Deafness is the complete loss of the ability to hear from one or both ears. Deafness may be inherited, or caused by complications at birth, certain infectious diseases, such as meningitis, use of ototoxic drugs, and exposure to excessive noise. Around half of all deafness and hearing impairment can be prevented.
Hearing loss, or deafness, can be present at birth (congenital), or become evident later in life (acquired). The distinction between acquired andcongenital deafness specifies only the time that the deafness appears. It does not specify whether the cause of the deafness is genetic (inherited).
Acquired deafness may or may not be genetic. For example, it may be a manifestation of a delayed-onset form of genetic deafness. Alternatively, acquired deafness may be due to damage to the ear from noise.
Congenital deafness similarly may or may not be genetic. For example, it may be associated with a white forelock, and be caused by a genetic disease called Waardenburg syndrome. In fact, more than half of congenital hearing loss is inherited. Alternatively, congenital deafness may be due to a condition or infection to which the mother was exposed during pregnancy, such as the rubella virus.
Hearing loss can also be classified based on which portions of the hearing system (auditory system) are affected. When the nervous system is affected, it is referred to as sensorineural hearing loss. When the portions of the ear that are responsible for transmitting the sound to the nerves are affected, it is referred to as conductive hearing loss.
Conditions affecting the cochlea, eighth cranial nerve, spinal cord, or braincause sensorineural hearing loss.
- Meniere's disease,
- hearing loss of aging (presbycusis),
- nerve injury from syphilis,
- hearing loss of unknown cause (idiopathic hearing loss),
- nerve tumors,
- drug toxicity (such as aspirin and aminoglycosides).
Conditions that affect the ear canal, eardrum (tympanic membrane), and middle ear lead to conductive hearing loss.
Examples of conductive hearing loss include:
- ear wax blocking the ear canal,
- otitis media,
Profound deafness is easy to recognize, since people will notice such a large change in hearing. Milder hearing loss may not be noticed right away, since it often comes on gradually and people "get used to it".
Age-related hearing loss often starts at the high frequencies, meaning that people may have trouble understanding women and children (whose voices are higher pitched) or telling the difference between similar sounds such as "th" and "sh". Many people are alerted by friends or relatives. The problem is initially most apparent in noisy environments.
All infants and children should be screened for hearing loss, as early diagnosis and intervention can have a dramatic impact on the child's future development and educational needs. Signs of deafness in young children include not responding to noises, responding slowly, or not learning to speak by the expected age. A deaf child may also lag behind in developing motor skills and coordination, or in learning how to balance, crawl, or walk.
Profoundly deaf children are usually diagnosed by the age of 2. The biggest obstacles to early diagnosis are typically a delay in a referral to a specialist (usually when the signs of hearing loss are not recognized), or a lack of access to appropriate infant hearing screening.
The following are some of the major causes of hearing loss.
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 asynergistic interaction.
People with allergies have a higher chance of increased hearing loss. Hearing loss can be caused by the sudden change in air pressure behind the eardrum in the Eustachian Tube during a sneeze. Though the amount of hearing loss seems negligible, roughly .0001% loss per sneeze dependent upon the strength and frequency of the sneeze(s), their effects are cumulative. Allergy sufferers typically have on average 1-3% more hearing loss than those without allergies, due to their above average number of sneezes.
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.
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.
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  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 and automobile exhaust, for example); andasphyxiants. 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:
- Antimalarial, antibiotics, anti-inflammatory (non-steroidal), antineoplastic, diuretics.
- Toluene, styrene, xylene, n-hexane, ethyl benzene, white spirits/Stoddard, carbon disulfide, fuels, perchloroethylene, trichloroethylene, p-xylene.
- Carbon monoxide, hydrogen cyanide.
- Lead, mercury, organotins (trimethyltin).
- Paraquat, organophosphates.
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. I. King Jordan lost his hearing after suffering a skull fracture as a result of a motorcycle accident at age 21.
The treatment of hearing loss depends on its cause.
- ear wax can be removed,
- ear infection can be treated with medications,
- medications that are toxic to the ear can be avoided;
- occasionally surgical procedures are necessary.
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.
The United States Food and Drug Administration reported that cochlear implant recipients may be at higher risk for meningitis. People who have hearing impairments, especially those who develop a hearing problem in childhood or old age, may need the 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 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 or total communication. This may be because not all audiologists are familiar with the benefits of sign language to the deaf child and family.
There are many arguments against the cochlear implant including the loss of deaf culture and the fact that the surgery does not always restore hearing.
Most parents and doctors tell children not to play sports or get involved in activities that can result in injuries to the head. Soccer, Hockey, and Basketball can be some examples. A child with a hearing loss may prefer to stay away from noisy places, such as rock concerts, Football games, airports, etc, as this can cause noise overflow (noise overflow is a type of headache that occurs in many children and adults when they are near loud noises).
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.
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.
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.
Real-time text technologies, involving streaming text that is continuously transmitted as it is typed or otherwise composed. This allows conversational use of text.
Instant messaging software. In addition, AOL Instant Messenger provides a real-time text feature called Real-Time IM.
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 (VRI) 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.
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.
For mobile phones, software apps are available to provide TDD/textphone functionality on some carriers/models to provide 2-way communications.
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.