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Electrophysiology > Lecture 8 > Flashcards

Lecture 8 Flashcards

1
Q

Explain the birth of the EcochG

A
  • 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
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2
Q

Explain the different parts of the EcochG

A
  • 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
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3
Q

Explain the different responses of the EcochG

A
  • 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)
  • Steady-State Response (cochlear)
    CM (< .5 uV)
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4
Q

Electrocochleography isn’t all ____

A

Cochlear

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5
Q

____ and ____ of the ABR are the same thing

A

CAP, wave I

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6
Q

What is the closet we can get to measure an EcochG without being invasive?

A

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)

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7
Q

Explain endolymph

A
  • 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)
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8
Q

Explain the intracellular fluid

A

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)

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9
Q

What is the potential difference?

A

Therefore, the potential difference is roughly 130 mV—this is the endocochlear potential (i.e., the battery that powers hearing)

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10
Q

What happens when the tiplinks open?

A
  • 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
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11
Q

What does the sum of the fluctuating fields produce?

A
  • The sum of these fluctuating fields produce the cochlear microphonic—it is the receptor potential of the cochlea
  • Predominantly reflecting OHCs
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12
Q

When you lose OHC you lose ____

A

CM

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13
Q

CM is primarily ____

A

OHC

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14
Q

What is the summating potential?

A
  • 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
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15
Q

What is the difference between CAP and wave I

A

Difference between CAP between wave I is that CAP goes negative

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16
Q

What are the 3 recording approaches?

A

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

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17
Q

Trans-tympanic measurement (TT)
-what is the size of CAP compared to ABR?

A
  • 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
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18
Q

Tympanic membrane electrode (TM)

A
  • 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
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19
Q

Extra-tympanic electrode

A
  • Ear canal electrode
  • Insert earphone wrapped in foil
  • These are a bit more expensive
20
Q

If you subtract alternating polarities to see the ____

A

CM

21
Q

If you add alternating polarities, you get rid of ____ and are left with ____

A

CM, SP & CAP

22
Q

____ is the negativity that starts before the CAP and lasts the whole stimulus

A

SP

23
Q

In the middle of the SP, you have the ____

A

CAP

24
Q

When we use the click, there is no ongoing ____

A

SP (can only see the CAP)

25
Q

Can you always see the SP?

A
  • 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
26
Q

What doesn’t respond to ABR at high rates?

A

Wave I

27
Q

What needs a slow rate to be seen?

A

CAP

28
Q

What 4 reasons do we measure the EcochG?

A
  1. Wave I enhancement (for neurodiagnostic)
  2. Assessment of severe-profound loss (ABR won’t work on those with severe-profound loss)
  3. Ménière’s
  4. Synaptopathy/neuropathy
29
Q

What does near field recording allow for?

A
  • Can be recorded without averaging
  • Enhancement of Wave I of the ABR
    • Wave I is useful for identification of tumours
30
Q

EcochG can pull ____ out (sometimes labelled N1)

A

Wave I

31
Q

How do you assess severe-profound loss?

A
  • 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
32
Q

What can the TT CAP be used for? Why?

A
  • 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
33
Q

Can get wave I right down to threshold when doing ____

A

TT CAP

34
Q

With a lot of HL (severe), you can get TT CAP where you would not get ____

A

ABR

35
Q

Match between ____ and TT CAP

A

Behavioural threshold

36
Q

Explain the diagnosis of meniere’s

A
  • 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
37
Q

What is the downside of meniere’s diagnosis?

A

Downside of Meniere’s diagnosis with EcochG is that Meniere’s fluctuates

38
Q

____ tracks meniere’s very well

A

SP/CAP ratio

39
Q

SP/CAP Ratios (what is abnormal)?

A
  • 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!
40
Q

Auditory Synaptopathy / Neuropathy

A
  • 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
41
Q

CAP/SP ratio of less than ____ provided a sensitivity and specificity ≈ 80–90%

A

1

42
Q

Etiology of neuropathy

A
  • 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)
43
Q

EcochG measurement paramters - filter settings

A

5 (SP)- 30 (CAP) Hz  1500 for SP and AP

44
Q

EcochG measurement paramters - # of averages

A

100-200 for TT
1000-2000 for ET

45
Q

EcochG measurement paramters - analysis window

A

5-12 ms for click, longer for tone burst

46
Q

EcochG measurement paramters - stimulus polarity

A

Alternating to remove CM

47
Q

EcochG measurement paramters - stimulus rate

A

24/sec max (better < 10/sec)

Electrophysiology (8 decks)

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