In one of the largest retrospective studies of its kind, researchers analyzed data from approximately 96,000 ears and created a word scoring model that can determine the amount of hidden hearing loss or cochlear nerve damage in people.
researchers from Massachusetts Ear and Eye.
Researchers at Eaton-Peabody Laboratories in Massachusetts Eye and Ear calculated average speech scores for age from the records of approximately 96,000 ears tested at Massachusetts Eye and Ear in a new study just published in Scientific reports. After that, they compared the results to previous research at Massachusetts Eye and Ear that had monitored typical cochlear nerve fiber loss over time. The researchers created an estimate of the relationship between speech scores and neural survival in humans by combining the two data sets.
The new model improves assessments of patients’ cochlear nerve damage and speech intelligibility deficits caused by neuronal loss, says Stéphane F. Maison, Ph.D., CCC-A, the study’s lead author and associate professor of Otorhinolaryngology. Head and Neck Surgery in Harvard Medical School. Maison is also the principal investigator for Eaton-Peabody Laboratories. The model also provides methods for estimating how well hearing loss interventions, such as the use of hearing aids and personal sound amplification devices, are working.
“Before this study, we could estimate neural loss in a living patient using a long battery of tests or measure cochlear nerve damage by removing their temporal bones when they died,” said Dr. Maison. “Using ordinary speech scores from hearing tests, the same ones that are collected in clinics around the world, we can now estimate the number of missing neural fibers in a person’s ear.”
Uncovering hidden hearing loss
The two main factors that determine a person’s hearing ability are audibility and intelligibility. Sensory cells known as hair cells in the inner ear have a role in sound audibility, or how loud a sound needs to be to be audible. The hair cells provide electrical impulses to the cochlear nerve in response to sound, and the cochlear nerve then sends those signals to the brain. The ability of the cochlear nerve to transmit these signals effectively affects the clarity or understanding of the central nervous system in processing sound.
For many years, researchers and medical professionals believed that the primary cause of hearing loss was hair cell degeneration and that cochlear nerve damage only became severe after hair cells were lost. The health of the hair cells can be determined using an audiogram, which has long been considered the gold standard of hearing tests. Patients with a normal audiogram received a clean bill of health and reported having trouble hearing in noisy environments, as 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 difficulty understanding a conversation in a busy bar or restaurant may have a ‘normal’ hearing test. It also explains why many hearing aid users who receive amplified sounds still struggle with speech intelligibility,” said Dr. Maison.
In 2009, M. Charles Liberman, Ph.D., and Sharon Kujawa, Ph.D., principal investigators at Eaton-Peabody Laboratories, turned the way scientists thought about hearing upside down when they discovered hearing loss hidden. 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, the audiograms had not actually assessed the full extent of the damage. In 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 those obtained during the patient’s hearing assessment.
Since the word list 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 deficiencies in intelligibility, Dr. Maison explained.
After considering a number of factors, including cognitive deficits that can 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 neuronal loss from existing histopathological data from human temporal bones to generate a predictive model based on a standard hearing test.
The results confirmed an association between worse speech scores and greater cochlear nerve damage. For example, the worst scores were found in patients with Ménière’s disease, consistent with studies of the temporal bone that showed a dramatic loss of cochlear nerve fibers. Meanwhile, patients with conductive, drug-induced, and normal age-related hearing loss, etiologies with the least amount of cochlear nerve damage, exhibited only 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 these people may not qualify as candidates for traditional hearing aids, especially if they have mild to moderate high-frequency hearing loss. Knowing the extent of neuronal damage should inform clinicians of the best ways to address a patient’s communication needs and offer appropriate interventions in addition to 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 cochlear nerve damage, Dr. Maison believes this model could help clinicians assess the efficacy of traditional and newer sound amplification products. The researchers also hope to introduce new audiometric protocols to further refine their model and deliver better interventions by assessing performance scores for words in noise, as opposed to quiet.
Reference: “Prediction of 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, study coauthors include Kelsie J. Grant, Aravindakshan Parthasarathy, Viacheslav Vasilkov, Benjamin Caswell-Midwinter, Maria E. Freitas, Daniel B. Polley, M. Charles Liberman of the Massachusetts Eye and Ear/School of Harvard Medicine and Victor DeGruttola of the Harvard TH Chan School of Public Health.
This study was funded by the National Institutes of Health.