multiple sessions in the same day spread 12–15 h apart12,31. No serious adverse events have been reported from 60 subjects participating in several series of experiments with subjects participating from 1 to 8 repeated visits spread a minimum of 24 hours apart.
taVNS, when administered as reported in this manuscript, has been demonstrated to modulate the autonomic nervous system, induce functional brain activity changes as measured by fMRI BOLD, and piloted to treat neuropsychiatric disorders and aid in rehabilitation.
Figure 1: Vagus Nerve Efferent Projections and Cross-section. (A) Efferent projections of the vagus nerve target every major organ on the body with wide effects on bodily function (B) Cross-section of the vagus nerve, demonstrating the inside anatomy of the nerve as a series of bundles of nerves all contained within one major pathway. Please click here to view a larger version of this figure.
Figure 2: taVNS Ear Targets. Targeting the ABVN can be accomplished by stimulating the anterior wall of the outer ear canal, landmarked notably by the tragus (A1), or cymba conchae (A2). Sham stimulation is administered to the earlobe (S). Please click here to view a larger version of this figure.
Figure 3: Key Components. The minimum required components for proper administration of taVNS are the following (A) ear stimulation electrodes, (B) conductive gel and alcohol prep pads, (C) computer capable of sending and receiving TTL pulses to a (D) constant current stimulator to trigger stimulation. Please click here to view a larger version of this figure.
Figure 4: Example Setup. This photo shows an individual receiving taVNS of the left ear while in position to undergo an experimental paradigm. Please click here to view a larger version of this figure.
Figure 5: Screenshot of the GUI used for stimulation. Please click here to view a larger version of this figure.
Figure 6: Electrical Stimulation Waveform Manipulations. Direct square wave electrical current can be delivered at various parameters. This figure demonstrates key properties of the waveform that can be changed in order to achieve desired biologic effects. Please click here to view a larger version of this figure.
Figure 7: Perceptual Threshold Values at Increasing Pulse Widths. As pulse width increases, perceptual threshold (PT) decreases. Most healthy individuals will have a PT within 2 standard deviations (SD) of these mean values. Please click here to view a larger version of this figure.
Table 1: Example of How to Determine Perceptual Threshold (PT). This table shows an example sequence of Yes/No responses used to parametrically determine PT.
Table 2: Stimulation Current Levels. Values of stimulation current in mA (200% PT) for each pulse width (n=15).
Table 3: PT, Stimulation Current, and Pain Values for Suggested Stimulation Parameters. Values of stimulation current in mA (200% PT) for each pulse width (n=25).
Supplemental File: Freeware GUI used in this protocol. Please click here to download this file.
As in all novel modalities, all the described steps are critical in the safe administration of taVNS. Of ultimate concern is subject safety, which includes not only mitigating risks before taVNS via proper screening, but also monitoring subjects during stimulation for discomfort, pain, or adverse events. Here are the three most important consideration for administering taVNS. Screening for taVNS contraindications – contraindications are as follows: any current or past history of cardiovascular disorders, facial or ear pain, recent ear trauma, metal implants above the level of the neck. For proper subject skin preparation, removing any surface oils, dirt, or makeup from the surface of the skin with alcohol helps with conductivity of the electrodes, reduces stimulation voltage required to drive the stimulator, and ultimately results in a more tolerable and safe stimulation session. It is encouraged to use a stimulator and electrodes meeting Low Output Transcranial Electrical Stimulation (LOTES) guidelines32. LOTES sets guidelines and industry standards for electrical stimulators that are built for stimulation of the head and neck and it is encouraged for groups to read this document before building their own systems. It is recommended to use either a FDA-cleared plug-in stimulator (see Table of Materials), or a low voltage (<50 V), battery powered, constant current stimulator with appropriate safety measures built-in to avoid unintended over-delivery of current to the stimulation site. Ensure that electrodes are manufactured and assembled for specific use in taVNS. Ensure that the current manufacturing and engineering guidelines are followed as a reference if lab-made customized systems are used.
One consideration for taVNS is to ensure that the voltage output of the constant current stimulator can surmount the resistance of the skin and deliver the current required for stimulation. Ohm's law (V=IR) demonstrates the relationship between current (I) and skin resistance (R). A minimum of a 20 V tabletop stimulator is recommended to avoid an underpowered system. Heat generated from the scalp or environment may degrade the conductive paste. If this occurs, it is recommended to stop stimulation and re-prep skin and electrodes with new conductive paste.
A limitation of taVNS is the vast parameter space. It is unknown as to which is more important — pulse width or frequency. There is a lacking data in recent taVNS trials that answer such questions. The various behavioral effects are derived from a variety of pulse widths, frequencies and stimulation currents13,33,34,35,36,37,38,39.
At this time, it is suggested that the 500 µs pulse width to be the most biologically active9. With respect to frequency, it has been demonstrated that 25 Hz is an effective frequency, although current investigations into optimal such as higher frequencies (>25 Hz), bilateral stimulation (left and right ears), and investigational burst paradigms are being conducted. Studies exploring different parameters of stimulation, alternative stimulation sites, and duty cycle optimization are needed to advance and refine the taVNS method.
taVNS is a promising non-invasive alternative to conventional VNS. taVNS provides an inexpensive (<$5,000 in the demonstrated experimental setup, cost heavily dependent on type of stimulator used) and straightforward method that can be used to translate positive findings in animal models exploring the use of VNS on a variety of disorders, noninvasively modulate the autonomic nervous system, and potentially miniaturized and optimized for at-home neuromodulation for the treatment of neuropsychiatric and other disorders.
The future potential and possible applications of taVNS are vast. taVNS can serve as a promising adjunct or standalone treatment for neuropsychiatric disorders such as depression and epilepsy, taVNS-paired rehabilitation training to restore or accelerate learning of a behavior40, decrease inflammatory response41,42, and can potentially be used to enhance performance and autonomic function8,10.
The authors have nothing to disclose.
Research reported in this publication was supported by funding from the National Institutes of Health National Center of Neuromodulation for Rehabilitation, NIH/NICHD Grant Number P2CHD086844 which was awarded to the Medical University of South Carolina. The contents are solely the responsibility of the authors and do not necessarily represent the official views of the NIH or NICHD.
70% Isopropyl Alcohol Wipes | Any | N/A | Any alcohol preparation pads used for skin in appropriate. |
Constant Current Stimulator (Triggerable) | Soterix Medical | N/A | Stimulator manufactured for custom use by Soterix Medical |
Disposable Conductive Electrodes | Custom Built | N/A | Stimulation electrodes are custom built at the City College Neural Engineering Lab (Badran/Bikson) |
Matlab Software w/ Stimulation GUI | MathWorks | N/A | MATLAB used for programing pulse pattern |
Ten20 Conductive Paste | Weaver and Company | N/A | Conductive paste used for administration of stimulation |