Definition:
Deafness is a condition wherein the ability to detect certain frequencies of sound is completely or partially impaired. When applied to humans, the term hearing impaired is rejected by the Deaf Culture movement, where the terms deaf and hard-of-hearing are preferred.
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Hearing sensitivity is indicated by the quietest sound that an animal can detect, called the hearing threshold. In the case of humans 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 within any given species are not the same for all frequencies. If different frequencies of sound are played at the same amplitude, some will be perceived as 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. All levels of the auditory system contribute to this sensitivity toward certain frequencies, from the outer ear’s physical characteristics to the nerves and tracts that convey the nerve impulses of the auditory portion of the brain.
A hearing loss exists when an animal has diminished sensitivity to the sounds normally heard by its species. In humans, the term hearing impairment is usually reserved for people who have relative insensitivity to sound in the speech frequencies. The severity of a hearing loss is categorized according to the increase in volume that must be made above the usual level before the listener can detect it. In profound deafness, even the loudest sounds that can be produced by an audiometer (an instrument used to measure hearing) may not be detected.
Another aspect to hearing involves the perceived clarity of a sound rather than its amplitude. In humans, that aspect is usually measured by tests of speech perception. These tests measure one’s ability to understand speech, not to merely detect sound. There are very rare types of hearing impairments which affect speech understanding alone.
Causes:
The following are some of the major causes of hearing loss:-
*Age:-
Presbycusis, the progressive loss of ability to hear high frequencies with increasing age, begins in early adulthood, but does not usually interfere with ability to understand conversation until much later. Although genetically variable it is a normal concomitant of aging and is distinct from hearing losses caused by noise exposure, toxins or disease agents.
*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 dB(A) 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 and other disruptive effects from noise, such as sleep disturbance, stress-related problems, learning detriment, etc. (EPA, 1974).
Noise-induced hearing loss (NIHL) typically is centered at 3000, 4000, or 6000 Hz. As noise damage progresses, damage starts affecting lower and higher frequencies. On an audiogram, the resulting configuration has a distinctive notch, sometimes referred to as a “noise notch.” As aging and other effects contribute to higher frequency loss (6–8 kHz on an audiogram), this notch may be obscured and entirely disappear.
Louder sounds cause damage in a shorter period of time. Estimation of a “safe” duration of exposure is possible using an exchange rate of 3 dB. As 3 dB represents a doubling of intensity of sound, duration of exposure must be cut in half to maintain the same energy dose. For example, the “safe” daily exposure amount at 85 dB A, known as an exposure action value, is 8 hours, while the “safe” exposure at 91 dB(A) is only 2 hours (National Institute for Occupational Safety and Health, 1998). Note that for some people, sound may be damaging at even lower levels than 85 dB A. Exposures to other ototoxins (such as pesticides, some medications including chemotherapy, solvents, etc.) can lead to greater susceptibility to noise damage, as well as causing their own damage. This is called a synergistic interaction.
Some American health and safety agencies (such as OSHA-Occupational Safety and Health Administration and MSHA-Mine Safety and Health Administration), use an exchange rate of 5 dB. While this exchange rate is simpler to use, it drastically underestimates the damage caused by very loud noise. For example, at 115 dB, a 3 dB exchange rate would limit exposure to about half a minute; the 5 dB exchange rate allows 15 minutes.
While OSHA, MSHA, and FRA provide guidelines to limit noise exposure on the job, there is essentially no regulation or enforcement of sound output for recreational sources and environments, such as sports arenas, musical venues, bars, etc. This lack of regulation resulted from the defunding of ONAC, the EPA’s Office of Noise Abatement and Control, in the early 1980s. ONAC was established in 1972 by the Noise Control Act and charged with working to assess and reduce environmental noise. Although the Office still exists, it has not been assigned new funding.
Many people are unaware of the presence of environmental sound at damaging levels, or of the level at which sound becomes harmful. Common sources of damaging noise levels include car stereos, children’s toys, transportation, crowds, lawn and maintenance equipment, power tools, gun use, and even hair dryers. Noise damage is cumulative; all sources of damage must be considered to assess risk. If one is exposed to loud sound (including music) at high levels or for extended durations (85 dB A or greater), then hearing impairment will occur. Sound levels increase with proximity; as the source is brought closer to the ear, the sound level increases.
*Genetic:-
Hearing loss can be inherited. Both dominant genes 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 first gene mapped for non-syndromic deafness, DFNA1, involves a splice site mutation in the formin related homolog diaphanous 1 (DIAPH1). A single base change in a large Costa Rican family was identified as causative in a rare form of low frequency onset progressive hearing loss with autosomal dominant inheritance exhibiting variable age of onset and complete penetrance by age 30.
#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.
#The congenital defect microtia can cause full or partial deafness depending upon the severity of the deformity and whether or not certain parts of the inner or middle ear are affected.
#Mutations in PTPRQ Are a Cause of Autosomal-Recessive Nonsyndromic Hearing Impairment
*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 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.
#AIDS and ARC patients frequently experience auditory system anomalies.
#HIV (and subsequent opportunistic infections) may directly affect the cochlea and central auditory system.
#Chlamydia may cause hearing loss in newborns to whom the disease has been passed at birth.
#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 commonly transmitted from pregnant women to their fetuses, and about a third of the infected children will eventually become deaf.
#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.
*Medications:–
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) and platinum based chemotherapeutics such as cisplatin.
Various other medications may reversibly affect hearing. This includes some diuretics, aspirin and NSAIDs, and macrolide antibiotics.
The 1995 Miss America Heather Whitestone lost her hearing after receiving strong antibiotics for haemophilus influenzae.[citation needed] Extremely heavy hydrocodone (Vicodin or Lorcet) abuse is known to cause hearing impairment. Commentators have speculated 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.
*Exposure to ototoxic chemicals:-
In addition to medications, hearing loss can also result from specific drugs; metals, such as lead; solvents, such as toluene (found in crude oil, gasoline[6] and automobile exhaust, for example); and asphyxiants. Combined with noise, these ototoxic chemicals have an additive effect on a person’s hearing loss.Hearing loss due to chemicals starts in the high frequency range and is irreversible. It damages the cochlea with lesions and degrades central portions of the auditory system. For some ototoxic chemical exposures, particularly styrene, the risk of hearing loss can be higher than being exposed to noise alone. Controlling noise and using hearing protectors are insufficient for preventing hearing loss from these chemicals. However, taking antioxidants helps prevent ototoxic hearing loss, at least to a degree. The following list provides an accurate catalogue of ototoxic chemicals:-
#Drugs
antimalarial, antibiotics, anti-inflammatory (non-steroidal), antineoplastic, diuretics
#Solvents
toluene, styrene, xylene, n-hexane, ethyl benzene, white spirits/Stoddard, carbon disulfide, fuels, perchloroethylene, trichloroethylene, p-xylene
#Asphyxiants
carbon monoxide, hydrogen cyanide
#Metals
lead, mercury, organotins (trimethyltin)
#Pesticides/Herbicides
paraquat, organophosphates
#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 an excessively loud music concert. I King Jordan lost his hearing after suffering a skull fracture as a result of a motorcycle accident at age 21
Diagnosis:
The diagnosis starts with verifying the medical and the family histories of the person. Ear examination is done to assess the hearing and the balancing ability of the person.
The following tests may be required to confirm the diagnosis and to find the cause:
Audiogram: The person sits in a sound proof room and wears a headphone attached to a machine. Different sounds of varying intensity or loudness are sent through the headphone and the person is asked to tell whether he hears the sound or not. Each ear is tested separately. An audiogram helps to assess any hearing loss.
Electrocochleography: This test measures electrical activity in the cochlea and in the nerves that take sensations from the ear to the brain. This is done by passing a thin needle into the ear that records the activity and sends it to an attached computer. In people who have SHL, the activity will be abnormal because of damage.
Caloric Testing: It is done to assess the functioning of the vestibular part of the inner ear, which maintains the body balance. The person is asked to sit in a chair. Cold and hot water is poured into the ear alternately and the associated involuntary eye movements are checked.
Computed Tomography Scan or Magnetic Resonance Imaging: Multiple images of the affected ear are taken and then they are assembled by a computer to generate a clear image of the internal body parts. These tests help to assess any damage in the inner ear. Also, any nerve tumor, such as Acoustic Neuroma, can be diagnosed.
Blood Tests such as fluorescent treponemal antibody absorption may be required to check for Syphili. Antinuclear antibodies may be checked to detect any autoimmune disorders.
Treatment:
The treatment of hearing loss depends on its cause. For example:
•ear wax can be removed,
•ear infection can be treated with medications,
•medications that are toxic to the ear can be avoided and;
•occasionally surgical procedures are necessary.
Gene therapy:-
A 2005 study achieved successful regrowth of cochlea cells in guinea pigs.[13] It is important to note, however, that the regrowth of cochlear hair cells does not imply the restoration of hearing sensitivity as the sensory cells may or may not make connections with neurons that carry the signals from hair cells to the brain. A 2008 study has shown that gene therapy targeting Atoh1 can cause hair cell growth and attract neuronal processes in embryonic mice. It is hoped that a similar treatment will one day ameliorate hearing loss in humans.
Assistive techniques and devices for hearing impairment:-
Many hearing impaired individuals use assistive devices in their daily lives:
#Individuals can communicate by telephone using Telecommunications Device for the Deaf (TDD). These devices look like typewriters or word processors and transmit typed text over regular telephone lines.Other names in common use are textphone and minicom.
#There are several new Telecommunications Relay Service technologies including IP Relay and captioned telephone technologies.
#Mobile textphone devices came onto the market as of 2004, allowing simultaneous two way text communication.
#Videophones and similar video technologies can be used for distance communication using sign language. Video conferencing technologies permit signed conversations as well as permitting a sign language-English interpreter to voice and sign conversations between a hearing impaired person and that person’s hearing party, negating the use of a TTY device or computer keyboard.
#Video Relay Service and Video Remote Interpreting services also use a third-party telecommunication service to allow a deaf or hard-of-hearing person to communicate quickly and conveniently with a hearing person, through a sign language interpreter.
#In the U.S., the UK, the Netherlands and many other western countries there are Telecommunications Relay Services so that a hearing impaired person can communicate over the phone with a hearing person via a human translator. Wireless, internet and mobile phone/SMS text messaging are beginning to take over the role of the TDD.
#Phone captioning is a service in which a hearing person’s speech is captioned by a third party, enabling a hearing impaired person to conduct a conversation with a hearing person over the phone.
#Hearing dogs are a specific type of assistance dog specifically selected and trained to assist the deaf and hearing impaired by alerting their handler to important sounds, such as doorbells, smoke alarms, ringing telephones, or alarm clocks.
#Other assistive devices include those that use flashing lights to signal events such as a ringing telephone, a doorbell, or a fire alarm.
#The advent of the Internet’s World Wide Web and closed captioning has given the hearing impaired unprecedented access to information. Electronic mail and online chat have reduced the need for deaf and hard-of-hearing people to use a third-party Telecommunications Relay Service in order to communicate with the hearing and other hearing impaired people;
Disclaimer: This information is not meant to be a substitute for professional medical advise or help. It is always best to consult with a Physician about serious health concerns. This information is in no way intended to diagnose or prescribe remedies.This is purely for educational purpose.
Resources:
http://www.bbc.co.uk/health/physical_health/conditions/deafness1.shtml
http://en.wikipedia.org/wiki/Hearing_impairment
http://www.medicinenet.com/deafness/article.htm
http://www.assiutknol.com/hearing-loss-and-deafness
http://healthscribes.com/disease/Hearing+Loss,+Sudden
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