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12 The Role of the Audiology Assistant in Assessing Hearing
Madell et al., Pediatric Audiology: Diagnosis, Technology, and Management, 3rd Ed. (ISBN 978-1-62623-401-7),
copyright © 2019 Thieme Medical Publishers. All rights reserved. Usage subject to terms and conditions of license.
in Children
Jane R. Madell
12
Summary
When working with young children, it is very helpful to have a
second person in the test room working with the child. The test
assistant may be a second audiologist or may be an audiology
assistant. The test assistant is responsible for engaging the infant
or child, keeping the child interested and attentive, teaching the
child the test protocols, and keeping family members at ease
and comfortable so they can cooperate with test protocols. This
chapter discusses the role of the test assistant in facilitating
testing using the dierent test protocols, including the dierent
behavioral evaluation and speech audiometry protocols, as well
as working with parents.
Keywords
behavioral observation audiometry (BOA), visual reinforcement
audiometry (VRA), conditioned play audiometry (CPA), speech
perception, supporting parents
Key Points
The audiology assistant is responsible for engaging the infant
•
or child, keeping the child interested and attentive, and
keeping family members at ease so they can cooperate with
test protocols.
In behavioral observation audiometry, the audiology assis-
•
tant is responsible for monitoring the positioning of the child,
observing responses and reporting observations to the audiologist, and monitoring the parent’s or caregiver’s behavior.
In visual reinforcement audiometry, the audiology assistant’s
•
rst responsibility is to assist in training the child to the task
and then to keep the child focused at midline so that the child
can make a conditioned head turn.
With conditioned play audiometry, the audiology assistant
•
teaches the child the listen-and-drop task and then assists the
child in completing the test activities.
An important responsibility for the audiology assistant is
•
keeping the test room in order.
12.1 The Audiology Assistant’s Role
in Testing Children
The role of the audiologist’s assistant (the term preferred by the
American Academy of Audiology [AAA]1) or audiology assistant
(the term preferred by the American Speech-Language-Hearing
Association [ASHA]2) is often undervalued. A good audiology
assistant can facilitate testing and increase the amount of
time an infant or young child will attend to test protocols. An
audiology assistant is a person who, after appropriate training
and demonstration of competency, performs delegated duties
and responsibilities that are prescribed, directed, and supervised by an audiologist. The role of the assistant is to support
he audiologist by performing routine tasks and duties so that
t
the audiologist is available for the more complex evaluative,
diagnostic, management, and required treatment services that
use the education and training of a licensed audiologist.
AAA Guidelines1 require the audiology assistant to have a mini-
mum of a high school diploma and competency-based training.
The audiologist has the responsibility to provide training and to
monitor the performance of the audiology assistant.
When evaluating infants and young children, testing is
frequently more easily and accurately accomplished with two
examiners. Both examiners may be audiologists, or one may be
an audiologist and the other an audiology assistant or a parent.
Whether one or two audiologists are participating in testing,
only one is in charge and “calling the shots.” The “managing
audiologist” (for want of a better term) will determine the test
protocol, presentation mode, order of testing, and timing of
presentations. The managing audiologist usually sits at the
audiometer; sometimes, however, with a dicult-to-test child,
the person working with the child may manage the session and
give directions to the second audiologist, who is sitting at the
audiometer and presenting test stimuli.
The purpose of this chapter is to describe the critical role of the
audiology assistant during various pediatric tests. Please refer to
Chapter 7 for detailed information about the specific behavioral
assessments of infants and children.
1,2, 3
The
12.1.1 Working with Parents
The audiology assistant is responsible for engaging the child (or
infant), keeping the child interested and attentive, managing
the patient’s behavior so that it does not interfere with testing,
and keeping family members at ease so that they can cooperate
with test protocols. The audiology assistant needs first to be
certain that family members understand exactly what is being
tested, how testing is accomplished, and what their role will
be. The child may be seated on a parent’s lap or in an infant
seat, a high chair, or a chair at a test table. The parent may be
seated next to or slightly behind the child so as to be able to
observe but not distract the child. In other cases, especially if
the child is uncomfortable with strangers, the parent may be
the best person to play with the child, with direction from the
audiologist.
The first responsibility of the audiology assistant is to explain
the test protocol to the family, describing what testing will consist
of, what will be expected of the child, and what will be expected
of the family.4 Family members need to understand that they must
not respond to any test stimuli before the child responds, to be
certain the responses obtained are measures of the child’s hearing
13
1
II Diagnosing Hearing Disorders in Infants and Children
Madell et al., Pediatric Audiology: Diagnosis, Technology, and Management, 3rd Ed. (ISBN 978-1-62623-401-7),
copyright © 2019 Thieme Medical Publishers. All rights reserved. Usage subject to terms and conditions of license.
and not his ability to receive cues from the parents. It is frequently
dicult for parents to sit still and not react. They may need to be
instructed not to say “Did you hear that?”; not to look expectantly
when sounds are presented; not to look at the reinforcing toy
during visual reinforcement audiometry (VRA), and not to suggest
the child put the toy in the bucket when the parent hears the
sound during conditioned play audiometry (CPA). When speech
stimuli are used, the parents may need to be reminded that they
must not repeat the tester’s stimuli for the child (e.g., “she said
baseball” if the tester said “baseball”) when the child fails to
respond. If the child is looking to the parent or audiology assistant
for encouragement, it may be best to look away from the child.
Once family members understand that their distracting behaviors
may make it dicult to obtain reliable results, they are usually
willing to do whatever is needed to obtain an accurate test. Some
audiologists like to use noise-canceling earphones to reduce what
family members can hear. However, allowing parents to hear
the same stimuli that their children hear can be very helpful in
counseling about test results.
12.2 Behavioral Observation
Audiometry
For behavioral observation audiometry (BOA), both the managing audiologist and the audiology assistant need to have a
good view of the infant so both can judge whether a response
is present. If the infant is in an infant seat, either the parent or
the audiology assistant may be holding the bottle, because, as
detailed in Chapter 7, sucking is the primary reliably observed
behavior. An infant who is being nursed or is not comfortable in
an infant seat will be in a parent’s arms instead. If the parent is
holding the infant or the bottle, the audiology assistant needs to
be certain the parent is not changing the way she is holding the
bottle or breast or moving it in or out of the infant’s mouth when
sound stimuli are presented.
The audiology assistant needs to be certain the infant is seated
comfortably and not fidgeting. If may be helpful for the audiology
assistant to hold and manipulate a bright toy or a light-emitting
diode (LED) display in front of the infant to keep the infant focused
straight ahead and to reduce fidgeting.4 If the audiology assistant
is using a toy or light to distract the infant, the object should be
placed so that the infant does not have to move the head up or
down to see it.
When a sound is presented, both the audiology assistant
and the managing audiologist need to judge the response. The
audiology assistant needs to be careful about how to inform the
managing audiologist about the observation. The testers need to
work out a signal system such as a minor head nod indicating yes
or no, or finger movement (one for yes, two for no). If the audiology assistant repeatedly says “No, I didn’t see anything,” such
speech compromises the quiet test environment and is likely to
be very disturbing to the family. Being right next to the baby, the
audiology assistant will be able to make suggestions about when
a rest or repositioning may be needed.
Pearl
Before testing, audiologists should work out a way to communicate with each other without communicating to the family. A
system to signal observation of responses or changes needed
during testing might consist of head nods or nger taps.
12.3 Visual Reinforcement
Audiometry
In VRA the audiology assistant’s first responsibility is to help
train the child to the task and then keep the child focused at
midline so that the child can make a conditioned head turn.4
Positioning is especially important for young children and for
children with neurologic or developmental delays. The audiology
assistant needs to be certain the child is comfortably seated,
has sucient neck support if needed, and is facing forward. If
the child is turned toward one side and sound stimuli are being
presented and reinforced from the other side, it may be dicult
for the child to make a sucient head turn to be counted as a
response. Focusing the child at midline will be most easily
accomplished if the audiology assistant is seated in front of the
child but in a position that permits the managing audiologist to
see the child as well. The distraction toys the audiology assistant
selects should be easily manipulated, bright, and entertaining.
As soon as the child starts to lose interest and look away, a new
toy should be presented.
When training the child to the VRA task, the audiology assistant
will keep the child’s attention focused front. If the child does not
autonomously turn and look when the VRA toy is turned on, the
audiology assistant will attract the child to the reinforcing toy by
waving at the toy or tapping the transparent box holding the toy
to get the child’s attention to the toy. Once the child is trained
for the task, the audiology assistant is responsible for keeping the
child focused forward and away from the reinforcing toy. The child
should be observing (not manipulating) the distracting toys, since
physical play may be too engrossing, especially for young children
and children with developmental issues. Older children may be
able to play with some simple toys and still respond to sound. The
audiology assistant needs to be alert and aware of how the child
is responding and whether playing with a toy is interfering with
attention. If it is, the tester will need to take the toys away from
the child.
12.4 Conditioned Play Audiometry
With CPA, the audiology assistant teaches the child the listenand-drop task and assists the child in completing the test
activities. The audiology assistant must make a judgment about
the child’s motor skills so as to be able to select toys the child
is capable of using, and must note when the child’s interest is
agging and determine when a new toy is needed. For children
who do not wish to cooperate, the audiology assistant will need
132
12
Madell et al., Pediatric Audiology: Diagnosis, Technology, and Management, 3rd Ed. (ISBN 978-1-62623-401-7),
copyright © 2019 Thieme Medical Publishers. All rights reserved. Usage subject to terms and conditions of license.
to present a “firm but kind” attitude to increase cooperation.
When the child is not cooperating, the audiology assistant has
to determine when it would be good to involve the parent in
obtaining cooperation and when it is best to leave the parent
out. As with VRA, the amount of interaction the tester and child
will have will depend partly on the personality of the tester and
partly on the child. However, because children who are able to
engage in CPA are older, more interaction with the audiology
assistant is to be expected.
12.5 Speech Audiometry
The use of an audiology assistant can be critical for speech audiometry. If a closed-set task is being used, the audiology assistant
will be responsible for turning pages and being certain the managing audiologist knows how the child responded. If an open-set
format is being used and the child is repeating the test stimulus,
the audiology assistant may need to be the audiologist’s “ears,”
especially if the child is responding in a very soft voice that is
typical of many young children. The audiology assistant needs
to find a way to let the audiologist know whether the child’s
response was correct and, if not, what the error was so that the
audiologist can accurately score the response, especially when
using phoneme scoring. It is important that the audiology assistant provide response accuracy information without making the
child feel as if he is doing poorly at the task. The assistant can
employ several strategies. The audiology assistant can simply
repeat what the child says in a suciently loud voice for the
audiologist to hear, or just repeat the error words. For example,
if the stimulus item is “Say the word ‘mouth’” and the child says
“mouse,” the audiology assistant can say “mouse” or “mouse,
good job.” The audiologist will know that an error was made and
will be able to record what the error was, but the child will not
know he has made an error.
12.6 Keeping Order in the Test
toys should be out of sight. One or two toys could be visible to
entice the child to enter the room and to sit in the test chair. Toys
should be sorted so that all parts of each toy are in the correct
box to facilitate moving quickly from one activity to the next. The
audiology assistant should oer the child just one toy at a time so
that there is little or no distraction during testing.
12.7 Conclusion
The audiology assistant is critical to obtaining accurate results
in a timely manner. An enthusiastic, cheerful audiology assistant
who enjoys children is likely to elicit good results. Observing the
testing video that comes with this text book will provide good
demonstrations about managing the test room.
When the audiology assistant and the managing audiologist
disagree or are not communicating well, it is probably best if
they take a moment to leave the test room and discuss how they
want to proceed out of earshot of the family. It hardly encourages
confidence if the testers cannot agree about what to do.
Pitfall
Audiology professionals should not disagree in front of families.
It can be distressing to the family and reduce trust about test
results.
Communication between the audiologist and audiology
assistant can be accomplished using a talk-back system with
the audiometer. If the audiometer does not have a talk-back
system, communication can be accomplished by using a remote
microphone (RM) system or developing hand signals. An experienced and professional team will make testing infants and
hildren fun and enable results to be obtained efficiently and
c
accurately.
Room
The test room does not have to be silent during testing, but it
should be quiet. The amount of interaction between the tester
and child will depend partly on the personality of the tester
and partly on the child. For some children, smiling, clapping,
and enthusiastic comments of “hurrah” will encourage longer
attention to the VRA task. For other children, “cheerleading” will
be intrusive, and better results will be obtained if the audiology
assistant is quieter or even silent. A silent audiology assistant is
frequently valuable with a child with autism spectrum disorder
(ASD) or other developmental disorders. The audiology assistant
will need to observe the child and determine what behavior
produces the best results.
An important responsibility for the audiology assistant is
keeping the test room in order. Bringing a young child, especially
a dicult-to-evaluate child, into a room that has toys all over the
oor will make it dicult to seat the child and have him focus on
the task. When a child is brought into a test room, most of the
Discussion Questions
1. What are some ways the audiologist and audiology assistant
can communicate during testing?
2. What are the responsibilities of the audiology assistant during
BOA, VRA, CPA, and speech audiometry?
References
[1] American Academy of Audiology. Audiologist’s Assistant. https://www.audiolo-
gy.org/publications-resources/document-library/audiologists-assistant. Updated
2014. Accessed December 16, 2017
[2] American Speech-Language-Hearing Association. Issues in ethics: audiology
assistants. http://www.asha.org/Practice/ethics/Audiology-Assistants/. 2017.
Accessed December 16, 2017
[3] Hamill T, Freeman B. Scope of practice for audiologists’ assistants: Survey results.
Audiol Today 2001;13(6):34–35
[4] Smith J, Wolfe J. “Oh, behave!” How to enhance behavioral audiometric assess-
ment. Hear J 2015;68(2):14–18
13
3
Madell et al., Pediatric Audiology: Diagnosis, Technology, and Management, 3rd Ed. (ISBN 978-1-62623-401-7),
copyright © 2019 Thieme Medical Publishers. All rights reserved. Usage subject to terms and conditions of license.
13 Auditory Evoked Response Testing in Children
Madell et al., Pediatric Audiology: Diagnosis, Technology, and Management, 3rd Ed. (ISBN 978-1-62623-401-7),
copyright © 2019 Thieme Medical Publishers. All rights reserved. Usage subject to terms and conditions of license.
Guy Lightfoot and Siobhán Brennan
13
Summary
This chapter summarizes the clinical role of this technique, with
special reference to auditory brainstem response (ABR) testing
of the postscreening newborn population. Based on the internationally acclaimed national guidance developed by the Newborn
Hearing Screening Programme (NHSP) in Britain, the chapter
details the ecient technical, strategic, interpretational, and
reporting aspects of ABR testing. On several counts the authors
have not shied away from suggesting achievable changes to commonly held methods, and there is much here to provoke thought
and comment. The promoted techniques draw on research and
clinical evidence and have at their heart a combination of technical
rigor and clinical eciency. Strictly defined criteria are oered for
both the presence and absence of an evoked response and previously ignored aspects of noise reduction, such as optimum artifact
rejection, take center stage. The uncertainty inherent in the use of
insert earphones and bone conduction in the newborn population
is highlighted and basic corrections oered, while the advantages
of narrowband chirp stimuli are discussed. The application of
threshold ASSR and its place in the test battery are also considered.
Keywords
evoked response, auditory brainstem response, newborn, clini-
cal eciency, evidence, test strategy, corrections, interpretation,
chirp, frequency, threshold
Key Points
1. For high-quality auditory brainstem response and auditory
steady-state response testing, infants must be well settled
and sleeping.
2. Dened criteria should be used for auditory brainstem
response presence and absence; inconclusive waveforms
must play no part in the denition of the auditory brainstem
response threshold.
3. Testers must be able to implement a strategy to deal with
the eects of noise through the informed choice of artifact
rejection level and number of sweeps in each recording.
4. In newborns tested using inserts or bone conduction (but
not using supra-aural earphones), the stimulus level is
higher than for adults; a correction for this eect should
be included in the conversion of auditory brainstem and
auditory steady-state response thresholds to estimated
hearing level.
5. The nature of any hearing loss should be investigated using
air and bone conduction testing rather than inferred from
the pattern of input-output functions.
6. A clinically ecient cochlear microphonic test, used in
conjunction with click auditory brainstem response testing,
enables auditory neuropathy spectrum disorder to be iden-
tied, but the two tests must use the same stimulus level.
13.1 Introduction to Auditory
Evoked Response Testing in
Children
In this chapter, we focus on the most clinically popular applications of auditory evoked potentials (AEPs): the auditory brainstem response (ABR) and auditory steady-state response (ASSR)
as used to estimate hearing sensitivity (the hearing threshold).
We also introduce the neurologic application of the ABR and the
cochlear microphonic (CM), which are helpful in their identification of auditory neuropathy spectrum disorder (ANSD) and con-
ditions aecting the auditory neural pathways. These techniques
have evolved from the research phase to become advanced yet
essential clinical tools, though we do believe they are often performed suboptimally. The goal of this chapter is recommend the
optimal way to perform these procedures in a clinically friendly
yet scientifically valid and evidence-based format.
We do not cover in detail other AEPs, but is it useful to mention
them, in the order they are generated. Table 13.1 summarizes the
main types of AEPs, giving the time course for full maturation,
typical latency and amplitude, primary use, and the stimuli used
for each. Some responses find clinical application prior to their
complete maturation; for example, the ABR wave V is recordable
at birth yet does not mature until around 2 years.
AEPs are most often recorded using scalp electrodes; pairs of
electrodes allow the dierence in their voltages to be amplified
(dierential amplification). This technique avoids the unwanted
recording of much larger “common mode” signals that are present
on both electrodes, such as power line interference, electrocardio-
gram (ECG), and other signals (e.g., from the patient’s muscles). To
record a single-channel ABR, we place the noninverting electrode
at the vertex (Cz) or, in newborns, as high on the forehead as
the fontanelle allows. The inverting electrode is placed on the
ipsilateral mastoid process, and the common (ground) electrode is
placed on the contralateral mastoid. For two-channel recordings,
we use both mastoids for the inverting electrode of each channel
and a lateral forehead as the common. When noninverting and
inverting electrodes are at similar distances from the anatomic
structure generating the response, they are considered to be
far-field electrodes, whereas if one electrode is relatively close to
the generator compared to the other, we refer to it as a near-field
electrode. Even with dierential amplification, the response we
seek (the “signal”) can be tiny in comparison to the overall voltage
from the electrodes. The overall voltage is a combination of signal
and noise, where noise is all unwanted information. Our challenge
in AEPs is to enhance the signal-to-noise ratio (SNR) to the extent
that any response can be identified with certainty, and we routinely adopt a number of methods to do this. The first is to apply
filters to the amplified voltage. Filters attenuate uctuation whose
spectrum lies beyond the frequency range of interest: the spectrum of the response. This sounds straightforward, but because
the spectra of the signal and noise often overlap, a compromise is
often necessary.
13
5
II Diagnosing Hearing Disorders in Infants and Children
Madell et al., Pediatric Audiology: Diagnosis, Technology, and Management, 3rd Ed. (ISBN 978-1-62623-401-7),
copyright © 2019 Thieme Medical Publishers. All rights reserved. Usage subject to terms and conditions of license.
Table 13.1 Characteristics and primary clinical applications of AEPs
Response Full maturation
CM At birth 0–5 0–1 Diagnosis of ANSD Clicks (separate
ABR wave I At birth 1–3 0–0.5 Neurodiagnosis Clicks
ABR wave V 2 years 6–15 0–0.7 Neurodiagnosis,
ASSR (80 Hz) 2 years 0–15 0–0.7 Threshold estimation Modulated tones/chirps
MLR 6 years 15–50 0–2 Neurodiagnosis Clicks
CAEP (various) Late teens 50–400 0–20 Threshold estimation,
Abbreviation: CM, cochlear microphonic.
a
Latency (ms) Amplitude (µV) Clinical application Stimulus
polarities)
theshold estimation
habilitation
Clicks, tone bursts,
chirps
Long tone bursts,
speech tokens
Even after filtering, the SNR will be too poor to identify any
response (signal), as it may be several orders of magnitude
smaller than the noise. Our main weapon in improving the SNR
is a process called time-domain averaging. All noise is assumed to
be random (not always entirely true) and unrelated to timing of
the stimulus. However, the signal is consistent in its relationship
to a given evoking stimulus; it will occur a fixed time after the
stimulus (the latency of the response) with a consistent pattern
and size (again, not always entirely true). By repeatedly stimulating and averaging the poststimulus voltage, the SNR improves by,
at best, the square root of the number of stimuli. So, to double
the SNR, we must average four stimuli; to improve the SNR by
a factor of 100, we must average 10,000 stimuli. This repeated
stimulation and averaging can be conducted as quickly as the
chosen timebase allows unless there is a good reason to reduce
the rate of stimulation. However, it is important to avoid a rate
that would unintentionally record unwanted noise, such as power
line signals. This means avoiding stimulus rates, multiples and
subharmonics of 60 Hz in the Americas and Japan or 50 Hz in
Europe and elsewhere. It is for that reason one usually employs
rather odd-looking stimulus rates, such as 49.1/s or 0.7/s. It is
commonly held that the use of a notch filter to attenuate power
line interference is to be avoided because the phase response will
lead to ABR waveform distortion or latency shifts. A recent study1
in the United Kingdom (UK), where the power line frequency is 50
Hz, failed to reveal any such eect on the newborn ABR except for
a small proportion of 500-Hz ABR waveforms. Notch filters should
not be used routinely, but when power line interference is prob-
lematic and cannot be eliminated, a notch filter may be applied in
threshold ABR testing for stimuli above 500 Hz.
A further tool in our AEP armory is artifact rejection; this is
related to amplifier gain but is far more relevant unless we possess
very fine technical details of the equipment design. We may spend
time getting the patient asleep or relaxed, but if the patient sneezes,
yawns, or twitches during recording, voltages far larger than any
response can enter and dominate the average. To ensure that we
record only when the SNR is relatively favorable, we reject individ-
ual samples that contain noise larger than a defined voltage. This
accept/reject recording dichotomy has a more sophisticated sibling:
weighted (or Bayesian) averaging,
2,3
in which quiet or moderately
noisy samples are not rejected, but their contribution to the final
average is diluted in proportion to the noise they contain; thus, the
average is dominated by periods of low noise.
13.2 The Input-Output Function
The size and latency of all AEPs are inuenced by the stimulus
level relative to the patient’s hearing threshold. At levels well
above threshold a large, well-defined response is recorded, but
as the stimulus level is reduced toward threshold, the response
size decreases and response latency increases, as illustrated in
Fig. 13.1. These relationships are known as input-output (I/O)
functions and are seen in all modalities of evoked potentials.
Knowledge of these functions is important in understanding
how we can utilize various tests to access dierent aspects of our
hearing pathways, but it is rarely appropriate to construct the I/O
4
Fi g . 13.1 A typical ABR input-output function, using 4-kHz tone bursts
in a 4-week-old normally hearing baby. Unreplicated responses are
obtained at a variety of stimulus levels, and the response is tracked to
lower levels until it is no longer apparent. A “response” is identied at
20 dB normal hearing level (nHL), but replication reveals it is simply
residual noise masquerading as a response.
function for an individual patient.
Controversial Point
In tests to estimate the hearing threshold (audiologic testing), it
is rarely appropriate to test over a wide range of stimulus levels
to construct the I/O function for an individual patient.
136
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Madell et al., Pediatric Audiology: Diagnosis, Technology, and Management, 3rd Ed. (ISBN 978-1-62623-401-7),
copyright © 2019 Thieme Medical Publishers. All rights reserved. Usage subject to terms and conditions of license.
13.3 The Distinction Between
Audiologic and Neurologic
AEP Testing
Our clinical objectives in these broad categories of tests dier,
and it is of no surprise that our test strategy, stimulus and test
parameters (Table 13.1), and the basis of our analysis also dier.
As the following sections describe, in audiologic testing, we are
in the business of response detection; the technical challenge
being to identify a tiny response buried in noise or to conclude
that no response is present. In neurologic testing, however, we
focus on response analysis and draw conclusions about the
status of the patient’s auditory pathways from response latency
and amplitude measurements.
13.4 Threshold ABR
13.4.1 The Basis of Threshold Estimation
In seeking to estimate the audiologic sensitivity (or hearing
threshold) objectively, we assume that if a response is evoked
by a particular stimulus, that stimulus lies above the hearing
threshold. The stimulus level is adjusted until two levels have
been identified: the lowest stimulus level at which a response
is present and the highest stimulus level at which a response is
absent. These two levels should be ideally only 5 or 10 dB apart.
We know from I/O functions that the response of most clinical
relevance, just above threshold, is likely to be one that is ver y tiny.
As a result, the technical challenge and our entire test strategy
must focus on being able to identify a response above the noise
oor and, in the case of response absence, to be confident that a
small response is not being obscured by noise. This latter point
is often given insucient importance. It is not enough to say,
“At this level I can no longer see a response.” For both response
presence and absence, we should aim to have a high degree of
confidence that a response is genuinely present or absent. We
do not favor the I/O approach to ABR threshold estimation, as we
believe it is clinically inecient and often fails to provide sucient precision. The I/O method exploits the change in response
latency and amplitude as stimulus level is varied. A range of
levels is used to “track” the response and, thus, determine the
ABR threshold. Fig. 13.1 illustrates this. In this method, ABR
wave V is identified at high stimulus levels without replication
and, using the “eye of faith,” responses at progressively lower
levels are sought. Fig. 13.1 appears to suggest a response at 20 dB
normal hearing level (nHL), yet further replications at this level
reveal that the response was not repeatable.
Unlike the I/O method, which invests time in recording
responses at stimulus levels considerably above the ABR thresh-
old, the SNR approach focuses on levels that are likely to define
the threshold with a high degree of confidence. For response
presence, the waveform should exhibit ABR-like characteristics,
be repeatable, and have an SNR that assures us that the likelihood
of misinterpreting noise as a response is low. The UK’s Newborn
Hearing Screening Programme (NHSP) guidance for ABR testing5
suggests that the response amplitude should be at least 40 nV and
have an SNR of at least 3:1. As not all ABR systems oer an objec-
tive calculation of residual noise, they suggest visually estimating
the noise as the average gap between superimposed replications
across the entire waveform. The authors of the NHSP Guidelines
believe this criterion (which they term a “Clear Response,” CR)
corresponds to a 95 to 97% confidence of response presence. Their
definition of “Response Absence” (RA) is that there should be no
evidence of a response, the waveform should be “appropriately
at,” and the residual noise (as just described) should be no more
than 25 nV, a value low enough to provide confidence that a small
response is not obscured by noise.
Pitfall
It is not enough to say “At this level I think I can see (or no longer
see) a response.” For both response presence and response
absence, we should aim to have a high degree of condence
that a response is genuinely present or absent, based on robust
dened criteria.
The NHSP authors estimate that their 25 nV “average gap”
noise criterion corresponds to a residual noise criterion of 15 nV
reported by current Bio-logic Navigator Pro (Natus Medical Inc.,
Pleasanton, CA) and Eclipse (Interacoustics, Eden Prairie, MN)
systems. Users of ABR systems not oering an objective noise estimate must use the subjective average gap method of noise estimation, and this approach requires replication at each stimulus
vel. It is important to acknowledge that not all waveforms will
le
meet the criteria of CR or RA, often because of excessive residual
noise, and the NHSP Guidelines label these “Inconclusive” (Inc).
No inconclusive test levels should contribute to the definition
of the ABR threshold. Fig. 13.2 illustrates waveforms using the
strategy recommended by NHSP. Note that a “clear response” has
been recorded 10 dB above the 50-dB nHL threshold. Doing this
Fig. 13.2 An example of an air conduction 4-kHz ABR threshold,
dened using “clear response” and “response absent” waveforms as
recommended by the English NHSP guidance.
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Madell et al., Pediatric Audiology: Diagnosis, Technology, and Management, 3rd Ed. (ISBN 978-1-62623-401-7),
copyright © 2019 Thieme Medical Publishers. All rights reserved. Usage subject to terms and conditions of license.
adds reassurance; if there was any interpretive uncertainty in the
responses at 50 dB, there should be none at the higher level where
we would expect the response to be larger, thus limiting any error
to 10 dB. NHSP refers to this combination of levels (CR, CR, RA)
as a “gold standard” threshold. For pragmatic clinical reasons, in
cases where thresholds at several frequencies are needed for the
purposes of amplification, this additional confirmation test level
can be omitted, thus balancing the need for statistical certainty
and clinical expediency.
Some ABR systems oer an objective indication of response
confidence, such as Fsp.6 Such measures are most successful for
relatively large, suprathreshold responses and can serve as a
guide to testers when it is safe to terminate an averaging run, thus
saving time. They are less helpful in identifying near-threshold
responses. Fsp-type measures can never be used to indicate
response absence; indeed, low Fsp values may be reported even
when a response is clearly evident. Likewise, objective residual
noise figures can be used to indicate when it is appropriate to terminate an averaging run when all the other criteria for response
absence are satisfied.
13.4.2 State of the Patient
ABR tests on newborns up to the age of 12 weeks are ideal when
tests conducted in natural sleep allow the use of a strict (±3 µV
to ±10 µV) artifact rejection level and ensure that low levels of
myogenic noise lead to high-quality results.
7
Controversial Point
We must move away from the concept of predetermined amplier
gain toward that of an artifact rejection level that is optimized for
the prevailing test conditions. A s a guide, for most systems a gain
of 240,000 corresponds to a default rejection level of around ±10
µV. Tests on newborns conducted in natural sleep allow the use
of a strict (typically ±5 µV) artifact rejection level and ensure that
low levels of myogenic noise lead to high-quality results.
Newborn hearing screening programs and associated services
for the follow-up testing of referred babies must be designed to
exploit this valuable window of opportunity, beyond which testing often becomes problematic. There is good evidence
8
that the
choice of a strict rejection criterion has a critical eect on residual
noise and the number of sweeps required; this in turn aects test
eciency. Tests on children over 3 months may be possible in
natural sleep, but with increasing age, the use of sedation or general anesthesia often becomes necessary. Awake children present
a significant testing challenge, and their level of myogenic noise
usually necessitates a rejection level above ±10 µV. To maintain an
acceptable level of residual noise (and therefore confidence in the
precision of results), averages employing over 8,000 sweeps may
be necessary, and this extension of test time usually forces the use
of larger stimulus level steps and other time-saving limitations in
the scope of testing. It is better to obtain a few results in which we
can rely than many results of doubtful accuracy.
13.4.3 Stimulus Options
The primary stimulus delivery option for ABR tests is air conduction (AC), using either inserts or supra-aural earphones.
The relative merits of these are outlined in Table 13.2. The
issue of stimulus level uncertainty with inserts has not been
widely acknowledged. This uncertainty9 is a consequence of the
variable, and generally far smaller, occluded ear canal volume
encountered when inserts are used in newborns compared to
adults from whom the calibration reference levels are derived.
In newborns, it is important not to test at very high stimulus
levels with inserts. The “lift” in stimulus level associated with
a newborn ear canal may be 10 to 20 dB.
sound pressure level (ppSPL) of a 100-dB nHL click in an adult
ear is 135.5 dB (occluded ear simulator) or 126.5 dB (HA-2 2cc
coupler [formerly supplied by Frye Electronics, Beaverton, OR]).
As clinical professionals, we have a duty of care to do no harm,
and it is for this reason, the English NHSP guidance
insert stimulus level in newborns to 85 dB nHL, which could
correspond to as much as 140 dB ppSPL in the newborn canal.
This issue would be largely overcome if manufacturers of ABR
systems included a probe microphone in the insert probe, as is
10, 11
The peak-to-peak
5,7
limits the
Table 13.2 Relative merits of supra-aural earphones and tubal insert earphones
Supra-aural earphones Insert earphones
Advantages Greater certainty of stimulus levels
Avoids inadvertent hair cell damage
Less disturbance of baby
Disadvantages Needs to be hand-held by sta
Small possibility of ear canal collapse if too much pressure
applied
Change in stimulus level with movement
Abbreviation: CM, cochlear microphonic.
Better interaural attenuation, reducing the likelihood of
requiring masking
Reduced stimulus artifact
Easier to block sound for blocked stimulus control runs and CM
testing
No need to change transducer if CM testing needed
Better attenuation of ambient noise
Better maintenance of ear canal patency
Facilitates use of thresholds for hearing aid prescription
Less electrical hazard when used in theater following
myringotomy
Stimulus level more uncertain due to individual variation in ear
canal volume
Possibility of tube blockage with wax
Often become dislodged
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copyright © 2019 Thieme Medical Publishers. All rights reserved. Usage subject to terms and conditions of license.
common in otoacoustic emission (OAE) systems, and we continue
to await this long-overdue facility.
Special Consideration
In the newborn ear canal the volume-related SPL lift associated
with inserts can take us well into the area of instantaneous hair
cell damage if maximum available levels are used.
In the meantime, age-related stimulus level corrections can be
applied to ABR thresholds obtained using inserts.
lated corrections are necessary for bone conduction (BC) ABR
thresholds.12 Newborns have smaller skulls than adults, and their
cranial plates are not fused, leading to a higher level of eective
BC stimulation. When raised AC thresholds are recorded, BC ABR
tests are the preferred method of establishing whether the loss is
conductive or sensorineural, with the transducer placed superior
to the mastoid electrode. Analysis of the AC ABR I/O function
has been used by many hearing professionals to infer the type of
hearing loss, but this method can be imprecise.
Infants considered to be at high risk of ANSD or neurologic
complications may initially be tested using AC clicks, followed, if
appropriate, by CM testing. Initial testing using tone bursts is also
acceptable and is probably the most ecient initial test option for
well babies when the discharge criterion uses a tone burst stimulus. Tone bursts used in ABR testing (sometimes called tone pips
to distinguish them from the longer bursts used in cortical AEPs)
are typically alternating polarity, 5-cycle Blackman enveloped
tones or are linear ramped tones with 2-cycle rise, 1-cycle plateau
7
Similar age-re-
4
13
and 2-cycle fall, as specified in IEC 60645-3.
by Gorga and colleagues,14 the conicting issues of frequency
specificity and neural synchrony are balanced for each test
frequency; this method has found favor with some testers. Chirp
stimuli are becoming increasingly popular as ABR stimuli, both as
a broadband (clicklike) stimulus and as narrowband (NB; tonelike)
stimuli. The temporal smearing of conventional stimuli resulting
from the cochlear traveling wave delay is avoided in chirps, whose
design compensates for this.15 As Fig. 13.3 shows, the greater
neural synchrony achieved with the chirp stimuli results in a larger
ABR, which has obvious advantages in reducing test time (typically
halving) and allowing more to be achieved in a test session.
Opinions dier on what initial preferred stimulus frequency
to employ; 2 kHz is popular in the United States and Canada,
whereas 4 kHz is used in the UK. Programs usually specify a
“discharge criterion” at the primary frequency. The English NHSP
uses a discharge criterion of 30 dB eHL; if clear ABR responses
are recorded at this level bilaterally, the baby may be discharged.
When ABR tests at the primary frequency fail to meet the criterion,
the extent and nature of the possible loss should be investigated.
Initially, BC testing should be conducted at the primary frequency
to identify any air-bone gap, and, if appropriate, AC testing should
be performed again at a lower frequency (typically 500 Hz in the
United States and Canada; 1 kHz in the UK). Additional frequen-
cies are helpful where amplification is indicated.
The eect of stimulus frequency on the ABR is shown in Fig.
13.4. The latency prolongation at lower frequencies is largely a
consequence of traveling-wave delay within the cochlea, but at
near-threshold levels, response morphology at lower frequencies
often becomes ill-defined and somewhat like power line interference, a consequence of the poorer neural synchrony associated
with low-frequency tone burst stimuli. In situations where a possible response could be nonphysiologic, it is helpful to conduct a
“blocked stimulus run.” Because one possible source of an artifact
is related to the stimulus, it is important to maintain presentation
of the stimulus, yet block it from reaching the ear. In the case of
an insert, the tube may be clamped. For supra-aural phones (Fig.
13.5), the tester occludes the phone orifice with a thumb but oth-
erwise keeps it in position. For BC, the tester lifts the transducer
half an inch from the mastoid. It is not appropriate to turn the
stimulus down to 0 dB or to withdraw the transducer, as these
actions might change the nature of the artifact and thus mislead
the tester. For this reason, we do not favor the term “no-sound
trial.” Blocked stimulus runs can be useful when needed, but it is
not ecient to use them as a matter of routine.
In an alternative
16,17,18
19,20
13.4.4 Recording Parameters and
Strategy
Table 13.3 summarizes the test parameters we consider optimal
for single-channel ABR threshold testing in the newborn popula-
tion. The use of a high-pass filter at around 30 Hz is preferred
rather than the 100 Hz more typically used in suprathreshold
Fig. 13.3 A comparison of narrowband CE-Chirp and 2:1:2-cycle tone
burst ABRs at 20 dB nHL in a normal 2-week-old baby.
neurologic ABR testing. Unfortunately, this does result in greater
noise, but a higher filter also attenuates near-threshold ABR
amplitude and could lead to a loss of precision, particularly in
21,22
13
9
II Diagnosing Hearing Disorders in Infants and Children
Madell et al., Pediatric Audiology: Diagnosis, Technology, and Management, 3rd Ed. (ISBN 978-1-62623-401-7),
copyright © 2019 Thieme Medical Publishers. All rights reserved. Usage subject to terms and conditions of license.
Fig. 13.5 Elevated ABRs at 500 Hz. Validity of the response at 90 dB
nHL (not seriously in doubt in this example) was conrmed by the
use of a “blocked stimulus run” at 90 dB nHL, where features of the
response have disappeared. Supra-aural phones were used, and the
stimulus was blocked by placing the tester’s thumb over the opening.
An ABR threshold of “= 90 dB nHL” was reported with condence.
should be as low as practicable and optimized for the prevailing
Fig. 13.4 The eect of stimulus frequency on the ABR. Stimuli are at
30 dB eHL in a 3-week-old baby.
test conditions, which can vary minute by minute. The number of
sweeps per average should be adapted to provide an acceptable
SNR or low noise oor. It is vital that testers understand the
relationship between artifact rejection level and the number of
sweeps needed to control residual noise; at near-threshold levels,
low-frequency tests. Stimulus repetition rates may be as fast as
the recording timebase allows unless there is reason to suspect
neurologic immaturity or pathology. The artifact rejection level
Table 13.3 Summary of test parameters for infant ABR5 and CM32 tests
Parameter Click, NB chirp, and 2–4-
kHz tone burst
Stimulus polarity Alternating Alt or Rar Rar & Con
Stimulus timing Click: 100 µs
Tone burst: 2-1-2 cycles (linear rise–plateau–fall) or
5-cycle Blackman
Stimulus rate 45.1–49.1 /s 35.1–39.1 /s < 20 /s 89.1 /s
Artifact rejection level ± 3 to ± 10 µV peak to peak
Begin as low as possible but increase in small steps if rejects > 30%
Amplier lters Low: 30 Hz
High: 1,500 Hz
Window length 20 ms 25 ms 10 ms 10 ms
Display scales Within range 25–100 nV ≡ 1 ms As required As required
Abbreviations: ABR, auditory brainstem response; CM, cochlear microphonic; NB, narrowband.
0.5–1-kHz tone burst Neuro ABR CM
where the response will be small, and at subthreshold levels,
where the response will be absent, it will usually be necessary to
use long averaging runs.
Click: 100 µs Click: 100 µs
Low: 100 Hz
High: 3,000 Hz
Low: 100–300 Hz
High: 3,000 Hz
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