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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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90 Disorders of the Auditory System
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2µV
P2
N1
Amplitude in µV
P2
N2
N1
100
200300 400500
Latency in msec
Figure 3–10. An example of a normal late auditory evoked response, includ-
ing the P3 (P300).
is an obvious difference in amplitudes
across the electrodes. Those employing the
late potentials clinically need to establish
their own norms. The following provides
some general guidelines for interpretation of LAER measures. The most sensitive index for the LAER in the opinion of
the present authors is an amplitude difference of 50% for the N1–P2 complex
between electrodes, similar to what was
discussed for the MLR. However, more
research needs to be done before general,
as opposed to clinic-specific, amplitude
norms can be routinely applied. Additional indices related to latency and amplitude measurements are as follows: (1) the
presence or absence of a response, (2) the
absolute latencies of the various responses
mentioned previously, and (3) the ampli-
Frequent
P3
Rare
N3
600
tude measures for the N2–P3 and P3–N3
in addition to the N1–P2 response. Abnormality can be based on the following criteria: (1) absent waves, (2) nonreplicable
waves, (3) N1 latency greater than 120
msec, (4) P2 latency greater than 228 msec,
and (5) P3/P300 latency greater than 350
msec for young adults with good hearing
sensitivity (Musiek et al., 1994; Musiek &
Lee, 1999; Picton, 2011). However, as noted
previously, more clinical investigation
is required before general norms can be
employed. Given the demyelination that
occurs in the CANS after the fourth decade
of life, it is recommended that approximately 10 msec be added to the P3/P300
latency for each additional decade of
life after the fourth decade (see Picton,
2011, for review). Again, these criteria are
3. Audiologic, Vestibular, and Radiologic Procedures 91
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intended as only a general guide and individual clinic norms should be obtained
for application of these evoked potentials.
Electrophysiologic tests of cochlear
and retrocochlear involvement provide
clinicians with both powerful and useful
diagnostic measures. The good sensitivity
and specificity of these tests certainly supports their clinical use. Table 3–4 provides
information regarding the sensitivity
and specificity data used by the authors
with respect to the use of auditory EPs
for patients with confirmed lesions of the
CANS (see Hall, 2007; Musiek, 1991; Musiek et al., 1999; Musiek & Lee, 1999).
The MLR and LAERs are not considered to be routine diagnostic procedures
but are underutilized in the opinion of the
authors. Although historically used primarily in the research arena, we believe
that these procedures, if used judiciously,
can have an important and critical role
in diagnostic audiology, as well as in the
(re)habilitation of individuals diagnosed
with neuroaudiologic disorders. To reiterate, the clinician needs to have in-depth
knowledge and experience to apply
these procedures accurately. If the reader
desires a more thorough review of auditory evoked potentials, they are referred
to Hall (2007), Musiek and Lee (1999), and
Picton (2011).
vestiBulaR
assessment
Although the focus of this book is on
auditory disorders, it is relevant to discuss tests of vestibular function and their
role in diagnostic audiology. Given the
relationship between auditory and vestibular structures and functions, there are
many disorders which affect both systems. Often, those individuals with auditory disorders have comorbid vestibular
involvement, making it necessary for the
audiologist to have knowledge of such
assessment tools. The following section
provides an overview of common vestibular measures used clinically (for a more
detailed review of vestibular assessment
the reader is encouraged to refer to Jacobson & Shepard, 2016, and McCaslin, 2013).
Electronystagmography and
Videonystagmography
By far the most widely utilized measures of
vestibular function are electronystagmography (ENG) and videonystagmography
(VNG). The primary difference between
these is the use of electrode (ENG) versus
video (VNG) recordings. These measures
Table 3 – 4. Sensitivity and Specificity Data Associated
with Auditory Evoked Potentials Based on the Authors’
Literature Review of Reports of Lesions of the CANS
Test Sensitivity Specificity
ABR High 80s to Low 90s 92%
MLR 50% to 85% 85%
N1–P2 70% 75%
P300 80% 68%
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assess the integrity of the vestibular system by evaluating both voluntary and
involuntary eye movements. Evaluation of
eye movements provides insight into the
function or dysfunction of both the peripheral and central vestibular systems. Specifically, ENG and VNG procedures provide
a direct measure of the vestibulo-ocular
reflex (VOR) and can provide information
regarding differential diagnosis for four
variables or domains: (1) normal versus
abnormal vestibular function, (2) peripheral versus central system involvement,
(3) right-sided versus left-sided involvement, and (4) compensated versus uncompensated function. Within the ENG and
VNG procedures, there are three primary
categories of tests, which include oculomotor, positional, and caloric measures.
Oculomotor tests include a subgroup
of tests that assess saccadic, smooth pursuit, and optokinetic tracking (see Leigh
& Zee, 2006). This subgroup of tests provides information regarding the contributions of the cerebellum and offers the
most insight into possible central involvement. However, the examiner should use
caution when administering these tests
because age, fatigue, and a variety of
medications can affect their results. The
first of these eye movements routinely
assessed are the saccades. Saccadic eye
movements are reflexive eye responses,
and testing involves examining the velocity, accuracy, and latency with which the
eyes can lock onto a target (Figure 3–11).
The smooth pursuit response, unlike
the saccades, is not a reflexive response,
Figure 3–11. An example of a normal saccadic eye movement tracing.
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but rather a measure of the ability of the
eyes to maintain gaze while tracking an
object of interest. This response, as seen
in Figure 3–12, is obtained by having the
patient fixate and track a target, which
moves smoothly between the left and
right visual fields, resulting in a sinusoidal tracing. The smooth pursuit response
is evaluated based on velocity, symmetry, and phase. Both slow and fast rates
of stimulation are typically employed in
this tracking test. The third test used in
the oculomotor battery of tests is optokinetic tracking. This evaluates the reflexive abilities of the vestibular system with
respect to moving stimuli in a visual
field. The patient is placed at a distance
so that stimuli fill 90% of the visual field.
The parameter for analysis is velocity
gain of the eye movement in both the left
and right directions for both slow and
fast rates of stimulation. An example of
a fast rate of optokinetic stimulation is
seen in Figure 3–13. Gaze testing is an
additional measure of ocular function
that can detect gaze-evoked nystagmus. It
allows for evaluation of the ocular range
and is completed by asking the patient
to look in the horizontal (left and right)
and vertical (up and down) directions.
Abnormalities on any of the previously
described oculomotor measures would
suggest possible central nervous system
involvement.
Figure 3–12. An example of a normal smooth pursuit tracking tracing. The target and the right
eye’s response have been labeled with arrows. Note that they show nearly perfect alignment consistent with a normal response.
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Figure 3–13. An example of a normal optokinetic tracing in both the left and right directions for
the right eye.
Positional and positioning tests constitute the second group of subtests
within the ENG/VNG evaluation. Positional tests examine the response of the
vestibular system when there are changes
in head and body positions (see Brandt,
1990). This is different than the positioning tests, which are intended to investigate the presence of benign paroxysmal
positional vertigo (BPPV). Both of the
measures, however, are intended to detect
nystagmus when nystagmus should not
be present. The positional tests typically
are performed by having the patient lay in
the supine, head left/right, and body left/
right positions. Initially, this is accomplished with vision denied. If nystagmus
is detected with vision denied, then a
vision-enabled condition is administered.
When positional tests induce nystagmus
in the vision-denied condition only, but
the nystagmus is suppressed with vision,
peripheral involvement is suspected.
However, if nystagmus is observed for
both conditions or is direction changing,
then possible central involvement should
be further investigated. As indicated earlier, positioning tests are primarily used
to investigate for possible BPPV (otolith debris in the posterior or horizontal
semicircular canals). This information
is obtained by performing several different maneuvers with the most widely
used being the Dix Hallpike maneuver, in
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which the examiner looks for nystagmus
when the posterior semicircular canal is
oriented vertically. This is achieved by
having the patient lie in a supine position with the neck extended and rotated
toward the affected ear.
Caloric testing is perhaps the hallmark vestibular test (Barber & Stockwell,
1980; Stockwell, 1997). This is the only
measure that can differentiate right-sided
versus left-sided involvement as it is the
only test among the vestibular tests discussed thus far that has the ability to evaluate only one vestibular system at a time.
The purpose of this test is to determine the
presence of a response (i.e., nystagmus),
and if a response is detected whether or
not there is symmetry in the eye movements with right and left ear stimulation.
Normal findings for the caloric test include
the presence of nystagmus and symmetric
vestibular system responses. The absence
of nystagmus and/or a finding of significant response asymmetry would constitute an abnormal test result. For this test,
patients are placed in a supine position
at a 30° angle, and each ear is stimulated
independently in both a warm and a cool
condition using either water or air that is
heated and/or cooled depending on the
test condition. This procedure offers a
direct measure of the function of the horizontal semicircular canals and is intended
to induce nystagmus that is evaluated for
the strength of the response and symmetry between the two ears. Two measures,
ear weakness and directional preponderance, can be extracted from the responses.
Unilateral weakness (UW) is assessed by
comparing the responses from the right
ear [right warm (RW) and right cool (RC)]
with those from the left ear [left warm
(LW) and left cool (LC)] (Figure 3–14). The
UW is calculated as follows: UW = [(RW +
RC) – (LW + LC) / RW + RC + LW + LC] ×
100. Directional preponderance (DP) can
also be derived from the caloric response
by comparing the amplitude of the rightbeating (RW and LC) to the left-beating
(RC and LW) nystagmus and is calculated
as follows: DP = [(RW + LC) – (LW + RC) /
RW + RC + LW + LC] × 100. If no response
is present or the response is severely
reduced when cool water is used, then ice
water stimulation is often attempted.
The battery of tests described previously are standard protocol for both ENG
and VNG assessments of vestibular (dys)
function. However, some variations of the
standard protocol are used in many clinics. Regardless of the fact that variations
in the protocols are often employed, these
tests are the most widely used tools for
the assessment of the vestibular system
in that they provide general information
regarding (ab)normality of the response,
peripheral versus central involvement,
and side of lesion.
Vestibular Evoked
Myogenic Potentials
Vestibular evoked myogenic potentials
(VEMPs) have been used to supplement
the vestibular evaluation. This test assesses
vestibular system function through the
use of evoked potentials (see Zapala &
Brey, 2004) and provides clinicians with
information that is not readily available
by means of ENG or VNG. It provides
additional information with respect to differential diagnosis of site of lesion. VEMP
testing can be conducted using either the
cervical technique, which involves placing
the active surface electrode on the sternocleidomastoid (SCM) muscle (referred to
as the cVEMP) or beneath the eye near
the inferior oblique muscle (referred to
as the oVEMP). The cVEMP evaluates
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Figure 3–14. An example of a normal bithermal caloric response.
the saccular and afferent inferior vestibular nerve functions. The normal cVEMP
response is characterized by two peaks
with a large positive peak (P1 or P13)
occurring around 13 msec, followed by
a large negative peak (N1 or N23) occurring around 23 msec (Figure 3–15). There
is still some debate regarding the generator sites of the oVEMP, but it is believed to
be generated by the utricle and superior
vestibular nerve. The oVEMP response is
characterized by a negative trough (N1)
occurring around 10 msec and a positive
peak (P1) occurring around 15 msec.
There are several VEMP measurement techniques, which have been reported. Typically, however, this response
is obtained with either a click or 500-Hz
tone-burst stimulus. A rarefaction stimulus is presented usually at around 100 dB
nHL and at a rate of around 5 stimuli per
sec. For the cVEMP, a noninverting electrode is commonly placed on the sternocleidomastoid muscle and the patient
must have sustained muscle contraction
during recording in order to observe a
waveform. Most clinicians will have the
patient sustain this contraction by having the individual lie in the supine position with the head turned in the opposite
direction of the test ear and slightly lifted
off the table. The electrode placement for
the oVEMP differs from that of the cVEMP
in that the noninverting surface electrode
is placed beneath the eye as opposed to on
the sternocleidomastoid muscle.
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Figure 3–15. Examples of normal (left panel) and abnormal (right panel) cervical vestibular
evoked myogenic responses obtained from the left (upper tracing) and right (lower tracing) sternocleidomastoid muscles. Note the differences in the amplitudes of the responses for the right versus
the left side and the abnormal amplitude asymmetry ratio displayed in the right panel of this figure.
Although there is variability among
patients with respect to the amplitude of
the response, the VEMP is typically very
large and robust in nature and is usually
on the order of 15 µV to 180 µV. The VEMP
response is unique in that the patients serve
as their own control with respect to determining the normality of the response. That
is to say, that the right-sided and left-sided
responses are compared to each other and
an asymmetry ratio is calculated, where the
asymmetry ratio equals 100(A
A
). This measure appears to be sensitive
R
– AR) / (AL +
L
to diseases that present with both vestibular and auditory involvement, including
Ménière’s disease, vestibular schwannoma,
superior canal dehiscence syndrome, and
multiple sclerosis. Amplitude ratios ≤ 40%
are considered to be within normal limits
(Akin & Murnane, 2008).
Rotational Chair
Rotational chair (RC) testing is another
tool sometimes used in vestibular assessment; however, the instrumentation is
quite expensive and is not available in
many clinics. Similar to the VNG, RC
also evaluates the VOR. There are three
primary measures obtained during RC
testing: (1) sinusoidal harmonic acceleration, (2) velocity step responses, and (3)
the VOR reflex and fixation. A significant
difference is that RC testing demonstrates
better sensitivity to vestibular abnormalities than caloric testing. However, with
that said, caloric testing has proven to be
more specific than RC testing (Arriaga,
Chen, & Cenci, 2005). Another significant
advantage of RC testing is its ability to
provide information regarding vestibular
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system compensation (i.e., the ability of
the central nervous system to reorganize
after dysfunction or destruction, leading
to eventual functional rehabilitation and/
or recovery) (Deveze, Bernard-Demanze,
Xavier, Lavieille, & Elziere, 2014). Finally,
it can also be used to assist in localizing
the site of the vestibular system deficit
to the left or right side. For a full review of
RC testing, the reader is referred to Jacobson and Shepard (2016).
Basics of comPuted
tomogRaPhy
and magnetic
Resonance imaging
Imaging has developed into a valuable tool
for diagnosing and characterizing congenital and pathologic conditions affecting the
auditory system. Computed tomography
(CT) and magnetic resonance imaging
(MRI) by far are the two most widely utilized imaging procedures in the evaluation
of the auditory system. Although there
are other imaging techniques available for
evaluating the anatomy of the auditory
system, the discussion in this chapter will
focus on these two imaging procedures as
they are the primary radiologic procedures
used to document structural abnormalities
within the peripheral and/or central auditory systems.
Computed Tomography
Computed tomography is an imaging
procedure that utilizes X-rays and their
absorption properties to generate twodimensional images of the body. Using
a loaf of bread as an example, a conven-
tional X-ray would allow inspection of
the bread as a whole, whereas a CT scan
would provide evaluation of the individual slices that make up the loaf of bread.
Unlike conventional X-rays, CT utilizes
a “beam” or “fan” of X-rays that pass
through the patient and then are detected
by a series of “detectors” located on the
other side of the patient. As the X-rays
pass through the patient’s body, they are
absorbed or attenuated to varying degrees
based on the different compositions of the
various body parts (e.g., bone, soft tissue,
air, etc.). To create the CT image, the X-ray
“beam” is rotated around the patient and
the varying amounts of X-ray attenuation are recorded by the detectors. This
process is then repeated multiple times as
the patient passes through the CT scanner (typically by automated movement
of the patient table), allowing for acquisition of several “slices” within the patient.
Through complex mathematical analysis,
the data collected by the detectors are
used to assign varying shades of gray to
different portions of the body and allow
for generation of a series of images. This,
of course, is a simplistic characterization
with most modern scanners able to assign
gray scale assignments to sample areas
(voxels) as small as 0.4 mm in thickness.
Compared with conventional X-rays,
CT provides very good contrast resolution and tissue discrimination, primarily
by reducing the interference caused by
overlapping structures. It is analogous to
having five vehicles lined up “bumper-to
bumper,” with the first vehicle being a
bus and the next four being compact cars.
Viewed from the front, the cars would be
obscured by the mass of the bus, but if one
were to walk in a circle (rotate) around the
vehicles, the five separate vehicles would
be readily apparent.
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Computed tomography scans employ
a black-white numerical scale measured
in Hounsfield units (HU). The name for
the CT values was chosen in honor of Sir
Godfrey Hounsfield, the “father” of CT,
who developed the first clinical scanner
in 1972 (see Wolbrast, 1993). Very dense
structures such as bone have very high
HUs (up to +1000), with less dense structures such as air having very low HUs (as
low as −1000). In addition to the natural
contrast provided by varying soft tissues
and their respective differences in X-ray
attenuation (densities), CT scans can often
be “enhanced” by the addition of intravascular or oral contrast agents. Oral contrast
is most commonly utilized for abdominal
and pelvic applications, but intravascular
contrast does have applications for imaging of the auditory system (Table 3–5).
Currently, nonionic, iodinated contrast
agents are the most frequently utilized
intravascular agents for CT applications.
These have the advantages of being readily available, relatively inexpensive (when
compared with gadolinium, see MRI sec-
tion), and have a reasonable safety profile with a low incidence of allergic reaction. Care, however, must be taken when
using these agents, as iodinated agents can
have a nephrotoxic effect, particularly in
patients with compromised renal function.
Dedicated imaging of the temporal
bone with CT should consist of a tailored
sequence combining a small field of view,
a high-resolution matrix (e.g., 512 × 512),
and a thin section acquisition. Current CT
scanners are capable of acquiring axial
image slices in the range of 0.4- to 1.0-mm
thickness, allowing for optimal multiplanar reconstructions (MPRs) in any imaging plane (Figure 3–16A). “True” coronal
images (Figure 3–16B) of the temporal
bone can be obtained by scanning the
patient in a prone position with the neck
extended, or in the supine position with
the neck extended (possibly requiring
hanging the patient’s head off the back of
the table). However, obtaining both axial
and “true” coronal CT images not only
requires patient cooperation for proper
positioning, but also essentially doubles
Table 3 – 5. Indications for Intravascular Contrast Administration
During Computed Tomography Imaging
Suspected Tumor
Intercranial Extension
or Involvement
Vascular Abnormalities/
Anomalies
Paragangliomas (glomus tumors)
Meningiomas
Schwannomas
Epidermoid and arachnoid cysts
Otitis media and mastoiditis
Dural sinus thrombosis
Vascular loops
Anterior inferior cerebellar artery
(AICA) syndrome
Aberrant internal carotid artery
Jugular bulb diverticulum, dehiscence
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