Week 8: ECochG Flashcards
what is ECochG
sound evoked cochlear and neural responses
what are the three parts of ECochG
- cochlear microphonic
- summating potential (SP)
- action potential (AP)
- –AP has N1 from the proximal auditory nerve and N2 from the distal auditory nerve
how long after the stimulus does ECochG occur
within 5 msec
what is the cochlear microphonic
- mimics the stimulus
- –sine wave of frequency of stimulus
- is the alternating current (AC) arising from outer hair cells
- –recorded response is mostly from the basal part of the cochlea
- best evoked by frequency specific signals, and can be evoked using a click
- latency does not change with intensity
what is the summating potential
- direct current (DC) arising possible from the inner hair cells (has 0 frequency)
- DC means doesnt change direction and therefore wont have frequency because cant measure alterations of the current
- SP may be viewed as a shirt in the baseline of an ECochG recording
- best recorded with higher frequency and can be evoked with click
- is seen by averaging alternating stimulus because CM and SP happen at the same time so need to use alternating presentation to get rid of the CM
- latency is not affected by intensity
what is the action potential
- most commonly used is N1
- –alternating current (AC) arising from the distal part of the VII nerve
- ——-FFT response frequency up to 1000 Hz
- –best recorded using click stimulus
- –same as wave 1 of ABR
- latency is a/b 1.5 ms
- AP amplitude increases and latency decreases with increased stimulus intensity
- less commonly used is N2
- –it is wave II of the ABR response and behaves similar to N1
three electrode types and amplitudes of N1
- TIPtrode N1 amp=0.1-1 microV
- TM electrode N1 >1 micro V
- trans-tympanic electrode Na amplityde= 15-25 micro V
electrode montage to record a negative voltage response
- non-inverting electrode near the ear (mastoid, TM, or promontory)
- inversting electrode on Cz/Fz/Fpz
electrode montage to record a positive voltage response
- non-inverting electrode on Cz/Fz/Fpz
* inverting electrode near the ear (mastoid TM promontory)
best type of transducer for ECochG
inserts allow for easier recording using the different types of electrode and less artifact
stimulus for SP/AP recordings
click stimulus of 100 micro volts (specifically used for AP recordings)
stimulus for CM recordings
controls for artifact
*longer duration is needed for better CM and SP recording
stimulus rate effect on CM/SP/AP
- doesnt affect SP or CM
- affects SP because speeding the rate makes the nerve fatigue so amp decreases and latency shifts to longer
- –use 7.1 or 9.1/second
stimulus polarity needed for SP/AP/CM
- SP/Ap need alternating to get rid of CM
* CM recording use rarefaction and condensation
stimulus intensity affects on SP/AP/CM
- as intensity increases, amplitude increases and latency decreases
- as intensity increases SP raises in amplitude but latency stays the same
Epoch settings for SP/AP and CM
- SP/AP is 5-10 msec
- CM recording (SP)= extended window
- –CM follows stimulus so if stim is 5 msec the response will be 5 msec
filter settings for SP/AP and CM
- SP/AP should be 10-3000Hz
* CM should be wide enough to include the frequency of interest
amount of amplification a ECochG signal needs
75,000-100,000X
what does electrode impedance need to be for ECochG
as low as possible and similar between the electrodes
what are the three clinical applications of ECochG
- CM helps with testing the cochlear function
- enhances detection of wave 1 which is needed in diagnostic testing
- SP/AP are useful for evaluating meniere’s disease
how are SP/AP used to evaluate meniere’s
looking at the ratio between the amplitude of SP and AP
- baseline (pre stimulus or at 0ms) ans then look at how far the SP is from baseline and how far the AP is from baseline
- –abnormal if the ratio is bigger than 0.45 (the endolymph makes the SP larger
- also use SP/AP area ratio
- –w/ increased endolymph, SP becomes larger and wider
- –abnormal if the ratio is larger than 2
what is the sensitivity to menirers disease if using both the SP/AP amplitude and area ratios
92%
