Hearing impairment pathophysiology

Hearing impairment Microchapters
Home
Patient Information
Overview
Historical Perspective
Classification
Pathophysiology
Causes
Epidemiology and Demographics
Risk Factors
Screening
Natural History, Complications and Prognosis
Diagnosis
History and Symptoms
Physical Examination
Laboratory Findings
CT
MRI
Other Imaging Findings
Other Diagnostic Studies
Treatment
Medical Therapy
Surgery
Primary Prevention
Secondary Prevention
Cost-Effectiveness of Therapy
Social Impact
Future or Investigational Therapies
Case Studies
Case #1
Hearing impairment On the Web
Most recent articles
Most cited articles
Review articles
CME Programs
Powerpoint slides
Images
American Roentgen Ray Society Images of Hearing impairment All Images X-rays Echo & Ultrasound CT Images MRI
Ongoing Trials at Clinical Trials.gov
US National Guidelines Clearinghouse
NICE Guidance
FDA on Hearing impairment
CDC on Hearing impairment
Hearing impairment in the news
Blogs on Hearing impairment
Directions to Hospitals Treating Hearing impairment
Risk calculators and risk factors for Hearing impairment

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor(s)-in-Chief:

Overview

Sound waves vary in amplitude and frequency. Amplitude is that the sound wave's peak pressure variation. Frequency is that the number of cycles per second of a sinusoidal component of a wave. Loss of the power to detect some frequencies, or to detect low-amplitude sounds, that an organism naturally detects, may be termed as a hearing disorder. Hearing sensitivity is indicated by the quietest sound that a person can detect, termed the hearing threshold. The normal hearing threshold is not the same for all frequencies of sounds. Long-term exposure to environmental noise, Genetics, Disease or illness, Medications, and Physical trauma are different biological mechanisms for hearing loss.

Pathophysiology

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

Loudness, frequency, and discrimination deficiencies

• Hearing sensitivity is indicated by the quietest sound that a person can detect, termed the hearing threshold. This threshold is often accurately measured by a behavioral audiogram, in humans and a few animals. A record is formed of the quietest sound that consistently prompts a response from the listener. The test is administered for sounds of various frequencies. There also are some electrophysiological tests that will be performed without requiring a behavioral response of the individual.

• The normal hearing threshold is not the same for all frequencies of sounds. If different frequencies of sound are played at an equivalent amplitude, some are going to be loud, quiet, or completely inaudible. Generally, if the amplitude is increased, a sound is more likely to be heard. Ordinarily, when animals use sound to speak, hearing therein sort of animal is most sensitive for the frequencies produced by calls, or, within the case of humans, speech. This tuning of hearing exists at many levels of the sensory system, all the way from the physical characteristics of the ear, to the nerves, and tracts that convey the auditory impulses to the portion of the brain that is sensitive to hearing sounds.

• A hearing disorder exists when a person isn't sensitive to the sounds normally heard by its kind. The term hearing impairment is usually reserved for folks that have relative insensitivity to sound within the speech frequencies. The severity of hearing impairment is defined as; how much louder a sound must be made over the usual levels before the listener can perceive it. In profound deafness, even the loudest sounds which will be produced by the instrument wont to measure hearing like an audiometer, might not be detected.

• There is a rare sort of hearing impairment that affects speech discrimination alone. There is another aspect to hearing that involves the quality of a sound instead of amplitude and frequency. This quality of sound is typically measured by tests of speech discrimination. These tests require that the sound isn't only detected but understood.

Different biological mechanisms

Long term exposure to environmental noise

• Populations of individuals living near airports or freeways are exposed to levels of noise typically > 65 dB. If the lifestyle of a person includes significant outdoor activities, these exposures over time can degrade hearing. Various states have set noise standards to guard people against these adverse sound risks.

• The EPA has identified the extent of 70 DB for twenty-four-hour exposure because of the level necessary to guard the general public against deafness (EPA, 1974).

• Noise-Induced deafness (NIHL) typically is centered at 4000 Hz.

• The louder the noise is, the shorter is the safe amount of exposure. Normally, the safe amount of exposure is reduced by an element 2 for each additional 3 dB. for instance, the safe daily exposure amount at 85 dB is 8 hours, while the safe exposure at 91 dB is merely 2 hours. Sometimes, an element 2 per 5 dB is employed.

• Personal electronic audio devices, like iPods, can produce powerful enough sound to cause significant Noise-Induced deafness, iPods often reaching 115 decibels or higher, as long as lesser intensities of even 70 dB also can cause deafness.

Genetic

• Hearing loss is often inherited. Both dominant and recessive genes exist which may cause mild to profound impairment. If a family features a gene for deafness it'll persist across generations because it'll happen within the offspring albeit it's inherited from just one parent. If a family had a genetic hearing disorder caused by a gene it'll not always be apparent because it will need to be passed onto offspring from both parents Dominant and recessive hearing disorders are often syndromic or non-syndromic. Recent gene mapping has identified dozens of non-syndromic dominant and recessive sorts of deafness.

• The most common sort of congenital hearing disorder in developed countries is non-syndromic recessive, also referred to as Connexin 26 deafness.

• The most common dominant syndromic sorts of hearing disorder include Stickler syndrome and Waardenburg syndrome.

• The most common recessive syndromic sorts of the hearing disorder are Pendred syndrome, Large vestibular aqueduct syndrome, and Usher syndrome

Disease or illness

• Measles can result in auditory nerve damage.

• Meningitis can damage the acoustic nerve or the cochlea of the inner ear.

• Autoimmune disease has only recently been recognized as a possible cause of cochlear damage. Although probably rare, autoimmune processes can focus on the cochlea specifically, without symptoms affecting other organs. Wegener's granulomatosis is one of the autoimmune conditions which will precipitate deafness.

• Mumps (Epidemic parotitis) may end in profound sensorineural deafness (90 dB or more), unilateral or bilateral.

• Presbycusis is deaf thanks to the loss of perception to high tones, mainly within the elderly people. It is caused by some to be a degenerative processes within the inner ear, although there is no proven link to aging.

• Adenoids that do not disappear by adolescence may still grow and should obstruct the Eustachian tube, causing conductive hearing disorder and nasal infections which will spread to the center ear.

• AIDS patients frequently experience sensory system anomalies.

• HIV and subsequent opportunistic infections can directly affect the cochlea and central sensory system.

• Chlamydia may cause deafness in newborns to whom the disease has been passed at birth through the birth canal during normal vaginal delivery.

• 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.

• Premature birth results in sensorineural hearing loss approximately 5% of the time.

• Syphilis is usually transmitted from pregnant women to their fetuses, and a few thirds of the infected children will eventually become deaf.

• Otosclerosis may be a hardening of the stapes within the tympanic cavity in the middle ear and it causes conductive deafness.

Medications

See also Ototoxicity

• Some medications cause irreversible damage to the ear and are limited in their use for this serious side effect. The foremost important group is that aminoglycosides (gentamicin).

• Various other medications may cause reversible hearing loss. This includes some diuretics, NSAIDs, aspirin, and macrolide antibiotics.

• Extremely heavy Vicodin and OxyContin abuse are understood to cause hearing disorders.

Physical trauma

• There is often external 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 susceptible to deafness or tinnitus, either temporary or permanent.

• Exposure to very bang (90 dB or more, like jet engines at close range) can cause progressive deafness. Exposure to one event of extreme bang (such as explosions) also can cause deafness. A typical source of acoustic trauma may be a too-loud music concert.