What is 'Hidden Hearing Loss'?

Hidden hearing loss is devilishly sneaky. Those who have it may not even know it. The condition eludes common hearing tests, reassuring the individual that his or her ears are functioning just fine…until the deficit pops up at the most inconvenient times. Say, when you’re in a noisy restaurant talking with an important client and can’t be sure if she just agreed with your point or disputed it. Or at a concert when you think your date said, “Let’s get more beer”…or was it “Let’s get outta here”?

SonicCloud sat down with Stéphane Maison, Ph.D., an assistant professor of otolaryngology at Harvard Medical School and principal investigator at Massachusetts Eye and Ear, a Boston-based research and treatment center. We asked him about his surprising discoveries about hidden hearing loss – and the exciting implications about solving this frustrating problem.

Q: What exactly is hidden hearing loss?A: Called cochlear synaptopathy, this hidden hearing loss is damage to the synapses, which are the connections between the auditory nerve fibers and sensory cells. This type of damage happens before the loss of sensory cells associated with typical hearing loss. Cochlear synaptopathy may also be related to tinnitus and hyperacusis, the increased sensitivity to sound.

Q: How does this type of hearing loss make everyday life more difficult?A: If you work in a noisy environment or in a place with a lot of reverberation, you’re going to struggle. The main problem will be understanding speech. Often, you’ll be able to hear someone talking, but you can’t figure out what it means.

Q: We’ve heard this can be a challenging type of hearing loss for medical professionals to detect. What do you recommend?A: In the typical audiogram, the frequencies tested range from 250 Hz to 8000 Hz. In the quiet testing room, there’s no difference between someone with hidden hearing loss and those with no damage within those ranges. However, if you ask that person to repeat words with background noise, reverberation or time-compressed words, such as when someone speaks super-fast, it’s going to be more challenging for those with cochlear synaptopathy. So, someone with hidden hearing loss has normal hearing thresholds and difficulties understanding speech in challenging environments. We suspect that the nerve fibers that code for louder sounds are damaged.

This kind of hearing loss was detected in the study of 34 college students. One group were musicians who were repeatedly exposed to loud music, the other protected their hearing and didn’t have prolonged contact with high-volume sounds. Our findings confirmed the existence of hidden hearing loss in the former group – when we tested them appropriately.

Q: If someone suspects hidden hearing loss, what’s should he or she do?A: Right now, there is no way that the medical community is diagnosing cochlear synaptopathy and unfortunately there is no cure. But if you struggle hearing in noisy environments, you know it’s real. There are a few tools and communication strategies that can improve your ability to understand. For example, face the person you’re talking to. You could also use a directional microphone or a phone application that can help in noisy environments.

Q: Can hearing aids help with hidden hearing loss?A: Hearing aids work by bringing in more sound. If anything, some people with hearing aids complain they’re too loud. A lot of people who have hearing aids say the problem isn’t the level of sound, but the clarity, especially the clarity of speech. Clarity is derived from the nerves, and if the nerve is missing, boosting the sound won’t provide clarity. This is why it’s important to establish a diagnostic to determine type of hearing loss to provide proper treatment.

Q: What were some of the revelations of your research?A: Using a different type of microscope and chemistry that binds proteins, we can see the actual synapses in the ear that are impacted. And we’ve seen from experiments in the lab that in the presence of loud noise, the very first thing to go is the connection between the sensory cell and the neuron. So that’s a complete change in our view of acoustic trauma. In the past, we’d go to a bar with loud music, hear buzzing or ringing then the next day think that hearing was back to normal. We thought that if there was no damage to the hair cells, that hearing wasn’t damaged. In fact, we now know that even though hearing threshold and density may be back to normal, there can be damage to the inner ear.

Q: What does your research mean for the future of hearing loss treatment?A: Through the research in mice, we found we can regrow the synapse. In laboratory mice, the hearing levels can go back to where it was before the trauma. This isn’t done through a hearing aid. We’re now talking about restoring hearing. This is truly exciting.

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