New research on human ear samples shows that loud noise exposure causes a tough-to-detect, yet common, form of damage called hidden hearing loss.
New research from Mass Eye and Ear hearing scientists shows that repeated noise exposure causes permanent damage to nerve fibers in the inner ear before it shows up on conventional hearing tests in the clinic.
As new therapies for different types of hearing loss enter clinical trials, knowing what damage has occurred in the inner ear is critically important in determining which patients will benefit the most from therapies.
A type of hearing loss that eludes diagnostic tests
Years ago, Mass Eye and Ear investigators M. Charles Liberman, PhD and Sharon G. Kujawa, PhD, showed in animals that noise overexposure causes inner ear damage that can’t be seen on an audiogram, the test that audiologists most commonly administer. They suggested this damage was at the root of one of the most common complaints of those with hearing loss: A person can hear that people are talking, but can’t understand what they are saying.
The audiogram – the gold-standard, hearing test – measures how loud a sound must be to be heard, but it doesn’t require any description of the sound. It’s like being asked in an eye exam whether there are any markings on the wall, without being asked to identify the letters. This is because the audiogram measures the health of the sensory cells in the inner ear, called hair cells, which convert sound into electrical signals.
But to hear clearly, those signals have to reach the brain via the many fibers of the auditory nerve. In animal studies a decade ago, the Mass Eye and Ear investigators showed that noise destroys nerve fibers before the sensory cells. “When this happens, it is like down-sampling a picture,” Dr. Liberman, the director of the Eaton-Peabody Laboratories at Mass Eye and Ear, explained to Focus. “When the number of pixels decreases, you can still tell there’s an image there, but you can’t tell what it is.”
Researchers called this “hidden hearing loss” – a kind of damage that causes an obvious hearing impairment, but one that “hides” behind the audiogram, because sounds can still be detected, just not understood.
Uncovering hidden hearing loss evidence in humans
Despite the evidence in animal studies, seeing this damage in human has proven difficult. The inner ear can’t be imaged in a living patient with enough resolution to see the sensory cells and their nerve fibers. The only way to see what kind of damage underlies a hearing impairment in humans is to look at inner ear specimens obtained at autopsy. Mass Eye and Ear maintains the largest collection of these preserved samples in the world that were donated by patients after they passed on along with their audiological and medical records of the hearing loss. This collection been growing since it was first established in the 1950’s by Harold Schuknecht, MD, former chair of Otolaryngology. Today, it is led by Felipe Santos, MD as part of the National Temporal Bone, Hearing and Balance Pathology Resource Registry.
In a new study published April 21 in the J Neurosci: The Journal of Neuroscience, a research team led by Dr. Liberman mined this collection to study the inner ears of 52 people with a noise-exposure history and compared them with an age-matched control group. Subjects from the exposed group had worked in noisy environments, such as factories, or had extensive military service including combat experience.
The researchers found a surprisingly large amount of age-related loss of auditory nerve fibers in all cases – an average of 50 percent loss by around age 60, even in the normal group. As predicted from the animal studies, the loss was significantly accelerated in those cases with a history of loud noise overexposure. Corresponding medical records showed the nerve loss did not affect the audiogram thresholds, just as the animal work had suggested, but that it decreased performance on word-identification tasks.
“This new study is important in two ways,” said Dr. Liberman. “First, it underscores the idea that repeated noise exposure causes permanent inner ear damage before it begins to change the audiogram. Second, it suggests that differences in word scores among those with similar audiograms is the best way to diagnose the disorder.”
This new study builds on research from this team published last year in J Neurosci that found that age-related hearing loss is mainly caused by damage to hair cells. Those findings also reinforced the importance of protecting your ears from noise damage.
Treatments on the horizon
Diagnosing this type of cochlear neural degeneration in living patients is important, because new therapies are on the horizon, with several already in clinical trials. Animal research in this area, much of it also done by Mass Eye and Ear researchers, has shown that local delivery of naturally occurring molecules called trophic factors, which are key to the normal development of the inner ear at birth, can reverse the damage to the nerve fibers if applied relatively soon after a sudden loud noise overexposure.
“Such therapies could have been effective after incidents like the Boston Marathon bombing,” said lead author Peizhe Wu, MD, a research fellow in the Liberman lab. She added that such treatments targeting specific forms of damage may benefit other tough-to-treat forms of hearing loss, like hidden hearing loss and tinnitus.
“In addition to causing problems with speech discrimination, this type of nerve damage may also be the trigger for tinnitus, another major complaint of those with noise-induced hearing loss,” she said.