In one of the largest retrospective studies of its kind, researchers analyzed data from nearly 96,000 ears and created a word-recording model that may quantify people’s hidden hearing loss or cochlear nerve damage.
A word registration model that may assess the degree of subtle hearing loss in human ears was created by researchers from Massachusetts Eye and Ear.
Researchers from Eaton Peabody Laboratories in Massachusetts Eye and Ear calculated average speech scores as a function of age from the records of nearly 96,000 ears assessed in Massachusetts Eye and Ear in a new study just published in Scientific reports. Next, they compared the results to previous research in Massachusetts Eye and Ear that monitored the typical loss of cochlear nerve fibers over time. The researchers created an estimate of the relationship between speech scores and nerve 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 neurological loss, claims Stephane F. Maison, PhD, CCC-A, lead author of the study and associate professor of otolaryngology-head and neck surgery at Harvard Medical School. Maison is also the principal investigator at Eaton Peabody Laboratories. The model also provides ways to calculate the success of hearing loss interventions – such as the use of hearing aids and personal sound amplifiers -.
“Before this study, we could either estimate nerve loss in a surviving patient using a prolonged test battery or measure cochlear nerve damage by removing their temporal bone when they died,” said Dr. Maison. “Using normal speech scores from hearing tests – the same results collected in clinics around the world – we can now estimate how many nerve fibers are missing in a person’s ear.”
Detect hidden hearing loss
The two main factors that determine a person’s ability to hear are the ability to hear and clarity. Sensory cells known as hair cells in the inner ear play a role in hearing sound, or how loud a sound is until it is audible. Hair cells deliver 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 effectively transmit these signals affects how the central nervous system processes sound clearly or understandably.
For many years, researchers and medical professionals believed that the main cause of hearing loss was hair cell degeneration and that cochlear nerve damage only became severe after hair cell loss. The health of hair cells can be determined by 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 while claiming to have trouble hearing in noisy settings as the nerve loss was thought to be secondary to the loss or weakening of hair cells. Experts now understand that the audiogram does not contain information about the state of the auditory nerve.
This explains why some patients who report difficulties understanding a conversation in a crowded bar or restaurant take a “normal” hearing test. Likewise, it explains why many hearing aid users who receive amplified sounds still struggle with intelligibility,” Dr. Maison said.
In 2009, M Charles Lieberman, Ph.D., and Sharon Kogawa, Ph.D., principal investigators at Eaton-Peabody Laboratories, changed the way scientists think about hearing when they discovered hidden hearing loss. Their findings revealed that cochlear nerve damage precedes hair cell loss due to ageing or noise exposure, and they suggested that by not providing information about the cochlear nerve, the audiograms did not assess the full range of damage to the ear.
Building a model to predict cochlear nerve damage
In the study, Dr. Maison and his team used the speech intelligibility curve to predict what an individual’s speech score should be based on an audiogram. They then measured the differences between the expected word recognition scores and those obtained during the evaluation of the patient’s hearing.
Dr. Maison explained that because the word list was displayed at a level well above the patient’s hearing limit – where hearing is not a problem – any difference between the expected result and the measured result would have reflected deficiencies in clarity.
After considering a number of factors, including the cognitive deficits that may accompany aging, the researchers argued that the magnitude of these discrepancies reflect the amount of cochlear nerve damage or subtle hearing loss a person has. They then applied measures of neurological loss from existing histopathological data from human temporal bones to come up with a predictive model based on a standard hearing test.
The results confirmed an association between poor speech scores and large amounts of 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 significant loss of cochlear nerve fibers. Meanwhile, patients with conductive hearing loss, drug-induced hearing loss, and normal age-related hearing loss — etiologies with minimal cochlear nerve damage — showed only moderate to small discrepancies.
Changing the nature 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 these people may not qualify as candidates for conventional hearing aids, especially if they have mild to moderate high frequency hearing loss. Knowing the extent of neurological damage should inform clinicians on the best ways to meet a patient’s communication needs, provide appropriate interventions, and use 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 greater amounts of cochlear nerve damage, Dr. Maison believes this model can help clinicians evaluate the effectiveness of traditional and newer sound amplification products. The researchers also hope to introduce new audiometry protocols to improve their model and provide better interventions by evaluating word performance scores in noise, rather than quiet.
Reference: “Prediction of Neurological Deficits in Sensorineural Hearing Loss from Word Recognition Scores” By Kelsey J. Grant, Aravindakshan Parthasarathy, Vyacheslav Vasilkov, Benjamin Caswell Midwinter, Maria E. Freitas, Victor de Grotola, Daniel Polley, M. Charles Lieberman, and Stefan F. Maison, 23 June 2022, Available here. Scientific reports.
DOI: 10.1038 / s41598-022-13023-5
In addition to Dr. Maison, study co-authors include Kelsey J. Grant, Aravindakshan Parthasarathy, Vyacheslav Vasilkov, Benjamin Caswell Medwinter, Maria E. Freitas, Daniel Polly, M. Charles Lieberman of Massachusetts Eye and Ear/Harvard Medical School and Victor Degrotola of Harvard College. TH Chan Public Health.
This study was funded by the National Institutes of Health.