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Nearly 10% of people in the US have some degree of hearing loss. About 1/800 to 1/1000 newborns are born with severe to profound hearing loss. Two to 3 times as many are born with lesser hearing loss. During childhood, another 2 to 3/1000 children acquire moderate to severe hearing loss. Adolescents are at risk from excessive exposure to noise, head trauma, or both. Older adults typically experience a progressive decrease in hearing (presbycusis—see Hearing Loss: Acquired Causes of Hearing Loss  ), which is probably related to aging and noise exposure. The prevalence of hearing impairment in people > 65 is 25 to 40%; in those > 75, 40 to 66%.
Hearing deficits in early childhood can result in lifelong impairments in receptive and expressive language skills. The severity of the handicap is determined by the age at which the hearing loss occurred; the nature of the loss—its duration, the frequencies affected, and the degree; and the susceptibilities of the individual child (eg, coexisting visual impairment, mental retardation, primary language deficits, inadequate linguistic environment). Children who have other sensory, linguistic, or cognitive deficiencies are affected most severely.
Hearing loss can be classified as conductive, sensorineural, or both (mixed loss). Conductive hearing loss occurs secondary to lesions in the external auditory canal, tympanic membrane, or middle ear. These lesions prevent sound from being effectively conducted to the inner ear. Sensorineural hearing loss is caused by lesions of either the inner ear (sensory) or the auditory (8th) nerve (neural—see Table 1: Hearing Loss: Differences Between Sensory and Neural Hearing Losses). This distinction is important, because sensory hearing loss is sometimes reversible and is seldom life threatening. A neural hearing loss is rarely recoverable and may be due to a potentially life-threatening brain tumor—commonly a cerebellopontine angle tumor. Mixed loss may be caused by severe head injury with or without fracture of the skull or temporal bone, by chronic infection, or by one of many genetic disorders. It may also occur when a transient conductive hearing loss, commonly from otitis media, is superimposed on a sensorineural hearing loss.
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Table 1
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Differences Between Sensory
and Neural Hearing Losses
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Test
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Sensory Hearing Loss
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Neural Hearing Loss
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Speech discrimination
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Moderate decrement
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Severe decrement
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Discrimination with increasing intensity
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Improves
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Deteriorates
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Recruitment
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Present
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Absent
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Acoustic reflex decay
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Absent or mild
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Present
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Waveforms in auditorybrain stem responses
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Well formed, with normal latencies
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Absent or with abnormally long latencies
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Otoacoustic emissions
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Absent
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Present
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Etiology
Hearing loss can be congenital (see Table 2: Hearing Loss: Congenital Causes of Hearing Loss) or acquired (see Table 3: Hearing Loss: Acquired Causes of Hearing Loss ), progressive or sudden (see also Hearing Loss: Treatment in Children), temporary or permanent, unilateral or bilateral, and mild or profound. Drug-induced ototoxicity is discussed elsewhere (see Inner Ear Disorders: Drug-Induced Ototoxicity).
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Table 2
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Congenital
Causes of Hearing Loss
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Type of Loss*
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Anatomic Area Affected
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Etiology†
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Conductive
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External and middle ear
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Genetic
Idiopathic (unknown) malformation
Drug-induced malformation ( thalidomide )
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Sensory
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Inner ear
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Genetic
Idiopathic (unknown) malformation
Congenital infection (eg, rubella, cytomegalovirus, toxoplasmosis, syphilis)
Rh incompatibility
Anoxia
Maternal ingestion of ototoxic drugs (eg, for TB or severe infection)
Drug-induced malformation ( thalidomide )
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Neural
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CNS
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Anoxia
Idiopathic (unknown) malformation
Genetic
Congenital infection (eg, rubella, cytomegalovirus, toxoplasmosis, syphilis)
Rh incompatibility
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*A number of the congenital hearing losses may be mixed losses—a combination of conductive and sensory and/or neural.
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†Listed in approximate order of greatest frequency first.
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Cerumen (earwax) accumulation is the most common cause of treatable hearing loss, especially in the elderly. Foreign bodies obstructing the canal are sometimes a problem in children both from their presence and from any damage inadvertently caused during their removal.
Infections, particularly otitis media and its sequelae, are common causes of conductive hearing loss, especially in children. Almost every child experiences mild to moderate transient hearing loss due to otitis media. However, repeated or severe infections can destroy the ossicles, particularly the long process of the incus, causing permanent hearing loss. Untreated otitis media may lead to development of a cholesteatoma, a benign tumor that can cause conductive hearing loss. Residual middle ear fluid (secretory otitis media) after infection commonly causes temporary hearing loss. Sensorineural hearing loss can result from various other infections, both congenital and acquired.
Noise can cause either sudden or gradual sensorineural hearing loss. In acoustic trauma, hearing loss results from exposure to a single, extreme noise (eg, a nearby gunshot or explosion). In noise-induced hearing loss, the loss develops over time from chronic exposure to noise > 85 decibels (dB—see Sidebar 1: Hearing Loss: Sound Levels ). Although people vary greatly in susceptibility to noise-induced hearing loss, nearly everyone loses some hearing if exposed to sufficiently intense noise for an adequate time. The loss is usually temporary, typically lasting several hours or a day after prolonged exposure to loud noise; some experience tinnitus as well. However, repeated exposure to loud noise ultimately results in loss of hair cells in the organ of Corti. Hearing loss typically occurs first at 4 kHz and gradually spreads to the lower and higher frequencies as exposure continues. In contrast to most other causes of sensorineural hearing losses, noise-induced hearing loss may be less severe at 8 kHz than at 4 kHz.
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Sidebar 1
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Autoimmune disease can cause sensorineural hearing loss at all ages and can cause other signs and symptoms as well.
Effects of aging: Presbycusis is sensorineural hearing loss that occurs with aging. It probably results from age-related changes and the chronic effects of noise exposure. Progressive deterioration and cell death of the sensory hair cells, stria vascularis, ganglion cells, and cochlear nuclei play a role. The hearing loss usually affects the highest frequencies (18 to 20 kHz) early on and gradually affects the lower frequencies; it usually becomes clinically significant when it affects the critical 2- to 4-kHz range around age 55 to 65 (sometimes sooner). The loss of high-frequency hearing significantly affects speech comprehension. Although the loudness of speech seems normal, certain consonant sounds (eg, C, D, K, P, S, T) become hard to hear. Consonant sounds are the most important sounds for speech recognition. For example, when “shoe,” “blue,” “true,” “too,” or “new” is spoken, many patients with presbycusis can hear the “oo” sound, but most have difficulty recognizing which word was spoken because they cannot distinguish the consonants. This inability to distinguish consonants causes affected people to often think the speaker is mumbling. A speaker attempting to speak more loudly usually accentuates vowel sounds (which are low frequency), doing little to improve speech recognition.
Evaluation
Evaluation consists of detecting and quantifying hearing loss and determining etiology (particularly reversible causes).
Screening:
Although most adults and older children notice a sudden hearing loss, progressive losses and all losses in infants and young children must be detected by screening. Screening should begin at birth (see Approach to the Care of Normal Infants and Children: Hearing) so that linguistic input can allow optimal language development. Suspected hearing loss at any time should prompt referral to a specialist. If screening is not performed, severe bilateral losses may not be recognized until age 2 yr, and mild to moderate or severe unilateral losses often are not recognized until the child reaches school age.
History:
Caregivers may suspect that a newborn has a severe hearing loss within the 1st week of life when the newborn does not respond to voices or other sounds. Any child with delays in speech or language development or difficulty in school should undergo evaluation for hearing loss. Mental retardation, aphasia, and autism also must be considered. Delayed motor development may signal vestibular deficit, which is often associated with a sensorineural hearing loss.
Older adults typically complain that other people are not speaking clearly rather than that their own hearing is decreased; often, family members prompt evaluation for hearing loss. Speech comprehension is particularly difficult when background noise is present. Screening in adults can be successfully carried out using the questionnaire from the Hearing Handicap Inventory for the Elderly—Screening Version. In this test, the patient is asked the following questions:
The patient responds to each question with “no” (0 points), “sometimes” (2 points), or “yes” (4 points). The points are then tallied, with higher scores suggesting a greater degree of hearing impairment. Scores > 10 suggest significant hearing impairment and necessitate follow-up.
Accompanying signs and symptoms, particularly neurologic ones (eg, dizziness, vertigo, nystagmus, headache, facial palsy), should trigger an immediate otologic evaluation, including a hearing test. History of CNS or ear infection, use of ototoxic drugs, exposure to loud noise, head trauma, sudden loss of hearing, ear pain (otalgia), a family history of hearing loss, or a combination may suggest a cause of hearing loss. Examples are a history of disorientation in the dark (loss of vestibular function), episodes of vertigo (the subjective feeling of rotation or movement in space), development of weakness or asymmetry of the face, and an abnormal sense of taste.
Physical
examination:
The physician evaluates the external ear for obstruction, infection, and congenital malformations and the tympanic membrane for perforation, otitis media, and cholesteatoma. In the neurologic examination, cranial nerve function, particularly balance, facial weakness, and taste functions, is important (see Approach to the Neurologic Patient: Cranial nerves).
Weber's test and the Rinne test use a tuning fork to differentiate conductive from sensorineural hearing loss. In Weber's test, the stem of a vibrating 512-Hz or 1024-Hz tuning fork is placed on the midline of the head, and the patient indicates in which ear the tone is louder. In unilateral conductive hearing loss, the tone is louder in the ear with hearing loss. In unilateral sensorineural hearing loss, the tone is louder in the normal ear, because the tuning fork stimulates both inner ears equally and the patient perceives the stimulus with the unaffected ear. In the Rinne test, hearing by bone and by air conduction is compared. Bone conduction bypasses the external and middle ear and tests the integrity of the inner ear, 8th cranial nerve, and central auditory pathways. The stem of a vibrating tuning fork is held against the mastoid (for bone conduction); as soon as the sound is no longer perceived, the fork is removed from the mastoid, and the still-vibrating tines are held close to the pinna (for air conduction). Normally, the fork can once more be heard, indicating that air conduction is better than bone conduction. With conductive hearing loss, the relationship is reversed; bone conduction is louder than air conduction. With sensorineural hearing loss, both air and bone conduction are reduced, but air conduction remains louder.
Audiologic tests:
Typical audiologic tests include measurement of pure-tone thresholds with air and bone conduction, speech reception threshold, speech discrimination, tympanometry, acoustic reflex testing, and, rarely, reflex decay testing. Information gained from these tests helps determine whether more definitive differentiation of sensory from neural hearing loss is needed.
Pure-tone audiometry quantifies hearing loss. An audiometer delivers sounds of specific frequencies (pure tones) at different intensities to determine the patient's hearing threshold (how loud a sound must be to be perceived) for each frequency. Hearing in each ear is tested from 125 or 250 to 8000 Hz by air conduction (using earphones) and up to 4 kHz by bone conduction (using an oscillator in contact with the mastoid process or forehead). Test results are plotted on graphs called audiograms (see Fig. 1: Hearing Loss: Audiogram of right ear in a patient with normal hearing. ), which show the difference between the patient's hearing threshold and normal hearing at each frequency. The difference is measured in dB (see Sidebar 1: Hearing Loss: Sound Levels). The normal threshold is considered 0 dB hearing level (Hl); hearing loss is considered present if the patient's threshold is > 25 dB Hl. When hearing loss is such as to require loud test tones, intense tones presented to one ear may be heard in the other ear. In such cases, a masking sound, usually narrow band noise, is presented to the non–test ear to isolate it.
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Fig. 1
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Audiogram of right ear in a patient with normal hearing.
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Normal audiogram of the right ear. The vertical lines represent the frequencies that are tested from 125 to 8000 Hz. The horizontal lines record the threshold at which the patient states that the sound is heard. Normal thresholds are 0 dB +/− 10 dB. Patients with a hearing threshold ≤ 20 dB are considered to have average or better-than-average hearing. The greater the dB, the louder is the sound and the worse the hearing. O is the standard symbol for air conduction of the right ear; X is the standard symbol for air conduction for the left ear. The < is the standard symbol for unmasked bone condition for the right ear; > is the standard symbol for unmasked bone conduction of the left ear.
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Speech audiometry includes the speech reception threshold (SRT) and the word recognition score. The SRT is a measure of the intensity at which speech is recognized. To determine the SRT, the examiner presents the patient with a list of words at specific sound intensities. These words usually have two equally accented syllables (spondees), such as railroad, staircase, and baseball. The examiner notes the intensity at which the patient repeats 50% of the words correctly. The SRT approximates the average hearing level at speech frequencies (eg, 500 Hz, 1000 Hz, 2000 Hz).
The word recognition score tests the ability to discriminate among the various speech sounds or phonemes. It is determined by presenting 50 phonetically balanced one-syllable words at an intensity of 35 to 40 dB above the patient's SRT. The word list contains phonemes in the same relative frequency found in conversational English. The score is the percentage of words correctly repeated by the patient and reflects the ability to understand speech under optimal listening conditions. A normal score ranges from 90 to 100%. The word recognition score is normal with conductive hearing loss, albeit at a higher intensity level, but can be reduced at all intensity levels with sensorineural hearing loss. Discrimination is even poorer in neural than in sensory hearing loss.
Tympanometry measures the impedance of the middle ear to acoustic energy and does not require patient participation. It is commonly used to screen children for middle ear effusions. A probe containing a sound source, microphone, and air pressure regulator is placed snugly with an airtight seal into the ear canal. The probe microphone records the reflected sound from the tympanic membrane while pressure in the canal is varied. Normally, maximal compliance of the middle ear occurs when the pressure in the ear canal equals atmospheric pressure. Abnormal compliance patterns suggest specific anatomic disruptions. In eustachian tube obstruction and middle ear effusion, maximal compliance occurs with a negative pressure in the ear canal. When the ossicular chain is disrupted, as in necrosis or dislocation of the long process of the incus, the middle ear is excessively compliant. When the ossicular chain is fixed, as in stapedial ankylosis in otosclerosis, compliance may be normal or reduced.
The acoustic reflex is contraction of the stapedius muscle in response to loud sounds, which changes the compliance of the tympanic membrane, protecting the middle ear from acoustic trauma. The reflex is tested by presenting a tone and measuring what intensity provokes a change in middle ear impedance as noted by movement of the tympanic membrane. An absent reflex could indicate middle ear disease or a tumor of the auditory nerve.
Advanced testing:
Gadolinium-enhanced MRI of the head to detect lesions of the cerebellopontine angle may be needed in patients with poor word recognition, asymmetric sensorineural hearing loss, abnormal neurologic examination, or a combination in whom the etiology is not clear.
The auditory brain stem response uses surface electrodes to monitor brain wave response to acoustic stimulation in people who cannot otherwise respond.
Electrocochleography measures the activity of the cochlea and the auditory nerve with an electrode placed on or through the eardrum. It can be used to assess and monitor patients with dizziness, can be used in patients who are awake, and is useful in intraoperative monitoring. Otoacoustic emissions testing measures sounds produced by outer hair cells of the cochlea in response to a sound stimulus usually placed in the ear canal. It is used to screen newborns and infants for hearing loss and to monitor the hearing of patients who are using ototoxic drugs (eg, gentamicin , cisplatin ).
Certain patients, such as children with a reading or other learning problem and elderly people who appear to hear but do not comprehend, should undergo a central auditory evaluation. It measures discrimination of degraded or distorted speech, discrimination in the presence of a competing message in the opposite ear, the ability to fuse incomplete or partial messages delivered to each ear into a meaningful message, and the capacity to localize sound in space when acoustic stimuli are delivered simultaneously to both ears.
Treatment
The underlying causes of a hearing loss should be determined and treated. Ototoxic drugs should be discontinued or the dose lowered unless the severity of the disease being treated (usually cancer or a severe infection) requires that the risk of additional ototoxic hearing loss be accepted. Blood levels of some ototoxic antibiotics (eg, gentamicin —see Bacteria and Antibacterial Drugs: Administration) can be measured.
Fluid from middle ear effusion can be drained by myringotomy and prevented with the insertion of a tympanostomy tube. Benign growths (eg, enlarged adenoids, nasal polyps) and malignant tumors (eg, nasopharyngeal cancers, sinus cancers) blocking the eustachian tube or ear canal can be removed. Hearing loss caused by autoimmune disorders may respond to corticosteroids.
Damage to the tympanic membrane or ossicles or otosclerosis may require reconstructive surgery. Brain tumors causing hearing loss may in some cases be removed and hearing preserved.
Many causes of hearing loss have no cure, and treatment involves compensating for the hearing loss. Most patients with moderate to severe loss benefit from hearing aids. Those with severe to profound loss usually benefit from a cochlear implant.
Hearing aids:
Amplification of sound with a hearing aid helps many people. Although hearing aids do not restore hearing to normal, they can significantly improve communication. Physicians should encourage hearing aid use and help patients overcome a sense of social stigma that continues to obstruct use of these devices, perhaps by making the analogy that a hearing aid is to hearing as eye glasses are to seeing.
All hearing aids have a microphone, amplifier, speaker, earpiece, and volume control, although they differ in the location of these components. An audiologist should be involved in selection and fitting of a hearing aid.
The best models are adjusted to a person's particular pattern of hearing loss. People with mainly high-frequency hearing loss do not benefit from simple amplification, which merely makes the garbled speech they hear sound louder. They usually need a hearing aid that selectively amplifies the high frequencies. Some hearing aids contain vents in the ear mold, which facilitate the passage of high-frequency sound waves. Some use digital sound processing with multiple frequency channels so that amplification more precisely matches hearing loss as measured on the audiogram.
Telephone use can be difficult for people with hearing aids. Typical hearing aids cause squealing when the ear is placed next to the phone handle. Some hearing aids have a phone coil with a switch that turns the microphone off and links the phone coil electromagnetically to the speaker magnet in the phone.
For moderate to severe hearing loss, a postauricular (ear-level) aid, which fits behind the pinna and is coupled to the ear mold with flexible tubing, is appropriate. An in-the-ear aid is contained entirely within the ear mold and fits less conspicuously into the concha and ear canal; it is appropriate for mild to moderate hearing loss. Some people with mild hearing loss limited to high frequencies are most comfortably fitted with post-auricular aids and completely open ear canals. Canal aids are contained entirely within the ear canal and are cosmetically acceptable to many people who would otherwise refuse to use a hearing aid, but they are difficult for some people (especially the elderly) to manipulate. The CROS aid (Contralateral Routing Of Signals) is occasionally used for severe unilateral hearing loss; a hearing-aid microphone is placed in the nonfunctioning ear, and sound is routed to the functioning ear through a wire or radio transmitter. This device enables the wearer to hear sounds from the nonfunctioning side, allowing for some limited capacity to localize sound. If the better ear also has some hearing loss, the sound from both sides can be amplified with the binaural CROS (BiCROS) aid. The body aid type is appropriate for profound hearing loss. It is worn in a shirt pocket or a body harness and connected by a wire to the earpiece (the receiver), which is coupled to the ear canal by a plastic insert (ear mold).
A bone conduction aid may be used when an ear mold or tube cannot be used, as in atresia of the ear canal or persistent otorrhea. An oscillator is held against the head, usually over the mastoid, with a spring band, and sound is conducted through the skull to the cochlea. Bone conduction hearing aids require more power, introduce more distortion, and are less comfortable to wear than air conduction hearing aids. Some bone conduction aids (bone-anchored hearing aids or BAHAs) are surgically implanted in the mastoid process, avoiding the discomfort and prominence of the spring band.
Cochlear
implants:
Profoundly deaf patients, including those with some hearing but who even with a hearing aid cannot understand speech without the assistance of vision (lip-reading or speech-reading), may benefit from a cochlear implant. This device provides electrical signals directly into the auditory nerve via multiple electrodes implanted in the cochlea. An external microphone and processor convert sound waves to electrical impulses, which are transmitted through the skin electromagnetically from an external induction coil to an internal coil implanted in the skull above and behind the ear. The internal coil connects to electrodes inserted in the scala tympani.
Cochlear implants help with speech-reading by providing information about the intonation of words and the rhythm of speech. Many if not most adults with cochlear implants can discriminate words without visual clues, allowing them to talk on the telephone. Cochlear implants enable deaf people to hear and distinguish environmental sounds and warning signals. They also help deaf people modulate their voice and make their speech more intelligible.
Brainstem
implants:
Patients who have had both acoustic nerves destroyed (eg, from bilateral temporal bone fractures, neurofibromatosis) can have some hearing restored by means of brainstem implants that have electrodes connected to sound-detecting and sound-processing devices similar to those used for cochlear implants.
Coping mechanisms:
Alerting systems that use light let people know when the doorbell is ringing, a smoke detector is sounding, or a baby is crying. Special sound systems transmitting infrared or FM radio signals help people hear in theaters, churches, or other places where competing noise exists. Many television programs carry closed captioning. Telephone communication devices are also available.
Lip-reading or speech-reading is particularly important for people who can hear but have trouble discriminating sounds. Most people get useful speech information from lip-reading even without formal training. Even people with normal hearing can better understand speech in a noisy place if they can see the speaker. To use this information the listener must be able to see the speaker's mouth. Health care personnel should be sensitive to this issue and always position themselves appropriately when speaking to the hearing-impaired. Observing the position of a speaker's lips allows recognition of the consonant being spoken, thereby improving speech comprehension in patients with high-frequency hearing loss. Lip-reading may be learned in aural rehabilitation sessions in which a group of age-matched peers meets regularly for instruction and supervised practice in optimizing communication.
Patients can gain control over their listening environment by modifying or avoiding difficult situations. For example, people can visit a restaurant during off-peak hours, when it is quieter. They can ask for a booth, which blocks out some extraneous sounds. In direct conversations, people may ask the speaker to face them. At the beginning of a telephone conversation, they can identify themselves as being hearing-impaired. At a conference, the speaker can be asked to use an assistive listening system, which makes use of either inductive loop, infrared, or FM technology that sends sound through the microphone to a patient's hearing aid.
People with profound hearing loss often communicate by using sign language. American Sign Language (ASL) is the most common version in the US. Other forms include Signed English, Signing Exact English, and Cued Speech.
Treatment
in Children
In addition to treatment of any underlying cause and the provision of hearing aids, children with hearing loss require support of language development with appropriate therapy. Because children must hear language to learn it spontaneously, most deaf children develop language only with special training, ideally beginning as soon as the hearing loss is identified (an exception would be a deaf child growing up with deaf parents who are fluent sign language users). Deaf infants must be provided with a form of language input. For example, a visually based sign language can provide a foundation for later development of oral language.
Children ≥ 6 mo with profound bilateral hearing loss who cannot benefit from hearing aids usually are candidates for a cochlear implant. Although cochlear implants allow auditory communication in many children with either congenital or acquired deafness, they appear to be more effective in those who already have developed language. Children who have postmeningitic deafness develop an ossified inner ear; they should receive cochlear implants early to maximize effectiveness. Children whose acoustic nerves have been destroyed by tumors may be helped by implantation of brain stem auditory-stimulating electrodes. Children with cochlear implants may have a slightly greater risk of meningitis than either children without cochlear implants or adults with cochlear implants.
Children with unilateral deafness should be allowed to use a special system in the classroom, such as an FM auditory trainer. With these systems, the teacher speaks into a microphone that sends signals to a hearing aid in the child's nonaffected ear, improving the child's greatly impaired ability to hear speech against a noisy background.
Prevention
Prevention of hearing loss consists mainly of limiting duration and intensity of noise exposure. People required to expose themselves to loud noise must wear ear protectors (eg, plastic plugs in the ear canals or glycerin-filled muffs over the ears). The Occupational Safety and Health Administration (OSHA) of the US Department of Labor and similar agencies in many other countries have standards regarding the length of time that a person can be exposed to a noise. The louder the noise, the lesser the permissible time of exposure.
Last full review/revision January 2007 by Robert J. Ruben, MD
Content last modified January 2007
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