In one of the largest retrospective studies of its kind, researchers analyzed data from about 96,000 ears and created a word-scoring model that can determine the amount of hidden hearing loss or cochlear nerve damage in humans.
Massachusetts Eye and Ear researchers have created a word-scoring model that can assess the degree of hidden hearing loss in human ears.
Researchers at the Eaton-Peabody Laboratories of Massachusetts Eye and Ear calculated mean speech scores as a function of age from data from about 96,000 ears evaluated by Massachusetts Eye and Ear in a new study just published in Scientific Reports. Then they compared the results with previous research at Massachusetts Eye and Ear that had tracked the typical loss of cochlear nerve fibers over time. Researchers have estimated the relationship between speech scores and nerve survival in humans by combining the two sets of data.
The new model improves the assessment of cochlear nerve damage in patients and the speech intelligibility deficits caused by neural loss, claims Stéphane F. Maison, Ph.D., CCC-A, the study’s lead author and associate professor of Otolaryngology-Head and Neck Surgery at Harvard Medical School. Maison is also the principal investigator of the Eaton-Peabody Laboratories. The model also provides methods to calculate how well hearing loss interventions, such as the use of hearing aids and personal sound reinforcement equipment, are working.
“Prior to this study, we could either estimate neural loss in a living patient using a long battery of tests or measure damage to the cochlear nerve by removing their temporal bones when they died,” said Dr. house. “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.”
Discover hidden hearing loss
The two main factors that determine a person’s ability to hear are audibility and intelligibility. The sensory cells known as hair cells in the inner ear play a role in the audibility of sound, or how loud a sound must be to be audible. Hair cells give electrical impulses to the cochlear nerve in response to sound, and the cochlear nerve then sends those signals to the brain. The cochlear nerve’s ability to transmit these signals effectively affects how clearly or intelligibly the central nervous system processes sound.
For years, researchers and medical professionals believed that the main cause of hearing loss was degeneration of hair cells, and that damage to the cochlear nerves only became serious after the hair cells were lost. Hair cell health can be determined from an audiogram, which has long been considered the gold standard for hearing tests. Patients with a normal audiogram were given a clean bill of health claiming they had trouble hearing in noisy environments, because nerve loss was thought to be secondary to hair cell loss or dysfunction. Experts now realize that the audiogram is not informative about the condition of the auditory nerve.
“This explains why some patients who report having difficulty 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 struggle with speech intelligibility,” said Dr. house.
In 2009, M. Charles Liberman, Ph.D., and Sharon Kujawa, Ph.D., principal investigators at the 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 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 results confirmed an association 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 people may not be candidates for traditional hearing aids, especially if they have 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.
Reference: “Predicting neural deficits in sensorineural hearing loss from word recognition scores” by Kelsie J. Grant, Aravindakshan Parthasarathy, Viacheslav Vasilkov, Benjamin Caswell-Midwinter, Maria E. Freitas, Victor de Gruttola, Daniel B. Polley, M. Charles Liberman, and Stéphane F. Maison, June 23, 2022, Scientific Reports.
In addition to Dr. Maison are co-authors of the study Kelsie J. Grant, Aravindakshan Parthasarathy, Viacheslav Vasilkov, Benjamin Caswell-Midwinter, Maria E. Freitas, Daniel B. Polley, M. Charles Liberman of Massachusetts Eye and Ear/Harvard Medical School and Victor DeGruttola from the Harvard T.H. Chan School of Public Health.
This study was funded by the National Institutes of Health.