Summary: A newly developed word-score model is capable of estimating hidden hearing loss and the effectiveness of hearing loss interventions.
Source: Mass Eye and Ear
Researchers from Mass Eye and Ear have developed a word-score model capable of estimating the amount of hidden hearing loss in human ears.
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.
Predicting neural deficits in sensorineural hearing loss from word recognition scores
The current gold standard of clinical hearing assessment includes a pure-tone audiogram combined with a word recognition task. This retrospective study tests the hypothesis that deficits in word recognition that cannot be explained by loss in audibility or cognition may reflect underlying cochlear nerve degeneration (CND).
We collected the audiological data of nearly 96,000 ears from patients with normal hearing, conductive hearing loss (CHL) and a variety of sensorineural etiologies including (1) age-related hearing loss (ARHL); (2) neuropathy related to vestibular schwannoma or neurofibromatosis of type 2; (3) Ménière’s disease; (4) sudden sensorineural hearing loss (SSNHL), (5) exposure to ototoxic drugs (carboplatin and/or cisplatin, vancomycin or gentamicin) or (6) noise damage including those with a 4-kHz “noise notch” or reporting occupational or recreational noise exposure.
Word recognition was scored using CID W-22 monosyllabic word lists. The Articulation Index was used to predict the speech intelligibility curve using a transfer function for CID W-22. The level at which maximal intelligibility was predicted was used as presentation level (70 dB HL minimum). Word scores decreased dramatically with age and thresholds in all groups with SNHL etiologies, but relatively little in the conductive hearing loss group.
Discrepancies between measured and predicted word scores were largest in patients with neuropathy, Ménière’s disease and SSNHL, intermediate in the noise-damage and ototoxic drug groups, and smallest in the ARHL group. In the CHL group, the measured and predicted word scores were very similar. Since word-score predictions assume that audiometric losses can be compensated by increasing stimulus level, their accuracy in predicting word score for CHL patients is unsurprising.
The lack of a strong age effect on word scores in CHL shows that cognitive decline is not a major factor in this test. Amongst the possible contributions to word score discrepancies, CND is a prime candidate: it should worsen intelligibility without affecting thresholds and has been documented in human temporal bones with SNHL.
Comparing the audiological trends observed here with the existing histopathological literature supports the notion that word score discrepancies may be a useful CND metric.