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figure 14–8. Results of vHIT testing on successive occasions over periods of around 11 months
on two patients who presented with acute right vestibular neuritis (Manzari, Burgess, MacDougall,
& Curthoys, 2013). At initial testing both patients show reduced VOR gain for head turns to their
affected (right) side and many corrective saccades, mostly overt. However the pattern of recovery
is totally different. A. In Patient 1 the VOR gain does not change over the next 11 months, although
the patient’s saccadic pattern changes — the saccades become more clustered and move earlier
in time. B. In patient 2 the VOR gain progressively improved over 5 months, and remained at normal
levels at 11 months, and correspondingly the corrective saccades become progressively smaller.
We attribute this improvement to the recovery from peripheral neuritis, rather than central vestibular
compensation, since our previous testing of patients after vestibular neurectomy show no recovery
of VOR gain to head impulses (Halmagyi et al., 1990). Interestingly, after one year both patients were
happy with their recovery.
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movement (Figure 14–8A). This pattern appears to be
associated with improved outcomes of that patient’s
recovery. In other patients the saccades may be temporally dispersed (Figure 14–8B). In some patients that
dispersion decreases over time, possibly related to
the progression of vestibular recovery. It may be that
patients who successfully recover after vestibular loss
use covert saccades to obscure the retinal slip that must
occur because of their inadequate VOR (MacDougall &
Curthoys, 2012).
For head impulse testing (both HIMPs and
SHIMPs) the direct diagnostic indicator is VOR gain.
In both paradigms the saccades are secondary — they
correct for inadequate semicircular canal function, but
they are indirect.
Normal vHIT Gain and Cutoffs for Patients
We stress that before even considering the VOR gain
values, it is absolutely essential to inspect the eye
velocity plots first. What does the eye velocity during
the head impulse look like? Is it low noise or contaminated by noise or blinks? If the records are noisy, the
test should be repeated. An outlying VOR gain value
can occur if a partial blink or an eyelid droop occurs
during a head impulse. In such cases it is important
to rely on the bulk of the actual traces and set aside
the aberrant gain value (which can be identified in the
eye velocity plots). Every head impulse is a separate,
independent test of the function of the semicircular
canal. So, in the usual vHIT test of 20 impulses, there
are effectively 20 separate tests of the function of each
semicircular canal.
DOING THE TEST
Introduction
Video HIT is a deceptively simple system — a pair of
glasses, the clinician’s hands, and some software; however, vHIT is not “plug and play”: clinicians have to
learn how to do the test properly. It is the clinician who
delivers the stimuli, records the results, and interprets
them, and the clinician must have a good understanding of how the vestibulo-ocular response works and
what artifacts can occur. There are a number of subtle
but very important factors that the clinician needs to
know in order to get acceptable data. With training
and practice, vHIT really does become as simple as it
appears, but it requires attention to every detail, and
practice and vigilant maintenance of the highest standards for this to happen.
The first step is to explain to patients that you will
be giving them small unexpected abrupt head turns
and measuring their eye movements during these
turns. It is absolutely essential that patients not have
any neck problems, so start by asking patients about
such issues and giving them a few “practice” small
head turns to give the patient some idea of what the
test will be. Discontinue testing if there is any issue
— obviously patient safety is the foremost criterion in
any test.
Where to Test?
The ideal video image for vHIT is a small pupil, and
so the testing should be done in a normally lit room
with the seated patient facing an evenly illuminated
wall about 1 m away from the patient (not closer). Do
not conduct the test in a sunlit room because the infrared component of sunlight causes reflections.
Perhaps surprisingly, the presence of visual stimuli does not affect the VOR measures from vHIT for
targets at 1 m (Chim, Lasker, & Migliaccio, 2013). Of
course this normally lit room is exactly the opposite
of the conditions for the caloric test, where any visual
fixation stimulus causes visual suppression of the vestibular nystagmus. In principle, visual suppression of
the VOR also occurs for head impulses in vHIT, but
with the accelerations used in the usual vHIT test, the
latency to the initiation of VOR suppression is about
80 to 100 ms (Crane & Demer, 1999), so VOR suppression by visual stimuli is just commencing at the very
end of the head impulse. In fact our new test protocol,
SHIMPs, takes advantage of that delay in suppression
to measure the VOR before suppression has time to
occur (discussed below).
Apart from the great convenience of testing in
light, there is the added advantage that visual stimuli
tend to suppress any spontaneous nystagmus that some
patients (e.g., neuritis patients) may have. In any case,
spontaneous nystagmus with slow phase eye velocities
of a few degrees per second or even tens of degrees
per second is a relatively minor consideration in vHIT
testing because the eye velocities to be measured are
around 150 to 200 deg/s. This is in sharp contrast to the
situation in caloric testing where the absence of visual
fixation acts to enhance any spontaneous nystagmus,
so the eye velocity of spontaneous nystagmus is large
in relation to the velocity of caloric nystagmus.
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Patient Position
There should be a small high-contrast fixation point (a
brightly colored spot about 1 cm in diameter) on the
wall directly in front of the patient at approximately
eye height and about 1 m from the subject (not closer).
The clinician must ensure that the patient can see this
fixation point without wearing prescription spectacles
because it is not possible for the patient to wear glasses
during the vHIT test. Patients who are color blind may
have difficulty in seeing the fixation point. Patients who
wear contact lenses can continue to wear them during
the test without affecting the results. Mascara must be
removed. For the vHIT test the clinician stands behind
the patient with his or her hands on the patient’s head
(Figure 14–9). The computer should be on an adjacent
table, so the clinician can deliver the stimulus and see
the screen, which should not be visible to the patient. In
the ICS Impulse system, every acceptable head impulse
is displayed and stored on the screen, so the clinician
can see if it is acceptable or what is being done wrong
in real time.
Overview: What Is Being Measured?
A small high-speed video camera in the glasses (see Figure 14–3) acquires images of the position of the eye in
the head at a high frame rate (250 frames/s). The position of the center of the pupil with respect to the headfixed camera is tracked from one image to the next.
Software calculates the velocity of the eye movement
and, for VOR gain calculations, identifies and removes
any saccades that occur during the head impulse (i.e.,
the software “desaccades” the records). This is because
the primary measure of VOR performance is the slow
phase eye velocity during the head impulse, without
any contribution from the saccades. The head velocity
is measured by a sensor in the glasses. What is being
acquired is a video image
at high speed — so blinks and drooping eyelids prevent
that image being acquired. Realize the precision — the
image of the pupil usually moves only a few millimeters during the usual head impulse, so it is imperative
to be as accurate as possible in every detail.
The Image
To get accurate measures of the pupil during the head
movement, the camera must be stationary with respect
to the head during the whole head movement, and the
image of the pupil must be clear and in sharp focus,
without shadows or a drooping eyelid obscuring the
pupil (Figure 14–10).
The camera used for vHIT in the ICS system
has a high frame rate in order to measure eye velocity accurately. Our direct comparison of simultaneous
measures of eye velocity by a video camera with a 250Hz frame-rate to eye velocity from scleral search coil
recordings (sampled at 1000 Hz) showed there was
little systematic difference in measured eye velocity of
the two systems (MacDougall et al., 2009), so we concluded that a 250 Hz frame rate is acceptable. Within
the goggles frame are low-level (i.e., safe) infrared
LEDs for illuminating the eye evenly, and the glasses
must be adjusted so there are minimal shadows around
the eye.
The Head Velocity Sensor
Also within the glasses frame is a sensor for measuring
the angular velocity of the head in three dimensions.
The head velocity sensor chip has long-term stability, and so there is no need for repeated calibration of
head velocity. Also in the ICS Impulse glasses are two
tiny, very low power (safe) red lasers for projecting
— a sharp image of the pupil
Figure 14–9. For testing horizontal head impulses, the clinician’s hands are placed on the vertex of the patient’s
head, well away from the glasses straps, and by pushing down slightly on the patient’s head, the clinician can
deliver the small, abrupt “turn and stop” head impulses with minimum of glasses slip.
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Figure 14–10. A. A good image of the eye from the video camera. The center of the pupil is tracked. To determine
this center, the system inverts the contrast of the image in the region of interest (like a photographic negative), so
all the black areas become white. B1.
contrast threshold is not set properly, extraneous shadows are seen and pupil finding is degraded, resulting in
noisy, unacceptable eye velocity records. Calibration check. C1.
passive head rotation back and forth, the eye velocity record is almost exactly superimposed on the head velocity
record. C2.
the head velocity record.
If the calibration is not correct, the eye velocity is systematically larger (or systematically smaller) than
In such a case the in vivo eye calibration should be done again.
The contrast threshold is ideal, so the pupil is a small sharp circle. B2. If the
The eye calibration is correct, so during the slow
calibration spots onto the wall, positioned to be 15 deg
apart on the wall for the subject to stare at, to allow
calibration of the eye movement measures. If a patient
is not able to see or to stare at these fixation spots (e.g.,
patients with acute vestibular neuritis and spontaneous
nystagmus, or patients with congenital nystagmus),
the clinician can use a default calibration setting that
is a good estimate for most subjects. Some color-blind
patients can have difficulty in seeing the projected laser
spots if the wall illumination is too bright.
Fitting the vHIT Glasses to the Subject
A most important consideration in the vHIT test is stability of the glasses on the patient’s head. The camera
is measuring eye in head and so if the camera moves
relative to the eye, then the system erroneously records
that camera slippage as an eye movement. And since
the whole eye movement during a head impulse is very
small, the camera only needs to slip by a very small
amount to cause a big measurement error. Every effort
must be made to minimize glasses slip. The strap holding the glasses on the head should be very tight. This
may be moderately uncomfortable for the patients, but
the test usually takes only a few minutes, so that discomfort does not last long. How is it possible to identify if glasses slip has occurred? A telltale sign is that
the onset of the eye velocity record occurs before the
onset of the head velocity. Checking for this is another
reason to inspect the records before believing the VOR
gain value.
Optimizing the Image of the Eye
Drooping eyelids must be avoided. This can be
achieved when fitting the glasses by lifting the glasses
slightly off the face while manually raising the eyebrows and eyelids and then allowing the glasses to sit
back on the face. This is not uncomfortable and it acts
to ensure that the image of the pupil is not obstructed
by a drooping eyelid. In some patients, we have found
that adhesive medical tape can be used to tape their
eyelids up. If there is any droop of the eyelid during the
head impulse, it results in a false reading to the pupil
center and so can generate a double peak of eye velocity, which can be mistaken for a sign of pathology. Similarly, eye lashes should not obscure the pupil image at
any time through the impulse.
Once the image of the pupil is obtained and displayed on the computer screen, the clinician must optimize it: first by using the software controls to select a
region of interest around the pupil, and then by setting
the threshold so the pupil is a sharp white circle on an
otherwise black field. The consequences of poor pupil
tracking are noisy eye velocity traces yielding spurious and unacceptable results. Then move the patient’s
head back and forth to ensure that the pupil image is
clear and unobstructed for the extent of the head turn
amplitude.
Calibration
The patient is asked to look at two red calibration spots
in turn, and when he or she is doing so, the computer
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logs the data and uses them for calculating eye velocity. If there is an error in the calibration, then all the eye
velocity measures will be in error, and so the VOR gain
calculations will be in error.
One simple technique we strongly recommend for
verifying that calibration has been successful is, after
the program has been started and is acquiring and displaying data, to slowly, passively rotate the subject’s
head horizontally from side to side through an angular range of about 20 deg at a very slow speed (about
0.5 Hz) while the subject is instructed to keep fixating
on the earth-fixed fixation target. If the calibration is
correct, the eye velocity trace should be exactly superimposed on the head velocity trace (Figures 14–10C1
and C2). If not, repeat the calibration process, and if
necessary, remove the glasses and start again. This procedure also verifies that the image of the pupil is clear
and sharp across the range of angles to be used in the
head turns.
Instructions
The clinician must make it very clear that the patient’s
task is to try to keep looking at the exact center of the
fixation spot the whole time during the head movement, to keep his or her eyes wide open, and, if the gaze
moves from that spot, to get back to that fixation spot
as quickly as possible. Make sure the patient can see the
fixation spot and understands what he or she has to do.
The patient should also be asked to try as hard as possible not to blink during the head movement, to keep
his or her eyes wide open, and to relax the neck muscles
to the greatest extent possible. Many patients find this
neck muscle relaxation difficult, and some gentle practice head turns by the clinician are useful.
Glasses movement may come about by direct contact of the clinician’s hands with the glasses themselves
or with the glasses strap or indirectly, by movement of
the patient’s hair or skin or headdress, which indirectly
moves the glasses strap. All of these must be avoided.
The clinician’s hands are placed on the vertex of
the patient’s head, well away from the glasses straps,
and the head is given a short, sharp turn to the left or
right (see Figure 14–9 and Video 14–1). The start and
the stop must be abrupt. It is the angular acceleration at
the onset which is activating the semicircular canal and
so generating the eye movement. It is not the extent of
the head movement which is most important but the
acceleration — how abrupt the start is. The actual extent
of movement of the head is quite small (about 10 or 15
deg) — smaller than carried out by physiotherapists or
chiropractors. We recommend giving the patient a few
practice head impulses before the actual data acquisi-
tion starts. This also helps the patient relax his or her
neck muscles. The head turns should be unpredictable as to timing, direction, and speed. Many patients
will try to predict what is coming next and will try to
“help.” But the clinician is all the time trying to beat
them at this prediction “game.”
Small head turns are to be preferred, since glasses
slip is less of a factor with small head turns. The ideal
head impulse is “turn and stop.” It is recommended
to start with the head in a central position with the
patient’s nose pointing straight ahead at the fixation
target, because from that central position the direction
of the turn is unpredictable. Then the abrupt “turn
and stop” head movement is carried out — essentially
aimed at directing the patient’s nose 10 deg to the right
or left of the central fixation target (see Video 14–1). It
is important that the direction and the timing of the
head turn be unpredictable. Do not fall into the habit
of giving alternate left and right head turns at a regular
interval, as such “metronoming” is to be avoided.
Some patients have difficulty relaxing their neck
muscles. They may resist the clinician turning their
head or anticipate the head turn and actively try to help
rotate their head. Both of these strategies are to be discouraged because we have shown that head impulses
actively performed by the patient do not reveal the VOR
deficit, probably because the patients who actively generate the head turn can also actively generate the compensatory eye movement to correct for it. So the active
contribution by the patient (“helping”) is to be avoided
in clinical vestibular testing — the head impulse should
be a passive, unpredictable, abrupt head turn.
How Many Impulses?
Each head turn is a separate test of vestibular function
of the semicircular canal on the side to which the head
is turned. Repetition is valuable for providing increased
confidence to the clinician about the functional state of
the semicircular canal. It is advisable to wait a variable
interval of about a second or so between the end of
one head impulse and the beginning of the next. Bring
the head slowly back to a central position ready for
the next impulses. Do not start from an extreme left or
right position, since then it is obvious which direction
the next impulse will be. With many patients the results
are very clear in the first two or three impulses, and the
extra impulses just add to the clinician’s confidence.
In some patients, in whom it is very hard to get
good results — for example, because of a stiff neck — it
is better to aim for a small number of quality head
turns than going after an impossible 20. It is the quality of the impulses, the abrupt start and stop, the peak
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head velocity, and not the number of impulses which
is important. We emphasize that if possible the head
velocity on some trials needs to exceed 150 deg/s,
because if only small head velocities are used, a UVL
may not be revealed. A data set illustrating this possibility is shown in Figure 14–11. We stress this because
since the glasses have been widely used, we find some
clinicians always simply give a few unacceptably lowvelocity impulses (they are easier to deliver) and so finish up with an inadequate test and so a false diagnosis.
TESTING THE VERTICAL SEMICIRCULAR
CANALS:
Video HIT can be used to test the vertical canals (left
anterior-right posterior, LARP; right anterior-left
posterior, RALP) in similar fashion to what we have
described above for horizontal canals (MacDougall,
McGarvie, Halmagyi, Curthoys, & Weber, 2013a). Once
again simultaneous recordings by vHIT and scleral
search coils of the same eye have validated that vHIT
detects vertical semicircular canal dysfunction as well
as search coils.
The vertical canals are oriented in planes about
45 deg to the median plane of the head (see Fig-
LARPS AND RALPS
ure 14–1) and form two matched pairs: left anteriorright posterior (LARP) and right anterior-left posterior
(RALP). As for the horizontal canals, each vertical canal
pair works in push-pull fashion. And to test vertical
canal function, the head impulse must be in or close
to the plane of the vertical canal pair being tested. The
head impulses are a pitch movement of the head but
aligned with the canal plane under test (see Figures
14–12 and 14–13, and Videos 14–2 and 14–3). However,
there is one extra very important factor to control: the
direction of gaze. If one tests the vertical canals with
gaze straight ahead, then it is necessary to measure
ocular torsion (rotation of the eye around the line of
sight) because with the eyes directed straight ahead
the eye movement response to vertical canal stimulation has both vertical and torsional components (Aw
et al., 1996). High-speed, accurate measures of torsional
eye velocity require a very high-definition image of
the iris and more computer processing, which acts to
slow down the image processing speed. This problem
can be overcome if, prior to the first impulse, the clinician repositions the patient by turning the patient’s
body (using a swivel chair) so the patient’s nose faces
30 to 40 deg to the left (for RALP) or right (for LARP)
of straight ahead (see Figure 14–12, and Videos 14–2
and 14–3). The patient must keep looking at the central fixation point out of the corner of his or her eyes.
Figure 14–11. To show why low-velocity head impulses give misleading results. These are data for the one left UVL
patient tested at a range of head velocities from about 60 deg/s to 300 deg/s. At low head velocities the eye
velocity matches head velocity, and so the patient’s left UVL is not detected — the VOR gains for low head velocities are close to 1.0. A, B . Superimposed records of head and eye velocity, for horizontal head impulses in a patient
recovering from left vestibular neuritis. C. Values of VOR gain for the same data set. VOR gains for impulses towards
the healthy right side (open circles) are within 2 standard deviations of the mean of the healthy population at
all velocities. Toward the lesioned left side, the VOR gains (filled circles) are lower than the ideal gain of 1.0, and
decrease with increasing head velocity. If this patient had been tested only at low head velocities (peak head
velocity around 100 deg/s), their leftward gains would have appeared close to normal. Testing at higher head
velocities disclosed their abnormally low VOR gain to the left side.
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figure 14–12. A. View looking down on a schematic head with enlarged canals to show the approximate planes
of the vertical canals. B.
with gaze aligned with the canal plane under test. C.
nent becomes much smaller (
are stills from the free iPhone app a
be used by anyone.) B, C.
Curthoys (2015d).
The optimal horizontal eye position for using vHIT to measure vertical canal function: it is
McGarvie, Martinez-Lopez, Burgess, MacDougall, & Curthoys, 2015d). (The images
VOR at the app store. The images in that app are not copyrighted and can
Reproduced with permission from McGarvie, Martinez-Lopez, Burgess, MacDougall, and
If gaze is straight ahead (0º), the measured vertical compo-
figure 14–13. The head movements for LARP (left anterior-right posterior) semicircular canal stimulation. For testing
the vertical canals, the person’s head is turned as shown and the movement of the head is a pitch movement in
the plane of the named canals. The clinician’s hand position is one hand placed under the patient’s chin, and
the other hand is on top of the patient’s head.
push the patient’s cheek since that will lead to movement of the glasses and result in artifactual eye velocity. Both
hands should be well away from the glasses and the strap.
Then the head impulse is a pitch rotation of the head
forward and back along the plane of the canal pair
under test and in the direction of the fixation point (see
Figure 14–12, and Videos 14–2 and 14–3). This is relatively simple and comfortable for the patient and the
clinician and tests the vertical canal response by measuring just the vertical eye movement component
(Migliaccio & Cremer, 2011). Head pitches like this activate the vertical canals and elicit compensatory slow
phase eye velocity, analogous to those for horizontal
head turns.
The fingers of the hand under the chin do not wrap around and
To test the LARP pair of canals, the patient is
rotated so that the body and head are pointed 30 to
40 deg to the right of the fixation point (Figure 14–13A).
The subject still fixates on the same fixation point, so the
eye is shifted to the left in the orbit. A head pitch forward, toward the target, activates the left anterior canal,
and a head pitch back (away from the target) activates
the right posterior canal. To test the RALP vertical pair
of canals, the patient is rotated so the head and body are
pointed about 30 to 40 deg to the left of the target so the
eye is shifted to the right of the orbit. In this position a
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head pitch forward activates the right anterior canal,
and a head pitch back activates the left posterior canal.
The results of testing all semicircular canals in a
healthy subject are shown in Figure 14–14A, and it can
be seen that eye velocity matches head velocity for all
canals. The advantage of testing the vertical as well as
the horizontal canals is that in a patient with unilateral
vestibular neuritis, it allows the clinician to identify
whether the neuritis is total or just affects the superior
division of the vestibular nerve and so leaves the posterior canal function, carried in the inferior division
of the nerve, intact. In rare patients, only the inferior
vestibular nerve is affected. The results of testing a
patient with total UVL are shown in Figure 14–14B. The
patient’s data show reduced eye velocity response for
stimuli testing right-sided semicircular canal function.
The Hex Plot of VOR Gain for
Every Semicircular Canal
Now that testing of all six semicircular canals is routine
with vHIT, we have developed a simple, convenient,
and intuitive way of communicating the results of these
multiple tests. It is called a hex plot, and it is a version
of a polar plot. Examples of hex plots for all six canals
are shown in Figure 14–14A for a healthy subject and
14–14B for a patient with a right UVL. The data for each
canal are shown on one face of a hexagon. So the results
for the right anterior (RA), right horizontal (RH), and
right posterior (RP) canals of one side are assigned
three lateral hexagonal faces, and similarly the left anterior (LA), left horizontal (LH), and left posterior (LP)
canals are assigned three lateral hexagonal faces on the
Figure 14 –14. A. Left panel. Time series results from testing horizontal and vertical canals in a healthy subject.
The plots show superimposed records of eye velocity responses to head velocity stimuli in each canal plane. In a
healthy subject the eye velocity matches head velocity, so the head-velocity and eye-velocity traces are almost
exactly superimposed in every plane. All gain values of the vestibulo-ocular reflex (VOR) are in the normal range.
B. Right panel. The time series results from testing all canals in a patient with a UVL — superimposed records of the
head velocity stimulus and the eye velocity responses to brief unpredictable head turns in the direction of activation of each semicircular canal. Eye velocity matches head velocity reasonably well for leftward head rotations
(toward the healthy ear), but not for rightward head rotations (toward the affected ear). As is the case for the
horizontal canal, reduced or absent slow phase eye velocity occurs during head turns toward the right anterior
and right posterior canals. There are many overt saccades. The central graph shows the VOR gain for each canal
in a variation of a radial or polar plot.
the hexagons. The bars are adjacent to the canal tested, so that it is quickly observed that all the right side canals
have low gains relative to all the canals on the left side. Reproduced with permission of Wolters Kluwer Health from
H. G. MacDougall, L. A. McGarvie, G.M. Halmagyi, I. S. Curthoys, and K. P. Weber, 2013, Application of the video head
impulse test to detect vertical semicircular canal dysfunction, Otology and Neurotology, 34(6), 974–979. https://
journals.lww.com/otology-neurotology. Published by the American Otological Society.
The bars show the value of the VOR gain re the 1.0 or 0.5 scale shown by
352 BALANCE FUNCTION ASSESSMENT AND MANAGEMENT
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opposite side of the hexagon. The VOR gain for each
canal is shown as a bar projecting from the center (VOR
gain of 0.0) to the outermost level corresponding to a
VOR gain of 1.0. As Figure 14–14 shows, this method
shows the results, both the absolute value of VOR gains
for all canals and the VOR asymmetry, intuitively, accurately, and immediately. This patient has suffered substantial loss of semicircular canal function for all canals
on the right side. Figure 14–15 shows the hex plots for
patients with various vestibular pathologies.
Vertical head impulses are more difficult to deliver
than horizontal head impulses because neck mobility is
Figure 14–15. “Hex plots” of VOR gain, comparing simultaneous search coils and video measures on 6 individuals. Bar plots show VOR gain for simultaneous search
coils (light gray
6 representative subjects and patients. Bars are plotted
side by side to facilitate comparison and arranged radially to indicate the results from head impulses delivered
in planes of the: right anterior (RA), left anterior (LA),
right horizontal (
(
RP), and left posterior (LP) semicircular canals. The data
shown are for a range of patient conditions: normal;
idiopathic bilateral vestibular loss, B
tibular deafferentation (lUVD) after surgery for vestibular
schwannoma; right lateral canal occlusion (rLCO) for
intractable benign paroxysmal positional vertigo; idiopathic right posterior canal dysfunction (rPCD); and
bilateral posterior canal occlusion (bPCO) for intractable benign paroxysmal positional vertigo. Results show a
range of responses from canals in these patients, each
with a pattern of canal responses that usually matches
the expectation based on previous literature, but importantly the pattern of response on coils and video measures remains similar across a broad range of canal
responses, thus validating video measures of vestibular
function. Reproduced with permission from MacDougall
et al. (2013b).
) and video measures (dark gray ) for
RH), left horizontal (LH), right posterior
VD; left unilateral ves-
more limited for pitch head movements than for horizontal head movements. In addition, eccentric eye position and limited vertical range by the upper and lower
eyelid make pupil tracking with video technically more
demanding. It is not usually possible to achieve a peak
head velocity as large as that delivered in the plane of
the horizontal canals. Training with a subject wearing
a headlight (aimed at the fixation point) will teach the
clinician how to do vertical head impulses with the requisite turn and stop and minimum rebound and ideally a peak velocity of around 150 deg/s. Especially in
testing the vertical canals, saccades provide valuable
confirmation of canal loss, since in some cases it may
appear that there is reduced VOR gain, but if there are
no corrective saccades, the apparently reduced gain is
more likely due to inappropriate fixation direction.
Interpreting the Results
At the end of the test, many clinicians simply look
at the average VOR gain and base their diagnosis on
that number. This is a serious mistake. What use is
that number if the impulses have not been delivered
properly? In such a case the VOR gain will be meaningless, just as caloric data are meaningless unless the test
has been carried out properly. It is vital to first ensure
the records, the time series data, are acceptable — not
too noisy, without large overshoot and rebound, and
including impulses with peak head velocity at least
150 deg/s. It is mandatory that the clinician inspect
the graphs of the time series of the individual head
impulses before putting any credence in the value
of VOR gain. If the records are very noisy, or if there
are large blinks or saccades, even before the peak head
velocity, if the eye velocity leads the head velocity (a
sign that the goggles have slipped), then the clinician
should be very skeptical of the calculated VOR gain.
The simple solution is to repeat the test ensuring that
the glasses are as tight as possible and the subject
understands his or her task. Even a complete retest will
only take a few minutes.
Repeatability
In healthy subjects our unpublished data have shown
that VOR gain is approximately constant from day to
day, but in patients this is not necessarily the case — it
has been shown in early Ménière’s disease patients that
there are fluctuations in VOR gain from occasion to
occasion, especially around the time of the attack (Manzari, Burgess, MacDougall, Bradshaw, & Curthoys,
14. THE VIDEO HEAD IMPULSE TEST (vHIT) 353
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2011). These fluctuations may be occurring because of
changes within the membranous labyrinth (Rey-Martinez et al., 2018). Different patient conditions cause very
different but highly repeatable response profiles during
a head turn. For example, some patients who may have
endolymphatic hydrops show a greatly enhanced eye
velocity at the onset of the head turn, but this enhanced
eye velocity response very rapidly disappears during
the turn and even reverses. As a result, these patients
can have a measured VOR gain close to 1.0, but they
have achieved this value by a totally different eye
velocity profile compared with the eye velocity profile
of healthy subjects. Only by examining the eye velocity records is it possible to identify this characteristic
response pattern which is very different from that of a
healthy subject but the cause of which is still uncertain.
ADVANTAGES OF VHIT
Previous tests of semicircular canal function using rotation have used large rotatory chairs with carefully controlled angular accelerations. The maximum angular
accelerations that could be achieved safely in this way
are only on the order of 10 to 100 deg/s2 and so are
far below accelerations encountered in everyday life,
which are on the order of up to 2000 to 5000 deg/s2.
Also the rotary chair stimuli were often highly predictable, low frequency (0.2–0.5 Hz) sinusoidal horizontal
rotations, to which even patients with BVL can generate eye movement responses (probably by predictive
pursuit) (Halmagyi & Curthoys, 1987). With a few very
expensive and clinically impractical exceptions, it was
not possible to test patients safely with very high (natural) accelerations using a mechanical device (Crane &
Demer, 1999).
We broke from that tradition when we showed
that a patient with zero semicircular canal function following bilateral vestibular nerve surgical removal for
the treatment of bilateral acoustic tumors could still
generate compensatory eye movements to low frequency, low acceleration, predictable sinusoidal horizontal rotational stimuli (Halmagyi & Curthoys, 1987).
But we found that this BVL patient did not generate a
compensatory eye movement response during the first
100 ms of an unpredictable, passive, high acceleration
head turn, and so we concluded that such stimuli are
specific probes of vestibular function. This was a very
important result because it showed that even when all
the other oculomotor control systems (e.g., pursuit,
optokinetic, cervical input) were available, they were not
capable of generating a compensatory eye movement
during the very earliest part of the response to the head
turn, so we concluded that the eye movement response
during this first 100 ms is a specific vestibular response.
VOR Suppression
Healthy people can almost totally override the vestibuloocular response. For example, if you are reading a road
map in a car as it goes around a corner, you want to
keep your eyes on the map rather than have them
driven off the map by the vestibular input automatically correcting for the angular turn by the car. In such
a situation, descending cerebellar inhibition acts to suppress the drive from the vestibular receptors to the eye
muscles, and this is referred to as VOR suppression. We
considered that this may affect the vHIT result, but by
restricting our measurements to just the start of these
brief unpredictable high acceleration head turns we
can selectively probe the function of the semicircular
canals, since with such stimuli it takes about 80 ms or
more for VOR suppression to start to operate (Crane &
Demer, 1999).
As a result, our approach to vestibular testing has
been totally different — to deliver passive head turns
with natural values of angular acceleration using the
safe means of the clinician’s hands, but to measure the
stimulus and the response exactly and to relate the response on each trial to the stimulus on that trial. We
have focused on the very earliest part of the response
because the results from our BVL patient show that at
later times other oculomotor control systems can operate and so the result is no longer a specific test of semicircular canal function. This has proved to be a very
effective way of assessing semicircular canal function
specifically and identifying deficits. Initially we used
scleral search coils for these tests but more recently we
have used vHIT, having shown that vHIT results are
similar to results using search coil recordings.
WHAT ARE THE SENSITIVITY AND
SPECIFICITY OF VHIT?
A number of studies have compared the results on vHIT
with the results on the caloric test. Both the caloric test
and vHIT are tests of vestibular function. The caloric
is not a definite indicator of vestibular loss — it is not a
“gold standard” because some perfectly healthy people
without any indication of vestibular dysfunction (such
as skaters, dancers, and aviators, and even one of the
authors) can have poor or even absent caloric responses
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