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118 Rotational Vestibular Assessment
A B
FIGURE 5–11. Patient talkback system. A. The clini-
cian audio headset with chin microphone and volume/
mute handset. B. The patient audio headset with chin
microphone. C. The same headset hanging next to the
volume adjustment dial mounted on the side of the rotational chair.
C
unaided), the talk-back system is often set aside
in favor of an alternative means of tasking, such as
reciting a story, recipe, or even manually signing a
predetermined topic.
Infrared Patient Video Monitor
With the exception of a boothless rotational chair,
each rotational chair also has an infrared video
camera that is mounted to the chair and allows
for visual monitoring of the patient during the
entire test (Figure 5–12). This can be important
for four reasons. First, it permits the detection
of any anxiety and/or allows the clinician full
observation of the patient’s behavior during the
test. At times, increased anxiety in a patient can
be detrimental to performing rotational testing,
and can often cause unwanted noisy, or spurious,
data in the VOR response. The onset of anxiety
may be secondary to claustrophobia experienced
from being in the lightproof enclosure, the fear of
having an increase in vertiginous symptoms, or
even nyctophobia (fear of the dark). Many times,
patients can overcome this anxiety and will at
least attempt, if not often complete, the test simply because they are comforted by the fact that
you can “see” them during the entire assessment,
even if they cannot see you. Second, having a
video monitor can also be critically important
for a select group of patients, particularly those
who are deaf and use American Sign Language to
communicate, or to alert the clinician to any problems they may be experiencing. The third reason
for the use of a video monitor is that the clinician
can monitor whether the patient has altered their
position in the chair. This addresses both a safety
issue as well as a standard of test-operations issue.
Patient safety is the number one priority during
rotational testing. Therefore, it is essential that
the clinician continuously monitor the patient
5. The Clinical Utility of Rotational Testing 119
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A
B
FIGURE 5–12. Patient visual monitoring system. A. The infrared
camera mounted to the arm of the rotational chair (yellow rectangle),
which provides a black and white image of the patient in the lightproof
enclosure on the video screen (B).
to determine whether they are slipping out of
position, or become agitated, confused, or disoriented. However, the concern for patients slipping
in the chair should be entirely ameliorated if the
patient is harnessed securely and properly in the
chair. Safety of the patient is always the primary
concern, particularly during higher (faster) rotational stimuli. Monitoring patient position also
addresses a standard-of-testing operations issue.
Confirmation that the patient’s head remains in
the proper test position is critical for reliable and
repeatable results. Patients have a tendency to slip
(slide) down in the chair during rotations, which
often places them in a slouched position and can
often alter the position of their head. Moreover,
movement of the head during angular rotations
120 Rotational Vestibular Assessment
can also incite an acute onset of severe vertigo
secondary to an abrupt change in the spatial positioning of the semicircular canals relative to their
pre-test orientation in space (Coriolis force). This
causes an immediate response from the semicircular canals that quickly respond to the ongoing
rotational stimulus. This, of course, also produces
a significant change in the VOR response (your
output measure) that no longer represents the
intentions of the test. Rather, the new head position introduces unwanted vertical VOR responses
and altered horizontal VOR responses that significantly contaminate the test data. The fourth
reason a visual monitor is important in rotational
testing is that it provides an alternative “recording” method by which the clinician can visualize
a patient’s eyes if VOG or EOG cannot be performed, such as with a baby or a toddler. Most
young children will not (and often should not)
perform rotational testing by themselves until they
are of sufficient age to maintain an appropriate
seated posture and understand their responsibility during the test. This, however, does not mean
that a baby or young child cannot participate in
rotational testing. In fact, they can and often do
complete rotational testing. Using a slightly modified test protocol, a young child or baby can sit
in a parent’s lap during rotations while the clinician simply observes the child’s or baby’s eyes
and confirms the presence of an appropriate VOR
in relation to the rotational stimuli. During such
testing, the video goggles are not being worn and,
consequently, an objective measure of nystagmus
cannot be determined. However, a binary decision
regarding the presence or absence of a functional
VOR can often be determined, which is sometimes
all the data you need when determining the functional aspects of a child’s or baby’s VOR. In this
sense, this “recording” method is more of a qualitative response rather than a quantitative one.
Headrest
Another basic component of the chair is the headrest (Figure 5–13). The headrest should be positioned so that it appropriately aligns the head in
a slight downward 30º position (see Figure 5–2).
This will properly align the horizontal semicircu-
lar canals in the horizontal (or yaw) plane and,
in accordance with Ewald’s first law, provide
maximum stimulation of the lateral canals during
upright rotation. However, slight-to-mild deviations in the orientation of the head have been shown
to have minimal effects on the intensity of the VOR
during rotational testing (Coats & Smith, 1967;
Fetter, Hain & Zee, 1986). We discuss the effects
of head position more when reviewing factors
that impact rotational testing later in this chapter.
Nevertheless, it is important the clinician make
every attempt to standardize test procedures in
order to maximize and homogenize the test results
across patients.
Booth Light and Circulating Fan
Within the booth itself are usually mounted a light
and a circulating fan. The fan should be strong
enough to periodically refresh the air during
each assessment. The light is usually red for two
reasons. First, a red-light environment provides
sufficient light to prepare the patient for electrooculography, if VOG is not available or appropriate,
thereby significantly reducing the adaptation time
needed to stabilize the corneoretinal potential
(CRP). Bartley (1951) effectively showed stabilization of the CRP to the required 1 mV in a little as 5
to 10 minutes using a red-light environment, compared to 15 to 20 minutes using a white-light environment (Brey et al., 2008b). That being said, the
time spent preparing your patient for EOG rotational testing can be applied to the time required to
effectively dark-adapt and stabilize the CRP prior
to testing. Obviously, this is less of an issue when
conducting videooculography testing, which has
become the standard for current rotational equipment. The second reason for the red light is that it
is generally less obtrusive to a patient who needs
to visually reacclimate to their surroundings prior
to opening the enclosure door following completion of test procedures (rotations). The circulating fan and booth light are, otherwise, functional
components of the equipment that have no effect
on the VOR response, unless the light is unintentionally left on during testing. Leaving the booth
light on during testing will invariably cause a suppression, (or even complete ablation), of the VOR
FIGURE 5–13. Rotational chair headrest. Note the
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slight 30° downward angle that each headrest promotes
in order to align the horizontal semicircular canal in
the yaw plane to maximize excitation and inhibition in
accordance with Ewald’s first law.
121
122 Rotational Vestibular Assessment
response secondary to visual fixation. In light of
this, failing to turn off the light should always be
considered a potential artifact when an absence
of VOR response is unexpected during rotations.
Rotational Chair Safety Harness
All rotational chairs will have some form of safety
restraint devices and/or harnesses. At a minimum,
all chairs will have a seatbelt and many now come
equipped with a four- or even a five-point harness
(Figure 5–14). In addition, other safety restraint
devices may be present such as knee, leg, and
even foot restraints (Figure 5–15). Such restraints
become increasing more important as certain tests
require higher acceleration and/or constant velocity stimuli. In fact, rotational stimuli greater than
200º per second can often produce a feeling of
centripetal force that can give the impression or
even produce a physical displacement of the body
away from the chair. During these rotations, having a snuggly-fit four-point harness restraint as
well as leg, knee, and foot restraints all contribute
to giving the patient an enhanced degree of safety
and feeling of security.
Rotational Chair Head Restraints
Another important restraint that all rotational
chairs have is a head restraint device (Figure 5–16).
This is one of the most critical components to rotational testing as it ensures that the angular acceleration and velocity stimuli generated by the chair’s
torque motor is precisely translated to the head,
and subsequently to the vestibular system. Without such head restraint devices, the head would
easily lag behind chair acceleration, which would
produce an aberrant delay in the stimulation of
the head and vestibular system relative to the
chair. This would almost certainly cause a significant decrease in the sensitivity (or gain) of the vestibular system, while concomitantly producing an
opposing VOR response that is not “in-time” with
the applied stimulus (referred to as the “phase” of
the VOR response). Without a doubt, this relationship between a poorly synchronized stimulus and
vestibular response gets exponentially worse as
rotational acceleration stimuli are increased (i.e.,
the quicker the acceleration stimulus, the greater
the chance and larger the error in head-to-chair
delay). As a result, a great deal of effort must be
given to ensure that the head is secured tightly
against the chair without sacrificing patient security, safety, and comfort. This is possibly the most
(or at least one of the most) critical setup procedures prior to performing any rotational test
protocol(s).
FIGURE 5–14. The rotational chair’s four-point seat
belt harness. Not pictured is an additional chest strap
that brings the shoulder straps close together for added
security and retention.
Types of Head Restraints. The most common
form of head restraint is achieved through articulating arms with soft head-posts, as pictured in
Figure 5–16. Such head posts are used to securely
“push” and hold the head against the headrest to
minimize the degree of head rotation and neck
5. The Clinical Utility of Rotational Testing 123
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A B
FIGURE 5–15. Foot restraints in both the open (A)
and closed (B) positions. C. Depicts the center leg sup-
port / cushion, which can also be used in conjunction
with a large Velcro strap that can help secure the legs
around the center cushion for increased safety and
support.
C
flexion during testing. It is important to note, however, that appropriate instructions to the patient,
like asking them to refrain from moving their head
and holding it as still as possible during testing,
can go a long way in improving your data acquisition. Other common forms of head restraints are a
head and/or chin strap (similar to a chin strap of a
football helmet) (see Figure 5–16). A chin strap can
help hold the head (and body) upright in the chair
to prevent slouching as well as assist in keeping
the head held tightly against the head restraint
while still maintaining an acceptable degree of
patient comfort. These additional restraints may
even be applied in addition to the articulating
arm soft head posts, particularly if higher rates
of stimuli are applied. Always remember that a
strong correlation exists between the potential for
head movement and stimulus intensity, such that
the greater the acceleration stimulus, the greater
the probability of inadvertent and unwanted
head movement or slippage. Such unwanted head
movements will always introduce noise and aber-
rant responses in your VOR outcome measures.
Subsequently, the measured response will less
accurately represent or reflect the true physiologi-
cal response that was originally intended by the
test stimuli. This deleterious relationship between
unintended head movement and measurement
artifact often exponentially worsens the quicker
the acceleration stimulus. This problem often
plagues test protocols such as quick head thrusts
secondary to either slippage of the video goggles
on the head or slippage of the head in the chair.
This challenge is again discussed in Chapter 8
when reviewing how newer rotational protocols
(crHIT) are attempting to apply extremely quick
chair accelerations in order to thrust the head
similar to that of head impulse test protocols. Less
124 Rotational Vestibular Assessment
A B
FIGURE 5–16. A–C. Various head restraints. Articulating arms
(a) serve as head restraints. The head posts are capped with soft
foam tips that can firmly be pressed against the patient’s scalp
for firm placement and restraint of the head against the headrest
without sacrificing comfort. Optional chin strap (b) can also be
used to secure the head for stability and safety. Optional self-centering head restraint (c) ensures head is positioned in the center
of angular rotation. Images A & C courtesy of Neuro Kinetics, Inc.
C
common head restraint devices are soft moldable
facemasks, self-centering head restraints (see Figure 5–16), and even bite blocks. Although these
are often found only in research labs, such devices
are critical if precise and reliable data are to be
obtained during rotational protocols involving
very high acceleration stimuli.
Computer/Software/Electrical Console
Ever since the 1960s, computer-controlled rotational chairs have permitted the control and
administration of extremely precise stimuli. Moreover, the recordings of vestibular ocular reflexes
through increasingly advanced eye tracking soft-
5. The Clinical Utility of Rotational Testing 125
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ware programs have significantly enhanced the
clinician’s ability to track more subtle eye movements (e.g., ocular torsion), and subsequently
identify more subtle pathology (e.g., internuclear
ophthalmoplegia, micro-saccadic abnormalities).
It goes without saying that the computer is the
single most important component to the rotational test suite. It is also the one component that
is constantly changing. Advances in computing
software are constantly improving eye tracking
recording techniques and analysis algorithms. In
fact, an in-depth discussion regarding the computer and its software would be futile as the discussion would likely be outdated by the time this
text was even one year old. Therefore, it is not the
intent of this text to review the software for each
manufacturer. Rather, the intent of this text is to
present a comprehensive understanding of the
rotational chair and its diagnostic tests and physi-
ologic results. With that being said, it is impor-
tant for the reader to understand that many of
the examples presented throughout this text are
from personal experience and, therefore, from a
single rotational system. The principles of analysis
and interpretation, however, can theoretically be
applied across all rotational data, regardless of the
software and manufacturer.
Figure 5–17 depicts the electrical console (or
the brains) behind the chair. It is a complex network of wires and switches that feed the appropriate electrical signals to the torque motor, the
videooculography goggles, and/or the ocular
A B
FIGURE 5–17. Electric console with the door open (A) showing the complexity of electrical engineering to control
the stimulus parameters of the rotational chair (B). The electrical console is supplied by a ceiling mounted remote
transformer (not shown) that “cleans” and “stabilizes” any fluctuations in power prior to delivering it to the console.
126 Rotational Vestibular Assessment
motor stimulus generator. Like any complex
mechanical network of wires, switches, and fuses,
problems can occasionally develop and it is important to establish a good working relationship with
the manufacturer to ensure the unit is maintained
in proper working order and any problems can be
troubleshooted when the need arises.
Videooculography Goggles
The second most critical component common to
all rotational chairs is the recording goggles (Figure 5–18). High-resolution goggles ensure that
accurate and precise eye movements are recorded,
from which a detailed analysis and video playback can be performed by the clinician. As previously alluded to, the goggles must also give the
patient a clear line of sight to see visual targets
that are presented at various times throughout
testing. This is accomplished through sophisticated cameras and mirrors mounted on the front
of the goggles (see Figure 5–18). There are many
properties of videooculography cameras that can
affect their performance. We will review some of
the more key components, such as their detection
of infrared light, their resolution and frame rate,
and various hardware components located on the
goggles. However, for an excellent and comprehensive review of videooculography, and the various aspects of ocular video recording, the reader is
encouraged to review “Vestibular Function Measurement Devices” by Miles and Zapala (2015).
Biocular Versus Monocular Cameras. Most rota-
tional chairs use biocular goggles rather than
monocular goggles, although both can be used
A B
C
FIGURE 5–18. Dichroic mirrored glass. A. Shows the
mounting angle of the dichroic glass (mirrors). B. Shows
the reflective coating of the dichroic glass (mirrors).
C. Shows the transparency of the mirrors, which allows
for the viewing of ocular motor signals presented directly
in front of the patient. A knob (a) is positioned on the
camera mount, which allows for slight adjustment in the
viewing angle of the dichroic lens. Smaller dials (b) allow
for focusing of the cameras to each eye image. Fixation
lights (c) can be presented through the goggle if the
vision-denied cover was in place.
5. The Clinical Utility of Rotational Testing 127
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effectively. Figure 5–19 depicts both types of
goggles. There may be times when there is only a
single eye to record from; however, this does not
preclude you from using a biocular recording and
only analyzing a single eye. However, it is essential to realize that when using a biocular goggle,
nearly all analysis software will report rotational
VOR data that have been combined or averaged
from both eyes. That being said, it is important to
appreciate that a single-eye VOR recording may
be more appropriate if there are significant disconjugate eye movements, or a unilateral ocular
pathology prohibits biocular recording. This is
particularly true if the analysis software does not
permit the ability to choose between a biocular
versus a monocular analysis. Keep in mind that
this rule applies only to VOR assessment and not
during analysis of ocular motor results, where
the identification of disconjugate eye movements
is always preferred and can be critical to the differential diagnosis. Finally, there are also subtle
distinctions between biocular and monocular goggles as it pertains to certain specialized tests, such
as subjective visual vertical (SVV) testing. These
distinctions are presented when discussing UCF
testing in Chapter 8.
Use of Infrared Light. Videooculography cam-
eras utilize infrared (IR) light to identify and capture eye movements by identifying pupil contrast
from the lighter surrounding iris (Figure 5–20). The
software’s capability of identifying and “locking
on” to the pupil allows it to track the movement
of each eye over time. However, various problems can limit the software’s eye-tracking algorithm. Specifically, problems with ocular tracking
are most commonly the result of eye makeup.
Mascara, in particular, is extremely sensitive to
A B
FIGURE 5–19. A biocular and monocular video
goggle. The biocular goggle (A) has independent right
and left infrared cameras mounted vertically above
each dichroic lens that record the eye image off the
mirrored glass. The monocular goggle (B) has a single
camera mounted horizontally, which provides a direct
in-line recording of the eye. The monocular camera can
be mounted on either eye. Inset images depict each
goggle with the “vision-denied” cover in place. C. Both
goggles are pictured in a side-by-side comparison with
their respective “vision-denied” covers placed in front
of each goggle.
C
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