The physiological bases and perceptual consequences of 'hidden' noise-induced hearing loss

Noise exposure is the main cause of preventable hearing loss worldwide. Noise exposure occurs in the workplace, such as in noisy factories, and recreationally, through the use of personal music players and attendance at nightclubs and live music events. Hearing loss is usually diagnosed using pure tone audiometry, which measures the sensitivity of the ear to quiet sounds by determining the levels of tones that can just be heard at several test frequencies. Until recently, it had been assumed that hearing loss results mainly from damage to the sensory hair cells in the cochlea, the part of the ear that converts acoustic vibrations into electrical impulses in the auditory nerve. However, recent results from animal studies suggest that moderate noise exposure can cause substantial damage to the auditory nerve, even when the hair cells are unaffected. Crucially, the results suggest that such damage does not affect sensitivity to quiet sounds, and hence is not detectable by pure tone audiometry. Hearing loss that is not detectable by conventional audiometry is sometimes called "hidden" hearing loss.

Auditory nerve damage degrades the information that is carried by the nerve from the ear to the brain. Some studies suggest that people with a history of noise exposure, but with normal hearing sensitivity as measured by pure tone audiometry, have problems with sound discrimination, including understanding speech in noisy environments. However, to date no direct link has been made between the physiological results and the perceptual deficits. It is also possible that damage to the auditory nerve leads to tinnitus (perception of sound in the absence of external sound: "ringing in the ear") and hyperacusis (diminished tolerance of moderate-to-high level sounds).

Hidden hearing loss is potentially a huge problem. Substantial numbers of people, probably millions in the UK alone, are routinely exposed to occupational and/or recreational noise levels similar to, or greater than, those used in the animal studies. A large UK study found that one in seven adults aged 17-30 reported "great difficulty" hearing speech in noisy backgrounds, while only one in fifty had impaired sensitivity as measured by pure tone audiometry. Hidden loss leads to a reduction in quality of life, and is likely to be predictive of more severe hearing loss in old age. Hence, hidden hearing loss is a major public health issue, which demands a comprehensive investigation.

Our programme is far-reaching and ambitious, involving three internationally renowned institutions across the UK and US, and a wide range of scientific methodologies. These include physiological and perceptual measures on both animals and humans. Our approach is to use overlapping methodologies across the animal and human studies so that we can understand the perceptual deficits experienced by humans in terms of the underlying physiological mechanisms. We will estimate the prevalence of hidden loss in young adults, and the impact of hidden loss on everyday tasks such as speech and music perception. We will also determine how hidden loss is related to tinnitus and hyperacusis. Finally, we will use our results to develop a sensitive diagnostic test that can be used to detect hidden loss, hence allowing the detection of hearing loss that is undetected by current clinical procedures.

Our research is expected to lead to a number of benefits. A diagnostic test for hidden loss in the clinic, and for monitoring the hearing of workers, will allow identification of at-risk individuals, and provision of personalised healthcare advice, regarding, for example, ways to reduce noise exposure. Our research could also result in a reduction in legal noise exposure limits. These measures will help prevent hidden loss, improve patient outcomes, and reduce usage of healthcare resources. Longer-term, there may be the possibility of reversing auditory nerve damage by replacing lost nerve fibres using stem cells.

Noise exposure is the main cause of preventable hearing loss worldwide. Dramatic results from recent animal experiments suggest that moderate noise exposure can cause substantial permanent damage to the auditory nerve that is not detected by standard hearing tests. A large number of people, probably millions in the UK alone, are exposed to occupational and recreational noise levels similar to, or greater than, those used in the animal experiments. There is some evidence that people with a history of noise exposure, but with normal hearing as measured by pure tone audiometry, have perceptual deficits that include tasks such as hearing speech in noisy backgrounds. This "hidden" loss may also be associated with tinnitus and hyperacusis. Together, these deficits lead to a reduced quality of life for the individuals concerned. It is vital that we understand the mechanisms and consequences of hidden hearing loss so that we can:

1. Adequately assess hearing ability, and provide appropriate treatments
2. Identify at-risk individuals and provide personalised healthcare advice to prevent further damage
3. Re-evaluate EU noise-exposure regulations

Our proposal is far-reaching and multidisciplinary, combining animal histology, neurophysiology, electrophysiology, and psychophysics, with human electrophysiology, psychophysics, and neuroimaging. By using overlapping methodologies across the animal and human experiments, it will be possible to make inferences regarding the physiological bases of noise-induced damage in the human auditory system; information that will be vital for the diagnosis and treatment of hidden hearing loss. The animal noise-exposure experiments will also be invaluable in the determination of safe noise levels for humans. The results will allow us to identify the perceptual effects of hidden damage, and provide a diagnostic test. We will also provide an estimate of the prevalence of this type of loss in young adults, and the risk factors involved.

Beneficiaries of the research include:

1. Individuals at risk through noise exposure.

These individuals will benefit from the diagnostic test, allowing more accurate and sensitive diagnosis of hearing loss, and personalised healthcare advice, leading to prevention of further loss and improved quality of life (~5-10 yr impact).

Accurate diagnosis and an understanding of the physiological mechanisms of hidden loss will pave the way for potential treatments such as noise reduction techniques in hearing aids (~10-20 year impact) and stem cell therapies (~15-30 year impact).

2. Workers at risk through occupational noise exposure.

These individuals will also benefit from the diagnostic test, allowing more sensitive health surveillance. There is also likely to be a re-evaluation of noise exposure regulations. Both outcomes will contribute to prevention of further loss (~5-10 yr impact).

3. Healthcare providers / clinicians

Benefit through improved diagnosis of hearing disorders, and ability to offer appropriate advice and counselling to those that report difficulties hearing in noisy situations but are currently told that they have "normal" hearing (~5-10 yr impact).

4. Policy makers concerned with noise exposure regulations and health surveillance in the workplace.

Benefit with more accurate information regarding the impact of environmental noise (~5-10 yr impact).

5. Industries and insurance companies

Benefit through more appropriate noise-exposure / surveillance regulations, and reduction in insurance claims for hearing loss (~10-15 year impact).

6. UK economy

Prevention of hearing damage will reduce usage of healthcare resources (~10-15 year impact).