What exactly is the vagus nerve?
The Vagus nerve is the 10th and longest cranial nerve that constitutes the main component of the parasympathetic nervous system. This nerve is responsible for a wide range of important bodily functions, including heart rate, control of mood, digestion, immune response, and heart rate variability. Through the afferent fibers, it sends information about the internal organs to the brain. The vagus nerve also establishes connections between the brain and the GI (gastrointestinal) tract. (1)
The vagus nerve is in control of the regulation of internal organ functions, such as certain reflex actions like vomiting, sneezing, coughing, swallowing, and vasomotor activity. Its activation leads to the release of acetylcholine (ACh) at the synaptic junction with intrinsic nervous fibers, secreting cells, and smooth muscles. ACh binds to nicotinic and muscarinic receptors and stimulates muscle contractions in the parasympathetic nervous system.
The parasympathetic nervous system represents one of the three branches of the autonomic nervous system (ANS) alongside the enteric nervous system (ENS) and sympathetic nervous system (SNS). The vagus nerve is the major benefactor of the parasympathetic nervous system. It’s principal function is to pass information from the inner organs, such as the heart, liver gut, and lungs to the brain and Vice versa. This indicates that the inner organs are major sources of sensory information to the brain. Researchers have identified a positive feedback loop between positive emotions, good physical health, and high vagal tone. This means the higher your vagal tone, the higher your mental and good physical health will improve. Before we discuss the treatment of anxiety using vagus nerve stimulation, we need to understand what Anxiety is and how to identify the disorder.
In recent years, the number of psychological distress disorders like depression and anxiety has increased enormously. They are one of the most common mental disorders diagnosed among the ages of 18 to 64 years old. General anxiety disorder (GAD) is characterized by both physical and psychological symptoms such as poor concentration, excessive anxiety, irritability, impaired social or occupational functioning, poor sleep, and muscle tension. Anxiety is your body’s natural reaction to stress. It’s a feeling of apprehension or fear of what’s to come. For example, giving a speech on the first day of work or going for a job interview may cause you to feel nervous and fearful. This anxiety becomes a disorder when it lasts for at least 6 months, and interferes with your life. This disorder alters how you behave and process emotions, also causing physical symptoms. Having mild anxiety might be unsettling and vague, while having severe anxiety may seriously impact one’s day-to-day living.
There are several types of anxiety disorders you might be experiencing. They include (6)
The feeling of anxiety varies and depends on the person experiencing it.
You may have a general feeling of fear and worry, or you may fear a specific event or place. Other symptoms of anxiety include
Vagus nerve stimulation (VNS) involves the use of a device that sends electrical charges to stimulate the vagus nerve. This treatment is used to alter the activity of the vagus nerves. VNS requires using a device that sends mild impulses of electrical charges through the vagus nerve in your neck to your brainstem. The electrical impulse is distributed to different parts of the brain when it reaches the brainstem, it alters how the brain cells work. Vagus nerve stimulation has exhibited the ability to treat anxiety, epilepsy, depression, cluster headache, and migraine(2)
The anatomy of the nervous system in the study of anxiety control indicates that manipulation of the vagus nerve affects anxiety. The sensory afferent nerve ﬁbers that vagus nerve stimulation excites, travels to the brain, and ends in the nucleus solitary tract (NTS). The afferent ﬁbers are the primary source by which the brain gets information from the organs within the gut. From there, information travels to the main site of all norepinephrine ﬁbers in the brain called the locus coureulus. Norepinephrine (NE) is an important neurotransmitter involved in the regulation and pathogenesis of anxiety. Reasonably, a device that directly stimulates this neurotransmitter control site might have important effects on anxiety.
Vagus nerve stimulation has been known to increase activity in the amygdala, thalamus, brainstem, and insula, which are the regions involved in anxiety perception and modulation. (4)
According to research, It has been estimated that between 25%- 45% of patients either failed to respond to conventional antidepressant/ anxiety treatment or withdrew from treatment due to intolerable side effects. Because of the wide influence of the Vagus nerve on multiple systems and its primary role in maintaining homeostasis, energizing the VN to modulate the function of related organs drew the attention of researchers.
In 2005, cervical vagus nerve stimulation as a slow-acting therapy was endorsed by the US FDA for the treatment of refractory epilepsy and treatment of chronic resistant depression. (5)
Vagus nerve stimulation (VNS), is known to cause changes in the neurotransmitters involved in the pathophysiology of anxiety and depression, which includes serotonin, norepinephrine, glutamate, and GABA.
This conventional vagus nerve stimulation involves surgical implants done by a neurosurgeon. This implanted device sends mild, regular pulses of electrical energy to your brainstem via your vagus nerve in the neck. After reaching the brainstem, an electrical impulse is released to different areas of the brain to change the way brain cells work. It is commonly done on an outpatient basis. As with all surgeries, there is a risk of infection. Other possible risks of an invasive VNS surgery include pain at the incision site, inflammation, nerve constriction, and damage to nearby nerves.
Transcutaneous vagus nerve stimulation (tVNS) was developed to reduce the risks of invasive vagus nerve stimulation (iVNS).
Through the skin, transcutaneous VNS stimulates afferent fibers of the vagus nerve using small electrical currents. This is primarily done at the outer ear, stimulating the ABVN (auricular branch of the vagus nerve). cervical stimulation of the vagus nerve is also used as a non-invasive VNS. The ABNV innervates the antitragus, cymba concha, and antihelix of the ear. There is notable variance with the right branch of the vagus nerve innervating the SA (sinoatrial) node of the heart than the left branch of the vagus nerve which preferentially innervates the AV (atrioventricular) node. Research has shown that tVNS can drastically reduce multiple symptoms of depression, including anxiety, psycho-motor deceleration, sleep disturbance, and hopelessness. Presently, there are two principal ways to use transcutaneous vagus nerve stimulators.
This stimulation is done by using two skin electrodes (by a hand-held device to the cervical portion of the vagus nerve).
This stimulation is mainly done at the auricular branch of the vagus nerve by using two surface electrodes to the ear.
The main rationale for ear stimulation (tVNS) is based on the anatomical studies that indicate afferent vagus nerve distribution in some parts of the ear (lower half of the back ear close to the mastoid process and concha). Research has shown that stimulation of the vagus nerve's auricular branch stimulates the inferior ganglion which projects to the nucleus tractus solitarii and thus produces the same therapeutic effects as invasive VNS.
Using VNS to decrease fear responses in recent technological developments, allows transcutaneous stimulation of the vagus nerve (tVNS) through a vagally innervated part of the outer ear. tVNS is a safe technique used to stimulate this auricular branch of the vagus nerve. Small impulses from the tVNS immediately reduce the activation of brain areas related to extinction learning eg, the amygdala, prefrontal cortex, and hippocampus. This increases performance in cognitive tasks that are dependent on norepinephrine activity.
The tVNS as a whole is a well-tolerated treatment option, but as with any medical treatment, has a few mild side effects.
Common side effects may include
Uncommon side effects you may experience include heart palpitations, gastrointestinal issues like nausea or vomiting, facial dropping, etc.