MIT Technology Review 05/24/2010
“Taming Tinnitus with Electrical Stimulation”,
Stimulating the vagus nerve may help block phantom sounds by reorganizing the brain.
Electrically stimulating the vagus nerve, which connects the brain and the visceral organs, could help temper the phantom sounds that plague tinnitus sufferers. Researchers from Microtransponder, a Dallas-based startup developing wireless stimulation technology, reported at a neurotechnology conference in Boston this week that the approach works in animals with auditory damage that mimics the disorder. The company is adapting its neurostimulation technology, currently being developed for chronic pain, to target the vagus nerve.
Tinnitus, the false perception of ringing or other sounds in the ear, affects millions of people worldwide. Most often associated with hearing loss, it has become an especially common problem in soldiers exposed to loud blasts. The severity of the disorder varies widely, from relatively benign to debilitating, and the few existing treatments tend to mask the intrusive sound rather than eliminate it.
While it's unclear exactly what causes tinnitus, research suggests it arises from the brain's attempt to compensate for hearing loss. Damage to the inner ear, which translates sound vibrations into neural signals for the brain, results in less input to the brain's auditory pathways. The brain appears to try to make up for this loss of input by increasing activity, which may in turn result in phantom sounds.
Michael Kilgard, a neuroscientist at the University of Texas, aims to reverse this maladaptive reorganization using a combination of electrical stimulation and sound. Kilgard has previously shown that stimulating part of the brain called the nucleus basalis while playing a particular tone triggers the auditory cortex to reorganize to become hyper-responsive to that tone. To treat tinnitus, the idea is to stimulate this area while playing all sound frequencies except the one corresponding to a patient's phantom sound, thus signaling to the brain to become more responsive to all these other frequencies. If successful, this would rebalance the auditory cortex.
Rather than targeting the brain directly in humans, Kilgard turned to the vagus nerve, part of the nervous system that connects the stomach, liver, and other organs to the brain. Implanted devices that stimulate the vagus nerve are currently approved to treat depression and epilepsy and are being tested for other disorders.
Researchers plan to test the concept in people with tinnitus in upcoming clinical trials in Belgium. Kilgard says the researchers will use simple electrodes, which are implanted at the neck and stimulated with an external device. While the exact parameters are still to be determined, patients will undergo treatment for half an hour to an hour each day, for days or weeks. Unlike vagus nerve stimulation for epilepsy, which involves chronic stimulation, treatment for tinnitus will likely be for a limited period of time, researchers say.
In conjunction with these clinical tests, Microtransponder is modifying its existing technology for tinnitus. Unlike other stimulation devices, Microtransponder's system is wireless and has no batteries. The implanted portion consists of small electrodes and a small coil. An external battery-powered coil worn like a cuff on the arm or leg powers the device. "The idea would be to inject the wireless device and then put a coil around the neck to activate it [during a treatment session]," says Kilgard. "If the tinnitus comes back five years later, the device is still there and you can do the treatment again.
Harvard's Melcher says the approach is very interesting, though "whether it works is an open question." She points out that "we are still trying to sort out what aspects of brain plasticity are involved in tinnitus. There may be different kinds of tinnitus, with different types of brain activity giving rise to the perception of sounds that aren't there." All of these may require different treatments.