Vagus nerve stimulation

Vagus nerve stimulation (VNS)

Vagus nerve stimulation (VNS) is a treatment for epilepsy which may be considered for people whose seizures do not respond to AEDs (sometimes called ‘drug resistant epilepsy’) and who are not suitable for epilepsy surgery.

But what are the vagus nerves, and how does VNS work?

The vagus nerves

As the name suggests, vagus nerve stimulation is a form of treatment that involves sending electrical stimulation to the vagus (or vagal) nerves.

The body has 12 pairs of nerves that start in the brain and link to other parts of the body: these are called the cranial nerves. Each pair of nerves is linked to a different part of the body, and each does different things. The vagus nerves are the 10th pair of cranial nerves.

In Latin ‘vagus’ means ‘wanderer’ and refers to the fact that these nerves connect to many different parts of the body and pass messages between the brain and various organs, including:

• the throat (where they connect to the muscles that control swallowing)
• the heart (as part of the autonomic nervous system that controls the way the heart  works), and
• organs in the chest and abdomen (including the stomach and intestine, where  they help to control the function of the bladder and bowels, and the lungs).

Around 80 per cent of the nerve fibres of the vagus nerve are afferent, meaning they relay messages from the body to the brain. In this way, the brain receives messages from parts of the body and these messages are, in turn, relayed to different areas within the brain.

VNS uses the afferent nerve fibres of the left vagus nerve to send electrical pulses to the brain. Because epilepsy is a condition that starts in the brain, sending electrical pulses in this way may help to prevent seizures. Looking at the areas of the brain that these fibres from the left vagus nerve connect to may hold the key to the way VNS works.

The VNS device

VNS therapy uses a small device, a bit like a heart pacemaker, which produces electrical impulses or ‘stimulation’. The device sends electrical impulses to the left vagus nerve in the neck, which then passes the impulses to the brain.

The VNS system is made up of a ‘pulse generator’ and a connecting ‘lead’. The battery-powered pulse generator is implanted (placed under the skin during an operation) in the upper chest, just under the collar bone. The lead is connected to the pulse generator at one end; the other end travels up the vagus nerve to where two small wires or ‘electrodes’ are coiled around the nerve I the neck.

Who can have VNS?

VNS therapy was first licensed in Europe in 1994 . It is used for people who have difficult-to-treat (‘drug-resistant’ or ‘refractory’) epilepsy: seizures that  are not completely controlled by AEDs. It is used alongside AEDs, so it is called an ‘adjunctive’ or ‘add-on’ treatment. VNS is very unlikely to cure or completely stop seizures; however, it may reduce the number or frequency of seizures. 

How does VNS work?

VNS is thought to do three things:

1. stop seizures once they have started
2. make seizures less likely to happen
3. increase in effect over time

Put simply, VNS aims to reduce the number of seizures an individual has.

The exact way VNS works to improve seizure control has not been established. However, we do know that the electrical pulses delivered by VNS can reach various parts of the brain which may be important in controlling seizures.

The electrical pulses produced by VNS follow the fibres of the vagus nerve as they enter the brain at the top of the spinal cord through the medulla (part of the brain stem). Many of the nerve fibres end in a synapse within the medulla at a point called the nucleus of the tractus solitarius (NTS). The NTS projects to various places within the brain including the thalamus, hypothalamus, locus coeruleus, reticular activating system, midline raphe, the limbic system and, secondarily, to the cortex. The electrical pulses from VNS travel into these areas of the brain and may interrupt seizures or stop them developing in the first place.

However, although we know the path of the left vagus nerve and its connections in the brain, we don’t know exactly how VNS affects neurons, and how this works against seizures. It is very possible that there is more than one mechanism of action and there are various theories about this.

How VNS may help

The VNS pulse generator is programmed to give regular bursts of electrical stimulation (this is because continuous stimulation of the vagus nerve could damage it). Initially, it is programmed to come on for 30 seconds every five minutes at a low current. The current is then increased over time in the same sort of way that AEDs are started at a low dose and gradually increased. (The programming is carried out by a computer and a programming ‘wand’ which is placed over the device. This is done in an outpatient clinic.)

It has been found that four in 10 people with VNS have at least 50 per cent fewer seizures, three in 10 have some improvement in their seizures (but not to the same extent), and the remaining three in 10 have no change. (However, for some people, even if VNS does not help with their seizures, it may have a positive effects on their mood ore quality of life.)

It can take up to two years to see any positive effect from VNS.

Side effects

VNS does not affect — and is not affected by — AEDs. But like many forms of treatment, including medication, VNS has some possible side effects. Most of these only occur while the electrical stimulation is happening. They vary from one person to another and usually improve over time. (Some side effect can be reduced by changing the frequency or strength of the stimulation.)

By looking at the areas of the body the vagus nerves connect to it is possible to understand why certain side effects might happen.

Side effects can include the following:

• coughing
• discomfort in the throat
• hoarseness (the stimulation can make the person’s voice sound different and ‘croaky’)
• shortness of breath (the stimulation can make the person short of breath, particularly if they have asthma or any lung problems — this is because the vagus nerve connects to the lungs)
• difficulty swallowing (the stimulation can make swallowing difficult — this is because the vagus nerve connects to the muscles in the throat that control swallowing. If the person is eating the device can be turned off so they can swallow their food.)

On the plus side, because VNS is not a medication it doesn’t affect cognition (a person’s awareness or speed of thought). In fact, VNS may help to make some people more alert and improve their concentration and mood.

Also, because VNS works directly on the vagus nerve it doesn’t have to ‘travel’ around the body to get to the nerve, which reduces the effect on other areas or organs (in contrast, drugs travel around the body to get their site of action, which means they can affect other organs and produce what are sometimes called systemic side effects).

Replacing the pulse generator

The battery in the VNS pulse generator usually lasts between six and 10 years, depending on how frequently the pulses are sent. If VNS helps the person to reduce their seizures the battery can be replaced, although it will need a second operation to do this. The connecting lead does not need to be replaced.

If VNS does not help to control the person’s seizures the pulse generator can be switched off and removed (also in an operation). Because it is difficult to remove them from the nerve, the lead and electrodes are usually left in place.

Cyberonics, the company that make the VNS device, has a number of booklets about VNS, including information for children and people with learning disabilities.

With special thanks to Anthony Linklater, epilepsy specialist nurse at the National Hospital for Neurology and Neurosurgery, for his help in writing this article.

Glossary

Afferent nerves are the sensory nerves that send messages from parts of the body to the brain. (Efferent nerves, the opposite of afferent nerves, are those that send messages from the brain to other parts of the body.)

Autonomic nervous system is the part of the nervous system that controls involuntary, unconscious actions of the body (for example, the beating of the heart and the activity of the bladder and bowels).

Cranial nerves are the 12 pairs of nerves that start in the brain and connect to different parts of the body. They include the optic nerves (responsible for vision) and the trigeminal nerves (responsible for, among other things, controlling the muscles for chewing and relaying information about pain from the mouth and face).

Limbic system is the part of the brain made up of the hypothalamus, hippocampus and amygdala. It controls instinct and self-preservation (for example, the drive to find food or to fight) and has an important function in memory. These are also areas where epilepsy often starts.

Nucleus of the tractus solitarius (NTS) is part of the medulla of the brain, where many of the fibres of the vagus nerve end.

Side effects are an unintended effect of any medication. Side effects are not the expected action of the medication (ie, the reason you take it). Also called 'adverse effects' (although they are not always unwelcome).

Synapses are the points at which two neurones connect. The messages from one neurone are transferred to the next neurone across the synapse using chemicals called neurotransmitters.

Systemic refers to the body as a whole rather than a particular part or organ.

Site of action is the place in the body where a particular drug works.

Vagus nerves refers to the 10th pair of cranial nerves, which start in the brain and connect to different areas of the body. ‘Vagus’ means ‘wanderer’ in Latin.

© Epilepsy Society
January 2009