Lecture 8 Flashcards
Explain the birth of the EcochG
- One year after Berger’s discovery of EEG, the first auditory evoked response was recorded from the auditory nerve of a cat (Wever & Bray, 1930)
- It likely wasn’t a neural response – it was the CM
- All early work with EcochG was invasive
Explain the different parts of the EcochG
- CM is following the fluctuations in the stimulus
- CAP is the little burst of activity in the beginning
- DC is the pedestal that the whole thing is riding on
Explain the different responses of the EcochG
- Transient Response (not cochlear)
- CAP (wave I of ABR)… also called N1 (first negativity)
- Sustained Response (cochlear)
- SP (.5-1 uV)
DC shift (basilar membrane motion)
- SP (.5-1 uV)
- Steady-State Response (cochlear)
CM (< .5 uV)
Electrocochleography isn’t all ____
Cochlear
____ and ____ of the ABR are the same thing
CAP, wave I
What is the closet we can get to measure an EcochG without being invasive?
We can get close enough to measure these in humans to not be invasive (just to the edge of the far field of the auditory nerve in the ear canal)
Explain endolymph
- Endolymph is different from all other extracellular fluids
- A very high concentration of potassium ions and low concentration of sodium ions
- Maintained by ion pumps in the vascular (and metabolically demanding) Stria Vascularis
- Very POSITIVE relative to the perilymph (+80 mV)
Explain the intracellular fluid
Normal intracellular fluids also high in potassium but NEGATIVE relative to extracellular fluids (a function of the permeability of the membrane to sodium (about –50 mV)
What is the potential difference?
Therefore, the potential difference is roughly 130 mV—this is the endocochlear potential (i.e., the battery that powers hearing)
What happens when the tiplinks open?
- When the tip links open the ion channels, potassium flows into the inner hair cells, depolarizing the membrane
- Repeatedly as the basilar membrane moves back and forth
What does the sum of the fluctuating fields produce?
- The sum of these fluctuating fields produce the cochlear microphonic—it is the receptor potential of the cochlea
- Predominantly reflecting OHCs
When you lose OHC you lose ____
CM
CM is primarily ____
OHC
What is the summating potential?
- This is a DC shift that occurs while the stimulus is present (a sustained potential)
- Primarily activity at the IHC synapse
- Negative for most stimuli, but can be positive for high-frequency tones when measured from the promontory
What is the difference between CAP and wave I
Difference between CAP between wave I is that CAP goes negative
What are the 3 recording approaches?
Easiest approach
1. Record in the ear canal
Harder approaches
2. Tympanic membrane electrode (put an electrode on the TM)
3. Transtympanic electrode (put a needle through the TM on the promontory, which is between the oval and round window)
- In the near field
Trans-tympanic measurement (TT)
-what is the size of CAP compared to ABR?
- Largest responses (e.g. CAP may be 5-10 µV when recorded trans-tympanically)
- Where as in the ABR the CAP is 0.2 uV
Tympanic membrane electrode (TM)
- These are the ones that go in the cotton ball, soaked in electrolyte fluid, and put it against the TM (half a microvolt bigger)
- A little bit more invasive than extra-tympanic electrode
Extra-tympanic electrode
- Ear canal electrode
- Insert earphone wrapped in foil
- These are a bit more expensive
If you subtract alternating polarities to see the ____
CM
If you add alternating polarities, you get rid of ____ and are left with ____
CM, SP & CAP
____ is the negativity that starts before the CAP and lasts the whole stimulus
SP
In the middle of the SP, you have the ____
CAP
When we use the click, there is no ongoing ____
SP (can only see the CAP)
Can you always see the SP?
- Sometimes the SP is difficult to identify
- Solution: raise the stimulus rate
- The receptor potential is not reduced at high rates
- Compound action potential is reduced at high rates
What doesn’t respond to ABR at high rates?
Wave I
What needs a slow rate to be seen?
CAP
What 4 reasons do we measure the EcochG?
- Wave I enhancement (for neurodiagnostic)
- Assessment of severe-profound loss (ABR won’t work on those with severe-profound loss)
- Ménière’s
- Synaptopathy/neuropathy
What does near field recording allow for?
- Can be recorded without averaging
- Enhancement of Wave I of the ABR
- Wave I is useful for identification of tumours
EcochG can pull ____ out (sometimes labelled N1)
Wave I
How do you assess severe-profound loss?
- CM can occur with severe-profound hearing loss
- Suggests OHC function (but useless if no IHC function)
- SP may be reduced or absent with cochlear hearing loss
- CAP tends to follow audiogram
- ABR generally only to moderately-severe
What can the TT CAP be used for? Why?
- TT CAP can be used to estimate hearing thresholds at levels that are beyond ABR—even TM CAP can often do this
- Why? because behavioural–physiologic differences are close to 0 (whereas they are ~30 dB for ABR)
- But standard deviations are still on the order of 10–15 dB (same as ABR)
- This is useful for infant cochlear implant candidates
Can get wave I right down to threshold when doing ____
TT CAP
With a lot of HL (severe), you can get TT CAP where you would not get ____
ABR
Match between ____ and TT CAP
Behavioural threshold
Explain the diagnosis of meniere’s
- The SP is often large in Ménière’s (or in other cases of endolympatic hydrops)
- Possibly mechanical diplacement of BM, or metabolic disturbance (Eggermont)
- This will diminish as hair cells die
- The CAP tends to be smaller
- The SP/CAP ratio is often enlarged
What is the downside of meniere’s diagnosis?
Downside of Meniere’s diagnosis with EcochG is that Meniere’s fluctuates
____ tracks meniere’s very well
SP/CAP ratio
SP/CAP Ratios (what is abnormal)?
- Generally, anything above .4 is unusual (note specificity)
- This ratio is consistent for males and females, and for different electrode montages
- It does vary as a function of CAP level though!
Auditory Synaptopathy / Neuropathy
- In Ménière’s, the SP is enlarged but the CAP is not
- so the SP/CAP ratio is large
- In synaptopathy/neuropathy, the active mechanisms in the cochlea are intact (e.g., OHCs), so the SP (OHC & IHC) and CM (OHC) should be less impacted than CAP (post-synaptic)
- The SP/CAP ratio should still be large, not because of an abnormally large SP, but because of an abnormally small CAP
- Although enlarged SP has been found in synaptopathy!
- Reasons are not understood
CAP/SP ratio of less than ____ provided a sensitivity and specificity ≈ 80–90%
1
Etiology of neuropathy
- Loss of sensory cells (IHCs) or presynaptic deficiency
- Summating potential and CAP affected
- Post-synaptic or neural deficency
- Summating potential intact, SP/CAP ratio large (or CAP/SP ratio small)
EcochG measurement paramters - filter settings
5 (SP)- 30 (CAP) Hz 1500 for SP and AP
EcochG measurement paramters - # of averages
100-200 for TT
1000-2000 for ET
EcochG measurement paramters - analysis window
5-12 ms for click, longer for tone burst
EcochG measurement paramters - stimulus polarity
Alternating to remove CM
EcochG measurement paramters - stimulus rate
24/sec max (better < 10/sec)
