VAGAL NERVE STIMULATION IN EPILEPSY

Vagus nerve stimulation (VNS) is designed to prevent seizures by sending regular, mild pulses of electrical energy to the brain via the vagus nerve. These pulses are supplied by a device something like a pacemaker.

The VNS device is sometimes referred to as a "pacemaker for the brain." It is placed under the skin on the chest wall and a wire runs from it to the vagus nerve in the neck.

The vagus nerve is part of the autonomic nervous system, which controls functions of the body that are not under voluntary control, such as the heart rate. The vagus nerve passes through the neck as it travels between the chest and abdomen and the lower part of the brain.

What is the surgery like?
The surgeon first makes an incision along the outer side of the chest on the left side, and the device is implanted under the skin. Then a second incision is made horizontally in the lower neck, along a crease of skin, and the wire from the stimulator is wound around the vagus nerve in the left side of the neck. The brain itself is not involved in the surgery.

The device (also called an implant) is a flat, round battery, about the size of a silver dollar—that is, about an inch and a half (4 cm) across—and 10 to 13 millimeters thick, depending on the model used. Newer models may be somewhat smaller.

	Fig. 1 Cyberonics batteries	Fig. 2 VNS Implant  diagram.

The procedure usually lasts about 50 to 90 minutes with the patient under general anesthesia. Sometimes a hospital stay of one night is required. Some surgeons have performed the procedure with local anesthesia and the patient has been discharged the same day.

How is VNS used?
The neurologist programs the strength and timing of the impulses according to each patient's individual needs. The settings can be programmed and changed without entering the body, just by using a programming wand connected to a laptop computer.

For all patients, the device is programmed to go on for a certain period (for example, 7 seconds or 30 seconds) and then to go off for another period (for example, 14 seconds or 5 minutes). The device runs continuously, usually with 30 seconds of stimulation alternating with 5 minutes of no stimulation. The patient is usually not aware that it's operating.

Holding a special magnet near the implanted device causes the device to become active outside of the programmed interval. For people with warnings (auras) before their seizures, activating the stimulator with the magnet when the warning occurs may help to stop the seizure. Many patients without auras also experience improved seizure control, however.

Settings (also called stimulation parameters) set by the neurologist typically include a stimulation amplitude of 1.0 to 3.0 mA (milliamperes), a stimulation frequency of 30 Hz (hertz), and a pulse width of 500 microseconds. By adjusting these settings, the doctor not only may be able to control more of the patient's seizures, but often can also relieve side effects. One study, for instance, found that changing the pulse width eliminated pain that some patients were experiencing.

The battery for the stimulator lasts approximately 5-10 years