Listening to loud music on personal devices could be leading to an epidemic of deafness among young people, a study shows.
Noisy sporting events, concerts and nightclubs is also expose them to increasing risk of ‘hidden hearing loss’ - which isn’t picked up by standard tests.
Known medically as cochlear synaptopathy if affects the ability to pick out sounds in a noisy environment.
Hearing tests typically measure the faintest sounds people can hear in a quiet environment which means they don’t identify the problem.
Now symptoms of difficulty understanding speech in loud places have been linked for the first time with college-age subjects with no apparent hearing problems.
A study of young adults who could regularly overexpose their ears to loud sounds found a strong association between the ‘speech-in-noise’ test and an electro-physiological measure of the health of the auditory nerve.
Scores on both tests were also much better among participants who regularly wore hearing protection when exposed to lots of noise, reports PLOS ONE.
Dr Stephane Maison, of Massachusetts Eye and Ear Infirmary in Boston, said: “While hearing sensitivity and the ability to understand speech in quiet environments were the same across all subjects we saw reduced responses from the auditory nerve in participants exposed to noise on a regular basis and - as expected - that loss was matched with difficulties understanding speech in noisy and reverberating environments.”
Hearing loss affects 11 million people in the UK - one-in-six of the population.
It can be caused by noise or ageing and typically arises from damage to the sensory cells of the inner ear or cochlea which convert sounds into electrical signals or the auditory nerve fibres that transmit those signals to the brain.
It is traditionally diagnosed by elevation in the sound level required to hear a brief tone as revealed on an audiogram - the gold standard test of hearing sensitivity.
But synaptopathy is damage to the connections between the auditory nerve fibres and the sensory cells - which happens well before the loss of the sensory cells themselves.
The breaking of these connections contributes to difficulties understanding speech in challenging listening environments and may also be important in causing tinnitus - ringing in the ears - or hyperacusis which causes increased sensitivity to sound.
So the researchers set out to develop more sensitive measures that can also test for cochlear synaptopathy.
The researchers have shown in animal models that under some conditions connections between the sensory cells and the auditory nerve can be successfully restored using growth factors such as neurotrophins.
Dr Maison said: “Establishing a reliable diagnosis of hidden hearing loss is key to progress in understanding inner ear disease.
“Not only may this change the way patients are tested in clinic but it also opens the door to new research - including understanding the mechanisms underlying a number of hearing impairments such as tinnitus and hyperacusis.”
In 2011 US researchers found older people with hearing loss are more likely to develop dementia - perhaps because a common brain pathway underlies both conditions.
Another theory is the strain of trying to decode sounds can overwhelm the brain - leaving people more vulnerable to dementia.
Research is continuing about whether hearing aids can reduce the risk. Studies suggest for every 25 decibels of hearing loss (decibels relate to volume of sound with normal speech around 60) a person experiences they age cognitively by seven years.
So a 59-year-old who has lost 25 decibels will perform like a 66 year-old in mental tasks.