TerpTopics: Deafness Classifications and Causes: Introduction to ASL and Sign Language Interpreting

Hearing health professionals generally group types of hearing deficit into categories according to the location (site) of its cause.

Sensorineural

This category, often called nerve-deafness, includes the most common causes of hearing loss. Here, hearing loss results from abnormality of the inner-ear, auditory nerve, or both. The use of a traditional hearing aid or cochlear implant might, but will not always, improve sound perception for persons with sensorineural hearing loss.

Conductive

Here, hearing loss results from damage to or obstruction of the middle or outer ear. Surgery to remove or correct the obstruction sometimes improves sound perception. The use of a bone-conductive hearing aid might, but will not always, improve hearing for persons whose hearing loss is this category. The use of a traditional hearing aid or cochlear implant typically does not improve hearing for persons with hearing loss from middle or outer ear abnormalities.

Cortical (also known as Central)

Once the ear has received sound, the auditory nerve sends sound impulses along brain pathways to eventually arrive at and be processed by the brain's hearing and language processing centers. When sound perception is disturbed at any point along brain pathways or at processing centers, the resulting hearing loss is categorized as cortical.

Grasping the implications of dB hearing deficit numbers can be a little tricky because a dB number may not tell the whole story.

Simply knowing a person has a deficit of, for example, 55 dB of sound perception is not as useful as also knowing the range and frequency (pitch) of the unheard dBs, as well as whether and how hearing deficit differs in each ear, as it often does. Hearing deficit does not always begin at zero and progress linearly to, using our example, 55. Think of Swiss cheese, the holes representing hearing deficiency: The holes may add-up to 55 dB, but this number is a total of several smaller numbers coming from various points.

Here's a film clip that will give you a sense for the differences between high- and low-frequency sound:

While the total dB of sound lost may be 55, the number could represent the sum of several dBs' hearing deficit at various points on an audiogram. For example, a person may be able to perceive some tones and not others, even though the loudness (dB) level is the same for each tone. In the case of two persons, each with mild hearing deficit, one might hear human speech, while the other might not. Or, one might hear human speech consonant sounds, while the other might not ... and so on.

Keeping in mind the trickiness of the thing, here is how audiologists tend to classify hearing deficit:

Mild (hard-of-hearing)

Less than 39 dB sound perception deficit.

Moderate (quite hard-of hearing)

Between 40 and 59 dB sound perception deficit.

Severe (deaf)

Between 60 and 85 dB sound perception deficit.

Profound (profoundly deaf)

More than 85 dB sound perception deficit.

Newborns are routinely screened in hospitals throughout the United States using BAER (Brainstem Auditory-Evoked Response), or another comparable procedure. BAER screening can be used on patients of any age, not only newborns. Before the widespread use of BAER in newborn screening, the average age for hearing deficit detection was 18-to-36 months.

BAER measures function of the cochlea, auditory nerve, and transmission time to the brainstem auditory nuclei. BAER can detect the presence of sensorineural and cortical sound transmission disturbances; however, hearing professionals emphasis that BAER is not a "hearing test" because it does not test the brain's ability to process and make sense of (understand) sound - both of which are essential to hearing.

According to the U.S. National Institutes of Health, BAER screening has revealed that:

about half of the children born deaf had previously been identified as at-risk for deafness because of genetic or other known factors; the other half had not been identified as at-risk for deafness.

by the time 1,000 of us reach adulthood, three of us are hard-of-hearing, and two of us are deaf.

We whipped out our calculator and found, using the above NIH numbers, that:

On reaching adulthood, one-half of one-percent of Americans
are deaf or hard-or-hearing.

1,536,511	Americans
.
28,167	Marylanders
91,641	Floridians
31,074	Tennesseans
4,020	South Dakotans
7,619	Idahoans
183,783	Californians
121,634	Texans

One of several acute illnesses can cause or contribute to hearing deficiency; among these are: mumps; measles (e.g. maternal rubella); maternal syphilis, meningitis, pneumonia, and influenza (flu). Deafness may result from such an illness because a high fever has damaged sensitive follicles within the cochlea. In the case of a pregnant woman who is exposed to or contracts such an illness, the unborn child's hearing may be damaged even if the expectant mother's is not.

Usher Syndrome and Cerebral Palsy are examples of "syndromic deafness." Syndromic deafness means deafness or hearing loss may be one of several symptoms resulting from a single cause. Conversely, "non-syndromic deafness" indicates that the resulting deafness or hearing deficiency stands alone as a symptom (e.g. trauma to the inner ear). Several hundred syndromes can involve hearing loss.

Congenital (birth) brain abnormalities, tumors, or lesions of the central nervous system can impact hearing ability in a variety of ways. Even if the ears, cochlea, and auditory nerves are in good working order, the brain stem or auditory/language processing centers within the brain may not be. In this instance, a person's ears are "working," but their brain is unable, for one reason or another, to receive or make sense of in-coming sound.

Brain injury (trauma) - to the brain stem, hearing center, language-processing center, or other areas of the brain - can cause the inability to perceive (detect) or process (understand) sound.

There are a number of birth (congenital) disorders that can create an obstruction to in-coming sound. For example, when one's outer ear is abnormally shaped, it may block (obstruct) sound from entering an otherwise perfectly functioning middle-ear, inner-ear, brain stem, and language and auditory processing centers. Sometimes, surgery is performed to correct such obstructions to sound.

When a person's eardrum is punctured, hearing may be permanently impaired due to scar tissue on the eardrum after it has healed. Punctures can come from outside intrusion, such as insertion of a stick, toy, or cotton swab, or from within, such as pressure build-up from an ear infection.

Ear wax build-up can block (obstruct) access to sound; but, can often be carefully removed by a medical care provider.

According to the American Speech-Language-Hearing Association (ASHA), exposure to painfully loud (111+ dB) or extremely loud (81 dB+) sound can cause or contribute to hearing loss. Repeated exposure to such harmful sound levels - above 80 dB (decibels) - will continue to compound the damage.

For comparison: The threshold for hearing in humans is about 10 dB; a whisper is about 30dB; conversational speech is about 60 dB; and a vacuum cleaner is about 70 dB. A 10 dB increase is experienced as twice as loud. For example, the loudness of a sound at 60 dB will be perceived as twice as loud at 70dB to a person with fully functioning hearing.

A single exposure to painfully loud sound (e.g. an explosion or combat) often causes permanent hearing loss. Repeated exposure to extremely loud sound (e.g. lawn mower; snow mobile; rock concert; industrial machine) will almost inevitably cause hearing loss. Even up-close exposure to the shrill whistle of a sports official can damage sensitive hearing organs. Image shows an example of pre-shaped foam earplugs.

Image shows an example of pre-shaped foam earplugs.

Damaged hearing organs do not regenerate (repair themselves). A person may believe s/he has "gotten used to" loud sound because hearing loss has already occurred, making loud sound seem no longer loud.

Humans nearly always experience some degree of hearing loss with age; some will experience profound deafness. Some experts report that half of all persons over the age of 60 experience hearing loss. For information about age-related hearing loss, the U.S. National Institutes of Health is a good place to begin.

Is there one "deaf gene?" Apparently not. Researchers have identified over 400 genes that, when altered (mutated), can cause syndromic deafness, and more than 100 that can cause non-syndromic deafness.

According to the U.S. National Institutes of Health, 50% of deafness in newborns is due to an inherited (genetic) form of deafness, with an additional 25% identified as likely inherited.

The most common form of non-syndromic genetic (inherited) deafness occurs in children whose parents are both carriers of a gene called "connexin 26 (GJB2)."

When both parents are carriers of a recessive gene for deafness, there is a one-in-four (25%) chance that their child will be deaf. When both parents carry a dominant gene for deafness, there is a much higher chance their child will be deaf.

This page was edited: 09/13/2009
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