AEP Exam 2

What are the pros and cons of surface electrodes and needle electrodes?

Surface Electrodes:
-Less invasive
-Cheaper
-Pain free
-Difficulties with skin prep/placement/location

Needle Electrodes:
-Stronger recordings
-Closer location
-Painful, requires doctor
-More expensive

How to clean the skin in AEP tests?

Clean with material (alcohol wipes, prep pads) on areas where electrodes will be placed. Rub and wipe thoroughly to remove unwanted material on the skin.

Why does the skin need to be cleaned in AEP tests?

To improve the electrical conductivity of the electrodes. Don't want anything impeding the signal.

Why do we need to apply gel, cream, or paste on the skin in placing certain types of electrodes in AEP tests?

This helps keep the electrode tightly placed on the skin to reduce the number of mediums involved in recording. Otherwise "air" might become a medium if not tight enough. Would cause recordings to be more difficulty. To improve the electrical conductivity.

Principles to improve electrical conductivity of electrodes:

(a) Electrodes should be made of materials of better conductance, such as gold or silver
(b) Have a large surface area for surface electrodes
(c) Have good contact on the skin

What is the basic three-lead electrode array in AEP tests?

Primary (Noninverting, +) electrode
Secondary (Inverting, -) electrode
Common (Ground) electrode

Why do we say there are two circuits in commonly used AEP tests protocols while there are only three electrodes being placed?

These circuits provide the electrical potentials:

1. Noninverting-Ground

2. Inverting-Ground. This is seen when you check impedances for the forehead circuit and mastoid electrodes.

Why do we have to get the interelectrode impedance low enough in AEP tests?

If there is a large difference (or high impedance) energy is reflected back which reduces the electrical signal being measured.
High impedance \= low conductance. Forces oppose transmission of the electrical signal which means it is reduced.

What does balanced electrode impedance for the three-lead array mean?

Means the impedances are at the same number

If you get a high impedance sign before conducting an AEP test, what are the possible causes?

1. Poor contact between the electrodes and skin; 2.Break in connection between electrodes and recording system;

2. Defect in system.

Why is an amplifier needed in an AEP recording system?

AEPs generated by auditory system and recorded in far-field (i.e. electrode on skull) responses are very weak. Amplitude for particularly short latency AEPs are very low and difficult to improve SNR. So we need the signal amplified.

What does the amplification gain mean?

The ratio of the voltage of a signal at the output of an amplifier relative to that of the input. Increasing signal level and reduce noise levels which improves SNR.

Output/Input - absolute measure ; S/N in dB is S-N; ex 60 dB - 30 dB \= 30 dB SNR

Is the same amplification gain needed for all types of AEP recordings? Why?

No, there is a range of about 10,000 - 500,000 ; the gain of the amplifier depends on the application

Is the amplifier in an AEP system only used to amplify the amplitude of the recorded electric signals? Why?

No - the electrical signals can be put through a subtraction or addition process or change polarity. It helps us reduce the noise which improves the overall SNR.

Why is the amplifier in an AEP system also called a differential amplifier?

It isn't just as simple as increasing the signal. Differential amplifiers increase the amplitude of some AEP components and have common mode rejection. The functions are amplification and CMR to reduce noise level.

What is the common mode rejection (CMR) procedure? How does it work?

It helps allow us to reduce noise with noise rejection. Noise from the EEG, power line, and other equipment. This takes 2 recordings with same polarity happening at the same time and with same amplitude and phase - it mixes them together and the noise cancels out.

What are the common-source electric noise sources in AEP recording?

(a) The non-response electrical activity of the brain
(b) Myogenic electrical activity
( c) Power line signals
(d) Electric signals from a computer

Would the noise in an AEP test be completely removed after the recorded signals go through a differential amplifier? Why?

No, in reality you cannot remove all noise. You can partially remove or reduce noise through a differential amplifier but you need more help controlling noise than just that.

What is the major function of a filter in an AEP recording system?

To further reduce or eliminate noise even more (non-AEP electrical activities).

Which type of filter is most commonly used in AEP tests?

Band-pass filters are most common. Usually from 30-3000 Hz.

If the frequency range of an AEP signal is mainly in between 50 and 1000 Hz, please design a filter for recording this signal.

30 - 1500 Hz

Would all non-AEP electric signals be filtered out if we set up an appropriate cut-off frequencies? Why?

No, a filter cannot removed all noise from background/myogenic noise. There is also a large variability between individuals.

What would be seen in recorded AEP waveform if the following filter parameters are used?

1. The high-pass cutoff frequency is too low:

2. The low-pass cutoff frequency is too high:

1. The high-pass cutoff frequency is too low: Latency would be prolonged and amplitude would increase; generally distorting the waveform and may remove some components of AEP. -Latency is shorter and amplitude is smaller as high-pass filter setting is raised.

2. The low-pass cutoff frequency is too high: Sharper peaks in waveform and increased amount of high frequency information (which leads to all the tiny peaks seen).

Why would the responses be enhanced and noise reduced after repeatedly recorded AEP signals are averaged?

Because responses are always in phase and occur at the same latency when evoked by stimulus; noise on the other hand is random in time from sources. Often times noise is partially or completely out-of-phase and cancel out. Responses are synchronous events evoked by stimuli (i.e. time locked).

Should the same number of sweeps be used for all types of AEP recordings? Why?

No, the number of sweeps should depend on
1-AEP amplitude: early responses weak, so we need more averages; or need well defined waveform can increase sweeps
2-Noise level: if in nice sound booth, can reduce number of sweeps
3-Desired SNR

Which of the measures will be significantly affected by the number of sweeps in AEP test: latency or amplitude? Why?

Latency is unaffected by number of sweeps, so it would be amplitude. As number of sweeps increases, amplitude is usually improved and vice versa. Weak signal will need more averaging.

Commonly, waveform is used. That is, AEP results are displayed in the time domain.

Time domain is a variation of amplitude over time. To show frequency components, can be converted into spectrum; however spectrum is not useful in most AEP recordings.

What does the analysis time mean in AEP recording?

The period of time in which a recorded AEP is displayed (time window)

Our decision on AEP analysis time (or time window in recording) depends on the expected latency or recorded AEPs. Why?

Those are the most reliable and consistent aspects of the recording, so we need to make sure we can observe them. Under certain conditions latency may be prolonged, so we need to change analysis time.

Understand the commonly seen conditions with which the analysis time needs to be longer (Hint: those conditions with which the AEP latency may be prolonged).

For low stimulus intensity or low-frequency tone bursts; certain non-pathological factors such as age; auditory or neurologic pathology; insert earphones; hearing loss will need to increase intensity.

Morphology : A subjective analysis, e.g. "good" or "poor"

This is based on the pattern or overall shape of the waves, or their repeatability.

Absolute latency

time from onset of stimulus until specific peak occurs

Inter-wave latency:

time from peak-to-peak; time gap in between 2 waves

Amplitude
Absolute amplitude
Measurement of amplitude: the voltage difference
Peak-to-trough : e.g. ABR
Trough-to-peak: e.g. AMLR; ALR
Peak-to-baseline: ECochG
Amplitude ratio: e.g. SP/AP; V/I - wave V should be \> 50% of wave I

Why may the following approaches reduce electric noise in AEP tests?

1- Separate the transducer from the electrode cables as much as possible. 2- Conduct test in shielded sound booth with electronic instruments placed outside. 3- Plug the AEP unit into an isolated socket equipped with a true-earth ground. 4- Use a grounded cable for the primary electrode. 5- Turn off the lights in the testing room. Insert earphones

Why would movement (body movement, chewing gum, etc.) of the subject affect AEP tests?

Muscular movements create their own EEGs, which can increase the amount of "noise" in the AEP recordings. You can reduce this noise by1- Instructing very well and detailed. Explain procedure with minimal technical terms
2-Minimize movement of body, particularly head and neck
3-Use chair, recliner, exam bed so they can relax and get as comfortable as possible
4-Remove glasses, earrings, no chewing.

Ipsilateral masking ?

to enhance frequency specificity

Contralateral masking

to ensure only the test ear is involved in AER measurement

Features of recording if crossover

1. Delayed latency

2. Absence of wave I component

Intensity of masker

50 dB nHL for clicks higher than 70 dB nHL

Basic Features of the ABR

Be recorded in far-field in humans
Stimulus-related AEP (exogenous responses) - no active involvement of subject
Neural potentials
Mainly arises from the auditory nerve and neurons of the brainstem [the term ABR is misleading]
Short latency AEP

Understand the estimated origins of ABR waves.

Wave I and II: the distal and proximal portions of the auditory nerve on the test side.
Wave III - IV: Pons
Wave V: Midbrain
Wave VI-VII: Undetermined

Stimulus Type and Duration:

Click: 100 usec (0.1 ms)

What does the 2/1/2 protocol mean for tone burst ABR test?

Indicates the rise/plateau/fall time of the tone burst stimuli

How would the ABR latency and amplitude be changed as the stimulus rate is too fast?

Amplitude would decrease if too fast of stimulus, latency is slightly prolonged, but overall fairly close (not significantly).
Slower repetition rates tend to preserve waveform morphology. As rate is increased, waveform morphology becomes poorer.

What is the commonly used stimulus rate in ABR recording for neurotological evaluation, threshold testing, and hearing screening?

Neurotologic evaluation (diagnostic) \= 5-10/sec (i.e. 7.1 or 11.3/sec)
Threshold testing \= 20-30/sec (21.1 or 33.3/sec)
Screening: faster rate to reduce screening time, up to 39.1/sec

Why is a slightly faster stimulus rate used in the ABR test for determining the detection of threshold or for hearing screening?

To reduce testing time.

How would the ABR latency and amplitude be changed as the stimulus intensity is decreased?

As intensity is reduced, waves become smaller, and latency increases.

Why the stimulus is usually presented at relatively high intensity in ABR test for neurotological evaluation? What is the commonly used intensity in this case?

Higher intensities are used to see waveforms. The commonly used intensity is 80-90 dB nHL (110-130 dB peSPL)

What is the commonly used stimulus polarity in ABR test?

For clicks, condensation and rarefaction. For tone bursts, alternating polarity.

The conventionally used electrode array for ipsilateral ABR tests.

Primary electrode (noninverting) placed on vertex/forehead
Secondary electrode (inverting) placed on ipsilateral mastoid/earlobe
Common electrode (ground) placed on contralateral mastoid/earlobe

The conventionally used amplifier gain for an ABR test.

10,000x - 200,000x

The conventionally used filter parameters for ABR tests.

Band-pass filter 30-3000 Hz

What can you do on the filter parameters if you suspect ABR tracing is contaminated by the power line signals?

Apply a notch filter (60 Hz power line signal is filtered out)

The conventionally used number of sweeps for ABR test.

1,000-2,000 with surface electrodes; if too small is hard to tell if problems caused by HL or insufficient amount of sweeps.

The conventionally used analysis time for ABR test.

15 ms

What are the special cases in which longer analysis time is needed? Why?

Can adapt to about 20-25 ms; for low stimulus intensities, young children, and in cases with hearing loss. In these cases latency is prolonged.

Does the click-evoked ABR represent the responses from the neurons innervating all locations along the basilar membrane? Why?

Click-evoked ABRs represent, to a large extent, responses from the neurons innervating the basal (high-frequency) region of the cochlea.

What is the signal frequency range that a recorded ABR waveform most likely represents?

High frequency

Why are the waves I, III, and V mostly used in the data analysis in clinical applications of the ABR test?

They are the most stable waves where you generally see individual peaks. They are also easily repeatable.

Why are two or more waveforms usually recorded for an ABR test?

Repeatability tells us those waves are more likely to actually be occurring, and not a result of artifact.

How to determine the peak-to-trough amplitude of ABR tests?

Mark on waveform at specified points.

Why is absolute amplitude not as popular as the latency in clinical application of the ABR test?

Amplitude is largely variable among individuals, whereas latency is a reliable measure and pretty consistent and stable among individuals.

How to determine the absolute latencies of ABR waves?

Wave I: 1.5 ms
Wave II: 3.5 ms
Wave V: 5.5 ms

Which inter-wave latencies are commonly used in application of ABR tests? Please estimate values in normal ears.

Wave I-V: 4.0 ms
Wave I-III: 2.0 ms
Wave III-V: 2.0 ms

The commonly seen cases with prolonged latency in ABR tests.

Low intensity stimulus; children; ears with hearing loss; tone bursts

What is interaural latency difference?

Comparison of absolute or interwave latency between 2 ears. If normal, they should be very similar

Normal Variations:

Extra peaks, fused peaks, missing peaks.

Why do short-latency AEPs mature at earlier ages, while long-latency AEPs at later ages?

This is because maturation occurs from the peripheral system first, then to the central system.

Why would the absolute latency and interwave latency of ABR waves be prolonged in newborns?

Due to the immature function of the nervous system. Neurons are not as effective as they are in adults. Interwave latency is increased due to prolonged absolute latency.

How do the latency and amplitude of ABRs change as infants grow?

Latency shortens, amplitude increases.

At what age do the ABR waves look like those of adults?

2 years old.

Do most of pre-term newborns have recordable ABRs?

Theoretically, ABRs should be recordable at 27 weeks. They are very small. However, it may be difficult to record because the newborn is not fully matured. Depends on preparation due to small ear canal, probe placement.

How do the latency, interwave latency, and amplitude change with age when people get old?

Latency and interwave latency increase, while amplitude decreases.

What are the differences between males and females in terms of latency and amplitude in ABR waves?

Males: have longer latencies and weaker amplitudes; Females: shorter latency and stronger amplitude

How do hypothermia and hyperthermia generally affect ABR waveform?

Hypothermia results in reduced amplitude and increased absolute latency and interwave latency.
Hyperthermia results in increased amplitude and shortened latency. System works faster.

Does the patients' state of arousal more significantly affect short or long latency AEPs?

Long-latency AEPs.

Does the patients' state of arousal significantly affect ABRs?

May have minimal effects on wave V; but we don't worry about it.

Get very familiar to the categories of clinical applications of ABR measurements.

A. Evaluation of auditory integrity in difficult-to-test populations; estimation of hearing sensitivity; hearing (auditory function) screening
B. Clinical diagnosis
Audiological diagnosis: site of lesion testing battery
Neurological diagnosis: retrocochlear pathologies

Why is the click-evoked ABR waveform typically normal in patients with low-frequency conductive hearing loss caused by middle ear disorders?

Energy 2-4 kHz for clicked evoked stimuli
Because Click-evoked ABRs represent, to a large extent, responses from the neurons innervating the basal (high-frequency) region of the cochlea; so if damage is in apical (low-frequency) region, ABR response may still appear normal.

What are the possible changes in terms of ABR latency and interwave latency in patients having conductive hearing loss with flat audiograms?

Latency of all waves are prolonged. Interwave latency is consistent. Amplitude may not be significantly affected.

Why is the click-evoked ABR waveform typically normal in patients with low-frequency hearing loss caused by cochlear disorders, for example Meniere's disease?

Similar to conductive hearing loss, energy of the click is from 2-4 kHz, so waveform would appear normal at those frequencies.

Why is click-evoked ABR waveform typically normal in ears with mild high-frequency hearing loss?

Due to high intensity stimulus presentation.

How do the latency, interwave latency, and amplitude of ABR waves change in patients with moderate to severe high-frequency hearing loss?

Latency increases, amplitude decreases, and threshold elevates. Interwave latencies are within the normal range.

What does ABR detection threshold mean?

The lowest intensity that a waveform can be detected

Which wave is usually used in determining ABR threshold? Why?

Wave V, because it's the largest wave, often the strongest in most cases.

Can ABR thresholds be used to replace behavioral thresholds? Why?

Click and tone burst ABR do correlate well with audiometry. They can replace behavioral due to this correlation

What is the frequency range in which ABR thresholds have better correlation with behavioral thresholds? Why?

2000- 4000 Hz (medium-high frequencies); due to the development of outer and middle ear

What are the advantages using ABR measures in newborn auditory screening?

Quicker procedure. Fairly accurate due to moderate stimulus intensity, not too many false positives or misses.

Why is a low-frequency tone-burst stimulus strongly recommended in addition to clicks in ABR newborn auditory screening?

Click evoked mostly represents responses from mid-high frequencies. So low frequency tone bursts give us an estimation of hearing sensitivity in the lows.

Why is a higher stimulus rate usually used for ABR recording in newborn auditory screening?

For faster test time.

What is the commonly used stimulus intensity for ABR recording in newborn auditory screening?

40 - 50 dB nHL

Why should the analysis time be longer for ABR recording in newborn auditory screening?

Latency will be long, so we use long analysis time (about 15-20 ms)

Criteria for pass/fail in screening

Response presences vs absence; or wave V latenc

What is the primary objective of using ABR in neurotological diagnosis?

Early identification of tumor involving the 8th cranial nerve and/or brainstem

What are the commonly seen changes in the waves of ABR waveform in ears with acoustic neuromas?

Absences of waves following wave I.
Normal wave I amplitude and latency.
Alterations of latency (prolonged)- prolonged latency for later waves and prolonged interwave latency for I-III and I-V; increased interaural wave differences for wave V.

Why is wave I mostly normal in patients with acoustic neuromas?

Wave I represents the distal portion of the auditory nerve.

How do the latency and interwave latency vary in patients with acoustic neuromas?

Prolonged latency for later waves, prolonged interwave latency between I and others. Increased ILD for wave V (and interwave latency)

What is the normal range of interaural latency difference in wave V for human ears without lesion in the auditory nerve?

0-0.4 ms

In identifying acoustic neuroma, ABR test shows high sensitivity but low specificity. What does this mean?

High sensitivity accurately identifies those with disease, while specificity identifies those without the disease. Sensitivity is about 90-95% and (1-Specificity) is about 10-20%. In this case there is a low false positive rate. The lower the number the better.