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352 Disorders of the Auditory System
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2001; Rowe & Carlson, 1980). Other studies have reported slightly poorer hit rates
(McClelland, Fenton, & Rutherford, 1994;
Schoenhuber, Gentilini, & Orlando, 1988).
The ABR diagnostic index that shows the
highest sensitivity is the I–V interwave
interval reflecting mostly central conduction time (Bergemalm & Borg, 2001).
Although high repetition rate ABRs have
not always reflected a diagnostic advantage for many clinical populations with
CANS compromise, in patients with
head injury, it appears that they could be
worthwhile (Soustiel, Hafner, Chistyakov,
Barzilai, & Feinsod, 1995). If the value of
the ABR for assessment of patients with
head injury is established, then maximum length sequences (MLS) techniques,
which permit high rates of stimulation
using the ABR, could be of value and
should be investigated.
Middle Latency Response. Generators
of the MLR are located in the thalamocortical auditory tracts of the CANS. This is
rostral to the ABR generators and, hence,
an important supplement to the ABR for
investigating the effects of head injury.
The MLR can be recorded simultaneously
with the ABR, which provides an efficient
diagnostic advantage in determining the
site of involvement. The MLR has not
been investigated as much as the ABR in
patients with head injury. However, significant differences for Na, Pa amplitude
and latency between controls and mild
head injury groups have been reported
(Drake, Weate, & Newell, 1996; Munjal,
Panda, & Pathak, 2010; Soustiel et al.,
1995). In addition, one study showed 12
out of 22 patients with head injury did not
have a Pa response (Ottaviani, Almadori,
Calderazzo, Frenguelli, & Paludetti, 1986).
Certainly, more research on the MLR and
head injury is indicated, especially when
it is combined with the ABR, as this could
yield a powerful diagnostic index.
Late Evoked Potentials (N1, P2, P300).
The N1 and P2 late potentials are generated by the primary auditory cortex and
regions close to it in humans. The P300
likely is generated by a number of areas
in the brain including the primary auditory areas. The N1, P2 track record for use
in head injury is somewhat mixed. Drake,
Weate, Andrews, and Castleberry (1995)
reported N1 to be significantly delayed
for patients with mild head injuries compared to controls. Jones et al. (2000) found
that almost 90% of post comatose patients
with head injury demonstrated abnormal
N1, P2 responses. Conversely, Harris and
Hall (1990) and Segalowitz, Bernstein,
and Lawson (2001) related that the N1
and P2 late responses from patients with
head injury did not differ significantly
from those of their control group. Other
studies have also shown this variance
in findings for TBI. The findings for the
P300; however, do not appear to have this
variability.
The P300 has been shown to be
highly sensitive to mild head injury/concussion in a number of studies (see Segalowitz et al., 2001). The P300, perhaps, has
been most frequently used in the evaluation of sport athletes with concussions. In
this regard, the P300 has been shown to discriminate athletes with concussion from
controls by either amplitude and/or
latency indices (De Beaumont, Brisson,
Lassonde, & Jolicoeur, 2007; Gaetz & Weinberg, 2000; Gosselin, Thériault, Leclerc,
Montplaisir, & Lassonde, 2006; Thériault,
De Beaumont, Gosselin, Filipinni, & Lassonde, 2009; see also Waryasz, 2017, for
review). In addition, the P300 may be the
most useful evoked potential procedure
for defining head injury and can be ap-
7. Disorders of the Central Auditory Nervous System 353
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plied across modalities, which can be
helpful in identifying the site of lesion
(Lew, Lee, Pan, & Date, 2004).
Case 7–6: Head Injury
History
This teenager was referred by his pediatrician for an audiologic evaluation. About
three weeks before he was seen for his
audiologic appointment he fell while
snowboarding, hitting the back of his
head and rendering him unconscious for
several minutes. He was taken to a local
hospital and released after medical evaluation. However, in a day or two, he began
noticing difficulty hearing. During his
audiologic evaluation, he reported that
music sounded strange and distorted
(described as being “robotic in nature”)
and that he was experiencing trouble
understanding speech. He claimed his
right ear was poorer than his left ear, but
that neither ear was “normal.” An MRI
performed at the hospital at the time of
his evaluation was normal.
forms at both low and high repetition
rates, but the response at 60 dB nHL was
abnormal with a questionable response
being observed. In the right ear, the
early waves were absent with a severely
delayed wave V noted at a low-repetition
rate. At a high-repetition rate, there was
essentially no response for the right ear.
Distortion product otoacoustic emissions
were essentially normal bilaterally (Figure
7–6C). Although many of the tests administered were normal, the ABR was consistent with the patient’s symptoms and
revealed poor auditory function. This case
portrays the value of using both behavioral and electrophysiologic approaches
to diagnostic testing in patients at risk for
CANS disorders.
Impression
This patient likely had central auditory
involvement secondary to head trauma.
He steadily improved following his audiologic evaluation, and in about a month, all
symptoms had disappeared and an ABR
test administered at that time showed
essentially normal waveforms bilaterally.
Audiology
A pure-tone audiogram showed normal
hearing thresholds at the octave frequencies from 250 to 8000 Hz bilaterally, and
speech recognition scores were 100% and
96% for left and right ears, respectively.
Tympanograms were of normal pressure,
shape, and compliance bilaterally.
Central auditory testing included
dichotic digits, frequency patterns, compressed speech, and dichotic rhymes, and
test results were all within normal limits
bilaterally (Figure 7–6A). The ABR, however, was clearly abnormal (Figure 7–6B).
The left ear showed normal ABR wave-
Case 7–7: Head Injury
History
This high-academic-performing, high
school senior presented with a significant
audiologic and otologic history. She had
chronic otitis media throughout childhood and her teenage years. Intervention
included multiple sets of PE tubes. She
underwent a tympanoplasty in her early
teens due to a right-sided tympanic membrane perforation and mild conductive
hearing loss. Surgical intervention was
successful and hearing returned to normal.
Figure 7–6. Central
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behavioral test results (A),
ABR tracings (B), and
continues
DPOAEs (C) for a teenager
who sustained a head injury
(Case 7–6).
A
354
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B
C
Figure 7–6. continued Key for (B): HR = high repetition rate of 79.3
clicks per second; top tracing shows latencies for waves I, III, V in msec;
4th tracing shows latency of wave V in msec. Key for (C): X = left ear, O =
right ear, n = averaged noise floor for both the right and left ears, dark line
= normative criteria used to differentiate normal versus abnormal DPOAE
amplitude measures.
Several years later, she returned due to
concerns regarding difficulty hearing,
particularly in noise. Remarkable history
included five sports-related head injuries,
one resulting in loss of consciousness. She
was seen by neurology as she reported
experiencing several migraines per week
along with chronic fatigue. Imaging was
found to be normal. Although she was
performing well academically, she felt
that she was having to work much harder
than she did prior to her head injuries.
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Audiology
A pure-tone audiogram showed normal
hearing thresholds at the octave frequencies from 250 to 8000 Hz bilaterally, and
speech recognition scores were 100% for
both ears. Tympanograms were of normal
pressure, shape, and compliance bilaterally. Based upon her significant medical
history, coupled with the normal findings
on the pure-tone, speech recognition, and
tympanometric tests administered (which
could not explain her reported hearing
difficulties), a central auditory processing assessment was recommended and
completed.
The auditory processing testing included the duration patterns, filtered
words, dichotic digits, competing sentences, and Listening in Spatialized NoiseSentences (LiSN-S) tests (Figure 7–7).
A right-ear deficit was noted on the filtered words, dichotic digits, and competing sentences measures. In addition,
a mild left-ear deficit was noted in addition to the right-ear deficit noted above on
the filtered words test. Results for the
LiSN-S test are not displayed in this figure, but were within normal limits across
all four conditions.
Medical Examination
An otologic examination was unremarkable.
Impression
This patient likely had central auditory
involvement secondary to head injury.
There also was a possible peripheral
contribution to the central findings due
to auditory deprivation effects secondary
to this patient’s history of chronic middle ear involvement years prior. While
there was no peripheral hearing loss
at the time of the central auditory processing evaluation, one cannot rule out
chronic middle ear involvement (with
conductive hearing loss) as a contributory
factor.
Audiologic Management
As her medical evaluation was unremarkable, it was recommended that the patient
undergo intense aural rehabilitation. She
and her family were in agreement with this
recommendation and were eager to proceed. Given the deficits documented on
the central test battery, the patient underwent intense aural rehabilitation in the
form of Dichotic Interaural Intensity Difference (DIID) training (3 times per week
for 4 weeks). In addition, she obtained
mild gain binaural amplification. Following DIID training, reevaluation demonstrated significant improvements in auditory skills (see Figure 7–7), specifically
in the areas of binaural integration and
separation as reflected on dichotic digits
and competing sentences, respectively.
Subtle improvements were also noted for
filtered words. Greater improvement in
this particular area (i.e., the processing of
degraded speech) was not anticipated as
this was not a skill specifically addressed
in therapy. The patient reported significant auditory improvements following
treatment. Specifically, she felt that she
did not need to exert the same degree of
listening effort as she did prior to therapy,
and she reported making fewer auditory
processing errors. In addition, she was no
longer experiencing migraines and had
improvement in terms of her fatigue. With
respect to the hearing aids, she appreciated a notable difference in her listening
abilities with them. The most significant
improvements were reportedly noted in
Figure 7–7. Pre- and
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posttherapy (retest) central
behavioral test results for
a teenager who sustained a
head injury and concussion
(Case 7–7).
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difficult listening environments and large
classroom situations.
Medical Recommendations
Continued use of hearing aids and monitoring of audiologic status.
temPoRal loBe
ePilePsy
Introduction
Temporal lobe epilepsy (TLE) is a specific type of epilepsy. The reason TLE is
addressed here is that it is a seizure of the
temporal lobe that is located in an anatomic region of the brain, which is likely
to lead to dysfunction of the central auditory system. Of the various seizure disorders, one that is especially relevant here
is Landau-Kleffner syndrome as it often
manifests auditory symptoms.
Epilepsy, in general, is considered a
brain disorder. This disease is characterized by the abnormal firing of clusters
of nerve cells, which results in unusual
sensations, perceptions, behaviors, and
motor activity. Temporal lobe epilepsy
can broadly be divided into two major
anatomic areas: (1) the mesial temporal
lobe, which includes the hippocampus
and amygdala; and (2) the lateral temporal
lobe, which is the surface of the temporal
lobe. A seizure focus in either of these sites
may affect the auditory areas as the abnormal firing of cells often spreads to adjacent
areas, resulting in auditory symptoms.
Symptoms
Three classifications of epilepsy are defined by the Commission on Classifica-
tion and Terminology of the International
League Against Epilepsy (1981). One classification is simple partial seizures, which
are defined by involvement of small areas
of the brain with seizures that do not result
in the loss of consciousness. This type is
associated with sensations that are dependent on the area affected (i.e., sounds,
images, unusual tastes and smells, etc.,
or a focal motor activity). A second category is complex partial seizures, which
impair consciousness to some extent.
The area involved here is larger than in
the previous classification. This category
often is characterized by motionless staring, automatic movements of both the
hands and mouth, and unusual speech
utterances. The third category is generalized tonic clonic seizures, which involve
the loss of consciousness, the stiffening of
the trunk, and subsequent jerking movements involving much of the body. This
type of seizure has a large spread of activity to adjacent and even remote regions of
the brain.
If the seizure activity affects auditory
areas, often auras (sensations preceding
the seizure) of sounds occur. Seizures do
damage to the underlying neural substrate; hence, if auditory tissue is damaged, then auditory symptoms such as
trouble hearing in noise can evolve.
Of special interest is Landau-Kleffner
syndrome (also referred to as acquired
epileptiform aphasia or acquired aphasia
with convulsive disorder). This syndrome
is a rare disorder that occurs when the seizure focus is at or near the auditory cortex (Pearl, Carrazana, & Holmes, 2001).
It is characterized by an arrest of language development and a deterioration
of speech. Toddlers with Landau-Kleffner
syndrome usually have inconsistent responses to sound and some are even
thought to be “deaf.” It is estimated that
70% of children with this syndrome have
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seizure activity of some degree (National
Organization of Rare Disorders, 2018).
The disorder usually manifests between
the ages of 3 and 7 and is twice as likely to
affect males than females (National Organization for Rare Disorders, 2018).
Incidence and Prevalence
In 2015 it was estimated that 1.2% of the
population (3 millions adults and 470,000
children) in the United States have epilepsy (Zack & Kobau, 2017). About 50%
have partial epilepsy and most of these
have TLE; however, the true prevalence (or
incidence) is not known. One extremely
rare epileptic disorder is Landau-Kleffner
syndrome. In a recent study, it was
reported that there were 350 children with
the syndrome documented worldwide
(Tuft, Arva, Bjornvold, Wilson, & Nakken,
2015). In addition, it has been reported that
about 10 new cases of Landau-Kleffner
syndrome are documented each year in
the United States; however, many more
cases of this disorder are believed to exist,
but just simply are not documented (Berg,
Shinnar, Levy, & Testa, 1999; Bronen, 2000;
Perkins, 2002).
Etiology and Pathology
Epilepsy can have many causes, including genetic mutations in ion channels,
focal brain injuries (due to prior insults
such as stroke or infection), and metabolic disease, although the cause is not
known in many cases. As stated earlier,
epilepsy results in the abnormal firing of
nerve cells. Heredity also may play a role
in regard to susceptibility to the disease.
Other conditions associated with this disorder are meningitis, encephalitis (such
as herpes encephalitis), vascular malfor-
mations, head injury, and mass lesions. It
is fair to say that the greater the involvement of a given lesion, the greater the
possibility of seizure disorder (Berg et al.,
1999; Bronen, 2000; Perkins, 2002). Sclerosis (scarring) of the medial temporal lobe
often is seen in TLE, but it is unclear if this
is a cause or an effect of the epilepsy.
Medical Diagnosis
Temporal lobe epilepsy or epilepsy in
general is medically diagnosed by using
some key procedures. Besides an in-depth
neurologic and family history, EEGs and
EEG monitoring over a 24- to 48-hr period
will determine if there are aberrant neural
discharges from the brain and where the
main epileptic focus is located if aberrant
discharges are detected. The EEG also can
provide insight as to the spread of epileptic activity. This is especially important
if the seizure activity spreads across the
corpus callosum to the opposite hemisphere. Radiologic procedures such as
CT, PET, MRI, fMRI, and SPECT are commonly used to document other possible
factors that may have contributed to the
epilepsy. Other factors could include
temporal lobe sclerosis, trauma, vascular
problems, and other temporal lobe conditions that could be triggering the seizure
disorder. A variety of blood tests can be
done to check for heavy metal poisoning,
anemia, and diabetes, which also can be
linked to epilepsy. In addition, developmental and genetic testing for children,
as well as a number of neurologic and
behavioral tests can be used to assist in
the diagnosis of epilepsy (Perkins, 2002).
Finally, individuals with TLE should
undergo central auditory testing to determine if the auditory system has been
compromised (Kwan & Brodie, 2000; Perkins, 2002).
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Medical Management
Medical management of TLE falls into
two main categories: medical and surgical. Medications are commonly used in
an attempt to decrease the excitation level
of the neurons in the brain by blocking
sodium or calcium channels. Examples of
some of these drugs include phenytoin,
carbamazepine, valpoate, phenobarbital, and some newer drugs such as gabapentin, lamotrigine, and topiramate. The
newer drugs have a better side effect profile; hence, they have gained in popularity
(Perkins, 2002).
Surgery usually is reserved for cases
where medications fail. The essence of the
surgical procedures is to excise the area of
the brain that is responsible for the abnormal EEG activity. Temporal lobectomies
are performed for TLE, and of course,
there is a possibility of central auditory
compromise in these patients that should
be checked. If the TLE has or could spread
to the other hemisphere, commissurotomy
(split-brain surgery) can be considered.
This may prevent the spread of epilepsy,
but can have consequences for proper
interhemispheric interaction, including
the interaction between the important
areas of the auditory system (Musiek et al.,
1994; Perkins, 2002).
Audiology
In terms of anatomic locus, the temporal lobe is a common site for the focus of
epilepsy. Because it also is the key lobe
involved in auditory processing, there is
a probable relationship between central
hearing deficits and epilepsy. Seizure
disorders can damage underlying neural tissue, resulting in degeneration and
even sclerosis (see earlier comments). If
the seizure focus is in auditory regions,
it is likely that central auditory dysfunction will result. An even bigger issue is
that sometimes epilepsy requires surgery
of the underlying tissue, such as temporal lobectomy and/or commissurotomy.
When surgery is to be performed, it is
critical that the CANS be tested before
and after this surgery, especially when
significant auditory neural tissue is to
be removed.
Behavioral Test Procedures
Dichotic Listening Tests. Dichotic
speech tests generally demonstrate significant deficits for patients with TLE
when compared to controls (Gramstad,
Engelsen, & Hugdahl, 2006; Roberts,
Varney, Paulsen, & Richardson, 1990.
Another trend is that, when the epilepsy
is confined to one hemisphere, the contralateral ear is most often affected (Gramstad et al., 2006). Various kinds of dichotic
tests including dichotic digits, staggered
spondaic words (SSW), and dichotic CVs
have been administered to patients with
epilepsy, with the results documenting
poorer scores in the experimental group
than in controls and left ear scores that
are typically poorer than right ear scores
(Collard, 1984). Dichotic listening has
been shown to reflect improvement when
patients are given anticonvulsant therapies or when the seizure activity was
reduced (Musiek, Bromley, Roberts, &
Lamb, 1990; Roberts et al., 1990).
Temporal Processing Tests. Brief tone
audiometry using Békésy tracking procedures did not show any significant shift
in thresholds between patients with seizures and controls in a study conducted
by Collard (1984). However, a difference
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in the excursions was noted for brief tones
(20
ms) compared to long duration tones
(500 ms) between the two groups in this
study. In addition, more recent studies
revealed that duration patterns test results
were reduced bilaterally for patients with
seizure disorders compared to controls
(Amaral et al., 2015; Meneguello, Leonhardt, & Pereira, 2006). Other recently
reported studies using the gaps-in-noise
(GIN) procedure have also shown poorer
performance for patients with epilepsy
than for control subjects (Aravindkumar
et al., 2012; Rabelo, Weihing, & Schochat,
2015).
Electrophysiologic Tests. Some impor-
tant evoked potential studies have been
performed on patients with TLE. Soysal
and colleagues (2002) demonstrated that
P2, N2, and P3 (P300) waves were all
delayed in latency for a large group of
patients diagnosed with epilepsy compared to a control group. Interestingly,
amplitudes were also lower for the epileptic group, but the differences did not reach
statistical significance. Verma, Twitty, and
Fuerst (1993) were interested in using
evoked potentials for lateralizing the seizure focus. They concluded that N1 and
P2 were better than the P3 (P300) for lateralizing information, but even these potentials were not consistently accurate for
lateralization of the epileptic focus. Other
studies have not shown the N1 and P2 or
P3 (P300) to be sensitive to seizure lesions
(Boutras et al., 2006; Chayasirisobhon et
al., 2007). Interestingly, the P50 response
(an auditory evoked potential that occurs
around 50 msec) has been shown to exhibit
delayed latencies and smaller amplitudes
for seizure patients compared to controls
in one large study (Drake et al., 1995), but
not so in another investigation (Boutros
et al., 2006).
Case 7–8: Temporal
Lobe
History
This young adult had a long history of
seizure disorder with its locus in the right
temporal lobe near Heschl’s gyrus. There
were minimal hearing complaints. The
patient was being considered for possible
surgery to control seizure activity and
therefore was being seen for a preoperative audiologic evaluation.
Audiology
Pure-tone thresholds were within the
normal range for the octave frequencies
between 250 and 8000 Hz and speech recognition scores were excellent bilaterally.
Behavioral central auditory tests, including dichotic digits and dichotic rhyme
tests, revealed reduced scores in the ear
contralateral to the lesioned hemisphere
when compared to the scores noted in
the ear ipsilateral to the lesion site (Figure 7–8A). These notable ear differences
were observed for both tests even though
the dichotic digits scores for both ears
fell below the normal range and the
dichotic rhymes scores fell within the
normal range. In addition to these dichotic test findings, abnormal performance
was noted for both ears on the duration
patterns test, which would be consistent
with the right hemisphere site of lesion in
this case. (Recall that regardless of the ear
being tested, normal function of the auditory cortex in the right hemisphere is necessary for accurate processing of auditory
patterns.) The late auditory evoked potentials (N1 and P2; traced from the originals)
showed poor morphology except for the
right ear at C3 (Figure 7–8B). The next best
recordings were from the left ear also at
Epilepsy
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