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Why Can't You Hear?

Many people struggle to understand conversation because of noise damage.

Next time you pass that drill on your street, cover your ears. Even better, bring ear plugs.

Daily noise pollution damages us more than we know.

Scientists have recognized for about a decade that a powerful blast of noise can shut down synapses in your brain that help you understand a conversation. This isn't ordinary hearing loss related to age. In fact, in a study of more than 100,000 patient records from a 16-year period, about 10 percent of patients who visited the audiology clinic at Massachusetts Eye and Ear had a normal audiogram. Many probably suffered from what audiologists call "hidden hearing loss," often related to loud noises.

A woman I'll call Alice has struggled with her hearing for her entire adult life. Recently, she told me, she dared to go to a party and stood in a cluster of three in a room of 15 people, a reunion of her college classmates. “I was right next to them and I couldn’t hear one word,” she said. “I eventually asked one to go into a room with just a few people, and we were sitting but I had to move my chair closer and lean forward. I said, ‘I’m sorry if I seem like I’m sitting in your lap.’”

Alice first went to an audiologist to check her hearing when she was only 20. Her audiologist told her that her hearing was normal, but suggested that she might have an “attention problem.”

But Alice can hear a whisper in a quiet place; she only has trouble understanding what audiologists call “speech in noise,” conversation in groups or noisy places. As background noise in restaurants became steadily louder, she looked for quiet restaurants. Large parties “lost their appeal,” she said. She became a therapist. She explained, “It’s a wonderful profession because it’s just me and another person and if I can’t hear them, I say 'What?'”

She had her second audiogram a year ago, decades after the first. Again, her hearing was normal and the audiologist explained her difficulties as an “auditory processing problem.” She took her audiogram to Costco’s hearing aid department but was told that because her hearing was in the normal range, the store wouldn’t sell her an aid.

So what's going on?

When we hear, movement in the cilia, or hair cells, in the inner ear send signals to the nerve of hearing, also called the “VIIIth” nerve, crossing over synapses. Ordinary hearing loss arises from damage to the hair cells or the nerve. Hidden hearing loss often arises because of a loss of synapses in between. The signal arrives incomplete, missing information we need to interpret words.

Audiologists have described patients like Alice for years. In 2009, a watershed mouse study documented that loud noises could destroy synapses. In the study, mice were forced to endure a 100-decibel noise—about the same level as using a lawnmower—for two hours. Later, the team discovered that although the mice’s hair cells had survived, half of their synapses were gone.

Humans with lost synapses may still hear the beep in a hearing test even at a low volume that stumps someone with cell or nerve damage.

Note that hidden hearing loss can be mistaken for attention deficit hyperactivity disorder (ADHD) as Alice's first audiologist did. It’s also not the same as “central auditory processing disorder,” which is often diagnosed in children and arises at a different level of the brain.

The team at Massachusetts Eye and Ear has developed two tests to catch hidden hearing loss. The first measures electrical signals from the surface of the ear canal to capture how well they encode subtle and rapid fluctuations in sound waves. For the second test, participants wear glasses that measure changes in the diameter of their pupils while listening to speech in noise. Our pupils reflect how much effort it takes to understand during a task.

When the team tried out the tests on 23 volunteers with clinically normal hearing, their ability to follow a conversation with babbling in the background varied widely. The two tests together predicted which people would have difficulty.

Noise pollution and aging combine to aggravate the problem. “Most researchers feel that long exposures to even low-level noise may cause hidden hearing loss and most agree that the aging auditory system reveals this problem. We lose some synapses as we age,” said Catharine Palmer, Director of Audiology and Hearing Aids at the University of Pittsburgh Medical Center.

Another possible cause, reported in 2017, could be problems with the cells that make myelin, a substance that insulates the neuronal axons in the ear. Autoimmune disorders like Guillain-Barré syndrome—linked to food poisoning, the flu, hepatitis, and the Zika virus—attack myelin.

Stay tuned here for any information I can find on what one can do.

But please, try not to deny the problem and withdraw. Like any kind of hearing loss, this “can have an effect on your psyche, creating avoidant behavior and social anxiety,” Alice noted. “You might not even know it.”

Research is underway to find medications that would prompt neurons to grow new synapses. In the meantime, although there’s no cure for hidden hearing loss, you can minimize its impact. If you’re exposed to noise, look for ways to protect yourself. If you already struggle to hear in groups, the first step is to get a baseline hearing test. State-of-the art hearing aids have “speech in noise” settings that use direction microphones to pick up the signal in front of you and reduce sound behind you or on your sides. You might place a microphone near the signal you need to hear and wear a Bluetooth receiver or hearing aid in your ear.

Like people with ordinary hearing loss, you’ll need to avoid noise. Eat earlier in the evening when restaurants are quiet, choose restaurants with carpeting and without bars, or sit in a booth. Arrive at lectures earlier so you can sit near the front. At gatherings, don’t be shy about creating smaller groups and learning in—even if you feel like you’re sitting in someone’s lap.

A longer version of this piece appears at Healthy Hearing.

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