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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_4455_Библиотеки_им_академика_М_И_Перельмана
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282 Disorders of the Auditory System
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Case 6–3: Acoustic Neuroma
History
This case is one of a 52-year-old woman
who had a history of tinnitus and progressive hearing loss on the left side for
several years. She also suffered for some
time with imbalance. No other significant audiologic or otologic history was
reported.
Audiology
An otoscopic check was unremarkable
bilaterally. Pure-tone thresholds showed
a mild to moderate sensorineural loss
with no measurable speech recognition at suprathreshold levels on the left
side (Figure 6–3A). A speech recognition
threshold was obtained in this ear, but
only at a high-intensity level, which was
inconsistent with the pure-tone findings.
The pure-tone and speech audiometry test
results for the right ear were normal. ABR
testing was completed. The right ear ABR
was normal except for poor morphology
of the IV–V complex (Figure 6–3B). In the
left ear, no response was observed after
wave I (see Figure 6–3B). Distortion product otoacoustic emissions were essentially
normal bilaterally (Figure 6–3C).
inconsistency between this speech recognition measure and the pure-tone findings
in this ear, along with the medical finding
of questionable facial nerve function. An
MRI was ordered to evaluate for retrocochlear involvement. Radiology revealed a
large acoustic neuroma in the left cerebellopontine angle (Figure 6–3D).
Impression
Left-sided acoustic neuroma.
Audiologic Recommendations
and Management
The patient was seen again following
treatment (detailed in the following medical recommendations and management
section). Postoperatively, there was no
improvement in hearing or word recognition (as would be expected due to the
size and involvement of the acoustic neuroma). The patient was provided with
the recommendation to consider a CROS
hearing aid as a traditional hearing aid
for the left ear would yield no benefit due
to the extremely poor word recognition.
The patient elected to proceed with the
CROS aid and reports significant benefit,
particularly hearing in difficult listening
environments.
Medical Examination
The otolaryngologic exam revealed balance difficulties. Tympanic membranes
were normal. However, facial nerve function was questionable for a possible slight
weakness. Radiologic follow-up was recommended based upon the audiologic
findings of an asymmetric hearing loss,
the extremely poor speech recognition
score (i.e., 0%) in the left ear, and the noted
Medical Recommendations
and Management
The patient was presented with management options, which included traditional
surgery or gamma knife radiosurgery.
The patient elected to have traditional
skull based surgery performed. This was
performed by the neurotology service in
conjunction with neurosurgery without
complications. The patient has done well
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A
Figure 6–3. Pure-tone audiogram and speech recognition scores (A), ABR tracings (B), DPOAEs
(C), and MRI (D) for a 52-year-old female with a left-sided acoustic neuroma (Case 6–3). continues
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B D
C
Figure 6–3. continued
postoperatively and continues to be monitored with routine MRIs to monitor any
reoccurrence of the acoustic neuroma.
auditoRy neuRoPathy
sPectRum disoRdeR
Introduction
Auditory neuropathy spectrum disorder (ANSD) is perhaps one of the most
difficult disorders to discuss as there is
considerable controversy that surrounds
this particular auditory nerve disorder.
This particular disorder has also been
referred to as auditory neuropathy/auditory dys-synchrony (AN/AD). Therefore, it is a challenge to present a fair and
balanced picture of this disorder. There
is no question that the present authors
have a perspective on this disorder, and
this perspective is likely biased toward
certain notions about ANSD. One of the
difficulties with ANSD is the way that it
is defined. The neurologic definition of
neuropathy is impaired function of the
peripheral nerves, and the only peripheral
nerve in the auditory system is the auditory nerve. Therefore, in a strict sense,
ANSD means dysfunction of the audi-
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tory nerve. If that is the case, then all of
the disorders mentioned earlier, including
acoustic neuromas, could be interpreted
as ANSD. It is difficult to determine if
those investigating ANSD would agree
or disagree with this interpretation. This,
therefore, presents a problem in terms of
classification and discussion of auditory
nerve disorders. Should all of the disorders mentioned previously be bundled
and discussed under ANSD, or should
they be treated separately? From a common usage or popular interpretation
standpoint, only discussing ANSD might
be best; but from a pathophysiology perspective, perhaps each disorder or type
of disorder should be individually presented. This is a dilemma in regard to presentation for which it is difficult to determine the best answer. The present authors
doubt if most people would say that the
acoustic neuroma or vascular loop is an
auditory neuropathy, yet both could meet
the audiologic criteria mentioned earlier. The same could be said for the rare
case of multiple sclerosis that attacks the
myelinated portion of the auditory nerve
rather than or in addition to the more rostral auditory structures (for discussion
of additional sites of lesions for multiple
sclerosis, please refer to Chapter 7, “Disorders of the Central Auditory Nervous
System”). Another disorder that presents
a challenge is hyperbilirubinemia, or in
its severe pathologic state — kernicterus,
which is often considered an ANSD (see
Dublin, 1986, and Rapin & Gravel, 2003).
Although this disorder can affect the
auditory nerve, the primary site of lesion
within the auditory system is the cochlear
nuclei located in the lower brainstem
(Dublin, 1986; Møller, 2000). This is why
this disorder is discussed in the central
auditory chapter and not in this chapter.
These questions with regard to the defini-
tion and classification of ANSD obviously
influence much of the discussion about
this particular auditory nerve disorder,
such as its incidence and prevalence, etiology, site of lesion, and so forth.
Symptoms
Hearing loss is the main symptom of ANSD.
However, depending on the etiology, the
hearing loss may take on various characteristics and degrees of involvement.
Individuals will offer complaints of severe
distortion of speech and extreme difficulty
hearing in noise, and yet will demonstrate
fairly good hearing sensitivity. Others will
have complete or near complete loss of
hearing sensitivity. Again, depending on
the etiology, there may be other associated
symptoms, such as vestibular problems,
tinnitus, and other sensory or motor difficulties. ANSD is frequently diagnosed
in newborns. Therefore, there is often limited information regarding the associated
symptoms as these patients are not able to
describe their experiences. Interestingly,
ANSD can present either unilaterally or
bilaterally.
Incidence and Prevalence
As alluded to earlier, the incidence or
prevalence of ANSD is difficult to determine because it depends on the breadth
of the inclusion criteria. For example, if
one includes hyperbilirubinemia, then the
incidence/prevalence would be higher.
Cone-Wesson and Rance (2000) in a review
article related the incidence of ANSD to
be slightly more than 2% for infants with
risk factors for hearing loss. However, this
figure includes those infants with hyperbilirubinemia. Others would argue that
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true ANSD is much rarer than 2% (Rapin
& Gravel, 2003).
Etiology and Pathology
As mentioned earlier, if one entertains
the neurologic definition of neuropathy,
then the etiology of ANSD could essentially be any disorder that damages the
auditory nerve. It has been proposed that
specific dysfunction of the auditory nerve
is the likely basis for ANSD. This dysfunction can include (1) injury to the synaptic
junctions between inner hair cells and the
dendrites of the spiral ganglion, (2) damage to spiral ganglion dendrites directly,
(3) direct injury to spiral ganglion neurons, and/or (4) axonal damage to the
auditory nerve, which in turn cascades
damage to more rostral (brainstem)
nuclei (Shaia, Bojrab, & May, 2018). These
types of auditory nerve damage are often
related to various disorders or conditions, such as anoxia, hypoxia, low birth
weight, prematurity, family history of
ANSD, viral disease, seizure, high fever,
Friedrich’s ataxia, Stevens-Johnson syndrome, Ehlers-Danlos syndrome, and
Charcot Marie-Tooth syndrome (Shaia
et al., 2018). Hyperbilirubinemia is commonly included in this group, but the
present authors would maintain that this
disorder is primarily one of the central
nervous system and the auditory nerve is
involved only secondarily, if at all.
Site of Lesion
By definition, ANSD implicates the auditory nerve as its site of lesion. However, it
has been shown that damage to the inner
hair cells, but not the outer hair cells, can
yield audiologic results consistent with
those noted in ANSD (Salvi, Wang, Ding,
Stecker, & Arnold, 1999).
It also is well known that some
researchers and clinicians advocate for
diagnosing dysfunction of the auditory
neurons in the lower brainstem as ANSD
(see Rapin & Gravel, 2003, for a review of
this particular topic). The problem simply
stated is, why would these disorders not
be classified as a central as opposed to
a peripheral auditory disorder? It is the
opinion of the present authors that disorders affecting the auditory neurons of
the brainstem should be classified among
the central auditory disorders, and not as
ANSD, which is a peripheral disorder. As
outlined by Rapin and Gravel (2003), there
are several possible “peripheral” sites of
lesion in the auditory nerve including the
myelin sheath and/or the axon, and the
neuronal cell body itself.
Audiology
Auditory neuropathy spectrum disorder
is most often defined by a constellation of
audiologic test findings, including variable degrees of hearing loss (most often
reduced), poor speech recognition ability
in relation to the degree of hearing loss,
and normal otoacoustic emissions (at least
in the vast majority of cases). In addition,
absent ABRs are typical; although in some
cases, the later waves may be present but
abnormalities of the earlier waves are
evident. This disorder is not currently
defined by anatomic or pathologic indicators. Certainly, vascular problems, infections, neural degeneration, and trauma
can damage the auditory nerve and will
likely yield abnormal ABRs, implicating
auditory nerve involvement. However,
within the present classification system,
these disorders may not provide the audi-
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ologic findings that would classify them
as ANSD disorders. The main point here,
however, is that a variety of disorders may
cause auditory nerve dysfunction that can,
and should, be detected by utilizing ABR
and not by imaging. This is a critical point
too often ignored in today’s diagnostic
world (see Musiek et al., 2007).
Pure-tone thresholds can be highly
variable in ANSD, ranging from normal
to a profound loss of hearing sensitivity.
When hearing loss is greater than 30 to 40
dB HL and OAEs are normal, the interpretation is that of retrocochlear involvement, such as in ANSD. A relatively recent
study reported that about 3% of ANSD
cases had normal pure-tone hearing and
15% had profound hearing loss (Berlin
et al., 2010). These results demonstrate the
wide range of hearing losses that can be
found in patients with ANSD.
Speech recognition performance, like
pure-tone thresholds, has been reported
as highly variable; however, the majority
of patients for whom speech testing can
be completed generally show reduced
scores and/or scores that are poorer
than expected based on the audiogram
(Hood, 2007). Also, particular difficulty
understanding speech in the presence of
background or competing noise is often
observed, if tested.
Acoustic reflexes are often absent
in ANSD cases (nearly 90% of the time),
but there are also exceptions to this. Otoacoustic emissions are generally present
in ANSD (approximately 75% of the time),
but there also have been reports showing
them to be absent and/or changing over
time (Berlin et al., 2010).
As mentioned earlier, ANSD is currently defined by patient performance
on a constellation of audiologic tests.
Perhaps the most important of these
tests is the ABR. The strictest interpre-
tation for ABR in ANSD is no response
but a recordable cochlear microphonic
(CM), and most reports on ANSD show
totally absent ABRs (Hood, 2007). It is our
view that when ABR waves are present
but delayed or of poor morphology, the
diagnosis of ANSD may not be as definitive as if there was no response. Often,
imaging procedures will not show anything abnormal in cases of ANSD; hence,
there is an emphasis on the ABR results
in these cases. At times, there can be confusion between the CM and a wave I of
the ABR. Changing polarity will “flip or
reverse” the CM, but not wave I. Also,
the CM does not change in latency as one
decreases stimulus intensity, but wave I
does (see Hood, 2007). Therefore, if there
is any question whether one is observing
the CM or wave I, one or both of these
strategies should be employed.
Recent reviews have promoted the
use of transtympanic electrocochleography for helping to determine if the ANSD
is related to presynaptic versus true neural (auditory nerve) dysfunction. This
technique is not used routinely in many
clinics, but its utilization may prove useful in better determining the underlying
mechanisms for ANSD (Berlin et al., 2010).
There appears to be a high incidence
of bilateral involvement in cases of ANSD.
A recent study revealed that 92% of 260
cases of ANSD had both ears involved.
Interestingly, in the small number of cases
that were unilaterally involved, there
were twice as many left ears involved
(Berlin et al., 2010).
Medical Examination
Because ANSD can have many causes,
an otologic consultation/examination is
necessary. A careful review of the patient’s
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medical records and a medical examination can help determine the cause and
whether there is any medical treatment
for the particular etiology of ANSD that
has been identified. Imaging is often
ordered to rule out other pathological
processes. Genetic testing, as well as various blood tests, may also be performed to
help determine the basis of the problem.
In many cases, however, the actual cause
of the problem will remain undetermined.
Medical and Audiologic
Management
If there is an ongoing underlying medical problem that is linked to ANSD, then
otologic management is key. Once this is
addressed, or if there is no known medical factor, then management will typically
involve a hearing aid fitting or a cochlear
implant. One of the main considerations
in the management of ANSD is the realization that some patients with this disorder will present with an overall auditory
performance that is often much poorer
than would be predicted by their puretone thresholds. In some patients with
ANSD, speech recognition ability can be
very poor even when hearing sensitivity is good (Berlin et al., 2010; Rance &
Barker, 2008). In these cases, the benefits
received from traditional amplification
may be limited.
A number of children with ANSD
have been managed with cochlear implants. Although the overall results have
certainly been worthwhile, there is great
variability in outcomes. This is related to
the highly heterogeneous nature of the
ANSD population. Some of the reasons
for the variability in patient outcomes
include various sites of lesion for the disorder; the duration, degree, and type of
hearing loss (especially the hearing status
of the nonimplanted ear); the duration of
implant use; and the age, linguistic and
cognitive abilities of the patient.
A key factor in predicting cochlear
implant success is the result of various
preimplant electrical stimulation procedures, such as electrical ABRs and/
or electrical promontory stimulation.
Clearly, those individuals that demonstrate robust responses to electrical stimulation perform better than those who
yield meager or no responses. A recent
study shows better speech understanding
for those with robust electrically evoked
action potentials (EEAPs) compared to
those with absent or poor EEAPs (Teagle
et al., 2010).
Most children with ANSD demonstrate improved speech recognition after
implantation, but this is not always the
case. The heterogeneous factors mentioned earlier make it difficult to identify
general and consistent trends for hearing
improvement. Generally, the longer the
patient wears the implant, the better the
speech recognition; however, again, this is
not always a consistent finding with many
notable exceptions (Teagle et al., 2010).
Those who do poorly after implantation are of the most interest in terms of
predictive and habilitative techniques. Research aimed at determining the reasons
why some individuals do not experience
positive outcomes following implantation
can help professionals establish better candidacy criteria. The findings from these
types of research efforts will advance the
success rate of cochlear implantation as a
habilitative intervention for individuals
diagnosed with ANSD.
Auditory training and counseling
can prove most helpful in the habilitation of the patient with ANSD who has
been implanted (Chute & Nevins, 2000).
Key factors are the age and the linguistic
level of the child or adult. Auditory train-
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ing approaches need to start at levels that
are consistent with the age and language
status of the individual and then progress to more advanced levels. Training
should start at the level of detection. This
can include presenting a wide variety of
sounds in varying contexts. This then can
be followed by auditory discrimination
tasks with the focus on same versus different decisions. Both various environmental
sounds and speech stimuli can be utilized.
Next, the identification of sounds/speech
can be targeted, again using a wide variety of stimuli in varying contexts. The
comprehension of language is the final
step and requires appropriate thinking
about the speech stimulus and responding verbally. There are a number of auditory training techniques presently used
in intervention for children with central
auditory processing disorders that would
seem to be excellent procedures for use
with children with cochlear implants (see
Musiek, Chermak, & Weihing, 2014, and
Musiek, Shinn, & Hare, 2002).
Hearing aids can be of help for at
least a subset of the children and adults
who present with ANSD. Because of
the wide range of abilities of those with
ANSD, individual monitoring and frequent follow-up is critical. Berlin et al.
(2010) reported that 61% of the patients
diagnosed with ANSD in their study
received no benefit from hearing aids.
Therefore, monitoring for the achievement of the appropriate language milestones with hearing aid use is essential
when working with young children. If
the child’s performance lags or does not
reach expected milestones, then a cochlear
implant becomes a consideration. Hearing aids can be fit on a trial basis when
the audiologist is attempting to decide
on a hearing aid(s) versus a cochlear
implant (Teagle et al., 2010). When hearing aids are fit, they should provide access
to and understanding of speech. If this is
not accomplished, then consideration for
cochlear implants becomes more viable.
Because in ANSD the audiogram may be
misleading, one must be careful in fitting
“power hearing aids” as they may damage peripheral hearing. This is where the
applications of OAEs and ABRs become
valuable as the finding of normal or near
normal test results should serve as a contraindication for the fitting of “power
hearing aids.”
Case 6–4: Auditory
Neuropathy Spectrum
Disorder
History
A 31-year-old female presented with
problems of bilateral facial nerve palsy,
severe balance disturbance, and auditory
symptoms of severe distortion of speech
and some loss of hearing sensitivity, all
with simultaneous onset approximately
10 years prior to her current audiologic
evaluation. The patient reported recovery
of the facial nerve palsy, but not the auditory symptoms, which reportedly became
progressively more severe following the
onset of symptoms.
Audiology
An otoscopic check was unremarkable
bilaterally. Results of a comprehensive
audiologic evaluation indicated a moderate to mild low-frequency sensorineural
hearing loss with normal tympanometric
findings bilaterally (Figure 6–4A). Speech
recognition thresholds were in agreement
with the pure-tone averages for the two
ears, whereas speech recognition scores
at suprathreshold levels were fair (74%)
in the right ear and very poor (4%) in the
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A
Figure 6–4. Pure-tone audiogram and speech recognition scores (A), TEOAEs (B), and ABR trac-
ings (C) for a 31-year-old female with auditory neuropathy spectrum disorder (Case 6–4). continues
B
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Figure 6–4. continues
291
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