Hearing health professionals have always known we hear better when our ears work together. Now a grant by the National Institutes of Health (NIH) will help researchers at the Washington University of Medicine in St. Louis study how it affects us when they don't.
Hearing loss can be the same in both ears (symmetrical) or different in each ear (asymmetrical). Because the brain uses signals from both ears to identify things like sound direction and distance from the listener, those with asymmetrical hearing may have difficulty hearing voices in a noisy environment or judging the distance of an emergency vehicle's siren. These individuals typically experience greater communication difficulties as a result.
"We have two ears for a reason," Jill B. Firszt, Ph.D., associate professor, audiologist, and director of the Cochlear Implant Program in the Department of Otolarnygology at the School of Medicine, said. "The auditory system is designed to be binaural - to receive input from both ears. It is a highly interactive system, rather than two independently operating ears."
Firszt and her research team will use the five year, three million dollar grant to continue the work she started in 2008. She and her colleagues studied patients who were deaf in one ear and had varying levels of hearing in the other, including those with normal hearing and those who used a hearing aid or had a cochlear implant. Those studies showed that individuals with normal hearing in one ear and deafness in the other perceived the same type of difficulties as those who used a hearing aid or cochlear implant and were deaf in the other.
Unilateral hearing loss can be caused by hereditary or genetic factors, trauma to the head, Meniere's disease, acoustic neuromas and exposure to excessive noise on one side of the head. Those with unilateral hearing loss often have trouble hearing in one ear and understanding speech when background noise is present, even with normal hearing in the other ear.
Firszt's research team will use neuroimaging to define changes the brain makes as it works to adapt to unilateral hearing loss so they can better understand the extra effort required by unilateral listeners.The new studies will help define the time period for restoring hearing when the loss is unilateral and identify patients who will benefit from the procedure.
"We know the brain copes with asymmetric hearing by reorganizing over time," she said. "We want to learn where and how quickly that reorganization occurs and how it impacts successful outcomes when hearing is restored years later through cochlear implants."
The studies will be conducted on both children and adults. According to Firszt, not much is known about how asymmetrical hearing affects infants or children. While Firszt and her colleagues have observed improved speech recognition and other benefits when adults were fit with a cochlear implant in their deaf ear and continued to use a hearing aid in their better ear, those outcomes were reduced in patients whose unilateral hearing loss occurred during childhood, before speech and language was learned.
In the United States, children and adults who are deaf or severely hard of hearing can be fit with a cochlear device. According to 2012 statistics by the Food and Drug Administration (FDA), approximately 58,000 adults and 38,000 children have received implants. These devices, along with postimplantation therapy, can help adults who have lost hearing later in life understand speech and young children acquire speech, language and social skills. According to the FDA, more than 90 percent of all commercial health plans, Medicare, Medicaid and the Veteran's Administration cover cochlear implants because they are recognized as standard treatment for severe-to-profound nerve deafness.
Washington University is also participating in clinical trials of the same nature. Participants enrolled at one of four U.S. sites will receive a cochlear implant in their non-hearing ear. Although only severe to profoundly hearing impaired individuals are traditionally considered as candidates to receive cochlear implants in the United States, the data from these trials may become the basis for a larger study with the goal of obtaining approval for the procedure from the Food and Drug Administration.