Several factors can impair hearing, including interference with transmission of vibrations to the inner ear (conductive deafness) or damage to the cochlea or the auditory nerve and its pathways (sensorineural deafness). Disease, injury, and heredity all can impair hearing. There are more than 100 forms of inherited deafness, many of which are part of other syndromes. In the U.S., 8% of the population, or 21 million people, have hearing loss.

About 95% of cases of hearing loss are conductive. One cause is accumulated dry wax or a foreign object in the ear, which plugs the auditory meatus. Changes in the eardrum or auditory ossicles can also block hearing. The eardrum may harden as a result of disease, becoming less responsive to sound waves, or an injury may tear or perforate it.

A common disorder of the auditory ossicles is otosclerosis, in which new bone is deposited abnormally around the base of the stapes. This interferes with the ossicles' movement, which is necessary to transmit vibrations to the inner ear. Surgery often can restore some hearing to a person with otosclerosis by chipping away the bone that holds the stapes fixed in position, or replacing the stapes with a wire or plastic substitute.

Two tests used to diagnose conductive deafness are the Weber test and the Rinne test. In the Weber test, the handle of a vibrating tuning fork is pressed against the forehead. A person with normal hearing perceives the sound coming from directly in front, while a person with a sound conduction blockage in one middle ear hears the sound coming from the impaired side.

In the Rinne test, a vibrating tuning fork is held against the bone behind the ear. After the sound is no longer heard by conduction through the bones of the skull, the fork is moved to just in front of the external auditory meatus. In middle ear conductive deafness, the vibrating fork can no longer be heard, but a normal ear will continue to hear its tone.

Very loud sounds can cause sensorineural deafness. If exposure is brief, hearing loss may be temporary, but when exposure is repeated and prolonged, such as occurs in foundries, near jackhammers, or on a firing range, impairment may be permanent. Pete Townshend, of the rock group The Who, suffers from hearing loss from many years of performing loud concerts.

Other causes of sensorineural deafness include tumors in the central nervous system, brain damage as the result of vascular accidents, and the use of certain drugs.

Because hearing loss and other ear problems can begin gradually, it is important to be aware of their signs, which may include:

• difficulty hearing people talking softly
• inability to understand speech when there is background noise
• ringing in the ears
• dizziness
• loss of balance

New parents should notice whether their infant responds to sounds in a way that indicates normal hearing. Before 1993, 50% of hearing impaired infants were not diagnosed until age 2. Since then, the federal government has advised hearing exams as part of a well-baby visit to a doctor. If the baby's responses indicate a possible problem, the next step is to see an audiologist, who identifies and measures hearing loss.

Often a hearing aid can help people with conductive hearing loss. A hearing aid has a tiny microphone that picks up sound waves and converts them to electrical signals, which are then amplified so that the person can hear them. An ear mold holds the device in place, either behind the outer ear, in the outer ear, or in the ear canal.

A cochlear implant enables people with sensorineural hearing loss to detect some sounds, although it usually remains difficult to discern distinct words. The device converts sound waves to electrical signals, which stimulate neurons in the cochlea.

Many persons over age 65 lose the ability to hear high-frequency sounds and can no longer discriminate speech sounds as well as they once did--a condition called presbycusis. Thus, it is important to use lower voice tones in speaking to an older person. Also, the speaker should face the listener so that lip-reading is possible.

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