audiology the secrets behind the squiggles dave pothier st marys, october 2003
TRANSCRIPT
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AUDIOLOGYThe Secrets Behind the
Squiggles
Dave PothierSt Mary’s, October 2003
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Ear examination
• Make sure ear is not occluded
• No other physical abnormalities
• Put audiological tests into context
‘Interesting’ technique
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Tuning fork tests
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• Rinne’s test• Weber’s test
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Striking the fork
• On bony prominence
• Not on hard surface
• Gives better single tone without overtones
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Rinne’s Test
Purpose• Compare AC to BC
How• Strike tuning fork• Place TF
alternately on mastoid process and EAM
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Rinne interpretation
• NormalPositive Rinne – louder at EAMAC > BC
• AbnormalNegative Rinne – louder on mastoid
processPositive Rinne – Bilateral SNHL
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Rinne interpretation
True Negative Rinne- Conductive Hearing loss- BC > AC
False Negative Rinne- Severe S/N loss on test side = tone
heard on contralateral side
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Rinne interpretation
If Rinne Negative, masking is essential
Types:
Tragal RubBaranay Noise Box
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Weber
Purpose: Conductive vs. SNHL in
unilateral losses
How: Strike fork
Place midline of headIncisors>Vertex>Foreh
ead
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Weber - interpretation
Normal - Midline sensation of hearing= equal hearing both sides of same type= equal loss of same type
Abnormal – Tone louder in on one side=Conductive loss – tone louder on affected
side=SNHL – tone louder on contralateral side
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Simple free field testing
• By ‘bedside’• Good adjunct to PTA• Helpful with children too young for
PTA• Fairly sensitive• Give some idea of significance of
hearing loss
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Concepts
• Turn patients head to side (so cannot see examiner's mouth)
• Apply tragal rub masking to non-test ear (furthest away)
• Whisper at arms length, then increase loudness of voice in increments
• Patient to repeat numbers/words
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What this tells you
Can hear whisper at arm’s length – Normal hearing
Can hear normal voice at arm’s length– mild / moderate loss
Can hear loud voice at arm’s length - moderate / severe loss
Can only hear loud voice close up- profound loss
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Pure Tone Audiometry
• Setup/physics• AC• BC• Masking
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Pure Tone Audiometry
Sound has 2 components:
• Frequency (pitch) cf. wavelength
Hz / kHz
• Intensity (loudness) cf. amplitude
dB
Setup/physics
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Pure Tone Audiometry
The Decibel
Sound intensity SPL measured in decibels- log of intensity of sound- NB.. A logarithmic scale- 20dB is 10 times 0dB, 40dB is 10 times
20dB, 60dB is 1000 times louder than 0dB
Setup/physics
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Very confusing…dB used as measures of SPL are
different to dB’s used as measure of hearing level
Pure Tone AudiometrySetup/physics
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Important concept…
A hearing level of 0 dB is an arbitrary level of hearing of a given SPL
Iowa State Fair (allegedly) in 1935, 10 000 young women had their hearing measured
This established the normal hearing levels for pure tone Audiometry (0 db Threshold)
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But, to confuse even more…
The cochlear does not hear all sounds equally at all frequencies
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And, we all know BC is not as good as AC, don’t we?
So, why does a normal PTA look like this?
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Calibration!
The audiometer accounts for the different hearing levels at different frequencies as well as the ‘natural’ reverse A/B gap.
Clever thing
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Those mysterious markings
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Legends for PTA’s
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Air conduction
Bone conduction
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How they do it (briefly)
1. Otoscopy + explanation2. Best ear?3. Start with AC on best ear4. Start at 1000Hz at 60dB5. Down by 10dB until no response6. Then up by 5dB until reponse (3 out of 5)7. Up and down frequencies8. Same for bone
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Normal PTA
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Conductive Loss
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Sensorineural Loss
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Mixed Loss
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Masking
Used to prevent non-test ear hearing stimulus presented to test ear
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Interaural attenuation
• BoneAssumed to be 0dB, but probably
nearer 4-6dB• AirAssumed to be greater the 40dB,
but varies between patients• Masking used to eliminate this
confounding factor
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PTA limitations
• PTA in NOT always a ‘Gold Standard’ and infallible
• Limited by : patient, audiologist and equipment
• Beware on NOHL• Try to supplement other simple
tests
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Tympanometry
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TympanometryMeasure of
compliance of TM at varying pressures in EAM
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Normal tympanogram (Type A)
Peak at 0dPa
Best movement of drum when no extra pressure on either side of TM
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Other Type A tympanograms
Peak at 0dPa, but unusually high
amplitude? Ossicular disruption
Peak at 0dPa, but unusually low
amplitude? Stapes fixation
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Flat tympanogram (Type B)
No Peak
No best TM movement at any pressure
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Flat tympanogram (Type B)
When tymp is flat, usually means 1 of 3
things:1. Artefact2. Fluid in ME3. Perforation
Look at EAM vol.If large = perfIf normal = fluid
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Negative tympanogram (Type C)
Peak at < 0dPa
Best movement of drum when no negative pressure in EAM thus middle ear pressure must be < atmospheric
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Negative tympanogram (Type C)
Can be further divided into:
C1 – peak between 0 and -200 daPa
C2 – peak less than -200daPa
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Final Thought
Tests are not infallible, they are only as good as those taking, administering and interpreting them…