February 15, 2024

Scientists discover biological mechanism of hearing loss caused by loud noise – and find a way to prevent it

Loud music is just one cause of noise-induced hearing loss.

  • Noise-induced hearing loss is a common condition in the United States that can lead to permanent hearing loss.
  • Despite being widespread, the mechanism that causes it is still not well understood.
  • Scientists have now discovered a potential root cause at the cellular level — and how to prevent it.

If you enjoy live music, you’ve almost certainly experienced noise-induced hearing loss (NIHL) — that nagging, muffled perception of sound or ringing in your ears at the end of the night.

Sometimes it’s gone by the next morning, sometimes it lingers longer. The condition can be transitory at first, but over time it can lead to more severe and even permanent hearing loss.

Despite being a common condition, millions of adults, up to one in four in the United States, are estimated to have some form of it.

Little is known about the exact mechanisms that cause NIHL to happen. This, consequently, also makes the condition more difficult to prevent and treat.

To address the pervasive condition, scientists are investigating how NIHL occurs on the cellular level in the body. In research published this week in the journal PNAS, scientists appear to be another step closer to understanding and potentially preventing NIHL.

In the study, scientists found that loud noises affected parts of the ear on a molecular level, disrupting hearing function specifically having to do with the mineral zinc in the cochlea, a spiral-shaped cavity in the inner ear that converts sound waves into electrical impulses that the brain interprets as sound frequencies.

Zinc plays an important role in the body, including supporting the immune system and chemical signaling in the brain.

Researchers reported that when mice were exposed to loud noises the rodents’ labile zinc levels spiked. Labile zinc is zinc that is “free” and has not bound to a protein. This dysregulation of zinc in turn led to damage and degeneration at the cellular level, manifesting as hearing loss.

The discovery also led the scientists to discover a potential cure as well. By using a chelating agent, a drug that was able to soak up that excess zinc, researchers reported that they could reduce hearing loss.

They said the findings could one day help to prevent NIHL in the future.

“Our study is the first to identify the precise location of labile zinc signaling in the inner ear. We also are the first to document the dysregulation of zinc signaling in the inner ear after loud noise exposure. Most importantly, we are the first to show that noise-induced hearing loss can be mitigated by administering compounds that chelate, or trap, excess free zinc,” Thanos Tzounopoulos, PhD, the director of the Pittsburgh Hearing Research Center at the University of Pittsburgh and senior author of the study, told Medical News Today.

“It’s a well-done study,” added Dr. Marc Feeley, an assistant professor of otolaryngology and head and neck surgery at Vanderbilt University Medical Center in Tennessee who was not involved in the study.

“The most significant contribution I believe is that it allows us to direct further studies and research regarding noise-induced hearing loss with zinc chelation,” he told Medical News Today.

Understanding the zinc link

Zinc is found throughout the body, but it is highly concentrated in the inner ear.

Dysregulation of zinc signaling has been investigated in other traumatic experiences, including stroke and optic nerve damage, where it’s been associated with tissue damage. It hasn’t been looked at as a mechanism for hearing loss.

A 2023 study found that in stroke patients, following a cerebral ischemia, “intracellular zinc accumulation has been shown to be associated with neuronal death.”

So, just as zinc is associated with cellular damage in other parts of the body, it is now believed to play a role in hearing loss.

“Under normal conditions, at least in the brain, but perhaps also in the inner ear, ‘free’ zinc functions by fine-tuning synaptic communications between nerve cells and helps sensory processing. But, as previous literature showed, dysregulation in free zinc signaling can cause cell degeneration and death,” said Tzounopoulos.

He and his team hope that this finding and the subsequent use of zinc chelators to prevent hearing loss could one day serve as an additional therapy for hearing loss. In the study, the zinc chelator was applied directly to the inner ear or into the belly of the mouse prior to a loud noise exposure.

“The fact that both options were effective at protecting mice from hearing loss suggests that, in the future, we could develop a pill that a person could take before a known loud sound exposure to protect oneself from hearing loss,” said Tzounopoulos.

Noise-induced hearing loss can be caused by a single incident of loud noise, such as an explosion, or progressively through exposure to loud noises, like working with machinery.

It can even be caused by things you experience in daily life, such as traffic or listening to loud music.

The National Institutes of Health(NIH) estimates that between 6% and 24% of adults in the United States under age 70 have some degree of NIHL. Teenagers are even more susceptible as 17% of them in the United States are estimated to have NIHL in one or both ears.

Hearing aids are the “gold standard” for treatment of NIHL, but there are also surgical and even some medicinal options. However, when it comes to hearing loss, the best thing to do is to prevent it in the first place.

“People do not realize how damaging loud noises can be. Our experiments show that even 100 decibels – the level of noise one can expect at a soccer match – is enough to trigger rapid zinc release, damage in the inner ear, and hearing loss. Prevention is the best way to protect your hearing,” said Tzounopoulos.

According to the NIH, regular exposure to sounds over 85 decibels can cause hearing loss. To better understand what that means, consider the following examples of sounds you might experience with some regularity:

  • Normal conversation: 60-70 decibels
  • Movie theater: 74-104 decibels
  • Motorcycles and dirt bikes: 80-110 decibels
  • Music with headphones at maximum volume or a live concert: 94-110 decibels
  • Sirens: 110-129 decibels
  • Fireworks: 140-175 decibels

Prevention just means common sense measures such as using earplugs if you know you’ll be exposed to loud noises.

“Current guidelines recommend that patients with noise exposure, continue to wear earmuffs, and/or earplugs to mitigate the hearing loss damage,” said Feeley.
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