In one of the biggest retrospective studies of its type, researchers analyzed data from roughly 96,000 ears and created a word-score model that may determine the amount of hidden hearing loss or cochlear nerve damage in people.
A word-score model that may assess the degree of hidden hearing loss in human ears has been created by researchers from Massachusetts Eye and Ear.
Researchers from the Eaton-Peabody Laboratories at Massachusetts Eye and Ear calculated the average speech scores as a function of age from the records of approximately 96,000 ears evaluated at Massachusetts Eye and Ear in a new study that was just published in Scientific Reports. After that, they compared the results to prior research at Massachusetts Eye and Ear that had monitored the typical loss of cochlear nerve fibers over time. Researchers created an estimation of the relationship between speech scores and nerve survival in humans by combining the two sets of data.
The new model improves assessments of cochlear nerve damage in patients and the speech-intelligibility deficits that are brought on by neural loss, claims Stéphane F. Maison, Ph.D., CCC-A, the study’s primary 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 for calculating how well hearing loss interventions—such as the use of hearing aids and personal sound amplification devices—are working.
“Prior to this study, we could either estimate neural loss in a living patient using a lengthy test battery or measure cochlear nerve damage by removing their temporal bones when they’ve died,” said Dr. Maison. “Using ordinary speech scores from hearing tests – the same ones collected in clinics all over the world – we can now estimate the number of neural fibers that are missing in a person’s ear.”
Uncovering 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 have a role in sound audibility, or how loud a sound must be to be audible. Hair cells provide electrical impulses to the cochlear nerve in response to sound, and the cochlear nerve subsequently sends those signals to the brain. The cochlear nerve’s capacity to transmit these signals effectively affects how clearly or understandably the central nervous system processes sound.
For many years, researchers and medical professionals believed that the major cause of hearing loss was hair cell degeneration and that cochlear nerve damage only became severe after the hair cells were lost. The health of hair cells may be determined by an audiogram, which has long been regarded as the gold standard of hearing tests. Patients with a normal audiogram were given a clean bill of health while claiming to have trouble hearing in noisy settings since it was thought that nerve loss was 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 difficulties understanding a conversation in a busy bar or restaurant may have a ‘normal’ hearing exam. Likewise, it explains why many hearing aid users who receive amplified sounds still struggle with the intelligibility of speech,” Dr. Maison said.
In 2009, M. Charles Liberman, Ph.D., and Sharon Kujawa, Ph.D., principal investigators in the Eaton-Peabody Laboratories, upended the way scientists thought about hearing when they uncovered hidden hearing loss. Their findings revealed that cochlear nerve damage preceded hair cell loss as a result of aging or noise exposure and suggested that, by not providing information about the cochlear nerve, audiograms had not actually 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 an individual’s speech score should be based on their audiogram. They then measured the differences between the predicted word recognition scores and the ones 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 deficits in intelligibility, Dr. Maison explained.
After considering a number of factors, including the cognitive deficits that may accompany 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 measures of neural loss from existing histopathological data from human temporal bones to come up with a predictive model based on a standard hearing exam.
The results confirmed an association between poorer speech scores and larger amounts of cochlear nerve damage. For example, the worst scores were found in patients with Ménière’s disease, consistent with temporal bone studies showing a 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 – only exhibited moderate-to-small discrepancies.
Changing the landscape of hidden hearing loss research and beyond
More than 1.5 billion people live with some degree of hearing loss, according to the World Health Organization. Some of those people may not qualify as candidates for traditional hearing aids, particularly if they present with a mild to moderate high-frequency hearing loss. Knowing the extent of the neural damage should inform clinicians about the best ways to address a patient’s communication needs and offer appropriate interventions besides the use of 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 higher amounts of cochlear nerve damage, Dr. Maison believes this model could help clinicians assess the effectiveness of traditional and newer sound amplification 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, as opposed to in quiet.
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, 23 June 2022, Scientific Reports.
In addition to Dr. Maison, co-authors of the study include 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 of the Harvard T. H. Chan School of Public Health.
This study was funded by the National Institutes of Health.