Week 11 objective measurements

Question 1

what are the 3 important uses of objective measurements of CI

Answer 1

• evaluate the CI function
• evaluate the auditory system
• CI programming

Question 2

electrophysiological objective measures for CI

Answer 2

• electrically evoked stapedial reflexes threshold (eSRT)
• electrically evoked compound action potential (eCAP)
• electrically evoked auditory brainstem response (eABR)
• electrocally evoked auditory cortical potential (eACP)

Question 3

electrical objective measures for CI

Answer 3

• impedance testing and voltage compliance
• electrical field imaging
• averaged electrode voltage–integrity test

Question 4

factors affection stapedial reflex threholds (SRT)

Answer 4

• 2% of normal hearing adults have absent reflexes
• the presence of minimal air bone gap could affect he probability of measuring SRT
• –average of 5 ABG results in 50% probability of measuring SRT
• measured reflexes in 85% of children with normal functioning middle ear
• grommits, ME effusion, affected ossicles, no seal, Ad or As tymps, 8th nerve dysfunction
• –history of middle ear issue could cause insignificant increase in ME stiffness, causing reflexes to disappear

Question 5

eSRT procedure

Answer 5

• stimulus is a 300-500msec pulse train
• relex decay with high frequency probe tone (678 or 1000Hz)
• –could start with 226 but lesser chance of measuring with 226 Hz because you have added mass to the system with the electrode lead
• contra ear gets the stimulus
• –or ear with better ME function for bilateral recipients
• present the pulse train 3-5 times to ensure you are getting a response
• start at a low level and go up
• –the response must be time locked
• make sure response is repeatable on two ascending runs

Question 6

clinical application of eSRT

Answer 6

• confirm functionality of CI
• strong correlation with C-level or USL
• –correlation coefficient (r) of 0.79 or 0.92
• –varies by electrode but not significant
• –congenitally deaf tend to need more and more power when going from HA to CI so run into problems with compliance and poor resolution because interference (compliance limit, battery life, current spread/low spectral resolution)
• –USL and eSRT increase with time, but not at the same rate
• similar performance with measured USL

Question 7

issues with eSRT

Answer 7

• different upper stimulation level definitions, therefore
• –Ab: M-level 10% below eSRT
• –Med-el: MCL at or just below (within 10%) of eSRT
• –cochlear: c-level 10-15 CL below eSRT
• not in all patients
• –up to 80% of patients have eSRT
• –measured in the ipsilateral ear, chance of measuring eSRT increases with time
• —–37% of electrodes measured (1 month after activation) to 74% (3 month after activation)

Question 8

when was eCAP 1st comercially available

Answer 8

*1998 with nucleus CI24M (nucleus 22 is therefore the only one that it wont work for)

Question 9

what is eCAP and where is it measured from

Answer 9

• N1 (0.2-0.4 ms) -P2 (0.6-0.8 ms)

- –25-30 micro volt at threshold, and as large as 1500 micro volt for a high current

Question 10

what is eCAP used for

Answer 10

• to check the integrity of the device and of the nerve as well as mapping
• measured in 95-96% of cases

Question 11

what are the names of eCAP for each manufacturer

Answer 11

*AB: neural response imaging (NRI)
Med-el: auditory nerve response telemetry (ART)
*cochlear: neural response telemetry (NRT)

Question 12

eCAP electrode coupling

Answer 12

• monopolar

- –bipolar can be used as well, but not as common

Question 13

eCAP montage

Answer 13

• stimulating electrode, recording is +2 in the apical direction, and ground is generally the extracochlear
• so if the stimulating is electrode 1, the recording would be electrode 4 and the extracochlear would b the ground, and so on depending on what electrode you are stimulating

Question 14

eCAP stimulus

Answer 14

• biphasic pulse
• rate of 30-80 pps
• pulse width of 25-40 microseconds

Question 15

eCAP advantages

Answer 15

• anesthesia does not affect
• recording site is very close because you are right on the nerve fibers, this makes the response very large so dont need many sweeps (about 100 sweeps)
• no myogenic noise (because right on the neuron)
• no maturation effect because myelination is not a factor because yet again stimulation does not need to travel because it is right there on the neuron

Question 16

what are the 3 methods of reducing stimulus artifact with eCAP

Answer 16

1) alternating polarity method
2) forward-masking subtraction method
3) artifact template subtraction method

Question 17

alternating polarity method of artifact reduction for eCAP

Answer 17

• the method of use for AB (NRI) and Med-el (ART)
• –is an optional method for cochlear (NRT)
• works like an alternating stimulus (for ecochg or abr) by sending a cathodic leading biphasic pulse which evokes an action potential and then sends anodic leading stimulus which also evokes an action potential
• –any artifacts in thses signals would be out of phase so when averaged this artifact is cancelled out (action potential is not because it always goes the same way)
• —–issue here is that the anodic and cathodic leading stimuli have slight differences in the AP generated so they are not perfectly in phase and can cancel parts of each other leading to a slightly smaller response
• disadvantage: latency and amplitude differ between anodic and cathodic leading pulses
• –elevates threshold

Question 18

forward-masking subtraction method of artifact reduction for eCAP

Answer 18

• available for NRT (cochlear) but not for NRI or ART
• do a run and measure the AP and artifact of that run
• second run have the same response and then do another tracing with masking from the first of the second runs so there will be no response, find the difference between A and B2 which is the action potential, then do a run with just the masker, then do a run with just with the noise from the amplifier and add this to cancell out

Question 19

what is the parameter settings for the forward-masking subtraction method

Answer 19

• MPI default is 400 microsecond (best 300-500 micosecond)
• –MPI is how close in time the masker and probe need to be, we dont want them to be too close or else you pick up activity and dont want them too far apart or you will miss the refractory peroid
• pulse width default 25 but can be 25-40
• amplifier gain (40-70 dB, default 50)
• amplifier delay (50-125 microsecond, default 122)
• –important because when the amplifier is activated they will try to capture a response, dont want this while you are stimulating
• recording electrode (+2 apical)
• **most effective method has larger amplitude and lower (better threshold)

Question 20

artifact template subtraction method of artifact reduction for eCAP

Answer 20

• optional for NRT
• present biphasic pulse at a level too soft to create a neural response and capture the stimulus artifact as a template
• when you present a stimulus, look at the relationship between stimulus presented and the template and increase the template by that amount and subtract from the response
• –idea is that the artifact will remain similar and when subtracted out will leave you with the action potential

Question 21

identifying the response with eCAP

Answer 21

• identified automatically but we cant completely accept that they are correct
• morphology and latency
• –compare morphology, amplitude, and latency
• scaling
• –open traces in separate window
• noise floor (important because they can loo like a flat line but when you open them you can see there is a response)
• –negative-to-positive amplitude compared to noise floor

Question 22

eCAP threshold and amplitude growth function

Answer 22

• conducting several recording can help construction and input-output function, or amplitude growth function
  1) slope: eCAP growth based on level (linear regression)= microvolt/CL
• –electrode coupling (mono or bipolar)
• —–broader stimulation leads to steeper slope
• –density of surviving neuron population
• —–more neurons leads to steeper slope
  2) threshold: lowest level to elicit a response
• –electrode coupling (mono or bipolar)
• —–broader stimulation–better threshold
• –density of surviving neuron population
• —–more neurons–better threshold
• –distance between electrode and neurons
• —–closer distance–better threshold

Question 23

threshold determination methods with ECAP

Answer 23

• linear regression method (LRM): T-NRT, t-NRI, no specific term for MED-EL
• –this is an estimation method
• –plot the input output function and extend this line to see the CL where the amplitude would be zero
• –looking for the intersection
• visual detection method
• –more subjective
• –looking at responses to find the lowest level that caused a peak
• **usually the threshold of LRM

Question 24

eCAP threshold determination methods for AB vs Cochlear

Answer 24

• cochlear had better correlation than AB
• if working with a device with a higher noise floor then want to use the linear regression method because alternating method will result in a little bit higher thresholds because response cancels out (cochlear has better measurement of actual amplitude of the response)

Question 25

disadvantages of the linear regression method of eCAP threshold estimations

Answer 25

* at least 3 supra-threshold measurements needed * invalid results with non-linear curves - --plateau/roll-over as a result of amplifier saturation, not enough masking - --shallow tail will cause the linear model to look different than the last place you visually see a response

Question 26

eCAP challenges

Answer 26

* excessive stimulus artifacts in case of ossified cochlea * amplifier saturation due to - --high gain setting - --high stimulus amplitude - --short delay between stimulating and recording - --recording electrode is too close to stimulating * solutions are to change these settings but change gain as the last resort

Question 27

eCAP clinical use

Answer 27

* device, auditory nerve integrity * programming - --method 1: measure thresholds across the array for all electrodes and then measure an electrode in the middle of the array behaviorally , see how behavioral compare to the threshold and USL and apply this relationship across the array - --method 2:measure eCAP thresholds and then turn all the levels down to where they arent audible (following the curve of the eCAP) then activate with live voice and set the t and c levels based off of this * correlation between behacioral and eCAP USL is poor

Question 28

eCAP threshold vs behavioral MAP levels

Answer 28

* eCAP is almost always higher than behavioral threshold * eCAP fall in the upper portion of the behavioral dynamic range, and may exceed the USL * eCAP profile follows C/T level more T level than USL * what causes poor correlation: - --orientation of electrode contact inside the cochlea, and their distance from nerve fibers could vary between subjects and within subjects - --the size and shape of the electrode could affect the eCAP threshold - --eCAP stimulus (averaged 100 pulses) at slow stimulus rate (worsens for higher rate map) -- difference between pulse rate of stimulus and speech signals of the stimulus used for thresholds * eCAP follow the T-level profile more than the c-level (c-level is more flat)

Question 29

when to use eCAP

Answer 29

* helps with peds and special pops * helps with cases of absent or untestable eSRTs (PE tubes, no seal) * should not substitute behavioral measurements * better predicted map when combining eCAP with a few behavioral measures

Question 30

eABR response basics

Answer 30

electrically evoked auditory brainstem response * I-V waves * 1-1.5 msec earlier * within 4-5 msec - --interpeak latencies * measured in 71-95% of pts

Question 31

eABR vs eCAP

Answer 31

* devices without telemetry capability * evaluate upper brainstem * reduced stimulus artifact effect (ossification) - --wave 5

Question 32

eABR protocol

Answer 32

``` *montage: G on nasion,+ on high forehead, - on opposite mastoid (opposite to the processor which is set up to the recording computer) *10-50Hz pulse rate *pulse width 25-400 microseconds *1000-2000 sweeps /alternating *100-3000 Hz *artifact rejection +/- 15 micro volts ```

Question 33

challenges of eABR

Answer 33

*very similar to eCAP but can have problems with muscle activities (facial stimulation)

Question 34

identifying the response of eABR

Answer 34

* resembles acoustical ABR * 1-1.5 msec earlier * waves I and II? * compare amplitude over different levels * repeatable * level should be audible (pt feedback)

Question 35

eABR clinical applications

Answer 35

* confirming functionality * programming * threshold with rate: - --slow rate - --fast rate (exceed C-levels, poor correlation) - --slow similar rate (behavioral and phsyiologic)

Question 36

eABR and performance

Answer 36

* weak to moderate correlation (behavioral vs measured) - --better correlation when combined with behavioral measures * significant + correlation between eABR AGF slope and speech perception * significant - correlation between eABR AGF slope and behavioral t-level - --steeper AF associated with robust eABR, lower t-level, lower eABR threshold - -----better performance