Добавил:
Sekretar
kiopkiopkiop18@yandex.ru
t.me/Prokururor I Вовсе не секретарь, но почту проверяю
Опубликованный материал нарушает ваши авторские права? Сообщите нам.
Вуз:
Предмет:
Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_4433_Библиотеки_им_академика_М_И_Перельмана
.pdf
264 BAlAnCE FunCtion AssEssmEnt And mAnAgEmEnt
https://t.me/medicina_free
calorics offer a better sensitivity and specificity over
cool monothermal calorics. Also, different studies have
adopted different rules for precluding the use of monothermal caloric testing in some patients. For example,
Barber, Wright, and Demanuele (1971) recommend
against using monothermal caloric tests when the nystagmus intensity from each ear is less than 11 deg/sec.
Jacobson and Means (1985) and Jacobson et al. (1995)
have added abnormalities in oculomotor tests and presence of any type of nystagmus in the static or dynamic
position tests as criteria for not using the monothermal
caloric test.
Some studies have used the same normative values for both monothermal and bithermal testing (e.g.,
Enticott et al., 2003; Keith et al, 1991). These studies
have produced relatively low false-negative rates for
monothermal testing (Enticott et al., 2003). That is,
there are very few instances where the monothermal
test indicates a normal finding and the bithermal test
does not. On the other hand, false-positive rates for the
monothermal test can be very high if the same normative values are used for both tests (Enticott et al., 2003).
That is, there are many instances where the monothermal test indicates an abnormal finding but the bithermal test does not. To address this shortcoming, other
studies have established distinct normative values for
the monothermal test (e.g., Jacobson & Means, 1985;
Jacobson et al., 1995; Murnane et al., 2009). Using this
approach, Jacobson et al. (1995) have demonstrated
high sensitivity and specificity for predicting bithermal test abnormalities from monothermal test results,
especially when additional restrictions were imposed
on the caloric response parameters.
Although methodological differences make it difficult to directly compare the results from different
studies, it is possible to develop a rational approach to
monothermal caloric testing if the outcomes of these
studies are considered collectively. The most effective
application of monothermal caloric testing seems to be
in predicting when the completion of bithermal caloric
testing is likely to result in a normal finding. That way,
the test can be terminated after the first two irrigations
if the examiner can predict with a high probability a
normal outcome for the bithermal caloric test.
The BSA (2010) has adopted a standard for predicting the normality of the bithermal caloric test based
on the results of the first two irrigations. The standard,
which is based on a study by Lightfoot et
recommends terminating the caloric test after the first
two warm irrigations if the asymmetry between the
right and left responses is less than 15%. Furthermore,
the patient should not have spontaneous nystagmus
greater than 4 deg/sec and the responses should be
al. (2009),
greater than 8 deg/sec for both irrigations (to exclude
patients with bilateral caloric weakness). For patients
who meet the above criteria, the outcome of normal
bithermal caloric testing can be predicted accurately in
95% of them, whereas 29% of the patients who do not
meet the criteria end up with normal results after completing the bithermal caloric test (Lightfoot et al., 2009).
Murnane et al. (2009), using a larger sample size
and a robust statistical approach, have developed criteria for monothermal caloric testing that are similar to
the BSA (2010) standard. They recommend an upper
limit of 10% for the asymmetry between the right and
left responses, which is a stricter criterion than the BSA
standard.
In summary, warm monothermal caloric testing
can be a useful screening test as long as certain exclusionary rules are applied and appropriate normal limits
are used. It is best to reserve the monothermal test for
patients who are unable to complete the bithermal test
or for those who have other test findings (e.g., vHIT)
that can supplement the monothermal caloric findings.
Minimum Age of Patients in Caloric Testing
Caloric testing requires considerable cooperation even
from adult patients. Very young children may not tolerate having an irrigator tip in their ears or being kept
in a dark room for an extended period of time (Cyr,
1980). In the literature, the youngest children who have
undergone caloric testing are reported to be 4 or 5 years
old (Melagrana, D’Agostino, Pasquale, & Taborelli,
1996; Melagrana, D’Agostino, Tarantino, Taborelli, &
Calevo, 2002). Generally, caloric testing is not recommended in children under the age of 6 and in developmentally delayed adults with a mental equivalent
age of 6 or under. The age limit can be modified in rare
cases where the child is very cooperative. It should be
noted that normal values for children are not the same
as those for adults (Melagrana et al., 1996).
analysis of CaloriC resPonses
A number of nystagmus parameters, including duration, frequency, and amplitude, have been used in the
past to quantify caloric responses (Jacobson & Newman, 1993). Currently, there is general agreement that
the nystagmus slow-phase velocity (SPV) is the most
practical measure of its intensity (ANSI, 2009; BSA,
2010). In computerized systems, the task of calculating
SPV is, for the most part, automated. Therefore, further
details about manual calculations are omitted here.
12. CALORIC TESTING 265
https://t.me/medicina_free
SPV Profile of Caloric Responses
Caloric responses can be quantified by measuring the
SPV of each nystagmus beat. Although the strength
and direction of caloric nystagmus can vary, the profile
of its SPV remains essentially the same from one irrigation to another (Figure 12–2). Caloric responses do not
start immediately at the onset of the irrigation. It takes
about 15 seconds from the onset of the irrigation before
nystagmus appears. The nystagmus intensity rises to
its peak approximately 60 to 90 seconds after the onset
of the irrigation and begins to decline thereafter. The
caloric nystagmus dissipates altogether after two to
three minutes.
The SPV profiles for all four irrigations are usually
displayed side by side to simplify the task of comparing the responses (Figure 12–2). In this type of display,
the combination of SPV profiles for warm and cool irrigations of each ear is called a caloric pod because of its
distinct shape.
When reviewing the caloric responses, there are
three different time intervals that require careful analysis of the nystagmus SPV:
1. As noted, caloric nystagmus from an ear with
intact TM usually has a latency of about 15 seconds. However, if the patient has pre-existing nystagmus without fixation in the supine position, this
nystagmus will be present during the first 10 to 15
seconds after the onset of irrigation (Figure 12–3A).
In that case, the baseline of the caloric response will
shift by the SPV of the pre-existing nystagmus and
will no longer be zero. In addition, the response
will return to this baseline once the caloric nystag-
mus subsides. When present, the intensity of the
pre-existing nystagmus should be calculated by
averaging the SPV of a few representative beats.
In the computerized systems, the baseline shift
(SPV of the pre-existing nystagmus) can be estimated by drawing a best-fitting horizontal line
through the SPV points within the first 10 to 15
seconds after the onset of the response. The estimates of spontaneous nystagmus SPV should be
approximately the same for all four irrigations and
should also match the SPV of the nystagmus without fixation in the supine position of the static position testing.
2. Caloric responses usually reach their maximum
level around 60 to 90 seconds after the onset of
irrigation. The peak caloric response for each irrigation should be calculated by averaging the SPV
of a few nystagmus beats that have the highest
velocities (Figure 12–3B). The peak responses are
represented by the symbols RW, LW, RC, and LC
for right warm, left warm, right cool, and left cool
irrigations, respectively. It is assumed that all of the
peak values are represented by a positive number
unless the peak is in the opposite direction of the
expected response. In that case, the value is represented by a negative number.
3. A 5- to 10-second interval around the time of fixation is used for the fixation suppression test. The
examiner should determine the SPV of nystagmus
beats right before fixation and nystagmus beats
right after fixation (Figure 12–3C). The beats that
occur within 1 second before and 1 second after
fixation should be avoided because they often contain artifacts.
Figure 12–2. Display of the caloric pods. The peak caloric response for each irrigation is identified by a box.
266 BALANCE FUNCTION ASSESSMENT AND MANAGEMENT
https://t.me/medicina_free
Figure 12–3. Analysis of caloric nystagmus in three time intervals: A. Around 10 to 15 seconds after the onset of the
irrigation to determine the baseline shift. B. Around 60 to 90 seconds after the onset of the irrigation to determine
the peak caloric response. C. Around 5 seconds before and 10 seconds after visual fixation (identified by gray
timeline) to determine fixation suppression of caloric nystagmus.
Total Caloric Responses
one ear and minimal response from the other ear will
be misidentified as bilateral weakness. Another criteThe first step in analyzing the caloric responses is to
calculate total responses from the right (TotRE) and the
left ear (TotLE):
rion, proposed by the British standard, requires all four
irrigation responses to be very small (less than 8 deg/
sec each) for bilateral weakness (BSA, 2010). Under that
criterion, some cases where there is a strong spontane-
TotRE = RW + RC
TotLE = LW + LC
ous nystagmus but no caloric responses from either ear
will not be identified as bilateral weakness. To avoid
these issues, one can use the criteria of TotRE < 12 deg/
The total response from each ear represents the
opening of the caloric pod (Figure 12–4). When the total
responses from both the right and left ears are very
small, the caloric test indicates the presence of bilateral
sec and TotLE < 12 deg/sec for bilateral caloric weak-
ness. The threshold value of 12 deg/sec is a composite
of the thresholds used by Barber and Stockwell (1980)
and Jacobson and Newman (1993).
caloric weakness. When a bilateral weakness exists,
the equations for the remaining response parameters
do not produce valid results because the denominator
Unilateral Caloric Weakness
approaches zero. Therefore, when the responses from
both ears are very small, the examiner must stop and
not proceed with the rest of the calculations.
The difference between the caloric responses from the
right and left ears is quantified by:
Traditionally, the criterion for bilateral weakness
has been based on the combined caloric responses from
both ears. For example, Barber and Stockwell (1980) use
UW% =
TotRE – TotLE
TotRE + TotLE
× 100,
TotRE + TotLE < 30 deg/sec and Jacobson and Newman
(1993) use TotRE + TotLE < 22 deg/sec. Under that crite-
rion, some cases where there is adequate response from
where UW% represents unilateral weakness in percent.
This parameter has also been called percent reduced ves-
12. CALORIC TESTING 267
https://t.me/medicina_free
Figure 12–4. Calculation of total caloric responses from each ear and unilateral caloric weakness.
tibular response (ANSI, 2009) and percent canal paresis
(BSA, 2010). Because unilateral weakness is proportional to the intensity of caloric responses and because
caloric stimuli are uncalibrated, the difference between
the responses of the right and left ears is normalized
by dividing it by the sum of responses from both ears.
This equation is equivalent to the formula that was first
proposed by Jongkees and Philipszoon (1964) and is
now referred to as the Jongkees equation.
A value of zero for UW% indicates that the
responses from the right and left ears are equal. A positive UW% indicates a unilateral weakness in the left
ear, and a negative UW% indicates a unilateral weakness in the right ear. A value of +100% or −100% for
UW% indicates total lack of caloric response from one
ear. As discussed in the previous section, when there
is no response from either ear, the equation becomes
undefined because the denominator is zero.
Unilateral weakness is commonly expressed relative to the weaker ear, such as, “UW% unilateral weakness in the weaker ear.” When there is minimal response
from one ear and UW% is approaching either +100%
or −100%, instead of using the percentage, it is best to
express the lack of response as “no response to the standard caloric stimulus from the nonresponsive ear.”
It is important to understand what UW% tells us
about vestibular function. One may assume that UW%
represents the loss of horizontal VOR function in the
weaker ear relative to the other ear. However, as Wexler (1994) has pointed out, this assumption is incorrect
because of the nonlinearity of Jongkees equation. That
is, a change in the horizontal VOR function does not
produce a proportional change in UW%. For example,
a 50% loss of hair cells in the lateral canal or their affer-
ent nerve fibers results in only 33% unilateral weakness
using Jongkees equation.
Directional Preponderance
In the standard bithermal caloric test, two irrigations
are expected to generate right beating nystagmus (right
warm and left cool) and two irrigations are expected
to generate left beating nystagmus (right cool and left
warm). In a normal individual, caloric responses in one
direction are approximately equal to those in the other
direction. However, some patients have a directional
preponderance where the responses in one direction
are stronger than the responses in the other direction
(Fitzgerald & Hallpike, 1942).
The difference between right beating and left beat-
ing caloric responses is commonly quantified by:
DP% =
where DP% represents directional preponderance
in percent. TotRB represents total responses from the
irrigations that are expected to generate right beating
nystagmus and TotLB represents total responses from
the irrigations that are expected to generate left beating
nystagmus:
A value of zero for DP% indicates that right beat-
ing and left beating responses are equal. A positive
TotRB – TotLB
TotRB + TotLB
TotRB = RW + LC,
TotLB = LW + RC.
× 100,
268 BALANCE FUNCTION ASSESSMENT AND MANAGEMENT
https://t.me/medicina_free
DP% indicates that right beating responses are stronger
than left beating responses, and a negative DP% indicates that left beating responses are stronger than right
beating responses. Directional preponderance is commonly expressed relative to the stronger nystagmus
direction, such as, “DP% directional preponderance to
the stronger nystagmus direction.”
There are several issues with the concept of directional preponderance that limit its usefulness. For
example, it is now clear that there are two types of
directional preponderance. The most common type is
due to the presence of pre-existing nystagmus without fixation in the standard caloric test position. For
this type, caloric responses are approximately equal
in both directions but the baseline is shifted by an
amount equal to the SPV of the pre-existing nystagmus. Therefore, peak SPVs in the direction of the shift
are higher than peak SPVs in the opposite direction
(Figure 12–5A). A second type of directional preponderance can occur when there is no pre-existing nystagmus but the caloric responses in one direction are truly
stronger (Figure 12–5B). This type of directional pre-
figure 12–5. Different types of directional preponderance: A. Baseline shift. B. Gain asym-
metry. Note that directional preponderance and gain asymmetry are expressed with
respect to the direction of nystagmus fast phases and baseline shift is expressed with
respect to the direction of nystagmus slow phases.
12. CAloriC tEsting 269
https://t.me/medicina_free
ponderance, which is called gain asymmetry, was first
identified by Halmagyi, Cremer, Anderson, Murofushi,
and Curthoys (2000). Gain asymmetry is extremely
rare and was reported in just 1% of the patients. On the
other hand, directional preponderance due to baseline
shift is quite common.
The above equation does not differentiate between
two types of directional preponderance. The criterion
for interpreting nystagmus in the position test is based
on its intensity in different head positions. Therefore,
when directional preponderance is due to pre–existing
nystagmus, it can be interpreted using the same criteria
for interpreting positional or spontaneous nystagmus
without the need for DP% (Stockwell, 1987). On the
other hand, DP% is appropriate for characterizing true
gain asymmetries if the effect of baseline shift is first
subtracted from the caloric response. However, a true
gain asymmetry is extremely rare and there is no established normative limit.
As a result of the above limitations, the clinical
value of directional preponderance has come under
question (Hain, 2014). Many clinicians now do not
incorporate directional preponderance in their interpretation of the caloric test.
Fixation Index
The fixation index (FI) is usually calculated as the ratio
of nystagmus SPV before and after fixation. The FI
should be determined for each direction of nystagmus
independently. When nystagmus is fully suppressed,
the fixation index will be 0%. When nystagmus is partially suppressed, the fixation index will be between
0% and 100%. When nystagmus is enhanced, the fixation index will be greater than 100%.
Analysis of Monothermal
Caloric Responses
Sometimes caloric responses are available only for one
temperature. This can be either by design, as in the
case of a monothermal screening test, or by necessity,
as in the case of a patient who is unable to complete
the test after two irrigations. The equation of UW%
can be modified for monothermal testing. First, if
there is any baseline shift due to pre-existing nystagmus, its effect on the peak caloric responses should
be taken into account. This is done by subtracting the
baseline shift from the peak SPV if they are in the same
direction or adding the baseline shift to the peak SPV
if they are in opposite directions. Then UW% can be
calculated as:
UW
UW
mono
mono
% =
% =
RW’ – LW’
RW’ + LW’
RC’ – LC’
RC’ + LC’
× 100, or
× 100,
where RW’, RC’, LW’, and LC’ are the peak caloric
responses after accounting for the baseline shift.
ARTIFACTS AND TECHNICAL
ERRORS IN CALORIC TESTS
Physiologic artifacts and technical errors can contaminate the caloric responses and affect the validity of the
test results (Becker, 1978). Some of the sources of artifacts
and errors for caloric tests are the same as those that
affect other vestibular tests — for example, eye blinks,
noisy tracings, and cross talk between the horizontal and
vertical channels. Other artifacts and technical errors are
more specific, and perhaps more critical to the caloric
test (Kileny & Kemink, 1986). They include: faulty or
poor irrigations, incorrect identification of peak caloric
responses, failure to maintain a constant level of alertness throughout the test, and faulty calibrations.
An invalid caloric test should be suspected when
the results cannot be explained by any known physiology or pathology of the VOR (Barin, 2006). There are
at least four types of valid caloric responses. In the
first type, all four caloric irrigations generate approximately equal responses (see Figure 12–2). This type of
response is seen in patients with normal VOR and in
patients with bilateral hypo- or hyperactive VOR function. In the second type, responses of each ear to warm
and cool irrigations are approximately equal but the
total responses from one ear are significantly different
from the total responses from the other ear (see Figure 12–4). This type of response is seen in patients with
unilaterally reduced VOR function. In the third type,
total responses from both ears are approximately equal
but the baseline of the caloric responses are shifted in
one direction, causing the nystagmus intensity in one
direction to be stronger than the nystagmus intensity
in the opposite direction (see Figure
of response is seen in patients who have nystagmus
without fixation in the supine position. In the fourth
type, the nystagmus intensity in one direction is truly
stronger than the nystagmus intensity in the opposite
direction without any baseline shift (see Figure 12–5B).
12–5A). This type
270 BALANCE FUNCTION ASSESSMENT AND MANAGEMENT
https://t.me/medicina_free
This type of response is seen in patients with the rare
abnormality of gain asymmetry (Halmagyi et al., 2000).
Any combination of these four types also represents a
valid caloric test. When the test results do not match
one of the valid caloric response types, the examiner
must not proceed with interpreting the test until all
possible sources of technical error have been ruled out.
There is no definitive method for identifying
invalid caloric tests but there is a simple observation
that can alert the examiner to that possibility. In almost
all of the valid caloric test types, the baseline shifts for
the right and left ears are approximately equal. Therefore, invalid test results should be suspected when
the baseline shifts from the right and left ears differ
significantly (Stockwell, 1994). The baseline shift for
each ear can be approximated by finding the midpoint
between the peak caloric responses. Figure 12–6 shows
an example of a significant difference between the
response midpoints of the right and left ears. The exact
amount of difference between midpoints that indicates
an invalid test is not known. Some commercial systems
have used a threshold of less than 5 deg/sec for valid
results. The threshold most likely depends on the overall strength of the response, with lower values for the
weaker overall response strengths and higher values
for the stronger overall response strengths.
When invalid caloric test results are suspected, the
first step is to verify that the SPV profiles are estimated
accurately. Computerized systems derive the SPV pro-
files using an algorithm that distinguishes between
the fast and slow phases of nystagmus. Even the most
sophisticated algorithms are not perfect in this task
and occasionally miscalculate nystagmus SPVs. This
can lead to misidentification of the response peak.
Therefore, if the system allows it, the examiner should
inspect the tracings for all irrigations and correct or
delete the SPV of outliers. Two key guidelines must be
kept in mind when identifying the outliers and cleaning the SPV profile. First, the shape of caloric responses
remains essentially the same, even though the strength
and direction can vary. Second, because the frequency
of caloric responses is very low, the intensities of adjacent nystagmus beats cannot differ significantly.
One of the most common examples of an invalid
caloric test is when the response from one irrigation
is significantly less intense than the responses from
the other three irrigations (see Figure 12–6). The most
likely cause of this error is poor RW irrigation but
faulty calibrations and lack of patient alertness are also
possible. Regardless of the cause, the examiner should
repeat one or more of the irrigations. If the error is
still present after repeating two irrigations, the entire
caloric test should be repeated at a later date. Performing more than six irrigations in the same setting is not
recommended because of the central adaptation issues,
as well as patient comfort.
Sometimes, one irrigation, usually the first one,
generates much stronger responses compared with
figure 12–6. Verifying the validity of the caloric test results by comparing the response midpoints from the right and left ears. In this caloric test, one irrigation generates much weaker
response compared with the other three irrigations.
12. CALORIC TESTING 271
https://t.me/medicina_free
the other three irrigations. The most likely cause of
this finding is hyperalertness during the first irrigation, but faulty calibrations and poor irrigation of the
same ear with the other temperature are also possible.
The examiner should first repeat the irrigation that has
produced the strong response, which is significantly
different from the responses of the other three irrigations. If this does not resolve the error, the examiner
should then repeat the irrigation of the same ear that
was performed with the other temperature. This allows
for the less likely event that the patient has a unilateral
weakness in the other ear.
A special case of a single irrigation producing
unusually strong responses occurs when the strong
response is produced by one of the warm irrigations
and repeating the irrigations as described before does
not resolve the issue. There have been several anecdotal reports that some patients, most commonly those
with vestibular migraine, produce responses for one
of the warm irrigations that are significantly stronger
than the responses for the other three irrigations (personal communication with several clinicians). A few
studies have reported rare cases of unilateral hyperactive caloric responses (e.g., Huygen, Nicolasen, Verhagen, & Theunissen, 1989). However, it is not clear if
the hyperactivity was present for both irrigation temperatures or limited to the warm irrigations. Also, a
few studies have reported unilateral caloric weakness
in a subset of patients with vestibular migraine (e.g.,
Celebisoy, Gökçay, Sirin, & Biçak, 2008). Again, it is not
clear if the caloric weakness could be attributed to a
strong response from the warm irrigation of the contralateral ear. As the evidence is lacking in these cases, it is
best to view the presence of strong caloric responses for
a single warm irrigation as a technical error until more
studies become available.
There are less common types of error that can
affect the caloric test. A careful review of the test as well
as a comprehensive knowledge of the equipment and
the VOR physiology can help the examiner to recognize and avoid technical errors.
INTERPRETATION OF CALORIC FINDINGS
Normative Values for Caloric
Test Parameters
Several studies have examined caloric responses of
healthy individuals and have established normal limits for many of the response parameters (see Barber &
Stockwell, 1980 and CHABA, 1992 for a review of early
studies). The key studies that have contributed to the
more common normal limits include those by Barber
and Wright (unpublished data as referenced by Barber and Stockwell [1980]), Sills, Baloh, and Honrubia
(1977), and Jacobson, Newman, and Peterson (1993).
Table 12–2 shows the most commonly used values as
well as alternative values used by some laboratories.
For unilateral weakness, almost all of the cited
studies agree that the upper normal limit for unilateral
Table 12–2. Normal Limits for Caloric Response Parameters (Common Values and
Alternative
Unilateral
weakness
Directional
preponderance
Bilateral weakness
Hyperactivity
Fixation
suppression
Values Used by Some Laboratories)
Common Alternative
| UW% | < 25% 20%–30%
| DP% | < 30% 25%–50%
TotRE > 12º/sec or
TotLE > 12º/sec
TotRE < 140º/sec and
TotLE < 140º/sec
FI% < 0.6 0.5–0.7
TotRE + TotLE > 22–30 deg/sec
RC or LC or RW or LW > 8 deg/sec
TotRE < 110º/sec and
TotLE < 110º/sec
RC and LC < 50–60 deg/sec and
RW and LW < 80 deg/sec
272 BAlAnCE FunCtion AssEssmEnt And mAnAgEmEnt
https://t.me/medicina_free
weakness is within the range of 20% to 25%. Each laboratory is free to choose the value within this range that
produces the best combination of sensitivity and specificity for its specific clinical settings.
There are more discrepancies among different
studies regarding the normative values for directional
preponderance. This is likely related to the limitations
associated with directional preponderance, which were
discussed previously. The most common normative
limit for directional preponderance is 30% but some
laboratories use a lower value of 25%.
As noted previously, it has now become clear that
directional preponderance is of limited clinical value
(Hain, 2014). As a result, some laboratories no longer
use it. Those who wish to continue using directional
preponderance should consider separating the contribution of baseline shift and gain asymmetry, because
they represent two distinct abnormalities.
The sum of peak caloric responses is used to determine hypoactivity (bilateral weakness) and hyperactivity of the vestibular pathways. It should be stated that
the caloric test is not a particularly useful test to detect
bilateral vestibular hypoactivity. Therefore, it should
not be surprising that there are considerable differences
among different studies about the normal limits for
total caloric responses. These differences are related to
the high variability of caloric stimulus and individual
differences in heat transfer from the external auditory
canal to the labyrinth.
The lower normal limit for total caloric responses
has been estimated to be anywhere from 22 to 30 deg/
sec (Barber & Stockwell, 1980; Jacobson et al., 1993).
It is stipulated that the normal limit should apply
only when caloric responses from the right and left
ears are symmetric. To address this issue, some have
recommended imposing a lower normal limit on the
peak caloric responses of individual irrigations (e.g.,
all irrigations less than 8
alternative approach is to define the threshold for bilateral weakness based on total responses from each ear.
Stockwell (1993) defines the caloric test to be bilaterally
weak when total responses from each ear are less than
12 deg/sec. Using this approach eliminates the possibility of misidentifying the test as bilateral weakness
when the responses of only one ear are very small.
Since the caloric test is of limited value in identifying patients with bilateral vestibular hypoactivity,
one can simplify the process. If a patient is suspected
of having bilateral caloric weakness using any of the
above criteria, another test, such as the rotation test or
the head impulse test, should be included in the evaluation protocol.
The normative limits for hyperactive caloric
responses have also been defined differently in dif-
deg/sec by BSA [2010]). An
ferent studies. Barber and Stockwell (1980) consider
caloric responses as hyperactive when peak slowphase velocities exceed 50 deg/sec for each of the cool
irrigations or 80 deg/sec for each of the warm irrigations. Jacobson et al. (1993) use the criteria of total cool
and warm responses of greater than 99 deg/sec and
146 deg/sec, respectively, and total caloric response
of greater than 221 deg/sec from both ears. Again, to
account for asymmetric caloric responses and presence
of spontaneous nystagmus, one can use total responses
from either ear as the criterion for hyperactivity (see
Table 12–2).
The fixation index (FI%) represents the patient’s
ability to suppress vestibular nystagmus. As with most
caloric response parameters, there is no general agreement as to the normal limit for the fixation index. Some
studies have suggested that any suppression of nystagmus (FI% <100%) indicates normal fixation suppression (Coats, 1970). In other studies, the normal range
includes any value less than 50% (Demanez & Ledoux,
1970). Alpert (1974) recommends a normal limit of less
than 60% to 70%. Today, most laboratories use FI% of
60% as the upper limit of normal fixation (Jacobson et
al., 1993). It should be noted that to generate a valid
fixation suppression test, the nystagmus intensity just
before fixation should not be lower than a critical limit
(~10 deg/sec). Otherwise, the fixation task may not be
challenging enough to reveal abnormalities even in
patients with defective fixation suppression.
It is important to recognize that normal caloric
test results do not necessarily indicate intact VOR
pathways. The caloric test is primarily a test of the lateral semicircular canals and their afferent pathways
and it does not adequately evaluate other vestibular
structures. Furthermore, the range of normal limits for
caloric response parameters is large. As a result, subtle abnormalities may not be detected. For example, a
unilateral weakness must be greater than 20% to 25%
before the caloric test can be classified as abnormal.
However, it is not clear what the threshold of asymmetry in the VOR pathways is before the patient begins
to experience symptoms. It is likely that the threshold is far less than 20%. In short, a normal caloric test
result does not mean normal vestibular function and
the patient diagnosis should still be established in the
context of history, physical exam, and other diagnostic
test findings.
Interpretation and Clinical
Significance of Unilateral Weakness
An abnormal unilateral caloric weakness exists when
the total responses from one ear are significantly
12. CAloriC tEsting 273
https://t.me/medicina_free
weaker than the total responses from the opposite
ear (see Figure 12–4). It indicates a lesion involving
the lateral (horizontal) semicircular canal or its afferent neural pathways in the weaker ear. This finding is
usually considered as the single most clinically useful
finding in the ENG/VNG test battery. Aside from the
benign paroxysmal positional type of nystagmus in the
Dix–Hallpike maneuver, an abnormal unilateral weakness is the only finding that localizes the lesion to the
peripheral vestibular system (labyrinth or vestibular
nerve) and identifies the damaged side.
Several important factors should be considered
about unilateral caloric weakness:
1. The finding of unilateral caloric weakness can
occur as a result of damage to the hair cells, damage to vestibular nerve fibers, or blockage of the
vestibular nerve at the root entry zone to the brainstem (Baloh & Honrubia, 2001). However, unilateral caloric weakness cannot differentiate between
damage to the hair cells and damage to the vestibular nerve fibers.
2. Damage to the central vestibular pathways does
not seem to produce a unilateral weakness. For
example, Uemura and Cohen (1973) found that
a focal lesion in the vestibular nuclei of monkeys
does not result in a unilateral caloric weakness
unless it involves the root entry zone of the eighth
nerve.
3. The damage to the hair cells or the nerve fibers has
to be confined to one side or at least should affect
one side more significantly to produce a unilateral
caloric weakness. Conditions that affect both sides
approximately equally, such as vestibulotoxicity
and aging, do not result in a significant unilateral
weakness.
4. The damage must involve the hair cells in the lateral canal or the superior portion of the vestibular nerve to generate a unilateral weakness in the
caloric test. Furthermore, the loss of function must
be substantial. As noted earlier, at least 30 to 40%
of the hair cells in the lateral canal or their nerve
fibers have to be damaged for unilateral caloric
weakness to exceed the normative limits.
5. The finding of abnormal unilateral caloric weakness is most common in otological diseases. However, any other type of disease that can damage
the hair cells or affect the function of vestibular
nerve can also result in a unilateral weakness in
the caloric test.
The underlying cause of hair cell damage can be:
(1) infection, (2) trauma, (3) ischemia affecting the labyrinthine blood supply, or (4) toxic agents. Other causes,
such as metabolic disorders, are possible but not well
understood.
One of the most common otologic diseases that
produce unilateral caloric weakness is vestibular neuritis. Baloh and Honrubia (2001) have suggested that
a complete or near complete loss of unilateral caloric
response is more common with vestibular nerve lesions
than those affecting the hair cell function. The underlying mechanism is considered to be an infection of the
vestibular nerve. It is now known that there are different types of vestibular neuritis and they result in different findings in the caloric test. One type of vestibular
neuritis is believed to cause degeneration of vestibular nerve and result in permanent loss of vestibular
function (Manzari, Burgess, MacDougall, & Curthoys,
2013). This type of vestibular neuritis results in a unilateral caloric weakness that persists indefinitely. Another
type of vestibular neuritis presumably causes inflammation of the vestibular nerve but does not permanently damage it. Once the inflammation subsides, the
vestibular function is restored and the unilateral weakness disappears. Finally, recent reports have identified
an uncommon form of vestibular neuritis that involves
only the inferior branch of the vestibular nerve (Kim
& Kim, 2012). This type of neuritis does not produce a
caloric weakness and will be missed without another
test such as VEMP testing.
An abnormal unilateral caloric weakness is also
seen in patients with Meniere’s disease, but it is not a
required criterion for the diagnosis. The presence and
the extent of caloric weakness usually depend on the
stage of the disease and the elapsed time since the last
attack (Mateijsen et al., 2001). In the early stages of the
disease, a unilateral caloric weakness is often present
shortly after the onset of the attack but dissipates after
a few days or weeks. In the later stages of the disease,
the weakness becomes persistent and is often progressive. These findings were assumed to be related to the
effect of Meniere’s disease on hair cells, but some investigators have attributed them to the hydropic expansion of the endolymphatic duct (McGarvie, Curthoys,
MacDougall, & Halmagyi, 2015).
Another example of vestibular nerve pathology that produces unilateral weakness is vestibular
schwannoma (acoustic neuroma). The mechanism is
the compression of the vestibular nerve by the tumor
arising from the schwann cells. Although the diagnosis of vestibular schwannoma is usually made through
imaging studies, the caloric test can provide useful
information about the proximity of the tumor to the
vestibular nerve. Absence or severe reduction of caloric
responses along with mild to moderate hearing loss
suggest that the tumor is impinging the vestibular portion of the eighth nerve more than the auditory portion.
Соседние файлы в папке Библиотека им академика М.И. Перельмана