Organ of Corti
o Sense organ of hearing, It is situated on the basilar
o Important components:
• Tunnel of Corti: Formed by inner and outer rods.
• Hair cells: Receptor cells of Hearing
Transfers sound energy into electrical energy.
Inner hair cells are supplied by afferent cochlear
Outer hair cells receive efferent innervation from
• Supporting Cells:
Dieters` cells: provide support to outer hair
Cells of Hensen: lie outside Dieters` cells.
• Tectorial Membrane: Overlies the organ of
corti. Shearing force between the hair cells
and tectorial membrane produces the
stimulus to hair cells.
Structure of organ of corti
Nerve supply of Hair cells
• 95% of Afferent fibres of spiral ganglion supply
the inner hair cells, while 5% supply the outer
• Efferent fibres to the hair cells come from
olivocochlear bundle whose cell bodies are
situated in superior olivary complex.
Dendrites of bipolar cells
of spiral ganglion
Axons of bipolar cells form
cochlear division of CN VIII
Superior olivary complex
Nucleus of lateral lemniscus
Medial geniculate body
Auditory cortex (area 41)
Mechanism of Hearing
• Mechanical conduction of sound
Pinna EAC Tympanic Memberane
• Chain of Ossicles Stapes Foot Plate
Pressure changes in inner ear fluid
Movement of basilar membrane
Stimulation of hair cells.
• Tranduction of mechanical energy to electrical
Movement of basilar membrane sets up
shearing force between tectorial membrane
and hair cells.
Distortion of hair cells gives rise to cochlear
microphonics which trigger the nerve impulse.
• Neural Pathways.
Frequently used terms
• Frequency: Number of cycles per second
• Pure tone: Single frequency sound
• Complex sound: Sound with more than one
• Decibel (dB): Unit to determine intensity of
sound. It is 1/10th of a bel, and named after
• Masking: Phenomenon to produce inaudibility
of one sound by the presentation of the other.
Types of Hearing Loss
• Conductive hearing loss: caused by
interference of sound conduction from
external ear to stapediovestibular joint.
• Sensorineural hearing loss: results from either
lesion of cochlea (sensory type) or from lesion
of VIIIth nerve and its connections (neural
type). Neural can be further divide as:
Retrocochlear: lesion of VIIIth nerve,
Central: Lesion of central auditory
• Mixed Hearing loss:
Elements of both conductive and
sensorineural deafness are present in the
No organic lesion present. It is either
psychogenic or due to malingering (usually
involved in litigation)
Assessment of Hearing
Tuning Fork tests:
• Performed with tuning forks of different
• Most Commonly used tuning fork has a
frequency of 512Hz.
• AC measures both the conducting mechanism
• BC measures cochlear function only.
• AC is compared with BC
• Vibrating tuning fork is placed on patient`s
mastoid and when he stops hearing, it is
placed erect and in line with the EAC, about
2cm from the orifice.
• If he still hears AC is more than BC.
• It is called positive when AC is longer and
louder than BC. It is seen in normal persons or
those having sensorineural deafness.
• Negative Rinne ( BC > AC) is seen in
• False negative Rinne test:
• patient has no hearing in test ear.
• BC stimulus may be perceived by contralateral
• No hearing by AC the result is labeled as Rinne
• This mistaken impression of function in a non
functioning ear is called false negative Rinne
• Tuning fork is struck and base placed on either
the forehead, vertex or upper incisor teeth.
• Patient is asked where the sound is heard the
• Normal hearing person: Sound is heard
equally in both ears.
• Conductive deafness: Lateralized to worse ear.
• SN deafness: Lateralized to better ear.
• Lateralized weber in conductive deafness may
indicate a hearing loss of 10-15 dB.
Modified Schawabach Test(ABC test)
• Compares the bone conduction of patient
with that of a normal hearing
• Tuning fork is placed on the patients mastoid
with the meatus blocked and when the
patient no longer hears it, it is placed on the
normal hearing person`s mastoid.
• Conductive deafness + Normal hearing
person: Patient and examiner hear the fork for
the same duration.
• SN deafness: Patient hears the fork for a
• Test of BC, and examines the effect of occlusion
of ear canal on hearing.
• Vibrating tuning fork is placed on the mastoid
while the examiner alternately closes and opens
the ear canal by pressing tragus inward.
• Normal person+ SN hearing loss: Hears louder
when ear canal is occluded and softer when canal
is open. (Bing +)
• Conductive hearing loss: No change appreciated
( Bing -).
• Test of BC and examines the effect of
increased air pressure in ear canal on hearing.
• Vibrating tuning fork is placed on the mastoid
while air pressure in ear canal is changed by
• Normal person + SN hearing loss: Hearing is
decreased ( Gelle`s +).
• Conductive deafness: No change (as ossicular
chain is fixed or disconnected) (Gelle`s -).
Pure Tone Audiometry
• An audiometer is an electronic device which
produces pure tones, intensity of which can be
increased or decreased in 5 dB steps.
• AC thresholds are measured for tones of
• BC thresholds are measured for tones of
• It is charted in the form of a graph called
• Patient should be visible to the tester, and
respond by signaling.
• No visible or tactile clues should be available
to the patient regarding the hearing stimulus.
• Test should be conducted in a sound proof
• Duration of presentation should be 1-3 sec.
• The head phones should be properly seated
over the external auditory canal.
• The audiometer should be properly checked
before performing the test.
• Before placing the ear phones on the patient,
the patient's ear should be examined for the
presence of wax. If present it should be
• Pure tone air conduction threshold is tested using
• “Up 5-down 10" method of threshold estimation:
This technique is based on Hughson - Westlake
• Tones of short duration is used for threshold
estimation. This method of threshold estimation
involves the following steps:
• Step I : The better ear is tested first in order to
determine the need for masking.
• Step II : Start with a 1000 Hz tone at a level above
the threshold to allow easy identification of the
tone. If the patient is suspected to be having a
profound hearing loss then the testing should be
started with 250Hz frequency. This is because of
the fact that the individuals with profound
hearing loss often have testable hearing only in
the low frequency range.
• Step III : The patient's understanding of the
listening task should be checked by using both
short and long duration test tones. The patient
should be instructed to raise the index finger as
soon as the sound is heard.
Step IV : During testing, the examiner should vary
the interval between tone presentations to avoid
telegraphing the stimulus.
• Step V : The starting intensity of the test tone is
reduced in 10 dB steps following each positive
response, until a hearing threshold level is
reached at which the subject fails to respond.
Then, the tone is raised by 5 dB until tone is
heard.The threshold is defined as the faintest
tone that can be heard 50% or more of the
time, and is established after several threshold
• Second ear is then test in the same way with
the retest beginning at 1K Hz.
• If there is difference in AC threshold exceeding
40dB at any frequency masking should be
• Bone Conduction level are measured between
250Hz and 4K Hz.
• BC is tested using a bone vibrator.
• The auditory threshold is assessed as
described for air conduction assessment. The
only difference is that the better hearing ear
should be masked using a masking tone
delivered via a head phone.
Scale of hearing threshold estimation in
dB loss Description
- 10 to 15 Normal limits
16 - 19 Mild hearing loss
30 - 44 Moderate hearing loss
45 - 59 Moderately severe hearing loss
60 - 79 Severe hearing loss
80 and above Profound hearing loss
Nomenclature on the pure tone
• O right air conduction
• X left air conduction
• [ masked right bone conduction
• ] masked left bone conduction
loss, low tone
Otitis media with
A conductive loss
maximal in the low
frequencies with now
Conductive hearing loss
Conductive hearing loss
across the audiometric
range with an increase in
the BC threshold
(caharts`notch) at 2kHZ.
Caharts` notch reflects a
loss in the inertial
component of stapes
Sensorineural hearing loss, low tone
Sensorineural hearing loss with a
notch at 6 kHz (noise induced hearing
• Patient`s ability to hear and understand
speech is measured.
• Two parameters are studied:
a. Speech Reception threshold
b. Discrimination score
Speech Reception Threshold(SRT)
• Minimum Intensity at which 50% words are
repeated correctly by the patient.
• Set of spondee words is delivered to each ear.
• Intensity is varied in 5dB steps till half of them are
• Normally SRT is within 10dB of the average of
pure tone threshold of 3 speech
freqeuncies(500, 1K, 2K Hz).
• SRT better than pure tone average by more than
10db suggests a functional hearing loss.
• Measure of patient`s ability to understand
• List of phonetically balanced (PB) words
(single syllable) is delivered to each ear
separately at 30 to 40 dB above his SRT and %
of words correctly heard is recorded.
• In normal persons and with CHL score of 90 to
100% can be obtained.
• Self recording audiometry where various pure
tone frequencies automatically move from low to
high while patient controls intensity through a
• Two tracings one with continuous and other with
pulsed tone are obtained.
• They help to differentiate a cochlear from
retrocochlear and organic from functional
• It is rarely done these days.
• The primary purpose of impedence
audiometry is to determine the status of the
tympanic membrane and the middle ear.
• It is divided into 2 parts:
b. Acoustic reflex measurements.
• Altering the pressure in the EAC results in
changes in compliance because the TM is tense
and ossicular chain stiffened.
• Point of maximum compliance occurs when the
pressure in the meatus is equal to that in the
• If the pressure exceeds or falls below the middle
ear pressure, compliance will be reduced.
• Indirectly middle ear pressure can be calculated
from point of maximum complaince.
• Probe is inserted into the external auditory canal till a air
tight seal is obtained. Probe tone is presented typically at
226Hz into the ear canal while the air pressure of the
canal is altered between +200 and - 400 decapascals. The
maximum compliance occurs when the pressure of the
external auditory canal and the middle ear becomes
• The compliance peak indicates the pressure of the
• The height of the compliance peak indicates the mobility
/ stiffness of the tympanic membrane or the middle ear
• By charting the compliance of tympanoossicular system
various pressure changes different types of
tympanograms are obtained.
Types of tympanograms:
• Type A: Normal Tympanogram
• Type As: Reduced compliance at or near
ambient air pressure, seen in fixation of
• Type Ad: Increased compliance at or near
ambient pressure seen in ossicular
discontinuity or thin or lax TM.
• Type B: Flat or dome shaped graph. No change
in compliance with pressure changes. Seen in
middle ear fluid or thick TM.
• Type C: Maximum compliance at pressures
more than -100mmH2O (negative pressure in
• A loud sound, 70-100dB above the threshold
of hearing of a particular ear causes bilateral
contraction of the stapedial muscle which can
be detected by Tympanometry.
• Reflex arc is the VIIIth nerve, cochlear nucleus
and complex brainstem internuclear
connections to the ipsilateral and contralateral
facial nuclei, facial nerve and the nerve to
• Minimal auditory stimulus that produces a
contraction of stapedius muscle is known as
acoustic reflex threshold and indicated on an
audiogram by the letter ‘Z’.
Physical volume of ear canal
• Acoustic immitance can measure the phsyical
volume of air between probe tip and TM.
• Normally it is upto 1ml in children and 2ml in
• If > 2ml in children and > 2.5ml in adults,
indicates perforation of TM.
Short Increment Sensitivity Index
• The SISI test is still widely used to determine
whether the patient is having cochlear
pathology. This test is based on a
phenomenon known as recruitment
(abnormal loudness growth).
• Recruitment: The ear which does not hear low
intensity sounds begins to hear greater
intensity sounds as loud or even louder than
normal hearing ear.
• A continous tone is presented 20dB above the
threshold and sustained for two minutes.
• Every 5 sec tone is increased by 1dB and 20
such blips are presented. Patient indicates the
• In conductive deafness SISI score is more than
15%, 70-100% in cochlear deafness, 0-20% in
Evoked Response Audiometry
• It measures electrical activity in the auditory
pathways in response to auditory stimuli.
• Two most important component of evoked
electrical response are:
b. Auditory brainstem response (ABR).
• It measures electrical potential arising in the
cochlea and CN VIIIth in response to auditory
stimuli within first 5 milliseconds.
• There are three classes of potentials that can be
recorded .They are compound action potential of
auditory nerve (AP), Summating potential
(SP), and cochlear potential (CP) also known as
• Recording electrode is usually a thin needle
passed through the TM onto the promontory.
• In Adults it can be done under LA, but in
children or anxious individuals sedation or GA
• It is useful:
a. To find threshold of hearing in young infants
b. To differentiate lesions of the cochlea from
those of CN VIII.
Auditory brainstem response (ABR)
• Also known as Brain stem evoked response
• BERA is resistant to the effects of
sleep, sedation, sleep and anesthesia.
• BERA is an objective way of eliciting brain
stem potentials in response to audiological
click stimuli. These waves are recorded by
electrodes placed over the scalp.
• Since the electrodes should be placed over
the head, the hair must be oil free.
• The standard electrode configuration for BERA
involves placing a non inverting electrode over
the vertex of the head, and inverting
electrodes placed over the ear lobe or mastoid
• One more earthing electrode is placed over
• In brain stem evoked response audiometry,
the impulses are generated by the brain stem.
• These impulses when recorded contains a
series of peaks and troughs.
• The positive peaks (vortex positive) are
referred to by the Roman numerals I - VII.
• These peaks are considered to originate from
the following anatomical sites:
1. Cochlear nerves - waves I and II
2. Cochlear nucleus - wave III
3. Superior olivary complex - wave IV
4. Nulclei of lateral lemniscus - wave V
5. Inferior colliculus - waves VI and VII
• Uses of BERA:
I. As a screening procedure for infants.
II. To determine the threshold of hearing in children and
adults who do not cooperate and in malingerers.
III. To diagnose retrocochlear pathology particularly
IV. To diagnose brainstem pathology e.g multiole
sclerosis or pontine tumors.
V. To monitor CN VIII intraoperatively in surgery of
Otoacoustic emissions (OAE)
• Sounds produced by motile elements of
cochlear outer hair cells.
• Can be elicited by a very sensitive microphone
placed in EAC.
• Sounds produced by outer hair cells travel in a
reverse direction: outer hair cells Basilar
Memberane perilymph oval window
ossicles TM EAC.
• OAEs are present when outer hair cells are
healthy and absent when they are damaged, thus
help test function of cochlea.
1. To screen children and neonates for hearing
2. Estimate hearing sensitivity within a limited
range of frequencies
3. To differentiate sensory and neural components
in sensorineural hearing loss
4. To rule out malingering (functional hearing loss).
Hearing assessment in Infants and
• They are employed to test hearing in high risk
infants and are based on infants behavioral
response to sound stimuli.
• Arousal test: High frequency noise is
presented for 2 secs to the infant when he is
in light sleep. Normal hearing infant can be
aroused twice when 3 such stimuli are
• Auditory response cradle: screening device for
newborns where baby is placed in a cradle
and his behaviour in response to auditory
stimulation are monitored by transducers.
Behaviour Observation Audiometry
• Auditory signal presented to an infant produces a
change in behaviour e.g alerting, cessation of an
activity or widening of eyes.
• Moro`s reflex: sudden movement of limbs and
extension of head in response to sound of 80-90
• Cochleopalpebral reflex: Child responds by a blink
to aloud sound.
• Cessation reflex: Infant stops activity or starts
crying in response to a sound of 90 dB.
Behaviour Observation Audiometry
• This test technique is suited to infants aged 7 or 8
months to 3 years developmentally.
• The child is taught (i.e., conditioned) to turn their head
when a sound is heard.
• Initial conditioning is achieved by the introduction of
stimuli at moderately high levels
• When child looks for source of sound they are shown a
colourful, moving puppet or toy under illumination as a
• This puppet or toy "reinforces" the child's
turning behaviour or orientation and gives rise
to the term Visual Reinforcement Orientation
Audiometry. Once this conditioned response is
reliably observed, the stimuli can be
presented at ever decreasing levels until
auditory threshold or minimum audible levels
have been reached.
• Play audiometry is suited to children aged around 3 to
7 years developmentally.
• The child is taught to respond, using a pre-determined
task, whenever they hear tonal stimuli that are
introduced through headphones or through a bone
conductor placed behind the ear on the mastoid.
• Given sufficient cooperation from the child it is usually
possible to produce a complete and accurate
"audiogram" that illustrates their threshold of hearing
for a pre-determined frequency range.
• Child is asked to repeat the names of certain
objects or to point them out on the pictures.
• Voice can be gradually lowered.
• In this way hearing level and speech
discrimination can be tested.
• Evoked Response Audiometry:
b. Brainstem Evoked Response Audiometry
• Otoacoustic Emmisions.
• Impedance Audiometry.