The device delivers small pulses of electrical current to the vagus nerve -- one of the major nerves leading to the brain. Though it is not yet understood precisely how it works, most patients see a reduction in the number and severity of seizures after the device is implanted. The median reduction in seizure frequency is about 50%. It can help control seizures that have been resistant to other treatments. Part of the evaluation for VNS is to be sure that some other surgical or medical therapy might not be more effective.

The device consists of two parts: a pulse generator -- a smooth metal disk about the size of a half-dollar, and the electrodes -- thin, plastic-coated wires which run from the pulse generator under the skin, up the left side of the neck and are wrapped carefully around the vagus nerve. A surgeon implants the pulse generator under the patient's collarbone or into the armpit area, then threads the wires under the skin to a second incision over the vagus nerve. The surgery usually takes less than an hour, and is often done on an outpatient basis.

Because the stimulator is implanted, it is an automatic means of seizure control and can be very helpful for patients who have trouble following a medication routine or have severe side effects from multiple medications. Few patients achieve complete seizure control with VNS, but many are able to reduce their medications, giving them greater flexibility and confidence.

The device is set to stimulate the nerve at regular intervals. One typical setting would be 30 seconds out of each 5 minute interval, but a neurologist will fine-tuned the settings in the weeks following surgery. Some patients experience unpleasant symptoms during stimulation such as hoarseness or a change in voice quality, trouble swallowing and a tingling sensation in the neck. Each patient is also given a small flat magnet which is used to start stimulation as a seizure comes on or to prevent stimulation for a period of time (such as while speaking or singing in public).