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 majority of deaf people 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.[1]
There are two different types of hearing impairments, conductive hearing impairment and sensorineural hearing impairment. A third type is a combination of the two called mixed hearing loss. 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).
A conductive hearing impairment is present when the sound is not reaching the inner ear, the cochlea. This can be due to external ear canal malformation, dysfunction of the eardrum or malfunction of the bones of the middle ear. The ear drum may show defects from small to total resulting in hearing loss of different degree. Scar tissue after ear infections may also make the ear drum dysfunction as well as when it is retracted and adherent to the medial part of the middle ear.
Dysfunction of the three small bones of the middle ear; hammer, anvil and stapes may result in conductive hearing loss. The mobility of the ossicles may be impaired of different reasons and disruption of the ossicular chain due to trauma, infection or anchylosis may also result in hearing loss.
Many of these conditions can be helped with surgery, and an air conduction hearing aid is often a good choice of treatment. However, in some cases such an aid is not possible to use. The most obvious reason is if the patient does not have any ear canals. Where to place the ear mould? A more common reason is in patients with chronic ear infections that drain continuously or start to drain when the ear canal is obstructed with an air conduction hearing aid mould. In these patients a direct bone conduction hearing device could be an excellent solution. An implant made out of titanium is placed in the bone behind the external ear and allowed to osseointegrate and an impedance-matched hearing aid can be attached. At present there are two such hearing aids on the market; the Baha 3 by Cochlear BAS and the Ponto by Oticon Medical.
A sensorineural hearing loss is one resulting from dysfunction of the inner ear, the cochlea, the nerve that transmits the impulses from the cochlea to the hearing centre in the brain or damage in the brain. The most common reason for sensorineural hearing impairment is damage to the hair cells in the cochlea. As we grow older the hair cells degenerate and lose their function, and our hearing deteriorates. Depending on the definition it could be estimated that more than 50% of the population over the age of 70 has an impaired hearing. Impaired hearing is the most common physical handicap in the industrialized world.
Another common reason for hearing loss due to hair cell damage is noise-induced hearing loss. These types of hearing loss are often most pronounced in the high frequency range. This will often interfere with speech understanding, as it is in the high frequency range that we find the consonant sounds that are most important especially in noisy surroundings. Head trauma, ear infections, tumours and ototoxic drugs such as gentamycine are other reasons for sensorineural hearing loss.
Hair cells that are damaged cannot be replaced with any surgical procedure, even if research with stem cell treatment is presently going on in many institutions. The clinical application of this will however not yet be available for many years. Protection from noise exposure is at present the only way to reduce the hair cell damage. Conventional air conduction hearing aids are often prescribed for patients with sensorineural hearing loss. The outcome with modern types of hearing aids is often excellent, but speech understanding could still be a problem in demanding situations.
Total or near total sensorineural deafness could be the result of congenital malformations, head trauma or inner ear infection. In patients with total or near total deafness, an air conduction aid could not be used even if the drum and middle ear are normal. For these patients a cochlear implant could be a treatment option. This means that a thin electrode is placed into the cochlea and is stimulated electrically through a small microprocessor under the skin behind that ear.
Mixed hearing loss is a combination of the two types discussed above. Chronic ear infection that is a fairly common diagnosis could result in a defect ear drum and/or middle ear ossicle damages. Surgery is often attempted but not always successful. On top of the conductive loss a sensory component is often added. If the ear is dry and not infected an air conduction aid could be tried, but if the ear is draining a direct bone condition hearing aid is often the best solution. If the conductive part of the hearing loss is more than 30–35 dB an air conduction device could have problems overcoming this gap. A direct bone conduction aid like the Baha or the Ponto could in this situation be a good option.
The severity of a hearing impairment is ranked according to the additional intensity above a nominal threshold that a sound must be before being detected by an individual; it is (measured in decibels of hearing loss, or dB HL). Hearing impairment may be ranked as mild, moderate, moderately severe, severe or profound as defined below:
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 a speech-in-noise test. As the name implies, a speech-in-noise test gives an indication of how well one can understand speech in a noisy environment. A person with a hearing loss will often be less able to understand speech, especially in noisy conditions. This is especially true for people who have a sensorineural loss – which is by far the most common type of hearing loss. As such, speech-in-noise tests can provide valuable information about a person's hearing ability, and can be used to detect the presence of a sensorineural hearing loss. A triple-digit speech-in-noise test was developed by RNID as part of a EU funded project Hearcom. The RNID version is available over the phone (0844 800 3838, only available in the UK), on the web and as an app on the iPhone.
The age at which hearing loss occurs is crucial for the acquisition of a spoken language.
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 is hearing impairment that is sustained after the acquisition of language, which can occur as a result of 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.
Patients with Unilateral Hearing Loss or Single-Sided Deafness (SSD) have difficulty in:
In quiet conditions, speech discrimination is approximately the same for normal hearing and those with unilateral deafness; however, in noisy environments speech discrimination varies individually and ranges from mild to severe.
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.
One reason for the hearing problems these patients often experience is due to the head shadow effect. Newborn children with no hearing on one side but one normal ear could still have problems[2]. Speech development could be delayed and difficulties to concentrate in school are common. More children with unilateral hearing loss have to repeat classes than their peers. Taking part in social activities could be a problem. Early aiding is therefore of utmost importance.
Deafness can be caused by environmental situations such as noise, trauma, or other ear defections. Dangerous Decibels, a group that is attempting to help reduce deafness, states that anything over 85 decibels, which is about the level of a busy city, will damage hearing.[3] Listening to loud noises for long periods of time make the eardrum less sensitive, which in turn makes it difficult to pick up quiet sounds more difficult to hear. Going through a trauma, like a car accident, can cause hearing loss due to the noise of the incident or an injury to the ear. If the middle ear or inner ear is damaged during the accident then hearing will degenerate because the sound wave cannot make it to the brain to be interpreted into sound. Other environmental factors that can cause deafness are nasal allergies. Nasal allergies cause mucus to build up in the throat and the nose, which will block the eustachian tubes and make it difficult for sound waves to make it into the inner ear.
Diseases such as Meniere’s, meningitis, and mumps can affect hearing. Meniere’s disease is a virus that makes it so the inner ear fluid is unable to drain.[4] This excess fluid increases pressure of the ear and makes it difficult for sound waves to travel to the brain. Because Meniere’s is a virus there is no cure at present. However, Meniere’s requires a trigger such as genetic predisposition or a traumatic event that changes the inner ear fluid.[4] Meningitis is the inflammation of protective membranes around the brain or the spine. This prevents the auditory nerve from transmitting signals to the brain. Meningitis is usually the result of an infection that went untreated and spread to the brain. Mumps is a virus that usually infects the parotid salivary gland.[5] A serious side effect of mumps is hearing loss due to the swelling of the parotid gland which blocks the sound waves from reaching the middle ear.[5] There are many diseases that affect hearing but Meniere’s and meningitis are the leaders with 3-5 million people suffering at present and more than 10,000 being diagnosed annually.[6] Mumps is not as common in the United States as it is in other areas of the world but is still relatively rare worldwide. On average there are only 14 deaths around the world annually.
Neurological disorders such as multiple sclerosis and strokes can have an effect on hearing as well. Multiple sclerosis, or MS, is an autoimmune disease where the immune system attacks the myelin sheath, a covering that protects the nerves. Once the myelin sheaths are destroyed there is no possible way at present to repair them. Without the myelin to protect the nerves, nerves become damaged, creating disorientation for the patient. This is a painful process and may end in the debilitation of the infected person until they are paralyzed and have one or more senses gone. One of those may be hearing. If the auditory nerve becomes damaged then the infected person will become completely deaf in one or both ears. There is no cure for MS.[7] A stroke occurs if there is a clot in the brain, and blood is unable to get to a section of the brain. Within minutes the oxygen-deprived cells begin to die, causing serious damage to the human body. Depending on what nerves are damaged, one of the side effects can be deafness.[8]
The most common hearing loss treatment is a type of surgical procedure that will remove a blockage in the outer ear. This is almost always effective and will treat conductive hearing loss however surgery is only possible if the cause of the hearing loss can be identified. If not, a hearing aid can be used to amplify sound to make it clearer and more defined. The newest treatment for ANSD and sensorineural hearing loss is a cochlear implant. The cochlear implant is surgically installed to connect to the auditory nerve. The implant has a membrane that vibrates when sound hits it and sends signals to the nerve which then transmits the stimulus to the brain. The cochlear implant has only one membrane compared to the human ear’s three membranes. This means that sound will not be as clear using the cochlear when juxtaposed to normal hearing, but sounds can still be identified and responses can be made.
Jack Gannon, a professor at Gallaudet University, said this about deaf culture. “Deaf culture is a set of learned behaviors and perceptions that shape the values and norms of deaf people based on their shared or common experiences.” Some doctors believe that being deaf makes a person more social. Dr. Bill Vicar, from ASL University, shared his experiences as a deaf person, “[deaf people] tend to congregate around the kitchen table rather than the living room sofa… our good-byes take nearly forever, and our hellos often consist of serious hugs. When two of us meet for the first time we tend to exchange detailed biographies.”[9] Deaf culture is not about contemplating what deaf people cannot do and how to fix their problems. That is called a "pathological view of the deaf". Instead deaf people celebrate what they can do. There is a strong sense of unity between deaf people as they share their experiences of suffering through a similar struggle. This celebration creates a unity between even deaf strangers. Dr. Bill Vicars expresses the power of this bond when stating, “if given the chance to become hearing most [deaf people] would choose to remain deaf.”[10] There is more to deaf culture than meets the eye and has to be “experienced” to full comprehend it.[10]
Abbe Charles Michel de l’Epee was the first person to open a deaf school. L’Epee taught French sign language to children, and started the spread of many deaf schools across Europe. Thomas Gallaudet was traveling to England to start a deaf school. His inspiration was a nine-year old girl who lived next door. Seeing her conquer her struggles made Gallaudet want to teach and see other children conquer their own disabilities. Gallaudet witnessed a demonstration of deaf teaching skills from Sicard, Massieu, and Clerc, the masters of teaching deaf children at the time. After the demonstration of Gallaudet studied under the French masters and perfected his own teaching skills. Once he was done learning Gallaudet and Clerc traveled to the United States and opened the first deaf school in Hartford Connecticut. American Sign Language, or ASL, started to evolve from primarily LSf, French sign language, and other outside influences.[11]
Sign language consists of different hand signals to mean different words. It also has signs for the alphabet which is primarily used to spell names. Sign language also uses: facial expressions, body language, hand shape, hand position, hand movement, and gestures, all of these things affect the meaning or sincerity of the signs that are used. Sign language is offered at most major schools and is recognized as its own language. Sign language makes deaf culture possible. Deaf people can communicate in their own way and do not have to rely on reading lips or technology to help them, if they choose not to.