Imagine that you’re out at a restaurant with an old friend. On the way in you were chatting with no problems, having a great time. Now that you’re seated, there is background music playing and a lot of blurred speech sounds coming from other tables. Your friend points out a song that is playing, an old favorite of both of yours, but you can’t quite make it out. More than usual, you have to ask your friend to repeat herself. You find yourself getting tired and just wanting the evening to be over.
Let’s say, after this experience, you suspect you might be suffering from hearing loss, so you make an appointment with a hearing healthcare professional and get your hearing tested. To your surprise, you pass with flying colors and are told that your hearing is perfectly normal. What gives? If you can hear so well, why weren’t you able to hear your friend at the restaurant?
Hidden Hearing Loss
The answer is “hidden hearing loss.” It is not known what percentage of people suffer from hidden hearing loss, but it does affect young people more than older people. Exposure to extremely loud sounds at a young age, such as loud music concerts or listening too loudly in earphones, can cause hidden hearing loss.
Where Does Hidden Hearing Loss Reside?
Normal noise-induced hearing loss (NIHL) is caused by damage to the cilia (tiny hairlike cells) in the cochlea. These are the cells that convert (transduce) the mechanical energy of sound into electrical energy for transport to the brain. When these cells are damaged, sound at certain frequency ranges is lost to the ear forever. Hidden hearing loss, however, is caused by damage to the cells that actually transmit this electrical information from the cochlea to the brain, called the auditory nerve.
Hearing Tests Do Not Measure Hidden Hearing Loss
The reason a person with hidden hearing loss can pass a hearing test is that their cochlea functions well enough to capture sound at all the usual frequencies, and hearing tests do not overtax the auditory nerves. When the auditory nerves are flooded with information, like at the restaurant, they effectively “leak” enough of the information being sent through them that hearing becomes difficult. Some things are sent to the exclusion of others, and what is lost might be the more desirable aspects of the environmental sound, like music or a single person’s speech. Hearing tests employ pure-tone audiometry and speech audiometry, but neither of these tests accounts for a person’s hearing ability in conditions with excessive background noise.
What Causes Hidden Hearing Loss?
From what is currently known, the “leaking” of auditory information is due to damaged myelin sheaths around the nerves. These sheaths separate the nerves from what is surrounding them and allow neurotransmitters to function effectively across the synapses from one nerve to the next.
Damage to myelin sheaths, generally, is referred to as “neuropathy.” Neuropathy in different parts of the body can result in tingling sensations in the limbs and loss of motor function. Incidences of neuropathy and hidden hearing loss are higher in people who suffer from Guillain-Barré Syndrome, which has seen an increase since the advent of the Zika virus.
Is There a Cure?
Currently there is no known cure for hidden hearing loss. Gene therapies are in studies that attempt to aid the regrowth of myelin sheaths, and there is increasing evidence that hidden hearing loss (and, indeed, all forms of hearing loss) occurs to a greater extent in those with a genetic predisposition. It also seems that young people who suffer from hidden hearing loss are at risk for greater NIHL as they age.
The wisdom here is the same as ever: if you’re in a noisy environment, protect your hearing. Young or old, loud sounds damage our hearing in ways that continue to be discovered, and our world is increasingly inundated with loud sounds. Be aware of what’s happening and always wear ear protection. Remember that while sound doesn’t become painful until it reaches a threshold of about 115 dBA (decibels A-weighted), levels as low as 80 dBA will cause hearing loss through repeated or extended exposure. Whenever possible, measure the sound level in your environment and wear ear plugs or other hearing protection appropriate to the sound level.