Vagal nerve stimulation is a treatment for epilepsy in which an electrode is implanted in the neck to deliver electrical impulses to the vagus nerve.
Purpose
Vagal nerve stimulation is an alternative to medication or surgical removal of brain tissue in controlling epileptic seizures. The seizures of epilepsy are caused by uncontrolled electrical discharges spreading through the brain. Anti-seizure drugs interrupt this process by reducing the sensitivity of individual brain cells to stimulation. Brain surgery for epilepsy either removes the portion of the brain where seizures originate, or cuts nerve fibers to prevent the nerve impulses that occur during a seizure from spreading to other parts of the brain. Vagal nerve stimulation uses a different approach; it provides intermittent electrical stimulation to a nerve outside the brain—the vagus, or tenth cranial nerve, which influences certain patterns of brain activity.
The vagus nerve is a major connection between the brain and the rest of the body. It carries sensory information from the body to the brain, and motor commands from the brain to the body. The vagus is involved in complex control loops between these destinations; its precise pathways and mechanisms are still not fully understood. It is also not known how stimulation of the vagus nerve works to reduce seizure activity—it may stimulate inhibitory pathways that prevent the brain's electrical activity from getting out of control, interrupt some feedback loops that worsen seizures, or act in some other fashion.
Vagal nerve stimulation has been effective in reducing seizure frequency in patients whose seizures are not controlled by drugs, and who are either not candidates for other types of brain surgery or who have chosen not to undergo these procedures.
Demographics
Epilepsy affects about 1% of people in the general worldwide population. Approximately 40% of patients do not respond well to medications, however, and so may be candidates for surgical treatment. Vagus nerve stimulation was first performed in the United States in 1988 and received final approval by the United States Food and Drug Administration (FDA) in July 1997. Approximately 10,000 people worldwide have had stimulators implanted as of 2003; about a fifth of these patients are children 12 years old and younger.
Description
The vagal nerve stimulator has two parts: an electrode that wraps around the left vagus nerve in the neck; and a pulse generator, which is implanted under the skin below the collarbone. The two parts are connected by a wire. Stimulation is performed only on the left vagal nerve, as the right vagal nerve helps control the heartbeat.
Surgery to implant a VNS device takes about two hours. A neurosurgeon implants the electrode and generator while the patient is under general anesthesia. A vertical incision is made in the left side of the neck, and the helical electrode is attached to the nerve itself. A second incision is made on the left side of the chest below the collarbone, and the pulse generator (a disc about 2 in [5 cm] in diameter) is implanted under the skin. The connecting wire is threaded around the muscles and bones to join the electrode and generator. The generator makes a small bulge under the skin but is hidden by clothing after the operation.
Before the neurosurgeon closes the incisions, he or she tests the VNS device to make sure it is working, and programs it to deliver the lowest amount of stimulation. The device is usually timed to stimulate the vagus nerve for 30 seconds every five minutes.
Diagnosis/Preparation
A candidate for vagal nerve stimulation will have had many tests already to determine the focal point of seizure activity. Preoperative tests include neuroimaging as well as psychological tests to determine the patient's cognitive (thinking) strengths and weaknesses.
The patient must be fully informed about VNS—how it works, its advantages and disadvantages, what will happen during surgery—before the operation is scheduled. A video as well as written material about VNS is available to view and discuss with the doctor.
Aftercare
Implantation of the stimulator in an adult may be performed as either an outpatient or inpatient procedure. In the latter case, the patient will remain in the hospital overnight for monitoring of heart function and other vital signs. Children who are receiving a VNS are usually scheduled for an overnight stay. Pain medication is given as needed.
The stimulation parameters are adjustable, and the neurologist may require several visits to find the right settings. Settings are adjusted with a magnetic wand that delivers commands to the stimulator's computer chip. The patient may be taught how to use a magnet to temporarily increase stimulation, to prevent a seizure, or to abort it once it begins.
The VNS generator is powered by a battery that lasts several years. It is replaced during an outpatient procedure under local anesthesia.
Risks
The most common adverse effects from vagal nerve stimulation are a hoarse voice, cough, headache, and ear pain. These side effects can be reduced by adjusting the stimulation settings, and may subside on their own over time. Infection and device malfunction are possible though rare.
Patients who have had a VNS implanted must avoid strong magnets, which may affect the stimulator settings. Areas with warning signs posted regarding pacemakers should be avoided. The patient should consult with the neurologist and the neurosurgeon about other hazards.
Normal results
Approximately half of all patients who have received vagal nerve stimulation experience about a 50% reduction in seizures. Another 9% of patients obtain complete relief from seizures. Most patients who continue to take antiseizure medications can reduce their dosage, however, which offers some relief from the side effects of these drugs.
Morbidity and mortality rates
Vagal nerve stimulation is a relatively safe procedure, with no deaths attributed to the stimulation as of 2003. Pilot studies of 300 patients that were done prior to FDA approval of VNS reported the following complication rates: hoarseness, 37% of patients; coughing, 14%; voice alteration, 13%; chest pain, 12%; and nausea, 2%.
Alternatives
Some candidates for vagal nerve stimulation are also likely to be candidates for a corpus callosotomy, temporal lobectomy, or other surgical procedures.
Devinsky, O. A Guide to Understanding and Living withEpilepsy. Philadelphia: E. A. Davis, 1994.
"Seizure Disorders." Section 14, Chapter 172 in The MerckManual of Diagnosis and Therapy, edited by Mark H. Beers, MD, and Robert Berkow, MD. Whitehouse Station, NJ: Merck Research Laboratories, 1999.
PERIODICALS
Rielo, Diego, MD, and Selim R. Benbadis, MD. "Vagus Nerve Stimulation." eMedicine, April 12, 2002 [June 10, 2003]. <www.emedicine.com/neuro/topic559.htm>.
ORGANIZATIONS
American Association of Neurological Surgeons (AANS). 5550 Meadowbrook Drive, Rolling Meadows, IL 60008. (847) 378-0500. <www.neurosurgery.org>.
