Is this the real reason you can't hear conversation round the dinner table? ...even though the standard test says nothing is wrong with your ears 

  • Pure tone audiometry test suggested Diana's hearing was normal
  • Then she heard about emerging phenomenon 'hidden hearing loss'
  • Decided to try very different type of hearing test using series of electrodes

By Diana Pilkington for the Daily Mail

Published: 17:09 EST, 31 August 2015 | Updated: 19:34 EST, 31 August 2015

Do you struggle to understand the person next to you above the background chatter at dinner parties? Or fret at the prospect of an evening in a pub that insists on blaring out loud music?

You're not alone - surveys suggest that more than a quarter of us have great difficulty deciphering speech in a noisy environment.

In my 20s, going to bars or restaurants with background music meant a stressful night of straining to work out what anyone said over the din. But when my GP referred me for a hearing test around five years ago, aged 29, I was surprised to receive a clean bill of health.

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Surveys suggest that more than a quarter of us have great difficulty deciphering speech

Surveys suggest that more than a quarter of us have great difficulty deciphering speech

The standard hearing assessment, known as the pure tone audiometry test - where tones of varying volumes and pitches were played to me through headphones while I pressed a button to indicate I had heard them - suggested my hearing was normal.

Then I heard about an emerging phenomenon known as 'hidden hearing loss', and wondered if this could apply to me.

Coined in 2011 by researchers at University College London, hidden hearing loss refers to the kind of difficulties not detectable via standard hearing tests, says David McAlpine, a professor of auditory neuroscience at the UCL Ear Institute in London.

'People with this problem often have difficulty with complex listening tasks, such as deciphering speech among background noise.

'This could be true of anyone with hearing loss. The difference with hidden hearing loss is that they don't necessarily have trouble hearing quiet sounds when there is no noise in the background, so the results of an audiometry test look fine.

'But it can make social and work life frustrating.'

Diana (pictured) was tested for hidden hearing loss. 'I was fitted with a sort of swimming cap with holes in, while dozens of electrodes were stuck to my scalp with gel. I then sat in a booth while sounds of various frequencies were played through headphones'

Diana (pictured) was tested for hidden hearing loss. 'I was fitted with a sort of swimming cap with holes in, while dozens of electrodes were stuck to my scalp with gel. I then sat in a booth while sounds of various frequencies were played through headphones'

And with our increasing exposure to noise - on personal music devices, for example - hidden hearing loss is potentially on the rise.

So last month I decided to have a very different type of hearing test: a series of electrodes were attached to my head so scientists could examine the electrical activity in my brain in response to various sounds.

It's not a definitive way to diagnose hidden hearing loss (there is no proper test yet), but it can give clues about problems in the brain and nerves that a standard hearing test does not reveal.

When we hear a noise, sound waves pass into the ear canal, making the ear drum vibrate. These vibrations are then passed through three small bones in the middle ear and on to the cochlea in the inner ear.

1 in 6

The proportion of adults with a hearing problem detectable by standard tests

This spiral-shaped structure contains 15,000 tiny sensory hair cells, which convert the sound waves into electrical signals, sending them along the auditory nerve to the brain, where they are perceived as sound.

There are two main types of hearing loss: conductive, where sounds cannot pass from the outer ear to the inner ear (ear wax is one cause); or sensorineural, where there is damage to the tiny hair cells or to the auditory nerve. For years, sensorineural hearing loss was taken to mean damaged hair cells, which are what allow us to detect pin-drop sounds, says Professor McAlpine.

These hair cells may wear out with age, but they can also become damaged by noise exposure. Sometimes it's just a temporary battering - your ability to detect faint sounds may be impaired for a few days after a concert, say, but it returns to normal as the hair cells recover.

Safe volume levels are 80 decibels (an average hairdryer) and below.

But noise may actually cause irreversible damage to the connections (called synapses) between hair cells and nerve cells in the cochlea, so that they cannot send information to the brain. This is thought to be what's going on in hidden hearing loss, says Chris Plack, a professor of audiology at Manchester University.

'Animal studies at Harvard found that in mice who were briefly exposed to loud noise, their sensitivity to quiet sounds was reduced for a few days. Though this recovered, the mice had permanently lost half the connections between the hair cells and the auditory nerve fibres in a large region of the cochlea.'

The damage is thought to occur because over- stimulated hair cells produce an excess of the chemical glutamate, which is toxic in large amounts. Scientists hope to one day be able to reverse this damage by treating the surviving neurons with neurotrophins (molecules which promote healthy nerve growth), injected through the eardrum.

A key issue is that there is not a test to diagnose this damage, though scientists are working on it.

In my 20s, going to bars or restaurants with background music meant a stressful night of straining to work out what anyone said

One promising approach is based on tests of the auditory brainstem response (ABR). This is a measure of the electrical activity in parts of the brain involved in hearing.

A pilot study by scientists at the University of Manchester, including Professor Plack, using a variant of the ABR test on students with normal results on their standard hearing tests, found poorer brain responses among those who had been exposed to loud noise in clubs.

Professor Plack has a £1.2 million grant from the Medical Research Council for further research.

I wondered if tests like these might reveal something about my hearing that a standard test could not. So, at the UCL Ear Institute, I underwent an ABR. I was fitted with a sort of swimming cap with holes in, while dozens of electrodes were stuck to my scalp with gel. I then sat in a booth while sounds of various frequencies were played through headphones.

Researchers also bounced a sound back and forth between my ears to check signals linked to my binaural sensitivity - the way my brain deals with input from both ears.

Signs you may have hidden hearing loss 

  • Difficulty in understanding speech when there is background noise, but you can pick out quiet noises in quiet environments. 
  • You can’t understand what people are saying on TV even when the volume is loud.
  • You struggle in social situations or meetings at work when there are lots of people talking.
  • Standard tests at a hearing clinic say your results are fine, but you are still struggling.

Next, I had the standard hearing test, the type I had five years ago. It showed that thought my right ear had mild loss of hearing at low frequencies, overall my hearing was 'generally normal'.

Then I did a speech-in-noise test, where you have to repeat back distinct sentences uttered amid an increasingly loud babble of chatter. The results, again, were fine.

But the electrode test showed my binaural sensitivity was 'extremely weak', said Professor McAlpine. 'Binaural sensitivity is what allows us to process and separate out noises coming from different locations at the same time. It's important for nearly every listening situation.'

Almost everyone will struggle to understand a speaker when there is loud background noise close by, but most people usually find it easier when any unwanted noise is coming from somewhere else in the room.

For some reason, this didn't seem to be happening in my case. Signals about the timing of sounds going from ear to ear were being scrambled in my brain.

It wasn't clear what caused it, but I was told it could have been a knock-on effect from some as yet undetectable damage to the auditory nerve.

And while some experts would argue this is probably not hidden hearing loss, in that it's unlikely to be down to damaged connections between hair cells and nerves, Professor McAlpine believes it still falls into this category as it's not detected by standard tests.

Tests like the one I had are not readily available outside the realm of scientific research.

Professor McAlpine recommends that those who do struggle with listening in everyday situations have their hearing tested in the standard way in case they could benefit from a hearing aid. 

 

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