9 June 2004--On 15 June, an expert advisory panel to the U.S. Food and Drug Administration is set to recommend whether an electronic implant should be approved for use in the United States to treat depression. Built by Cyberonics, of Houston, the device is an implanted nerve stimulator that the company says helps alleviate the symptoms of chronic recurrent depression in people for whom drug therapies fail. In the United States depression strikes about 19 million adults. Of those, about 20 percent are not helped by drug therapies such as antidepressants like Prozac and Paxil.
The FDA usually follows the advice of its panels. If the agency sanctions the stimulator, it will become the first device-based therapy for a psychiatric illness to make it through FDA's approval process.
About the size of a pocket watch, the nerve stimulator looks and acts much like a cardiac pacemaker, but instead of sending electric pulses to the heart, it sends them to the left vagus nerve in the neck. This nerve, otherwise known as the cranial nerve X (ten), regulates functions like heart rate and the muscle tone in the gut. But about two decades ago, scientists discovered that vagus nerve stimulation (VNS) sends signals up into the brain that can quell epileptic seizures. In 1988 the first VNS device was implanted in an epileptic human, and today more than 25 000 people rely on them. The implantation can be done as an outpatient procedure.
During the first ten years experience with the device, doctors began to notice improvements in their epilepsy patients' moods that seemed unrelated to the fact that they were having fewer seizures. Looking more closely at the phenomenon, scientists found, among other things, that VNS changed the way blood flows in particular areas of the brain in a pattern similar to that seen in treatment with Prozac-like drugs. Cyberonics began its first depression trials in 1998, and so far about 350 patients have received the experimental treatment.
"There is a clinical need for new treatments for depression that does not respond to medications," says Mark S. George, an associate professor of radiology, psychiatry, and neurology at the Medical University of South Carolina in Charleston. Along with psychotherapy and drugs the only other approved treatment for depression is electroconvulsive therapy (ECT), in which seizures are induced by shocking the brain through electrodes placed on the scalp. (ECT was approved by FDA through a grandfather clause, because its use predated the agency's control over such devices.) Like VNS, electroconvulsion is seen as a treatment of last resort, but the two treatments are used differently. ECT's strength is as a way out of acute episodes of depression. Once the treatment is over, however, doctors have little option but to put the patients back on the medications that were not working before, according to George. In contrast, VNS seems to work best as a long-term therapy.
How VNS works to alleviate depression is still under study. "We have very little idea about the mechanisms [by which VNS therapy works], but that is also true of Prozac and the other medications we use, as well as electroconvulsive therapy," says George, who has treated patients using both VNS and ECT. And while all therapies may ultimately play on the same, still unknown, neurological mechanism to control depression, they get to it by very different pathways. George points out that VNS is commonly used to quell seizures, while electroconvulsion is used to produce them intentionally.
A third electrically mediated treatment for depression is undergoing clinical trials as well. Transcranial magnetic stimulation is the application of strong, focused magnetic fields through the skull to parts of the brain where they induce electric currents that alter brain activity. Neuronetics Inc., of Malvern, Pa., is spearheading the trials, which are expected to be complete in 2005.
Cyberonics's VNS device is already approved for treatment resistant depression in Canada and the European Union. In addition to its use in depression, it is also being experimented with as a treatment for anxiety disorders, Alzheimer's disease, bulimia, and migraine headaches.