Hearing Impairment

Hearing impairment or deafness refers to conditions in which individuals are fully or partially unable to detect or perceive at least some frequencies of sound which can typically be heard by members of their species. Use of the term impaired implies that deafness presents an inherent disadvantage to an animal, a view that is rejected within the Deaf culture movement, where the terms Deaf and hard of hearing are preferred.

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 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. 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.

Classification

Hearing impairments are categorized by their type (conductive, sensorineural, or both), by their severity, and by the age of onset. Furthermore, a hearing impairment may exist in only one ear (unilateral) or in both ears (bilateral).

Conductive and sensorineural hearing impairments

A conductive hearing impairment is an impairment resulting from dysfunction in any of the mechanisms that normally conduct sound waves through the outer ear, the eardrum or the bones of the middle ear.

A sensorineural hearing impairment is one resulting from dysfunction in the inner ear, especially the cochlea where sound vibrations are converted into neural signals, or in any part of the brain that subsequently processes these signals. The vast majority of human sensorineural hearing loss is associated with abnormalities in the hair cells of the organ of Corti in the cochlea. This dysfunction may be present from birth due to genetic or developmental abnormalities, or arise through trauma or disease during the lifetime of an individual. There are also very unusual sensorineural hearing impairments that involve the VIIIth cranial nerve, the Vestibulocochlear nerve or, in rare cases, auditory cortex. Damage to parts of the brain that process auditory signals can lead to a condition in which sounds may be heard at normal thresholds, but the quality of the sound perceived is so poor that speech cannot be understood. Sensorineural hearing loss associated with abnormalities of the auditory system in the brain is called Central Hearing Impairment.

Quantification of hearing loss
 
The severity of a hearing impairment is ranked according to the loudness (measured in decibels (dB)) a sound must be before being detected by an individual. Hearing impairment may be ranked as mild, moderate, severe or profound as defined below:

Mild:
for adults: between 25 and 40 dB
for children: between 20 and 40 dB
Moderate: between 41 and 55 dB
Moderately severe: between 56 and 70 dB
Severe: between 71 and 90 dB
Profound: 90 dB or greater

Hearing sensitivity varies according to the frequency of sounds. To take this into account, hearing sensitivity can be measured for a range of frequencies and plotted on an audiogram.

For certain legal purposes such as insurance claims, hearing impairments are described in terms of percentages. Given that hearing impairments can vary by frequency and that audiograms are plotted with a logarithmic scale, the idea of a percentage of hearing loss is somewhat arbitrary, but where decibels of loss are converted via a recognized legal formula, it is possible to calculate a standardized "percentage of hearing loss" which is suitable for legal purposes only.

Another method for quantifying hearing impairments is the Hearing in Noise Test (HINT). HINT technology was developed by the House Ear Institute, and is intended to measure an ability to understand speech in quiet and noisy environments. Unlike pure-tone tests, where only one ear is tested at a time, HINT evaluates hearing using both ears simultaneously (binaural), as binaural hearing is essential for communication in noisy environments and for sound localization.

Age of onset

The age at which hearing loss occurs is crucial for the acquisition of a spoken language.

Pre-lingual deafness

Prelingual deafness is hearing impairment that is sustained prior to the acquisition of language, which can occur as a result of a congenital condition or through hearing loss in early infancy. Prelingual deafness impairs an individual's ability to acquire a spoken language, but children born into signing families rarely have delays in language development. Most pre-lingual hearing impairment is acquired via either disease or trauma rather than genetically inherited, so families with deaf children nearly always lack previous experience with sign language.

Post-lingual deafness

Post-lingual deafness where hearing loss is adventitious after the acquisition of speech and language, usually after the age of six. It may develop due to disease, trauma, or as a side-effect of a medicine. Typically, hearing loss is gradual and often detected by family and friends of affected individuals long before the patients themselves will acknowledge the disability. Common treatments include hearing aids and learning lip reading.

Post-lingual deafness is far more common than pre-lingual deafness.

Unilateral and bilateral hearing impairment

People with unilateral hearing impairment (single sided deafness/SSD) have an impairment in only one ear. This can impair a person's ability to localize sounds (e.g., determining where traffic is coming from) and distinguish sounds from background noise in noisy environments.

A similar effect can result from King-Kopetzky syndrome (also known as Auditory disability with normal hearing and obscure auditory dysfunction), which is characterized by an inability to process out background noise in noisy environments despite normal performance on traditional hearing tests. See also: "cocktail party effect," House Ear Institute's Hearing In Noise Test.

Causes

The following are some of the major causes of hearing loss.

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 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.

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 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.
• 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).

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

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; and asphyxiants.  These are mostly industrial chemicals, uncommon in residencies. 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

anti-malarial, 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.

Treatments, management and prevention

Approaches

If the hearing loss occurs at a young age, interference with the acquisition of spoken language and social skills may occur. Hearing aids, which amplify the incoming sound, may alleviate some of the problems caused by hearing impairment, but are often insufficient. Cochlear implants artificially stimulate the VIIIth Nerve by providing an electric impulse substitution for the firing of hair cells. Cochlear implants are not only expensive, but require sophisticated programming in conjunction with patient training for effectiveness. People who have hearing impairments, especially those who develop a hearing problem in childhood or old age, require support and technical adaptations as part of the rehabilitation process. Recent research shows variations in efficacy but some studies  show that if implanted at a very young age, some profoundly impaired children can acquire effective hearing and speech, particularly if supported by appropriate rehabilitation such as auditory-verbal therapy .

Views of treatments

There has been considerable controversy within the culturally Deaf community over cochlear implants. For the most part, there is little objection to those who lost their hearing later in life or culturally Deaf adults (voluntarily) choosing to be fitted with a cochlear implant. Many in the culturally Deaf community strongly object to a deaf child being fitted with a cochlear implant (often on the advice of an audiologist; new parents may not have sufficient information on raising deaf children) and placed in an oral-only program that emphasizes the ability to speak and listen over other forms of communication such as sign language. Another issue is the fact that the implanted deaf child has to avoid team or full-contact sports to minimize the chances of a head injury, which carries a greater risk where the implant is involved.

Gene therapy

A 2005 study achieved successful regrowth of cochlea cells in guinea pigs. 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.

Adaptations to 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 an 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.
• 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.