Hearing impairment pathophysiology

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Editor-in-Chief: Angela Botts, M.D., Beth Israel Deaconess Medical Center Geriatric Medicine [1]

Pathophysiology

Sound waves vary in amplitude and in frequency. Amplitude is the sound wave's peak pressure variation. Frequency is the number of cycles per second of a sinusoidal component of a sound wave. Loss of the ability to detect some frequencies, or to detect low-amplitude sounds, that an organism naturally detects, is a hearing impairment.

Loudness, frequency, and discrimination deficiencies

Hearing sensitivity is indicated by the quietest sound that an individual can detect, called the hearing threshold. In the case of people and some animals, this threshold can be accurately measured by a behavioral audiogram. A record is made of the quietest sound that consistently prompts a response from the listener. The test is carried out for sounds of different frequencies. There are also electro-physiological tests that can be performed without requiring a behavioral response.

Normal hearing thresholds are not the same for all frequencies in any species of animal. If different frequencies of sound are played at the same amplitude, some will be loud, and others quiet or even completely inaudible. Generally, if the gain or amplitude is increased, a sound is more likely to be perceived. Ordinarily, when animals use sound to communicate, hearing in that type of animal is most sensitive for the frequencies produced by calls, or, in the case of humans, speech. This tuning of hearing exists at many levels of the auditory system, all the way from the physical characteristics of the ear to the nerves and tracts that convey the nerve impulses of the auditory portion of the brain.

A hearing impairment exists when an individual is not sensitive to the sounds normally heard by its kind. In human beings, the term hearing impairment is usually reserved for people who have relative insensitivity to sound in the speech frequencies. The severity of a hearing impairment is categorized according to how much louder a sound must be made over the usual levels before the listener can detect it. In profound deafness, even the loudest sounds that can be produced by the instrument used to measure hearing (audiometer) may not be detected.

There is another aspect to hearing that involves the quality of a sound rather than amplitude. In people, that aspect is usually measured by tests of speech discrimination. Basically, these tests require that the sound is not only detected but understood. There are very rare types of hearing impairments which affect discrimination alone.[1]

Different biological mechanisms

Long term exposure to environmental noise

Populations of people living near airports or freeways are exposed to levels of noise typically in the 65 to 75 dbA range. If lifestyles include significant outdoor or open window conditions, these exposures over time can degrade hearing. The 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).

  • 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.
  • Personal electronic audio devices, such as iPods (iPods often reaching 115 decibels or higher), can produce powerful enough sound to cause significant Noise-Induced Hearing Loss, given that lesser intensities of even 70 dB can also cause hearing loss.

Genetic

Hearing loss can be inherited. Both dominant and recessive genes exist which can cause mild to profound impairment. If a family has a dominant gene for deafness it will persist across generations because it will manifest itself in the offspring even if it is inherited from only one parent. If a family had genetic hearing impairment caused by a recessive gene it will not always be apparent as it will have to be passed onto offspring from both parents 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 DFNB1, also known as Connexin 26 deafness or GJB2-related deafness.
  • The most common dominant syndromic forms of hearing impairment include Stickler syndrome and Waardenburg syndrome.
  • The most common recessive syndromic forms of hearing impairment are Pendred syndrome, Large vestibular aqueduct syndrome and Usher syndrome.

Disease or illness

  • Measles may result in auditory nerve damage
  • Meningitis may damage the auditory nerve or the cochlea
  • Autoimmune disease has only recently been recognized as a potential cause for cochlear 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 dB or more), unilateral (one ear) or bilateral (both ears).
  • Presbycusis is deafness due to loss of perception to high tones, mainly in the elderly. It is considered by some to be a degenerative process, although there has never been a proven link to aging. (See impact of environmental noise exposure above.)
  • 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.
  • AIDS and ARC patients frequently experience auditory system anomalies.[2]
  • HIV (and subsequent opportunistic infections) may directly affect the cochlea and central auditory system.[2]
  • Chlamydia may cause hearing loss in newborns to whom the disease has been passed at birth.[2]
  • Fetal alcohol syndrome is reported to cause hearing loss in up to 64% of infants born to alcoholic mothers, from the ototoxic effect on the developing fetus plus malnutrition during pregnancy from the excess alcohol intake.[2]
  • Premature birth results in sensorineural hearing loss approximately 5% of the time.[2]
  • Syphilis is commonly transmitted from pregnant women to their fetuses, and about a third of the infected children will eventually become deaf.[2]
  • Otosclerosis is a hardening of the stapes (or stirrup) in the middle ear and causes conductive hearing loss.

Medications

See also Ototoxicity

Some medications cause irreversible damage to the ear, and are limited in their use for this reason. The most important group is the aminoglycosides (main member gentamicin).

Various other medications may reversibly affect hearing. This includes some diuretics, aspirin and NSAIDs, and macrolide antibiotics.

Extremely heavy Vicodin abuse is known to cause hearing impairment. There has been speculation that radio talk show host Rush Limbaugh's hearing loss was at least in part caused by his admitted addiction to narcotic pain killers, in particular Vicodin and OxyContin.

Physical trauma

  • 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 tinnitus, either temporary or permanent.
  • Exposure to very loud noise (90 dB or more, such as jet engines at close range) can cause progressive hearing loss. Exposure to a single event of extremely loud noise (such as explosions) can also cause temporary or permanent hearing loss. A typical source of acoustic trauma is a too-loud music concert.[3]

References

  1. eBook: Current Diagnosis & Treatment in Otolaryngology: Head & Neck Surgery, Lalwani, Anil K. (Ed.) Chapter 44: Audiologic Testing by Robert W. Sweetow, PhD, Jennifer McKee Bold, AuD, Access Medicine
  2. 2.0 2.1 2.2 2.3 2.4 2.5 "Frequently Asked Questions: Etiologies and Causes of Deafness". Retrieved 2006-12-02.
  3. http://www.4hearingloss.com/archives/2005/05/sonic_tonic.html



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