The most common injury to American troops is silent and invisible. And I don't mean PTSD: hearing loss and tinnitus—ringing in the ear—top the list of service-related disabilities for veterans. They are an unsung consequence of prolonged exposure to roaring environments, such as the deck of an aircraft carrier, or, increasingly, to the sudden blast of a roadside bomb. One fifth of veterans of the Iraq and Afghanistan wars are affected, according to a 2017 analysis. Hearing loss has enduring social and economic impacts, harming one's ability to earn a living and the quality of relationships. The Department of Defense is in the process of calculating the enormous financial costs.
Noise trauma is a civilian problem, too. Up to 24 percent of U.S. adults have a hearing loss consistent with damage from noise, and, shockingly, 20 percent of teenagers have hearing issues, although whether it is caused by blaring earbuds or something unrelated to booming sounds is unknown. Now a major step has been taken toward understanding the precise mechanisms of injury from loud blasts. Along with that discovery comes an intriguing opportunity to intervene and preserve hearing.
The inner ear, which processes sound, is protected by one of the densest bones in the body, the otic capsule, making it difficult to visualize its tiny structures with conventional imaging. But a tool developed a few years ago by John Oghalai, then at Stanford University and now chair of otolaryngology at the University of Southern California's Keck School of Medicine, uses a laser-based technology called optical coherence tomography (OCT) to get the picture. OCT is already used to look at the retina of the eye. “We built this into a special microscope so that we could look inside the cochlea, the auditory portion of the inner ear,” Oghalai explains.
Using OCT in mice, Oghalai and his colleagues were able to see for the first time what happens when the ear is exposed to an explosive blast—akin to a roadside bomb—and reported the results in a recent paper. First, the shock wave overwhelms the tiny hair cells that line the snail-shaped cochlea. The delicate hairs of these cells “can detect very quiet sounds,” Oghalai says, “and when you have a big blast wave, it's just going to break them.” In the wake of the destruction, potassium ions build up in the inner ear fluid called endolymph, pulling in more liquid by osmosis. The resulting swelling begins to damage the synapses linking surviving hair cells to auditory neurons. In the mouse model, the hair cells lose about half their connections to auditory nerve fibers, which means they cannot send proper signals for the brain to interpret as sounds.
When the brain loses sound input, it fills the gap with the buzzing din known as tinnitus. At least that is the leading theory. Oghalai likens tinnitus to phantom limb pain. It can be temporary, as often occurs after an earsplitting rock concert, or infuriatingly constant.
In his mouse studies, Oghalai saw a chance to intervene in the window between the instant harm to hair cells and the delayed destruction of nerve synapses. His team was able to protect the latter by injecting a very salty solution through the eardrum, which reversed the buildup of fluid in the cochlea.
Could this approach lead to a battlefield intervention? A lot more research is needed, but saving neural connections, even if some hair cells are lost, could potentially make a functional difference in hearing. Past research suggests that lost synapses may lead to the common conundrum of being able to detect faint sounds on a hearing test and yet not being able to distinguish speech in a noisy environment—an issue that hearing aids do not fix very well.
“It would be great to have the ability to intervene in the minutes or hours or days after an exposure,” says Sharon Kujawa, director of audiology research at Massachusetts Eye and Ear. Currently the remedy for sudden hearing loss—such as after a firecracker mishap on the Fourth of July—is to treat with corticosteroids.
Better treatments are a priority for the U.S. military, as reflected in the 2012 founding of the Department of Defense's Hearing Center of Excellence. A variety of new therapies are in early stages of development to prevent and even reverse damage, says Tanisha Hammill, who coordinates research at the center.
Perhaps the most exciting thing about Oghalai's work, Kujawa says, is the advent of a “new and powerful technique” to peer into the living ear and watch hidden events unfold. The audiology world is all eyes and ears for the opportunities it will open.