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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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322 Disorders of the Auditory System
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changes in their amplitude as well as
latency. Electrodes should be placed laterally as was described previously for
the MLR. A study on patients with left
hemisphere strokes interestingly demonstrated reduced N1 amplitudes over both
hemispheres for right but not for left ear
stimulation (Ilvonen et al., 2001). The N1
also has been shown to have good differential capability. Studies have shown
the N1 to be reduced in temporal lobe
strokes significantly more often than
when the parietal or frontal lobes are similarly affected (Knight, Hillyard, Woods,
& Neville, 1980; Knight, Scabini, Woods,
& Clayworth, 1988). As with the MLR,
N1, P2 responses are generally smaller in
amplitude for the electrode that is near
the lesion site. In addition, the latency of
the response for the electrode nearer the
site of lesion is likely to be delayed compared to that observed for the other hemispheric electrode (see Musiek & Lee, 1999,
for review).
P300 Response
The P300 generators are not well known,
but it seems logical that there are many
throughout the brain. Auditory regions of
the temporal cortex play a role, as probably do the frontal lobe and the hippocampus (see Musiek & Lee, 1999). The P300
does not appear to provide distinct laterality information like the MLR and N1, P2.
This may be because of the multiple generator sites for the P300 response (Musiek, Baran, & Pinheiro, 1992). The P300 is
highly sensitive to aging and dementia. It
is also sensitive to lesions of the auditory
cortical areas of the brain. Both amplitude
and latency effects are noted for P300s
in individuals with mass and/or vascular lesions (Musiek et al., 1992; Obert &
Cranford, 1990). The P300 has been shown
to differentiate parietal from temporoparietal lesions with the latter lesions severely
compromising latency and amplitude
measures (Knight, Scabini, Woods, &
Clayworth, 1989). The oddball paradigm
that is used to acquire the P300 is a powerful approach and allows the simultaneous
recording of the N1, P2 and P300 evoked
potentials. The electrode arrangement for
the P300 classically is midline anterior to
posterior (Fz, Cz, Pz). However, because
the N1 and P2 can be recorded with the
P300 electrodes in lateral positions (C3,
C4, and/or T3, T4), the use of these types
of electrode arrays could prove useful.
Case 7–1: Temporal
Lobe Tumor
History
This patient was a young adult who developed seizures secondary to a temporal
lobe tumor. She reported left arm numbness for years and difficulty with motor
movements on the left side. In addition,
she complained of light-headedness and
often feeling as if she was about to pass
out. She stated that the hearing in her left
ear was worse than that in her right ear,
yet hearing sensitivity was better in her
left ear on the audiogram.
Audiology
The patient had normal hearing sensitivity for pure tones in the left ear and a very
mild conductive loss for the right ear with
excellent speech recognition bilaterally.
Central auditory behavioral tests showed
rather marked left ear deficits for dichotic
listening tests (digits and rhyme tests) and
a bilateral deficit for duration patterns
(Figure 7–1A).
behavioral tests results
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(A) and computerized
Figure 7–1. Central
tomography (CT) scan (B)
7–1). continues
for a young adult with a
right temporal lobe tumor
(Case
A
323
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B
Figure 7–1. continued Note the temporal lobe tumor
at 8 o’clock.
Medical Examination
Neurologically, there was documentation
of a seizure disorder. Radiologically, a CT
scan showed a right posterior temporal
lobe tumor that can readily be seen in Figure 7–1B. This tumor was approximately
2 × 2 × 4 cm in size. Note how the tumor
has affected the right lateral ventricle by
compressing it (see Figure 7–1B).
Impression
This patient has central auditory involvement secondary to a right temporal lobe
tumor and the patient’s related subjective complaints of hearing difficulty for
the left ear. A very mild conductive loss is
present in the right ear. Interestingly, the
left ear (i.e., contralateral to the involved
hemisphere) was reported by the patient
as being poorer than the right ear (i.e., the
ear with the mild conductive loss). These
central test findings are consistent with
expectations for a right posterior temporal
lobe lesion in that they revealed a classic
contralateral ear effect.
Case 7–2:
Temporoparietal Stroke
History
At the time of evaluation, this middleaged woman was several months post
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right temporoparietal stroke. She was
bothered by dizziness episodes, difficulty
hearing soft voices, and difficulty hearing in the presence of background noise
to the point that audiologic consultation
was sought.
Audiology
This patient’s pure-tone thresholds were
normal and her speech recognition ability
was excellent bilaterally. Behavioral central auditory test results showed the classic contralateral effect on dichotic speech
tests and a bilateral deficit on frequency
patterns (Figure 7–2A). Central auditory
electrophysiologic tests included ABRMLR testing performed simultaneously
using a click stimulus (Figure 7–2B). The
ABR was normal bilaterally, whereas the
MLR showed an ear effect with the right
ear Pa responses essentially present at all
electrode sites and the left ear showing an
absent Pa response with a meager Na that,
if present, would be delayed at all recording sites.
The ENG test results for this patient
showed some mild smooth pursuit tracking abnormalities and a stronger caloric
response for the right side that did not
reach clinical significance. The CT scan
revealed a lesion site in the right hemisphere (temporoparietal area) (see arrows
in Figures 7–2C and 7–2D).
Impression
Central auditory involvement consistent
with a right temporoparietal stroke.
Medical Examination
and Management
This patient was being followed and managed neurologically poststroke. She also
had weakness in the left extremities but
was ambulatory.
Audiologic Management
At the time of her evaluation, the patient
was counseled on the use of assistive listening devices, enhancing the listening
environment, and home auditory training
techniques and arrangements were made
for follow-up and consultations closer to
her home.
degeneRative
disoRdeRs
Introduction
A variety of degenerative disorders can
compromise the CANS. However, in this
chapter, the focus is on two relatively
common degenerative disorders that can
affect the CANS: multiple sclerosis (MS)
and Alzheimer’s disease (AD). Multiple
sclerosis is a chronic, progressive autoimmune disease that results in myelin
destruction, which primarily affects young
adults. The severity is highly variable,
and some patients have prolonged periods of total remission. Patients with MS
can have flares (i.e., transient episodes of
focal demyelination) that can last days to
weeks. Thus, the symptoms can be relapsing and remitting over time, but eventually an overall progressive decline is
typically observed. Because it is a disease
affecting the myelin sheathing of nerve
fibers, MS can affect any motor or sensory system or region of the nervous system that has myelinated nerves. Of note is
the fact that only a portion (medial) of the
auditory nerve is myelinated. Therefore,
Figure 7–2. Central
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behavioral test results (A),
combined ABR-MLR trac-
ings (B), and computerized
tomography (CT) scans (C
and D) for a middle-aged
woman with a right-sided
7–2). continues
temporoparietal stroke
(Case
A
326
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B
C
Figure 7–2. continued Key for (B): V = ABR wave V; Pa = MLR wave; Cz, C3, C4 = electrode
sites.
D
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this disease more commonly involves the
CANS rather than the peripheral auditory
system.
Alzheimer’s disease (AD) is the most
common cause of dementia and is classified as a degenerative cognitive disorder,
but it also affects other systems in the
central nervous system. Generally, older
adults are affected by the disease, but it
can attack middle-aged individuals as
well. There is a tendency for the disease
to run in families (see Zabar, 2005).
Symptoms
In MS, the symptoms are related to the
locus of the disease. That is, wherever
the myelin is attacked and plaques (a
type of scarring of myelin) form, symptoms related to that area of the brain can
emerge. As mentioned previously, the
motor and sensory symptoms associated with MS can come and go. When
the symptoms become worse, it is called
an exacerbation. This situation is accompanied by swelling around the plaque(s)
and a related increase in dysfunction. All
neural systems can be affected. Auditory
symptoms are often overlooked by both
the patient and the clinician because they
tend to be more subtle than other motor or
sensory symptoms. Initial onset is usually
in the 20- to 40-year-old age range and
common symptoms include paresthesia,
motor weakness, diplopia, blurred vision,
ataxia, vertigo, balance disturbances, and
hearing difficulties (Jerger & Jerger, 1981;
Merritt & Antunes, 1979).
In patients with AD, a variety of
symptoms emerge, and in the early stages
of this neurodegenerative disease process,
it is quite likely that the symptoms may go
unnoticed. However, in time, short-term
memory loss, loss of organizational skills,
decreased language function, the need
for frequent repetition of conversational
statements and verbal requests, impaired
visual spatial skills, and hearing difficulties will become obvious and progress to
the point where the patient requires significant care (see Zabar, 2005).
Incidence and Prevalence
It is interesting to note that MS is more
common in cold as opposed to warm geographic regions. For example, in Southern
states, the incidence is 6 to 14 per 100,000,
whereas in Northern states, it is 40 to 60
per 100,000. MS is not found in nativeborn Africans and is rare in the populations of Japan, Taiwan, and India. It is
more common in women than men, and
the mean age of onset is usually between
20 to 30 years of age (Poser, 1979).
Alzheimer’s disease is present in
about 50% of the population over 85 years
of age. There are about 4 million cases in
the United States and the risk of developing the disease doubles every 5 years after
age 65 (Zabar, 2005).
Etiology and Pathology
The etiology for MS is unknown but
results in the demyelination of nerves
(with secondary axonal degeneration
occurring as a result). MS is classified as
an autoimmune disease, and it appears
that lymphocytes and macrophages play
a role in the disease process by attacking
myelin (Calabresi, 2006). Demyelination
due to inflammatory plaques results in
reduced and/or delayed neural impulses.
In turn, this affects the specific system that
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is involved. The brain is capable of repairing areas of demyelination, which is why
periods of exacerbation and remission
(i.e., a period of time when the symptoms
are not manifested) are noted in many
patients with MS, especially those in the
earlier stages of this disease. However, if
the inflammation is severe, recurrent, or
prolonged, it can cause axonal damage
that becomes permanent.
The exact etiology of AD is not known.
However, a common finding with this disease is an excess of beta-amyloid protein,
which can impair synaptic function. It
appears that the excess of beta-amyloid
protein is a factor in the development of
amyloid plaques, which are commonly
seen in brains affected by AD. However, it
is unclear if beta-amyloid alone is enough
to trigger AD. Inflammation and free
radical damage also appear to play a role.
One hallmark of AD is the presence of
neuronal inclusions called neurofibrillary
tangles that contain tau, a protein that can
cause cell damage. Another factor in AD
is the reduction in acetylcholine, which
is observed in patients with AD and may
be related to some of the conditions mentioned previously (Zabar, 2005).
Risk factors associated with AD include age, family history, hypertension,
stroke, diabetes, and head injury (Zabar,
2005). In addition, patients with Down syndrome also commonly develop AD, with
the onset of the degenerative processes
frequently occurring in these individuals
when they are in their 40s (Zabar, 2005).
Site of Lesion
In MS, there is not a relationship between
the disease and any particular nerve tract.
Because MS is a disease affecting myelin,
it will often be found where there is an
abundance of white matter, such as in the
corpus callosum, the medial longitudinal
fasciculus, and a periventricular region
of the brain referred to as the trigone
(Muriello, Jones, & Chaves, 2005; Rubens,
Froehling, Slater, & Anderson, 1985).
In AD, the frontal, temporal, and
parietal lobes often are the first structures
to become involved. However, with time,
the entire cerebrum eventually undergoes
degenerative processes. When this cortical
degeneration occurs, the gyri become thin,
the sulci become large, and the subcortical
areas also degenerate (Zabar, 2005).
Medical Diagnosis
In MS, a neurologic history will check for
intermittent and relapsing symptoms,
such as ocular, vestibular and/or auditory problems, “electric shock” sensations,
numbness, tingling, weakness, motor
problems, and fatigue. Several tests are
used to make the diagnosis including MRI
(to visualize plaques), lumbar puncture,
and evoked potentials. Usually, evidence
of lesions in at least two areas of the central nervous system and a minimum of
two separate neurologic episodes must
have occurred for the diagnosis to be
made (Muriello et al., 2005).
There are no definitive tests for AD.
However, careful evaluation of mental status is critical. Trends of cognitive
impairment and decline are important to
document. Neuropsychologic studies in
regard to cognitive (reasoning) and memory functions are central in the evaluation
of AD. In advanced AD, CT or MRI can
show cortical atrophy. In addition, other
studies such as positron emission tomography (PET), cerebrospinal fluid analysis
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for beta-amyloid and tau, single photon
emission computed tomography (SPECT),
functional MRI (fMRI), electroencephalography (EEG), and P300 evoked potentials occasionally are used for diagnosis
(Musiek et al., 1994; Zabar, 2005).
Medical Management
In the treatment of MS, pharmaceutical management is the main course. The
mainstay of MS therapy is treatment with
injectable interferon-beta or glatiramer
acetate, which has been shown to be
effective in reducing exacerbations. Several new drugs that modulate the immune
system are becoming available. In severe
cases, azathioprine, cyclosporine, cyclophosphamide, and methotrexate have
been recommended (Muriello et al., 2005).
Alzheimer’s disease is managed by
initiating good all-around health habits
such as exercise, good nutrition, and social
activities. Current medications such as
Aricept, Exelon, and Razadyne seem to
focus on arresting the breakdown of acetylcholine. These medications are not a cure
for AD, but there are some indications of
improved mental function for patients
taking these drugs (WebMD, 2005–2019).
Audiology
A wide variety of degenerative disorders
can affect the CANS. However, in this section of this chapter, we focus on MS and
AD. It is important to realize that hearing difficulties occur much more often
in patients with MS than was originally
believed (Jerger, Oliver, Chmiel, & Rivera,
1986). Musiek, Gollegly, Kibbe, and Reeves
(1989) reported that approximately 40% of
the patients with MS in their study had
some type of hearing complaint, but only
18% had abnormal pure-tone audiograms.
In another study, Luxon (1980) showed
abnormal audiograms in over 50% of the
MS patients she studied.
Although not a common occurrence,
sometimes hearing difficulties can be the
first sign of MS. Usually, the hearing problems are subtle and become more obvious
in noisy listening situations (Luxon, 1980;
Musiek et al., 1989). Tinnitus and vestibular symptoms can also be experienced by
patients with MS.
Alzheimer’s disease has come to
the forefront of audiology in the past 10
years or so due to the interesting findings that have been reported by George
Gates. Gates and his colleagues have
demonstrated that central auditory test
results are often abnormal in patients
with AD compared to matched controls
(Gates, Anderson, Feeney, McCurry, &
Larson, 2008). In addition, it appears that
poor performance on central auditory
tests may precede the onset of AD, or at
least the diagnosis of AD. This in turn has
raised the question that perhaps central
auditory testing may have some predictive value in regard to AD. Based on their
findings, Gates and colleagues (2008)
have argued strongly for including central
auditory tests in the hearing evaluation of
the elderly.
Behavioral Test
Procedures in MS
Dichotic Listening Tests. There is a
generous amount of literature pertaining to MS and dichotic listening. It is well
known that dichotic listening relies on
normal function of the corpus callosum,
a structure that is highly myelinated. As
myelin is the target of MS, this is a likely
locus for the manifestation of the disease.
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Both recent as well as early reports support the claim that the corpus callosum is
a common lesion site in MS, as do studies that document the classic finding
of left ear deficits on dichotic listening
in patients with MS (Gadea et al., 2009;
Rubens et al., 1985). The left ear deficit can
be variable and depends on the amount
and locus of the interhemispheric neural
disruption caused by the MS plaques.
Dichotic listening to speech stimuli is one
of the best procedures for detecting auditory dysfunction in MS (Gadea et al., 2009;
Musiek et al., 1989).
Temporal Processing Tests. Temporal
processing tests have not been utilized
as much as dichotic speech tests in MS.
At best, auditory pattern perception and
gap detection procedures have revealed
only moderate sensitivity for detecting dysfunction related to MS (Hendler,
Squires, & Emmerich, 1990; Musiek et al.,
1989; Musiek & Weihing, 2011). However,
recent research has shown these temporal
processing tests to be of significant value
in defining auditory dysfunction in MS
patients (Valadbeigi et al., 2014). Given
these findings, it appears more testing of
temporal processing procedures needs to
be completed before a final determination
regarding the value of these tests in the
assessment of central auditory function in
MS patients can be made.
Monaural Low Redundancy Speech
Tests. Low redundancy speech tests
such as filtered speech have not shown
particularly notable hit rates for identifying auditory involvement in patients with
MS (Musiek et al., 1989). However, in a
study evaluating a mixed population of
patients with MS and brainstem tumors,
Karlsson and Rosenhall (1995) demonstrated that both filtered and compressed
speech tests separated the involved group
from a group of normal controls. They also
noted a greater deficit for the ear ipsilateral to the lesion site than for the contralateral ear in the patients with brainstem
involvement. In another study, Jerger and
his colleagues (1986) found that the synthetic sentence identification with ipsilateral competing message (SSI-ICM) was
moderately sensitive to auditory involvement related to MS.
Binaural Interaction Tests. Two bi-
naural interaction tests will be mentioned
here: the MLD test and an interaural timing task. The MLD procedure has a long
history of use in patients with MS and
has been shown to be reasonably sensitive to central auditory involvement (see
Hendler et al., 1990; Jerger et al., 1986;
and Musiek et al., 1989). Although both
speech and tonal stimuli can be used, lowfrequency tonal stimuli (usually 500 Hz)
have been more commonly utilized with
patients with MS. Interaural timing tasks
also have been shown to be highly sensitive to auditory involvement from MS. In
fact, this procedure has been shown to be
even more sensitive than the ABR in MS
(Levine et al., 1994). Unfortunately, this
procedure has not made its way into common clinical use.
Electrophysiologic Test
Procedures in MS
Auditory Brainstem Response. Per-
haps the most utilized audiologic test in
the MS population is the ABR. Hit rates
vary widely because without detailed
radiology, one cannot be sure if the auditory tracts are involved. When the brainstem auditory tracts are involved, the
ABR is abnormal an extremely high percentage of the time (Levine et al., 1994).
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