Vagus nerve stimulation (VNS) is a type of neuromodulation that involves the use of a device that sends electrical impulses to stimulate the vagus nerve. This type of treatment is used to alter the activity of the nerves. Vagus nerve stimulation(VNS) requires the implantation of a device that sends mild pulses of electrical energy through the vagus nerve in your neck to your brainstem. After it reaches the brainstem, the electrical impulse is discharged to various parts of the brain to alter how the brain cells work. VNS has demonstrated efficiency in the treatment of epilepsy, migraine, depression, and cluster headache (1).
The vagus nerve is the main component of the ANS (autonomic nervous system). It plays a key role in the control of metabolic homeostasis and plays an important function in the neuroendocrine-immune system to sustain homeostasis through its efferent and afferent pathways (2). The vagus nerve is the 12th cranial nerve that passes through the neck as it travels through the chest, abdomen, and the lower part of the brain. It is connected to both motor and sensory functions in the sinuses and esophagus. In fact, "vagus" means “wanderer” in Latin, which precisely represents how the nerve wanders throughout the body and reaches different organs. It majorly constitutes the parasympathetic nervous system and serves many internal body functions like saliva and mucus production, immune response, orgasm, control of mood, heart rate, and muscle sensations.
Due to the wide influence of the Vagus nerve on multiple systems and its primary role in maintaining homeostasis, stimulating the VN to modulate the function of related organs has long drawn the attention of researchers. Cervical vagus nerve stimulation (VNS) as a slow-acting therapy, is approved by the US FDA for the treatment of refractory epilepsy and treatment of chronic resistant depression in 2005 (3).
There are two methods by which the Vagus nerve can be stimulated:
-Invasive vagus nerve stimulation
-Noninvasive vagus nerve stimulation
In conventional vagus nerve stimulation, surgical implants are carried out by a neurosurgeon and it takes roughly 50-100 minutes with the patient under general anesthesia. It is commonly done on an outpatient basis. As with all surgeries, there is a minor risk of infection. Other possible risks of VNS surgery include pain at the incision site, inflammation, nerve constriction, and damage to nearby nerves. The surgery requires two small incisions. The first incision is done on the upper left quadrant of the chest wall where the pulse generator (about the size of a silver dollar) is implanted. The second incision is made horizontally on the left side along a crease of the skin. Two thin flexible wires are then inserted to connect the pulse generator to the vagus nerve.
The stimulator contains a battery that can last for up to 15 years. The stimulator is usually replaced with a less invasive procedure when the battery is low. It is activated three to four weeks after implantation, although in a few cases it is activated at the time of implantation in the operating room. The surgeon programs the stimulator with a tiny hand-held computer, programming wand, and programming software. It is at this point the duration and strength of the electrical impulses are programmed. The amount of stimulation is initiated at a low level depending on the case and slowly increased to an appropriate level for the individual. The device is programmed to run continuously for specific periods (Turning on and off at intervals). For example, 40 seconds on and 5 minutes off.
With the number of conditions that stimulating the vagus nerve can benefit, researchers have found a simpler, less invasive approach that would enable the treatment of remarkably larger numbers of diseases. Over the years, different VNS devices have been manufactured to improve safety and efficacy (4).
Transcutaneous vagus nerve stimulation (tVNS), a noninvasive technology is used to modulate vagal and brain activity through afferent vagal pathways using electrical current. These new noninvasive VNS devices do not require any surgical implantation. It has been approved in Europe to treat depression, epilepsy, and pain. It has also been endorsed for the treatment of cluster headaches by the FDA in the United States. The device was developed to achieve a similar result as with invasive VNS but without the downside of a surgical procedure and continuous stimulation.
This prospective non-invasive route for VNS is electrically stimulating the auricular branch of the vagus nerve, which is administered to the external ear. This stimulation is done transcutaneously by adding acupuncture needles or surface electrodes to the external ear (TVNS). Anatomical investigation of the ear indicates that the concha, tragus, and cymba concha are the points on the body where there is a cutaneous afferent vagus nerve and that stimulation of these afferent fibers produces therapeutic results similar to those of regular VNS.
Transcutaneous auricular VNS (tVNS) selects the cutaneous responsive area of the ABVN (auricular branch of the vagus nerve) located in the outer part of the ear. Studies have shown that anatomically, the cymba conchae is the only part of the outer ear that is solely innervated by the ABVN, which makes it the most appealing contender for stimulation (5). Noninvasive stimulation of this area causes notable activation of the ipsilateral nucleus tractus solitarii, which is the first central relay of vagal afferents in addition to intracranial structures equally affected by invasive VNS.
The auricular branch of the vagus nerve is a mixed nerve consisting of the glossopharyngeal, facial nerves, and vagus nerve. Anatomically, the ABVN is distributed to the posterior wall of the external auditory canal. It has been investigated that transcutaneous vagus nerve stimulation works for different disorders, including migraine, schizophrenia, tinnitus, pain, post-error slowing, atrial fibrillation, prosocial behavior, associative memory, and cardiac disorders, by targeting the vagus nerve at sites in both the neck and ear.
Another method of noninvasive VNS is stimulation of the transcutaneous anterolateral part of the neck is superficial to the route of the vagus nerve.
tVNS has also been endorsed for the treatment of drug-resistant epilepsy, a brain disorder characterized by abnormal discharge of neurons in the brain. When compared to the invasive VNS, tVNS has more advantages, including the efficiency of VNS but with lower cost, fewer side effects, no secondary tissue damage, lack of the need for an invasive procedure, and the simplicity of performing the tVNS procedure. This device has improved the quality of life of patients with refractory epilepsy and helped reduce seizure frequency.
Transcutaneous vagus nerve stimulation works for epilepsy by;
- Increasing the levels of some key neurotransmitters responsible for controlling seizures in the brain.
- Increasing blood flow in key brain areas.
- Changing the electroencephalogram (EEG) patterns when a seizure occurs. EEG is a device used to measure electrical activities in the brain.
- Monitoring any sharp increase in the heart rate. 85% of epileptic people notice an increase in heart rate before experiencing the onset of a seizure. A TVNS can be used to immediately send an extra burst of electrical stimulation to help impede the impending seizure (6).
In July 2015, Vagus nerve stimulation was approved by the FDA for the treatment of severe unipolar and bipolar depression in adults 18 years of age and above who have long-term depression that hasn’t been responding to antidepressant medications.
Researchers have found that tVNS can remarkably lessen multiple symptoms of depression, including sleep disturbance, psychomotor deceleration, anxiety, and hopelessness. Vagus nerve stimulation has been shown to elicit changes in neurotransmitters involved with the pathophysiology of depression, including norepinephrine, serotonin, glutamate, and GABA.
Cluster headache attacks are one-sided, with severe pain located in retro-orbital, temporal, and periorbital regions. Pain spikes within minutes may last up to four hours if untreated and are mostly backed by cranial autonomic symptoms (e.g. nasal congestion, lacrimation, periorbital oedema) and agitation. Research has shown that non-invasive nerve stimulation supports the rationale that strong parasympathetic activation that occurs during Cluster headache attacks (7). It is a very painful medical condition that can substantially reduce patients’ quality of life by diminishing their functional abilities in work, domestic, and social activities. Significant decreases in attack frequency, duration, and severity are observed in patients with cluster headaches who did not respond to acute treatments.
Autism is a neurodevelopmental disorder characterized by restricted, repetitive behavior and continuous deficits in social communication. Research has shown that children with autism treated with tVNS showed increased improvement in cognitive behavior, function, and seizure frequency. Vagal stimulation modulates the cortical and subcortical (particularly the thalamus and amygdala) functions, which in turn regulates the perturbed brain function of ASD (8).
Tinnitus also known as ringing in the ear is the misconception of sound in the absence of actual external sound and it affects 9-16% of the general population. Research has shown that chronic tinnitus is connected to a dysfunction in the auditory system that causes abnormal neuronal behavior. tVNS improves mood and decreases the severity of tinnitus (8).
Research has shown that tVNS can influence the way pain is processed and offer an inhibitory effect on various pain modalities. Examination of these different sub-modalities suggests that transcutaneous vagus nerve stimulation has an impact on the central pain processing centers and not just the peripheral nociceptor activity. Another mechanism involved may be linked to the minimized production of inflammatory cytokines by the spleen macrophages after a vagus nerve stimulation (10).
Although tVNS as a whole is a well-tolerated treatment option, a few mild side effects have been noted. Common side effects include voice hoarseness, soreness, tingling or pain around the site of stimulation, pharyngitis, nausea, redness, or itching. Rare side effects include gastrointestinal issues like nausea or vomiting, facial dropping, heart palpitations, etc.
After getting a TVNS device, you should monitor your overall health closely. Take note of the following and talk to your doctor immediately after you notice the following;
⦁ Stimulation which becomes painful.
⦁ Any irregular Stimulation that causes a change in heart rate, choking, swallowing, or breathing difficulties.
⦁ increased drowsiness.
⦁ Constantly hoarse voice.
⦁ Signs that the battery is depleted.
⦁ Signs that the device has stopped stimulating properly.
⦁ Any unusual or new changes related specifically to the stimulation.
⦁ Do not apply stimulation while sleeping or in the shower.
⦁ Do not place electrodes near the thorax because introducing electrical current may
⦁ elevate the risk of cardiac fibrillation.
⦁ Do not apply stimulation while operating machinery or driving.
⦁ Do not apply stimulation across the chest region as this may cause disturbances to the rhythm of the heart.
⦁ Do not wrap the wire around your neck.
⦁ Do not apply stimulation over rashes, swollen, red, infected areas, or skin eruptions (e.g., varicose veins, thrombophlebitis) or over open wounds.
⦁ Do not get the tVNS device wet.
⦁ Do not stimulate during any activity where electrical stimulation can put you at risk of injury.
⦁ Do not apply stimulation over the mouth or neck because it could lead to severe
⦁ muscle spasms resulting in difficulty in breathing, closure of the airway, or adverse effects on blood pressure and heart rhythm
⦁ Women who are pregnant.
⦁ Pediatric patients.
⦁ Patients with a diagnosed irregular heart rhythm.
⦁ Patients who already have a functioning implantable medical device, such as a hearing aid, cardiac pacemaker, or any implanted electronic or metallic device.
⦁ Patients who have had a cervical vagotomy (surgery done to cut the vagus nerve in the neck).
⦁ Patients with a history of atherosclerotic cardiovascular disease, congestive heart failure (CHF), or recent myocardial infarction.
⦁ Patients diagnosed with carotid atherosclerosis (narrowing of the arteries).
5. https://pubmed.ncbi.nlm.nih.gov › ...The nerve supply of the human auricle - PubMed