This document provides information on hearing tests and deafness. It begins by classifying deafness into conductive, sensorineural, and mixed types. Conductive deafness occurs when sound transmission to the cochlea is impaired, while sensorineural deafness results from damage to the cochlea or auditory nerve. Mixed deafness affects both the middle and inner ear. Common causes of each type are described. The document then explains various hearing tests, including tuning fork tests, pure tone audiometry, and brainstem auditory evoked responses, which assess the functional status of the auditory pathway. Treatment options for deafness such as hearing aids and cochlear implants are also summarized.

1. D R . G . R E E N A P R A S O O N A
A S S T P R O F E S S O R
PATHOPHYSIOLOGY OF DEAFNESS
AND HEARING TESTS

2. The student will be able to: (MUST KNOW)
1. Classify hearing defects and name the causes for each defect.
2. Give the physiological basis of each test to assess hearing defects.
3. Understand physiological basis of treatment of deafness.
4. Name the hearing tests, briefly describe the procedure, and interpretation
of result of each test.
5. Understand the principle of audiometry and BAEP.
LEARNING OBJECTIVES

3. Deafness
Deafness is divided into the following types:
conductive deafness
and sensorineural deafness.
When both conductive and sensori neural deafness are present, it is
known as mixed deafness.
HEARING DEFECTS

4. Conductive Deafness
Conduction deafness occurs due to impaired transmission or amplification
of sound to the cochlea due to disease of the external or middle ear. The
sound waves are conducted to the cochlea by the skull bones (bone
conduction) and produce vibration of the basilar membrane. So, the hearing
loss is usually partial. The common causes are:
1. Blockade of external auditory canal: This occurs as a result of
impaction of wax, or foreign bodies.
2. Thickening of the tympanic membrane following repeated
middle ear infections. Sometimes perforation may occur due to infection
or trauma.
3. Acute and chronic otitis media

5. 4. Immobility of the ossicles. This may occur as a result of
adhesions due to repeated middle ear infections or bony ankylosis.
When the stapes is fixed to the oval window in an abnormally rigid
manner, it is known as otosclerosis.
5. Destruction of the auditory ossicles.
6. Eustachian tube obstruction

6. Sensorineural Deafness
This occurs as a result of
diseases of the cochlea or of the 8th cranial nerve. The degree of
hearing impairment depends on the extent to which the structures are
affected.
When deafness occurs due to lesion of the cochlear nuclei or their
central connections, it is known as central deafness. It occurs rarely and
is associated with marked neurological deficits.

7. The common causes of sensorineural deafness are:
1. Toxic degeneration of the hair cells caused by chronic treatment with
drugs such as aminoglycoside antibiotics (streptomycin, kanamycin, and
gentamycin).
These drugs block the mechanosensitive channels in the stereocilia of hair
cells, subsequently leading to their degeneration.
They also affect the hair cells of the vestibular apparatus, so nerve deafness
along with abnormal vestibular functions occurs.
Some other drugs like salicylates, quinine, cytotoxic drugs, and certain diuretics
also produce deafness.
2. Senile degeneration of the hair cells: There occurs gradual cumulative loss
of hair cells and neurons due to effect of aging. This is known as presbycusis.
It is seen in about one-third of population over 75.

8. 3. Damage to the outer hair cells by prolonged exposure to
loud noise, e.g. loud music, revved up motorcycles, and engine roar
of jet planes. Usually, deafness begins with loss of sensitivity for
high-intensity sounds.
4. Tumors of the vestibulocochlear nerve (acoustic neuroma) or
tumors in the cerebellopontine angle pressing on the cochlear nerve.
5. Trauma or fracture of the base of the skull.
6. Labyrinthitis.
7. Meniere’s disease is characterized by episodic vertigo,
deafness, and tinnitus.

9. Mixed Deafness
Mixed deafness can be due to trauma, infection or tumor that affects both middle
ear and inner ear. Deafness due to genetic mutations is classified into syndromic
(when it is associated with abnormalities in other systems) and nonsyndromic
(when deafness is the only abnormality). The mutations can occur in the following
proteins and cause nonsyndromic deafness.
• Connexin 26, which helps in normal recycling of K+ through supporting cells
• Myosin-VIIa, which is associated with actin in the hair cell processes
• Myosin-Ib, which causes movement of the cation channels at the tip links
• Myosin-VI, which plays a role in the formation of the cilia
• α-tectin, which is the major protein in tectorial membrane.

10. Mutations in the following proteins cause syndromic deafness.
• Sulfate transport protein: Its mutation results in Pendred’s syndrome
(deafness and goiter).
• KVLQT1, a K+ channel protein that is present in the stria vascularis and
maintains the high K+ concentration of the endolymph and a normal QT
interval in the heart.
Its mutation produces long QT syndrome (deafness and a longer QT interval that
predisposes to ventricular arrhythmias).
• Barttin, the mutation of which causes deafness and renal manifestation of
Bartter syndrome.
Tinnitus: It is the perception of sound in the absence of any sound in the
environment. It may be buzzing, roaring or ringing type, and can be pulsatile. It is
usually associated with conductive or sensorineural deafness.

11. Treatment of Deafness
Complete restoration of hearing is possible in cases, where the cause
can be treated successfully, for example removal of wax or tumor and
treating otitis media with antibiotics.
HEARING AIDS: Hearing aids are useful to restore the hearing
capacity to some extent in cases of deafness due to partial decrease in
functions of middle ear or cochlea. These are electronic devices
attached to the ear that amplifies the sound waves.

12.  COCHLEAR IMPLANTS: These are tiny electronic devices implanted
under the skin over the mastoid bone.
 They translate the sound waves into electrical signals that are directly
transmitted to the auditory pathways bypassing the damaged structures.
 As they take up the role of hair cells in sound transduction, they are also
called artificial ears.
 The implants are used for people with deafness due to disease or injury
that has destroyed hair cells of organ of Corti.
 These days cochlear implants are quite successful to restore almost normal
hearing.

13. In a patient with complain of deafness, the external ear is checked for
wax or foreign body and if present, they are removed.
The tympanic membrane is examined and if there is any infection in
the middle ear, it is treated with suitable antibiotics.
The hearing distance of a conversational voice is tested in each ear
separately at a distance of about 3.5 m.
This is followed by tuning fork tests (using forks vibrating at 256 or 512
Hz), named after the men who developed them, Rinne, Weber, and
Schwabach.
TESTS FOR HEARING

14. Tuning Fork Tests
Rinne Test
Normally, sound conducted through air and the ossicular system is louder than
that conducted through skull bones to the inner ear.
1. First, the bone conduction (BC) is tested by placing the base of the vibrating tuning fork
on the mastoid process.
The subject hears the sound and indicates by raising his finger, when he no longer
hears the sound.
2. The vibrating tuning fork is then held near the ear and a normal person continues to
hear the sound for some time. This tests the air conduction (AC).
3. So, AC is more than BC in a normal person or in partial nerve deafness.
4. In case of conduction deafness, bone conduction is normal but he does not hear any
sound when the fork is placed near the ear. So, BC is more than AC.
5. In case of total nerve deafness, AC and BC both are absent and no sound is heard.

16. Weber Test
Normally, environmental noise is transmitted to the inner ear by
ossicular conduction along with the sound the person listens to, and
tends to mask it; but it has no effect on sound conducted through the
bone of the skull.
1. If the base of a vibrating tuning fork is placed at the center of the
forehead, or the vortex of the skull, a normal person will hear it
equally in both ears, but a person with conduction deafness
will hear it louder in the diseased ear, because the environmental
noise does not reach the inner ear of that side.
2. On the other hand, the subject with nerve deafness hears the
sound louder in the normal ear.

18. Schwabach Test
This test employs the same principle as the Weber test. The bone
conduction of the patient is compared with that of a normal person. If
the bone conduction of the patient is better than normal, he has
conduction deafness, but if his bone conduction is less than
normal, he has nerve deafness.

19. Audiometry
The assessment of hearing using electronic equipments is known as audiometric
test. The tests can be subjective (results are based on patient’s answer) and
objective.
PURE TONE AUDIOMETRY: This is one of the most generally used subjective
tests for assessing hearing. The machine is called audiometer and the recording is
called audiogram. This test is useful to detect or rule out conduction deafness.
1. The audiometer emits pure tones of various frequencies, which the
subject listens to through the earphones (for air conduction) or by an electronic
vibrator fixed to the mastoid process (for bone conduction).

20. 2. The test is performed in a soundproof room and each ear is tested separately
beginning with the better ear. A total of 6–10 different tones with frequencies
ranging from 250 Hz to 8,000 Hz are tested and the threshold decibels against
each tone are plotted for each ear.

22.  SPEECH AUDIOMETRY: Other subjective hearing tests are
speech audiometry (measures the patient’s ability to recognize
and repeat correctly lists of words that are presented to him),
 alternate binaural loudness balance test,
 loudness discomfort level test,
 and tone decay test.

24. Brainstem Auditory Evoked Responses (BAERs)
The potentials recorded from the auditory pathway in response to a
brief auditory stimulation are known as BAERs. As the stimulus travels
from the cochlea to higher up, it generates action potentials in all the
nerve fibers. BAERs assesses functional status of the auditory pathway
up to the midbrain.

25. 1. The waveforms of BAEP are named I, II, III, IV, and V (Fig. 173.1).
2. Wave I originates from peripheral portion of 8th cranial nerve adjacent to the
cochlea.
3. Wave II arises from cochlear nucleus. Action potentials in the superior olivary
nucleus generate wave III.
4. Wave IV arises from lateral lemniscus.
5. Wave V originates from inferior colliculi. Absence or reduced amplitude of a
waveform indicates lesion of the area that gives rise to that wave.
BAEP is useful in assessing hearing loss in infants and small children.
It is also useful in localizing brainstem lesions, diagnosing tumors at various
levels of the auditory pathway, coma, brain death, and strokes affecting the
brainstem.