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Researchers discover hidden hearing loss mechanism

Contributed by , staff writer for Healthy Hearing

We've all experienced times when sound has been so loud that our ears actually hurt as a result. Whether it was a loud rock concert or fireworks display - maybe even an evening of cheering on the home team - it's common for our ears to ring for several hours afterward. Most of the time our hearing seems to return to normal; however, new research indicates these noisy events may be causing permanent damage to our hearing health.

According to the National Institute on Deafness and Other Communication Disorders (NIDCD), more than 36 million American adults report some degree of hearing loss. That's almost as many people as live in the state of California. The NIDCD estimates 26 million adults have hearing loss related to exposure to loud noise, or noise induced hearing loss (NIHL). Age-related hearing loss, or presbycusis, is another cause for hearing loss, with an estimated 18 percent of American adults age 45-64, 35 percent age 64-74, and 47 percent 75 years of age and older reporting hearing loss.

hearing loss research
Researchers are uncovering a
hidden mechanism behind 
hearing loss. 

Until now, hearing health professionals believed the small hair cells in the inner ear were the most vulnerable to damage. Recently; however, researchers from the Massachusetts Eye and Ear Infirmary (MEEI) Peabody Laboratory at Harvard Medical School were able to study the synapse that occurs between these inner ear hair cells and cochlear nerve fibers. Because of their discovery, they now believe this synapse is the heart of the process that occurs in our inner ear when the noise our ears collect is translated into the impulses that travel through the auditory nerve for our brains to interpret as sound. These synapses, they believe, are a hidden hearing loss mechanism even more susceptible to damage from noise than the hair cells themselves. The researchers recently presented their findings at the 167th meeting of the Acoustical Society of America in Providence, Rhode Island. 

One of the reasons this mechanism stayed hidden for such a long time is because it's difficult to see. To study the process, MEEI researchers stained the synapses and viewed them in a light microscope. This allowed them to count the synapses in a normal hearing ear and compare them to those from an ear that had been exposed to noise. The researchers discovered that even noise that causes a "transient elevation of thresholds" reduced the number of synapses.

Not only is this mechanism hidden, it's also silent. Because there's usually no pain involved when synapses are lost, individuals aren't aware their hearing has been irreversibly damaged.

What is noise-induced hearing loss (NIHL)?

Individuals diagnosed with noise-induced hearing loss have lost hearing due to exposure to loud noises either from work-related or leisure activities. The damage can occur suddenly and permanently from an explosion or other loud noise or over a period of time from continuous exposure to excessive noise.

Currently, hearing health professionals say that exposure to sound levels higher than 80 decibels (dB) for extended periods of time can cause NIHL. For reference, normal conversation registers at 60 dB, while emergency sirens can register 120 dB. The louder the sound, the faster it can cause NIHL.

What is age-related hearing loss?

As we age, changes to the sensory receptors in our inner ear often causes presbycusis, or age-related hearing loss. Presbycusis usually affects higher-pitched sounds first and occurs gradually in both ears. It can be hereditary or caused by various health conditions such as heart disease, high blood pressure and other circulatory problems.

Symptoms include perception of mumbled or slurred speech in others, difficulty in hearing high-pitched sounds and difficulty understanding speech in the presence of background noise, or the presence of a hissing or ringing in one or both ears (tinnitus).

How can you protect your hearing?

Now that you know how destructive excessive sound can be to your hearing, take care to conserve your hearing health. Start by having your hearing examined by a hearing health professional. Turn the volume down on your car stereo, television and personal electronic device. Keep a set of inexpensive foam ear plugs close at hand and take them with you if you'll be attending an noisy event. Wear appropriate ear protection whenever you're operating machinery that emit loud noise, such as lawn mowers, snowmobiles or hunting equipment.

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