Noise-induced hearing loss (NIHL) is a true problem in the United States. An estimated 10 to 40 million adults across the country show signs of this condition. Many of these are due to exposure to noisy areas such as the loud burst of gunfire found in the military or at shooting ranges, the clang of heavy machinery often heard in factories or the constant buzz of power tools used in automotive or construction industries.
These occupational and recreational environments are hazardous to the hearing health of both people who show up daily as well as those who are only occasional exposed. Sudden, extreme noises can damage the delicate inner ear just as seriously as the constant exposure to these sounds can. It’s been proven that sudden impulse noises have a more adverse effect than a steady noise.
Different people have different reactions to noises. Some may be more susceptible, yet others are not as responsive to them. While studies show that certain lifestyle habits such as smoking, drug use, work environment, and even age can have an effect on the body’s reaction to NIHL, the effects of genetics are now being studied as well.
Though the effects of NIHL in humans is tough to study thanks to the variations of lifestyles people live as well as their individual genetics, studies performed in animal models is proving much more controlled. A combination of both genetic and environmental elements, the genetic susceptibility has been definitively proven in mice.
More than 140 gene variations have been considered in the cause of hearing loss when there is a lack of additional indications. Variations in 34 genes have been found to be linked to the likelihood of increased auditory thresholds for people who have exposure to occupational noise. This type of noise can cause two different types of injury to the sensitive inner ear, though they depend on the duration and the intensity of noise exposure.
One type is transient attenuation of hearing acuity or temporary threshold shift (TTS), which hearing often returns within 24-48 hours after TTS. Testing in mice, however, shows that instance of TTS at younger ages can speed up the process of age-related hearing loss even with the short-term recovery.
The second type of injury is a permanent threshold shift (PTS). These can be brought on by dramatic, loud sounds such as being near a jet engine. This type of injury can lead to problems understanding speech in areas where there is a lot of loud background noise.
NIHL symptoms can be associated with the injury of certain parts of the ear. The tympanic membrane, which is affected by the acoustic waves transmits soundwaves to the inner ear. Damage to these structures from blasts such as explosions, gunshots, jet engines, or even the blare of the siren from an emergency vehicle is highly possible. These sensitive areas can be ruptured or even destroyed completely, resulting in permanent hearing loss.
Once soundwaves enter the cochlea, the outer hair cells begin to expand and contract quickly in an effort to pick up the acoustic vibrations produced within the inner hair cells. This is an area where the body’s potassium levels must be up in order to fuel the energy requirements of the process. This is where excessive noise can cause a great deal of harm, by damaging these outer hair cells. As with the tympanic membrane, any damage to these hair cells can also cause a threshold shift, resulting in permanent damage.
While there is no known correlation between tinnitus and human genetics, hearing loss itself is subject to the genetics and susceptibility of the individual. Though preventable, NIHL is permanent and there is no way to reverse the condition. As the second-highest reason for hearing loss, it’s only been viewed as a problem since the 20th century.
Approximately twelve percent of the population of the U.S. is exposed to noise levels severe enough to cause hearing damage through at least half of their workday. Genetics can play a factor in that some people will develop NIHL in this environment while others will not.
Approximately 38 percent of alternative NIHL genes were found to be associated with families who experience hearing loss. Oxidative stress, or the imbalance between free radicals and antioxidants found in the body, endangers auditory function. Twenty-three percent of NIHL variants have been found in the oxidative stress response gene, which are proteins that are encoded to neutralize radical peroxide byproducts produced from the mitochondrial electron transport chain, according to an October 2019 article in the Hearing Health Journal.
While each person has a different susceptibility level to NIHL, researchers are finding that many are based on differences in their specific genetic code. Future studies into the relationship between noise-induce hearing loss and genetics have the potential to reveal more surprising connections.