New Implanted Receiver Helps Restore Hearing
“Last week, I was actually able to recognize a sound on the radio on my way to Los Angeles,” said Christine P. Arcia, 27, a baker from Fresno. Such an assertion might sound mundane. But for Arcia, who is deaf, the statement borders on the miraculous.
The miracle in this case has been made possible by the development of new technology known as the multichannel auditory brain stem implant (ABI), a collaborative effort between the Cochlear Corp. and the House Ear Institute, a private research organization that has been working with related auditory implant technologies for the last 15 years.
The new device was developed for patients with neurofibromatosis (NF2), a genetic condition characterized by the growth of tumors along the spinal cord and the auditory nerves. In most cases, the tumors become life threatening at some point and must be removed, which usually requires severing the auditory nerves.
When the nerves are cut, complete deafness in both ears results, with no hope of help from either hearing aids or cochlear implants. About 4,000 people in the United States are afflicted with NF2, and researchers say the work on the NF2 implant will help in developing treatments for many other kinds of hearing impairment.
“One surprising thing that has resulted from this research is that we are discovering that people can understand speech with much less information than we thought was necessary,” says Dr. Robert V. Shannon, director of the auditory implants department of the House Ear Institute. “Once we’ve understood this process, we’ll be able to design more efficient hearing aids and cochlear implants,” which will benefit a far larger number of hearing-impaired people.
*
The NF2 device includes a tiny radio receiver implanted underneath the skin and eight 1-millimeter platinum electrodes implanted in the part of the hearing mechanism known as the cochlear nucleus.
Sounds, amplified and clarified by a special, shirt-pocket computer and a coil that slips onto the ear like a conventional hearing aid, are collected by the receiver, converted to electrical pulses and transmitted to the electrodes. From there the signals travel by a wire and connect to the brain stem at the same place the auditory nerve would normally connect.
In 1992, the first NF2 patient was implanted with the device, and 26 patients have received it to date. There is great variability in terms of what and how much they can hear.
“Most patients can hear some sound, although it doesn’t resemble at all what they remember,” Shannon says. “The sounds are often muffled, or buzzing, or electronic-type sounds.”
“In the patients that do very well, we’ve noticed that they seem to have more of a range of pitch across the eight electrodes,” Shannon says.
Based on these observations, Shannon and his colleagues, under contract from the National Institutes of Health, are working on the next generation of auditory devices, which will utilize electrodes that penetrate into the brain stem in order to stimulate directly specific pitch areas.
