In a new study published June 23 in Scientific Reports, a team of researchers at Mass Eye and Ear’s Eaton-Peabody Laboratories determined average speech scores as a function of age from the records of nearly 96,000 ears examined at Mass Eye and Ear.
They then compared the data to a previous study at Mass Eye and Ear that had tracked the average loss of cochlear nerve fibers as a function of age. By combining both sets of data, researchers constructed an estimate of the relation between speech scores and nerve survival in people.
According to lead study author Stéphane F. Maison, Ph.D., 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 the cochlear nerve damage in patients and the associated speech-intelligibility deficits that come with the neural loss.
The model also offers ways to estimate the effectiveness of hearing loss interventions, including the use of personal sound amplification products and hearing aids.
“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
Two main factors determine how well a person can hear: audibility and intelligibility. Hair cells, the sensory cells inside the inner ear, contribute to the audibility of sounds – or how loud a sound must be to be detectable. Upon receiving a sound, hair cells pass electrical signals to the cochlear nerve, which then passes those signals to the brain.
How well the cochlear nerve relays these signals contributes to the clarity, or intelligibility, of sound processed within the central nervous system.
For years, scientists and clinicians believed hair cell deterioration was the primary cause of hearing loss and that cochlear nerve damage was widespread only after the hair cells were destroyed. Audiograms, long considered the gold standard of hearing exams, provide information about the health of hair cells.
Because it was believed nerve loss was secondary to hair cell loss or dysfunction, patients with a normal audiogram were given a clean bill of health despite reporting difficulties hearing in noisy environments. Experts now understand why the audiogram is not informative about the health 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 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 one 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 findings 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 individuals 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 beside 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.
Most hearing impairment in adults is sensorineural in origin. It is caused by damage to the inner ear, where the cochlear hair cells normally convert mechanical vibrations into electrical signals that are transmitted via glutamatergic synapses to the sensory fibers of the cochlear nerve. Each human cochlea contains only ~15,000 hair cells and ~40,000 nerve fibers. Once destroyed, neither cell type regenerates in any mammalian ear .
Decades of research on noise-exposed humans and animals have shown that acoustic overexposure leads to hair cell damage, which in turn causes threshold elevation (e.g. [2, 3]). The dogma has been that hair cells are the primary targets of noise and that cochlear neurons only die as a result of hair cell degeneration .
This view arose because hair cell loss can be detected within hours post noise exposure, while loss of spiral ganglion cells is not detectable for months to years after the insult [5, 6]. According to this view, a noise exposure that only causes a temporary elevation of cochlear thresholds is benign, because there is no permanent hearing impairment. This assumption underlies the damage-risk criteria for noise in the workplace set by several federal agencies .
Recent animal studies showing that noise exposure can lead to cochlear neuronal degeneration, even when hair cells recover and thresholds return to normal  have challenged this view. In noise-exposed ears showing no acute or chronic hair cell loss, there can be up to a 50% loss of the synapses between inner hair cells (IHCs) and cochlear neurons. The same primary loss of cochlear synapses occurs in the aging ear [9, 10]. This cochlear synaptopathy has remained “hidden” because, although loss of synapses is immediate, the synapses are not visible in routine histological material, and the subsequent loss of spiral ganglion cells takes months to years . Cochlear synaptopathy is also “hidden” because cochlear neural degeneration does not elevate behavioral or electrophysiological thresholds until it becomes extreme [12, 13].
Part of the reason for the relative insensitivity of threshold measures to cochlear synaptopathy is that, near threshold, a small increase in sound level can compensate for a large loss of neurons, by increasing discharge rates in remaining fibers and by spreading activity to additional fibers along the cochlear spiral . Another part of the explanation is that the most vulnerable cochlear neurons, to both noise and aging, are those with high thresholds and low spontaneous rates (SRs) [15, 16]. These low-SR fibers do not contribute to threshold detection in quiet, but, by virtue of their high thresholds, are key to the coding of transient stimuli in the presence of continuous background noise  that saturates the responses of the sensitive high-SR fibers. These observations have suggested that low-SR fiber loss is a major contributor to the classic impairment in sensorineural hearing loss (SNHL), i.e. difficulties with speech discrimination in challenging listening environments .
Cochlear synaptopathy may also be key to the genesis of other perceptual anomalies associated with noise damage, including hyperacusis and tinnitus [19–23], which may arise via an induction of central gain adjustment secondary to loss of afferent input to the auditory central nervous system .
Based on the animal work, we hypothesized that cochlear synaptopathy is widespread among young people who regularly abuse their ears, despite the presence of a normal audiogram. For the present study, we recruited young adult subjects and divided them according to noise-exposure history into high-risk and low-risk groups. We found significant deficits in difficult word-recognition tasks in the high-risk group that were associated with significant elevation of pure-tone thresholds at frequencies higher than those normally tested and with changes in auditory evoked potentials consistent with the presence of cochlear synaptopathy, also known as hidden hearing loss.
reference link :https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0162726
More information: Kelsie J. Grant et al, Predicting neural deficits in sensorineural hearing loss from word recognition scores, Scientific Reports (2022). DOI: 10.1038/s41598-022-13023-5