Auditory defects

TYPES AND CAUSES OF AUDITORY DEFECTS

Auditory defects may be either partial or complete.

Auditory defects are of two types:

1. Conduction deafness

2. Nerve deafness.

Auditory defects

CONDUCTION DEAFNESS

Conduction deafness is the type of deafness that occurs due to impairment in transmission of sound

waves in the external ear or middle ear.

Causes for Conduction Deafness

i. Obstruction of external auditory meatus with dry wax or foreign bodies

ii. Thickening of tympanic membrane due to repeated middle ear infection

iii. Perforation of tympanic membrane due to inequality of pressure on either side

iv. Otitis media (inflammation of middle ear)

v. Otosclerosis (fixation of footplate of stapes against oval window) due to ankylosis.

Ankylosis means the abnormal immobility and consolidation of a joint.

NERVE DEAFNESS

Nerve deafness is the deafness caused by damage of any structure in cochlea, such as hair cell, organ of

Corti, basilar membrane or cochlear duct or the lesion in the auditory pathway.

Causes for Nerve Deafness

i. Degeneration of hair cells due to some antibiotics like streptomycin and gentamicin

ii. Damage of cochlea by prolonged exposure to loud noise

iii. Tumor affecting VIII cranial nerve.

TESTS FOR HEARING

There are various tests to assess the sensation of hearing. However, some simple tests called bedside

tests are usually carried before doing conventional types of hearing tests. Such simple tests are useful to

know whether the hearing is normal or less.

Bedside tests:

1. Whispering test

2. Tickling of watch test.

Whispering Test

The examiner stands about 60 cm away from the subject at his side and whispers some words. If the

subject is not able to hear the whisper, then hearing deficit is suspected.

Tickling of Watch Test

Wrist watch with tickling sound is kept near the ear of the subject. The subject suffering from hearing defects cannot hear the tickling sound of watch.

Routine Tests for Hearing

Routine tests for hearing are of three types:

1. Rinne test

2. Weber test

3. Audiometry.

First two tests are done by using a tuning fork with high frequency. Mostly, a tuning fork with 512 cycles per second is used. By turning fork tests, only the nature of auditory defect is determined. By audiometry, both nature and severity of auditory defects can be determined.

RINNE TEST

Base of a vibrating tuning fork is placed on mastoid process, until the subject cannot feel the vibration and

cannot hear the sound. When the subject does not hear the sound any more, the tuning fork is held in air in front of the ear of same side. Normal person hears vibration in air even after the bone conduction ceases because, in normal conditions, air conduction via ossicles is better than bone conduction. But in conduction deafness, the vibrations in air are not heard after cessation of bone conduction. Thus in conduction deafness, the bone conduction is better than air conduction. In nerve deafness, both air conduction and bone conduction are diminished or lost.

WEBER TEST

Base of a vibrating tuning fork is placed on the vertex of skull or the middle of forehead. Normal person hears the sound equally on both sides. In unilateral conduction deafness (deafness in one ear), the sound is heard louder in diseased ear. In unaffected ear, there is a masking effect of environmental noise. So, the sound through bone conduction is not heard as clearly as on the affected side. In affected side, the sound is louder due to the absence of masking effect of environmental noise. During unilateral nerve deafness, sound is heard louder in the normal ear.

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