Exposure to loud noise, such as a firecracker or an ear-splitting
concert, is the most common preventable cause of hearing loss. Research
suggests that 12% or more of the world population is at risk for
noise-induced loss of hearing.
Loud sounds can cause a loss of auditory nerve cells in the inner
ear, which are responsible for sending acoustic information to the
brain, resulting in hearing difficulty. However, the mechanism behind
this hearing loss is not fully understood.
Now, a new study from Keck Medicine of USC
links this type of inner ear nerve damage to a condition known as
endolymphatic hydrops, a buildup of fluid in the inner ear, showing that
these both occur at noise exposure levels people might encounter in
their daily life.
Additionally, researchers found that treating the resulting fluid
buildup with a readily available saline solution lessened nerve damage
in the inner ear.
“This research provides clues to better understand how and when
noise-induced damage to the ears occurs and suggests new ways to detect
and prevent hearing loss,” said John Oghalai MD, an otolaryngologist with Keck Medicine, chair of the USC Caruso Department of Otolaryngology – Head and Neck Surgery and lead author of the study.
A previous study
by Oghalai conducted on mice exposed to blast pressure waves simulating
a bomb explosion linked nerve damage with fluid buildup in the inner
For this study, Oghalai and colleagues wanted to explore the effect
of common loud sounds ranging from 80 to 100 decibels on the ear. After
the exposure, they used an imaging technique known as an optical
coherence tomography to measure the level of inner ear fluid in the
cochlea, the hollow, spiral-shaped bone found in the inner ear.
Up until exposure to 95 decibels of sound, the inner ear fluid level
remained normal. However, researchers discovered that after exposure to
100 decibels — which is equivalent to sounds such as a power lawn mower,
chain saw or motorcycle — mice developed inner ear fluid buildup within
hours. A week after this exposure, the animals were found to have lost
auditory nerve cells.
However, when researchers applied hypertonic saline, a salt-based
solution used to treat nasal congestions in humans, into affected ears
one hour after the noise exposure, both the immediate fluid buildup and
the long-term nerve damage lessened, implying that the hearing loss
could be at least partially prevented.
These study results have several important implications, according to
Oghalai, especially as the loss of nerve cells in the inner ear is
known as “hidden hearing loss” because hearing tests are unable to
detect the damage.
“First, if human ears exposed to loud noise, such as a siren or
airbag deployment, can be scanned for a level of fluid buildup — and
this technology is already being tested out — medical professionals may
have a way of diagnosing impending nerve damage,” he said. “Secondly, if
the scan discovered fluid buildup, people could be treated with
hypertonic saline and possibly save their hearing.”
He also believes the study opens a new window into understanding
Meniere disease, a disorder of the inner ear that causes vertigo,
ringing in the ears (tinnitus) and hearing loss.
“Previously, inner ear fluid buildup was thought to
be primarily linked to Meniere disease. This study indicates that
people exposed to loud noises experience similar changes,” he said.
Oghalai hopes this study will lead to further research on the reasons
ear fluid buildup occurs, and encourage the development of better
treatments for Meniere disease.
The study was supported by the National Institute on Deafness and Other Communication Disorders.
Source: News Release
University of Southern California - Health Sciences
November 11, 2021