Types and Causes of Hearing Loss Notes

Auditory Defects Types And Causes Of Auditory Defects

The auditory defects may be either partial or complete. The auditory defects are of two types:

1. Conduction deafness
2. Nerve deafness.

1. Conduction Deafness: Conduction deafness is the type of deafness that occurs due to impairment in the transmission of sound waves in the external ear or middle ear.

• Causes of conduction deafness:
  • Obstruction of external auditory meatus with dry wax or foreign bodies
  • Thickening of eardrum due to repeated middle ear infection

Read And Learn More: Medical Physiology Notes

• • • Perforation of eardrum due to inequality of pressure on either side
    • Otitis media (inflammation of the middle ear)
    • Otosclerosis (fixation of the footplate of stapes against the oval window) due to ankylosis. Ankylosis means the abnormal immobility and consolidation of a joint.

2. Nerve Deafness: It is the deafness that is caused by damage of any structure in the cochlea such as hair cells, organs of Corti, basilar membrane or cochlear duct or the lesion in the auditory pathway.

• Causes for Nerve Deafness:
  • Degeneration of hair cells due to some antibiotics like streptomycin and gentamicin
  • Damage of the cochlea by prolonged exposure to loud noise
  • Tumor affecting 8 cranial nerves.

Tests For Hearing

There are various tests to assess the sensation of hearing. However, some simple tests called bedside tests are usually carried out 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 a watch test.

1. 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, a hearing deficit is suspected.

2. Tickling of Watch Test: A wristwatch with a tickling sound is kept near the ear of the subject. The subject suffering from hearing defects cannot hear the tickling sound of a watch.

• Routine Tests for Hearing: The routine tests for hearing are of three types.

1. Rinne’s test
2. Weber’s test
3. Audiometry.

The 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 the auditory defect is determined. By audiometry, both the nature and severity of the auditory defects can be determined.

1. Rinne’S Test:

• The base of the vibrating tuning fork is placed on the mastoid process until the subject cannot feel the vibration and cannot hear the sound. When the subject does not hear the sound anymore, the tuning fork is held in the air in front of the ear of the same side.
• A normal person hears vibration in the 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 the air are not heard after cessation of bone conduction.
• Thus, in conduction deafness, bone conduction is better than air conduction. In nerve deafness, both air conduction and dv. Conduction is diminished or lost.

2. Weber’S Test:

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

3. Audiometry:

• Audiometry is the technique used to determine the nature and severity of auditory defects. An instrument called an audiometer is used. The instrument is an electronic function generator or oscillator connected to an earphone.

• This instrument is capable of generating sound waves of different frequencies from lowest to highest. The intensity (loudness or volume) of sound at each ‘once is adjusted on the basis of previous studies in normal persons.
• Thus, before calibrating the instrument, the minimum (threshold) volume or intensity or loudness for each frequency of sound heard by normal persons is determined. The minimum intensity is set in the instrument as zero. Now, while testing the patient, the loudness is increased above zero level. The intensity of sound is expressed in decibels (dB).
• At a particular frequency, if the patient hears a sound with a loudness of 30 dB above zero level, the person is said to have a hearing loss of 30 dB for that particular frequency. During the tests by an audiometer, the subject’s ability to hear the sounds with 8-10 different frequencies is observed and, the hearing loss is determined for each frequency.
• By using these values, the audio gram is plotted. The audiometer also has an electronic vibrator. It is to test the bone conduction from the mastoid process into the cochlea.