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D
Figure 6–6. continued
302
6. Auditory Nerve Disorders 303
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ear response was abnormal with the
absence of a wave I; however, waves II
through V were present at normal latencies and a normal III–V interwave interval was noted. No significant abnormalities were observed for either ear at the
high repetition rate of 77.7 clicks/sec).
Medical Examination
The patient presented with an essentially
normal otolaryngologic examination.
Imaging revealed a very prominent anterior inferior cerebellar artery on the right
side (Figure 6–6D).
Impression
Right-sided vascular loop syndrome.
Audiologic Recommendations
and Management
It was recommended that the patient
undergo aural rehabilitation to include
intense training exercises to build skills
in binaural integration and the processing of degraded speech. This was particularly important for this patient as she is
employed as an operating room nurse.
In addition, a mild gain hearing aid was
recommended in order to assist with the
auditory processing deficit. Although the
patient considered both recommendations, she opted not to proceed with any
intervention at the time of her evaluation;
however, she continues to be monitored
annually.
Medical Recommendations
and Management
benefits of surgery were discussed with
the patient. The patient continues to be
seen for otologic evaluations annually as
she elected to forego surgery.
summaRy
Acoustic neuromas (vestibular schwannomas), auditory neuropathy (ANSD), and
vascular loops (compression) are all disorders that can compromise auditory nerve
function. Acoustic neuromas require
early detection so they can be managed
optimally. Early detection is dependent
on interaction between audiologists
and otologists. Audiologists also can be
involved in monitoring and managing
these patients after surgery. The main
treatments, however, are surgery, radiation, and monitored observation. ANSD
remains, at least from some perspectives, somewhat controversial. It affects
newborns as well some older children
and adults. It is defined by behavioral,
electroacoustic, and electrophysiologic
audiologic test results. Cochlear implants
have provided significant help to many of
the patients with this disorder. Vascular
loops or vascular compression syndrome
results from pressure on the eighth and/
or adjacent cranial nerves resulting from
a misplaced blood vessel in the CPA.
Hearing loss, hemifacial spasm, tinnitus,
vestibular problems, and even facial pain
can be symptoms. Although commonly
diagnosed and treated, the nature and
characterization of this disorder remain
controversial.
The option of microvascular decompression of the vascular loop away from the
facial nerve and the vestibulocochlear
nerve complex along with the risks and
Acknowledgments. The authors grate-
fully acknowledge Eric Smouha, MD, Neurotologogist, ENT and Allergy Associates,
LLP, for his contributions to this chapter.
304 Disorders of the Auditory System
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7
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Disorders of the Central
Auditory Nervous System
intRoduction
When considering disorders of the central
auditory nervous system (CANS), it must
be understood that it is not necessarily
the type of disorder but rather the location of the lesion and the specific effects
of the disorder on the CANS (i.e., the
disease mechanisms) that are the crucial
issues. Anatomically, the CANS begins
at the level of caudal pons, specifically at
the cochlear nucleus. At a similar level,
but located deep in the pons, is the next
major group of nuclei: the superior olivary complex (SOC). The SOC projects
fibers along the lateral lemniscus, a major
brainstem pathway that also has a group
of nuclei in the upper half of the pons. The
next nucleus in the CANS is the inferior
colliculus. This nucleus is located in the
midbrain and receives input from practically all of the more caudally located auditory nuclei and projects to the underside
of the thalamus to the medial geniculate
body (MGB). The MGB sends fibers to
the auditory cortex, specifically Heschl’s
gyrus and secondary auditory areas such
as the insula. The corpus callosum connects the right and left hemispheres of the
brain and has a specific auditory region
where impulses are exchanged between
the two hemispheres.
Many disorders can result in audiologic deficits if there is insult to the auditory neural substrate within the CANS.
However, the effects of CANS damage
often result in audiologic findings that
are not unique to a particular disorder but
rather to the site of the CANS lesion. Some
of the more significant disorders that
can result in central auditory deficits are
discussed in this chapter. Many of these
disorders are often overlooked by healthcare professionals due to the fact that
other comorbid conditions (e.g., paralysis,
vertigo) often overshadow the auditory
symptoms. In some cases, these disorders
may not result in hearing deficits, but in
others, they certainly can and do. When
they do, it is important to determine the
nature and degree of the deficit(s) so that
optimal medical and/or audiologic treatment can be realized.
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Audiology
The audiology sections in this chapter are
organized in a slightly different manner
than in previous chapters. This was done
for efficiency. We discuss the audiologic
profiles associated with various types of
CANS disorders as has been done in other
chapters; however, due to the similarity
of the audiologic findings in mass and
vascular lesions, rather than dedicating
a separate section to the audiologic findings for each of these lesion sites, the discussion is combined. Audiologic findings
are provided separately for the following
lesion types: degenerative disorders, neurotoxicity, traumatic brain injury, temporal lobe epilepsy, surgical compromise of
the CANS, and learning difficulties.
Anatomic Factors
In the assessment of various disorders
of the CANS, it is useful to understand
that various tests have anatomic limitations. Some central tests are efficient for
the assessment of brainstem involvement,
whereas others may be better suited for
use with cortical or interhemispheric compromise. Also, some disorders may manifest their dysfunction primarily in one of
these three anatomic regions, whereas
others may affect multiple areas. This,
of course, influences the types of tests
selected for administration during evaluation of the patient. For example, a small
tumor of the low brainstem would have a
focal effect in the pons. On the other hand,
heavy metal neurotoxicity could involve
the entire auditory system. Therefore, the
selection of the tests to be administered
to a particular patient will depend on
the patient’s case history and presenting
symptomatology, as well as the results
of any tests that are initially administered during the patient’s evaluation, as
these may indicate that additional testing
is needed to explore different (or potentially additional) auditory processes and/
or sites of lesion.
Types of Tests
Two main categories of central tests are
discussed in this chapter. These include
psychophysical (i.e., behavioral) tests and
electrophysiologic tests. The psychophysical test category includes dichotic listening, temporal processing, low redundancy
speech, and binaural interaction tests,
whereas the electrophysiologic procedures
to be discussed include the auditory brainstem response (ABR), the middle latency
response (MLR), the N1 and P2 late potentials, and the P300 (also referred to as the
P3 potential). On occasion, a brief discussion of other auditory evoked potentials
also is included. Finally, there are some
electroacoustic tests (e.g., otoacoustic
emissions) that are often used to help differentiate a central site of lesion from a
peripheral one. Although the otoacoustic
emissions test does not assess the integrity of the CANS, it can be used to rule out
a significant peripheral hearing loss.
In cases of CANS involvement, the
goal of testing generally is not to make the
diagnosis as in many instances, the disorder is already known and many medical procedures are better at defining the
lesion than are the central auditory tests.
However, there are three major goals of
audiologic testing when CANS involvement is either suspected or confirmed. The
first is as a screener. The audiologist may
be the first professional to see a patient
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with a CANS disorder and therefore has
the responsibility to make the appropriate referral for medical and/or psychological/psychiatric follow-up. This, of
course, cannot be done unless central
dysfunction is determined. Although not
a common occurrence, this situation does
happen and when it does, appropriate
follow-up and management of the patient
is essential. The second major goal in utilizing central tests in CANS disease is to
determine if the central auditory system
is involved. For example, a patient with a
long-standing diagnosis of multiple sclerosis might seek audiologic assessment
because of a new symptom of hearing difficulty. The key here is to determine if this
new symptom is really auditory in nature
or not, and if it is, whether it is due to
peripheral or central system compromise.
The third goal of central auditory assessment is to corroborate the medical and
communicative symptoms and/or complaints of the patient with test measures
and to determine the degree of the patient’s
functional deficit(s). All three of these goals
lead to another important aspect of audiology; that is, the appropriate management
of the patient. Without the appropriate
diagnostic information, proper management of the patient with confirmed or suspected CANS involvement is difficult.
Brain Plasticity
Plasticity of the CANS is a factor in all
brain lesions. Natural compensation by
the brain for central dysfunction often
occurs over time. Therefore, some lesion
effects may not be as severe if the patient
is assessed some time after the initial
occurrence of the CANS disorder or insult
as they would have been had the patient
been assessed closer in time to the original
disease process or CANS compromise. On
the other hand, some CANS lesions may
progress, creating greater problems over
time, as potentially would be the case in
progressive CANS disorders such as multiple sclerosis. Given these considerations,
central auditory testing can be used not
only to initially document the auditory
deficits associated with CANS involvement, but also to monitor subsequent
changes in the patient’s audiologic profile that may result as a function of brain
plasticity, audiologic intervention, and/or
disease progression.
mass lesions
Introduction
Mass lesions are space occupying lesions
located within the brain. Lesions that
are located within or close to the auditory areas of the brain, of course, are at
risk for influencing central auditory function. Sometimes, vascular lesions such
as aneurysms and hematomas could be
viewed as mass lesions in that they can
be space occupying as they often are quite
large. However, these types of lesions
are classified as vascular disorders. In
the brainstem, mass lesions are divided
into intra-axial and extra-axial categories
depending on whether the tumor resides
primarily within (intra-axial) or outside
(extra-axial) the brain tissue. The first
reports on central auditory disorders by
Bocca, Calearo, and Cassinari (1954) were
on patients with temporal lobe tumors. In
many cases, these patients reported varying auditory symptoms, depending on the
characteristics of their tumors.
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