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Hearing impairment comes from different biologic causes. Most commonly, the ear is the affected part of the body.
▲</ref>. Since sound can be picked up by a normally sensitive inner ear even if the ear canal, ear drum, and ear ossicles are not working, conductive hearing loss is often only mild and is never worse than a moderate impairment. Hearing thresholds will not rise above 55-60 dB from outer or middle ear problems alone. Generally, with pure conductive hearing loss, the ''quality'' of hearing (speech discrimination) is good, as long as the sound is amplified loud enough to be easily heard.
A conductive loss can be caused by any of the following:
A sensorineural hearing loss is due to insensitivity of the
The great majority of human sensorineural hearing loss is caused by abnormalities in the
▲A sensorineural hearing loss is due to insensitivity of the [[inner ear]] (the [[cochlea]]), or to impairment of function in the auditory nervous system. It can be mild, moderate, or severe, including total deafness.
▲The great majority of human sensorineural hearing loss is caused by abnormalities in the [[hair cells]] of the [[organ of Corti]] in the cochlea. There are also very unusual sensorineural hearing impairments that involve the VIIIth [[cranial nerve]] (the [[Vestibulocochlear nerve]]) or the auditory portions of the brain. In the rarest of these sorts of hearing loss, only the auditory centers of the brain are affected. In this situation, [[central hearing loss]], sounds may be heard at normal thresholds, but the quality of the sound perceived is so poor that speech can not be understood.
Most sensory hearing loss is due to poor hair cell function. The hair cells may be abnormal at birth, or damaged during the lifetime of an individual. There are both external causes of damage, like noise trauma and infection, and intrinsic abnormalities, like deafness genes.
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Typical causes are discussed in following subsections.
Populations of people living near airports or freeways are exposed to levels of noise typically in the 65 to 75 dB(A) range. If lifestyles include significant outdoor or open window conditions, these exposures over time can degrade hearing. The [[United States Environmental Protection Agency|U.S. EPA]] and various states have set noise standards to protect people from these adverse health risks. The EPA has identified the level of 70 dB(A) for 24 hour exposure as the level necessary to protect the public from hearing loss (EPA, 1974).▼
▲Populations of people living near airports or freeways are exposed to levels of noise typically in the 65 to 75 dB(A) range. If lifestyles include significant outdoor or open window conditions, these exposures over time can degrade hearing. The
* Noise-Induced Hearing Loss (NIHL) typically is centered at 4000 Hz. ▼
* The louder the noise is, the shorter the safe amount of exposure is. Normally, the safe amount of exposure is reduced by a factor 2 for every additional 3 dB. For example, the safe daily exposure amount at 85 dB is 8 hours, while the safe exposure at 91 dB(A) is only 2 hours (National Institute for Occupational Safety and Health, 1998). Sometimes, a factor 2 per 5 dB is used.▼
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* Personal electronic audio devices can produce powerful enough sound to cause significant Noise-Induced Hearing Loss, given that lesser intensities of even 85 dB can also cause hearing loss.
If one is exposed to loud music consistently (i.e. 110 decibels+), then hearing impairment will occur. This is especially true when the listener is listening through headphones, as the headphones are in closer proximity to the ear drum than a loudspeaker. With the invention of in-ear headphones, these dangers are increased.
Hearing loss can be inherited. Both
Dominant and recessive hearing impairment can be syndromic or nonsyndromic. Recent gene mapping has identified dozens of nonsyndromic dominant (DFNA#) and recessive (DFNB#) forms of deafness.
*The most common type of congenital hearing impairment in developed countries is
*The most common dominant syndromic forms of hearing impairment include
*The most common recessive syndromic forms of hearing impairment are
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*AIDS and AIDS-related complex|ARC patients frequently experience auditory system anomalies.
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▲====Disease or illness====
▲*[[Measles]] may result in [[Vestibulocochlear nerve|auditory nerve]] damage
▲*[[Meningitis]] may damage the auditory nerve or the [[cochlea]]
▲*[[Autoimmune disease]] has only recently been recognized as a potential cause for [[cochlea]]r damage. Although probably rare, it is possible for autoimmune processes to target the cochlea specifically, without symptoms affecting other organs. [[Wegener's granulomatosis]] is one of the autoimmune conditions that may precipitate hearing loss.
▲*[[Mumps]] (Epidemic parotitis) may result in profound [[sensorineural hearing loss]] (90 [[Decibel|dB]] or more), unilateral (one ear) or bilateral (both ears).
▲*[[Presbycusis]] is a progressive hearing impairment accompanying age, typically affecting sensitivity to higher frequencies (above about 2 kHz).
▲*[[Adenoids]] that do not disappear by [[adolescence]] may continue to grow and may obstruct the [[Eustachian tube]], causing conductive hearing impairment and nasal infections that can spread to the [[middle ear]].
▲*[[HIV]] (and subsequent opportunistic infections) may directly affect the cochlea and central auditory system.<ref name="faqGal"/>
▲*[[Chlamydia]] may cause hearing loss in newborns to whom the disease has been passed at birth.<ref name="faqGal"/>
▲*[[Fetal alcohol syndrome]] is reported to cause hearing loss in up to 64% of infants born to [[alcoholism|alcoholic]] mothers, from the [[ototoxic effect]] on the developing fetus plus malnutrition during pregnancy from the excess [[ethanol|alcohol]] intake.<ref name="faqGal"/>
▲*[[Premature birth]] results in [[sensorineural hearing loss]] approximately 5% of the time.<ref name="faqGal"/>
▲*[[Syphilis]] is commonly transmitted from pregnant women to their fetuses, and about a third of the infected children will eventually become deaf.<ref name="faqGal"/>
▲* [[Otosclerosis]] is a hardening of the stapes (or stirrup) in the middle ear and causes conductive hearing loss.
▲* [[Superior canal dehiscence]], a gap in the bone cover above the inner ear, can lead to low-frequency [[conductive hearing loss]], [[autophony]] and vertigo
Some medications cause irreversible damage to the ear, and are limited in their use for this reason. The most important group is the
▲====Medications====
▲Some medications cause irreversible damage to the ear, and are limited in their use for this reason. The most important group is the [[aminoglycoside]]s (main member [[gentamicin]]).
Various other medications may reversibly affect hearing. This includes some
Extremely heavy
* There can be damage either to the ear itself or to the brain centers that process the aural information conveyed by the ears.
* People who sustain head injury are especially vulnerable to hearing loss or
* Exposure to very loud
==Categories of hearing impairment==
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