Small Animal Referral Hospital Vagus Nerve Stimulator

A good description of the device is available on the website of the manufacturer, alongside information for human patients.

The system is made up of:

• A circular generator that is implanted under the skin
• A wire lead that connects the generator to the nerve and is secured to muscles under the skin with plastic clips to prevent it being dislodged with exercise
• A small magnet that can be worn on an owner's wrist or easily carried and can be used to generate extra stimulations from the generator if a seizure is starting

The generator sends electrical impulses up the vagus nerve at regular intervals, all day every day. These impulses are then carried by the vagus nerve to the brain. This regular stimulation alters the neurotransmitter balance in the brain to inhibit generation and spread of seizures. Specifically, it is thought to increase the levels of noradrenaline and serotonin.

This inhibitory neurotransmitter effect is thought to become more pronounced with prolonged stimulation (over weeks to months) but is active immediately too. This means that whilst the repetitive stimulations can decrease the likelihood of seizures, seizures that are just starting may be abolished by stimulation of the vagus nerve too. For this reason, the generators can be activated by passing a magnet over them and this gives an immediate stimulation to the vagus nerve that can stop the seizure that is about to start from developing fully.

We have published the results from our initial dogs here:

Long‐term outcome of epileptic dogs treated with implantable vagus nerve stimulators - Harcourt‐Brown - 2023 - Journal of Veterinary Internal Medicine - Wiley Online Library

We found that:

• Some dogs cannot tolerate the device (around 1 in 15)
• In dogs who can tolerate the device, seizures start to significantly decrease in frequency after 18-24 months
• After 3-4 years, most dogs had changed from a seizure frequency of more than one per month to none over 6-18 months.

So, we suspect that this is an effective long-term treatment for dogs, but the long time to start to see a positive effect might not make it suitable for all dogs.

Dogs need to be referred by their vet for VNS assessment. We can implant these devices in any dog that has been diagnosed with epilepsy following appropriate confirmation of seizures, blood samples and an MRI (Magnetic Resonance Imaging) scan of the brain.

We have trialled the devices in dogs with severe epilepsy and had encouraging results. We are now opening it to all epileptic dogs.

Because the positive effects can take 18-24 months to appear, VNS therapy is best suited to dogs with a moderately high seizure frequency despite medications (e.g. more than one day with seizures every 3 months). For dogs with more frequent seizures, we would recommend VNS in combination with other changes that we can discuss with clients during their consultation.

We implant the devices under general anaesthetic by making two incisions; one just next to the vagus nerve by the trachea where the lead is connected, and one just behind the shoulder blade where the generator is inserted. Surgery takes 1-2 hours, and we can start stimulation immediately after. Dogs will normally stay with us for 2-3 days whilst we adjust the initial settings and can then go home where they will have lead exercise only until the surgery site is fully healed (in 4-5 weeks).

We can offer VNS implantation for a fixed price of £3500. This includes initial assessment, placing the VNS and optimisation of the settings. This optimisation can be achieved with several days in our hospital or as an outpatient with several revisits here - but the cost is the same.

We require dogs to have had an MRI to help confirm their diagnosis prior to implantation. For cases who have not had MRI but are highly likely to have idiopathic canine epilepsy, we can perform the MRI for an additional fixed fee of £1000, bringing the total cost for the VNS to £4500 in these cases. If dogs have previously had MRI, we only need to see the original images.

The main side effect of VNS implantation is a cough when the device is active. This is usually self-limiting in people and is a function of how much stimulation is given, so is more severe in dogs who need treatment urgently, as we will use higher doses sooner. There are multiple strategies to limit this though and we are working with a very experienced team who have implanted many devices in children so we should be able to keep the cough to a manageable level that does not affect the dog or owner's quality of life.

There is a small risk of the stimulator affecting the heart. This occurs in around 1% of humans and is not persistent. The rate is unknown in dogs but has not been observed in the cases previously described.

The main risk is self-limiting swelling around the generator (seroma); infection of the implant or damage to the leads. seromas will resolve without treatment, but infections mean the generator needs to be replaced. wearing a Halti muzzle or soft harness whilst on the lead should reduce the risk of lead damage.

Once we have the initial settings, dogs can go home and be monitored there. The stimulation will increase until there is no more benefit to increasing it (this takes several weeks) and then the settings adapted to maximise battery life (this takes several months). We will need to see dogs here to do this as it requires specialist equipment to do.

The device may interfere with MRI scans unless certain precautions are taken and not all MRI scanners can do this. If a scan is necessary, we would recommend it before implantation.

At some point, the generator will need replacing when the battery runs low. The generator battery can last between 1 and 16 years, depending on the model and settings used. We would expect to get between 8-10 years use with the settings we will use.