Hearing disorder

Hearing impairment or hard of hearing 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. When applied to humans, the term impaired is rejected within the Deaf culture movement, where the terms Deaf and hard of hearing are preferred.

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.

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:

1. for adults: between 27 and 40 dB
2. 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 a Speech-in-Noise test. As the name implies, a Speech-in-Noise test will give you an indication of how well you 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

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 congenitalcondition 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 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.

Causes

• Age
• Genetic factors
• Long term exposure to environmental noise
• Diseases or illness
• Ototoxic materials
• Physical trauma

Management
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 treatmentsThere 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 therapyA 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 impairmentMany 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 orword 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 conferencingtechnologies 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/SMStext 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.