The vagus nerve is one of 12 cranial nerves that pass information to and from the brain. The vagus nerve is the longest of the cranial nerves, with connections to the brain and many of the organs of the body. Intermittent electrical stimulation of the vagus nerve seems to reduce or prevent seizures.
Vagus nerve stimulation involves a surgical procedure to implant an electrical pulse generator in the chest and attach electrodes to the vagus nerve in the neck. A pulse generator is similar to a pacemaker. It is about the size of a pocket watch and weighs about 25 grams (g).
How does vagus nerve stimulation work?
The pulse generator is surgically implanted in the chest just under the skin. This generator is programmed to intermittently transmit small electrical signals via a wire to the vagus nerve in the neck. We do not understand why electrical stimulation of the vagus nerve is effective at preventing seizures. It may be related to changes in the brainstem and thalamus, or the electrical stimulation may intercept the abnormal brain activity associated with seizures.
The generator functions automatically 24 hours a day. An external programming wand and software are used by the doctor, to read and alter the stimulation strength and frequency. After the generator is first implanted, the usual starting settings are 30 seconds of stimulation every 5 minutes. The strength of the electrical signal usually starts at 0.25 or 0.5 milliamperes (mA) and is then increased every few months by 0.25 mA. The frequency of the stimulation can also be adjusted, to come on and off more frequently.
Patients are also given a magnet, which acts as a switch for the generator. When a patient senses the beginning of a seizure, she can pass the magnet over the pulse generator to deliver extra stimulation. For some people, this extra stimulation can stop a seizure from progressing, shorten a seizure, or reduce the intensity of the seizure. The magnet can also be used to turn the stimulator off temporarily.
The battery life of the generator is about six years, after which the device can be replaced. This will require another surgical procedure, similar to the first.
What and who is vagus nerve stimulation best used for?
Vagus nerve stimulation has been used to treat partial and generalized seizures in both adults and children. It is considered in the following cases:
Anti-epileptic medication does not control seizures effectively.
The child is not a good candidate for brain surgery, either because of high risk or because he has various epileptogenic (seizure-generating) areas in his brain.
The child is large enough to support the implant. This is usually decided by the neurologist and surgeon.
Vagus nerve stimulation should not be used in children with certain cardiac, respiratory, or gastrointestinal problems. The vagus nerve serves the heart, lungs, and stomach, and some existing problems in these areas may lead to complications with vagus nerve stimulation.
Vagus nerve stimulation usually reduces seizures but does not often eliminate them completely. Usually it is used together with medication.
What is the procedure?
A medical evaluation is necessary to determine if this is a case that may be successfully treated with vagus nerve stimulation.
Once it has been decided that your child may be helped by vagus nerve stimulation, the surgeon, anaesthetist, and others on the medical team will explain the procedure to you in detail. It's important that you understand the procedure, its risks, and your child's future care; feel free to ask them any questions you may have. With the help of the treatment team, you should then explain the procedure to your child and help him understand what to expect after the implantation.
Your child will need to be admitted into the hospital to install the device. Your child will probably need to be put to sleep under general anaesthetic, and therefore will need to stop eating eight hours before. The operation can be done with a local anaesthetic, but because it is hard for children to stay still, most require a general anaesthetic.
The procedure itself takes about one hour. One cut about 5 cm long is made on the left chest wall to implant the pulse generator. Another cut about 5 cm long is made in the neck, through which flexible platinum electrodes are attached to the vagus nerve. Because the vagus nerve also goes to the heart, the generator will be turned on in the operating room so that the surgeon can make sure the device does not interfere with the heart. For the same reason, the electrodes are almost always placed on the left vagus nerve, which has less critical influence on the heart.
Your child may be sent home on the same day or kept overnight for observation. You and your child will be taught how to care for the incisions before discharge.
Using vagus nerve stimulation
After the pulse generator is implanted, the doctor will start the stimulation, using a wand attached to a laptop computer. The doctor will begin with a low dose of current, given for 30 seconds every five minutes. The strength and duration of the stimulation will be slowly increased, at subsequent doctor visits, to achieve optimum seizure control. The stimulation is delivered automatically, 24 hours a day.
Your child will also be given a special magnet for use with the pulse generator. Children who experience auras (a feeling that signals the onset of a seizure) can use the magnet to give an extra burst of stimulation, which may reduce or eliminate an imminent seizure. You can also use the magnet if you see that your child is about to have a seizure. The magnet can also be used to turn off the pulse generator temporarily and to check its operation. The doctor will make sure that you and your child understand how to use the magnet before you leave the hospital.
Your child will need to continue taking his medications. The dosage will be kept constant for several months while the generator's output current is slowly increased. Once the current has been increased to its optimal level, your child's doctor will consider reducing medication and will discuss a new schedule with you.
Regular monitoring is required to check the generator and battery and to make sure the treatment is working well and is not uncomfortable. The battery life of the generator is about six years, after which the device may be replaced. This will require another surgical procedure, similar to the first.