deafness, partial or total lack of hearing. It may be present at birth (congenital) or may be acquired at any age thereafter. A person who cannot detect sound at an amplitude of 20 decibels in a frequency range of from 800 to 1,800 vibrations per second is said to be hard of hearing. The ear normally perceives sounds in the range of 20 to 20,000 vibrations per second. There are two principal kinds of deafness, conductive deafness and sensorineural deafness. In some cases of deafness both the conductive and the nerve mechanisms are disturbed.
Conductive problems are those that disrupt the conduction of sound through the outer and middle ear (see ear), affecting hearing before the sound reaches the cochlea and the nerve receptors of the inner ear. Disturbances of the conductive mechanism are often temporary or curable. Most such cases are caused by otitis media, an infection that spreads to the middle ear from the upper respiratory tract; the condition usually responds to antibiotic therapy, but serious cases may require drainage of collected fluids through an incision in the eardrum (tympanum) or insertion of a tiny drainage tube. Foreign bodies or impacted wax can cause hearing loss and must be removed by a physician. In adults a predominant cause of conductive deafness is otosclerosis, a chronic hereditary condition in which spongy bone formation results in fixation of the stapes (the bone that connects the middle ear to the inner ear) and restricts its vibration. Important advances in surgical techniques have led to successful treatment of otosclerosis by replacing the stapes with a combination of grafted tissue, plastic, and wire appliances. Deafness can also be caused by perforation or rupture of the eardrum by a sudden loud noise, by physical puncture, or as a result of an infectious disease. In some such cases the eardrum can be repaired by grafting. Today there are many advanced medical techniques for treating infection of the mastoid and congenital malformations of the outer and middle ear that, if neglected, might result in deafness.
Sensorineural deafness results from damage to the neural receptors of the inner ear (the hair cells, organ of Corti), the nerve pathways to the brain (notably the auditory nerve), or the area of the brain that receives sound information. Deafness of this type is usually permanent. It can be congenital or accompany other birth-related problems such as erythroblastosis fetalis (Rh incompatibility) or anoxia (lack of oxygen during delivery). Before vaccines were available, German measles (rubella) and common measles (rubeola) were leading causes; maternal cytomegalovirus and genital herpes simplex continue to be threats.
Tumors, injury, stroke, toxic substances (e.g., mercury), and certain over-the-counter and prescription drugs (e.g., streptomycin) are additional factors that can affect auditory pathways and the brain and lead to sensorineural deafness. Continued exposure to loud noise, as in certain industries or from loud music (see noise pollution), can result in damage to the inner ear, causing irreversible hearing loss. Presbycusis, or changes in hearing, especially of high frequencies, in adults has long been accepted as inevitable, but study of cultures where the phenomenon does not exist is bringing this into question. The hearing of patients with sensorineural deafness can sometimes be improved if the patient discontinues harmful medications or avoids exposure to loud noise, e.g., by wearing protective earplugs. In some cases, limited hearing has been restored by cochlear implants, tiny devices implanted into the inner ear that translate sound waves into electrical impulses that are then transmitted to the auditory nerve.
Mechanical and Educational Aids
Persons whose deafness cannot be relieved by medical or surgical means may be greatly helped by various types of electronic hearing aids. Those with hearing loss that cannot be relieved even by mechanical devices (i.e., those with sensorineural deafness) can have special training in speechreading (see lip reading). When deafness is present at birth or develops before a child has learned to speak, it is necessary also to provide specialized speech training and education in sign language, in which fingers and hands are the instruments of expression and communication. Schools and trained teachers for the hearing-impaired are now found in every large city in the world. Other aids for the hearing-impaired include specially trained "hearing dogs," customized telephones, and closed-caption television.
History of Education for the Deaf
Except for sporadic attempts by clerics in past centuries, there was no well-organized effort to help the hearing-impaired until the Abbé Charles Michel de l'Epée founded a school for the deaf in Paris in 1755. Samuel Heinicke established another one in Germany in 1778. The first public school for the deaf in the United States was founded (1817) in Hartford, Conn., by Thomas Hopkins Gallaudet; it is now called the American School for the Deaf. Alexander Graham Bell and his father, Alexander Melville Bell, did much to establish the study of speech on a scientific basis and to improve the methods of teaching the hearing-impaired. Educational and employment opportunities for the deaf have improved since passage of legislation in 1973 that prohibited discrimination against the handicapped by any institution receiving federal money and of the Americans with Disabilities Act in 1990.
See L. DiCarlo, The Deaf (1964); R. V. Harrison, The Biology of Hearing and Deafness (1988); A. P. Freeland, Deafness (1989).