Hearing loss

HOW DO WE HEAR? ​

Hearing depends on a series of events that change sound waves in the air into electrical signals. Your auditory nerve then carries these signals to your brain through a complex series of steps.

1. Sound waves enter the outer ear and travel through a narrow passageway called the ear canal, which leads to the eardrum.

2. The eardrum vibrates from the incoming sound waves and sends these vibrations to three tiny bones in the middle ear these bones are called the malleus, incus, and stapes.

3. The bones in the middle ear couple the sound vibrations from the air to fluid vibrations in the cochlea of the inner ear, which is shaped like a snail and filled with fluid. An elastic partition runs from the beginning to the end of the cochlea, splitting it into an upper and lower parts. This partition is called the basilar membrane because it serves as the base, or ground floor, on which key hearing structures sit.

4. Once the vibrations cause the fluid inside the cochlea to ripple, traveling wave forms along the basilar membrane. Hair cells, sensory cells sitting on top of the basilar membrane, ride the wave.

5. As the hair cells move up and down, microscopic hair-like projections (known as stereocilia) that perch on top of the hair cells bump against an overlying structure and bend. Bending causes pore-like channels, which are at the tips of the stereocilia, to open up. When that happens, chemicals rush into the cells, creating an electrical signal

6. The auditory nerve carries this electrical signal to the brain, which turns it into a sound that we recognize and understand.

I. Genetic Hearing Loss

Genetics is one of the most common hearing loss causes, accounting for approximately 60 percent of deafness occurring in infants. An average of three in every 1,000 babies is born with hearing loss or born deaf. Genetics is also one of the leading hearing loss causes in the elderly. Inherited genetic defects can occur at any point in a person’s life, leading to sudden hearing loss. These same defects can also cause a person to become deaf.

Hearing loss is the most familiar sensory defect present in humans, affecting more than 10 percent of people aged 65 and older. In many cases, hearing loss is caused by multiple factors, including genetic and environmental causes. Single-gene mutations can often lead to hearing loss. Genetic factors make some people more susceptible to hearing loss as their genes make them more predisposed due to aging, loud noises, infections, or drugs. It is estimated that between 30 and 60 percent of age-related hearing loss is due to genetics.

There are two primary forms of hearing loss. These include sensorineural hearing loss and conductive hearing loss – as well as a combination of the two.

Onset

  • Prelingual hearing loss is present before speech develops. All congenital (present at birth) hearing loss is prelingual, but not all prelingual hearing loss is congenital.
  • Post-lingual hearing loss occurs after the development of normal speech. 
  • The different gene loci for nonsyndromic deafness are designated DFN (for DeaFNess). Loci are named based on the mode of inheritance
  • DFNA: Autosomal dominant
  • DFNB: Autosomal recessive
  • DFNX: X-linked

Examples of clinical manifestations and molecular
genetics of known genes causing hearing impairment

II. Age-Related Hearing Loss

Age-related hearing loss (presbycusis) is the loss of hearing that gradually occurs in most of us as we grow older. It is one of the most common conditions affecting older and elderly adults.

Approximately one in three people in the United States between the ages of 65 and 74 has hearing loss, and nearly half of those older than 75 have difficulty hearing. Having trouble hearing can make it hard to understand and follow a doctor’s advice, respond to warnings, and hear phones, doorbells, and smoke alarms. Hearing loss can also make it hard to enjoy talking with family and friends, leading to feelings of isolation.

Age-related hearing loss most often occurs in both ears, affecting them equally. Because the loss is gradual, if you have age-related hearing loss you may not realize that you’ve lost some of your ability to hear.

There are many causes of age-related hearing loss. Most commonly, it arises from changes in the inner ear as we age, but it can also result from changes in the middle ear, or from complex changes along the nerve pathways from the ear to the brain. Certain medical conditions and medications may also play a role.

– Age is the strongest predictor of hearing loss among adults aged 20-69, with the greatest amount of hearing loss in the 60 to 69 age group.

– About 2 percent of adults aged 45 to 54 have disabling hearing loss. The rate increases to 8.5 percent for adults aged 55 to 64. Nearly 25 percent of those aged 65 to 74 and 50 percent of those who are 75 and older have disabling hearing loss.

Why do we lose our hearing as we get older?

Many factors can contribute to hearing loss as you get older. It can be difficult to distinguish age-related hearing loss from hearing loss that can occur for other reasons, such as long-term exposure to noise.

Noise-induced hearing loss is caused by long-term exposure to sounds that are either too loud or last too long. This kind of noise exposure can damage the sensory hair cells in your ear that allow you to hear. Once these hair cells are damaged, they do not grow back and your ability to hear is diminished.

Conditions more common in older people, such as high blood pressure or diabetes, can contribute to hearing loss. Medications that are toxic to the sensory cells in your ears (for example, some chemotherapy drugs) can also cause hearing loss.

Rarely, age-related hearing loss can be caused by abnormalities of the outer or middle ear. Such abnormalities may include reduced function of the tympanic membrane (the eardrum) or reduced function of the three tiny bones in the middle ear that carry sound waves from the tympanic membrane to the inner ear.

Most older people who experience hearing loss have a combination of both age-related hearing loss and noise-induced hearing loss.

III. Noise-Induced Hearing Loss

Every day, we experience sounds in our environment, such as the sounds from television and radio, household appliances, and traffic. Normally, these sounds are at safe levels that don’t damage our hearing. But sounds can be harmful when they are too loud, even for a brief time, or when they are both loud and long-lasting. These sounds can damage sensitive structures in the inner ear and cause noise-induced hearing loss (NIHL).

Most NIHL is caused by the damage and eventual death of inner ear hair cells. Unlike bird and amphibian hair cells, human hair cells don’t grow back. They are gone for good.

NIHL can be immediate, or it can take a long time to be noticeable. It can be temporary or permanent, and it can affect one ear or both ears. Even if you can’t tell that you are damaging your hearing, you could have trouble hearing in the future, such as not being able to understand other people when they talk, especially on the phone or in a noisy room. Regardless of how it might affect you, one thing is certain: noise-induced hearing loss is something you can prevent.

Exposure to harmful noise can happen at any age. People of all ages, including children, teens, young adults, and older people, can develop NIHL. Based on a 2011-2012 CDC study involving hearing tests and interviews with participants, at least 10 million adults (6 percent) in the U.S. under age 70—and perhaps as many as 40 million adults (24 percent)—have features of their hearing test that suggest hearing loss in one or both ears from exposure to loud noise.
Researchers have also estimated that as many as 17 percent of teens (ages 12 to 19) have features of their hearing test suggestive of NIHL in one or both ears (Pediatrics 2011, the link is external), based on data from 2005-2006.
About 18 percent of adults aged 20-69 have speech-frequency hearing loss in both ears among those who report 5 or more years of exposure to very loud noise at work, as compared to 5.5 percent of adults with speech-frequency hearing loss in both ears who report no occupational noise exposure2.

Exposure to harmful noise can happen at any age. People of all ages, including children, teens, young adults, and older people, can develop NIHL. Based on a 2011-2012 CDC study involving hearing tests and interviews with participants, at least 10 million adults (6 percent) in the U.S. under age 70—and perhaps as many as 40 million adults (24 percent)—have features of their hearing test that suggest hearing loss in one or both ears from exposure to loud noise.
Researchers have also estimated that as many as 17 percent of teens (ages 12 to 19) have features of their hearing test suggestive of NIHL in one or both ears (Pediatrics 2011, the link is external), based on data from 2005-2006.
About 18 percent of adults aged 20-69 have speech-frequency hearing loss in both ears among those who report 5 or more years of exposure to very loud noise at work, as compared to 5.5 percent of adults with speech-frequency hearing loss in both ears who report no occupational noise exposure2.

Exposure to harmful noise can happen at any age. People of all ages, including children, teens, young adults, and older people, can develop NIHL. Based on a 2011-2012 CDC study involving hearing tests and interviews with participants, at least 10 million adults (6 percent) in the U.S. under age 70—and perhaps as many as 40 million adults (24 percent)—have features of their hearing test that suggest hearing loss in one or both ears from exposure to loud noise.
Researchers have also estimated that as many as 17 percent of teens (ages 12 to 19) have features of their hearing test suggestive of NIHL in one or both ears (Pediatrics 2011, the link is external), based on data from 2005-2006.
About 18 percent of adults aged 20-69 have speech-frequency hearing loss in both ears among those who report 5 or more years of exposure to very loud noise at work, as compared to 5.5 percent of adults with speech-frequency hearing loss in both ears who report no occupational noise exposure2.

Exposure to harmful noise can happen at any age. People of all ages, including children, teens, young adults, and older people, can develop NIHL. Based on a 2011-2012 CDC study involving hearing tests and interviews with participants, at least 10 million adults (6 percent) in the U.S. under age 70—and perhaps as many as 40 million adults (24 percent)—have features of their hearing test that suggest hearing loss in one or both ears from exposure to loud noise.
Researchers have also estimated that as many as 17 percent of teens (ages 12 to 19) have features of their hearing test suggestive of NIHL in one or both ears (Pediatrics 2011, the link is external), based on data from 2005-2006.
About 18 percent of adults aged 20-69 have speech-frequency hearing loss in both ears among those who report 5 or more years of exposure to very loud noise at work, as compared to 5.5 percent of adults with speech-frequency hearing loss in both ears who report no occupational noise exposure2.

Exposure to harmful noise can happen at any age. People of all ages, including children, teens, young adults, and older people, can develop NIHL. Based on a 2011-2012 CDC study involving hearing tests and interviews with participants, at least 10 million adults (6 percent) in the U.S. under age 70—and perhaps as many as 40 million adults (24 percent)—have features of their hearing test that suggest hearing loss in one or both ears from exposure to loud noise.
Researchers have also estimated that as many as 17 percent of teens (ages 12 to 19) have features of their hearing test suggestive of NIHL in one or both ears (Pediatrics 2011, the link is external), based on data from 2005-2006.
About 18 percent of adults aged 20-69 have speech-frequency hearing loss in both ears among those who report 5 or more years of exposure to very loud noise at work, as compared to 5.5 percent of adults with speech-frequency hearing loss in both ears who report no occupational noise exposure2.

Tinnitus

Loud noise exposure can also cause tinnitus—a ringing, buzzing, or roaring in the ears or head. Tinnitus may subside over time but can sometimes continue constantly or occasionally throughout a person’s life. Hearing loss and tinnitus can occur in one or both ears.

Sometimes exposure to impulse or continuous loud noise causes a temporary hearing loss that disappears 16 to 48 hours later. Recent research suggests, however, that although the loss of hearing seems to disappear, there may be residual long-term damage to your hearing.

TINNITUS

Loud noise exposure can also cause tinnitus—a ringing, buzzing, or roaring in the ears or head. Tinnitus may subside over time but can sometimes continue constantly or occasionally throughout a person’s life. Hearing loss and tinnitus can occur in one or both ears.

Sometimes exposure to impulse or continuous loud noise causes a temporary hearing loss that disappears 16 to 48 hours later. Recent research suggests, however, that although the loss of hearing seems to disappear, there may be residual long-term damage to your hearing.

This figure shows age and sex-specific trends in chronic tinnitus, i.e., ringing, roaring, or buzzing in the ears or head that has lasted for 3 months or longer in the past year. This information was collected in household interviews conducted as part of the 1994–1995 Disability Supplement to the U.S. National Health Interview Survey; the data are self-reported and exclude proxy responses.

The overall trend with age shows a marked increase for both sexes, beginning at age 40, peaking between 65 to 79 years, and then declining after 80 years of age. Males show significantly higher rates of tinnitus (p<0.0001) compared to females. The male prevalence is consistently higher from age 40 through 79 years, with the largest differences between males and females occurring from 50 through 69 years of age. One explanation for these differences is that males are more likely to be exposed to loud environmental noise, whether through work or leisure activities. Strong associations between occupational noise exposure and tinnitus are well documented in the literature (Hoffman and Reed, 2004).

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