Better Hearing in Real-Life Situations: Advances in Cochlear Implants
Background: Using state-of-the-art technology, cochlear implants (CIs) provide electrical stimulation directly to the auditory nerve, bypassing the damaged cochlea, which is usually the cause of deafness. Although the vast majority of CI users can understand speech in a quiet room, and report improvement in quality of life, they still report having continued difficulties locating sounds and understanding speech in everyday noisy environments. Given the growing population of CI users (approximately 96,000 worldwide), a key challenge is to improve CI users’ ability to localize sound and understand speech in noisy environments. In the past year, several groups of NIDCD-supported scientists have addressed these issues by improving the quality of the sound input into the brain and promoting the health of the existing auditory nerve cells and fibers.
Advances: One group of NIDCD-supported scientists is clinically testing a new CI design with a shorter electrode that is inserted into the base of the cochlea to restore hearing at high frequencies, while preserving residual low frequency hearing in the top of the implanted cochlea. The scientists observed that residual hearing is preserved with the use of this electrode. In addition, individuals implanted with the short electrode showed better speech recognition in a noisy environment than individuals with the traditional long electrode.
Another group investigated the benefits of bilateral cochlear implantation (Bi-CI) (a cochlear implant in each ear) in both adults and children. Results showed that particpants were significantly better at localizing sounds and hearing speech in a noisy room when they wore Bi-CIs compared with a single CI. In addition, within 1 to 2 years, children with Bi-CIs learn how to locate sounds, and the majority of Bi-CI children localized sounds better with two ears than one.
Finally, another group developed a new CI design capable of delivering drugs to the inner ear. A drug called brain derived neurotrophic factor (BDNF) provides significant enhancement of auditory nerve function when delivered into the inner ear of guinea pigs. This finding suggests that BDNF delivery might be used to counteract the degeneration of the auditory nerve typically seen following hearing loss. Neural sensitivity to electrical stimulation (ES) was significantly improved in animals receiving this drug as compared to those receiving electrical stimulation alone; sensitivity to ES is an important factor for the successful function of a cochlear implant. Based on these promising results, future cochlear implant designs could include a drug delivery system in order to improve the long term health of the auditory nerve and thus maximize the individual’s ability to hear with this device.
Implications: Individuals who are deaf and have a CI work hard in order to be able to hear, learn, and play in a hearing world. Improving sound localization (with Bi-CIs) and preserving residual hearing and nerve health can boost quality of life for hearing-impaired individuals who use a CI.
Citations: Gantz BJ, Turner C, Gfeller KE, Lowder MW, Preservation of Hearing in Cochlear Implant Surgery: Advantages of Combined Electrical and Acoustical Speech Processing. Laryngoscope 115: 796-802, 2005.
Litovsky R, Johnstone P, Godar S, Agrawal S, Parkinson A, Peters R, Lake J, Bilateral Cochlear Implants in Children: Localization Acuity Measured with Minimum Audible Angle. Ear and Hearing, 27(1): 43-59, 2006.
Shepherd RK, Coco A, Epp SB, Crook JM, Chronic Depolarization Enhances the Trophic Effects of Brain-Derived Neurotrophic Factor in Rescuing Auditory Neurons Following a Sensorineural Hearing Loss. J Comp Neurol 486: 145-158, 2005.
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