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A hearing impairment is a decrease in the ability to perceive sound. This article discusses the biologic basis of hearing impairment in humans and other animals. Explanation Hearing impairment can affect any creature that hears. Sound varies in amplitude (loudness) and frequency (pitch). Most animals are able to sense sound energy, but no individual animal species can detect all frequencies of sound at all amplitudes (levels of loudness). In fact, although there is much overlap, each species has a different set of audible sounds. Within that range of audible sound, each species is attuned to an even narrower range of pitches that it hears best — usually the sorts of sounds that are important for survival. In social animals, like humans, who use sound as a primary means of communication (spoken language), hearing is most sensitive for the pitches that are produced in speech. The medical term hearing impairment has become controversial due to very strong objections from the deaf community who refute the implication of a disability. Those who consider themselves part of Deaf Culture feel offended by the term "impaired". However, not all people, especially those who become deaf in later years, may make this distinction. (See Models of Deafness) Hearing deficiencies: loudness, pitch, and discrimination Hearing sensitivity is generally 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 behavioral tests. That measurement is called a behavioral audiogram. A record is made of the quietest level of sound that consistently prompts a response from the listener. The test is carried out for sounds of different frequencies. Behavioral audiograms require a certain amount of co-operation and training on the part of the individual being tested that other types of hearing tests do not require. Instead of observing behavior these tests rely on technical measurements of activity in the auditory nervous system. These electro-physiological tests of hearing include Auditory Evoked Potentials and Oto-acoustic emissions. Normal hearing thresholds are not the same for all frequencies in any species of animal. If different pitches of sound are played at the same amplitude (loudness), some will be loud and clear, and others muffled or even completely inaudible. Generally, if the gain is increased, a pitch is more likely to be perceived. Ordinarily, when animals use sound to communicate, hearing in that type of animal is most sensitive for the pitches produced by calls, or, in the case of humans, speech. This tuning of hearing exist at many levels of the body, 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 reach threshold. 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. References: eBook: Current Diagnosis & Treatment in Otolaryngology: Head & Neck Surgery, Lalwani, Anil K. (Ed.) Chapter 44: Audiologic Testing by Robert W. Sweetow, PhD, Jennifer McKee Bold, AuD, Access Medicine Types of hearing impairment Hearing impairment comes from different biologic causes. Most commonly, the ear is the affected part of the body. Conductive Conductive hearing loss occurs when sound is not normally conducted through the outer or middle ear (or both). Since sound can be picked up by a normally sensitive inner ear even if the ear canal, ear drum, and ear ossicles are not working, conductive hearing loss is often only mild and is never worse than a moderate impairment. Hearing thresholds will not rise above 55-60 dB from outer or middle ear problems alone. Generally, with pure conductive hearing loss, the quality of hearing (speech discrimination) is good, as long as the sound is amplified loud enough to be easily heard. Sensory Hearing loss due to insensitivity of the inner ear, the cochlea, can also be only mild or moderate but can also be much more severe, causing insensitivity to even the loudest sounds (total deafness). Most cases of human deafness (severe to profound hearing impairment) are due to insensitivity of the cochlea at the level of the hair-cell, which is the sound receptor cell that actually transduces sound vibration into the nerve impulses that stimulate the auditory portion of the VIIIth Cranial Nerve. Hearing loss caused either by problems in the cochlea ,or by the auditory portion of the central nervous system, is categorized medically as sensory or sensorineural hearing loss. The great majority of human sensory hearing loss is caused by abnormalities in the cochlea. There are also very unusual sensorineural hearing impairments that involve the VIIIth cranial nerve or the auditory portions of the brain. In the rarest of these sorts of hearing loss, only the auditory centers of the brain are affected. In this situation, central hearing loss, sounds may be heard at normal thresholds, but the quality of the sound perceived is so poor that speech can not be understood. Age of onset of hearing impairment 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. Causes There are many causes of hearing loss. Conductive hearing loss Sensory hearing loss Most sensory hearing loss is due to poor hair cell function. The hair cells may be abnormal at birth, or damaged during the lifetime of an individual. There are both external causes of damage, like noise trauma and infection, and instrinsic abnormalities, like deafness genes. Sensory hearing loss (also called sensorineural hearing loss) may also result from abnormalities of the VIII cranial nerve. Sensory hearing loss that results from abnormalities of the central auditory system in the brain is called Central Hearing Impairment. Since the auditory pathways cross back and forth on both sides of the brain, deafness from a central cause is unusual. 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 dbA range. If lifestyles include significant outdoor or open window conditons, 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 (EPA, 1974). Genetic Hearing loss can be inherited. Both dominant 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 Disease or illness Medications See also Ototoxicity 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 Vicodin abuse is known to cause hearing impairment. There has been speculation 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. Physical trauma Categories of hearing impairment Hearing loss is categorized by its severity and by the age of onset. Two persons with the same severity of hearing loss will experience it quite differently if it occurs early or late in life. Furthermore, a loss can occur on only one side (unilateral) or on both (bilateral). Types There are three major types of hearing loss: neural/sensorineural, conductive, or a combination of both. Treatment depends upon the type of hearing loss that is present. Sensorineural Sensorineural hearing loss is caused by damage/malfunction of the inner ear (cochlea, eighth cranial nerve) or auditory brainstem. This can be divided further into a sensory hearing loss (inner ear) or a neural hearing loss (brainstem). Conductive Conductive hearing loss is caused by damage/malfunction of the middle or outer ear system (external ear canal, ear drum, or structures in the middle ear space including the malleus, incus and stapes bones). Combination Mixed hearing loss is caused by both conductive and sensorineural causes. Quantification of hearing loss The severity of hearing loss is measured by the degree of loudness, as measured in decibels, a sound must attain before being detected by an individual. Hearing loss may be ranked as mild, moderate, severe or profound. It is quite common for someone to have more than one degree of hearing loss (i.e. mild sloping to severe). The following list shows the rankings and their corresponding decibel ranges: The quietest sound you can hear at different frequencies is plotted on an audiogram to reflect your ability to hear at different frequencies. The range of normal human hearing (from the softest audible sound to the loudest comfortable sound) is so great, that the audiogram must be plotted using a logarithmic scale. This, and the different amount of hearing loss at different frequencies, make it virtually impossible to accurately describe the amount of hearing loss in simple terms such as percentages or the rankings, above. Pre- or postlingual The age at which the hearing impairment develops is crucial to spoken language acquisition. Post-lingual hearing impairments are far more common than pre-lingual impairments. Pre-lingual deafness Main article: Prelingual deafness Prelingual hearing impairment exists when the impairment is congenital or otherwise acquired before the individual has acquired speech and language, thus rendering the disadvantages more difficult to treat because the child is unable to access audible /spoken communication from the outset. It is important to note that those children born into signing families have no delay in language development and communication. Most pre-lingual hearing impairment is due to an acquired condition, usually either disease or trauma; therefore, families commonly have no prior knowledge of deafness. Post-lingual hearing impairment Main article: Post-lingual hearing impairment Post-lingual hearing impairment 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 the people so affected long before the patients themselves will acknowledge the disability. Common treatments includes hearing aids and learning lip reading. Loneliness and depression can arise as a result of isolation (from the inability to communicate with friends and loved ones) and difficulty in accepting their disability. Hard-of-hearing People who are hard of hearing have varying amounts of hearing loss but usually not enough to be considered deaf. Many people who are deaf consider spoken language their primary language and consider themselves "hard of hearing". How one classifies themselves relative to hearing loss or deafness is a very personal decision and reflects much more than just their ability to hear. The phrase hard of hearing, normally used as an adjective or adverb, can also be used as a noun, referring to people with hearing impairment as the hard of hearing. People who consider themselves culturally deaf, prefer the term "hard of hearing" or "deaf", and perceive "hearing impaired" as an insult. Hearing impaired persons with partial loss of hearing may find that the quality of their hearing varies from day to day, or from one situation to another or not at all. They may also, to a greater or lesser extent, depend on both hearing-aids and lip-reading. They may perhaps not always be aware of it, but they do admit to it being important to see the speaker's face in conversation. Many people with hearing loss have better hearing in the lower frequency ranges (low tones), and cannot hear as well or at all in the higher frequencies. Some people may merely find it difficult to differentiate between words that begin with consonantal sounds such as the fricatives or sibilants, z, or th, or the plosives d, t, b, or p. They may be unable to hear thin, high-pitched or metallic noises, such as birds chirping or singing, clocks ticking, etc. Often, they are able to hear and understand men's voices better than women's. Others will find their condition so much worse if circumstances in their immediate environment affect the way they are able to use their hearing aids, or prevent them from employing their speech reading skills. A room with a high ceiling and a lot of reverberation will affect the sound of a speaker's voice adversely. The position of the listener, too, sitting at a right angle to the speaker at a long seminar table, thus being able to hear only with one, maybe the ineffectual ear, can make a difference. Difficulties can also arise for the listener trying to lip-read, if the speaker is sitting with his back against the light-source and is in this way obscuring his face. A rule of thumb is that bright lighting is to the hearing-impaired what noise is to the hearing; a source of distraction. The speaker's accent; the topic under discussion, possibly with many unfamiliar words; the softness of his voice; possibly his having a speech impediment; a habit of holding a hand in front of his mouth or turning his face away at times: all these tendencies cause problems to the hard-of-hearing, especially when they have to rely on lip-reading. The rustling of papers, and notebook pages being turned are precisely the noises that will be the first thing hearing-aids pick up. Noisy situations are especially difficult, because hearing loss not only affects the ability to hear sounds, but also to localize and filter out background noise. Unilateral hearing loss People with unilateral hearing loss (single sided deafness/SSD) can hear normally in one ear, but have trouble hearing out of the other ear. Problems with this type of deficit is inability to localize sounds (ie. unable to tell where traffic is coming from) and inability to process out background noise in a noisy environment, such as in a restaurant. Pre-lingual impairment In children, hearing loss can lead to social isolation for several reasons. First, the child experiences delayed social development that is in large part tied to delayed language acquisition. It is also directly tied to their inability to pick up auditory social cues. This can result in a deaf person becoming generally irritable. A child who uses sign language, or identifies with the deaf sub-culture does not generally experience this isolation, particularly if he/she attends a school for the deaf, but may conversely experience isolation from his parents if they do not know sign language. A child who is exclusively or predominantly oral (using speech for communication) can experience social isolation from his or her hearing peers, particularly if no one takes the time to explicitly teach her social skills that other children acquire independently by virtue of having normal hearing. Finally, a child who has a severe impairment and uses some sign language may be rejected by his or her deaf peers, because of an understandable hesitation in abandoning the use of existent verbal and speech-reading skills. Some in the deaf community can view this as a rejection of their own culture and its mores, and therefore will reject the individual preemptively. Post-lingual impairment Those who lose their hearing later in life, such as in late adolescence or adulthood, face their own challenges. For example, they must adjust to living with the adaptations that make it possible for them to live independently. They may have to adapt to using hearing aids or a cochlear implant, develop speech-reading skills, and/or learn sign language. The affected person may need to use a TTY (teletype), interpreter, or relay service to communicate over the telephone. Loneliness and depression can arise as a result of isolation (from the inability to communicate with friends and loved ones) and difficulty in accepting their disability. The challenge is made greater by the need for those around them to adapt to the person's hearing loss. Many relationships have suffered because of the anger that occurs when there is general miscommunication between family members. Generally, it's not only the person with a hearing disability that feels isolated, but others around them who feel they are not being "heard" or paid attention to, especially when the hearing loss has been gradual. Many people opt not to choose hearing aids for fear of looking old, since hearing loss is usually associated with old age and that in turn equals ineffectiveness in our society. Family members then feel as if their hearing loss partner doesn't care about them enough to make changes to reduce their disability and make it easier to communicate. Approaches In addition to hearing aids there exist cochlear implants of increasing complexity and effectiveness. These are useful in treating the mild to profound hearing impairment when the onset follows the acquisitions of language and in some cases in children whose hearing loss came before language was acquired. Recent research shows variations in effacacy but some promising studies* show that if implanted at a very young age, some profoundly impaired children can acquire effective hearing and speech. Views of treatments There is controversy in the culturally deaf community as to whether cochlear implants address wellness concerns, the overall health and psycho-emotional well-being of prelingually deaf children at all. Gene therapy In 2005, there was success of the regrowth of cochlea cells in test subjects by a research team led by Dr. Yehoash Raphael from the University of Michigan. This study was conducted using Guinea Pigs as test subjects. * 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 team led by Dr. Stefan Heller from Stanford University are pioneering stem cell research in the prospect of regrowth in cochlea cells.* Adaptations to hearing impairment Many hearing impaired individuals use certain assistive devices in their daily lives. Individuals can communicate by telephone using telecommunications devices for the deaf (TDD) This device looks like a typewriter or word processor and transmits typed text over the telephone. Other names in common use are textphone and minicom. In 2004, mobile textphone devices came onto the market for the first time allowing simultaneous two way text communication. In the U.S., the UK, the Netherlands and many other western countries there are telephone relay services so that a hearing impaired person can communicate 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. Other assistive devices include those that use flashing lights to signal events such as a ringing telephone, a doorbell, or a fire alarm. Video conferencing is also a new technology that permits signed conversations as well as permitting an ASL-English interpreter to voice and sign conversations between a hearing impaired and hearing person, negating the need to use a TTY or computer keyboard. Resources Bibliography U.S. Environmental Protection Agency press release, April 2, 1974 See also Quotations | |||||||
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