Roadside bombs injure soldiers in many ways, including causing hearing loss. The loud blasts can damage hair cells in the inner ear as well as destroy the connections between auditory neurons. This damage is irreversible.
But a new study in mice suggests that triggering osmosis in the ear could prevent this noise-induced hearing loss (Proc. Natl. Acad. Sci. USA 2018, DOI: 10.1073/pnas.1720121115).
John S. Oghalai, a head and neck surgeon at the University of Southern California, and colleagues found the possible treatment while studying inner ear damage in mice. The researchers subjected anesthetized mice to loud blasts equivalent to the sound soldiers would experience when a roadside bomb detonates near them. The team then used an imaging technique called optical coherence tomography, which they had previously developed to follow what happened to structures inside the mice’s ears in the hours after the blast.
They saw that membranes that separate different ducts within the inner ear started to bulge in the first three hours after the blast, which suggested that one of the chambers inside the cochlea was filling up with a fluid called endolymph. The cochlea is a spiral structure in the inner ear containing the moving parts responsible for hearing. In the images, the researchers also observed that hair cells and synapses, the connections between auditory neurons, were increasingly damaged over the following seven days. The team concluded that the fluid buildup was responsible for the damage to the cells and synapses.
To reduce the amount of endolymph, the researchers turned to osmosis. They injected a solution that resembled perilymph, the fluid surrounding the cochlear duct, onto the opening to the inner ear. This changed the concentration of sodium and potassium ions in the perilymph and increased the rate of osmosis, causing the endolymph to flow out of the duct.
After two months, mice treated with the most concentrated synthetic perilymph had lost 45 to 64% fewer synapses compared with untreated mice. These results suggested that noise-induced hearing loss could be halted if doctors could reverse the endolymph buildup quickly enough.
“Blast-level sound exposure is a major health burden to armed forces personnel carrying out combat missions,” says Philine Wangemann of Kansas State University. Oghalai’s work, she says, is an important step toward developing a treatment to prevent hearing loss from this kind of trauma.
Such a treatment could help people exposed to many types of damaging loud noises, such as those from firecrackers, concerts, or sirens, Oghalai says. He thinks the time window for receiving treatment would be 12 to 24 hours after exposure.
“Treatment could be either an injection into the ear, or if we can develop it, an intravenous injection or possibly even a pill,” he adds.