Home > Encyclopedia of Nursing & Allied Health > Hearing Loss

Definition

Hearing loss is any degree of impairment of the ability to comprehend sound.

Description

Sound can be accurately measured. The term decibel (dB) refers to an amount of energy moving sound from its source to the ears of one or more listeners or to a microphone. A decrease of more than 10 dB in the level of sound a person can hear is significant.

Sound travels through a medium, such as 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 vibrations to the oval window, increasing their amplitude 20 times along the way. There, the energy creates 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 shape 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. Motions in the nerve endings are converted into electrical impulses. These are transmitted to the brain via a nerve. Specialized areas of the brain interpret 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 the acoustic nerve, 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 inside the organ of Corti
• nerve conduction into the brain
• interpretation by the brain

Causes and symptoms

Hearing can be interrupted in several ways at each of the five steps noted above.

The external ear canal can be blocked with ear wax (cerumen), 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 among northern Californian surfers and is aptly called "surfer's ear."

The ear drum is so thin that a physician can see through it, into the middle ear. Sharp objects, pressure from an infection in the middle ear, or even a firm cuffing or slapping of the ear, can rupture the ear drum. The eardrum is also susceptible to pressure changes, such as those that occur when one is 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 is pus or mucus, dampens the motions of the ossicles. A disease called otosclerosis can bind the stapes in the oval window, thereby cause deafness.

All of the problems and conditions 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, or the acoustic nerve. Prolonged exposure to loud noise is the leading cause of sensory hearing loss. More than a million people have been identified as having this condition. The cause is often believed to be prolonged exposure to loud music. Occupational noise exposure is the other leading cause of noise induced hearing loss (NIHL) and is an ample reason for wearing ear protection on the job. Jobs in construction and loud offices may contribute to ear damage, as may noises of recreation, such as loud music or the engine of a motorcycle. Both types of noises may lead to loss of hairs in one's inner ear. Thousands of these tiny hairs are attached to nerve cells in the cochlea (a snail-shaped structure in the inner ear). These tiny hairs aid the conversion of sound vibrations into electrical signals, which are then transmitted to the brain. When hairs become broken or bent, the transmission of the electric signals isn't as good, and sound may become muffled. Words may become difficult to distinguish against background noises.

One-third of people over 65 have presbycusis (gradual loss of hearing that occurs as one ages) and one-half of those older than age 75 have a hearing impairment. Both NIHL and presbycusis are primarily high frequency losses. The human speech frequencies are in relatively low ranges. People with presbycusis hear noise but cannot easily make sense of it. They have particular trouble distinguishing speech from background noise. Brain infections, such as meningitis, drugs such as the amino-glycoside antibiotics (e.g., streptomycin, kanamycin, tobramycin), and Menière's disease may also cause permanent sensory hearing loss. Menière'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).

DECIBEL RATINGS AND HAZARDOUS LEVELS OF NOISE
Decibel Level	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
30	Soft whisper
35	Noise may prevent the listener from falling asleep
40	Quiet office noise level
50	Quiet conversation
60	Average television volume, sewing machine, lively conversation
70	Busy traffic, noisy restaurant
80	Heavy city traffic, factory noise, alarm clock
90	Cocktail party, lawn mower
100	Pneumatic drill
120	Sandblasting, thunder
140	Jet airplane
180	Rocket launching pad

Sudden hearing loss—at least 30dB in less than three days—is most commonly caused by cochleitis, an inflammation of the cochlea. The source of this process is thought to be viral, but as of 2001, no causative virus has been identified.

The final category of hearing loss is neural. Damage to the acoustic nerve and the parts of the brain that integrate and interpret sounds 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 hearing loss. These sounds are referred to as tinnitus and can be ringing, blowing, clicking, or anything else that no one but the affected person hears.

Diagnosis

An examination of the ears and nose, combined with simple hearing tests that can be conducted in a physician's office, detect many common causes of hearing loss. Analysis of an audiogram often concludes the evaluation, since these simple imaging often enables a diagnosis. If the defect is in the brain or the acoustic nerve, further neurologic testing and imaging will be required.

An 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, indicates a brain problem rather than a sensory or conductive deficit. Loudness perception is distorted by disease in certain areas of the brain, but not in others. Acoustic neuromas often distort the perception of loudness.

Treatment

Conductive hearing loss can almost always be restored to some degree, if not completely.

• Matter in the ear canal can be easily removed. This results in a dramatic improvement in hearing. In cases of earwax blockage, wax may be removed by the physician, who may loosen it and drain the ear, scoop it out, or use a suction device to removed softened wax.

• Surfer's ear gradually regresses if cold water is avoided or a special ear plug is used. In advanced cases, excess bone can be ground away by surgeons.
• Middle ear infection with fluid is also relatively easy 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 procedures available, and substitutes tiny, artificial parts for the original ossicles.

Sensory and neural hearing loss, on the other hand, cannot readily be cured. Fortunately, the loss 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 with a physician's prescription. For complete conduction hearing loss, there are now bone conduction hearing aids available, as well as 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 many organizations that provide assistance. Special language training, both in lip reading and signing, and special schools and camps for hearing-impaired 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.

Prognosis

The prognosis for conductive hearing loss is quite good. Since there is no cure for sensory or neural hearing loss, the prognosis is poor.

Health care team roles

Hearing examinations are usually conducted by physicians. Audiologists are trained to evaluate hearing. Speech language specialists are trained to provide treatment and rehabilitation for persons with impaired hearing.

Prevention

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. Wearing specially designed earmuffs that resemble earphones may be of use where the noise is still too loud. Surfers should use the right kind of ear plugs.

One should have his or her hearing tested on a regular basis if a noisy environment cannot be avoided. Early detection will enable one to take steps to prevent further hearing loss. Avoiding exposure to loud noise and using hearing protection are the best ways to prevent hearing loss and slow the onset of presbycusis.

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KEY TERMS

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Compression—Narrower than average distances between wave peaks.

Decibel—A unit of the intensity of sound, a measure of loudness.

Meniere's disease—The combination of vertigo and decreased hearing caused by abnormalities in the inner ear.

Multiple sclerosis—A progressive disease of brain and nerve tissue.

Otosclerosis—A disease that scars and limits the motion of the small conducting bones in the middle ear.

Rarefaction—Wider than average distances between wave peaks.

Stroke—A sudden loss of blood supply to part of the brain.

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L. Fleming Fallon Jr., M.D, Ph.D, Dr.PH