Researchers from Harvard Medical School, Massachusetts Eye and Ear Infirmary, and Northeastern University have, for the first time, found evidence in people of noise-induced damage to the connections between the sound-sensing hair cells and the auditory nerves, which may be one of the causes of difficulty in understanding speech in background noise. Nicola Robas from our Biomedical Research team tells us more.
Hidden hearing loss
The most common cause of hearing loss in adults is damage to the sensory cells of the inner ear. When sound enters the ear, it is detected by the hair cells (so called because of the hair-like projections on their surface). The hair cells then send sound signals to the auditory nerves which, in turn, send these signals to the brain. In a human ear there are approximately 15,000 hair cells and 40, 000 nerve fibres. A single hair cell has connections (called synapses) to several nerve fibres which can respond to different levels of sound.
Until recently it was thought noise that caused only a temporary hearing loss (like after a loud concert) did not cause lasting effects as no damage to the hair cells or nerves was immediately visible. However, research in animals has now shown that this kind of noise can damage the synapses between the hair cells and the auditory nerve fibres, and that this leads to difficulty hearing in background noise. The researchers called the loss of synapses “hidden hearing loss” because the animals had normal hearing when measured using standard hearing tests (simple sounds in a quiet environment).
Following-up on the animal studies, the scientists wanted to investigate if loss of synapses was involved in difficulty hearing in background noise in people. However, while there is a lot of scientific evidence showing this effect in animals, it’s been very difficult to find a way to measure hidden hearing loss in people.
Developing a new test
The team led by researchers at Harvard developed a new test to see if they could find evidence of synapse damage in people and what effect this would have on a person’s hearing. The researchers hypothesised that young people who regularly expose themselves to loud noise without ear plugs or other hearing protection would have synapse damage in their inner ears, even though their hearing was normal using standard tests. The university students who volunteered for the study were divided in high-risk and low-risk groups depending on their level of previous noise exposure and whether they regularly used hearing protection in noisy environments. The researchers used electrodes placed in the ear canal to measure the electrical waves produced by the hair cells and by the auditory nerve – the idea being that if the connections between the two are damaged, the difference between the hair cell signal and the nerve signal (called the SP/AP ratio) would be much greater compared to undamaged connections where signals can pass effectively between the two.
The researchers found that the students in the high-risk group had high SP/AP ratios suggesting that they did have synapse damage, and this correlated with them having difficulty in understanding speech in background noise. These students had normal hearing using standard hearing tests, but did have a degree of hearing loss at sound frequencies higher than are normally tested. The high-risk students also showed an increased sensitivity to certain sounds suggesting they might be at risk for hyperacusis.
Why is this important ?
This is the first time that difficulty in understanding speech in noise has been linked to inner ear synapse damage in people. The researchers now want to repeat the study on larger numbers of people to confirm the results and see if the SP/AP ratio could be developed as a diagnostic test for hidden hearing loss. This is important as new diagnostic tests will help increase our understanding of how noise-induced and age-related hearing loss develop and allow damage to be detected earlier. It will also be essential for the assessment of new medicines to repair nerve damage in the ear. It could also open up new areas of research as scientists believe that synapse loss might be involved in hyperacusis and some types of tinnitus. Having a diagnostic test in people would allow this to be investigated further and could help in the development of new medicines for these conditions.
This research was published in the journal PLoS One
Full reference: Liberman MC, Epstein MJ, Cleveland SS, Wang H, Maison SF.Toward a Differential Diagnosis of Hidden Hearing Loss in Humans. PLoS One, 2016 Sep 12;11(9):e0162726.