Check Up: Concert junkies could rock their hearing away

New research suggests that sound at 100 decibels - less than what can be encountered at concerts by Metallica (above) and other rock bands - can permanently damage hearing.
New research suggests that sound at 100 decibels - less than what can be encountered at concerts by Metallica (above) and other rock bands - can permanently damage hearing. (FELIPE DANA / AP)
Posted: July 07, 2014

It turns out your mother may be right - one rock concert could permanently damage your hearing.

That is the implication of new research from the Massachusetts Eye and Ear Infirmary, in which mice and guinea pigs were exposed to sound at 100 decibels for two hours.

That amount of noise is within federal workplace guidelines, and well below what one can hear at some concerts. Yet in the lab animals, it was enough to permanently disrupt transmission to some of the neurons in the inner ear.

The findings are worrisome, as the inner ear in the lab animals is close to our own, said physiologist M. Charles Liberman, director of the Eaton-Peabody Laboratories at the hospital. "There's every reason to believe that it applies to humans," said Liberman, who worked with colleague Sharon G. Kujawa.

The damage occurs to the dendrites, the delicate branches on the end of neurons in the inner ear. Ordinarily, these dendrites receive auditory signals from the ear's adjacent "hair cells" and carry them on to the brain.

But with sustained exposure to loud noise, the dendrites pull away from the hair cells and can no longer receive that signal - like unplugging the earbuds from an iPod.

Once the dendrites retract, they do not grow back, said Liberman, also a Harvard Medical School professor. His team measured the number of connections between the hair cells and the adjacent dendrites, and found that after two hours of exposure to 100 decibels, half of these connections were gone. Thomas Willcox, director of Thomas Jefferson University's Balance and Hearing Center, said the findings are cause for concern. "It could reshape the way we think about noise-induced hearing loss," said Willcox.

People may think they can recover from a noise-induced hearing loss. Indeed, the hair cells, which convert sound into an electrochemical signal for transmission, can recover from temporary harm caused by loud noise.

But even if the hair cells recover, any lost connections with the adjacent neurons do not, Liberman said. A person with this type of damage can still perceive soft noise as well as before. But with fewer connections to the cochlear neurons, that person will do so with less resolution - almost like with a blurry photograph - and will be less able to interpret it, he said.

This loss of resolution explains why someone who seems to have otherwise normal hearing can have a hard time understanding speech in restaurants and other loud places.

There may yet be hope. Using lab animals, Liberman's team is testing the use of neurotrophic proteins to spur the regrowth of dendrites.

Meanwhile, wear earplugs.


tavril@phillynews.com

215-854-2430

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