Hearing Loss and Deafness
Hearing loss is deterioration in hearing; deafness is profound hearing loss.
More than 28 million people in the United States are deaf or have hearing loss. Older people are the most affected: 30 to 40% of people aged 65 and older have significant hearing loss. Children also develop hearing loss (see Section 23, Chapter 276), which can be detrimental to language and social development. Every year, about 1 of 5,000 people develops sudden deafness. Sudden deafness is severe hearing loss, usually in only one ear, that develops over a period of a few hours or less.
Causes
Hearing loss has many causes. It may be caused by a mechanical problem in the external ear canal or middle ear that blocks the conduction of sound (conductive hearing loss). Blockage of the external ear canal can be due to something as mundane as an accumulation of wax or something as uncommon as a tumor. The most common cause of conductive hearing loss in the middle ear, especially in children, is an accumulation of fluid. Fluid can accumulate in the middle ear as a result of ear infections or conditions, such as allergies or tumors, that block the eustachian tube, which drains the middle ear.
Hearing loss also may be due to damage to the sensory structures (hair cells) of the inner ear, auditory nerve, or auditory nerve pathways in the brain (sensorineural hearing loss). These sensory structures may be damaged by drugs, infections, tumors, and skull injuries. Hearing loss is often a mixture of a conductive and sensorineural loss.
 See the sidebar Causes of Hearing Loss.
Age: Age-related hearing loss is called presbycusis. As some people age, structures of the ear become less elastic and undergo other changes that make them less able to respond to sound waves, contributing to hearing loss. In many people, exposure to noise over many years worsens the changes caused by aging. Age-related hearing loss begins early, starting some time after age 20. However, it progresses very slowly, and most people do not notice any changes until well after age 50.
Age-related hearing loss first affects the highest pitches (frequencies) and only later affects lower pitches. Loss of the ability to hear high-pitched sounds often makes it more difficult to understand speech. Although the loudness of speech appears normal to the person, certain consonant sounds--such as the sound of letters C, D, K, P, S, and T--become hard to hear, so that many people with hearing loss think the speaker is mumbling. Indeed, some people complain more that others are not speaking clearly than that they cannot hear well. Women and children, whose voices tend to be higher in pitch than those of men, are particularly difficult to understand. Many people also notice a change in the vibrancy of certain musical sounds, such as those of violins and flutes.
Otosclerosis: In otosclerosis, a hereditary disorder, the bone surrounding the middle and inner ear grows excessively. This exuberant growth immobilizes the stirrup (the ear bone attached to the inner ear) so that it cannot transmit sounds properly. Sometimes the bone's growth also pinches and damages the nerves connecting the inner ear with the brain. Otosclerosis tends to run in families and may develop in someone who had a childhood measles infection. Hearing loss first becomes evident in late adolescence or early adulthood. About 10% of adults have some evidence of otosclerosis, but only about 1% develop hearing loss as a result.
Noise: About 30 million people in the United States are exposed to levels of noise that can cause hearing loss. Noise destroys the hair cells in the inner ear. Although people vary greatly in their sensitivity to loud noise, everyone loses some hearing if exposed to sufficiently loud noise long enough.
Both the loudness and duration of exposure are important--the louder the noise, the less time it takes to produce hearing loss. Extremely loud noise can cause hearing loss with even a single, brief exposure. Although brief exposure to loud noise usually produces only temporary hearing loss lasting a few hours to a day or so (called a temporary threshold shift), loss can be permanent, especially when the person is exposed many times. The person may have high-pitched ringing in the ears (tinnitus (see Section 19, Chapter 220)) and problems comprehending speech. When a person experiences these symptoms, it is a warning that a sound is too loud and must be avoided.
Common sources of potentially damaging noise include highly amplified music, power tools, heavy machinery, and many types of powered vehicles, such as snowmobiles. Many people are exposed to injurious levels of noise during the course of their jobs, and hearing loss is a significant occupational hazard for many people. Explosions and gunfire also damage hearing.
Ear Infections: Young children commonly have some degree of conductive hearing loss after an ear infection (otitis media), because infection may lead to accumulation of fluid (effusion) in the middle ear. Most children regain normal hearing in 3 to 4 weeks after the infection resolves, but a few have persistent hearing loss. Chronic, long-standing infections of the middle ear often result in both conductive and sensorineural losses. Hearing loss is more likely in children who have recurring ear infections.
Autoimmune Disorders: Autoimmune disorders are sometimes a cause of hearing loss. The hearing loss may occur in people who have rheumatoid arthritis, systemic lupus erythematosus, Paget's disease, and polyarteritis nodosa. A fluctuating hearing loss, which may be progressive, occurs in both ears. The cause is an attack by the immune system on the cells of the cochlea.
Drugs: Drugs sometimes cause hearing loss. The aminoglycoside family of intravenous antibiotics are the drugs most commonly implicated, particularly when given in high doses. Some people have a rare hereditary disorder that makes them extremely susceptible to hearing loss due to aminoglycosides. Other drugs include vancomycin, quinine, and the cancer chemotherapy drugs cisplatin and nitrogen mustard. Hearing loss can be caused by aspirin (salicylate), but the hearing can come back when the drug is discontinued.
Diagnosis
All hearing loss needs to be evaluated by an otolaryngologist--a doctor who specializes in the care of the ear. An audiologist is a trained professional who tests hearing and performs hearing evaluation tests that measure the degree of hearing loss and the particular sound frequencies that are impaired. If hearing loss is present, other tests help determine how much the hearing loss affects the person's ability to understand speech and whether the hearing loss is sensorineural, conductive, or mixed. Some hearing tests also help identify possible causes of hearing loss. Although many hearing tests require the person's active participation, some do not.
See the sidebar Measurement of Loudness.
Audiometry is the first step in hearing testing. In this test, a person wears headphones that play tones of different frequency (pitch) and loudness into one ear or the other. The person signals when he hears a tone, usually by raising his hand on the side the tone was heard. For each pitch, the test identifies the quietest tone the person can hear in each ear. The results are presented in comparison to what is considered normal hearing. Because loud tones presented to one ear may also be heard by the other ear, a sound other than the test tone (usually noise) is presented to the ear not being tested.
Speech threshold audiometry measures how loudly words have to be spoken to be understood. A person listens to a series of two-syllable, equally accented words (spondees), such as "railroad," "stairway," and "baseball"; presented at different volumes. The volume at which the person can correctly repeat half of the words (spondee threshold) is recorded.
Discrimination, the ability to hear differences between words that sound similar, is tested by presenting pairs of similar one-syllable words. The percentage of words correctly repeated is the discrimination score. People with a conductive hearing loss usually have a normal discrimination score, although at a higher volume. People with sensorineural loss often have abnormal discrimination at all volumes.
Tympanometry tests how well sound can pass through the eardrum and middle ear. This procedure does not require the active participation of the person being tested and is commonly used in children. A device containing a microphone and a sound source is placed snugly in the ear canal, and sound waves are bounced off the eardrum as the device varies the pressure in the ear canal. Abnormal tympanometry results suggest a conductive type of hearing loss.
The Rinne tuning fork test is a screening test that helps distinguish between conductive and sensorineural hearing loss. This test compares how well a person hears sounds conducted by air with how well the person hears sounds conducted by the skull bones. To test hearing by air conduction, the tuning fork is placed near the ear. To test hearing by bone conduction, the base of a vibrating tuning fork is placed against the head so the sound bypasses the middle ear and goes directly to the nerve cells of the inner ear. If hearing by air conduction is reduced but hearing by bone conduction is normal, the hearing loss is conductive. If both air and bone conduction hearing are reduced, the hearing loss is sensorineural or mixed. People with sensorineural hearing loss may need further evaluation to look for other conditions, such as Meniere's disease or brain tumors.
Auditory brain stem response is a test that measures nerve impulses in the brain stem resulting from sound signals in the ears. The information helps determine what kind of signals the brain is receiving from the ears. Test results are abnormal in people with some sensorineural types of hearing loss and in people with many types of brain tumors. Auditory brain stem response is used to test infants and also can be used to monitor certain brain functions in people who are comatose or undergoing brain surgery.
Electrocochleography measures the activity of the cochlea and the auditory nerve by means of an electrode placed on, or through, the eardrum. This test and the auditory brain stem response can be used to measure hearing in people who cannot or will not respond voluntarily to sound. For example, these tests are used to find out whether infants and very young children have profound hearing loss (deafness) and whether a person is faking or exaggerating hearing loss (psychogenic hypacusis).
Otoacoustic emissions testing uses sound to stimulate the inner ear (cochlea). The ear itself then generates a very low intensity sound that matches the stimulus. These cochlear emissions are recorded using sophisticated electronics and are used routinely in many newborn nurseries to screen newborns for congenital hearing loss. This test is also used in adults to help determine the reason for a hearing loss.
Other tests can measure the ability to interpret and understand distorted speech, understand a message presented to one ear when a competing message is presented to the other ear, fuse incomplete messages to each ear into a meaningful message, and determine where a sound is coming from when it is presented to both ears at the same time. Depending on the person's symptoms and the results of the hearing tests, some people need computed tomography (CT) or magnetic resonance imaging (MRI) to look for tumors invading structures of the ear or blocking the eustachian tube.
Prevention and Treatment
Age-related hearing loss and most other causes of hearing loss are not preventable. However, many measures can be taken to help prevent noise-induced hearing loss, such as limiting exposure to loud noise, reducing noise levels whenever possible, and staying away from the source of the noise. The volume of music played through headphones should always be kept at a reasonable level. The louder the noise, the less time a person should spend near it. For occupational or firearm exposure, the use of hearing protectors, such as plastic or foam rubber plugs in the ear canals or glycerin-filled muffs over the ears, is essential. Plastic plugs can also be used in other loud environments.
Treatment of hearing loss depends on the cause. When the cause is fluid in the middle ear, children and adults may need to have a small tube placed in the eardrum (tympanostomy (see Section 23, Chapter 276)). The tube helps prevent fluid from accumulating. Some children also need to have their adenoids removed (adenoidectomy), which helps keep the eustachian tube open. Tumors blocking the eustachian tube are removed. Hearing loss caused by autoimmune disorders is treated with corticosteroids, such as prednisone.
Damage to the eardrums or the bones in the middle ear may require reconstructive surgery. For some people with otosclerosis, hearing may be restored by removing the stirrup surgically and replacing it with an artificial one. Brain tumors causing hearing loss may, in some cases, be removed and the hearing preserved.
Most other causes of hearing loss have no cure. In these cases, treatment involves compensating for the hearing loss as much as possible. Most people with moderate to severe loss use hearing aids. Those with severe to profound loss are greatly helped by a cochlear implant.
Hearing Aids: Sound amplification with a hearing aid helps people who have either conductive or sensorineural hearing loss. Unfortunately, a hearing aid does not restore hearing to normal. A hearing aid should, however, significantly improve a person's ability to communicate and enjoy sounds.
 See the figure Hearing Aids: Amplifying the Sound.
All hearing aids have a microphone to pick up sounds, a battery-powered amplifier to increase their volume, and a means of transmitting the sound to the person. Most hearing aids transmit the sounds through a small speaker placed in the ear canal. Other hearing aids, which require surgical implantation, transmit sounds directly to the bones of the middle ear (ossicles) or the skull instead of through a speaker. Hearing aids differ in how big the components are and where they are located. As a general rule, larger hearing aids are more noticeable and less attractive but are easier to adjust. Larger aids can often accommodate features that are not available in small ones.
Hearing aids have different electronic characteristics that are chosen to suit the person's particular type of hearing loss. For example, people whose hearing loss affects mainly higher frequencies do not benefit from simple amplification, which merely makes the mumbled speech they hear sound louder. Hearing aids that selectively amplify the high frequencies markedly improve speech recognition. Other hearing aids contain vents in the ear mold, which facilitate the passage of high-frequency sound waves into the ear. Many hearing aids use digital sound processing with multiple frequency channels so that the amplification can even more precisely match the person's hearing loss. People who cannot tolerate loud sounds may need hearing aids with special electronic circuitry, which keeps the maximum volume of sound at a tolerable level.
Telephone use can be difficult for people with hearing aids. With typical hearing aids, placing the ear next to the phone handle causes squealing. Some hearing aids have a phone coil: With the flip of a switch the microphone is turned off, and the phone coil links electromagnetically to the magnet in the phone handle. As long as the hearing aid has the proper features, this setup can be arranged by the phone company with simple changes to the phone. Hearing aids with complex features tend to be the most expensive but are often essential to meet hearing needs.
Cochlear Implants: Most profoundly deaf people who cannot hear sounds even with a hearing aid benefit from a cochlear implant. Cochlear implants provide electrical signals directly into the auditory nerve by means of multiple electrodes inserted into the cochlea, the inner ear structure containing the auditory nerve. An external microphone and processor pick up sound signals and convert them to electrical impulses. The impulses are transmitted electromagnetically by an external coil through the skin to an internal coil, which connects to the electrodes. The electrodes stimulate the auditory nerve.
 See the figure Cochlear Implant: Aid for the Profoundly Deaf.
A cochlear implant does not transmit sounds as well as a normal cochlea but provides different benefits to different people. It helps some people read lips. Others can distinguish some words without reading lips. Some people can hear on the telephone.
A cochlear implant also helps deaf people hear and distinguish environmental and warning signals, such as doorbells, telephones, and alarms. It helps them modulate their own voices to make their speech easier for others to understand. A cochlear implant is more effective in a person whose hearing loss is recent or who had successfully used a hearing aid before the implant.
Other Means of Coping With Hearing Loss: Several other types of devices are available for people who have significant hearing loss. Light alerting systems enable these people to know when the doorbell is ringing or a baby is crying. Special sound systems help people hear in theaters, churches, or other places where there is competing noise. Many television programs carry closed captioning, with the dialog shown as visible text. Telephone communication devices are also available.
Lip reading (speech reading) is an important skill for people who have decreased hearing. It is particularly important for people who can hear but have trouble discriminating sounds, typically those with age-related hearing loss. Observing the position of a speaker's lips allows people to recognize which consonant is being spoken. Because people whose hearing loss affects high frequencies are unable to understand consonant sounds, lip reading can significantly improve the comprehension of speech.
Lip reading and other strategies for coping with hearing loss are sometimes taught by hearing professionals in a program called aural rehabilitation (see Section 1, Chapter 7). In addition to training in lip reading, people are taught to gain control over their listening environment by learning to anticipate difficult communication situations and modifying or avoiding them. 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. They can request that "specials of the day" be written rather than spoken. In direct conversations, people may ask the speaker to face them. At the beginning of a telephone conversation, people can identify themselves as being hearing-impaired.
People with profound hearing loss often communicate using sign language. American Sign Language (ASL) is the version most widely used in the United States. Other forms include Signed English, Signing Exact English, and Cued Speech.
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