Hearing loss is any degree of impairment of the ability to apprehend sound.
Sound can be measured accurately. The term decibel (dB) refers to an amount of energy moving sound from its source to our ears or to a microphone. A drop of more than 10 dB in the level of sound a person can hear is significant.
Sound travels through a medium like air or water as waves of compression and rarefaction. These waves are collected by the external ear and cause the tympanic membrane (ear drum) to vibrate. The chain of ossicles connected to the ear drum—the incus, malleus, and stapes—carries the vibration to the oval window, increasing its amplitude 20 times on the way. There the energy causes a standing wave in the watery liquid (endolymph) inside the Organ of Corti. (A standing wave is one that does not move. A vibrating cup of coffee will demonstrate standing waves.) The configuration of the standing wave is determined by the frequency of the sound. Many thousands of tiny nerve fibers detect the highs and lows of the standing wave and transmit their findings to the brain, which interprets the signals as sound.
To summarize, sound energy passes through the air of the external ear, the bones of the middle ear and the liquid of the inner ear. It is then translated into nerve impulses, sent to the brain through nerves and understood there as sound. It follows that there are five steps in the hearing process:
- air conduction through the external ear to the ear drum
- bone conduction through the middle ear to the inner ear
- water conduction to the Organ of Corti
- nerve conduction into the brain
- interpretation by the brain.
|Decibel Ratings And Hazardous Levels Of Noise
||Example Of Sounds
|Above 110 decibels, hearing may become painful
||Above 120 decibels is considered deafening
||Above 135 decibels, hearing will become extremely painful and hearing loss may result if exposure is prolonged Above 180 decibels, hearing loss is almost certain with any exposure
||Noise may prevent the listener from falling asleep
||Quiet office noise level
||Average television volume, sewing machine, lively conversation
||Busy traffic, noisy restaurant
||Heavy city traffic, factory noise, alarm clock
||Cocktail party, lawn mower
||Rocket launching pad
Hearing can be interrupted in several ways at each of the five steps.
The external ear canal can be blocked with ear wax, foreign objects, infection, and tumors. Overgrowth of the bone, a condition that occurs when the ear canal has been flushed with cold water repeatedly for years, can also narrow the passageway, making blockage and infection more likely. This condition occurs often in Northern Californian surfers and is therefore called "surfer's ear."
The ear drum is so thin a physician can see through it into the middle ear. Sharp objects, pressure from an infection in the middle ear, even a firm cuffing or slapping of the ear, can rupture it. It is also susceptible to pressure changes during scuba diving.
Several conditions can diminish the mobility of the ossicles (small bones) in the middle ear. Otitis media
(an infection in the middle ear) occurs when fluid cannot escape into the throat because of blockage of the eustachian tube. The fluid that accumulates, whether it be pus or just mucus and dampens the motion of the ossicles. A disease called otosclerosis
can bind the stapes in the oval window and thereby cause deafness.
All the conditions mentioned so far, those that occur in the external and middle ear, are causes of conductive hearing loss. The second category, sensory hearing loss, refers to damage to the Organ of Corti and the acoustic nerve. Prolonged exposure to loud noise is the leading cause of sensory hearing loss. A million people have this condition, many identified during the military draft and rejected as being unfit for duty. The cause is often believed to be prolonged exposure to rock music. Occupational noise exposure is the other leading cause of noise induced hearing loss (NIHL) and is ample reason for wearing ear protection on the job. A third of people over 65 have presbycusis—sensory hearing loss due to aging
. Both NIHL and presbycusis are primarily high frequency losses. In most languages, it is the high frequency sounds that define speech, so these people hear plenty of noise, they just cannot easily make out what it means. They have particular trouble selecting out speech from background noise. Brain infections like meningitis
, drugs such as the aminoglycoside antibiotics
(streptomycin, gentamycin, kanamycin, tobramycin), and Meniere's disease also cause permanent sensory hearing loss. Meniere's disease combines attacks of hearing loss with attacks of vertigo. The symptoms may occur together or separately. High doses of salicylates like aspirin
and quinine can cause a temporary high-frequency loss. Prolonged high doses can lead to permanent deafness. There is an hereditary form of sensory deafness and a congenital form most often caused by rubella (German measles
Sudden hearing loss—at least 30dB in less than three days—is most commonly caused by cochleitis, a mysterious viral infection.
The final category of hearing loss is neural. Damage to the acoustic nerve and the parts of the brain that perform hearing are the most likely to produce permanent hearing loss. Strokes, multiple sclerosis, and acoustic neuromas are all possible causes of neural hearing loss.
Hearing can also be diminished by extra sounds generated by the ear, most of them from the same kinds of disorders that cause diminished hearing. These sounds are referred to as tinnitus
and can be ringing, blowing, clicking, or anything else that no one but the patient hears.
An examination of the ears and nose combined with simple hearing tests done in the physician's office can detect many common causes of hearing loss. An audiogram often concludes the evaluation, since these simple means often produce a diagnosis. If the defect is in the brain or the acoustic nerve, further neurological testing and imaging will be required.
The audiogram has many uses in diagnosing hearing deficits. The pattern of hearing loss across the audible frequencies gives clues to the cause. Several alterations in the testing procedure can give additional information. For example, speech is perceived differently than pure tones. Adequate perception of sound combined with inability to recognize words points to a brain problem rather than a sensory or conductive deficit. Loudness perception is distorted by disease in certain areas but not in others. Acoustic neuromas often distort the perception of loudness.
Conductive hearing loss can almost always be restored to some degree, if not completely.
- matter in the ear canal can be easily removed with a dramatic improvement in hearing.
- surfer's ear gradually regresses if cold water is avoided or a special ear plug is used. In advanced cases, surgeons can grind away the excess bone.
- middle ear infection with fluid is also simple to treat. If medications do not work, surgical drainage of the ear is accomplished through the ear drum, which heals completely after treatment.
- traumatically damaged ear drums can be repaired with a tiny skin graft.
- surgical repair of otosclerosis through an operating microscope is one of the most intricate of procedures, substituting tiny artificial parts for the original ossicles.
Sensory and neural hearing loss, on the other hand, cannot readily be cured. Fortunately it is not often complete, so that hearing aids
can fill the deficit.
In-the-ear hearing aids can boost the volume of sound by up to 70 dB. (Normal speech is about 60 dB.) Federal law now requires that they be dispensed only upon a physician's prescription. For complete conduction hearing loss there are now available bone conduction hearing aids and even devices that can be surgically implanted in the cochlea.
Tinnitus can sometimes be relieved by adding white noise (like the sound of wind or waves crashing on the shore) to the environment.
Decreased hearing is such a common problem that there are legions of organizations to provide assistance. Special language training, both in lip reading and signing, special schools and special camps for children are all available in most regions of the United States.
Conductive hearing loss can be treated with alternative therapies that are specific to the particular condition. Sensory hearing loss may be helped by homeopathic therapies. Oral supplementation with essential fatty acids such as flax oil and omega 3 oil can help alleviate the accumulation of wax in the ear.
— A unit of the intensity of sound, a measure of loudness.
— The combination of vertigo and decreased hearing caused by abnormalities in the inner ear.
— A disease that scars and limits the motion of the small conducting bones in the middle ear.
— Sudden loss of blood supply to part of the brain.
Prompt treatment and attentive follow-up of middle ear infections in children will prevent this cause of conductive hearing loss. Control of infectious childhood diseases such as measles has greatly reduced sensory hearing loss as a complication of epidemic diseases. Laws that require protection from loud noise in the workplace have achieved substantial reduction in noise induced hearing loss. Surfers should use the right kind of ear plugs.
Alexander Graham Bell Association for the Deaf. 3417 Volta Place NW, Washington, DC 20007. (202) 337-5220. http://www.agbell.org.
Auditory-Verbal International. 2121 Eisenhower Ave., Suite 402, Alexandria, VA 22314. (703) 739-1049. 〈firstname.lastname@example.org〉 〈http://www.auditory-verbal.org/contact.htm〉.
Better Hearing Institute. 515 King Street, Suite 420, Alexandria, VA 22314. (703) 684-3391.
Central Institute for the Deaf. Washington University. St. Louis, Missouri. 〈http://cidmac.wustl.edu〉.
League for the Hard of Hearing. 71 West 23rd St., New York, New York 10010-4162. (212) 741-7650. http://www.lhh.org.
National Association of the Deaf. 814 Thayer Ave., Silver Spring, MD, 20910. (301) 587-1788. 〈http://nad.policy.net〉.
Self Help for Hard of Hearing People, Inc. 7800 Wisconsin Ave., Bethesda, MD 20814. (301) 657-2248. http://www.shhh.org.
The Sight & Hearing Association (SHA). http://www.sightandhearing.org.
World Recreation Association of the Deaf (WRAD). http://www.wrad.org.
Gale Encyclopedia of Medicine. Copyright 2008 The Gale Group, Inc. All rights reserved.
partial or complete loss of the sense of hearing
; called also deafness
. The number of hearing impaired individuals has steadily increased over the past few decades. While improved detection and reporting of impaired hearing can account for some of this increase, other contributing factors include an aging population and increased noise levels in the environment.
Types of Hearing Loss. There are three broad categories of hearing loss: conductive, sensorineural, and central. Conductive hearing loss is associated with impaired transmission of sound waves through the external ear canal to the bones of the middle ear. A blockage of the external ear or dysfunction of the middle ear will produce conductive loss of hearing. Examples of common causes of conductive hearing loss are obstruction of the ear canal by cerumen or a foreign object, perforated eardrum, otitis media, otosclerosis, and congenital malformations of the outer or middle ear.
Sensorineural hearing loss is associated with some pathological change in structures within the inner ear or in the acoustic nerve. Normally, sound waves received by the external and middle ear are conveyed to the fluid in the cochlea of the inner ear. On the surface of its basilar membrane lies the organ of Corti, which contains mechanically sensitive cells. These minute structures act as end-organs that generate nerve impulses in response to sound vibrations. Thus the mechanical energy of sound vibrations is transformed into electrical energy that stimulates the nerve fibers of the acoustic (eighth cranial) nerve. The impulses are then transmitted to the brain, where the cerebral cortex decodes or interprets the sound. A sensorineural hearing loss results when there is dysfunction in either the perception or the interpretation of sound waves. Common causes of sensorineural hearing loss include hereditary disease, aging (presbycusis), noise damage, viral childhood infections, skull fractures and intracranial tumors, ototoxic drugs, and Rh incompatibility during fetal life.
Central hearing loss occurs when there is a pathologic condition above the junction of the acoustic nerve and the brainstem. Brain tumors, vascular changes that suddenly deprive the structures of the inner ear of their blood supply, stroke syndrome, and erythroblastosis fetalis are examples of pathologic conditions that can produce central hearing loss.
Preventing Hearing Loss
. Not all cases of hearing loss can be prevented, but there are measures that are known to be effective in curtailing the incidence and severity of many types of hearing loss. For example, excessive environmental noise is a major contributing factor that is avoidable or at least manageable. Permanent damage to hearing can result when the structures of the inner ear and nerve cells are repeatedly bombarded with loud sound waves which reduce their blood supply. The cells thus lost are replaced by scar tissue that cannot transmit sound waves and impulses. If one's occupation requires continued exposure to loud noises, ear protection devices should be worn to mitigate the harmful effect.
Excessive noise in one's environment often can be greatly reduced by lowering the volume on radios and other music appliances, especially those with headphones. Authorities recommend that one should avoid continual exposure to music amplified to more than 104 to 111 decibels. It is believed that the loss of hearing associated with aging may not be an inevitable outcome of growing older, but rather a lasting effect of living in an increasingly noisy environment.
Efforts to reduce the risk of atherosclerosis
can help prevent hearing loss by lowering the possibility of atherosclerotic plaque in the blood vessels supplying the delicate structures of the inner ear, and also by minimizing the risk of stroke.
There are some drugs that are particularly toxic to the ear. Examples are aspirin
, which can produce tinnitus and temporary deafness; the antibiotics erythromycin
, and streptomycin
; the nonsteroidal antiinflammatory drugsfenoprofen
; the diureticfurosemide (Lasix)
; and several drugs used in the chemotherapeutic treatment of malignancies. Any person taking a prescribed medication should be alert for hearing-related problems while taking the drug and report such problems as soon as they appear.
Prompt attention to and successful treatment of ear infections is another means by which hearing loss can be prevented. Symptoms such as ringing in the ears, a feeling of pressure in the ear, or increasing hearing difficulty call for medical consultation.
Great progress has been made in the treatment of conditions that once almost always resulted in impaired hearing. One example is the use of antibiotics to manage ear infections. Another is the development of microsurgery, which enables the surgeon to operate freely in the crowded inner chambers of the ear. Two such surgical procedures are stapedectomy
. In a stapedectomy to correct otosclerosis the diseased stapes or stirrup is removed and replaced by a prosthesis that allows the chain of sound transmission to function again. Tympanoplasty is useful in correcting some types of conductive hearing loss. If chronic ear infection or injury has destroyed one or more of the ossicles, they can be rebuilt or replaced surgically.
Detecting Hearing Loss
. In the infant and very young child hearing loss is evidenced by a lack of response to the sounds in the immediate environment. As the infant matures and begins to approach the age at which talking should begin, there may be either a failure to talk at all by age two or scarcely intelligible speech after age three.
Emotional and behavioral disorders can be signs of hearing loss in children and adults. The frustration they feel in trying to cope with their disability can be manifested by irritability, hyperactivity, hostility, and withdrawal. Other signs include speaking in either a very loud or very soft voice; habitually saying “What?” and failing to follow instructions; facial expressions indicating difficulty in understanding what is being said; and inappropriate responses to questions asked or statements made during a conversation.
Among the tests used to evaluate hearing are tuning fork tests such as the weber test
and the rinne test
. A tympanogram
provides information about the movement of the eardrum. Evaluation of hearing acuity by audiometry
uses a special machine for testing sound perception.
. For those cases of hearing loss that cannot be corrected, some form of rehabilitation is prescribed to make the best of whatever hearing remains and to improve the person's quality of life and socialization. The hearing aid
is helpful for persons with certain kinds of hearing loss. It should be selected with the help of an audiologist or an otologist because different types of hearing loss require different types of hearing aids. Careful training in the proper use of the hearing aid is also necessary to assure that the person wearing it will achieve maximum benefit.
An important tool in rehabilitation of the profoundly hearing impaired is training in signing
and lip reading
. For lip reading, the patient is taught to use visual clues, such as facial expression and body movements, as well as movements of the lips and tongue.
A third component of rehabilitation of the hearing impaired is speech therapy
. Profoundly deaf persons cannot hear their own voices, and those who have never heard spoken language have difficulty learning to speak coherently. Those who have lost their hearing over a period of time often suffer a gradual deterioration in their speech so that communication becomes increasingly awkward.
Patient Care. Communicating with the partially hearing impaired person can be enhanced by following a few simple rules. Speak slightly more loudly than normal but do not shout, as this can distort your speech and will not make your message any clearer. Speak slowly and distinctly. Get the person's attention before speaking. The best distance for speaking to a hearing-impaired person is 1 to 2 meters (3 to 6 feet). Face the person to whom you are speaking and place yourself at eye level so that other visual clues can be seen. Never speak directly into the person's ear. This can distort the message and hide all visual clues. Be aware of nonverbal communication; facial expressions, gestures, and lip and body movements all give clues to the meaning of the message you are trying to send. Use short, simple sentences. If the person does not appear to understand or responds inappropriately, rephrase your statement. Give the person time to respond to what you have asked or stated.
Sources of Help for Those with Hearing Loss
. Among the resources available to hearing impaired persons and their families are universities and medical centers with departments of speech and hearing that provide lip-reading classes, rehabilitation, and hearing aid clinics. The national institute on deafness and other communication disorders
(NIDCD), a part of the National Institutes of Health, has a clearinghouse for information on hearing, balance, language, and related issues. Their mailing address is NIDCD Information Clearinghouse, 1 Communication Ave., Bethesda, MD 20892, and their web site is http://www.nidcd.nih.gov. They can be reached by telephone at (voice) 1-800-241-1044 or (TDD/TTY) 1-800-241-1055. Other societies and organizations devoted to helping the hearing impaired include The Better Hearing Institute, 515 King Street, Suite 420, Alexandria VA 22314, which operates a toll-free “Hearing Helpline” at 1-800-327-9355; and The Alexander Graham Bell Association for the Deaf, 3417 Volta Pl. NW, Washington, DC 20007. Web site http://www.hear-it.org is a worldwide hearing information web site.
conductive hearing loss
that due to a defect of the sound-conducting apparatus, i.e., of the external auditory canal or middle ear. See hearing loss
ototoxic hearing loss
deafness caused by ingestion of toxic substances or medications that affect the eighth cranial nerve
paradoxic hearing loss that in which the hearing is better during loud noise.
sensorineural hearing loss
that due to a defect in the inner ear or the acoustic nerve. See hearing loss
Miller-Keane Encyclopedia and Dictionary of Medicine, Nursing, and Allied Health, Seventh Edition. © 2003 by Saunders, an imprint of Elsevier, Inc. All rights reserved.