May
19
2014

Noise and hidden hearing loss

Researcher_microscope_290x170 AoHLEven moderate levels of noise (similar to a busy street) may damage hearing. Tracey Pollard is Research Programme Manager in the Biomedical research Team at UK charity Action on Hearing Loss. She tells us about ‘hidden hearing loss’ and some of the research Action on Hearing Loss is funding into noise-induced hearing loss.

Hidden hearing loss
Exposure to very loud sounds (such as a chainsaw) can permanently damage your hearing and cause significant hearing loss. It has recently been realised that even moderate levels of noise (similar to a busy street) may damage hearing, but in a different way. The damage doesn’t affect someone’s ability to hear quiet sounds, but rather makes it harder to pick out important sounds, such as someone’s voice, from competing background noise. This damage is permanent, but because it doesn’t show up in standard audiological tests, which measure our ability to hear quiet sounds, it’s often missed and is therefore called ‘hidden hearing loss’.

What causes hidden hearing loss?
Damage to the sensory hair cells that detect sound causes permanent hearing loss (or a permanent increase in hearing thresholds) – extremely loud noise kills hair cells, and once they’ve died, they cannot be replaced. With hidden hearing loss, however, damage is sustained by nerve cells in the cochlea – they don’t die immediately, but instead they quickly lose their connections with hair cells, so they cannot send information to the brain. As a result, the brain receives degraded information from the ear, and struggles to interpret it correctly.

One particular kind of nerve cell in the ear is especially susceptible to noise damage; these nerve cells only respond to sounds above a particular volume. This makes them less sensitive to background noise, so they’re important for picking out one specific sound from background noise – like following the voice of a particular person who’s talking when lots of other people are too. The hearing loss caused when these cells are damaged is significantly under-diagnosed, and likely to affect younger people, who go to loud music concerts or frequently listen to loud music through headphones. It’s also likely that people who sustain this kind of hearing damage when they’re younger will suffer more severely with hearing loss as they get older.

Research into noise damage and hearing loss
At Action on Hearing Loss, we’ve been funding research looking at the problem of noise and hearing from a variety of angles. We funded work in Belgium which led to the identification of genes that are involved in noise-induced hearing loss – we hope that this will help to develop drugs to treat or prevent this type of hearing loss and diagnostic tests to predict whether someone might be at increased risk of hearing damage from noise. We also funded research in Leicester which uncovered more about the nature of the damage sustained by our hearing, showing a specific type of damage to auditory nerve cells after exposure to loud noise.

Our current projects in this area include research, also in Leicester, investigating the impact of noise exposure on the brain itself. It is thought that noise doesn’t just damage the cells in the inner ear like hair cells or cochlear nerve cells, but can also affect how the brain processes sound. In Manchester, we’re funding researchers assessing the extent of hidden hearing loss in people who work in noisy environments, and to determine how to measure this damage clinically. This work could have implications for the regulations protecting the hearing of workers who are regularly exposed to loud noise and the levels they can be safely exposed to. We’ve also given a small grant to a group in America who are running a clinical trial of a drug that they hope can be used to protect people’s hearing, when they are regularly exposed to loud noise, such as members of the military. Our funding is helping them to find out how the drug works at the cellular level.

In October, two new projects will start looking at noise-induced hearing loss. The first, based in Manchester, will look in more detail at the link between hidden hearing loss and damage to the auditory nerve in people with this kind of hearing loss, to try to find out more about possible ways of treating it. And at University College London, researchers will develop a model of noise-induced hearing loss in fruit flies, to tell us more about the genes and processes involved in the damage caused to our hearing by noise.

We hope that through funding this research, we will obtain a better understanding of the effects of noise on our hearing, which will lead to the development of treatments to protect hearing from and restore hearing after exposure to loud noise.

Tracey Pollard

To find out more about Action on Hearing Loss and research they’re funding, visit www.actiononhearingloss.org.uk/biomedicalresearch

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