Researchers from Mass Eye and Ear have developed a word score model that can estimate the amount of hidden hearing loss in human ears.
In a new study published June 23 in Scientific Reports, a team of researchers from Eaton-Peabody Laboratories at Mass Eye and Ear determined average age-specific speech scores from recordings of nearly 96,000 ears examined at Mass Eye and Ear. They then compared the data to a previous Mass Eye and Ear study that tracked average cochlear nerve fiber loss as a function of age. By combining the two data sets, the researchers constructed an estimate of the relationship between speech scores and nerve survival in people.
According to lead study author Stephane F. Maison, PhD, CCC-A, principal investigator of Eaton-Peabody Laboratories and associate professor of otolaryngology-head and neck surgery at Harvard Medical School , the new model leads to better assessments of cochlear nerve damage in patients and the associated speech intelligibility deficits that accompany 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.
“Before this study, we could either estimate neural loss in a living patient using a long battery of tests, or measure cochlear nerve damage by removing his temporal bones when he died,” said the Dr. House. “Using ordinary speech scores from hearing tests – the same ones collected in clinics around the world – we can now estimate the number of missing neural fibers in a person’s ear.”
Uncover hidden hearing loss
Two main factors determine a person’s ability to hear: audibility and intelligibility. Hair cells, the sensory cells inside the inner ear, contribute to the audibility of sounds – or how loud a sound has to be to be detectable. When receiving a sound, the hair cells transmit electrical signals to the cochlear nerve, which then transmits these signals to the brain. The way the cochlear nerve relays these signals contributes to the clarity or intelligibility of the sound processed in the central nervous system.
For years, scientists and clinicians believed that hair cell damage was the primary cause of hearing loss and that cochlear nerve damage was only prevalent after hair cell destruction. Audiograms, long considered the gold standard for hearing tests, provide insight into hair cell health. Because nerve loss was believed to be secondary to hair cell loss or dysfunction, patients with a normal audiogram received good health despite reported hearing difficulties 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 difficulty understanding a conversation in a busy bar or restaurant may undergo a ‘normal’ hearing test,” House says.
In 2009, Mr. Charles Liberman, PhD, and Sharon Kujawa, PhD, Principal Investigators at Eaton-Peabody Laboratories, upended the way scientists thought about hearing when they discovered hidden hearing loss. Their results revealed that cochlear nerve damage preceded hair cell loss due to aging or noise exposure and suggested that by not providing information about the cochlear nerve, audiograms had no effect. 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 that 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 differences between the predicted score and the measured score would have reflected deficits in intelligibility , explained Dr. House.
After looking at 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, that a person had. They then applied measures of neural loss from existing histopathological data of human temporal bones to come up with a predictive model based on a standard hearing test.
The results confirmed an association between lower speech scores and greater amounts of cochlear nerve damage. For example, the worst scores were found in patients with Ménière’s disease, which is consistent with temporal bone studies showing dramatic loss of cochlear nerve fibers. Meanwhile, patients with conductive, drug-induced, and normal age-related hearing loss — etiologies with the least cochlear nerve damage — had only moderate to small deviations.
Changing the research landscape for hidden hearing loss 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 be eligible for traditional hearing aids, especially if they have mild to moderate high frequency hearing loss. Knowing the extent of neural damage should inform clinicians on the best ways to meet a patient’s communication needs and suggest 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 patients most likely to have greater 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 provide better interventions by assessing word performance scores in noise, as opposed to silence.
In addition to Dr. Maison, study co-authors include Kelsie J. Grant, Aravindakshan Parthasarathy, Viacheslav Vasilkov, Benjamin Caswell-Midwinter, Maria E. Freitas, Daniel B. Polley, M. Charles Liberman of Mass. Eye and Ear/Harvard Medical School, and Victor DeGruttola of Harvard TH Chan School of Public Health. This study was supported by the National Institutes of Health (NIDCD P50-DC015857).