Comprehensive Guide to Vagus Nerve Stimulation: Applications & Benefits

Vagus nerve stimulation (VNS) has been widely used for nearly 20 years in various scientific and practical applications. The vagus nerve has been known to play a role in maintaining homeostasis, controlling stress, and regulating the immune system. VNS can be performed electrically and invasively from the neck, but also non-invasively from the neck or ear. Due to the variable character of the autonomic nervous system activity, it is seen that the application of personalized current parameters is more important in VNS compared to other neuromodulation methods. In order to increase the effectiveness of the method and reduce possible side effects, stimulation parameters (frequency, duration, pulse width, etc.) can be shaped online with closed-loop stimulation according to user data. In order to create personalized stimulation parameters, measurements such as electroencephalography, functional magnetic resonance imaging, heart rate variability, pulse, blood pressure, and evoked potential can be collected continuously from the individual. Stimulation parameters can be determined more clearly, thanks to the follow-up of the effects on the individual as a result of the stimulation. 

The vagus nerve is the most important and largest part of the parasympathetic nervous system. This nerve, which has nuclei in the brain stem, descends from the neck and makes connections with almost all internal organs (1). This method, which starts with the stimulation of the cervical vagus nerve, is often performed invasively by placing a programmed electrical generator on the chest wall and is unilateral (2). Apart from its role in numerous complications, the vagus nerve provides the brain-intestinal connection and is also responsible for establishing homeostasis. Vagus nerve stimulation can also affect the immune system through the cholinergic anti-inflammatory pathway. The method has shown promising results in helping with long-term inflammatory issues of the digestive tract. It is also used to ease pain in fibromyalgia and migraine (3,4,5), aiding the application of existing treatment methods. Stimulation of vagal afferent fibers affects the monoaminergic system in the brain stem, which plays an important role in mood and stress fluctuations. The vagus nerve plays an important role in the regulation of nutrition, satiety, and energy homeostasis. Stress can trigger the sympathetic system and decrease vagal tone, resulting in impairment of the mentioned functions. The vagus nerve has connections with brain regions (locus coeruleus, orbitofrontal cortex, insula, hippocampus, and amygdala) that are activated under stress. VSU can reduce the effects of stress on the body by modulating the activity of the vagus nerve (6). The connections of the vagus nerve with the cardiovascular system form the basis of the use of vagus nerve stimulation in cardiac well-being as well. The high-frequency component of heart rate variability is considered an indicator of vagal tone (7). Vagus nerve stimulation is a method with indications and potential for use in diverse applications, as studies have indicated.

Vagus Nerve Stimulation Methods

In addition to cervical invasive stimulation methods, vagus nerve stimulation can be performed by cervical noninvasive and auricular transcutaneous or percutaneous methods. Electrical stimulation is the main form of stimulation. In addition to its wide distribution in the body, VNS can be used in many different indications thanks to the connections of its nuclei in the brain stem (8). Some of the invasive VNS devices have a closed-loop system and include a cardiac-based seizure detection algorithm. Automatic stimulation is provided by triggering an increase in heart rate due to the seizure, although the optimal stimulation parameters are still unclear. Invasive VNS requires surgery; however, surgical complications, side effects caused by stimulation of efferent fibers, and its cost limit its use (9).

Although invasive VNS is clinically beneficial, side effects such as hoarseness, sore throat, shortness of breath, and cough are common. Auricular Vagus Nerve Stimulation, as a method targeting the ear branch of the vagus, can activate similar neuronal systems in the brain, just like the invasive stimulation. The projection of vagus nerve afferents to the nucleus tractus solitarius is known from histochemical and electrophysiological experiments (10). When the existing literature is examined, it is seen that there is no clear consensus on the regions where the vagus nerve is most concentrated in the ear. However, it is reasonable to point out that the inner part of the concha and tragus are suitable sites for vagal modulation. When evaluated in terms of efficacy, there is no consensus in the literature about the optimum parameters of auricular VNS, similar to invasive stimulation (11).

In different situations, it remains unclear what the wavelength, intensity, and frequency of the excitation should be. A systematic approach is required to determine the optimal stimulation intensity, pulse width, waveform, and frequency that confers the greatest benefit. This may require setting participant-specific parameters in a closed-loop setup where stimulation parameters are set online. All current stimulation strategies for noninvasive VNS devices are based on open-loop control of stimulation parameters, where levels are set at the start of the stimulation protocol and do not change in response to any continuous measurement of neuronal activation level. It is reasonable to expect different results in response to open-loop electrical stimulation between participants and between trials, due to the different brain activities that go on during stimulation. In a system in which personalized stimulation parameters are shaped according to the collected data, it can be expected that efficacy will increase and possible side effects will decrease.

With VNS, a system in which changes in the organism are constantly measured should be targeted. Measurements such as electroencephalography, functional magnetic resonance imaging, heart rate variability, pulse, blood pressure, and evoked potential can be collected both during stimulation and during non-stimulation (to determine the most appropriate session length). Collecting data outside the stimulation time can also help to understand how long the stimulation’s effectiveness lasts after the stimulation ends. Many studies have shown that Vagus Nerve Stimulation participants who complete their sessions throughout the study respond better than those who quit halfway. Also, it was noted that longer durations correspond to better outcomes. The mechanism of VNS is still not fully clarified. A closed-loop stimulation method can help us better understand the functioning of the autonomic nervous system. Although it is an easily applicable method with few side effects, finding the right stimulation parameters for each purpose and user remains a critical challenge in auricular VNS. Professionals should customize their applications for different functional outcomes (12,13).

Lateral or Bilateral Auricular Vagus Nerve Stimulation

Another issue in Auricular Vagus Nerve Stimulation is about which ear the stimulation should be made from. Invasive cervical VNS is done from the left side to reduce cardiac side effects because it also stimulates efferent fibers. For this reason, auricular VNS is similarly often performed on the left side. However, stimuli from the right or left ear eventually go to the same place, the nucleus tractus solitarius. Simultaneous activation of the left and right auricular vagus nerves can potentially increase stimulation effects due to increased sensory input to the brainstem (14). There are very few documented studies in the literature in which bilateral auricular VNS is performed (15). Using artificial intelligence through sensors, it may be possible in the near future to decide whether stimulation should be done in the right, left, or both ears. 

Stimulation Optimization

Due to the complex physiology of the body, continuous and intermittent stimulation, as well as strong or moderate stimulation, can even cause opposite physiological effects. Certain stimulation patterns may cause compensatory increased sympathetic activity along with increased parasympathetic activity. The easiest aspect of providing closed-loop stimulation is to obtain subjective data from the person being administered. However, the absence of objective physiological data may result in the inaccurate or incomplete evaluation of treatment efficacy. Correct selection and processing of physiological signals as recorded by the sensor or sensors is crucial for closed-loop auricular vagus nerve stimulation because the feedback must include information about the characteristics of physiological reactions occurring in response. Auricular vagus nerve stimulation systems with closed-loop or biofeedback can be integrated with telemedicine applications (16).

The vagus nerve is a complex nerve that provides afferent and efferent innervation of the pharynx, larynx, heart, lungs, esophagus, stomach, liver, pancreas, small intestine, and proximal colon. This widespread distribution allows vagus nerve stimulation to have an effect over a wide window. Since cervical vagus nerve stimulation also includes efferent fibers, selective stimulation of these fibers may prevent off-target effects (17,18). In auricular stimulation, on the other hand, different stimulation points may cause different effects (11). In vagus nerve stimulation, a personalized program is very important, as in other areas of health. Based on the results of studies in animals and humans, an experimental idea about the ideal stimulation can be obtained. In light of these results, guidelines can be created (19). However, it can be expected that a closed-loop stimulation, in which the activity of the vagus nerve is personally recorded, subjective and objective data are collected through sensors, the data is processed, and the optimal stimulation parameters are determined with artificial intelligence (20). Although there is currently no device that can do all of these features, there are devices and clinical studies in which the sensor-stimulation algorithm is used as a precursor (21,22,23). New method proposals for bidirectional neuromodulation, in which simultaneous recording and stimulation are performed on the same nerve, signal processing, and system control on an external computer, are also encountered in the literature (24). Although vagus nerve stimulation is a method that prioritizes regulation in the autonomic nervous system due to its nature, it was found that the treatment led to different results in users (25). From this, it can be deduced as a result of the diversification of the data to be collected from the people who have VNS. Somatosensory evoked potential, pupil diameter, and salivary alpha-amylase levels can be used to evaluate VNS activity (26). In noninvasive methods, it may also be necessary to quantify vagus nerve stimulation (27).

Vagus Nerve Stimulation Methods

Although closed-loop stimulation systems are used for various applications, it can be stated that the number of available devices, the type of sensors, and the data used is low (28). The automatic stimulation feature triggered by an increase in ictal heart rate by at least 20% is used to transmit vagus nerve stimulation in a closed-loop manner in epilepsy patients (21). A closed-loop stimulation according to heart rate to the cervical vagus nerve of pigs under anesthesia was successfully performed invasively, and stabilization of heart rate under VNS was achieved (30). Different VNS closed-loop control systems are also proposed for real-time regulation of heart rate. In the proposed model, multiple VNS parameters can be modulated according to the applied object, and the model can be integrated into implanted devices (31,32). In the study of Cork et al., the extracellular pH in the subdiaphragmatic vagus nerve of rats under anesthesia was measured, and it was stated that this method could be used in closed-loop VNS (33). Diffusion tensor imaging and machine learning algorithms can be used to select patients with epilepsy who are more likely to benefit from VNS therapy outside the closed-loop (34). Vagus nerve stimulation has emerged as a method that can be used in overcoming different challenges; it can be applied invasively or non-invasively, and has gained importance in recent years. Recently, approaches that include closed-loop algorithms and artificial intelligence have also started to come to the fore. It can be expected that the success rate of VNS will increase and its use will become widespread in situations where noninvasive, easy-to-apply, personalized stimulation parameters and detailed follow-up with data collected from individuals are available.

References

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