BOSTON, MASSACHUSSETTS — 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.
Hidden Hearing Loss
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.
**read the full story on the Mass Eye & Ear website here.
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