NEW YORK, May 26 (UPI) -- A new implant restores a degree of hearing by venturing into revolutionary territory and stimulating nerves directly inside the brain, experts told United Press International.
The Penetrating Auditory Brainstem Implant is a neuroprosthetic device and currently is in clinical trials after 15 years of work by U.S. researchers and Australian company Cochlear in Sydney, with funding from the National Institutes of Health in Bethesda, Md.
"We began with our first patient last July and just did our fifth one two weeks ago," researcher Bob Shannon, an auditory neuroscientist at the House Ear Institute in Los Angeles, told UPI.
Deafness often is caused by damage to the cochlea, the spiral cavity of the inner ear that contains nerve endings vital for hearing. Implants in the cochlea can bypass its sound-detecting hair cells, taking electrical signals from a microphone, pulsing them directly at the auditory nerve and connecting that ear with the brain using up to 22 electrodes.
One in 40,000 people in the United States suffers from a profound, hereditary form of deafness known as neurofibromatosis type II, or NF2, which cannot be treated by any hearing aid or cochlear implant. The disorder is a genetic condition where benign tumors impinge on one or both of the auditory nerves, Shannon said. Doctors often remove the tumors, because there is a danger they can grow to pinch off vital nerves, such as those controlling breathing.
"Some people have tumors up to 6 centimeters long, with the record being 7. The average is still 4," Shannon said. "They're pretty big tumors."
Once known as "the wasting disease" for its eventual paralytic and muscle deteriorating effects, until the 1960s, the mortality rate just for removing NF2 tumors was 50 percent, although nowadays it is virtually zero. However, removing the growths usually severs both auditory nerves.
Until now, the only way to return any hearing to people with NF2 was the Auditory Brainstem Implant, or ABI. The device transmits electrical signals to the cochlear nucleus, the hearing portion of the brainstem. The brainstem connects the brain to the spinal cord and directs the vital, automatic bodily functions.
Developed at the House Ear Institute after two decades of research, and approved by the Food and Drug Administration in 2000, the ABI has been implanted in more than 300 patients worldwide. The ABI helps a user hear sounds but cannot enable understanding speech without lip reading. Cochlear implants users can understand speech over the phone because they can make out up to eight distinct audio frequency ranges known as channels of pitch information. The ABI only gives users one channel.
"It takes at least four channels of pitch information to understand speech," Shannon said. Normal hearing has at least 100 channels, he added.
The reason the ABI does not work as well as the cochlear implant is because its electrodes are only implanted on the surface of the brainstem.
"The pulses from the electrodes on the surface are probably smearing out," Shannon said. This means the ABI cannot selectively transmit electrical signals to different areas of the cochlear nucleus to produce distinct pitch channels.
The more sophisticated Penetrating Auditory Brainstem Implant, or PABI, actually inserts eight microelectrodes directly into the brainstem in the hopes of producing selective electrical stimulation. This could give users the ability to comprehend speech to a degree experienced by those with advanced cochlear implants.
"This is the first implant into the brainstem," Shannon said.
Still, surgery in such a vital area of the human brain is risky.
"You don't want to cause any kind of damage there at all. That's why it took us 15 years, to be absolutely sure we could do it with complete safety," Shannon explained.
The implant uses eight electrodes made of iridium, chosen for its strength and excellent electrical conductivity. It is only 75 microns wide, thinner than a human hair, with a tip only 5 microns wide, less in diameter than a blood cell. Each electrode tip stimulates a region of brain only 100 microns across.
"The most interesting thing is the electrode technology developed by our partners at the Huntington Medical Research Institutes (in Pasadena, Calif.) that could penetrate the brainstem without any ill effects," Shannon said. "The secret is in the tip."
If the electrodes are too sharp, he said, they can slice right through the microvasculature, causing little hematomas around the shaft. If they are too blunt, they can cause blunt tissue injury -- or, they cannot penetrate.
"We've controlled the exact shape of the tip, and it seems to go in without causing any damage," Shannon said. "The tissue is always in perfect condition around the electrodes, and for some reason it's even self-sealing."
After performing surgery to remove the tumors, doctors implanted PABIs along with ABIs into the cochlear nucleus, a target about 8 millimeters by 2 millimeters in size. The penetrating electrodes are 1 millimeter to 2 millimeters long.
"We want different depths in the nucleus to reach different pitches," Shannon said.
The benefits have proved encouraging, he said. In the best result so far, the patient could recognize 14 percent of words and sentences in initial tests, "which we've never seen before with surface electrodes right away," he said.
"The fact she could get that immediately is a very good sign," Shannon added.
The researchers hope the PABI recipients experience improvements over time, as it can take up to two years before a person adapts to a normal ABI. If the PABI approach continues to work, the researchers would eventually like to add more electrodes in devices for future patients.
"This is breakthrough research. It's giving hearing back to people who don't have it at the present time, who have no alternative," audiologist Honor O'Malley at Columbia University in New York told UPI. "I would like the FDA to permit further research on this particular implant device. I'm very excited about this."
Side effects occasionally seen include "a tickling, tingling sensation, along the side, or arm, or leg, or even head," Shannon said. "If we see them, we just turn those electrodes off."
The research team is now hoping the FDA will authorize tests on up to 25 more patients.
Although NF2 is a rare disease, this research "gives us leverage on understanding how the normal acoustic system works by artificially stimulating it in ways that can't occur normally," Shannon said. "And along the way, we can help these people. The better we understand how the system works, the more we can feed it back to the next generation of devices and improve them."
There is also the possibility the electrodes can find use in other, future neuroprosthetics, such as "in the spinal cord for pain control or to help quadriplegics to help in their motion," Shannon said.
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Charles Choi covers research for UPI Science News. E-mail [email protected]
