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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_4382_Библиотеки_им_академика_М_И_Перельмана
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218 Rotational Vestibular Assessment
beyond the cupular return rate of 4 to 7 seconds
(Stockwell & Bojrab, 1997a). This can be seen in
Figure 7–6 where the right-beating nystagmus
in response to a rightward step stimuli is clearly
identifiable to 50 seconds. In reality, a healthy
vestibular system will produce an enduring nystagmus that continues well beyond cessation of
afferent signaling from the h-SCC peripheral
end organ. What is the reason for this perpetuation of the VOR beyond that of cupular dynamics
alone? As cited earlier, the persistence of the VOR
response beyond afferent input is largely due to
velocity storage mechanisms and the neural integrator (Curthoys & Halmagyi, 1996; Highstein,
1996). The persistence and slow decay of the VOR
response beyond cupular mechanics is of significant clinical interest and has been termed the
VOR time decay constant, often abbreviated as TC
(Raphan, Matsuo, & Cohen, 1979). We discuss the
properties of the VOR time decay constant later
during our discussion of the low velocity step test.
Low Versus High Velocity Step Stimuli
The primary difference between low and high
velocity step testing is the target velocity. In gen-
eral, velocity step testing is performed twice,
using both a low and high target velocity. This
would suggest there exists a clinically significant
difference in the outcome measure between the
two target velocities, and this would be correct.
The fundamental difference lies in the degree of
cupular deflection, which is fundamentally determined by the target velocity. As mentioned earlier, it is the target velocity that determines how
long the acceleration period persists; the longer
the period of acceleration, the greater the degree
of cupular deflection. As previously stated, with
a greater degree of cupular deflection comes a
greater the degree of afferent drive. Subsequently,
this creates a greater degree of labyrinthine asymmetry between the depolarized labyrinth (excited
or leading ear) and polarized labyrinth (inhibited
or trailing ear). For the lower target velocity step
testing, the degree of cupular deflection causes
a concomitantly lower excitatory and inhibitory response as both cupulae are only modestly
deflected. However, during high velocity step testing, the period of acceleration is significantly longer allowing for a near-to-complete deflection of
the cupulae to their maximally displaced position.
This effectively causes a near-to-complete satura-
FIGURE 7–6. 60°/second step velocity stimulus depicting VOR in response
to rightward acceleration and constant velocity rotation. Top chart depicts
decay of raw nystagmus. Bottom graph depicts a temporally locked plot of
the decaying (leftward) slow-phase eye velocity response in relation to the
decaying raw right-beating nystagmus shown in the top chart. Solid black line
represents chair velocity.
7. Velocity Step Testing 219
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tion of the inhibitory afferent response from the
trailing ear allowing for the resulting nystagmus
response to more solely represent the prevailing
excitatory response from the leading ear. In doing
so, the excitatory response from each step stimulus (ear) can be compared to determine a gross
measure of labyrinthine symmetry, or, conversely,
asymmetry.
Due to the significant differences in the outcome measures between each test (low versus
high velocity), each measure will now be discussed separately.
LOW VELOCITY STEP TESTING
Nystagmus Response
The primary purpose of a low velocity step test
is to measure the rate of nystagmus decay in
response to an abrupt angular acceleration (and
deceleration) to the right (clockwise) and left
(counterclockwise) (Brey et al., 2008b). Target step
velocities of 60° per second are often considered
standard for low velocity step testing, largely
because normative reference data for nystagmus
decay have been determined for this velocity. Figure 7–7 depicts a 60° per second rotational test
paradigm. A step velocity to the right is followed
by a step velocity to the left. Each period of per-
rotation and post rotation contains a 60 second
interval during which the nystagmus response is
recorded. Figure 7–8 depicts a normal nystagmus
response to each step velocity. Figure 7–9 shows
an enhancement of the nystagmus response in
response to both rightward and leftward acceleration to the target velocity of 60°/sec (an enhancement of the deceleration response periods are
shown later in Figure 7–10). A right-beating nystagmus can clearly be seen in response to rightward
acceleration. Using rightward acceleration (top
panel A) for this discussion, the left slow-phase
component of each nystagmus beat is measured
and plotted directly below the raw nystagmus
tracing. Note how the strongest slow-phase component occurs almost immediately after or even
during the final moments of the acceleration
stimulus. In this example focusing on rightward
step stimuli, (Figure 7–9) the strongest slow-phase
component occurs at approximately one second
and is measured at −41.67° per second. This is a
vital data point to the velocity step analysis, and
is known as the peak slow phase eye velocity.
Following this data point, each subsequent slowphase velocity component is plotted for the
reaming nystagmus beats within the 60-second
constant-velocity rightward interval. Note how
the slow-phase components of the VOR response
slowly decrease in intensity and are commensurate with the decline in the intensity of the raw
nystagmus tracing. In this example, the nystagmus and corresponding vestibular slow-phases
FIGURE 7–7. 60°/second step velocity paradigm showing each 60-second per and post, rightward and leftward
step stimuli. Time (in seconds) is plotted on the x-axis and velocity (in degrees/second) is on the y-axis. Acceleration and deceleration stimuli are held constant at 200°/second2, which produce an acceleration and deceleration
period equal to 0.3 seconds. Total test time including the acceleration, deceleration, and constant velocity stimuli
periods of 60 seconds each, equals 241.2 seconds.
220 Rotational Vestibular Assessment
FIGURE 7–8. Normal 60°/second velocity step test (after correcting/deleting for
noise). Bottom chart shows relative response parameters (e.g., peak slow phase
eye velocity) for each stimulus condition. Solid black line represents chair velocity.
have essentially fully abated by approximately
50 seconds (see Figure 7–8), despite the ongoing constant rotational velocity. This nystagmus decay is the second vital data points to the
velocity step analysis and has been termed the
nystagmus time decay. A similar response
is observed in response to leftward step acceleration, although the VOR response is now appropriately reversed.
Figure 7–10 now shows an enhancement of
the nystagmus response following rightward and
leftward deceleration back to 0°/sec. A left-beating response can clearly be seen following deceleration from rightward rotation. Plotting of the
vestibular slow-phase components is performed
in the same manner as post acceleration. Note a
similar decaying of the nystagmus raw tracings
and the corresponding plot of each slow-phase
vestibular component. The same is true for the
right-beating response following deceleration
from leftward rotation.
Low Velocity Step
Response Parameters
Following an abrupt acceleration (usually 120° to
200° per second squared) to a low velocity target
of 60°/second, a burst of peak nystagmus is generated and noted to slowly deteriorate in its slowphase velocity over time. During a low velocity
step test, the primary response parameter is the
timed decay of the nystagmus response from
the peak slow-phase component post acceleration/deceleration. We will explore this parameter
shortly.
A
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B
FIGURE 7–9. Right (A) and left (B) step acceleration stimuli and nystag-
mus responses. There is a right-beating nystagmus in response to a rightward step stimulus and a left-beating nystagmus in response to a leftward
step stimulus. The respective slow-phase component for each nystagmus
beat is plotted below each raw tracing of nystagmus. A leftward (negative)
slow phase component decay is plotted in A, whereas a rightward (positive) slow phase component decay is plotted in B. The peak (maximum)
slow-phase nystagmus component is identified by the pink vertical line
immediately post acceleration. A best-fit (yellow) line is plotted through the
decaying slow-phase component plots for each step stimuli.
221
A
B
FIGURE 7–10. Right (A) and left (B) step deceleration stimuli and nys-
tagmus responses. There is a left-beating nystagmus in response to deceleration from a rightward step stimulus and a right-beating nystagmus in
response to deceleration from a leftward step stimulus. The respective
slow-phase component for each nystagmus beat is plotted below each raw
tracing of nystagmus. A rightward (positive) slow phase component decay
is plotted in A, whereas a leftward (negative) slow phase component decay
is plotted in B. The peak (maximum) slow-phase nystagmus component is
identified by the pink vertical line immediately post acceleration. A best-fit
yellow line is plotted through the decaying slow-phase component plots for
each step stimuli.
222
7. Velocity Step Testing 223
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In addition to the slow-phase nystagmus
time decay, the peak gain of the VOR response
can be calculated, but less emphasis is given to
this parameter. The peak VOR response gain is
determined in the same manner as gain is calculated during SHA testing. The peak slow-phase
eye velocity response is divided by the target
chair velocity (in this case, 60°/sec). For example,
if the peak slow-phase component was 40°/sec,
40°/sec
the response gain would be
= (0.67), or 67%.
60°/sec
Similar to the response gain during SHA testing,
determination of VOR response gain during VST
is important to determine if the response was sufficiently robust to validate calculation of the time
of the decaying nystagmus response. As with SHA
testing, if there is insufficient VOR gain per or post
acceleration, calculation of a VOR time decay constant may be invalid or unreliable. This is largely
due to the fact that the insufficient presence of
nystagmus is likely inadequate to identify and
calculate an appropriate decay of the response.
Similar to SHA testing, peak VOR gain values near or below 15% should be performed with
caution only after reviewing the response spectral
purity. There exist no clear agreed upon normative reference ranges for what constitutes normal
low velocity step gain, and, as a consequence, little
attention is given to this parameter. Some facilities
have used low velocity step gain in conjunction
with high velocity step gain to judge compensation status in unilateral peripheral vestibulopathies, but more will be discussed regarding this
interpretation after we discuss high velocity step
testing. Finally, like any near absence of VOR
response, when significantly reduced VOR gain
is identified following one or both acceleration
step stimuli, and is free of any noise or artifact,
any analysis of the response parameters (time
decay constant) should be done with extreme care
and precision - although the time decay response
would also likely be abnormal in the presence of
such low VOR gain.
VOR Time Constant
The low-velocity step test is primarily concerned
with the rate of nystagmus decay; specifically the
time, in seconds, for the nystagmus response to
deteriorate by 63% from the peak slow phase eye
velocity, or alternatively said; the time, in seconds,
for the response to decline to 37% of its peak value
(Stockwell & Bojrab, 1997a). Time decay constants
are usually discussed in terms of a decline to 37%
of the peak value
equivalent.
— but the two interpretations are
What Is the Significance of 37% When Discussing
VOR Time Decay Constants?
Many students will often ask where the number 37% comes from when describing
time decay constants. The reason for this is fairly straightforward. Time decay constants are used to define the deterioration of energy (or sometimes gain) of a logarithmic
function such as those often encountered in physics, pharmaceutical, meteorological, neurophysiologic, and even electrical science. Linear decay is easy to define
and even visualize. It is simply the rise over the run, which we know as the slope
of the function (Figure 7–11). However, the decay of a natural logarithmic function
(Figure 7–12) is more difficult to visualize and must be defined differently. As can
be clearly seen in Figure 7–8, the decay of the nystagmus response is non-linear
(curved). The decline (or “slope”) of such non-linear functions is termed the time
decay constant and is defined by the inverse of the base of the natural logarithmic
function. Recall that the base of a natural log function is “e” or Euler’s number (logX
is actually written log
X, where the subscript “e” if often implied and often excluded
e
224 Rotational Vestibular Assessment
FIGURE 7–11. Calculation of the slope of
a linear equation.
FIGURE 7–12. Calculation of the slope
of a non-linear equation does not follow the
same rules when determining the slope of a
linear function.
when writing natural log equations). So the time decay of a natural logarithmic system (our nystagmus decline) is best defined by the period of time that a non-linear
system declines (or decays) to the inverse of the base of the log, or simply:
1
e
Where e (Euler’s number) = 2.718; therefore:
1
2.718
= 0.37, or 37%
Natural log decay functions are commonly used to define such non-linear functions and are, in principle, very similar to the slope of a linear function (
rise
⁄run
).
One example of a non-linear time decay response that you may be familiar with is
the half-life of a drug, which describes the non-linear decay constant of a drug’s
concentration levels in the body. Although the two decay processes are similar, in
reality, one time decay constant is slightly longer than a pharmaceutical half-life
decay constant. All things considered, one time decay constant does not describe
the complete loss of energy within a nonlinear system, only a portion of it (specifically 63% of it). Most physiological systems natural time decay from the peak of the
response to a near complete depletion of the response takes approximately three
time decay constants (Leigh & Zee, 2006). However, most reports of time decay
often imply “one” time decay constant and use of the word “one” when discussing any time decay constant is usually omitted, unless more than one time decay
constant is being discussed.
7. Velocity Step Testing 225
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Calculating the Time Decay Constant
Most computer software algorithms will calculate
the VOR time constant automatically. However,
prior to this, it is absolutely critical that the data
be analyzed for its signal to noise ratio and any
spurious data deleted.
To begin the analysis process, most software
algorithms are designed to identify appropriate
beating nystagmus by “looking” for the expected
slope of the vestibular slow-phase, whether it is
negative (in the case of right-beating nystagmus)
or positive (in the case of left-beating nystagmus)
(Figure 7–13). For example, when a step acceleration is performed to the right (clockwise), the
software “expects” to analyze right-beating nystagmus (i.e., leftward moving slow phases) and
subsequently filters out any and all portions of the
raw nystagmus response that exhibits a positive
slope, including all the fast-phases of the nystagmus response (as the fast-phase will invariably
have an extreme positive slope as well). In turn,
the software identifies and calculates the remain-
FIGURE 7–13. Simple cartoon of nystagmus illustrat-
ing the direction of slow-phase eye velocity component
in relation to the nomenclature for which right-beating
and left-beating nystagmus is identified. The slow phase
component of nystagmus is always the portion of nystagmus that is analyzed during (vestibular) rotational
testing.
ing portion of the raw nystagmus tracing that
exhibits a negative slope. Although most software
“filters” are quite refined at identifying and measuring the negative slow-phase of a right-beating
nystagmus, at times, the software will mistake
“noise” that contains a negative slope and interpret it as a potential slow-phase of a right-beat
nystagmus. Figure 7–14 is an example of such an
analysis. This can wreak havoc on your analyses
as it significantly weakens the signal-to-noise ratio
of your response, (not to mention such “analyzed
noise” usually have relatively higher or lower
degree values then the actual degree of the surrounding true nystagmus beats). Any spurious or
extraneous noise that is being erroneously analyzed and subsequently included in the overall
(decay) response should be modified or deleted.
The same rules and principles apply to a
step acceleration to the left (counterclockwise),
although the software “expects” to analyze left-
beating nystagmus (i.e., rightward slow phases).
During this analysis, the software filters out any
and all portions of the raw nystagmus response
that exhibits a negative slope including all the fastphases of the nystagmus response (as, again, the
fast-phase will invariably have an extreme negative slope as well). In turn, the software identifies
and calculates the remaining portion of the raw
nystagmus tracing that exhibits a positive slope.
Prior to interpreting the velocity step data,
there is one additional key data analysis point
that needs to be investigated, or, more precisely,
needs to be confirmed. Because the time decay constant is calculated from the peak slow phase-eye
velocity response, it is critical to determine that
the correct peak eye velocity response is properly
identified and selected. The peak VOR response
usually occurs immediately after the acceleration
(or deceleration) stimulus is finished. In the case
of a 60° step acceleration, this would likely be near
0.2 to 0.5 seconds after the onset of acceleration (or
deceleration). It is vital that the response during
this time period have as high (good) a signal-tonoise ratio as possible. Peak nystagmus can often
be obscured during this time period by spurious
data caused by blink artifact, eye closure, or ocular noise. Identification of the correct and true
slow-phase eye velocity peak response should be
226 Rotational Vestibular Assessment
FIGURE 7–14. Example illustrating the deleterious effects of noise on the determina-
tion and calculation of the slow phase eye velocity response. A. Depicts the entire VOR
response to leftward step acceleration. B. A magnified view from 123 to 140 seconds of
the response. The circled portion of the response in B provides evidence to suggest that
a VOR response is valid; however, the inclusion of the erratic noise (saccadic intrusions
and ocular jitter) throughout severely limits the algorithms ability to determine what constitutes actual slow phase components versus positive sloping noise.
scrutinized closely and any cleaning or correction
of the data should be performed at this point prior
to further data analysis. We review some examples of noisy step responses in the next section.
Recall our discussion in Chapter 5 regarding clinician intervention into data analysis as being a vital
and critical factor to the correct interpretation of
rotational data. This is never truer then when analyzing step data. Some critical errors during step
analysis are discussed in the following section.
Once the VOR peak response has been confirmed, the time decay constant can be calculated
(Figure 7–15). Simply subtract 63% from the peak
response. As an example, let us say that the peak
VOR nystagmus response immediately following
rightward acceleration is 40°/second (for a VST
gain to rightward acceleration of 0.666 or 67%).
Subtracting 63% from 40°/sec equals 14.8°/sec.
Therefore, the time lapse at which the nystagmus
response decays from the peak response (40°/sec)
to 14.8°/sec equals one time decay constant. Using
the example presented in Figure 7–15, this time
would equate to 18.47 seconds. This process is
then repeated for the three remaining step stimuli;
rightward deceleration, leftward acceleration, and
leftward deceleration.
To help identify or indicate the time decay
through the declining nystagmus, most software
algorithms plot a best-fit line from the point of
peak slow-phase eye velocity through the remaining slow-phase velocity plots of the decaying nystagmus (this is shown in Figure 7–15 as the yellow
7. Velocity Step Testing 227
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Peak
0
)
(-40.14
OneTimeConstant
VelocityStorage
63%TimeDecayConstant
Best-fitlinethroughslowphaseeyevelocitydata
Ex nc onofVORresponse
(≈3TimeConstants)
FIGURE 7–15. 60°/second step velocity stimulus depicting VOR in response to rightward
acceleration and constant velocity rotation. Top chart depicts decay of raw nystagmus. Bottom graph depicts plotting of slow-phase eye velocity response. Inset chart shows peak eye
velocity response and decay time constant for this step response. Response is parsed and
identified by: the peak response, one VOR time-decay constant, and the extinction of the
nystagmus response (which usually occurs after three time constants).
line). From this best fit line, the time between the
peak response and 37% of the peak response is
automatically determined and the time decay
constant is reported. Figure 7–15 illustrates how
the best-fit line approximates the decaying slow
phase eye velocity data points.
Critical Errors in Calculating
Time Decay Constants
There are a few critical errors that can occur when
determining time decay constants. The first two have
already been alluded to. First, extraneous noise in
the slow-phase velocity plot can lead to a best-fit
line that either underestimates or overestimates
the proper decay. Figure 7–16 is an example of
extraneous undeleted data points that aberrantly
“elevates” the best-fit line above the primary slow
phase eye velocity data points, which erroneously
extends the decay. This can be problematic, particularly if the “uncleaned” response initially
identifies the response as “normal” when, in fact,
the “cleaned” response would otherwise calculate
as abnormal. Although we have yet to discuss
what exactly constitutes an “abnormal” 60° step
test, Figure 7–16 is such an example. Figures 7–17
and 7–18 are two more examples illustrating how
deleting any extraneous noise and “cleaning” the
overall data response can change the decay time
constant. Moreover, Figure 7–18 is yet another
example where the “cleaning” of the data changes
a “normal” to an “abnormal” 60° step response.
A second error in the calculation of the VOR
time constant is an inaccuracy in identifying the
absolute peak of slow-phase eye velocity response,
which can often significantly shorten (or lengthen)
the time decay constant. One example of this can
occur when the peak response is incorrectly identified later in the time domain rather than at the
appropriate, “earlier,” time just postcompletion
of the acceleration or deceleration stimulus. In
this case, the time decay constant would likely
be incorrect (often shortened), as the logarithmic
decay tends to calculate as a much steeper decline
from the aberrant peak response and result in a
much shorter time decay constant. Figure 7–19
illustrates this point. The opposite can also occur
when the VOR step gain is significantly low.
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