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The Dangers of a Misspent Youth

Researchers uncover the crescendoing costs of noise exposure.

GOING GAGA: Although the flash and sparkle of pop star Lady Gaga may dazzle, the frazzle to auditory structures caused by the ear-splitting volume of rock concerts may provide the more lasting memory.<br/><br/>© Darrin Zammit<br/>Lupi/Reuters/Corbis Lady Gaga mounts the stage in a sequin-studded corset and grabs the mike. Bleached-blonde bangs hide her eyes; short black boots accentuate long, bare legs. She pauses briefly before a spray of pyrotechnics and an explosion of drums send her hips gyrating. With a gloved fist pump, the high-kitsch diva begins belting out the opening verse to “LoveGame.” Teen nirvana.

That night, a young fan returns home with dreams of life as a pop phenomenon—and ringing ears. By the next day, though, her hearing returns to normal, and the impact of her evening of auditory excess has begun to fade, or so it seems. Research now suggests that ears hold grudges: Damage may progress for months, even years, after an assault.

“Until recently, we accepted the dogma that noise has a time-limited effect,” says Sharon Kujawa, HMS associate professor of otology and laryngology and director of audiology at the Massachusetts Eye and Ear Infirmary (MEEI). “We assumed that ongoing changes were related to other causes—to aging, for example. We also assumed that the return of hearing thresholds in the hours and days after exposure signaled a recovered ear.” Studies by Kujawa and colleagues are now challenging such assumptions about noise-induced hearing loss—and promising to inform future treatments.

The need for such treatments is growing. In the United States, an estimated ten million people have permanent hearing loss from noise, other forms of trauma, and aging. New work shows that hearing loss is a growing problem in young adults, and noise exposure is the primary suspect.

Kujawa collaborates with M. Charles Liberman, the Harold F. Schuknecht Professor of Otology and Laryngology at MEEI, to probe the relationship between noise-induced and age-related hearing loss, processes long assumed to be simply additive in a given ear. These investigators made a surprising discovery when they monitored genetically identical adult mice for years after subjecting them to loud sounds. Fleeting auditory insults—roughly akin to a blaring rock concert—changed the way the animals’ ears aged, long after the noise had stopped.

Additional research has since provided a possible explanation for these observations. Kujawa and Liberman have detected changes in cellular elements that convey messages from the sensory receptors of the inner ear—the hair cells—to the auditory nerve and brain. The animals still had picture-perfect hair cells, but shortly after noise exposure, the ability of those cells to communicate with the auditory neurons was interrupted—first by loss of structures that store and release chemical messengers called neurotransmitters, next by loss of the spindly extensions of neurons that reach the hair cells, and finally, over months to years, by loss of the auditory neurons themselves.

“These mice could still detect soft sounds in a quiet setting,” says Kujawa, “but they lost processing power. Translating to humans, they might hear you, but not understand you.”

In people, processing power comes in handy when we need to sort specific sounds from a jumble surrounding us. Kujawa suspects that processing deficits exacerbate other hearing problems, including those related to aging. Perhaps the teen concertgoer will pay for her exposure in a few decades, when a full complement of auditory neurons would have helped her understand the conversations of friends in a noisy restaurant.
Prevention in the form of ear protection is key. But even when protection is available, life does not always allow time for it. An improvised explosive device, for example, could detonate near soldiers, leaving them with permanent threshold shift, which impairs communication and changes the quality of their lives forever. What would Kujawa’s findings mean for them?

“Researchers at Harvard and around the world are working to develop drugs that could be administered before exposure to protect the ears or after exposure to rescue the ears and preserve hearing,” says Kujawa. “The goal is to interfere with the processes that destroy auditory neurons and demolish delicate hair cells.”

A morning-after pill for your ears. Imagine the possibilities.

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