Audiometry is one of the essential topic in MBBS.
This presentation helps students to learn about basic audiometry for MBBS level and shall equally be useful for postgraduate ENT students, too.
2. • Pure Tone Audiometry : Measuring relative hearing threshold
using pure tones
• Hearing threshold : lowest (softest) sound level needed for a
person to detect a signal approximately 50% of the times
• Audiometer : device used in the measurement of auditory
threshold
• Patients’ hearing threshold is measured in comparison to
ideal fixed normal hearing level (0 dB) and thus is relative
hearing threshold measurement
• PTA is subjective test of hearing
3. Why Pure Tone thresholds?
• The auditory system is organized tonotopically in the cochlea :
High frequencies represented at the basal end and low
frequencies at the apical end of the basilar membrane
• Damage to sensory cells of the cochlea at a specific place
along the basilar membrane can result in a loss of hearing that
corresponds to the frequencies coded by that place
• Pure tone threshold tests provide details that would otherwise
remain unknown if a broadband stimulus such as speech were
used
6. Hearing Threshold Estimation
• Hughson and Westlake technique (5 up 10 down)
− Better ear tested first
− Air conduction measured for 1K, 2K, 4K, 8K, 500, 250 and
125 Hz via head phone
− Bone conduction measured for 1K, 2K, 4K, 500 and 250 Hz
via bone vibrator with masking of other ear
− If difference between these octaves is >20dB then half
octaves i.e. 750, 1500/3000/ 6000 Hz tested
7. • 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, if the subject hears this
increment, the tone is reduced by 10 dB and if the tone is not
heard then it is raised by another 5 dB increment
• This 5 dB increment is always used if the preceding tone is not
heard, and a 10 dB decrement is always used when the sound
is heard
• 2 correct responses out of 3 is acceptable and plotted on the
graph
15. Diagnosis of type of deafness
Type Air
Conduction
Bone
Conduction
Air bone gap
Conductive Worsened Normal Present
Sensori-
neural
Worsened Worsened Absent
Mixed Worsened Worsened Present
22. Uses of pure tone audiogram
1. To find type of hearing loss
2. To find degree of hearing loss
3. For prescription of hearing aid
4. Predict hearing improvement after ear surgery
5. To predict speech reception threshold
6. A record for medico-legal reference
23. Speech Audiometry
• Speech Reception Threshold (S.R.T.)
– Minimum intensity at which 50% of spondee
(disyllable with equal stress) words are correctly
identified
– Falls normally within 10 dB of Pure Tone Average
• Speech Discrimination Score (S.D.S.)
– Percentage of phonetically balanced (single
syllable) words correctly identified at 40 dB above
S.R.T.
24. •Uses of Speech Audiometry
–Differentiate between cochlear and retro-
cochlear lesions
–Volume of hearing aid fixed at PB max
score
–In functional deafness : SRT > + 10 dB of
pure tone average
26. Speech Discrimination
Hearing loss Speech understanding
0 – 25 dB No difficulty with faint speech
26 – 40 dB Difficulty with faint speech only
41 – 55 dB Difficulty with faint + normal speech
56 – 70 dB Difficulty even with loud speech
71 – 91 dB Only understands amplified speech
> 91 dB Can’t understand amplified speech
28. Tests for Recruitment
• Recruitment is abnormal growth in loudness with
increasing frequency of sound
• Tests of recruitment are done to diagnose cochlear
pathology
• Tests used:
– Short Increment Sensitivity Index (SISI) Test
– Alternate Binaural Loudness Balance (ABLB) Test
29. S.I.S.I. Test (Jerger, 1959)
• Continuous tone given 20 dB above hearing threshold
and sustained for 2 min
• Every 5 sec, tone intensity increased by 1 dB and 20
such blips are given
• SISI score = % of blips heard
• 70-100 % in cochlear deafness
• 0-20 % in conductive & nerve deafness
30. A.B.L.B. Test (Fowler, 1936)
• Pure tone is presented alternately to deaf & normal ear
• Intensity heard in normal ear is adjusted to match with
deaf ear
• Test started 20 dB above threshold in normal ear &
repeated with 10 dB raises till loudness is matched in
both ears
• Initial difference is maintained, decreased & increased
in conductive, cochlear and retro-cochlear lesions
respectively
32. Threshold Tone Decay Test
• Olsen and Noffsinger (1974)
• Detects abnormal auditory adaptation due to nerve
fatigue caused by a retro-cochlear lesion
• Pure tone presented 20 dB above hearing threshold
continuously for 1 min
• If patient stops hearing earlier, intensity increased by
5 dB and restarted
• Test continued till pt hears tone continuously for 1
min or intensity increment (decay) > 25 dB
35. Tympanometry
• Based on the principle of impedance
• When a sound strikes the tympanic membrane, some of the
sound energy is absorbed while the rest is reflected
• A stiffer tympanic membrane would reflect more sound energy
than a compliant one
• A compliant T.M. gives equal pressure in E.A.C. and middle ear
• By changing the pressures in a sealed external auditory canal
and measuring the reflected sound energy, it is possible to find
the compliance or stiffness of the tympano –ossicular system
and thus find the healthy or diseased status of the middle ear
36. • The equipment consists of a probe which snugly fits into the
external auditory canal and has three channels
– Oscillator: to deliver a tone of 220 Hz
– Microphone: to pick up the reflected sound
– Air pump : to bring about changes in air pressure in the ear
canal from positive to normal and then negative
• By charting the compliance of tympano-ossicular system
against various pressure changes, different types of graphs
called tympanograms are obtained which are diagnostic of
certain middle ear pathologies
40. Types of Tympanogram
Type Pressure Compliance Seen in
A Normal Normal Normal ME
As Normal Decreased Otosclerosis
Ad Normal Increased Ossicular
discontinuity
B Nil (flat curve) Nil (flat curve) Fluid in ME, TM
perforation
C Negative Normal ET obstruction
48. Acoustic Reflex
Loud sound > 70 dB above hearing threshold causes
B/L stapedius muscles contraction, detected in
tympanometry as decrease in compliance
49. Acoustic Reflex
• Principle:
– A loud sound, 70–100 dB above the threshold of
hearing of a particular ear, causes bilateral
contraction of the stapedial muscles which can
be detected by tympanometry
– Tone can be delivered to one ear and the reflex
picked from the same or the contralateral ear
50. Clinical uses of Acoustic Reflex
• To test the hearing in infants and young children
• To find malingerers
– A person who does not respond on pure tone audiometry but
shows a positive stapedial reflex is a malingerer
• To detect cochlear pathology
– Presence of stapedial reflex at lower intensities (e.g. 40–60
dB) than the usual 70 dB indicates recruitment cochlear
type of hearing loss
• To detect VIII th nerve lesion
– If a sustained tone of 500 or 1000 Hz, delivered 10 dB above
acoustic reflex threshold, for a period of 10 s, brings the
reflex amplitude to 50%, it shows abnormal adaptation and is
indicative of VIII th nerve lesion (stapedial reflex decay)
51. • To diagnose the lesions of facial nerve and its
prognosis
– Absence of stapedial reflex when hearing is
normal indicates lesion of the facial nerve
proximal to the nerve to stapedius
– The reflex can also be used to find prognosis of
facial paralysis as the appearance of reflex, after it
was absent, indicates return of function and a
favourable prognosis
• Lesion of brainstem
– If ipsilateral reflex is present but the contralateral
reflex is absent, lesion is in the area of crossed
pathways in the brainstem
61. Otoacoustic Emission (Kemp echoes)
• Sounds generated within normal cochlea due to
activities of outer hair cells
• Types:
– Spontaneous: absent in > 25 dB HL
– Evoked: transient; distortion product
• Applications:
– Objective non-invasive test for hearing screening in
neonates & evaluation of non-organic hearing loss
64. Brainstem Evoked Response Audiometry
(BERA/ABR)
• Auditory evoked neuro -electric potentials recorded
within 10 msec from scalp electrodes
• An Objective test for
– Hearing threshold for uncooperative pt / malingerer
– Hearing threshold in sleeping / sedated / comatose
– Diagnosis of retro-cochlear pathology
– Diagnosis of C.N.S. maturity in newborns
– Intra-op monitoring of auditory function
