April 14, 2014 | 3
A single exposure to loud but not deafening noise may be enough to precipitate irreparable harm to nerves in the auditory system. This is the take-home from a new line of research that may help explain why many people, particularly as they age, have difficulty in picking out a conversation from the wall of background noise that is a requisite accompaniment to any football game or meal at a family-style restaurant.
Studies over the past five years in animals—with some evidence now coming from human research—are starting to overturn conventional wisdom about hearing loss. It was previously thought that the downside of exposure to the raucous sounds of an afternoon sporting event might do nothing more than leave you with the sensation that your ears were filled with cotton for a while, but then hearing would more or less return to normal.
Only years, perhaps decades, of assaulting the sensitive recesses of the inner ear would be enough to kill off the minute hair cells in the fluid-filled cavity where vibrations of sound waves are converted into electrical signals for processing inside the brain. It was thought that only with the hair cells gone would your ability diminish to hear clearly through the busyness of the daily aural clamor. (Of course, standing unprotected beside a jet engine for even a short period will do the trick of getting rid of hair cells immediately.)
The textbook account of what happens when things get loud may be a wholly inadequate depiction of what is really going on for many of the millions in the U.S. who suffer from noise-induced hearing loss. M. Charles Liberman and Sharon G. Kujawa, neuroscientists at Harvard Medical School and the Massachusetts Eye and Ear Infirmary who study the auditory system, have found that hair cells can survive an Animal House-style frat event, but the connecting nerve fibers that channel electrical signals into the brain may sustain permanent damage.
Their work in mice, guinea pigs and chinchillas has confirmed that a one-time exposure to a loud sound can cause withering of some of the uninsulated tips of nerve fibers that make their way into the brain. Their disappearance severs the connection to the hair cells across the tiny gap, or synapse. “There’s every reason to think that exactly the same thing is going on in our ears,” Liberman says. “At the level of the inner ear, all mammalian ears look the same,” He conjectures that, in noisy conditions, an excess of a signaling molecule, the neurotransmitter glutamate, is released from the hair cells into the synapse. Over a period ranging from months to years, the disconnected fiber leads to the death of the entire nerve cell—of which the synaptic nerve fiber is just a long extension.
There are up to 25 nerve fibers for each of the up to 4,000 signal-converting hair cells in humans. When a few of them die, it may at first produce a minimal impact on the ability to hear but, if the losses continue with repeated exposures, there is a slow decline in the acuity of what your ears can pick up. “You can create a visual analogy that if you downsample the pixels in an image, you can tell if there’s something there but you can’t tell what it is,” Liberman says.
The loss in auditory resolution doesn’t turn up on the traditional audiogram that measures the functioning of hair cells by determining whether they can detect a sound of a particular loudness and frequency. The threshold at which a sound can be discerned goes up after exposure to loud sounds but then, given a few hours or days, often returns to where it was. Some 90 percent of the nerve cells may have died but an audiogram can still be completely normal. At this point, you might be able to still hear sounds emanating from a dinner partner across the table, but have no capacity to distinguish individual words.
Liberman did seminal work in the 1980s on how noise kills hair cells. He had always wanted to find out what happens to the nerve fibers that connect to the hair cells, but until recent years, the needed staining agents to highlight the nerve endings under a microscope simply did not exist.
Research is now broadening beyond lab animals. Walter Reed National Military Medical Center has begun to do studies on Iraqi War veterans who have have impaired hearing despite normal readings on audiograms. Liberman and Kujawa have inspected 100 specimens of temporal bones that enclose the middle and inner ear from a collection at the Massachusetts Eye and Ear Infirmary. The bones came from individuals ranging from newborns to centenarians. For older individuals, there was a significant loss of the nerve cells that carry an electrical representation of sound into the brain, whereas the hair cells remained intact.
Liberman and Kujawa have started contemplating therapeutic measures to restore synapses. They are investigating whether injection of growth factor proteins through a membrane in the middle ear may allow the truncated nerve fibers to make new synapses and restore normal hearing—an idea that a Boston-based venture firm has shown interest in. They are also interested in determining whether a similar loss of nerve fibers may play a role in tinnitis (ringing in the ear) and what effect this may have on the ear’s vestibular system, and whether it might compromise balance.
Others have also started to take notice. “Charlie and Sharon’s work at Harvard has been extremely compelling in demonstrating in animal models how loud sounds can have a detrimental and cumulative impact on the auditory system and affect hearing,” says Frank Lin, a professor of otolaryngology and head and neck surgery at Johns Hopkins. “Their research emphasizes the importance and need for greater hearing conservation efforts focused on minimizing cumulative noise exposure over one’s lifetime.”
Paul Fuchs, a professor of neuroscience and biomedical engineering at Johns Hopkins adds: ”Synaptopathic’ hearing loss is an important addition to our understanding of hearing and deafness, especially since evidence shows that such damage can be caused by sound exposures previously thought to be harmless,”
If the evidence continues to mount, it could have implications for public-health policy. Liberman thinks repeated hits of noise exposure can be compared to the experiences of professional football players who sustain multiple, small concussions throughout their careers before being diagnosed decades later with a form of dementia, chronic traumatic encephalopathy (CTE). “There are analogies to CTE,” he says. “You get woozy from a concussion, you get better and think you’ve dodged a bullet and then you go out and do it again. Thirty years later, your brain is cheesecloth and you’ve got all sorts of problems.” Hearing is similar, he says. “The body’s little imperceptible deficits later catch up to you.”
Any sustained exposure to sound above 100 decibels may potentially put you at risk for one of those little imperceptible deficits, Liberman thinks. Some of the inter-city rivalry last fall between fans at Seattle Seahawks and Kansas City Chiefs home games to attempt the Guinness world record for the loudest stadium noise left fans boasting that their ears rang for days after. “Some percentage of people did irreparable damage,” Liberman says. It may be that the NFL may have to start worrying about the long-term brain health of their fans along with that of their players.
Image Source: David Benbennick
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