The FDA has approved a first-of-its-kind implantable electrical nerve stimulator device to treat severe depression.

The device, called the Vagus Nerve Stimulation (VNS) System, was approved for adult patients with long-term or recurrent major depression that has not responded adequately to four or more antidepressant treatments.

VNS consists of a stopwatch-sized device that's surgically implanted in the upper chest. Tiny wires attach the device to the vagus nerve, which runs from the neck to the brain. The electrical stimulation is thought to alter the chemical transmitters that carry messages between nerve cells involved in regulating mood. The maker, Cyberonics, is a WebMD sponsor.

Read WebMD's "FDA Panel Recommends Device for Depression"

Approximately 20 percent of depressed Americans, or approximately 4 million people, experience chronic or recurrent depression that has failed to respond to multiple antidepressant treatments including antidepressant medications, talk therapy, and in some cases, ECT (electroconvulsive therapy), according to Cyberonics.

Vagal nerve stimulation is already used to treat epilepsy.

More than 32,000 patients worldwide have used the VNS Therapy System, according to a Cyberonics news release.

The device was previously submitted for approval in August 2004 for depression, but the FDA rejected it at that time, asking for additional information.

According to Cyberonics,the most common side effects from VNS therapy include hoarseness, a prickling feeling on the skin, and increased coughing. These side effects diminish over time, they add. As with any surgery, there is a risk of infection.

Take WebMD's "Quiz: Could You Be Depressed?"

By Michael W. Smith, MD, reviewed by Brunilda Nazario, MD

SOURCES: News release, FDA. News release, Cyberonics. WebMD Medical News: "FDA Rejects Implanted Device for Depression."