Researchers at Mass Eye and Ear have developed a word-scoring model that can estimate the amount of hidden hearing loss in the human ear.
In a new study published June 23 in scientific reports, a team of researchers from Mass Eye and Ear’s Eaton-Peabody Laboratories determined mean speech scores as a function of age from data from nearly 96,000 ears examined at Mass Eye and Ear. They then compared the data with a previous study at Mass Eye and Ear that tracked the average loss of cochlear nerve fibers as a function of age. By combining both data sets, researchers have estimated the relationship between speech scores and nerve survival in humans.
According to lead study author Stéphane F. Maison, PhD, CCC-A, principal investigator of the Eaton-Peabody Laboratories and associate professor of Otolaryngology-Head and Neck Surgery at Harvard Medical School, the new model leads to better evaluations of cochlear nerve damage in patients and the associated speech intelligibility disorders associated with the neural loss. The model also provides ways to estimate the effectiveness of hearing loss interventions, including the use of personal sound reinforcement products and hearing aids.
Prior to this study, we were able to estimate neural loss in a living patient using a long battery test or measure cochlear nerve damage by removing their temporal bones when they have died. Using common speech scores from hearing tests — the same ones collected in clinics around the world — we can now estimate the number of neural fibers missing from a person’s ear.”
dr. Stéphane F. Maison, PhD, CCC-A, lead author of the study
Discover hidden hearing loss
Two important factors determine how well a person can hear: audibility and intelligibility. Hair cells, the sensory cells in the inner ear, contribute to the audibility of sounds — or how loud a sound has to be to be detectable. When receiving a sound, hair cells transmit electrical signals to the cochlear nerve, which then relays these signals to the brain. How well the cochlear nerve transmits these signals contributes to the clarity or intelligibility of sound processed in the central nervous system.
For years, scientists and clinicians believed that hair cell decline was the leading cause of hearing loss and that damage to the cochlear nerves was not widespread until after the hair cells were destroyed. Audiograms, long considered the gold standard for hearing tests, provide information about hair cell health. Because nerve loss was believed to be secondary to hair cell loss or dysfunction, patients with a normal audiogram received a clean bill of health despite having trouble hearing in noisy environments. Experts now understand why the audiogram is not informative about auditory nerve health.
“This explains why some patients who report problems understanding conversation in a crowded bar or restaurant may undergo a ‘normal’ hearing test. Likewise, it explains why many hearing aid users who receive amplified sounds still have difficulty with intelligibility of speech,” says Dr. said Maison.
In 2009, M. Charles Liberman, PhD, and Sharon Kujawa, PhD, lead researchers at Eaton-Peabody Laboratories, changed the way scientists thought about hearing when they discovered hidden hearing loss. Their findings revealed that cochlear nerve damage preceded hair cell loss due to aging or noise exposure and suggested that, by failing to provide information about the cochlear nerve, audiograms had not assessed the full extent of damage to the ear.
Building a model to predict cochlear nerve damage
In the study, Dr. Maison and his team used a speech intelligibility curve to predict what a person’s speech score should be based on their audiogram. They then measured the differences between the predicted word recognition scores and those obtained during the patient’s hearing evaluation.
Since the list of words was presented at a level well above the patient’s hearing threshold — where audibility is not an issue — any difference between the predicted and the measured score would have reflected a lack of intelligibility, explained Dr. House out.
After considering a number of factors, including the cognitive deficits that can come with aging, the researchers argued that the size of these discrepancies reflected the amount of cochlear nerve damage or hidden hearing loss a person had. They then applied measurements of neural loss from existing histopathological data from human temporal bones to devise a predictive model based on a standard hearing exam.
The findings confirmed a link between poorer speech scores and greater amounts of cochlear nerve damage. For example, the worst scores were found in patients with Ménière’s disease, consistent with studies of temporal bones showing dramatic loss of cochlear nerve fibers. Meanwhile, patients with conductive hearing loss, drug-induced, and normal age-related hearing loss — etiologies with the least amount of cochlear nerve damage — showed only moderate to minor discrepancies.
Changing the landscape of hidden hearing loss research and beyond
According to the World Health Organization, more than 1.5 billion people live with some degree of hearing loss. Some of those individuals may not qualify as candidates for traditional hearing aids, especially if they present with mild to moderate high frequency hearing loss† Knowing the extent of neural damage should inform clinicians of the best ways to meet a patient’s communication needs and offer appropriate interventions in addition to using effective communication strategies.
This new research was part of a five-year $12.5 million P50 grant from the National Institutes of Health to better understand the prevalence of hidden hearing loss.
By identifying which patients are most likely to have cochlear nerve damage, Dr. Maison that this model can help clinicians assess the effectiveness of traditional and newer sound reinforcement products. The researchers also hope to introduce new audiometric protocols to further refine their model and offer better interventions by evaluating word performance scores in noise, rather than in silence.
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