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Standard Hearing Tests Miss Common Form of Hearing Loss

By Jamie Santa Cruz

Without proper testing, many older adults fail to have hearing loss diagnosed.

Hearing loss is very prevalent among older adults, with some research suggesting it affects as many as two-thirds of the US population older than age 70.1 However, a new study from the University of Buffalo suggests that an audiogram—the gold standard test of hearing—is unable to detect one common form of hearing loss, meaning that many older adults with significant hearing deficits are likely going undiagnosed.2

Hidden Cause of Hearing Loss
Hearing loss can be caused by damage to either the ear's outer hair cells or its inner hair cells, both of which are in the inner ear. Most research in the last century has focused on damage to the outer hair cells, says Benjamin Auerbach, PhD, a postdoctoral fellow at the University of Buffalo's Center for Hearing and Deafness and one of the study authors. The new research, however, focuses on inner hair cells and the type I neurons that transmit information from the inner hair cells to the brain, a topic that has begun to receive research attention only in the last five to 10 years.

The new research, published in Frontiers in Neuroscience, is based on a series of studies of chinchillas in which the animals were treated with the ototoxic anticancer drug carboplatin to selectively destroy inner hair cells and their associated type I neurons. Inner hair cells synapse with upwards of 90% of auditory nerve fibers and are thus crucially important for the transmission of acoustic information to the brain. However, the results of the chinchilla research show that significant damage to inner hair cells and type I neurons produces a hearing loss that's quite different from damage to outer hair cells.

In the case of outer hair cells, if every one of these cells were severely damaged, an individual would experience a reduction in hearing capacity of 40 to 50 decibels, says Richard Salvi, PhD, a professor of communicative disorders and sciences at the University of Buffalo and the study's lead author. This would qualify as moderate to severe hearing loss and would be obvious on a standard audiogram in which the listener is presented with a series of pure tones in a quiet room.

By contrast, however, the chinchilla studies show that inner hair cells can sustain far more damage without the damage showing as hearing loss on an audiogram. Even if there are only a few inner hair cells left, the brain will have no problem detecting a pure tone in an environment devoid of other noise. "You only need a very few inner hair cells and type I neurons to hear, as long as they are functionally normal," says Salvi, who is also director of the University of Buffalo's Center for Hearing and Deafness. "You can actually destroy up to between 75% and 80% of the inner hair cells and type I neurons and your hearing threshold in a clinic would look like it was normal, as long as you had the outer hair cells there."

An 'Internal Hearing Aid'
According to Auerbach, the reason an individual continues to be able to hear even with a severely limited number of inner hair cells is that the brain has a mechanism for magnifying the auditory stimulus it is receiving from even the few undamaged cells and their associated neurons. "As this diminished input ascends through the auditory system, there is a progressive amplification of this diminished input," Auerbach says. "So there is a brain plasticity involved that compensates for this loss of input by increasing the gain in the central auditory system."

A good analogy, Salvi says, is to think of the brain as having a sort of internal hearing aid that boosts the auditory signals it is receiving. The problem, he says, is that this "hearing aid" magnifies all sound indiscriminately—the sounds the individual wants to focus on as well as all background noise.

To use another analogy, Salvi says, the effect is similar to what happens if someone turns up the volume on a radio for a station whose signal is weak. "I like to listen to National Public Radio, and if I drive from Buffalo to Rochester, as I get 10 or 15 miles out of town, the signal becomes very weak," he says. "And I reach over to my volume control on my radio, and turn up the volume control. Now I can hear the signal, but I hear all the static and background noise."

The issue in this case is one of a low signal-to-noise ratio. Thus, while an individual with inner hair cell damage can hear well in a quiet environment—the kind of setting in which an audiogram is conducted—he or she will have far more difficulty hearing in an environment with more auditory distractions. And this is exactly the sort of hearing difficulty that many older adults report, Salvi says. "When you're talking to [older people] in a quiet room, they hear pretty well, but if you go to a restaurant and there's some background noise, they typically have great difficulty understanding," he says.

"There's a limit to how much your brain can compensate," Auerbach adds. "It can reestablish rudimentary sound detection, but when you challenge listeners with a noisy environment or more complex stimuli, they may have a more difficult time."

It is currently unknown how much of a role long-term exposure to low-level noise plays in inner hair cell damage, Auerbach says. However, he says, some evidence in animal models suggests that such noise can preferentially damage inner hair cells and their auditory type I synapses, and if so, it is possible that younger adults in high-risk environments, such as those exposed to high levels of occupational noise, could also be affected.

Significantly, according to the two researchers, inner hair cell damage and the brain's compensatory magnification of auditory signals explain not only certain forms of hearing loss, but may also help to elucidate other hearing disorders—for example, the ringing in the ears characteristic of tinnitus and the loudness intolerance characteristic of hyperacusis. Again, Auerbach says, the hypothesis related to both conditions is that the brain is amplifying auditory signals, but it is simultaneously amplifying noise.

Tests to Measure Inner Hair Cell Loss
In addition to audiograms, there is another common means of measuring hearing deficits: a distortion product otoacoustic emission test. This is the form of hearing test often conducted on newborns before they're discharged from the hospital, but unfortunately it, too, fails to show hearing loss related to inner hair cell damage. This is because the test relies on the fact that outer hair cells generate their own sound, which can be detected with a microphone inserted into the ear canal; inner hair cells, however, do not generate such sounds.

While these standard hearing tests miss hearing loss resulting from inner hair cell damage, there are two types of tests that could be used to detect this particular kind of deficit. The first is an audiogram under less-than-ideal conditions, wherein the listener would be challenged to detect tones in a noisy environment rather than in a quiet room. A speech-discrimination-in-noise test, which measures how much conversational language the listener can understand in a relatively noisy environment, could also be effective. While the latter is occasionally used currently, it is not routine, Salvi says.

Anecdotal evidence suggests that the percentage of older adults suffering from inner hair cell and/or type I neuron damage could be high. However, Auerbach says, since neither of the tests that could measure such hearing loss is commonly used at present, it is currently unknown how many older adults actually have such damage. Both Auerbach and Salvi recommend that alternative tests be adopted into routine use to promote a better understanding of the number of adults affected.

Treatments for Inner Hair Cell Loss
Currently, the treatments for inner hair cell loss are unclear. The best solution, according to Auerbach, would be to either reverse the damage or compensate at the periphery. But without any current method of accomplishing either of those, he suggests a combination of behavioral training and regenerative pharmacotherapy may be useful in the future. "What our study shows is how dynamic the central auditory system can be and how much it can compensate," Auerbach says. "If there is a way to harness that plasticity through a combination of behavioral training and possibly drug therapy, there may be ways to have the brain better compensate for the hearing loss even if we can't reverse the damage."

The most readily available treatment is a hearing aid with a directional microphone—one that picks up signals coming only from directly in front of the user, the signals the listener wants to hear, but not signals coming from the back or side. Although a directional microphone is not always desirable (such as in a quieter environment where the user will actually want to hear sounds in all directions), a directional microphone can help older adults experience a better signal-to-noise ratio in an environment with auditory distractions, Salvi says. Fortunately, most moderately priced and high-end hearing aids today already include the option to switch the microphone from an omnidirectional mode into a directional mode.

In lieu of other available treatments, Salvi says, health care providers simply need to be alert to the fact that hearing deficits resulting from inner hair cell and type I neurons damage are likely going unrecognized among many older adults, and providers should be prepared for an increasing incidence of this form of "hidden" hearing loss.

"As people get older, this is going to become more and more of a problem, as the average lifespan increases," Salvi says. "Especially in noisy Western environments where we get a lot of early noise exposure, this could predispose you to having hearing problems as you get older."

— Jamie Santa Cruz is a freelance writer based in Englewood, Colorado.

1. Lin FR, Thorpe R, Gordon-Salant S, Ferrucci L. Hearing loss prevalence and risk factors among older adults in the United States. J Gerontol A Biol Sci Med Sci. 2011;66(5):582-590.

2. Salvi R, Sun W, Ding D, et al. Inner hair cell loss disrupts hearing and cochlear function leading to sensory deprivation and enhanced central auditory gain. Front Neurosci. 2017;10:621.