Adults with severe hearing loss benefit from pairing a cochlear implant in one ear with a hearing aid in the other ear, even though the sound signals from each device are very different, according to a study at Washington University School of Medicine in St. Louis published in the June issue of the Journal of the American Academy of Audiology.
The patients were better able to hear spoken words and to locate the direction of a sound with both devices turned on compared with either device alone. Additionally, the patients liked the fuller, richer sound they heard when using both devices.
"It is increasingly common to place cochlear implants in both ears when patients have profound hearing loss on both sides, but the majority of these bilateral implants are done in children," says lead author Lisa Potts, Ph.D., research instructor in otolaryngology. "Many adults lose their hearing as they age, and it may not be financially or physically possible for them to undergo surgery for two cochlear implants. So it is important to know if there is a benefit to using a hearing aid plus a single cochlear implant."
Each of the 19 study participants received a cochlear implant in one ear and a hearing aid for the other ear. Washington University surgeons at Barnes-Jewish Hospital performed the implantations. The participants were seen at the Adult Cochlear Implant and Aural Rehabilitation Division at the School of Medicine for cochlear implant programming and hearing aid fitting.
Because the participants were profoundly hearing impaired, the hearing aid restored only partial hearing in one ear, while the cochlear implant gave them a greater level of hearing in the other ear. In addition to the imbalance in sound levels, each device processes sound information in a unique way: a cochlear implant translates sounds into electrical impulses that directly stimulate the hearing nerves of the inner ear, while a hearing aid amplifies sounds so the ear can sense its acoustic vibrations. Specialists have questioned whether patients could adequately integrate the asymmetric signals from implants and hearing aids.
This study showed that when the participants used both a cochlear implant and a hearing aid, speech recognition improved by an average of 14 percent over when they used just an implant or just a hearing aid. When both devices were active, participants also made fewer mistakes in determining sound direction — they were better able to say which loudspeaker emitted sound in a semicircular array of 15 loudspeakers placed 10 degrees apart.
Interestingly, when the participants wore both devices, speech recognition and localization was equally good, no matter the direction of the sound source. That was surprising because of the lower sound correction in the hearing aid ear.
"That result really got our attention," Potts says. "It shows that even when patients have minimal hearing with a hearing aid, it still helps them get input and helps them catch important sound cues. The two inputs are complimenting each other. Hearing aids are better at giving temporal speech cues, while implants supply a fuller spectrum of sound frequencies."
Temporal information is important for music appreciation. "Patients report that they actually prefer music with a hearing aid and cochlear implant together. There's something more natural about that tone," Potts says.
Potts adds that the brain learns to integrate these two separate signals. The sound signals meet in the brainstem and cross all along the auditory pathway up to the brain's hearing centers, which interpret the signals as one sound.
When asked about their subjective sense of how well they heard with the devices, most patients said they felt they heard sound better with both devices turned on. When they were both on, they described the sound as "louder, clearer and more natural," "more complete," and having "a little extra depth, richness and volume."
The participants, eight men and 11 women, ranged in age from 26 to 79, with an average age of 50. Almost half had some hearing impairment before age six. But nearly all were adults when diagnosed with severe to profound deafness. The patients' ages or hearing history had no statistically significant effect on the results of the hearing tests conducted in the study.
The researchers are now studying whether a second cochlear implant gives additional benefit over that of an implant plus hearing aid in some of the same patients who participated in this study. But Potts indicates that for some adults with profound deafness, a single implant with a hearing aid may be the best solution.
"A high-power hearing aid may cost a couple thousand dollars, while a cochlear implant costs tens of thousands of dollars," Potts says. "Implantation is a relatively low-risk surgery, but as you grow older, elective surgery is always something that has to be carefully considered. Our studies are providing information that will help determine the best treatment options for these patients."
Potts LG, Skinner MW, Litovsky RA, Strube MJ, Kuk F. Recognition and localization of speech by adult cochlear implant recipients wearing a digital hearing aid in the nonimplanted ear (bimodal hearing). Journal of the American Academy of Audiology. 2009 Jun;20(6):353-73.
The Widex Corporation donated the hearing aids used in this study.
Washington University School of Medicine's 2,100 employed and volunteer faculty physicians also are the medical staff of Barnes-Jewish and St. Louis Children's hospitals. The School of Medicine is one of the leading medical research, teaching and patient care institutions in the nation, currently ranked third in the nation by U.S. News & World Report. Through its affiliations with Barnes-Jewish and St. Louis Children's hospitals, the School of Medicine is linked to BJC HealthCare.