Hearing loss can come from decades of exposure to loud music at concerts or on earphones, or from a single loud bomb blast or industrial explosion.
Damage starts with sustained noise levels of about 85 decibels, equal to the racket of a hair dryer or food blender. Improvised explosive devices pack something like 170 decibels. So a single blast can deliver as much damage as results from lower levels of noise exposure over time.
But researchers studying mechanisms of hearing loss in mice are discovering that it may be possible to reverse or prevent damage to the microscopic hair-like cells, or cilia, inside the ear that make hearing possible.
Loud explosions don't actually destroy the cochlea -- the auditory portion of the inner ear -- but instead damage the cilia and nerve cells that help convert and send along to the brain the nerve impulses representing sounds, Stanford University researchers noted in findings published in the journal PLOS One in July.
The work was supported mainly by the Defense Department. More than 60 percent of service members wounded in action have injuries to their eardrums, tinnitus -- ringing in the ears -- hearing loss or a combination of the three, according to the researchers.
They exposed anesthetized mice to loud blasts and then looked at the inner workings of the rodents' ears. Inside the cochlea, they found some hair cells and nerve cells were damaged, the rest destroyed.
Once scar tissue forms, it limits the ability of any remaining hair cells to vibrate and pick up sounds. But if the body's immune response and inflammation around the damage could be stopped quickly or slowed, it might limit the damage and retain some hearing, the researchers said.
A second study showed that an experimental drug was able to at least partially regenerate cilia in mice that were completely deaf, researchers at Massachusetts Eye and Ear Hospital and Harvard Medical School reported in the journal Neuron in January.
The drug, labeled LY411575, works by blocking a protein. Called Notch, it ordinarily prevents stem cells, which normally function to support the hair cells, from becoming new cilia within the cochlea.
The study confirmed that the cells were generated from differentiating stem cells. And brain scans showed the new cells partially restored hearing in the mice. Though the their hearing fell far short of the normal range, the rodents could detect loud noise at low frequency, like a door slamming or a thunderclap.
Finally, in September, researchers from the Hearing Research Center at the Oregon Health and Science University, again working with mice, found that injections of a protein prior to noise exposure at 120 decibels caused cells within the ear to change how they arranged themselves in response to loud noise.
For the study, published in the Journal of the Federation of American Societies for Experimental Biology, researchers used a protein called pigment epithelium-derived factor, found in most vertebrates and already being studied for treatment of heart disease and cancer.
The scientists speculate that it might be possible to treat soldiers, emergency workers and others with the protein before they move into environments with a high risk of explosions.
(Contact Scripps health and science writer Lee Bowman at BowmanL@shns.com. Distributed by Scripps Howard News Service, www.shns.com.)