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.
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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