Even during the simple act of commuting to work, we can’t escape the ubiquitous noise that invades our lives: Traffic rumbling down the street, a jackhammer at a construction site, the roar of the subway—not to mention music and podcasts we pump directly into our ears.
Yet for many years conventional wisdom has held that as long as we could pass a hearing test, everything was fine. That is until 2009 when researchers uncovered a phenomenon called “hidden” hearing loss—a likely contributor to the cumulative loss typically associated with aging. Now, scientists at Massachusetts Eye and Ear hospital have developed a suite of tools for detecting hidden hearing loss, an important step in figuring out the magnitude of the problem and what we can do to fix it. They reported their findings this week in PLoS ONE.
Hearing loss is most often associated with damage to so-called hair cells inside our ears, which detect vibrations in the environment and translate them into chemical signals that prompt our auditory nerves to fire. During an audiogram—considered the gold standard of hearing tests—patients listen to sounds of different frequencies at different volumes to determine their thresholds for sound detection. Those with damaged hair cells tend to have trouble detecting low-volume sounds at certain frequencies.
Damage to our hearing, however, actually starts long before our hair cells are affected, research suggests. First there is damage to the fibers in the auditory nerve that connect to hair cells and help us hear in noisy environments—hence the “hidden” in hidden hearing loss. This phenomenon remained obscure for so long because an audiogram administered in a quiet room does not activate these auditory nerve fibers, which means patients can pass a hearing test even when the fibers have been damaged.
Previous research has found signs of such hidden hearing loss in mice, guinea pigs, chinchillas and postmortem human ear tissue. Now auditory physiologist and study co-author Stéphane Maison and his colleagues have taken the next logical step: investigating hidden hearing loss in human patients. “In the end, the main goal is to have a measure of hidden hearing loss that is currently not available in [the] clinic,” Maison says.
To test the tools they developed, the researchers divided 34 volunteers ranging from ages 18 to 41 into two groups: a low-risk one composed of college students studying “quiet subjects” (such as communication science) and a high-risk group mostly consisting of music students exposed to loud sounds for four to six hours per day.
Both groups passed their audiograms with flying colors. When the researchers extended the test to include higher frequencies than are normally evaluated, however, the high-risk group had trouble hearing the sounds at low volumes.
The researchers also outfitted subjects with electrodes to measure how each volunteer’s auditory nerve responds to sound (which they compared with the response of the hair cells for each individual to reduce variability). The auditory nerves of subjects in the high-risk group did not respond to sound as robustly as those of subjects in the low-risk group. Perhaps most striking, when subjects were asked to listen to words and repeat them back while noise played in the background, the high-risk group performed significantly worse than the low-risk group. The same was true when the words were sped up and played with a background echo.
On a questionnaire that accompanied the tests, the high-risk group indicated they were more “bothered” by everyday sounds like a dog barking or a baby crying than the low-risk group was. Cumulatively, these results not only reveal hidden hearing loss can start at a young age but also that it is not so hidden after all—it adversely affects our hearing in numerous ways, including the ability to detect high-frequency sounds and hear in noisy settings. In a quiet place, Maison says, “you're not going to notice it, but when you're going to go to a bar or a restaurant, and there's a lot of background noise, you're going to struggle.”
Gabriel Corfas, a researcher at the University of Michigan’s Kresge Hearing Research Institute who was not affiliated with the study, thinks this work is an important step in the field of hidden hearing loss. “It raises the issue of how exposing ourselves as young people to noise levels that we don’t think of being extreme—through education, through entertainment, through work—could actually impact our hearing throughout our life,” he says, “and brings to the forefront the need to discuss how much we expose ourselves to noise, and how to protect ourselves.”
On an individual level, protecting ourselves from hidden hearing loss could entail limiting noise exposure and using ear protection. On a society-wide scale, it may necessitate revisiting current standards regulating noise in the workplace.
Fortunately, even after hidden hearing loss has occurred, the picture is not entirely bleak. Corfas and his team have previously reported that treating the ears of mice exposed to loud environments with a growth factor called neurotrophin 3 can reverse hidden hearing loss by repairing some of the damage to auditory nerve fibers.
Corfas is hopeful that a similar therapy could ultimately be used in humans and could, in theory, be an important step in preventing age-associated hearing loss. He points out that tools to diagnose hidden hearing loss—such as those developed by Maison’s team—would be essential for testing such therapies and detecting damage at an early stage when it is more treatable.