LOS ANGELES - It's well known that exposure to extremely loud noises -- whether it's an explosion, a firecracker or even a concert -- can lead to permanent hearing loss. But knowing how to treat noise-induced hearing loss, which affects about 15 percent of Americans, has largely remained a mystery. That may eventually change, thanks to new research from the Keck School of Medicine of USC, which sheds light on how noise-induced hearing loss happens and shows how a simple injection of a salt- or sugar-based solution into the middle ear may preserve hearing. The results of the study were published today in PNAS.
To develop a treatment for noise-induced hearing loss, the researchers first had to understand its mechanisms. They built a tool using novel miniature optics to image inside the cochlea, the hearing portion of the inner ear, and exposed mice to a loud noise similar to that of a roadside bomb.
They discovered that two things happen after exposure to a loud noise: sensory hair cells, which are the cells that detect sound and convert it to neural signals, die, and the inner ear fills with excess fluid, leading to the death of neurons.
"That buildup of fluid pressure in the inner ear is something you might notice if you go to a loud concert," says the study's corresponding author John Oghalai, MD, chair and professor of the USC Tina and Rick Caruso Department of Otolaryngology - Head and Neck Surgery and holder of the Leon J. Tiber and David S. Alpert Chair in Medicine. "When you leave the concert, your ears might feel full and you might have ringing in your ears. We were able to see that this buildup of fluid correlates with neuron loss."
Both neurons and sensory hair cells play critical roles in hearing.
"The death of sensory hair cells leads to hearing loss. But even if some sensory hair cells remain and still work, if they're not connected to a neuron, then the brain won't hear the sound," Oghalai says.
The researchers found that sensory hair cell death occurred immediately after exposure to loud noise and was irreversible. Neuron damage, however, had a delayed onset, opening a window of opportunity for treatment.
A simple solution
The buildup of fluid in the inner ear occurred over a period of a few hours after loud noise exposure and contained high concentrations of potassium. To reverse the effects of the potassium and reduce the fluid buildup, salt- and sugar-based solutions were injected into the middle ear, just through the eardrum, three hours after noise exposure. The researchers found that treatment with these solutions prevented 45-64 percent of neuron loss, suggesting that the treatment may offer a way to preserve hearing function.
The treatment could have several potential applications, Oghalai explains.
"I can envision soldiers carrying a small bottle of this solution with them and using it to prevent hearing damage after exposure to blast pressure from a roadside bomb," he says. "It might also have potential as a treatment for other diseases of the inner ear that are associated with fluid buildup, such as Meniere's disease."
Oghalai and his team plan to conduct further research on the exact sequence of steps between fluid buildup in the inner ear and neuron death, followed by clinical trials of their potential treatment for noise-induced hearing loss.
About the Keck School of Medicine of USC
Founded in 1885, the Keck School of Medicine of USC is among the nation's leaders in innovative patient care, scientific discovery, education and community service. It is part of Keck Medicine of USC, the University of Southern California's medical enterprise, one of only two university-owned academic medical centers in the Los Angeles area. This includes Keck Medical Center of USC, composed of Keck Hospital of USC and USC Norris Cancer Hospital. The two world-class, USC-owned hospitals are staffed by more than 500 physicians who are faculty at the Keck School. The school has more than 1,750 full-time faculty members and voluntary faculty of more than 2,400 physicians. These faculty direct the education of approximately 800 medical students and 1,000 students pursuing graduate and postgraduate degrees. The school trains more than 900 resident physicians in more than 50 specialty or subspecialty programs and is the largest educator of physicians practicing in Southern California. Together, the school's faculty and residents serve more than 1.5 million patients each year at Keck Hospital of USC and USC Norris Cancer Hospital, as well as USC-affiliated hospitals, Children's Hospital Los Angeles and Los Angeles County + USC Medical Center. Keck School faculty also conduct research and teach at several research centers and institutes, including the Eli and Edythe Broad Center for Stem Cell Research and Regenerative Medicine at USC, USC Cardiovascular Thoracic Institute, USC Institute of Urology, USC Mark and Mary Stevens Neuroimaging and Informatics Institute, USC Norris Comprehensive Cancer Center, USC Roski Eye Institute and Zilkha Neurogenetic Institute.
In 2018, U.S. News & World Report ranked the Keck School among the top 35 medical schools in the country.
For more information, go to keck.usc.edu.
This study was supported by the National Institutes of Health under award numbers DC014450, DC013774, DC010363 and UL1TR001085 and the U.S. Department of Defense under award number W81XWH-11-2-0004 (DM090212). Approximately 90 percent of the project's funding was federally funded. Ten percent of the project's funding was not federally funded. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.