CNS Hearing Loss

Imagine you’re out to dinner with friends at a popular local spot. Everyone is talking and laughing around you but you’re having a hard time understanding what they’re saying. You've asked "What?" about one thousand times. This eventually becomes so frustrating that you tune out of the conversation altogether. What’s unusual is that you just had a hearing test last month and passed with flying colors. What’s the deal?

This is “hidden hearing loss,” and if you relate to this scenario at all, you could very well be suffering from it. Harvard researcher Dr. M. Charles Liberman explains that this type of hearing loss can go undetected by a routine hearing test.

To put it simply, sound enters the inner ear, making the eardrum and chain of tiny bones and hair cells vibrate. These hair cells turn the vibrations into electrical signals and send them off to auditory nerve fibers that then deliver the information to the brain. Voila, hearing.

An audiogram is the standard exam to test how well that process is working. An audiologist presents tones at different frequencies and you say when you can hear them. If you can’t hear them, that indicates hearing loss.

When it comes to hearing loss, it’s always been thought that hair cells, or sensory cells, were the most vulnerable element. But seven years ago, what Liberman discovered in mice was that well before the sensory cells were damaged, the connections to the nerve fibers could disappear through exposure to loud sounds. This goes completely undetected by the audiogram and also explains why information isn’t easily delivered to the brain, making it difficult for you to hold a conversation in settings with background noise.

“We always used to think if the audiogram came back normal, your ear was undamaged,” Liberman says. “And what we and a bunch of other people have shown in animals is that that’s not true. Just detecting whether sound is there is much easier to do than to understand it.”

After seeing the same results in studies with mice, rats, guinea pigs, chinchillas and even monkeys, Liberman’s team wanted to find human evidence of this phenomenon. They succeeded. Their recently published study showed that, when they separated college-aged students with normal audiogram scores into groups of those who always wear ear plugs to concerts and those who don’t, the latter group scored more poorly on a difficult wordsin- noise recognition test. These students had done more previously undetected damage to their ears.

“The idea is if you compromise the ear at a young age, then it’s possible subsequent age-related problems will be worse,” he says. “Your ear will deteriorate at a faster rate.”

To avoid this, Liberman advises wearing earplugs at loud events, as well as while mowing the lawn or using power tools. For concerts, he recommends musician’s earplugs. which don’t compromise the sound of the music.

It’s not all bad news. According to Liberman, in age-related hearing loss, the major complaint is detecting sound but not being able to understand it in a noisy environment. Unfortunately for the elderly, people often write this off as cognitive decline or problems with the brain. But these hidden hearing loss findings might help to better explain— and even prevent—this issue. Because experts know the chemicals required to stimulate and reconnect nerve fibers, treatment can be developed to fix the problem. In fact, they’ve successfully accomplished it in mice, already.

“For the first time, there’s a glimmer of hope on the horizon that sometime in the next 10-20 years they’re going to figure out how to really do this in people and bring some hearing back,” he says.