A sound idea helps partial hearing loss

Hybrid cochlear implant clarifies - not amplifies - sound for users

December 15, 2006|By Regina Nuzzo | Regina Nuzzo,Los ANgeles Times

Jeanne Yeoman had been dealing with her hearing loss for a couple of decades, but listening still exhausted her. And technology wasn't really helping her patience. She remembers driving down the road one day and coming close to just hurling her hearing aids out the window.

"Hearing aids made everything louder, not clearer," she says. "I didn't need amplification. I needed clarification."

Yeoman wasn't deaf. So she was surprised to learn that she was an ideal candidate for an experimental type of cochlear implant. Unlike hearing aids, cochlear implants communicate directly with the brain by converting sounds into electrical impulses and shooting them along the auditory nerve.

Until now these devices have been used only for profoundly deaf people. But this new hybrid cochlear implant was designed specifically for partial hearing loss -- so that users could enjoy their own natural hearing plus bionic hearing for sounds where they need an extra boost.

Five years after surgery that implanted the device in her inner ear, 34-year-old Yeoman of Humboldt, Iowa, sometimes even forgets it's turned on. "Everything sounds so crystal-clear," she says.

Good solutions are scarce for many people with hearing loss, including growing numbers of aging baby boomers. A large number of the 28 million hard-of-hearing Americans have what is known as a "ski-slope" loss, in which their ability to hear high-pitched sounds plummets.

They can hear sounds such as "aah" and "ooh" quite plainly, but not "ssss" or "shhh." Unfortunately, the latter types of sounds give speech the lion's share of its understandability. Speech doesn't necessarily sound quiet; it sounds muddy.

Even at full blast, hearing aids often can't help enough, says Dr. James Battey, director of the National Institute on Deafness and Other Communication Disorders at the National Institutes of Health. "This type of hearing loss can become extremely socially isolating," he says.

Traditional cochlear implants aren't a good answer. By bypassing damaged inner ears to stimulate auditory nerve fibers directly, these devices can be a boon for some deaf people. But the procedure -- which involves threading a tiny bundle of electronics into the inner ear through a hole in the skull -- aims to replace a patient's entire range of hearing. Any natural abilities usually get wiped out by the surgery.

With the new hybrid implant, however, surgeons hope simply to supplement natural hearing without destroying it, says Dr. Bruce Gantz, professor of otolaryngology at the University of Iowa and developer of the device.

The secret lies in the inner ear's design. Normal hearing is sort of a Rube Goldberg process. First, sound waves enter the ear as rhythmic pulses, which set the eardrum vibrating in sync.

This triggers quivering in three tiny bones, with the last bone hammering against the entrance to the inner ear. In response, fluid sloshes in rhythmic waves throughout the corridors of the snail-shaped cochlea, which alerts sensory cells to electrically stimulate auditory nerve fibers.

Strangely enough, the cochlea is laid out like a coiled piano keyboard: Cells along the corridors are tuned to particular frequencies entering the ear. In the case of a low-pitched sound, cells tucked away deep inside the cochlea alert the auditory nerve; cells that respond to high notes sit close to the cochlea's entrance. That's fortunate -- because cochlear regions where "ski-slope" patients need a boost are those most accessible to surgeons.

Compared with traditional implants, hybrid systems use a thinner, shorter bundle of electronics (10 millimeters in length compared with up to 28 millimeters for traditional implants). This short electrode is positioned just at the opening end of the cochlea, stimulating the auditory nerve only when high-frequency sound waves enter the ear.

Since surgeons don't need to probe as deeply into the delicate cochlea, tissue trauma is reduced. Preserved natural hearing, amplified with a hearing aid if necessary, gives patients an easier time in tough situations, such as crowded restaurants or concert halls. The added high-frequency electronic hearing clears up muddy speech.

Since 1999, about 80 patients have received the hybrid device, Gantz says, and clinical trials are under way at 15 U.S. sites. Preliminary results, released last month, reported that surgeons in the trial have been able to retain hearing in about 96 percent of the patients. Before surgery, patients were able to understand about one-third of words on standard hearing tests. After one year or more with the implant, scores increased to an average of 75 percent.

Hybrid implant users also function better than traditional implant users in noisy situations, says Christopher Turner, audiology professor at the University of Iowa and a study investigator. They are far more able to follow and appreciate music.

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