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Implant retrains the brain to hear sounds

January 25, 2010 12:00:00 AM PST
Doctors are turning to a new type of implant that helps retrain the brain and open up a new world of sound.Danny Sanborn wishes he could go back to a time before his world went silent.

He suffers from a disease that causes brain tumors. In order to get the tumors out, doctors had to sacrifice his auditory nerve.

"Before I had the tumors I had hearing, but as soon as they took the tumors out I didn't have any," said Sanborn.

A cochlear implant wasn't an option, so surgeons put in an auditory brainstem implant or ABI to restore certain sounds.

"We are bypassing the auditory nerve directly to stimulate the next level of sound processing in the brain," said Daniel Lee, M.D., a surgeon at Massachusetts Eye and Ear Infirmary in Boston, Mass.

Surgeons place 21 electrodes on the bundle of nerves that sit on the brain stem, the lower part of the brain.

"The electrodes provide electrical current," said Dr. Lee. "It stimulates those nerves that are responsible for continuing a signal of sound to the rest of the brain."

A tiny microphone is positioned by the ear. It picks up sounds from the environment and digitally transmits them to a decoding chip placed under the skin. The chip stimulates the electrodes, allowing the patient to hear sounds.

It doesn't restore complete hearing, but Sanborn's now able to hear the phone, an alarm clock and even his best friend.

While the device can restore some hearing, it can't restore a patient's balance that's damaged by the tumors. Up to one in 25,000 Americans suffer from the condition that causes hearing loss like Danny Sanborn's. It's called NF2.

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An auditory brainstem implant (ABI) is a device that is surgically implanted into the brain of a deaf person who lacks auditory nerves or with a damaged auditory nerve. The auditory nerve is responsible for sending signals from the ear to the brain.

The implant consists of a small electrode or multiple electrodes applied to the brainstem where it stimulates sound-producing nerves with electrical signals. The device also includes a small microphone in the outer ear to pick up sounds from the environment and a decoding chip placed under the skin. A wire connects the decoding chip to the electrode at the brainstem.

While an ABI doesn't restore a full hearing sense, it enables a deaf person to hear some sounds like sirens and alarms and makes it easier to lip read. An ABI is different from a cochlear implant, which is implanted in the inner ear, converts speech and other sounds into electrical signals, and sends these signals to the hearing nerve.

According to the Mayo Clinic, the placement of an ABI is only the first step in restoring partial hearing. After the surgery, a patient visits an audiologist for several sessions to test and adjust the sound processor and learn new sounds. While some patients implanted with an ABI develop word recognition, others combine general sound cues with lip reading to improve conversation skills.


Most often, candidates for an ABI suffer a condition called neurofibromatosis type II, or NF2. This condition affects as many as one in 25,000 people, according to the National Institutes of Health, and is characterized by brain tumors that grow on the hearing and balance nerves. Removing the tumors often damages affected nerves and leads to deafness. According to the National Institutes of Health, more than 200 mutations have been identified with the condition, which are often inherited from an affected parent.

Thirty-five-year-old Danny Sanborn, who has NF2, was the first patient in New England to get an ABI implant. In an open-brain surgery, surgeons implanted 21 electrodes to replace his damaged hearing nerves. After 10 years of deafness, Sanborn's doctor says he can now hear sounds including doorbells, sirens, phones and voice sounds that help him lip read.