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Using Objective Measures to Estimate Stimulation Levels Flashcards

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1
Q

What are the objective measures?

A

ESRT
EABR
ECAP

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2
Q

What is ESRT?

A

Electrically-Evoked Stapedial Reflex Threshold (ESRT) is a contraction of the stapedial muscle in response to intense electrical stimulation from the cochlear implant
Preferable method for setting upper-stimulation levels in children as precise loudness scaling and balancing are often unreliable before age 8

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3
Q

How can ESRTs be measured?

A

Using the standard impedance bridge and cochlear implant programming equipment

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4
Q

What is the procedure for ESRT?

A

Place acoustic immittance probe into canal contralateral to the CI
Record acoustic admittance while presenting the programming stimulus to the implant in an ascending manner
When the stimulus is strong enough to elicit the reflex, a time-locked decrease in admittance will be observed
Upper stimulation levels should typically not exceed ASRT threshold
Once the ESRT is recorded for a sufficient number of electrodes across the array, the upper stimulation level profile may be set to the ESRT levels
Decrease stimulation levels to avoid loudness discomfort before activating the sound processor in live speech mode
Gradually increase upper stimulation levels
Adjust levels until optimal loudness and sound quality are achieved

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5
Q

Can ESRTs be used for children and adults?

A

Yes
Good for anyone who cannot produce reliable responses
Can be the standard procedure depending on the manufacturer

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6
Q

Why do we want to extend the window view on the immittance equipment during ESRT?

A

Yes
Set to reflex decay so the window expands (10 sec)
Look for changes in immittance measures due to the stimulation of the CI

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7
Q

Will ESRT always have the reflex pattern that we expect?

A

No
Why we start really low and establish what a no response looks like
Not looking for big reflexes either, just looking for threshold

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8
Q

Should you measure tymps first before ESRT?

A

Yes
Make sure you are able to get responses and ensure that the seal is good

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9
Q

What frequencies should be used for ESRT?

A

667 or 1000 Hz
Better chance of getting it than the standard 226 Hz

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10
Q

Why does ESRT not look like standard acoustic reflex decay?

A

The stimulus is not continuous

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11
Q

What should the stimulus be set to on the immittance system?

A

Contralateral or external
Because we are measuring the electrical stimulation through the implant instead

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12
Q

What is the clinical utility of ESRT in estimating upper stimulation levels?

A

Typically, the ESRT is within an average of 9 clinical units of upper-stimulation levels for Nucleus recipients
Typical ranges for upper-stimulation levels relative to ESRT:
10 to 15 CL below ESRT for Nucleus recipients
5 to 10% below ESRT for Advanced Bionics recipients
At or near ESRT for MED-EL recipients
ESRT is lower than levels set by behavioral measures, making it a safer starting point

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13
Q

What are the limitations of ESRT?

A

Measurable in approximately 65-85% of implant users
In about 30% of recipients, subtle ME anomalies may interfere with obtaining a measurable response
If a contralateral ESRT is not measurable, testing may be attempted in the ipsilateral ear
ESRT measurements may be more problematic to measure in bilaterally implanted recipients because both ears have undergone surgery

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14
Q

What are factors that can preclude ESRT measurement?

A

PE tubes
ME effusion
ME dysfunction

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15
Q

Is there better success in measuring contralateral ESRT?

A

Yes
But it may be worth trying if you don’t get anything in the contralateral side

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16
Q

What is EABR?

A

Electrically-evoked auditory brainstem response
A neurophysiological test measuring auditory brainstem activity in response to electrical stimulation from a CI
Recorded from scalp electrodes in response to electrical stimulation from the cochlear implant
Reflects synchronous firing of neurons in the auditory brainstem
Wave V most prominent
Waves are generated in the pons and midbrain regions

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17
Q

Will EABR latencies be earlier?

A

Yes
Because it is a lot closer than acoustic ABR signals

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18
Q

How do you record EABR?

A

Interfacing the cochlear implant programming system with a clinical auditory evoked response system
Electrical stimulation is delivered from the programming software to the CI via an external sound processor or diagnostic transmitting coil
The interface must send a trigger pulse to the evoked response system to mark stimulus onset and begin EEG recording
EEG recorded using surface electrodes on the head

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19
Q

How do you set the stimulation in the ABR system?

A

Set to external stimulation

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20
Q

How should you interpret waveforms for EABR?

A

Two distinct peaks: wave III about 2 ms and wave V about 4 ms
Wave I and II often not seen due to stimulus artifact
Shorter latencies than traditional ABR due to direct electrical stimulation
No latency shift with increasing intensity

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21
Q

What are the clinical uses of EABR?

A

Used to confirm implant function or auditory responsiveness in recipients without telemetry systems
With the development of telemetry and ECAP, the routine clinical use of EABR has declined
ECAP is easier to record, does not require sedation, is less affected by movement, and does not need a separate evoked response system
Still be useful when ECAP cannot be recorded, such as in cases of cochlear abnormalities (e.g., cochlear ossification)
Unlike ECAP, EABR reflects neural activity at the level of the brainstem, not just the cochlear nerve
A present EABR suggests the signal is audible at that stimulus level, but EABR is not a reliable predictor of T levels or upper-stimulation levels

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22
Q

What is ECAP?

A

Electrically-evoked compound action potential
Wave I of ABR
Represents a synchronous response from electrically stimulated auditory nerve fibers
A neural response from the distal cochlear nerve fibers and/or spiral ganglion cell bodies of the cochlear nerve in response to electrical stimulation from the cochlear implant
Typically recorded by stimulating the auditory nerve with a series of pulses applied to a single electrode at a relatively slow rate

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23
Q

Is ECAP known as different things based on manufacturer?

A

Yes
Med-El - auditory response telemetry (ART)
AB - neural response imaging (NRI)
Cochlear - neural response telemetry (NRT)

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24
Q

What does the ECAP look like?

A

Negative peak (N1) at about 0.2-0.4 ms following stimulus onset
Followed by a much smaller positive peak (P2) or plateau occurring at about 0.6-0.8 ms

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25
What is the most likely generator of the ECAP?
Spiral ganglion neurons
26
Is ECAP essentially the electrical version of wave I of the ABR?
Yes, with greater amplitude, shorter latency, and no myogenic artifact
27
Is ECAP unaffected by sleep and anesthesia?
Yes No need for sedation when measured in children
28
Is ECAP successfully measured in a lot of patients?
Yes, about 95% Exceptions in which the ECAP cannot be recorded most often include recipients with cochlear nerve deficiency and those with auditory neural dysfunction
29
What is the setup for ECAP?
Stimulus - biphasic electrical pulse delivered to one intracochlear electrode via monopolar electrode coupling at a rate of 30-80 pulses/sec at stimulation levels similar to those used for measuring stimulation levels during the implant mapping process The ECAP response is recorded on an intracochlear electrode contact that is typically located one to two electrode contacts away (in the apical direction) from the stimulated electrode and referenced to an extracochlear ground electrode *stimulated and recorded with different electrodes
30
How is ECAP measured?
Signal is delivered to a recording amplifier (in the electronics package housed inside the titanium case of the CI) The amplified signal is delivered from the recording amplifier to the DSP of the CI The DSP converts the signal to a digital code which is converted to an electromagnetic signal that is delivered from the coil of the CI to the coil of the external sound processor Signal is then delivered to the programming computer where it is processed, analyzed, and displayed
31
What are some challenges for ECAP recordings?
Current source saturation Poor reference contact Stimulation artifact
32
What is current source saturation?
The internal voltage of the implant depends on the type of coil and implant, but mainly on the distance between the coil and the implant A greater distance results in lower voltage If the supply voltage is too low, the current source provides less current than needed This is called saturation or compliance limit
33
What is poor ECAP reference contact?
Poor contacts (high impedance) make the recording more difficult and the artifacts bigger During surgical recordings, achieving adequate impedance and obtaining accurate results can be challenging if sutures are placed directly over contacts
34
What are some artifact concerns for ECAP?
ECAP is an early-latency evoked potential; subjected to electrical artefact The electrical artefact from the stimulus delivered from the cochlear implant is much larger than the auditory evoked response Requires artifact reduction techniques for accurate interpretation
35
What are artifact reduction techniques?
Forward masking subtraction method Alternating stimulus polarity method Scaled template subtraction reduction
36
What is forward masking subtraction artifact reduction?
Exploits refractory properties of the auditory nerve to isolate stimulus artifact Four stimulus conditions: probe stimulus alone, masker stimulus followed by probe, masker stimulus alone, and zero-current pulse to measure system artifact (subtract the latter two from the first two)
37
What are advantages of the forward masking subtraction?
Most effective artifact reduction technique Largest amplitude ECAP response Lowest ECAP threshold Most robust morphology
38
Is the forward-masking subtraction technique available for all manufacturers?
No Default for cochlear, but unavailable for AB and Med-El
39
What is the alternating polarity method?
Responses are measured for negative-leading (cathodic) and positive-leading (anodic) biphasic pulses Switching stimulus polarity changes the artifact polarity but not the neural response Averaging responses from both polarities cancels out most of the artifact, leaving the neural response
40
What are the limitations for the alternating polarity method?
There may be minor differences in amplitude and latency of the cochlear nerve’s response to cathodic- and anodic-leading biphasic electrical pulses
41
Is the alternating polarity method used for all manufacturers?
Default method in AB and MED-EL devices, may also be used to measure the ECAP with Cochlear Nucleus cochlear implants
42
What is scaled template subtraction artifact reduction?
A method for reducing artifacts in Cochlear Nucleus NRT and MED-EL ART measurements Two phases of stimulation are used: In the first phase, a weak stimulus is applied, which creates an artifact but no neural response In the second phase, a stronger stimulus is used, capturing both the neural response and the artifact By scaling up the artifact from the first phase, it's subtracted from the second phase's data, effectively removing the artifact and preserving the neural response
43
What parameters optimized ECAP recordings?
Amplifier gain Amplifier delay Recording electrode contact
44
What is the typical amplifier gain for ECAP?
The amplifier gain is measured in dB and typically ranges from 40 to 70 dB Setting the amplifier gain to a higher level will improve the potential of recording low-level ECAP responses but will increase the risk of saturating the recording amplifier Setting the amplifier gain at a lower value reduces the risk of amplifier saturation but may not offer enough amplification to allow for successful recording of the ECAP
45
What is the amplifier delay for ECAP?
The amount of time that elapses between the presentation of the signals and the “switch on” of the amplifier Typically ranges from 50 to 125 microseconds
46
What is the recording electrode contact for ECAP?
Can be adjusted in an attempt to avoid amplifier saturation while also allowing for the recording of a robust ECAP Can be set to be as close as one electrode contact away or several electrode contacts away from the stimulating electrode contact Can be located basally from the stimulating electrode contact
47
Does ECAP amplitude increase with increasing stimulus level?
Yes
48
What will increasing the number of averages do for ECAP?
Increasing the number of averages will reduce the amount of noise in the recorded ECAP response Fewer averages are typically used in the operating room due to limitations in testing time
49
What are commonly used recordings for ECAP?
Amplitude growth sequence Recovery sequence Spread of excitation
50
What is amplitude growth sequence?
Serial ECAP measurements on the same intracochlear electrode contact but at different stimulation levels AGF typically includes measures completed at a stimulation level or stimulation levels that are below the ECAP threshold and several ECAP measures completed at suprathreshold stimulation levels The amplitude of the single recordings is raised from a minimum value to a maximum value not to exceed the patient comfort level
51
What are the two ways of showing results of amplitude growth sequence?
Visual analysis Regression analysis
52
How do you perform a visual analysis on ECAP?
As the stimulus level is increased, what happens to the: Amplitude? Morphology? Latency? (latency will not shift)
53
How do you perform a regression analysis for ECAP?
Based on calculating a linear regression line that best fits the ECAP amplitude points obtained in the AGF The regression-derived ECAP threshold is determined by identifying the stimulus level value at which the linear regression line crosses the x-axis To get an accurate threshold, the AGF should have measurements from four to five levels of stimulation, and an ECAP response close to or at the visual ECAP threshold *calculating difference in amplitude at different stimulation levels
54
Is visual or regression more accurate?
Visual
55
What is recovery sequence?
Consists of independent signal sequences that have two stimulation pulses followed by an ECAP measurement The time between the two pulses (inter-pulse interval or IPI) can be varied, and a typical IPI would be between 0.3 ms (this is within the absolute refractory time) and 10 ms *response reduces with shorter IPIs, due to refractory responses
56
What is the spread of excitation?
To estimate intracochlear regions that individual stimulation electrodes excite Normally this is expressed as distance along the electrode array and is also a measure of channel interaction If contact is further from stimulation, then the response will spread further
57
What are the reasons for recording ECAP?
It provides confidence that electrical stimulation leads to action potentials Longitudinal monitoring of ECAP signals can be used to discover changes in the peripheral auditory system It gives additional information helpful for implant fitting, especially in difficult cases It offers a possibility to glean information about the placement of electrodes in the cochlea It offers objective data for research in areas such as physiology and coding strategies It is one of several tests of functionality of the implant and includes information about stimulation of neural tissue
58
What are the clinical applications of ECAP?
Interoperative monitoring Postoperative applications
59
Are ECAPs not as good as ESRT?
Yes, they do not address upper stimulation levels ECAP cannot be reliably used as a predictor of T Levels or upper-stimulation levels Research has shown that electrically evoked auditory response thresholds have a moderately strong correlation to behavioral electrical threshold (i.e., T level) when a similar stimulus is used to elicit each type of response There is not a one-to-one relationship between the ECAP threshold and T level or upper-stimulation level
60
What is the off-set method?
Used for estimating T and C levels from ECAP thresholds ECAP threshold is measured on all active intracochlear electrode contacts Behavioral T and upper-stimulation levels are measured for one electrode contact (typically in the middle of the array) An offset (i.e., difference value: difference between T level and ECAP threshold and upper-stimulation level and ECAP threshold for the channel on which the behavioral measures are obtained) is determined between the ECAP and the measured T and upper-stimulation levels for that channel The offset measured at that electrode is used to estimate T levels and upper-stimulation levels for the rest of the active channels

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