Navy Medical Breakthrough Could Repair Hearing Loss
By Staff Sgt. Alicia K. Borlik
American Forces Press Service
WASHINGTON, Sept. 17, 1998 Hearing-impaired service members and those whose jobs put their hearing at risk may benefit from a recent Navy medical breakthrough.
An Army-Navy research team at the Navy Medical Center in San Diego developed an innovative way to rescue a person's hearing from the deadly effects of loud noises, said Navy Capt. Dennis McBride, program manager, Biomedical Science and Technology, Office of Naval Research, Arlington, Va.
The inner ear is home to thousands of tiny hair cells that vibrate to different frequencies or tones, he explained. As the hairs vibrate, they send a message the brain interprets as sound. Loud noise causes the hair cells to produce a toxin that makes them "commit suicide," McBride added.
Gun blasts, "which happen a lot in the services," McBride said, and other painfully loud noises start the death process -- ringing in the ears is a warning. Hair cells start releasing suicide toxin about four hours after exposure, he said. Hearing losses occur as hairs weaken and die; the body doesn't replace these cells.
Lower, constant noise levels can injure hearing as well -- even through hearing protectors, he said. Service members around aircraft carrier deck operations, gunnery ranges and flightlines and other loud work areas can be at risk.
The researchers were trying to save hearing and hair cells by testing an oral steroid drug, but they learned the amount needed to deliver even a limited dose to the ear caused undesirable side effects. They decided to try applying a related drug, methylprednisolone, directly to the dying cells.
The team's treatment involves implanting a tiny tube through the eardrum up to a membrane that begins the inner ear, McBride explained. The tube is attached on the outside of the ear to a reservoir and pump that release the drug into the inner ear chamber. The drug coats the hair cells and protects them from the toxin, he added.
McBride said test patients wear the Navy researchers' device for 10 to 14 days while the medicine is released. Research shows this time period is optimal for protection and the healing of injured hair cells, he said.
DoD spends about $1.5 billion a year treating noise-induced hearing loss. "It's an awful problem for human beings when they lose their hearing, and we are compelled to try to treat it, McBride said. We are a very noisy environment. We are very aggressive about working the environmental aspects in reducing the noise, but nothing is perfect."
DoD also has an aggressive hearing protection program, but the problem with current ear plugs and phones is that they block all sounds, which could cause safety hazards. "On a flight deck of an aircraft carrier, the chief says, 'Hey sailor!'" McBride supposed. "You don't hear him, and you get run over by an airplane. You didn't lose your ears, but you lost your life."
DoD needs safe, effective hearing protection and a medical procedure to reverse noise-induced hearing losses, he said. "We're working on protection that might allow certain tones in and keep others out," he said. "At the same time, we have a backup plan [that says], 'Soldier or sailor, if you get hurt, we're going to fix you.'"
McBride said three patients who'd suffered sudden hearing losses have been treated to complete recovery, but that's not enough. More patients are needed for a large enough sample to determine statistical significance, he said.
"If you get 15 [completely recovered], then you can say, 'I doubt this is chance alone,'" he explained.
Clinical tests continue at the San Diego center. "We need to temper the enthusiasm over the initial results," said researcher Dr. Richard Kopke. "There's a lot we don't know, so we need to go forward carefully in a controlled fashion." Among other tasks, the team is still devising a treatment protocol, he said.
Kopke said the research team invites calls from the attending physicians of any interested DoD healthcare beneficiaries who develop sudden, severe hearing losses from any cause -- the sooner, the better. The team can be reached at the Defense Spatial Orientation Center in San Diego at 1-619-532-9563.
McBride is optimistic. He projects more clinical use in the next six months and believes the method could be common medical practice in as little as three years if the drug proves safe and effective. If his prediction is right, he said, the Navy Bureau of Medicine and Surgery would set the military's standards, policy and medical doctrine.
In addition to continuing its promising clinical tests with noise-trauma patients, the team is researching animal-test evidence that the drug can protect hearing and prevent long-term losses if given in advance in controlled oral dosages, McBride said. The drug hasn't been tested this way in humans, he remarked.
The potential is staggering, he suggested. For instance, he said, a special operations unit could use the medication to prepare in advance for work in a dangerous, noisy environment. The drug would protect unit members' hearing while allowing them to hear everything going on around them.
"The intensity of research in this area is very high," McBride said.