Conductive hearing loss

Definition

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

Description

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 twenty 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.

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, 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 or 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.

Diagnosis

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.

Treatment

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

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.

• 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.

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.

Alternative treatment

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.

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. Surfers should use the right kind of ear plugs.

Key Terms

Decibel

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

Otosclerosis

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

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.

Stroke

Sudden loss of blood supply to part of the brain.

Further Reading

For Your Information

Books

• Alberti, R.W. "Occupational hearing loss." In Disorders of the nose, throat, ear, head, and neck. Edited by John Jacob Ballenger. Philadelphia: Lea & Febiger, 1991, pp. 1053-68.
• Austin, David F. "Non-inflammatory diseases of the labyrinth." In Disorders of the nose, throat, ear, head, and neck. Edited by John Jacob Ballenger. Philadelphia: Lea & Febiger, 1991, pp. 1209-13.
• Bennett, J. Claude and Fred Plum, ed. Cecil Textbook of Medicine Philadelphia: W. B. Saunders, 1996, pp. 2021-24.
• Niparko, John K. "Hearing loss and associated problems." In Principles of Ambulatory Medicine. Edited by L. Randol Barker, et al. Baltimore: Williams & Wilkins, 1995, pp. 1403-15.
• Rakel, Robert E., ed. Current Therapy. Philadelphia: W. B. Saunders, 1998, pp. 459-460.
• Tierney, Lawrence M., et al., ed. Current Medical Diagnosis and Treatment. Stamford, CT: Appleton & Lange, 1998, pp. 215.

  Periodicals

• Cohen, N. and S. Waltzman. "The Department of Veterans Affairs Cochlear Implant Study Group." New England Journal of Medicine 328 (1993): 233.
• Nadol, J.B. "Hearing loss." New England Journal of Medicine 329 (1993): 092.

  Organizations

• Alexander Graham Bell Association for the Deaf. 3417 Volta Place NW, Washington, DC 20007-2778. (202)337-5220. http:/www.agbell.org.
• Auditory-Verbal International. 2121 Eisenhower Avenue, Suite 402, Alexandria, VA 22314. (703)739-1049, (703)739-0874. avi@auditory-verbal.org. http://www.auditory-verbal.org/contact.htm.
• Better Hearing Institute. Washington, DC. 800-EAR-WELL. mail@betterhearing.org. http://www.betterhearing.org.
• Central Institute for the Deaf. Washington University. St. Louis, Missouri. http://cidmac.wustl.edu.
• The League for the Hard of Hearing. 71 West 23rd Street, New York, New York 10010-4162. (212)741-7650. http://www.lhh.org.
• National Association of the Deaf. NADHQ@juno.com. http://www.nad.org.
• National Institute on Deafness and Other Communication Disorders. National Institutes of Health, Bethesda, Maryland 20892. webmaster@ms.nidcd.nih.gov. http://www.nih.gov/nidcd.
• Self Help for Hard of Hearing People, Inc. 79 O Woodmon Avenue, Suite 120C, Bethesda, MD 20814. (301)657-2248. http://www.shhh.org.
• The Sight & Hearing Association (SHA). http://www.sightandhearing.org.
• The World Recreation Association of the Deaf (WRAD). http://www.wrad.org.

  Other

• Vessel, B. "Deaf Source." http://home.earthlink.net/~drblood (April 26, 1998).

Gale Encyclopedia of Medicine. Gale Research, 1999.