Immittance Battery Flashcards
Who was Otto Metz? (3)
Otto Metz was a Jewish German doctor
wanted to distinguish sensorineural from conductive loss (tuning forks still popular!)
began reading about sound absorption in constructing materials
What did Otto Metz publish?
He published the basic principles of immittance audiometry in 1946.
5 years later, he published the first work on acoustic reflexes
What does Tympanometry provide us? (2)
rich information about the middle ear
easy to obtain
What is the use of AR measuring? (3)
- rich information about the auditory and facial nerves and lower brainstem
- some information about hearing
- just as easy to obtain
What are findings of Otto Metz’s legacy? (3)
learning about something by it’s impedance to sound
the ossicular chain is connected
this is a powerful tool for studying the middle ear
What is acoustic impedance?
Acoustic impedance is the degree to which sound energy is NOT absorbed.
ex: a good paper towel absorbs moisture well i.e., low impedance
if I covered a paper towel in butter, impedance would increase!
What absorbs more sound?
Carpeting or hardwood floor?
Small room or large room?
Carpeting
Large room
How does impedance relate to the middle ear? (2)
The sound is absorbed from the ear canal into the ossicular chain when acoustic energy sets the ossicular chain in motion
low impedance means that the ossicles move well with the sound
Does low impedance mean better hearing?
What are the three components of acoustic impedance?
mass, stiffness and resistance (no relation to impedance)
What would increase any of these 3 components do to impedance (intuitively speaking)?
What would increase any of these 3 components do to impedance (intuitively speaking)?
Increase mass results in the loss of high frequencies
Increase stiffness results in the loss of low frequencies
What is resistance?
R is resistance — friction in this example
loss of energy that is not dependent on frequency (fairly minimal in the middle ear system)
so we are mainly looking at the effects of mass and stiffness
the mass and stiffness effects are both called “Reactance”
What is reactance?
the mass and stiffness effects are both called “Reactance”
Low reactance means that it doesn’t move well with the sound
As mass reactance increases, frequency ___________________
Mass reactance increases as frequency go up
Explain Mass Reactance: (4)
mass reactance increases as frequency goes up
it’s difficult to move large things back and forth quickly
think of how long it takes to accelerate or decelerate a tractor trailer
this is an acceleration issue
high mass interferes with high frequencies (more reactance and thus more impedance)
high mass moves better at low frequencies (less reactance and thus less impedance)
Explain Stifness Reactance: (4)
stiffness reactance decreases as frequency goes up
something stiff will want to return to its resting position faster
faster = higher frequency
high stiffness interferes with low frequencies (more reactance and thus more impedance)
highly stiff systems move better at high frequencies (less reactance and thus less impedance)
What does this graph show? (3)
mass reactance and stiffness reactance are inversely related to frequency:
increasing frequency will increase mass reactance (Xm) and decrease stiffness reactance (Xs)
decreasing frequency will decrease mass reactance (Xm) and increase stiffness reactance (Xs)
these are in opposite directions so the sum determines total reactance (Xm–Xs)
How do we find the total impedance?
the hypotenuse is the square root of the sum of the squares (Pythagoras)
What are the effects of mass and stiffness in the ME system? (2)
- the effects of stiffness and mass are perfectly balanced at around 800–1200 Hz — the resonant frequency of the middle ear
- this is a fairly high frequency resonant peak! This is because the middle ear has low mass and high stiffness. It’s small and stiff!
What is important from changes in impedance to the ME?
Changes in middle ear impedance are primarily changes in its stiffness
Why aren’t higher frequencies not transmitted as well? (3)
Higher frequencies are not transmitted as well because the system has some mass (although only a small amount)
mass of the ossicles and tympanic membrane
And because it’s not even stiffer than it is
You could improve its high freq response by reducing mass or making it stiffer
Why aren’t lower frequencies not transmitted as well? (2)
lower frequencies are not transmitted as well because of the high stiffness of the system (and because it has low mass)
- stiffness of tympanic membrane
- stiffness of ossicular chain (with ligaments)
you could improve low frequency response by making it less stiff
What is impedance measured at clinically?
Measured at 226 Hz, where the mass of the ossicular chain is neglible (so changes in impedance here are mostly due to changes in stiffness)
What is impedance measured at clinically?
Measured at 226 Hz, where the mass of the ossicular chain is neglible (so changes in impedance here are mostly due to changes in stiffness)
What do Clinical audiological systems generally measure?
Clinical systems generally measure admittance, which is the inverse of impedance
Impedance [how much is ________________], measured in ________
Impedance [how much is NOT absorbed], measured in ohms
Admittance [how much _________________], measured in_____________
Admittance [how much IS absorbed], measured in mhos (ohms backwards)
What is immittance?
Immittance is a general term that refers to impedance and admittance
Acoustic admittance is equal to: (3)
- sound flow (volume velocity) / sound pressure i.e., how much sound flows into the system for a given amount of pressure
- Sound flow is measured in m3/s
- Sound pressure is measured in pascals (N/m2)
How can acoustic admittance be estimated?
acoustic admittance can be estimated by measuring reflected sound (i.e., how much sound isn’t reflected)
What causes low admittance?
(i.e., high impedance) when TM/ossicular chain cannot be set in motion easily, energy is reflected back
Would admittance be Higher or lower than normal?
Otitis Media with effusion:
Ossicular discontinuity:
Ossicular fixation:
No middle ear (i.e. only an outer ear):
Otitis Media with effusion:
Ossicular discontinuity:
Ossicular fixation:
No middle ear (i.e. only an outer ear):
How does admittance relate to volume?
- Admittance also relates to volume
- more volume, more admittance
more sound is absorbed into a large cavity… so less is reflected (higher admittance)
Why do we use 226 Hz tone?
For a 226 Hz tone, at sea level, air in a cavity that is 1 cubic centimetre (or 1 ml) has approximately 1 millimho of admittance -> this is why we use 226 Hz!
How could you assess ONLY middle ear admittance? (3)
- When putting sound into ear, you need to move the air in the external canal, then move the eardrum + ossicles
- The admittance that you measure includes the middle and outer ear
sound is absorbed by air and the tympanic membrane + ossicular chain - To get the middle ear impedance, you either need to find a way of subtracting the outer ear impedance
What happens to admittance if the eardrum is very stretched?
When the eardrum is very stretched it doesn’t move as well, and admittance of the middle ear goes down
example: pushed out (e.g. full of fluid) or retracted (negative middle ear pressure)
What does it mean when the OE becomes hard-walled cavity? (2)
- If the TM can’t move, the middle ear admittance becomes zero, and the outer ear becomes hard-walled cavity—like a cup!
- This gives you the admittance of outer ear
subtracting this from total admittance leaves just the middle ear!
What does it mean when the OE becomes hard-walled cavity?
- If the TM can’t move, the middle ear admittance becomes zero, and the outer ear becomes hard-walled cavity—like a cup!
- This gives you the admittance of outer ear
subtracting this from total admittance leaves just the middle ear!
Figuring out the Middle Ear Portion: what do both of these graphs show?
ME + OE – OE = ME
Top: Atmospheric Pressure (mobile eardrum)
TM = 0 daPa
Bottom: High Pressure (tight eardrum)
+200 daPa
What does this graph show?
Tympanometry: Vary the Pressure
Terkildsen and Thomson decided to try manually changing the pressure in the ear
with negative or positive pressure, the eardrum became stiff, the middle ear admittance became negligible
this gave them outer ear admittance
when this was subtracted from overall admittance, they were left with the middle-ear admittance
Who is this dude?
Poul Madsen
created the first clinical tympanometer – the Madsen ZO61 while in Copenhagan
moved to Oakville in 1969
sold the company and became a professor at UofT in 1991
died in 1997
How does the typanometer work? (2)
- system plays a 226 Hz tone and records level of reflected sound
- pump controls pressure in ear canal (sucking or blowing)
automatic gain control in system keeps constant sound level in ear canal…
What does this point represent?
Admittance when lots of positive pressure in the ear (ear drum stiff)
… this is external ear canal admittance (or the volume of the ear canal)
What does this peak represent? (2)
Peak admittance. The TM and ossicles are maximally set in motion.
It occurs here with atmospheric pressure.
What is static compliance?
Peak admittance minus the ear canal admittance (or volume).
This is the admittance (often called static compliance) of the middle ear.
What is Peak Pressure?
The peak pressure is the pressure at which the system is maximally compliant. Measured relative to x axis (daPa)
What does this represent?
Tympanogram Width, measured relative to x axis
What are the two types of tympanograms?
What does this tympanogram show?
A Complete Tympanogram
What are the 5 tympanogram shapes?
A: Normal
AS: Shallow
AD: Deep
B: Nothing
C: -ve
Which of the 5 shapes could occur with OME? (3)
As B C
Which of the 5 Tympanogram shapes could occur with TM pathology or Ossicular discontinuity?
Which of the 5 Tympanogram shapes could occur with TM perforation?
Which of the 5 Tympanogram shapes could occur with blocked probe/cerumen?
How can you differentiate OME, perforation and
blocked probe/cerumen?
these will look similar on a compensated tympanogram
the difference will be the ear canal volume!
Which of the 5 Tympanogram shapes could occur with Otosclerosis? (2)
A As
Which of the 5 Tympanogram shapes could occur with ET blockage?
What is Otto Metz’s discovery on AR?
loud sounds evoke a reflex that reduces admittance
The stapedius muscle
- thresholds between 70 and 100 dB HL
What does the AR do to the ossicular chain?
stiffens the ossicular chain, reduces intense low frequency sound
What are vestibular schwannomas?
Schwann cells form myelin sheath in PNS
unilateral in 95% of cases
usually slow growing, benign
(1-2 mm per year)
usually, create unilateral high-frequency loss
can grow out of IAM into CPA
The Acoustic Reflex Pathway
The Acoustic Reflex: BILATERAL
How do we label sound presentation?
We always label the response by the ear of sound presentation
(e.g., Left ipsilateral and Leftcontralateral means sound went to Left ear)
How do we test for the AR?
ear canal pressure adjusted to peak pressure (for best sound transmission)
In the AR, we can see that the reflex strength grows with _______
Reflex Strength Grows with Level
What does this graph represent?
Reflex Thresholds with Hearing Loss (90th percentile)
A “Right” reflex means the sound went to the right ear. What is a right contralateral?
Left reflex with sound presented to the right
Remember, sound to one ear causes reflex in both ears.
What is a probe?
“Probe” is the reflex measurement device (i.e., the tympanometry probe)
The stimulus can be presented by the probe or by a plain earphone on the other side of the head from the probe
What are the AR pathways in the brainstem?
What will happen to the AR with VIIIth nerve disorder?
abnormal reflex on stimulus side
elevated or absent (i.e., beyond 90th percentile)
What will occur to the Ar from VIIth (facial) nerve disorder?
abnormal reflex on probe (measurement) side
i.e., elevated or absent
How can brainstem anomaly affect AR?
- a centralized tumour may eliminate/make abnormal both contralateral reflexes but preserve both ipsilateral
- or both contras and one ipsi (depending on location)
How can conductive loss affect AR? (3)
1) will obliterate reflex on the probe side
can’t measure reflex in ear with conductive loss
2) Will elevate the reflex on the stimulus side
sound needs to be raised to overcome conductive loss – more than with cochlear loss (i.e., beyond 90th percentile)
3) Bilateral conductive loss
Absent everywhere
- it generally isn’t worth measuring reflexes with conductive loss
What is reflex decay?
- present tone 10 dB above ART at 500 and 1000 Hz (if safe)
- half-life time: time after stimulus onset when admittance deflection decreased by 50%
if reaches this before 10 s, considered to be abnormal
What is considered an abnormal adaptation?
half-life time of less than 10 seconds is abnormal
Left CHL Obliterates will obliterates reflex on the probe side (right contra)
Elevated or Absent reflex on the stimulus side with CHL
Bilateral CHL absent AR on both sides
