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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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158 Rotational Vestibular Assessment
+ 60°
Rotaon
Rightward
Leward
ChairVelocity
Rotaon
Time
0°
- -60°
chair velocity
Target
One Cycle
One Cycle
FIGURE 6–3. Rotational plot showing two cycles of rotation and the corresponding direction of chair rotation
during each one-half cycle of rotation.
+ 60°
Rotaon
Rightward
Leward
ChairVelocity
Rotaon
Phase
Time
0°
- -60°
90° 180° 0° 270° 360° / 0°
0 sec 100 / 0 sec 50 sec
One Cycle; 0.01 Hz One Cycle; 0.01 Hz
90° 180° 270°
360° / 0°
100 sec 50 sec
FIGURE 6–4. Rotational plot showing two cycles of rotation for 0.01 Hz. The time and degree of chair rotation
are plotted for each cycle of rotation.
Angular Acceleration
As already mentioned, acceleration is frequency
dependent and increases as frequency increases.
When discussing acceleration is terms of a rotational stimulus, it is known as angular acceleration
(α). Angular acceleration can be thought of as
how many degrees the chair rotates in a certain
period of time. Angular acceleration can be mathematically defined as the quotient of the change
in angular velocity (ω) divided by the change in
time (t), and is expressed by the equation:
∆ω
α =
∆t
FIGURE 6–5. Comparison of rotational characteristics for 0.01 Hz and 0.32 Hz. Note: The time scales between
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both examples are not relative to one another.
FIGURE 6–6. Angular velocity of chair rotation; (d) Equals distance in degrees traveled (A → B)
in a known period of time (usually one second); ω = angular velocity; when R = 1; d = ∠a. Note:
cartoon of chair rotates at point “c”. Chair is depicted off center of axis for illustrative purposes.
159
160 Rotational Vestibular Assessment
In this equation, change in angular velocity is
equal to the target velocity of chair rotation during SHA testing (a constant value either 50°/s or
60°/s), and change in time is equal to 1/4 duration of one full cycle of rotation, which is the time
needed to accelerate to the stimulus target velocity before decelerating back to 0°/sec (see Figure
6–5). Using 0.01 Hz as an example, the angular
acceleration of the chair is as follows;
∆ω
α =
∆t
60°/sec
α =
25 sec
2.4°
α =
sec
2
The angular acceleration for each of the stimulus
frequencies performed during SHA testing is provided in Table 6–1.
Number of Required
Cycles of Oscillation
One can quickly determine that the number of
cycles conducted can have a significant impact on
the overall time required to complete each rotational frequency, and ultimately impacting the
total time required to conduct a complete range of
frequencies during SHA testing. It stands to reason that a greater number of oscillations (or cycles)
will invariably produce more compensatory VOR
data during the test, which would undoubtedly
result in greater reliability, validity, and repeatability of data. If one were to randomly assign a
minimum number of ten cycles to be conducted
for each rotational frequency, the total test time to
complete 0.32 Hz would be 31.25 seconds (3.125
seconds times ten cycles). This is not unreasonable
in a clinical setting. However, the time needed to
complete ten cycles of 0.01 Hz would be 1,000 seconds, or 16 minutes and 40 seconds for a single
frequency, which is clinically unreasonable. So
how does one determine the best number of oscillations to administer?
To answer this question, we must first be
cognizant that collection of a “bad cycle” of VOR
response data has traditionally been highly prob-
lematic. This was due to the forced “deletion”
of the entire cycle of data when “bad” data was
collected, even if only a portion of a cycle was
“bad.” This was often secondary to the limitations of some analysis algorithms. This limitation has traditionally plagued rotational testing,
as the deletion of an entire cycle of “bad” data,
when only two cycles of data are collected, can be
disastrous for data analysis and subsequent clinical interpretation, particularly if both cycles have
periods of bad data and you are forced to delete
both cycles or, worse, analyze “bad” data. Thankfully, this problem is becoming less of an issue as
data analysis algorithms continue to improve in
their ability to detect and remove discrete spurious data points/regions within distinct portions of any particular cycle. This eliminates the
“forced deletion” of an entire cycle of data due to
a small portion of the cycle containing “bad data.”
Second, and perhaps the most important factor when determining the number of oscillations
needed for reliable data, is that the test time must
be long enough to allow for the characterization
of the steady-state response of the vestibular system (Wall, 1990). For the lower frequencies of
rotation (0.01 to 0.02 Hz), this will generally occur
after 45 seconds of slow oscillatiory stimulation
(Wall, 1990). In general, the minimum number of
cycles for any frequency should be two full cycles
for frequencies less than 0.04 Hz, with a higher
number of cycles for frequencies above 0.02 Hz.
For the lower two frequencies, this would equate
to a testing time of 200 seconds for 0.01 Hz and
100 seconds for 0.02 Hz. If you are to add in the
45 seconds of test time needed prior to initiation
of the steady-state response, this would calculate
to 245 seconds for 0.01 Hz (or 4 minutes, 5 seconds) and 145 seconds for 0.02 H (or 2 minutes,
25 seconds). Although more data during these low
frequencies of rotation is seldom discouraged, one
must be cognizant of the added test time required
and subsequent mental tasking demand placed on
the patient, which could potentially lead to poorer
data, secondary to reduced mental alertness. The
tradeoff between a limited amount of robust
clean data, and more data that are less robust, is
undoubtedly important and can often be patientdependent with respect to how adept they are performing mental tasking as well as each patient’s
6. Sinusoidal Harmonic Acceleration (SHA) Testing 161
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relative fatigue. Most SHA rotational protocols
typically present two to three cycles of rotation
for 0.01 Hz and 0.02 Hz, and up to 10 cycles for
the remaining frequencies between 0.04 and 0.64
Hz. To account for adaptation of the steady-state
response, most current analysis paradigms delete
a portion of, or the entire first and last half of, the
beginning and ending cycle, respectively.
Chair Rotation Versus Chair Revolution
Finally, it is important to make a distinction
between the rotational chair completing one full
cycle of rotation versus making one full revolution.
This distinction is relevant as one quickly realizes
that the rotational chair no longer completes a full
360° revolution for stimulus frequencies above
0.04 Hz. This can clearly be seen when critically
examining the video for each SHA frequency on
the companion website. The reason for this lies in
the test’s terminal target velocity of 60° per second. Recall that as soon as the acceleration of the
chair reaches the target velocity of 60° per second
(which is achieved at 1/4 of one full cycle), the
chair immediately begins to decelerate back to 0°
per second (full stop at 1/2 cycle) before changing
directions and completing the second half of the
cycle (Figure 6–7). For SHA test frequencies greater
than 0.04 Hz, the increased acceleration stimulus
achieves this target velocity rather quickly and is
able to both accelerate to the target velocity and
decelerate back to full stop prior to completing a
full 360° revolution. That is, the time needed to
complete one-half a cycle of oscillation (acceleration and deceleration in the rightward or leftward
direction) at a rotational frequency of 0.08 Hz is
approximately 180° of rotational displacement, or
just one-half a full revolution.
When considering the example using a rota-
tional frequency of 0.01 Hz, where the rotational
2
acceleration is slow (2.4°/sec
), the chair will take
25 seconds to accelerate to the target velocity of
60° per second (1/4 cycle) and another full 25 seconds to decelerate back to 0° per second (full stop
at 1/2 cycle) before reversing direction of rotation.
During these 50 seconds of rightward (or leftward)
rotation, the chair will actually travel (or rotate) a
1
full 1500°, or 4
⁄3 revolutions in 1/2 cycle before
coming to a full stop and reversing directions. During the next 50 seconds of acceleration and deceleration in the leftward direction, the chair will once
1
again make 4
⁄3 revolutions during this second,
1/2 cycle. However, during a rotational frequency
of 0.16 Hz, because the rotational acceleration is
2
much faster (38.4°/sec
), the chair will only take
3.125 seconds to accelerate to the target velocity of
60° per second and only another 3.125 seconds to
decelerate back to 0° per second (full stop) before
reversing direction of rotation. During these 6.25
seconds of rightward (or leftward) rotation, the
chair will actually travel (or rotate) only 93.75°,
or approximately ¼ of one revolution during this
½ cycle before coming to a full stop and reversing
directions. The angular displacement and revolutions for each of the frequencies performed during
SHA testing is provided in Table 6–1.
FIGURE 6–7. One cycle of rotation showing peak target chair velocity for both rightward and leftward rotations.
Chair velocity always peaks at predetermined target velocity (60º/second in the example).
162 Rotational Vestibular Assessment
NORMAL PHYSIOLOGICAL
RESPONSE DURING SHA TESTING
In response to chair rotations, specifically chair
acceleration and deceleration, an intact vestibular system will generate a slow compensatory eye
movement (i.e., the VOR) in the opposite direction to that of chair rotation. The constant-varying
cupular deflection creates the excitatory and
inhibitory peripheral afferent response that is integrated by the central vestibular nuclei, cerebellum, and various brainstem structures reviewed
in earlier chapters. Central efferent signals are sent
to the appropriate efferent ocular motor neurons
resulting in the compensatory slow-phase vestibular nystagmus that is generated in the opposite
direction of chair rotation (Figure 6–8).
The vestibular slow-phase occurs until the
eye reaches an eccentric position within the orbit
and is then followed by a fast resetting of the eyes
back to the primary position (fast-phase), which is
initiated by the brainstem. This pattern continues
as long as the chair is accelerating or decelerating in a single direction. The resultant nystagmus
(named for the fast-phase) is, therefore, always in
the same direction of chair rotation: right-beating
nystagmus with rightward rotation and left-beating nystagmus with leftward rotation. As the chair
decelerates back to a velocity of 0° per second (full
stop), the chair immediately reverses direction and
begins accelerating and then decelerating in the
opposite direction. The nystagmus switches directions in response to the change in the direction of
chair rotation. This process is repeated until the
predetermined number of oscillations has been
FIGURE 6–8. Relationship between direction slow and fast phases of nystagmus versus the direction of chair
rotation. Direction of nystagmus fast phase is always in the direction of chair rotation. Vestibular slow phase is
always in the opposite direction of chair rotation.
6. Sinusoidal Harmonic Acceleration (SHA) Testing 163
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completed for each respective frequency. The
observed nystagmus is captured and “traced,” or
“charted,” in real time through a computer software program. A series of rightward and leftward
chair oscillations and the respective right-beating
and left-beating nystagmus can be seen in Fig-
6–9. Notice how the nystagmus increases in
ure
its slope as the rotation of the chair increases to its
peak target velocity, at which time the slope of the
nystagmus begins to diminish as the chair decelerates back to a velocity of 0°/second.
From the resulting nystagmus, software programs delete the fast-phase components of the VOR
response leaving only the slow-phase vestibular
components remaining (Figures 6–10 and 6–11).
The reason for this is simple but should, nevertheless, be stated. It is the vestibular response that is of
interest during rotational testing and therefore only
the slow phase that is of primary concern. After
the slow-phases are parsed from the response, the
degree of each slow-phase is measured and plotted in relation to chair oscillation. Because the plot
of slow phase velocity of vestibular nystagmus is
in the opposite direction to chair rotation and the
degree of nystagmus increases and decreases with
the acceleration and deceleration of the chair rotation, the resultant nystagmus velocity and chair
velocity data always appear as opposing (or mirror) sinusoids; that is, 180 degrees out of phase
from one another (Figure 6–12). The peak velocity
of the slow phase component of the vestibular
nystagmus will generally increase and peak near
the same time the chair reaches peak velocity prior
to beginning deceleration (Figure 6–12). Once the
degree of the slow-phase nystagmus has been plotted, various algorithms are applied to the data in
order to characterize the vestibular response with
respect to its response gain, phase, and symmetry.
SHA ANALYSIS PARAMETERS
Analysis of the VOR in response to sinusoidal
chair rotations produces three principle measurement parameters: gain, phase, and symmetry
(Brey et al. 2008a; Shepard, Goulson, & McPher-
FIGURE 6–9. Relationship between direction of chair rotation versus slow and fast
phase of nystagmus. Raw tracing of nystagmus (top plot ) shows right-beating and left-
beating nystagmus in relation to rightward and leftward chair rotation, respectively.
FIGURE 6–10. Close up view of slow and fast phases of the rota-
tional nystagmus during one-cycle of rotation (top graph). Fast phases
are deleted from the nystagmus (orange), leaving only the slow phase
component of the nystagmus remaining for analysis.
FIGURE 6–11. Fast phase components of the vestibular nystagmus are deleted
(orange), and the degree for each slow phase component of the vestibular nystagmus is
plotted below on the eye velocity plot (green arrows to the lower graph) for each one-half
cycle of rotation.
164
6. Sinusoidal Harmonic Acceleration (SHA) Testing 165
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son, 2016; Shepard & Telian, 1996). Comparison
of the peak ocular response to that of peak chair
rotational velocity can be easily determined (Figure 6–13). This ratio of peak eye velocity to peak
chair velocity is known as the sensitivity (or gain)
of the vestibular system and, through a series of
rotations (accelerations and decelerations), the
gain of the vestibular system can be effectively
determined across a wide range of frequencies
during SHA testing.
In addition, as the chair begins to accelerate in one direction and the eye begins to
slowly deviate in the opposite direction due to
the vestibular response, the timing relationship
between the exact moment chair rotation begins and the exact moment the eyes begin to
move in the opposite direction can also be determined. This timing relationship is known as the
phase of the VOR response, and is simply the
temporal (timing) relationship between the move-
FIGURE 6–12. Plot of chair velocity and corresponding vestibular slow phase velocity data. Arrows
identify the peak target velocity of the chair rotation in relationship to the peak vestibular slow phase
component of the eye velocity data. Data show a strong temporal relationship between peak eye data
and peak chair data. As dictated by the vestibular ocular reflex, the slow phase velocity data will always
be in the opposing direction of chair rotation, thus forming a mirror eye velocity sinusoid in relation to
the chair velocity.
FIGURE 6–13. Single cycle of chair rotation, illustrating how the various analysis parameters of SHA testing
(gain, phase, and symmetry) are determined.
166 Rotational Vestibular Assessment
ment of the eyes in relation to the movement of
the head (chair).
Finally, the degree of peak eye response can
be compared from rotations in the clockwise direction to those from the counter-clockwise direction.
The ratio between these two peak responses is
known as the symmetry of the VOR. Therefore,
three primary measures are specifically analyzed
during rotational testing; VOR gain, phase and
symmetry (see Figure 6–13). Each parameter has
unique characteristics, strengths, and limitations.
VOR Response Gain
Gain defines the relationship between peak eye
velocity and peak chair velocity (Brey et al. 2008a;
Shepard & Telian, 1996; Shepard et al., 2016). It
is simply the sensitivity, or responsiveness, of
the vestibular system to a particular stimulus
(or rotational frequency). A perfect compensation
of eye movement to that of chair rotation would
produce a VOR response that was truly equal (and
opposite) with respect to chair movement. That
is, the relationship between peak eye and peak
chair velocity would be exactly the same, which is
expressed as the ratio 1:1, or simply a gain of 1.0.
A perfect response is often referred to as response
unity. VOR response gain is calculated for each
stimulus rotational frequency performed during
SHA testing.
Because the exact acceleration and velocity
stimulus being delivered to the vestibular system
is known, simple calculations of the slow-phase
eye velocity in relation to chair velocity can be
precisely determined. If the VOR were truly an
equal and opposite response to head acceleration,
then the degree of ocular reflex (slow-phase velocity slope calculation) would exhibit an equivalent
increase or decrease in relation to an increase or
decrease in chair rotational frequency. That is, as
the frequency of chair rotation is increased, so
too would the response velocity of the peak eye
response. Although this is generally the case, we
will discover that for the frequencies assessed during rotational testing (0.01 to 0.64 Hz) the response
of the VOR is always opposite the direction of head
(chair) rotation; however, it is seldom, if ever, truly
equal to the intensity of the chair rotation. This
concept is especially true for the slower rotational
frequencies below 0.08 Hz and, to a lesser extent,
also true for the mid-to-high rotational frequencies above 0.04 Hz. This is not surprising as the
frequency stimuli used during SHA testing remain
below the vestibular system’s optimum operating
range of 1 to 5 Hz. Most physiological systems
fail to exhibit sufficient responses for stimuli that
do not adequately stimulate their ideal operating
range, (visual acuity in darkness is one such example). However, if one could adequately and reliably assess the vestibular system using rotational
stimuli between 1 to 5 Hz, the data would show
that the degree of VOR response would not only
continue to be opposite (this is always the case)
but would also be truly equal (or nearly equal) to
the degree of stimuli input. That is, response unity
would be present within our functional operating
range of 1 to 5 Hz.
Analyzing Raw VOR Gain Data
Figure 6–14 depicts the raw nystagmus response
of the VOR during a rotational stimulus. The slowphase responses are plotted against the rotational
stimulus for 0.04 Hz. The figure depicts right-beating nystagmus in response to rightward rotation
and left-beating nystagmus in response to leftward rotation. It can clearly be seen that the VOR
nystagmus response (i.e., the slope of the nystagmus) crescendos and decrescendos in relation
to chair acceleration and deceleration. A healthy
VOR nystagmus response will, therefore, have
a peak response that is often associated with, or
occurs near, the peak response of chair rotation.
The gain of the VOR is determined by comparing the peak eye velocity response to peak
chair velocity response. The peak velocity of chair
rotation is always held constant, most often 50°
or 60°/second, depending on the predetermined
stimulus parameters of the rotational paradigm or
chair protocol. The peak eye (VOR) response, however, will often vary with respect to the frequency
of chair rotation and physiology of the vestibular
system. For lower frequencies of rotation, like that
in Figure 6–15, the peak VOR response crescendos to approximately 30°/second in response to a
0.02 Hz rotational stimulus, whereas the peak eye
(VOR) response crescendos to a much more robust
peak response at 35° to 38°/second in re-sponse to
a 0.32 Hz rotational stimulus (Figure 6–16). Identify-
FIGURE 6–14. Data plot for two cycles of chair rotation and the corresponding slow
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and fast phases of the vestibular nystagmus.
FIGURE 6–15. Complete
data for 0.02 Hz rotation.
Top graph shows raw
nystagmus tracing. Middle
graph shows a plot of the
slow phase component
of the vestibular data in
relation to chair velocity.
Bottom graph shows
averaged slow phase data
in relation to chair velocity.
Peak eye velocity data is
determined and VOR gain
for rightward and leftward
rotation is calculated.
167
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