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Audiology Review: Preparing for the Praxis and Comprehensive Examinations
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TABLE 9–9. Predictive Factors for Cochlear Implant Performance in Pediatrics
PREDICTIVE
FACTOR
Age of
identification,
BETTER OUTCOMES
(RELATIVE)
Timely diagnosis and
treatment
management
Mode of
Communication
Listening and spoken
language
Etiology Genetic (particularly
GJB2, SLC26A4,
Usher syndrome)
Family variables Higher maternal
education level and
socioeconomic status
(SES)
POORER OUTCOMES
(RELATIVE) SPECIAL CONSIDERATIONS
Delayed diagnosis
and/or treatment
Most important factor. Critical for
identification as soon as hearing
changes, and management with
amplification and appropriate
intervention as soon as possible.
Signed or manual
communication
Influenced by child characteristics,
family need, expected outcomes.
Highest performers may be more
likely to use listening and spoken
language but may have initially used
signed language.
Cochlear nerve aplasia
or deficiency
Etiology-less influence on pediatric
outcomes; congenital ANSD better
performance than acquired ANSD
Lower maternal
education level and
Greater financial flexibility allows
for access to therapies and services
SES
Child-specific
factors
Preoperative
hearing
Age of
implantation
(for prelingually
deaf children)
Quality of life, access
to environmental
sounds, ability to
connect with family
and peers
Residual hearing can
be useful
Earliest is best; 9 to
12 months of age at
implantation yields
best outcomes
Later implantation
exhibits poorer
outcomes
Childhood hearing loss may be the
result of a syndrome or co-occur with
a number of factors that can cause
developmental delay. Comorbidities
affect the outcomes of CI use.
Preoperative residual hearing not a
necessity for children
Poorer outcomes if implanted
after 2 to 3 years of age. Auditory
maturation as measured by late
evoked potentials cannot achieve
normal latencies if implanted after
age 7 (Sharma et al., 2005).
Source: Key factors identified via Wolfe (2020).
Surgery
A CI surgery completed by an experienced surgeon is considered a routine procedure. Adults are typically discharged the same day and children may undergo a 23-hour stay (23 hours to avoid charges for
a full-day admission). The surgical procedure involves an incision behind the ear, the lifting of the skin
flap, and drilling into the mastoid cavity. Ideally, the round window should then be identified by the
surgeon. There are two primary surgical approaches: a cochleostomy, which involves drilling into the
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cochlea, and the round window approach, which has significantly less drilling. Regardless of surgical
approach, the ideal placement of the electrode array is in the scala tympani (Finley et al., 2008).
Internal Devices
The electrode array is the component of the internal device that consists of the intracochlear electrode
contacts and is typically housed in silicone. This is connected to the receiver-stimulator, which is the
silicone and titanium device that receives the signal from the external sound processor and drives
the electrode array.
n
A perimodiolar electrode array is typically a precurved array that is designed to hug the
internal wall of the scala tympani and position close to the modiolus. This type of array has
the benefit of reduced current needed to elicit a response from the auditory nerve due to being
closely and tightly placed to the modiolus. This can in turn provide greater battery life and
more efficient loudness growth.
n
A lateral wall electrode array is designed to lie against the lateral wall of the scala tympani.
Theoretically, this better preserves the organ of Corti and scala media structures.
As there are advances in atraumatic surgical techniques and thinner electrode arrays, there is no current
evidence to indicate that lateral wall or perimodiolar arrays are preferable for hearing preservation.
Post-Implantation and Device Management
Once the CI team approves a candidate and surgery is complete, the patient is considered a CI recipient. The device can be activated 2 to 4 weeks postsurgery, depending on recipient age and surgeon
recommendation.
Cochlear Implant Signal Processing
CI signal processing is manufacturer specific and often proprietary. Table 9–10 identifies key features
for the CI companies’ signal processing strategies and internal devices. Please note, the list is not
exhaustive but can be used as a guide to differentiate components of their internal devices and features.
Note, some features are common between companies.
Programming
Programming CIs, also called mapping or MAPping, is completed by the audiologist. Programming
will occur throughout a recipient’s life, with the most substantial changes during the first year following implantation. After the initial activation period, substantial changes are seen during other times
secondary to changes in the body such as puberty, pregnancy, hormone therapy, menopause, and aging.
The clinician can evaluate and adjust a variety of parameters within a recipient’s device. Appendix9–B
contains definitions of parameters that can be adjusted by the clinician and descriptions on their
clinical implication when changes are made.
Goals of Device Programming
The goals of programming are dependent on the timing of the visit. The audiologist will have different
goals whether it is during the initial stimulation period or during subsequent visits.
Audiology Review: Preparing for the Praxis and Comprehensive Examinations
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TABLE 9–10. Key Features of Manufacturer-Specific Cochlear Implant Internal Components and
Signal Processing
PARAMETER ADVANCED BIONICS MED-EL COCHLEAR
Number of
electrodes
Numbering of
electrodes
Electrode coupling Monopolar Monopolar Monopolar
Electrode arrays Lateral wall and
Current default
strategy in U.S.
Signal analysis Fast Fourier transform:
Method of electrode
stimulation
Channel selection
and stimulation
16 12 22
Apex to base Apex to base Base to apex
Longest available, all
perimodiolar
HiRes Optima FS4-P ACE
splits signal into envelope
and spectral peak
information, frequency
bands
Current steering; up to
120 virtual channels
HiResS: completely
sequential; HiResP:
partially simultaneous
lateral wall; hybrid
Hilbert transform:
separates slow moving
envelope from fine
temporal structure
Electrodes 6–12 (envelope
information): virtual
channels
Electrodes 1–5 (fine
temporal structure): pulse
packets with channel
interaction compensation;
allows stimulating two
electrodes simultaneously
Lateral wall and
perimodiolar; hybrid
Band-pass filters:
breaks signal down into
frequency bands
No virtual channels or
current steering
Stimulation of maxima
(channels with the
highest amplitude);
continuously interleaved
n
Goals of initial stimulation
Recipient will tolerate full-time use of the devices.
Recipient can detect auditory stimuli at a comfortable level.
n
Goals of follow-up programming appointments
The electrical dynamic range is different depending on the manufacturer of the
device being programmed. Due to proprietary differences in current level and
presentation method, the labeled electrical dynamic range will vary greatly. The
clinician should refer to the manufacturer recommendation regarding preferred
electrical dynamic range.
KNOWLEDGE CHECKPOINT
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Provide the recipient with an electrical dynamic range that the recipient finds comfortable
and provides them access to soft sounds at a perceptually soft level and loud sounds at a
perceptually loud level.
Assess the recipient’s performance with behavioral and objective measures.
Subjective Measures of Cochlear Implant Programming
At a fundamental level, changes are made to upper and lower stimulation levels by changing the amplitude of the current. Changes to pulse width can also affect recipient perception of loudness and may
be used when the audiologist wants to change loudness percept without constraints of current level.
n
Measurement and considerations for threshold levels: threshold is the lowest amount of
electrical signal that is transmitted to the recipient. The specific formula or specifications for
which the threshold level is set are dependent on the CI manufacturer.
Advanced bionics (AB): the recipient should be able to detect the threshold on 50% of
presentations. It is labeled the T level. T level measurement is not required and may be set
as a percentage of the upper stimulation level, typically between 5% and 10%.
Cochlear: the recipient should be able to detect the threshold on 100% of presentations.
It is labeled the T level. T levels are regularly measured by many audiologists who program
cochlear devices. It can be programmed using the traditional audiologic Hughson-Westlake
procedure. To ensure the recipient is hearing it at threshold level and to obtain that 100%
mark, it may be obtained by having the recipient count the number of stimulation beeps
they hear, often between two and five beeps.
MED-EL: the threshold level should be just below the level of detection. It is labeled the
THR level. THR-level measurement is not required and is set as a percentage of the upper
stimulation level, automatically set at 0% to 10%.
n
Measurement and considerations for upper stimulation levels: upper stimulation level is the
highest level of stimulation electrically provided to the recipient. The exact definition of the
upper stimulation level and whether that level is the maximum electrical output is dependent
on the manufacturer.
Appropriate upper stimulation levels are essential for recipients and affect performance and
acceptance.
There are a variety of methods used to set upper stimulation levels, with psychophysical
loudness scaling being the most common.
●
Psychophysical loudness scaling is when the recipient reports when the sound is soft,
medium, perfect, loud, uncomfortably loud, and so on.
Despite its importance, behavioral setting of loudness by recipients is a skill that requires
practice as the user adapts to electric stimulation.
AB: upper stimulation levels are called M levels.
Cochlear: upper stimulation levels are called C levels.
MED-EL: upper stimulation levels are called MCL.
n
Loudness balancing: this measure is completed with the recipient’s upper stimulation levels
after they are set.
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Psychophysical loudness balancing is completed by playing upper stimulation levels
at adjacent electrodes. The recipient then reports whether the two are equally loud.
Adjustments should be made to ensure equal loudness across the electrode array. Upper
stimulation levels that are equally loud result in better speech perception due to better
transmission of the natural loudness relationships in the speech signal.
Recipients have difficulty adequately reporting loudness. Electrically evoked stapedial reflex
threshold (ESRT), discussed below, is a more reliable measure to assess equal loudness and
should be used when possible.
n
Sweeping upper stimulation levels
This is a quick way to confirm equal loudness and appropriate pitch transitions. Each
electrode is stimulated across the frequency range at the upper stimulation level. The
recipient will then report unequal loudness or abnormal pitch transitions.
n
Pitch scaling
It should also be confirmed that the recipient has appropriately ascending pitch. This can
be done in a similar fashion to loudness balancing where two adjacent electrodes are played
and the person confirms that the more basal electrode is higher in pitch. If the electrode is
not higher in pitch or if it sounds like the same pitch, that electrode should be deactivated
to ensure the auditory signal is transmitted optimally.
Q & A
Question: Which programming parameter is the single most important for
speech understanding and subjective sound quality?
Answer: Electrical dynamic range (threshold and upper stimulation levels).
Specifically, optimal upper stimulation levels that are equally loud between
channels allow for the natural transmission of loudness relationships that exist
in the speech signal (Wolfe, 2020).
Objective Measures of Cochlear Implant Programming
There are a variety of options for objectively verifying stimulation levels for a recipient.
n
ESRT
This is the only objective measure that demonstrates a strong correlation (0.79–0.92) to
upper stimulation levels (e.g., Walkowiak et al., 2011; Wolfe, 2020).
This measure assesses the lowest level of electrical stimulation that elicits a stapedial reflex
(middle ear muscle/acoustic reflex).
A reflex can be electrically stimulated in most recipients. Recipients with ANSD or nerve
deficiency may not have measurable ESRT values.
ESRT is considered current best practice measure for objective assessment.
ESRT assists with programming upper stimulation at levels that are equally loud between
channels.
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KNOWLEDGE CHECKPOINT
Acoustic reflex measurements are a standard component of the audiologic test
battery and are typically completed during immittance measures. ESRT activates
the reflex pathway using electrical stimulation directly from the implant. For
review on the reflex pathway, refer to Chapters 2 and 5.
Because recipients have a difficult time with loudness ratings, it is recommended to utilize
this measure during programming sessions, even if the recipient can complete loudness
scaling.
ESRT does not typically change over time. If the recipient cannot tolerate upper
stimulation levels at their ESRT level, they may need time to acclimate to their device. Over
time, upper stimulation levels should be near ESRT.
Upper stimulation levels should not be set over ESRT, as the stapedial reflex will be
activated too frequently.
457
The number of clinical units below the ESRT to set the upper stimulation level varies by
manufacturer. Consult with currently available research.
n
Measurement of ESRT
Measurement is obtained via a middle ear analyzer system. Some manufacturers have a
dedicated ESRT setting, or the clinician may use reflex decay mode.
Tympanometry should be completed first to assess middle ear function in both ears.
ESRT cannot be measured in the presence of a PE tube, perforation, or middle ear
dysfunction.
ESRT should be measured in the recipient’s nonimplanted ear or the ear with the highest
compensated acoustic admittance if both ears are implanted.
If ESRT cannot be measured with the traditional 226 Hz probe tone, a 678 or 1000 Hz
probe tone may be utilized.
After the immittance probe is placed in the ear canal, the CI software is utilized to present
three to four pulses at any one channel.
Any ESRT response is time-locked to the stimulus, similar to the traditional acoustic. The
ESRT is the lowest stimulation that initiates a reflex response on two ascending trials.
n
Electrically evoked compound action potential (ECAP)
ECAP is a group of auditory nerve fibers exhibiting a synchronous response to a brief
electrical pulse elicited from an intracochlear electrode. The response is recorded on a
nearby intracochlear electrode.
This is another objective measure that can be utilized to confirm and set recipient electrical
dynamic range.
●
ECAP values more closely relate to upper stimulation levels, but the configuration is
more reflective of threshold levels (Wolfe, 2020).
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ECAP has less objective utility than the ESRT and a weaker correlation (described as
weak to poor) than ESRT to behaviorally measured stimulation levels (e.g., Wolfe, 2020;
Zimmerling & Hochmair, 2002).
The ECAP can be used to confirm that a response can be elicited from the auditory nerve
via the recipient’s CI. It can also be used to indicate that there is a synchronous auditory
nerve response to electric stimulation. Most recipients have a recordable ECAP. ECAP may
not be present with ANSD or cochlear nerve abnormalities.
ECAP may be used intraoperatively to test auditory system response to the CI.
Since ECAP measures do not significantly change over time, they also can be used as a
reference of auditory system function that can be reassessed if concerns for performance or
underlying physiologic function exist.
Each CI manufacturer has a different name and recording mechanism for their ECAP
measure.
●
AB: neural response imaging (NRI)
●
Cochlear: neural response telemetry (NRT)
●
MED-EL: auditory response telemetry (ART)
n
Measurement of ECAP
ECAP measurements are completed within the manufacturer software.
●
The exact procedure varies by manufacturer, but it typically involves a series of increasing
or decreasing electric stimuli with responses recorded on an amplitude growth function
(AGF). The AGF is then used to derive the ECAP threshold.
●
Some manufacturers offer the ability to manipulate some ECAP parameters such as pulse
width if a response is not obtained via the default settings.
The recipient can be in any state during measurement — moving, talking, asleep, under
anesthesia, or awake.
An 18-month-old with CHARGE syndrome presents with bilateral profound
SNHL. After evaluation and approval by the CI team, the child undergoes
implantation in one ear. Imaging reveals normal cochlear anatomy. Due to significant developmental delays, this child is unable to participate in behavioral
programming methods but does exhibit changes in movement and eye gaze in
response to stimulation. The child presents with an abnormal tympanogram in
the implanted ear. In the contralateral ear, the child has a normal tympanogram.
ESRT is therefore attempted as the acoustic reflex is a bilateral response. Unfortunately, the child exhibits too much movement, causing artifact in recording of
the ESRT response. In this scenario, the next option is to utilize ECAP measures
to estimate appropriate stimulation levels. This should be used with caution
in combination with behavioral observation of the recipient’s reactions to live
speech. Over time, ESRT and behavioral assessments of loudness should be reattempted to achieve optimal programming levels.
CASE EXAMPLE
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Electrical auditory brainstem response (EABR)
459
EABR is another assessment of the auditory neural pathway in response to the electric
stimulus.
EABR may be utilized when ESRT or ECAP cannot be elicited, for example, when the
person has an abnormal cochlea.
The setup and administration of EABR are comparable to the traditional ABR. The
stimulus is elicited via a connection from the ABR system to the cochlear implant
programming software.
As with traditional ABR, the recipient needs to be asleep or very still and calm with eyes
closed for optimal recording.
EABR does not have a direct correlate to any one component of the electric dynamic range.
Considerations for Cochlear Implant Recipients
Facial Nerve Stimulation
Facial nerve stimulation is considered the most common side effect of cochlear implantation. This is
when electrical stimulation of the auditory nerve causes additional stimulation of the facial nerve (due
to close anatomical proximity). Facial nerve stimulation may present as any of the following during
electric stimulation:
n
Tingling sensation in the face
n
Watering of the eye(s)
n
Itching of the face or eye
n
Facial spasm
n
Pain in the face or at the eye
Facial stimulation can occur due to a wide array of factors that include proximity of the facial
nerve to the cochlea, changes in the cochlea secondary to disease process, and the amount of electrical
stimulation needed for individual loudness percepts. Abnormal cochlear and/or skull anatomy can also
increase the likelihood of facial nerve stimulation. Facial nerve stimulation is abnormal and a recipient
should never have active facial stimulation with CI use. The following process can be used to identify
and eliminate facial nerve stimulation:
n
Identify the electrode(s) that are exhibiting facial nerve stimulation.
Consider sweeping upper stimulation levels to identify the electrode or region that is
causing facial nerve stimulation.
n
Make changes to the recipient’s map at the affected electrode(s).
If minimal reductions in upper stimulation levels eliminate facial stimulation, no further
adjustments need to be made.
●
With AB devices, activate clipping, which restricts the maximal power output at that
electrode, thereby preventing active stimulation at the level that causes facial nerve
stimulation.
If changes of more than a few clinical units are required to eliminate facial nerve
stimulation, increase the pulse width and remeasure upper and lower stimulation levels.
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●
Increasing pulse width allows for greater loudness percept with less energy.
If facial nerve stimulation is confined to a small number of electrodes and programming
adjustments will not yield an adequate loudness percept without stimulation, deactivate the
affected electrodes.
Vestibular Considerations
Patients with SNHL are at greater risk for vestibular dysfunction before and after cochlear implantation.
n
Vestibular testing may be completed before and after cochlear implantation and may be
included as part of the candidacy evaluation. On studies of vestibular function of patients
compared to baseline, Rasmussen et al. (2021) found that although recipients had significantly
poorer overall scores on a vestibular battery 3 to 6 months after they underwent implantation,
results of vestibular function continued to decline until 14 months after surgery. The authors
noted that despite changes in testing scores, participants did not report significantly different
scores of the Dizziness Handicap Inventory, which may have indicated little to no change in
their subjective dizziness.
n
A systematic review of studies that evaluated vestibular function before and after cochlear
implantation, Hansel et al. (2018) found that <20% of recipients had new instances of vertigo
after CI surgery, and <25% of recipients had vertigo prior to surgery with <8% resolving after
surgery and <8% having no change postsurgery.
Age was a significant factor; the older the patient was, the more likely they were to have
postsurgery vertigo.
In total, less than 10% of recipients have postsurgery vertigo. It is important to note
that there are significant differences in studies, with some reporting higher incidence
of subjective vertigo and others reporting fewer changes in subjective vertigo and more
changes in postsurgical vestibular function testing.
n
Overall, it is important to consider the potential impact of CI surgery on a recipient’s
vestibular system and to complete pre- and posttesting if possible. For concerns about
a recipients’ vestibular systems, referrals to a vestibular-specialized physical therapist are
encouraged.
n
Results of pediatric assessment are much more varied (Koyama et al., 2021) due to challenges
with children appropriately participating in vestibular tasks and ability to follow directions.
Again, if vestibular function is an area of concern, recipients should be referred to a specialized
physical therapist to adequately address their needs. This is particularly important for young
children to address areas such as late walking.
Preservation of Residual Hearing
Hearing preservation in cochlear implantation occurs when the patient has useable low-frequency (<1500
Hz) hearing thresholds prior to surgical placement of the electrode array and they still have measurable
hearing on postoperative audiometric testing. Hearing preservation should be considered and attempted
for recipients with low-frequency hearing thresholds in the moderately severe range or better.
n
While preservation of preoperative hearing thresholds can be present across the entire
frequency range, preservation is most clinically and functionally important in the lower
frequency range (i.e., <1500 Hz).
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n
Hearing preservation can be achieved with a shorter internal array, the Hybrid™ or electric-
acoustic stimulation (EAS™) array, which is available with Cochlear or MED-EL, respectively,
for recipients 18 years of age or older. Due to advances in technology and thinner, more flexible
internal arrays, hearing preservation can also be achieved with full-length (traditional) electrode
arrays. Figure 9–6 demonstrates the audiogram range when the CI team should consider a
hybrid array or hearing preservation, based on the FDA criteria for Cochlear and MED-EL.
n
Recipients with residual hearing have better sound quality, better performance on speech-in-
noise, and better music appreciation.
n
Some recipients with hearing preservation have low-frequency hearing that does not need
amplification; for others, an acoustic component can be added to their processor to amplify
the low frequencies, so they are hearing with electroacoustic stimulation.
AUDIOLOGY NUGGET
With advances in technology creating more flexible, atraumatic electrode arrays,
and with greater surgeon experience, hearing preservation is possible with fulllength electrode arrays (Dillon et al., 2020). Some surgeons are choosing to place
full-length electrode arrays to accommodate the full length of the cochlea even
when there is preservable hearing for natural or amplified acoustic or electric
low-frequency hearing, should the hearing change postoperatively.
461
FIGURE 9–6. Audiogram representation of preservable hearing thresholds based on FDA
criteria for Cochlear Hybrid™ and MED-EL EAS™ devices, with the least restrictive audiogram criteria being chosen.
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