deafness(redirected from toxic deafness)
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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 decibelsdecibel
, abbr. dB, unit used to measure the loudness of sound. It is one tenth of a bel (named for A. G. Bell), but the larger unit is rarely used. The decibel is a measure of sound intensity as a function of power ratio, with the difference in decibels between two sounds being
..... Click the link for more information. 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 earear,
organ of hearing and equilibrium. The human ear consists of outer, middle, and inner parts. The outer ear is the visible portion; it includes the skin-covered flap of cartilage known as the auricle, or pinna, and the opening (auditory canal) leading to the eardrum (tympanic
..... Click the link for more information. ), 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 fetaliserythroblastosis fetalis
, hemolytic disease of a newborn infant caused by blood group incompatibility between mother and child. Although the Rh factor is responsible for the most severe cases of erythroblastosis fetalis, the disease may be produced by any of the other blood
..... Click the link for more information. (Rh incompatibility) or anoxia (lack of oxygen during delivery). Before vaccines were available, German measles (rubellarubella
or German measles,
acute infectious disease of children and young adults. It is caused by a filterable virus that is spread by droplet spray from the respiratory tract of an infected individual.
..... Click the link for more information. ) and common measlesmeasles
, highly contagious disease typically contracted during childhood, caused by a filterable virus and spread by droplet spray from the nose, mouth, and throat of individuals in the infective stage.
..... Click the link for more information. (rubeola) were leading causes; maternal cytomegalovirus and genital herpes simplexherpes simplex
, an acute viral infection of the skin characterized by one or more painful, itching blisters filled with clear fluid. It is caused by either of two herpes simplex viruses: Type 1, herpes labialis,
..... Click the link for more information. continue to be threats.
Tumors, injury, stroke, toxic substances (e.g., mercurymercury
[from the Roman god Mercury], metallic chemical element; symbol Hg [Lat. hydrargyrum=liquid silver]; at. no. 80; at. wt. 200.59; m.p. −38.842°C;; b.p. 356.58°C;; sp. gr. 13.55 at 20°C;; valence +1 or +2.
..... Click the link for more information. ), and certain over-the-counter and prescription drugs (e.g., streptomycinstreptomycin
, antibiotic produced by soil bacteria of the genus Streptomyces and active against both gram-positive and gram-negative bacteria (see Gram's stain), including species resistant to other antibiotics, e.g.
..... Click the link for more information. ) 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 pollutionnoise pollution,
human-created noise harmful to health or welfare. Transportation vehicles are the worst offenders, with aircraft, railroad stock, trucks, buses, automobiles, and motorcycles all producing excessive noise. Construction equipment, e.g.
..... Click the link for more information. ), 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 aidshearing aid,
device used in some forms of deafness to amplify sound before it reaches the auditory organs. Modern hearing aids are electronic. They contain a tiny receiver and a transistor amplifier, and are usually battery powered.
..... Click the link for more information. . 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 readinglip reading,
method by which the deaf are able to read the speech of others from the movements of the lips and mouth. It is sometimes referred to as speech reading, which technically also includes the reading of facial expressions and body language.
..... Click the link for more information. ). 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 languagesign language,
gestural communication used as an alternative or replacement for speech. Sign languages resemble oral languages in every way other than their modality. As with oral languages, sign languages are acquired spontaneously and have highly intricate, rule-governed
..... Click the link for more information. , 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'EpéeEpée, Charles Michel, Abbé de l'
, 1712–89, French pioneer teacher of deaf-mutes. A Jansenist priest, he developed a manual system of communication for deaf-mutes and founded a school for their instruction in 1755.
..... Click the link for more information. 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 GallaudetGallaudet, Thomas Hopkins
, 1787–1851, American educator of the deaf, b. Philadelphia, grad. Andover Theological Seminary. In England and France he studied methods of education in schools for the deaf, and in Hartford, Conn.
..... Click the link for more information. ; it is now called the American School for the Deaf. Alexander Graham BellBell, Alexander Graham,
1847–1922, American scientist, inventor of the telephone, b. Edinburgh, Scotland, educated at the Univ. of Edinburgh and University College, London; son of Alexander Melville Bell.
..... Click the link for more information. and his father, Alexander Melville BellBell, Alexander Melville,
1819–1905, Scottish-American educator, b. Edinburgh. Bell worked out a physiological or visible alphabet, with symbols that were intended to represent every sound of the human voice.
..... Click the link for more information. , 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).
the complete absence of hearing or a degree of diminution of it so that discrimination of speech sounds becomes impossible. Complete deafness is found rarely; in most deaf persons there are remnants of hearing that permit perception of very loud sounds, including also some speech sounds, and sometimes even a few very familiar words and phrases pronounced loudly near the ear (deafness bordering on hardness of hearing).
The cause of deafness is most often a disease process in the inner ear and the auditory nerve, arising either as a complication of a middle-ear infection or as a consequence of certain infectious diseases (cerebrospinal meningitis, influenza, mumps, measles, and scarlet fever). In some cases a progressive diminution of hearing owing to otosclerosis may lead to deafness. Sometimes deafness arises with prolonged action of loud noise and vibration, and also with intoxication by certain substances such as arsenic, mercury, or lead. In prerevolutionary Russia, as a result of poor organization or the absence of protection for laborers, deafness developed especially frequently among boiler-makers and weavers (noisy industries).
Deafness may also be congenital. It may arise under the influence of genetic (hereditary) factors, as a result of the effect of infection on the developing fetus, or as a result of intoxication of the mother’s body. Congenital deafness, as well as deafness acquired in early childhood, deprives the child of the ability to master speech independently. With deafness that develops at a later age, voice modulation changes and pronunciation defects appear, but the speech as a whole does not suffer.
Speech communication of deaf persons with those around them may be significantly facilitated by means of mastering the skills of visual perception of speech (lip reading) and, when there are substantial remnants of hearing, by the use of sound-amplifying devices. Treatment of deafness in most cases is not very effective. In otosclerosis, and also in deafness associated with the aftereffects of inflammatory processes in the middle ear, improvement of hearing is sometimes achieved by surgical treatment. Prophylaxis of deafness consists in prevention and timely treatment of diseases that lead to persistent disruption of hearing. Principally significant in prevention of congenital deafness is pregnancy hygiene. Marriage between congenitally deaf persons is not recommended.
REFERENCETemkin, Ia. S. Glukhota i tugoukhost’. Moscow, 1957.
L. V. NEIMAN