What does the device look like?
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
How does VNS work to stop seizures?
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 vagus nerve that can stop the seizure that is about to start from developing fully.
Which dogs can have VNS therapy?
Dogs need to be referred by their vet for VNS assessment. We recommend it for dogs with generalised seizures, a seizure frequency >1 per 3 weeks for the preceding 3 months and are resistant to drug therapy (ie they have been on phenobarbital and potassium bromide at a therapeutic level for that 3 month period, or they can not take one of those medications because of side effects).
What does VNS surgery involve?
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).
How much does VNS implantation cost?
We are able to 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 MRI to help confirm their diagnosis prior to implantation. For cases who have not had MRI but are very 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.
What are the risks and side effects?
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 appears to be 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.
What happens after VNS surgery?
Once we have the initial settings, dogs can go home and be monitored there. The stimulation will be increased 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.
What are the benefits of VNS?
The benefits of VNS can include the following:
- Having fewer seizures - complete resolution of seizures is uncommon
- Having less severe seizures or shorter seizures - this is the most common effect
- Being able to reduce anti-seizure medication - we would aim for most dogs to be stable on two drugs alongside the VNS
- Having improved quality of life - this is reported in people very frequently and is also described in dogs that are treated too.
How long does the generator last?
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.