General Principles of AERs Flashcards
1
Q
What are auditory evoked responses?
A
Represent activity within the auditory system that is stimulated or evoked by sound
2
Q
How are AERs typically described?
A
In terms of either the region of the auditory system where they are generated or their temporal relation to other responses
3
Q
When were EEGs discovered?
A
1929 by Berger
4
Q
When were the earliest AEPs discovered (cochlear microphonic)?
A
1930 by Weaver and Bray
5
Q
When was the first recorded alternation in EEG with auditory stimulation in humans?
A
1939 by P. Davis
6
Q
When were summating potentials first described in animals?
A
1950 by Davis
7
Q
When was the ABR first described in animals?
A
1971 by Jewett
Response first shown in 1967 but reported as ECochG
Then described in humans that same year
8
Q
When was the relationship between ECochG abnormalities and meniere’s disease discovered?
A
1974 by Eggermont
9
Q
When did the description of ABRs in infants and young children described?
A
1974 by Hecox and Galambos
10
Q
Can electrical potentials in the human nervous system be recorded both in response to external stimuli and without external stimuli?
A
Yes, can either be evoked or non-evoked
11
Q
Are EEGs considered a non-evoked potential?
A
Yes
ECochG and ABR are considered to be evoked
12
Q
How are AEPs classified?
A
According to whether their characteristics are determined by external or internal processes (exogenous vs endogenous)
According to the time epoch following the stimulus in which they occur (latency)
According to the relation of the recording electrodes to the actual generator sites (near vs far field)
According to what structures in the auditory system generates them (receptor potentials vs neurogenic potentials)
13
Q
What is exogenous?
A
Do not have to hear the signal
Could be performed when the patient is asleep or sedated
14
Q
What is endogenous?
A
The patient must hear the signal
15
Q
What are the timing groups for AEPs?
A
Very early - 0-1.5 ms (CM, SP, N1) (ECochG)
Early - 1.5-12 ms (nerve and brainstem) (ABR)
Middle - 12-50 ms (thalamus and auditory cortex) (MLR)
Slow - 50-300 ms (1st and 2nd areas of the cortex) (ALR)
P300 - 300+ ms (1st and association areas)
16
Q
Is AP of the ECochG the same thing as wave 1 of the ABR?
A
Yes
17
Q
Most AEPs are recorded in the far-field (extracranially), what are the two exceptions?
A
During intraoperative monitoring (when recording electrodes mat be placed directly on the VIIIth Nerve)
During transtympanic membrane ECochG (when a recording electrode may be placed on the promontory)
18
Q
What are receptor potentials vs neurogenic potentials?
A
Receptor potentials are generated from the cochlear hair calls
Neurogenic potentials are generated by the VIIIth nerve and/or brainstem
19
Q
What are the stimulus factors that could affect the recording and measurement of AEPs?
A
Stimulus type
Stimulus duration and rise time
Stimulus polarity
Stimulus intensity
Stimulus rate
20
Q
What stimulus are early latency AEPs best generated with?
A
Very brief (trasnient) stimuli having an almost instantaneous onset
21
Q
What stimuli are simultaneous responses from a large number of neural units best generated with?
A
An electrical pulse, which sounds like a brief click
22
Q
What is a click?
A
A click, or pulse, is characterized by an abrupt or rapid onset and a broad frequency bandwidth, theoretically containing all frequencies
A click causes stimulation of a broad portion of the cochlear partition simultaneously, which in turn causes a response from a large number of neurons and more neurons that discharge within a very brief time (the larger the amplitude of the recorded peaks will be)
The standard clikc duration used in clinical ECochG and ABR recordings is 100 microseconds or 0.1 ms
23
Q
Why are abrupt stimuli more desirable?
A
because slowly changing stimuli may not elicit responses from a sufficiently large number of neurons at one time to see a surface recorded ECochG or ABR
24
Q
Do evoked responses directly depend on temporal synchronization or neuronal activity?
A
Yes
25
Should the duration of the stimulus (click) alter the ABR response?
No, because ABR is an onset response
26
Is the stimulus more frequency specific the longer the duration?
Yes (pure tone)
The shorter the stimulus, the less frequency specific it will be
27
Is the primary frequency emphasis of a click determined by the resonant frequency of the transducer?
Yes, because it is a broad band signal
Standard TDH-39 or TDH-49 supra-aural earphones, or ER-3 insert earphones have maximum energy peaks in the frequency range between 1000 and 4000 Hz.
Thus, a click, though a broadband stimulus, is nonetheless somewhat frequency specific based primarily on the frequency response of the earphones (transducer)
28
Where is the greatest agreement between clicks and pure tone thresholds?
In the 2000 to 4000 Hz frequency range
29
What are the other (more frequency specific) stimuli that have been proposed?
Filtered clicks
Clicks with high-pass noise
Tonebursts
Tonebursts with high-pass noise
Tonebursts in notched-noise
Amplitude modulated (AM) tones
Frequency-modulated (FM) tones
30
Do the onset and offset (envelope) characteristics of the stimulus also affect the spectral energy or frequency characteristics of the stimulus?
Yes
If an envelope is used that has more gradual onset and offset function, such as a cosine-squared, Blackman, or Hanning function, then there is less spectral spread of energy into other frequency ranges
So, it is more desirable to use other than a linear envelope to maintain as much frequency specificity as possible
31
What is polarity?
The initial direction of the pressure wavefront in the stimulus waveform, measured at the face of the transducer
32
What are the three types of polarities?
Condensation
Rarefaction
Alternating
33
What is a rarefaction polarity?
A polarity that initially causes the pressure wavefront of a transducer to move away from the eardrum
Pushing away
An initial outward movement of the earphone diaphragm that generally leads to an outward movement of the stapes footplate and an upward motion of the basal-most structures of the organ or corti
Upward motion of the BM is generally depolarizing for the hair cells, this means that latency is slightly shorter and amplitude is higher for the early components of the AEPs rarefaction compared to condensation
34
What is condensation polarity?
A polarity that initially causes the pressure wavefront of a transducer to move toward the eardrum
Pushing toward
Produces an initial inward movement of the earphone, followed by an outward movement and depolarization of the hair cells
The early components of AEPs may be slightly longer than those produced by rarefaction - the latency difference is not significant in normal-hearing subjects
35
What is alternating polarity?
The polarity of the stimulus pressure wavefront is alternating on successive trials
Sum of rarefaction and condensation
Sometimes used at high intensities to reduce stimulus artifact (done after ruling out ANSD)
Not recommended for AC testing ABR because spuriously abnormal recordings may result from cancellation of responses that are out of phase - but for the most part, you will be ok
36
Does the amplitude of wave V tend to be larger in response to condensation stimuli than for rarefaction stimuli?
Yes, specifically for normal-hearing subjects
But not really clinically relevant
37
When are alternating polarities commonly used?
ECochG
38
Do all waves in early AEPs show a systematic increase in latency and a decrease in amplitude?
Yes, as the stimulus intensity decreases
All waves of an early AEP show a systematic decrease in latency and increase in amplitude as stimulus intensity increases
39
What is the unit of measure for stimulus intensity?
Decibel (dB)
As many as five references may be used to describe stimulus intensity (dB SPL, dB peSPL (peak-equivalent SPL), dB HL, dB SL, dB nHL (normal hearing level))
40
What reference is used to describe stimulus intensity for ABRs?
dB nHL
dB relative to normal behavioral hearing threshold level for a click stimulus
ABR threshold is not the true threshold, need to compare to normative data and correction factors (collected from a number of normal-hearing listeners)
41
What is the average of dB nHL?
0 dB nHL
This is the reference level for indicating clinical intensity level
42
What is the actual intensity level and frequency information reaching the cochlea dependent on?
The acoustic properties of the transducer
The volume of the ear canal
Middle ear transmission characteristics
43
Does the rate (times per second) at which test stimuli are presented affect both the latency and amplitude?
Yes
There is no single correct rate, or on that is appropriate for all test circumstances
Cannot use to same rate for all testing (early and late) - due to the different neurons that are involved; moving from small fast neurons to large slower neurons
In general, stimulus rates above approx 30/sec, the latency of all components increases and the amplitude of the earlier components decreases - in a normal system, you can go up to rates of 90/sec and get decent results
44
What are interstimulus intervals (ISI)?
Defined as the time interval between successive stimuli presentation
For transient stimuli (like clicks), the interval between successive stimuli can be determined by dividing a discreet time period by the number of stimuli presented (1 sec/rate = ISI)
To see it in seconds, you need to multiple by 1000
45
Do fast responses, such as ECochG or ABR, occur with a relatively brief time period?
Yes, 5-6 ms or less
Require relatively brief ISIs and permit more rapid stimulus rates
46
Is the effect of ISIs on AEPs related to basic neurophysiologic mechanisms?
Yes
Following every neural event, there is a recovery or refractory period during which the neural unit is either incapable of being activated or has a higher threshold for activation
If the ISI time period exceeds the refractory period, the neural unit can fully recover and will be responsive to the next stimulus
If the ISI is shorter than the refractory period, some stimuli will not contribute fully to the response because the are presented during the refractory period
47
What may result if the ISI is too short?
Alteration of the response such as increase latency or decreased amplitude (and therefore morphology)
48
Do later responses require a longer refractory period?
Yes, this is why you cannot use clicks for the mid- to late-responses
They generally move slower
49
What are the common components of instrumentation that are used when recording AEPs?
Stimulus generators
Electrodes
Filters (to take out artifacts and establish a stimulus)
Amplifiers (to make the signal large enough to project on the screen and interpret)
A signal averager with artifact rejection (average out the responses that you get)
Response delay (trigger) - when do you start measuring the response
Response processing
A means to print or display test results
50
Are the stimulus generation and recording systems generally connected by a time-lock trigger?
Yes
51
What are the non-pathologic subject factors for AEPs?
Age
Gender
Body temperature
State of arousal
Drugs
Muscle activity - heavily affects mid- and late-responses
*these factors can affect the responses that are collected, even when no pathology is present
52
Why are insert earphones recommended for acquiring early AEPs?
Separation of the stimulus artifact from the onset of the response through a 0.9-ms delay line in the earphone makes Wave I of the ABR more visible in most instances
These earphones prevent ear canal collapse
Increase interaural attenuation
Attenuate environmental noise
53
Can inserts affect the amplitude of wave I of ABRs?
Yes
Generally, wave I amplitude will be lower with inserts than supra-aural headphones
This may be due to the inclusion of more stimulus artifact in the response obtained with supra-aural earphones, where there is less time separation between the stimulus and the response
The difference in Wave I amplitude will affect the Wave V-to-Wave I amplitude ratio obtained with different types of earphones - not typically clinically significant
If amplitudes are measured, information about the type of earphones used should be provided
54
Will insert earphones allow for a better view of the cochlear microphonic and the summating potential of the ECochG?
Yes
55
What is the purpose of the electrode when recording AEPs?
The sensing device that detects bioelectric activity and sends it to the pre-amplifier
Makes use of a specialized metal plate through which electrical stimuli are measured or applied to the body
Recorded by attaching electrodes to scalp, mastoid, earlobes, external ear canal, or TM
56
Are there several types of electrodes that can be used in AEP measurement?
Yes
The overall objective in selecting an electrode type and specific placement site is to consistently record complete, clear, and sufficiently large evoked responses
Electrode effects depend on the complex interaction between electrode location and location of neural generators of the responses
Electrode effects may alter the AERs, such as latency, amplitude, morphology, and polarity
57
How is the electrode connected?
It is connected to an insulated lead wire (typically one meter or 36 inches in length) that is connected to a DIN pin that is about 2 mm in diameter
58
What do you do if the lead wire is not long enough?
There are electrode cable extension wires available
Typical length for cable extensions is about 2 meters
59
Do electrodes essentially function as antennae in electrically hostile environments?
Yes, and longer leads can lead to greater electrical interference
A shorter wire (one foot) reduces electrical interference, but is not commonly used
60
What are the most commonly used electrodes used in AER measurements?
Disc or cup electrode (metal ones)
Disposable electrode
61
What are the other types of electrodes that are used for specific applications?
Earclip electrodes (ABR) - typically used for pediatrics
Canal (tiptrode/tymptrode) electrode for ECochG
62
What are disc-type electrodes?
Available in adult (10 mm) or peds (6 mm) sizes
Metal cup coated with tin, silver, gold, or platinum
A hole of about 2 mm in the center to inject conducting paste/gel
Silver-chloride coated electrodes are particularly useful for very low frequency (slow) electrical responses
63
Why is it best to use electrodes made of the same metal in one recording?
To avoid imbalances caused by different electrical properties of different metal compositions of electrodes
64
What are disposable electrodes?
Available in different shapes and sizes
Disc area is small about 20 to 25 mm
Self-adhesive; backing is removed from the adhesive side prior to application
Commonly used for infants
Not sterile, but maybe more hygienic than metal electrodes
Skin needs to be prepared but conducting gel/paste/cream is not required as electrodes already have it on
There is typically an expiration date because of the gel
Either the electrode is connected to a reusable snap lead wire or alligator clip lead wire at the top of the electrode; the entire electrode and wire can also be disposable
65
What are the advantages for disposable electrodes?
Application to the skin without application of conducting gel, paste, or cream
No tapes required to adhere to skin
Contributes to infection control
66
What are the disadvantages for disposable electrodes?
Replacement costs
Limited selection of materials
Limitations of design (no ear clip style)
They have an expiration date and should not be used past the expiration date
67
What is the electrode montage?
Ground (Fpz) on forehead between eyes
Vertex (Cz) on point equidistant between the left and right ear canals on the coronal plane and equidistant between the nasion and inion in the sagittal plane
Left ear (A1)
Right ear (A2)
68
What are non-inverting electrodes?
Located at Cz or midline of forehead near Fz
The polarity of the signal coming from this electrode is not inverted
Will take the signal that is gathers and will be left alone - how it comes in is how it stays (no manipulation)
69
What are inverting electrodes?
On the earlobe or mastoid of the stimulus side (A1 or A2)
The polarity of the signal coming from this electrode is inverted
When it comes in, it gets inverted by 180 degrees - flipped over into the mirror image
The sum of the inverted and non-inverted signal will filter our anything that is common between the electrodes, because they will cancel out
70
What do filters do?
Selectively remove part (or parts) of something from the whole
In AEP measurement, filters reject electrical activity at certain frequencies and pass energy at other frequencies
In AEP measurement the filter settings describe the filter band through which the physiological response is recorded from the electrodes
71
What are filters of physiological responses used for?
Eliminate as much internal noise (electrical activity coming from the patient) as possible
Eliminate as much of external electrical noise (e.g., 60Hz) as possible
72
Is physiological filtering different from any filtering of a stimulus transduced through an earphone?
Yes
73
What is noise in terms of filter settings?
Any electrical activity detected by the electrodes (from the patient or from external sources) which is not AEP
Anything that is not part of the signal we are interested in
74
What are the filter types?
High-pass filter - rejects lower frequency energy and allows high frequency energy to pass
Low-pass filter - rejects higher frequency energy and allows lower frequency energy to pass
Bandpass filter - rejects energy below a curtain cutoff and above a certain cutoff passing energy for a band of frequencies between the two cutoffs
Band-reject or notch - rejects very specific frequencies between a low and high cutoff frequency; not typically used in AER recordings
75
Are ABRs obtained with three different filter settings?
Yes
May show differences in amplitude, generally with increased amplitude and more low-frequency energy is included
Generally will not mess with the filters though - only when there is interference
76
Is filtering another technique used to enhance signal detection in the presence of electrical activity?
Yes
Filters selectively remove parts from a whole
In AER measurements, filters reject electrical energy at some frequencies and pass energy at other frequencies
77
Can electromyogenic activity (neuromuscular) be completely filtered out?
No
Electromyogenic activity shares a portion of the ABR spectrum (100 to 500 Hz region)
Electromyogenic activity at higher frequencies (up to 5000 Hz) can be filtered out
78
What are filter slopes?
Cutoff at specified frequencies doesn't happen abruptly, but at a given slope
The slope represents how fast or slow the gain measured in dB is reduced
Quantified in dB per octave
One can define how steep the “cliff” at the cutoff point is going to be by choosing a steep or gradual slope
Filter slopes can play a critical role in AER recordings, particularly the mid and late AERs
79
Why are right choices for filters important?
Too much filtering can eliminate the AER response
Too little filtering can lead to high noise levels and poor AER recordings
Filtering can cause distortion in the response
Can also affect the latency of the response
Consistency of filter settings is important to ensure reliable comparisons across waveform recordings
*band-pass filters are commonly used
80
Why would a band-pass filter setting of 3 to 50 Hz be inappropriate for ABR recordings?
Because the spectral energy of the ABR waveform is most dominant between 30 to 3000 Hz
Most ABR responses would be eliminated with such a filter setting
81
Why are amplifiers used in AEPs?
A device increases the strength of the signal
Amplifiers are crucial components of any AEP system
AEPs generated by the cochlear or VIIIth Nerve are very small in strength (1µV – a microvolt – a millionth of a volt)
The average amplitude for ABR wave V is 0.5 µV - need to amplify it a lot to see it
82
Do ABR responses need to be amplified?
Yes
Because they are too small
An ABR amplifier gain is typically set at X 100,000
Amplification is crucial to evoked potentials because AERs generated by the CANs are very small in amplitude compared to the EEG
83
What are the two characteristics of amplifiers that have direct influence on successful AER recordings?
Input impedance
Common mode rejection (CMR)
84
What is input impedance?
There is opposition to current flow, specifically impedance, across the amplifier input i.e., the electrodes
For AER recordings, optimally, the input impedance of the amplifier should equal or be higher than electrode impedance
Serious recording problems occur with imbalance (asymmetry) between interelectrode impedance or high impedance for each electrode
3-5 kohms or less
85
What is common mode rejection?
Vital amplifier function
CMR allows the electrodes to pick up what is common to each electrode (noise) and cancel it out
Two electrodes placed at different locations on the head, such as forehead and earlobe, will both detect the same electrical interferences/activity that is not occurring in response to the stimuli
That interference will be canceled out
The evoked response at the two electrode sites will be different and those responses will be retained
86
What is a differential amplifier?
A component of the evoked potential CMR system
It reverses polarity (+ve or -ve) of the inverting electrode’s input voltage and adds it to the non-inverting electrode
This is, in fact, a subtraction process
The same activity in each electrode is eliminated (rejected)
The electrical interference is subtracted out and what remains is the AER
87
What would happen if two electrodes were placed next to each other on the same location?
Result would be a nearly flat line
Because similar AER activity is recorded by each electrode and eliminated by the CMR process
Recordings made from electrodes placed close to each other is ill advised due to the CMR process
Subtracting activity from the vertex or forehead and the ear lobe electrode reduces noise interference
It also may increase amplitude of some components of the AER waveforms (but not the early ones, particularly wave I of ABR)
88
Is artifact rejection a necessary feature of AER instrumentation?
Yes, this allows us to view the incoming waveform in its crude states before its averaged or filtered
We are looking at a combination of the AER, the EEG, and unwanted electrical interference
Even at this stage, the input has been exposed to CMR and amplification
Periodic review of the un-averaged signal is helpful to detect quality of incoming waveform
Clinicians can judge if the data coming in is acceptable or has too much unwanted interferences
89
What are some sources of artifacts?
Electrical
Electromagnetic (generated from an external or non-patient sources, such as an x-ray view box, phone, or computer)
Electrophysiologic (originating from the patient; such as neuromuscular potentials or myogenic activity)
90
What are the three main approaches to reducing the negative influence of artifacts on the ABR?
Remove source of artifact - turn off x-ray box or phone, ask the patient to sleep or relax, have a dedicated electric circuit for AERs
Modify test parameters - increase intensity, change filters, increase number of sweeps
Artifact rejection - any voltage that exceeds the designated pre-set voltage is not sent to onto the signal averaging computer
91
In theory, are only signals within a certain voltage range accepted and averaged?
Yes
92
What is a signal averager?
Converts the analog electrical activity from the amplifiers (the physiological response) into a series of numerical (digital) values
These digital values are processed by the computer to generate the summed or averaged response
The time period in which the response of interest occurs determines the time window set for collection of digital values (about 10-15 msec for ABR and 2-5 msec for ECochG)
This time window is broken down into a series of time units, time bins, or data points after the occurrence of the stimulus
Over successive stimulus presentations, positive values consistently accumulate in some time bins and negative values in others, resulting in the familiar response waveform
*further reduces the noise so the desired response can be seen and increases the size of the AER
93
Does signal averaging make AERs clinically possible?
Yes
94
Is the voltage of the waveform over the course of the analysis period sampled?
Yes
It is averaged at a certain number of points and expressed as a number
Typically, 256 sample points are used
512 or even 1024 points can be used
The greater the sample points, i.e., higher the sampling rate, the better the waveform resolution
95
Is noise detected by the electrodes also averaged?
Yes
Noise is random with different phases that alternate between positive and negative
Adding together this activity or greater signal averaging will eventually result in the noise being cancelled or “averaged” out or reduced in size leaving behind mostly the AER
96
What are artifact rejections?
When a sweep occurs that contains excessive voltage amplitudes, excessive noise is included in the average
Artifact rejection excludes these excessive voltages
The artifact rejection level is set so that sweeps containing voltages well above the voltages of interest are rejected, and
Not averaged
For the ABR, which is in the range of 0.1 to 1.0 µV, a typical artifact rejection level might be 10 µV
This will serve to exclude large electrical responses, such as those related to muscle activity or the environment, while including the response of interest
97
What is a response delay/trigger?
To extract a time-locked waveform form background noise, the computer must “know” when the stimulus occurs and when to begin a recording epoch
A trigger pulse that marks the beginning of the recording epoch can be set to occur in conjunction with the stimulus
Usually the recording epoch begins with the onset of the stimulus and the trigger pulse is coupled to the onset of the stimulus
You can also set the trigger to occur just before the onset of the stimulus which could allow a recording of a “pre-stimulus interval”, which may be useful in evaluating noise
98
What are the factors that affect AEPs?
Stimulus Factors
Acquisition Factors
Non-Pathologic Subject factors (gender, age, etc.)
Waveform Analysis
99
What are subject factors that are known to influence AEPs and must be considered clinically in the interpretation of the findings?
Subject Age
Subject Gender (male vs. female differences)
Subject’s Body Temperature
Subject’s State of Arousal
Subject Muscular Artifact
Drug Effects
100
Does the effect of subject age vary considerably among AEPs?
Yes
No AEP is fully mature and adult-like in a preterm infant
The general principle relating age and AEPs is that shorter-latency responses mature at an earlier age than longer-latency responses (mature from distal to medial)
Age also interacts in a complex fashion with other subject characteristics (e.g., sensorineural hearing loss), with stimulus parameters (e.g., rate and intensity), and with acquisition parameters (e.g., filter settings)
101
If the patient is normothermia (normal) at the time of testing, is there a need to account for temperature in the interpretation of AEPs?
No
Temperature exceeding 1oC from this value must be considered as a possible factor in AEP outcome
Patients at risk for temperature aberrations include those with infection (high temperature) and those in coma, or under the effects of alcohol or anesthesia (low temperature)
102
What are the things we are looking at when we are interpreting AEPs?
Presence vs Absence of response or landmark
Absolute latency (what time interval does something come in)
Inter-peak latency or relative latency (useful when trying to figure out where lesion is)
Amplitude (not as important with ABR)
Inter-aural difference or ratio
Morphology and reproducibility
103
Are most AEP waveforms in the time domain?
Yes
That is, the amplitude of a response (microvolts) is displayed over time (ms)
AEP waveforms shown in the time domain are a sequence of peaks, amplitude of greater voltage) and valleys (amplitude of lower voltage) occurring within a specific time period (the analysis time or epoch)
104
What is latency?
The time interval between the exact moment of stimulus presentation and the appearance of a change in the waveform
Expressed in ms
105
Do we usually mark waves on the peak or trough?
Yes
However, other clinicians do not always define AEP waves by peak. Instead, some other portion of the wave, such as the shoulder, is used for selected components
This is common practice for identification of ABR wave V
106
How is amplitude measured?
Preceding valley to peak
Not used much unless its ECochG
Usually described in microvolts
107
Can you calculate the voltage difference between the peak and the preceding trough?
Yes
108
What is morphology?
The pattern or overall shape of these waves
Usually, morphology is described with reference to an expected normal appearance, even though wave component latency and amplitude values may be within normal limits, morphology is often judged "poor"
In clinical AEP measurement, morphology currently remains a rather subjective analysis parameter
Need to be able to replicate (usually replicate near threshold)
109
Why may AERs be useful?
Evaluation of hearing sensitivity (threshold ABR, ASSR)
Evaluation of CANS for pathology (neurodiagnostic ABR)
Evaluation of the CANS including auditory processing (ALR, MLR, P300, MMN)
Evaluation of children with language, cognitive, and other developmental disorders
Monitoring effectiveness of intervention, such as with CIs or HAs and auditory training, because of the plasticity of the CANS
110
Do AERs show a high correlation to physiological changes in the auditory pathway?
Yes
111
Why AERs?
Early detection and accurate diagnosis of auditory disfunction, and to provide effective intervention for hearing loss and other types of auditory disorders
Automated auditory evoked response measurement and analysis makes possible universal newborn hearing screening, i.e., hearing screening of large numbers of babies by non-professional personnel
Cortical auditory evoked responses can be used in documentation of central nervous system maturation and development in infants following intervention with hearing aids or cochlear implantation
Auditory evoked responses offer greater sensitivity to certain types of auditory dysfunction than behavioral audiometry, including neural dysfunction
Auditory evoked responses offer greater specificity for detection and diagnosis of dysfunction in specific regions of the auditory system than behavioral audiometry
112
Are AERs feasible in patients who cannot be assessed validly with behavioral audiometry?
Yes, including newborn infants, difficult-to test-children, patients with developmental disorders, patients with cognitive impairment, persons with false or exaggerated hearing loss, very Sick Patients or Sleeping or unconscious patients, patients who are anesthetized and undergoing surgery that puts the auditory system at risk, comatose patients with severe head injury who have central nervous system damage
113
Can the overall success of the procedure be increased when certain pre-test practices are carried out?
Yes, such as patient instructions, patient arousal state, patient medications, makeup, hair, and clothing (the more relaxed and still they are, the better)
Also preparation of the electrodes (such as scrubbing the skin to remove dead skin, debris, and oil that might interfere with conductance of electrical activity - gives it a clean signal)
114
What is electrode impedance?
A material's resistance to flow of electrical current
Measured in ohms
Need to measure how good the electrode contacts are connected to the skin
3-5 or less kohms are ideal
Interelectrode ones are generally 2 or less kohms
115
How do you measure interelectrode impedance?
A small current, generally in the region of 30 Hz is applied automatically by the machine to one of the electrodes
The amount of current crossing over to the second electrode is determined
From this data interelectrode impedance is calculated
Often testing cannot proceed till certain criterion for interelectrode impedances are met
*want them to be low
116
When should impedances be measured?
Before and during the AER recording is there is reason to suspect change
Excessive patient movement, increased electrical artifact, and when switching between ears
117
If electrical interference is too severe, can it obliterate AERs?
Yes
118
What do low and balanced absolute electrode impedances represent?
High quality AER recording
Limiting internal amplifier noise
Reducing effects of external electrical interference (noise)
Maintaining higher common mode rejection (CMR) ratios
The convention for maximum desirable impedances is between 1 to 5 kOhms
An impedance of 0 kOhms is not desirable because it may reflect a direct connection between two closely spaced electrodes and it may lead to a short circuit at the amplifier output
119
While low absolute impedances are desirable, are balanced impedances among electrodes necessary?
Yes
Interelectrode impedance differences should be as small as possible, ~ 2 kOhms
A high impedance electrode in combination with a low impedance electrode can create an electrical imbalance at the input to a differential amplifier
Impedance imbalance can cause excessive interference from sources of the electrical artifact
Low (within 2 Kohms) inter-electrode impedance is very important for common mode rejection (CMR) to work well
120
What happens if the impedance is reading "open" in the software?
It will read 99.9 on the preamp
"Open" means the impedance is greater than 80kOhm
This can happen if there is a faulty electrode
Or if no electrode is plugged into that receptor on the pre-amp, for example, selecting a two-channel recording but selecting a set-up for a one-channel montage in the computer
121
What is the major determinant of electrode impedance?
The tester is the contact between the skin and the electrode
122
How can impedances be lowered?
Pressing on the electrode for several minutes
Moving the electrode slightly to get a better contact with the prepped skin
Adding more electrode conducting paste/gel
Securing the electrode snugly with additional tape
Removing electrodes with higher impedances and re-prepping the skin
*if they remain high, consider the possibility of a break in the electrical conduction (broken wire)
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How do you prepare the skin?
Always use an abrasive preparation gel (e.g., NuPrep) to ensure that the top layer of skin (epidermis) is cleaned and oil is removed
The skin may become a little red after an appropriate preparation
Aim to get impedances below 5kOhm (some advise 3kOhm)
Be careful not to damage the skin
Alcohol wipes/pads only can be used to remove vernix prior to ABR recording on neonates (age 0-3 months) and for preparing the skin
For sensitive/allergic skin, may be best to use only a soft dry cloth since alcohol can dry out the skin
Alcohol takes time to dry, impedances may be slightly higher (make sure alcohol is completely dry before applying conductive gel and electrodes)
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When mounting a pre-gelled disposable electrode, should you press on the middle of the electrode?
No, the gel will be dispersed to the adhesive outer edge causing the electrode to loosen from the skin resulting in very high impedances
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Do reusable electrodes usually have higher impedance than disposable electrodes?
Yes
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What should the test environment conditions be like?
The pre-amplifier (pre-amp) of the ER equipment should not be placed near the isolation transformer, a computer monitor, or other electronic equipment
Turn off any unnecessary computer monitors and fluorescent lights (do not just put them on “dim” mode)
If possible, ensure you are using a designated and grounded electrical outlet (help eliminate unwanted noise)
Unplug the chair if the patient is sitting in an electrically operated chair
Turn off cell phones (not just turning them to silent)
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How do you reduce impedance and improve recordings?
Explain the test procedure in detail before placing the earphones since many patients may have hearing loss (they may be more relaxed if they know what to expect)
Always choose the largest size ear tip to reduce the risk of stimulus leakage (an ear tip that is not inserted properly will cause a reduction in dB SPL at the TM and artificially elevated hearing thresholds)
Braid electrodes or tape them (if they are separated, you can get more noise)
Do not mix electrode types/materials
Do not place ground electrodes near the heart such as on the shoulder of a baby (may pick up EKG responses)
For bone conduction, the electrodes and bone oscillator should be as far apart as possible (place the electrode on the front of the earlobe instead of the back)
Make sure the electrodes are clean
Turn the equipment on before the patient is connected and off after the electrodes have been removed
Avoid the stimulus transducer box touching the patients skin (more likely to pick up artifact)
Do not let the tubing of the inserts touch the electrode wires (causing noise and stimulus artifact)
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Should electrodes on earlobes or mastoid be symmetrical in placement?
Yes
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If using a true Cz placement, why should you place an alcohol pad on top of the hair while scrubbing other sites?
The alcohol will dissolve the hair products giving better impedance
Use a Q-tip with a small amount of Nu-Prep to prepare site
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Where should all electrode leads run?
Toward the top of the patient's head
It is critical that the electrode leads are completely separated from the transducer (earphones) cable and tubing
If the electrode leads are placed towards the top of the head, transducers can be clipped to the front of the body
This is especially important with unsedated babies because if they wake up, they cannot pull off the electrodes
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Are electrode sites usually defined according to the international 10-20 system?
Yes
Number and letter combination telling you where the electrodes go
Midline ones don't typically have a number (because it isn't on the right or left)
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What is the nasion site?
The most anterior-inferior site
Bridge of nose
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What is Fpz?
Low forehead
Fz is high forehead
*z is typically used at midline running from anterior-to-posterior along the center of the head
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What is the inion site?
Most posterior interior site
Site of the occipital protuberance
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Are some auditory evoked responses (like ABR) recorded with only 3 or 4 electrode sites?
Yes
whereas cortical auditory evoked responses are sometimes recorded simultaneously with 20 to 30 electrodes located over the scalp
Precise placement and consistent attachment of multiple scalp electrodes is facilitated by the use of an “electrode cap” available from most manufacturers of evoked response systems
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How are AERs recorded?
With combination or sets of two electrodes located in two different and designated locations, plus a common o ground electrode that can be located anywhere on he body
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What are active and reference?
Other terms for inverting (reference) and non-inverting (active)
Only used by one manufacturer
Do not use
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What does the ground do?
Indispensable in order to record consistently optimal responses in varied test conditions
A well applied ground electrode is one of the most important ways of ensuring high quality AER recordings
A single ground electrode may be shared by more than one pair of non-inverting/inverting electrodes in ER recordings
A ground electrode can be placed anywhere on the body
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Where are electrodes plugged into?
An electrode box or preamp which is connected to the amplifier
An electrode box at this stage has minimally three receptacles or jacks - one each for non-inverting, inverting, and ground electrode (this constitutes a one channel recording)
Most electrode boxes allow for at least two channel recordings with five or six electrode receptacles
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What are jumpers?
For two channel recordings with a Cz or Fz site, to avoid placing two non-inverting electrodes on a patient (one for each channel), the receptacles are linked by a jumper cable
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What can two channel ABR recordings be used for?
Ipsilateral and contralateral ear recordings can be performed simultaneously
Easy to recognize ispi and contra because there will be absent/reduced waves I and II in the contralateral response
Waves III and V are generated from a complex interaction of both contralateral and ipsilateral brainstem anatomy
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