Hearing loss
Hearing loss is a partial or total inability to hear.[5] Hearing loss may be present at birth or acquired at any time afterwards.[6][7] Hearing loss may occur in one or both ears.[2] In children, hearing problems can affect the ability to acquire spoken language, and in adults it can create difficulties with social interaction and at work.[8] Hearing loss can be temporary or permanent. Hearing loss related to age usually affects both ears and is due to cochlear hair cell loss.[9] In some people, particularly older people, hearing loss can result in loneliness.[2]
This article is about the medical aspects of hearing loss. For the cultural aspects, see Deaf culture.Hearing loss
Deaf or hard of hearing; anakusis or anacusis is total deafness[1]
Decreased ability to hear
Conductive, sensorineural, and mixed hearing loss, central auditory dysfunction[3]
Genetics, aging, exposure to noise, some infections, birth complications, trauma to the ear, certain medications or toxins[2]
Immunization, proper care around pregnancy, avoiding loud noise, avoiding certain medications[2]
1.33 billion / 18.5% (2015)[4]
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Hearing loss may be caused by a number of factors, including: genetics, ageing, exposure to noise, some infections, birth complications, trauma to the ear, and certain medications or toxins.[2] A common condition that results in hearing loss is chronic ear infections.[2] Certain infections during pregnancy, such as cytomegalovirus, syphilis and rubella, may also cause hearing loss in the child.[2][10] Hearing loss is diagnosed when hearing testing finds that a person is unable to hear 25 decibels in at least one ear.[2] Testing for poor hearing is recommended for all newborns.[8] Hearing loss can be categorized as mild (25 to 40 dB), moderate (41 to 55 dB), moderate-severe (56 to 70 dB), severe (71 to 90 dB), or profound (greater than 90 dB).[2] There are three main types of hearing loss: conductive hearing loss, sensorineural hearing loss, and mixed hearing loss.[3]
About half of hearing loss globally is preventable through public health measures.[2] Such practices include immunization, proper care around pregnancy, avoiding loud noise, and avoiding certain medications.[2] The World Health Organization recommends that young people limit exposure to loud sounds and the use of personal audio players to an hour a day in an effort to limit exposure to noise.[11] Early identification and support are particularly important in children.[2] For many, hearing aids, sign language, cochlear implants and subtitles are useful.[2] Lip reading is another useful skill some develop.[2] Access to hearing aids, however, is limited in many areas of the world.[2]
As of 2013 hearing loss affects about 1.1 billion people to some degree.[12] It causes disability in about 466 million people (5% of the global population), and moderate to severe disability in 124 million people.[2][13][14] Of those with moderate to severe disability 108 million live in low and middle income countries.[13] Of those with hearing loss, it began during childhood for 65 million.[15] Those who use sign language and are members of Deaf culture may see themselves as having a difference rather than a disability.[16] Many members of Deaf culture oppose attempts to cure deafness[17][18][19] and some within this community view cochlear implants with concern as they have the potential to eliminate their culture.[20]
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Research[edit]
Stem cell transplant and gene therapy[edit]
A 2005 study achieved successful regrowth of cochlea cells in guinea pigs.[137] However, 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. Some hope that a similar treatment will one day ameliorate hearing loss in humans.[138]
Recent research, reported in 2012 achieved growth of cochlear nerve cells resulting in hearing improvements in gerbils,[139] using stem cells. Also reported in 2013 was regrowth of hair cells in deaf adult mice using a drug intervention resulting in hearing improvement.[140] The Hearing Health Foundation in the US has embarked on a project called the Hearing Restoration Project.[141] Also Action on Hearing Loss in the UK is also aiming to restore hearing.[142]
Researchers reported in 2015 that genetically deaf mice which were treated with TMC1 gene therapy recovered some of their hearing.[143][144] In 2017, additional studies were performed to treat Usher syndrome[145] and here, a recombinant adeno-associated virus seemed to outperform the older vectors.[146][147]
Audition[edit]
Besides research studies seeking to improve hearing, such as the ones listed above, research studies on the deaf have also been carried out in order to understand more about audition. Pijil and Shwarz (2005) conducted their study on the deaf who lost their hearing later in life and, hence, used cochlear implants to hear. They discovered further evidence for rate coding of pitch, a system that codes for information for frequencies by the rate that neurons fire in the auditory system, especially for lower frequencies as they are coded by the frequencies that neurons fire from the basilar membrane in a synchronous manner. Their results showed that the subjects could identify different pitches that were proportional to the frequency stimulated by a single electrode. The lower frequencies were detected when the basilar membrane was stimulated, providing even further evidence for rate coding.[148]
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