What is deafness?

Causes and Symptoms

To understand deafness, it is first necessary to understand how sound is heard.
The sound waves produced by any noise travel through the air and are funneled down the ear canal by the external ear, which is specially shaped for this function. The sound waves then cause the tympanic membrane
to vibrate, which in turn causes the chain of tiny ossicles to vibrate. This mechanical energy of vibration is then transformed by the cochlea into nerve impulses that travel along the eighth cranial nerve to the spinal cord. These impulses are transmitted to the auditory cortex (center) of the brain, where they are interpreted. The ability to hear depends on all these elements working properly.

Deafness can occur when any particular part of this hearing pathway is not functioning as it should. If the ear canal is blocked with cerumen (earwax), a foreign body, fluid, or the products of infection or inflammation, then the sound waves are unable to travel to the eardrum. If the eardrum has ruptured or become stiff (sclerosed), then it cannot vibrate. If the ossicles have been damaged in any way, then they cannot vibrate. If the middle ear is filled with fluid from inflammation or infection, then the eardrum and ossicles cannot work properly. If the cochlea, auditory nerve, or both have been damaged through trauma, disease, or use of an ototoxic drug, then they cannot do their job of converting vibration into nerve impulses and transmitting them to the brain. If the auditory center of the brain is damaged, then it cannot interpret the nerve impulses correctly.

Hearing loss is classified by the cause: conductive, sensory, or neural. Conductive losses are those that affect the conduction of sound waves; they involve problems with the external ear, ear canal, tympanic membrane, and ossicles. The sensory and neural causes are usually classified together as “sensorineural”; these losses affect the cochlea, auditory nerve, or auditory cortex of the brain.

The most common cause of hearing loss in children is otitis media, or middle-ear infection, which causes fluid to build up behind the tympanic membrane. It is usually reversible with time and treatment. This is a type of conductive loss, as is hearing loss attributable to cerumen impaction (excessive buildup of earwax), which can occur in both children and adults. Sensorineural losses are caused by such things as excessive noise exposure, ototoxic drugs, exposure to toxins in the environment, and diseases such as rubella (German measles). The type of sensorineural loss that many people experience as they get older is called presbycusis, which means “a condition of elder hearing.” Many people with sensorineural losses have a genetic predisposition for hearing loss.

Congenital deafness is deafness that is present at the time a baby is born. Before the widespread availability of immunization against rubella, mothers who contracted the disease during pregnancy were at great risk of having a baby with congenital deafness. Congenital deafness may also be genetic. For example, if a child inherits a defective copy of the GJB2 gene from each parent, then that child will be deaf even if both parents can hear.

Likewise, deafness that occurs after birth may be either acquired or genetic. Deafness associated with exposure to loud noises is acquired, for example, while many forms of deafness that occur in older age are genetic. About one-third of people over the age of sixty-five have hearing loss.

Treatment and Therapy

According to the World Health Organization, as of early 2013, about half of all cases of hearing loss worldwide were preventable. Indeed, the best treatment for deafness is prevention. Immunization against rubella, protection of ears from excessive noise, and avoidance of too much aspirin are all examples of preventative measures.

Once hearing impairment has occurred, determination of the cause is essential. The primary care provider, an otorhinolaryngologist (ear, nose, and throat doctor), an audiologist (hearing specialist), and a neurologist may be involved in this assessment. Reversible causes of deafness or hearing loss can be addressed medically or surgically, as in the removal of earwax. For nonreversible causes, hearing aids
and other assistive hearing devices may be helpful. These devices have become increasingly sophisticated. Some allow the wearer to adjust the hearing aid to specific circumstances, while older versions amplified all sounds equally.

Cochlear implants
are electronic devices that give profoundly deaf persons a sense of sound that helps them understand speech and other noises. A microphone picks up sounds, which are processed, converted to electric impulses, and sent to different areas of the auditory nerve.

In addition to these products, many assistive devices are available to help persons with severe hearing impairment function independently. For example, special telephones, alarm clocks, and doorbells that flash a light or shake the bed in addition to ringing are readily available.

Bibliography

A.D.A.M. Medical Encyclopedia. "Hearing Loss." MedlinePlus, May 22, 2012.

American Medical Association. American Medical Association Family Medical Guide. 4th rev. ed. Hoboken, N.J.: John Wiley & Sons, 2004.

Carmen, Richard, ed. The Consumer Handbook on Hearing Loss and Hearing Aids: A Bridge to Healing. 3d rev. ed. Sedona, Ariz.: Auricle Ink, 2009.

Dillon, Harvey. Hearing Aids. New York: Thieme, 2001.

Komaroff, Anthony, ed. Harvard Medical School Family Health Guide. New York: Free Press, 2005.

National Center on Birth Defects and Developmental Disabilities. "Hearing Loss in Children." Centers for Disease Control and Prevention, March 22, 2013.

National Institute on Deafness and Other Communication Disorders. "Hearing, Ear Infections, and Deafness." National Institutes of Health, September 8, 2011.

Romoff, Arlene. Hear Again: Back to Life with a Cochlear Implant. New York: League for the Hard of Hearing, 1999.

Stoppard, Miriam. Family Health Guide. London: DK, 2006.

World Health Organization. "Millions Have Hearing Loss That Can Be Improved or Prevented." World Health Organization, February 27, 2013.