A study by the Stanford University School of Medicine has challenged the belief that the damage to the cochlea caused by loud blasts results in irreversible hearing loss. The study was published in PLOS ONE on July 1, 2013.
Researchers found that the damage done to the cochlea, the auditory part of the inner ear, is hair-cell and nerve-cell damage, and not structural.
“It means we could potentially try to reduce this damage,” said John Oghalai, MD, associate professor of otolaryngology and senior author of the study. Earlier studies determined that the cochlear had been ripped apart and shredded by loud blasts, creating irreversible damage. Stanford researchers speculated that these conclusions may be based on the use of older, less sophisticated imaging technology. The damage done to the cochlea by loud blasts is currently not well understood by scientists.
Previous studies on larger animals had found that the cochlea was torn apart and shredded after exposure to a loud blast, causing irreversible, long term hearing loss. Stanford scientists felt that damage done to the cochlea can potentially be treated and speculate that conclusions of previous research may have occurred as a result of older research and imaging techniques.
Scientists hope the study’s results can lead to treatments for people with long-term, noise-induced hearing loss, particularly veterans. “The most common issue we see veterans for is hearing loss,” said Oghalai, a scientist and clinician who treats patients at Stanford Hospital & Clinics and directs the hearing center at Lucile Packard Children’s Hospital. Many veterans suffer long term hearing loss after loud blasts as well as tinnitus. Other loud noises that can cause hearing loss are gunfire, air bags, or airplane engines. Damage to the eardrum after blasts can be detected during a clinical exam and either the eardrum heals itself or it can be repaired through surgery.
- More than 60 percent of wounded-in-action veterans had tinnitus, eardrum injuries or hearing loss, a combination of one or more of these
- 28 percent of military personnel experience some hearing loss
- The most damaging effect of a blast injury to the ear is permanent hearing loss because of trauma to the cochlea
The scientists created a mouse model for the study. Anesthetized mice were exposed to loud blasts. Then the researchers examined the inner workings of their ears from the eardrum to the cochlea for a three month period. A micro-CT scanner took images of the ear.
Permanent hearing loss occurs when the ear is exposed to loud noises beginning at about 85 decibels, the equivalent of the sound of a food blender or hair dryer. Improvised explosive devices have noise levels of nearly 170 decibels.
“When we looked inside the cochlea, we saw the hair-cell loss and auditory-nerve-cell loss,” Oghalai said. “With one loud blast, you lose a huge number of these cells. What’s nice is that the hair cells and nerve cells are not immediately gone. The theory now is that if the ear could be treated with certain medications right after the blast, that might limit the damage.”