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Hearing Journal:
doi: 10.1097/01.HJ.0000315827.05734.be
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TAKING ON TWO TOXINS: AGE AND NOISE TRAUMA

Pallarito, Karen

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The most meaningful hearing research taking place in laboratories today, in terms of the sheer number of Americans who could benefit, isn't about curing deafness. It's about helping people regain hearing that's diminished due to age or noise trauma.

“Arguably these are much more important because these affect many, many, many more people than deafness,” says Lawrence E. Lustig, MD, director of the Division of Otology, Neurotology and Skull Base Surgery at the University of California San Francisco.

Age-related hearing loss, or presbycusis, affects roughly one in three adults aged 65 to 75 and as many as half of all people over 75, according to the National Institute on Deafness and Other Communication Disorders (NIDCD).

In addition, people of all ages are prone to noise-induced hearing loss due to excessive or prolonged noise exposure. About 10% of Americans aged 20 to 69 already have sustained permanent damage to their hearing from harmful noises, the NIDCD notes, adding that 30 million are at risk of it because of exposure to excessive noise loss in the workplace, in recreational settings, and at home.

So it's no surprise that many scientists are drawing a bead on these villains of hearing. Here's how two researchers are taking on the challenge:

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PUMPING UP HEARING POTENTIAL

What happens in the ear as people age may not be what you think. “I think there's a misconception out there that age-related hearing loss is caused primarily by the loss of sensory hair cells,” asserts Bradley A. Schulte, PhD, professor and vice-chair of research in the Department of Pathology and Laboratory Medicine and professor in the Department of Otolaryngology/Head and Neck Surgery at the Medical University of South Carolina.

In recent studies involving gerbils born and raised in a low-noise environment and subjected to very controlled conditions, Schulte and colleagues observed very little hair cell loss as the animals aged. Rather, old gerbils exhibited degeneration of the stria vascularis, the layer of tissue found in the lateral wall of the cochlea that produces a potassium-rich fluid called endolymph. They also saw a decline in “endocochlear potential” (EP), the battery power that drives hearing sensitivity.

Figure. Bradley Schu...
Figure. Bradley Schu...
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Schulte suspects these two events are intertwined and represent the major culprit in age-related hearing loss. Although the mechanisms responsible for this lateral wall pathology are unknown, he hypothesizes that the drop in EP leads to the shrinkage and loss of spiral ganglion neurons.

Now how do you fix that problem? Animal studies show that by using electrical current to boost EP, it is possible to re-establish hearing. Schulte thinks the same idea may apply to humans. “If we can pump up the EP and get that battery back to a near-normal level, we should be able to restore at least some hearing in older people” he says. A treatment, though, could be a decade away.

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DOSING AGAINST NOISE-INDUCED DAMAGE

Scientists studying noise-induced hearing loss, on the other hand, are beginning to see some early results. One of the researchers leading the charge is Richard D. Kopke, MD, chief executive officer of the Hough Ear Institute in Oklahoma City, OK.

Kopke's team conducted a randomized, double-blind, placebo-controlled study involving roughly 600 U.S. Marines in mandatory weapons training in San Diego. Half of them received N-acetylcysteine (NAC), an amino acid needed to make the antioxidant glutathione, while the other half received a placebo. All of them wore hearing protection, but those who took NAC had about a 25% decrease in permanent hearing loss compared with those on the placebo. The data were presented last year at a meeting of the American Otologic Society (AOS).

Figure. Richard Kopk...
Figure. Richard Kopk...
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Separately, Kopke and colleagues are interested in whether or not a combination of drugs might be more effective than a single medication in reducing permanent hearing loss after loud noise exposure. In a study using chinchillas, they found that combining NAC with other compounds was “very effective” when it was administered within 24 hours, and even more so when given closer to the time of injury. That paper also was presented at the AOS meeting.

“NAC is used by the body to form cysteine, which is used by the cells to form glutathione, and glutathione is one of the key antioxidants in the inner ear,” explains Kopke, a clinical professor in the Department of Otorhinolaryngology—Head and Neck Surgery at the University of Oklahoma Health Sciences Center. “So we believe that NAC eventually increases the glutathione levels in the inner ear, and that works against some of the oxidative stress that's going on.”

Drugs, of course, are no substitute for hearing protection. “We're not saying take a pill instead of using earplugs,” Kopke stresses. But in the military, in particular, damaging noise can occur without warning, and at levels that even protective devices can't mask.

“If you can't avoid the damage and you can't prevent it, then you have to treat it,” he says. The trick is administering the treatment before hair cells die and permanent hearing loss occurs. “That,” he adds, “is where hair cell regeneration comes in.”

© 2008 Lippincott Williams & Wilkins, Inc.

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