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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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behavioral test results (A)
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and P1, N1, and P2 cortical
auditory evoked potentials
(smoothed and retraced)
(B) for a young adult with
Figure 7–8. Central
temporal lobe epilepsy
7–8). continues
(Case
A
362
7. Disorders of the Central Auditory Nervous System 363
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B
Figure 7–8. continued Note: C3 and C4 = electrode
sites.
the C3 electrode site (this electrode site
was over the healthy or unaffected hemisphere), and the poorest recordings were
from the C4 site over the involved hemisphere (see Figure 7–8B). As can be seen,
all of these recordings are noisy with poor
replicability.
Medical Management
At last contact with the patient, he had not
undergone surgery as he and his parents
were still considering various options for
the control of the seizure disorder.
Case 7–9: Temporal
Lobe Epilepsy
History
This 40-year-old female presented with
increased difficulty understanding speech
in background noise for several years
prior to her evaluation. She admitted to
difficulty understanding speech even in
one-on-one conversations, following multistep directions, and needing extra time
to process conversations. She worked
as a medical technician and expressed
364 Disorders of the Auditory System
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increased difficulty at work. She was diagnosed with epilepsy at the age of 13.
Audiology
The patient underwent an audiologic evaluation. Pure-tone air- and bone-conduction
audiometry revealed normal peripheral
hearing sensitivity through 4000 Hz in the
left ear sloping to a mild to moderate sensorineural hearing loss from 6000 to 8000
Hz. Test results for the right ear indicated
essentially normal peripheral hearing sensitivity through 6000 Hz sloping to a very
mild sensorineural hearing loss at 8000
Hz. Word recognition scores were 100%
in the right ear and 84% in the left ear at
40 dB SL re: SRT. Due to the asymmetry
noted in the pure-tone thresholds and the
speech recognition scores, the patient was
referred to neurotology for evaluation to
rule out retrocochlear involvement.
Medical Evaluation
The patient was evaluated by neurotology due to the asymmetric hearing loss
noted during her audiologic evaluation.
Imaging was negative for retrocochlear
involvement.
Impression
Auditory processing deficit secondary to
temporal lobe epilepsy.
Audiologic Recommendations
and Management
Due to the auditory complaints, an auditory processing evaluation was recommended. Results from the auditory processing evaluation are seen in Figure 7–9.
Abnormal performance was noted on all
tests administered including the following tests: dichotic digits (moderate left
ear deficit and a very mild right ear deficit), filtered words (bilateral deficits), and
duration patterns assessed in the sound
field (severe deficit). Results of the Speech
in Noise–Revised (SPIN-R) Test (administered in the sound field) demonstrated
a large discrepancy between high- versus
low-probability sentences.
As a result of the deficits on the auditory processing battery, the patient underwent DIID training, which included 12
sessions during a 6-week period. Results
demonstrated a significant improvement
in binaural integration (see dichotic digits retest scores) as well as speech-innoise (SPIN-R scores) test performance
(see Figure 7–9). The patient noted overall improvement in her ability to hear,
particularly in challenging listening
environments.
Medical Recommendations
and Management
Continued monitoring and management
by audiology.
suRgical
inteRventions that
alteR centRal
auditoRy function
Temporal lobe epilepsy leads into another
topic of interest. As mentioned earlier,
epilepsy sometimes is treated by temporal lobectomy (removal of the anterior
portion of the temporal lobe) or commissurotomy (sectioning the corpus callosum). These procedures can have rather
marked consequences for central auditory
function. Considerable data have been
generated on central auditory deficits
associated with temporal lobectomy and
posttherapy (retest) central
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behavioral test results for
Figure 7–9. Pre- and
an adult with temporal lobe
epilepsy (Case 7–9).
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366 Disorders of the Auditory System
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split-brain surgery. The closer the surgical
sectioning to the primary auditory areas,
the greater the deficit(s) and vice versa,
and this needs to be kept in mind when
reviewing these types of studies (see
Oxbury & Oxbury, 1969).
Temporal Lobectomy
Behavioral Tests
Dichotic Listening Tests. In an inves-
tigation of central auditory function in
individuals with temporal lobectomy,
dichotic listening results showed deficits (often severe) in the ear opposite the
side of the lobectomy (Oxbury & Oxbury,
1969). In some situations, excision of part
of the temporal lobe does not create large
deficits. In one study utilizing the SSW
and dichotic CVs, there were no significant differences between pre- and postoperative results for the ipsilateral ear, but
there were for the contralateral ear and
for the difference between ears (Collard et
al., 1986). There is some evidence that left
temporal lobe excisions result in greater
deficits than those in the right hemisphere
(Bougeard & Fischer, 2002). These types
of comparisons, however, suffer from the
high probability that the lesions studied
were not equivalent and, therefore, the
results must be interpreted with caution.
Temporal Processing Tests. There is
a paucity of information on temporal
processing functions and temporal lobectomy. There was, however, a key study
reported on anterior temporal lobectomy
(ATL) and gap detection. In this study,
gap intervals in noise were significantly
longer for individuals with ATL than for
the control group. In addition, it appeared
that the greater deficit was contralateral
to the lesioned hemisphere (Efron et al.,
1985). This study raises the issue that these
patients may have temporal resolution
problems and that the anterior temporal
lobe possibly may play a role in auditory
processing. Another study revealed problems in musical timbre perception in general, and more specifically, on a subtest of
multidimensional scaling among patients
with temporal lobectomies compared
to controls. This procedure is complex
and a full description of the procedure is
beyond the scope of what can be reported
here. However, it is interesting to note that
depending on which particular subtest
was given, the right hemisphere (when
compromised) seemed to have considerable influence on test performance (Samson, Zatorre, & Ramsey, 2002).
Electrophysiologic Tests
Like temporal processing, there is limited information on the results of evoked
potential testing in individuals who have
undergone temporal lobectomies. The
MLR Na wave has been shown to be
delayed across Cz, C5, and C6 electrode
sites in patients with ATL compared to
controls. Also, interestingly, the NaPa
amplitude was found to be greater after
surgery (at the same electrode positions)
than it was prior to surgery (Jacobson,
Privitera, Neils, Grayson, & Yeh, 1990). In
a case study, P300 amplitude was found
to be decreased after temporal lobectomy
but returned to near preoperative values
2 weeks after surgery (Hirayasu, Ohta,
Fukao, Ogura, & Mukawa, 1995). In
another case study, N1, P2, and P3 (P300)
evoked potentials actually increased in
amplitude and decreased in latency after
surgery (Musiek et al., 1990). In this case,
it was hypothesized that reducing the
seizure disturbance allowed the evoked
7. Disorders of the Central Auditory Nervous System 367
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potential responses to become more synchronous, allowing more of the generators to respond appropriately.
Commissurotomy
The next major surgical intervention, also
for intractable seizures, is commissurotomy or split-brain surgery. This surgical procedure is done to prevent epilepsy
from spreading from one hemisphere to
the other. Much has been written about
these cases and their deficits, but only a
thumbnail sketch is offered here. Of note
is that only certain audiologic tests will
show deficits in these kinds of patients.
The most heralded is dichotic listening.
The other, although not as profound, is
certain types of pattern perception. As
one would expect, other diagnostic tests,
which do not engage (at least to a high
degree) the corpus callosum do not show
deficits and can be utilized to rule in or
out hemispheric or subcortical involvement (see Baran & Musiek, 1999).
Behavioral Tests
Dichotic Listening Tests. As is now
well known, commissurotomy results in
severe left ear deficits on dichotic listening tests in these patients. Although not
a consistent finding, in some patients, the
right ear performance actually improves
after surgery (Musiek et al., 1984). Auditory deficits can be demonstrated only if
the posterior half of the corpus callosum
is sectioned as that is where the auditory
fibers cross from one hemisphere to the
other (Baran, Musiek, & Reeves, 1986).
Severe dichotic left ear deficits and normal
performance on other central tests, such as
filtered speech and speech in noise, help
make the diagnosis of callosal involvement.
Temporal Processing Tests. The fre-
quency pattern perception test yields a
bilateral deficit (for verbal report) in splitbrain patients (Musiek et al., 1980). This
is because both hemispheres are needed
to decode and verbally report the patterns. Often, there can be asymmetry, but
both ears will yield performance below
the normal range. Although data for the
duration pattern test are not available, it
is likely that similar results would also be
noted for this temporal patterning test.
Monaural Low Redundancy Speech
Tests. Tests such as filtered speech, com-
pressed speech, and speech-in-noise are
typically not affected by sectioning of the
corpus callosum. These tests usually yield
normal or near-normal performance bilaterally. This is important to know because
these kinds of tests help in determining if left ear deficits are related to right
hemisphere involvement or corpus callosum compromise. That is, if the monaural speech test (as well as dichotic tests)
shows a left ear deficit, then it is likely
that the right hemisphere is involved.
However, if the monaural speech test is
normal and the dichotic test shows a left
ear deficit, then it is likely that the corpus
callosum is not transferring information
appropriately.
Electrophysiologic Tests
Few studies have been conducted in
patients with commissurotomy. Probably
the most cited is a study by Kutas, Hillyard, Volpe, and Gazzaniga (1990). This
study showed that for N2 and P3 (P300)
evoked potentials, split-brain patients
showed smaller potentials over the left
hemisphere compared to the right for the
P3 (P300). Also, when compared to the
control subjects, the evoked potentials
368 Disorders of the Auditory System
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were smaller for the patients who had
undergone commissurotomy. However,
as mentioned previously, these smaller
responses also could be related to damage to other auditory areas of the brain
in these subjects. In our own experience
using tonal stimuli, there has been essentially little or no change in N1, P2, or P3
(P300) potentials before compared to after
commissurotomy. Of course, the utilization of speech stimuli, especially in some
type of dichotic paradigm, may result in
much different results.
Case 7–10: Commissurotomy
History
This young adult underwent surgical
commissurotomy for intractable grand
mal seizures. This patient had previously
been under a neurologic protocol for seizure management as daily activities were
becoming difficult to carry out.
Audiology
Pure-tone thresholds were normal with
excellent speech recognition bilaterally
before and after surgery. Central auditory test results obtained prior to surgery
were within the normal range bilaterally. However, following surgery, there
was a marked left ear deficit for the
dichotic digits and the staggered spondaic word (SSW) tests with only mildly
depressed scores noted for the right ear
on both of these tests. In addition, the
patient’s scores for the frequency patterns
test were severely depressed bilaterally
(Figure 7–10). Speech-in-babble scores
remained normal bilaterally. These results
were consistent with expected postcommissurotomy audiologic results.
Medical Examination
and Management
The focus of the seizure activity was localized by EEG in both hemispheres. Computerized tomography scans were normal
except for slightly enlarged lateral ventricles. There was no history of any auditory
problems. The surgery went well, and
the patient was able to resume many normal everyday activities due to a marked
decrease in the number of seizures. Tasks
requiring interhemispheric interaction, of
course, were difficult for the patient.
centRal auditoRy
disoRdeR associated
with leaRning
difficulties
Introduction
One of the most popular uses of central
auditory tests is to identify individuals
(mostly children) with auditory processing disorder that may either precipitate or coexist with learning difficulties.
Although the basis for this kind of problem is not known, there are several good
theories as to what may cause these problems. Children with normal hearing and
intelligence but who complain of hearing difficulties (or whose family and/or
teachers raise concerns regarding potential hearing difficulties in their child or
student) and who also have difficulty with
learning (especially in the verbally related
subjects such as reading and spelling) are
candidates for a central auditory processing evaluation and a diagnosis of CAPD
if supported by test results. Other factors,
such as language proficiency and attention deficits, may be comorbid conditions
Figure 7–10. Central
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behavioral test results for a
young adult who underwent
a commissurotomy to control
intractable grand mal sei-
zures (Case 7–10).
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that often enter into this complex picture.
However, many cases of central auditory
processing disorder (CAPD) do not have
either language or attention issues. The
combining of behavioral and electrophysiologic tests of central auditory function is
a prudent approach to defining this problem. Evaluation of speech-language and
attention also is key in making an accurate
diagnosis.
Symptoms
As with other disorders of the CANS,
children with learning-related central
auditory disorder complain of difficulty
hearing in background noise, trouble
understanding people who talk fast, problems hearing in highly reverberant rooms,
problems following multistep directions,
difficulty in interpreting verbal messages,
and sometimes even having trouble localizing sound sources. Academically, these
individuals often have trouble reading
and spelling, but do fine in math and science. At times, some students appear to
be inattentive, whereas others seem to
attend closely in watching people speak
(see Chermak & Musiek, 1997).
Incidence and Prevalence
The incidence or prevalence of CAPD
associated with learning disability is unknown. There are some “educated guesses”
that place the prevalence around 2% to 3%
of the school-age population (Chermak
& Musiek, 1997). Of course, this is not
based on any prevalence study but does
seem to be a reasonable estimate based
on our experience in seeing large numbers of school-age children with this type
of problem and reviewing the prevalence
of related problems such as learning disabilities, attention deficits, and so on.
Etiology and Pathology
The etiology of CAPD related to learning difficulties is not known, but more
information is emerging that supports
our early theoretical categories of what
may cause or play a role in this type of
CAPD (Chermak & Musiek, 2011; Musiek, Gollegly, & Ross, 1985). One category
is neurologic. That is, a small percentage
have CAPD (and possibly learning difficulties) secondary to a frank neurologic
problem (see Chermak & Musiek, 2011).
Although any of a wide range of neurologic disorders, such as those discussed
earlier in this chapter, can play a role in
this kind of CAPD, absence seizure disorder may be the most common. Another
category is delayed maturation of myelin
within the auditory system. Myelination
of the higher auditory regions does not
mature until the teenage years (Yakovlev
& LeCours, 1967). This long maturational
course lends itself to great variability
over the years of growth and some children may “lag” behind in the maturation
of their CANS. This neuromaturational
lag means less myelin and slower transfer velocities and potential central deficits
that may manifest as CAPD. The third
category is that of neuromorphological abnormalities (see Galaburda, Sherman, Rosen, Aboitz, & Geschwind, 1985).
Polymicrogyria, heterotopias, and ectopic areas of the brain have been shown
to exist primarily in the left hemisphere
and in the auditory regions in individuals with dyslexia. In addition, the planum
temporale, which is usually larger on the
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left than right side, often is symmetrical in
children with dyslexia/learning disabilities. These abnormalities are anatomic
abnormalities that could easily disrupt
appropriate functions, especially auditory functions. In fact, a recent report has
shown this to be the case in children with
polymicrogyria who do poorly on central
auditory tests (Boscariol et al., 2009).
Site of Lesion
Depending on what may be the underlying condition that results in the CAPD
related to learning disability, the site of
lesion will vary. However, it seems that
most individuals with this problem have
auditory cortex or corpus callosum dysfunction (normal ABRs are common in
this particular population). If heterotopias and polymicrogyria are players,
then obviously the auditory cortex will
be involved. Myelination of the corpus
callosum is the last to mature within the
auditory system; hence, this could be a
site for “delayed” maturation. A study
by Skoe and Kraus (2010) indicates that
abnormalities on speech ABRs possibly
could implicate the brainstem as a site
for dysfunction in these kinds of cases,
but further work must be done to solidify
this possibility. Another point of consideration in this population as well as other
populations discussed in this chapter is
that of neural connectivity. Heterotopias,
polymicrogyria, and myelin maturational
delay could all contribute to poor neural
connectivity within the CANS. This could
decrease function of the neural substrate
as neurons and pathways cannot communicate optimally, which is critical for
both high-level learning and auditory
processing.
Medical Diagnosis
Central auditory processing disorder
related to learning disability is primarily
an audiologic diagnosis. Only when there
is an underlying medical condition is
there medical input to the diagnosis. The
same can be said for treatment.
Audiology
In the assessment of learning difficulties
linked with CAPD, tests with reasonable
sensitivity and specificity (based on cases
of confirmed lesions of the central auditory system) should be considered first.
Consistent with this thinking, behavioral
tests of dichotic listening, temporal processing, and degraded monaural speech
tests are useful. Also, binaural interaction
procedures such as masking level differences can be of value. In addition, auditory
evoked potentials like the combination of
ABR-MLR (or others) are suggested. It is
important to attempt to evaluate as many
processes as possible without making the
test battery too long and tiring.
Audiologic Management
Treatment can include several approaches.
One is auditory training (AT), which capitalizes on auditory plasticity. By training
on tasks that are related to the deficit(s),
the deficit(s) can be ameliorated in many
cases (see Chermak & Musiek, 2014).
Assistive listening devices, which provide
improved signal-to-noise ratios in settings such as the classroom, can also help
with the problems. These approaches, as
well as metacognitive strategies, can well
serve those with learning disabilities and
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