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\begin{document}

\title{Analysis on the optimal horizontal eye position as well as the
effectiveness of a new system for video head impulse test}



\author[1]{Tao Jiang}%
\author[2]{Fei Li}%
\author[1]{Jing Yu}%
\author[1]{Ruonan Huang}%
\author[2]{Chenyong Shang}%
\author[2]{Rui Gao}%
\author[2]{Jianhua Zhuang}%
\author[1]{Wenyan Li}%
\affil[1]{Fudan University Eye Ear Nose and Throat Hospital}%
\affil[2]{Changzheng Hospital}%


\vspace{-1em}



  
  \date{May 18, 2024}


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\selectlanguage{english}
\begin{abstract}
Objective: To identify an optimal gaze angle for precisely evaluating
the function of vertical semicircular canals in video head impulse test
(vHIT) and draw a comparison on the performances among three different
systems for vHIT. Design: Two-center prospective study. Setting:
Participants were sit 1.2 meters away from the wall in a noise-proved
room that dedicated for vHIT experiments. For the gaze-angle projects,
targets were placed on the wall sequentially at the pre-marked lines for
different angles. During the comparison experiments, similar settings
were ensured in both hospitals, like distance to wall and angle of
staring. Same examiner performed the comparison between two systems in
one location. Participants: 26 healthy participants were recruited in
the gaze-angle experiments, 16 of which were further involved in
inter-examiner tests. For the comparison projects, 9 and 13 participants
were recruited, respectively. Any participant with otologic or
vestibular disorders was excluded. Results: Our research showed that
instead of right ahead at 0 degree, when torsion applied at vertical
semicircular canal planes, 25-degree is a better place to set the
targets. At this angle, a more accurate VOR gain was easier to be
obtained. Besides, our evaluations of three different systems showed
that a new vHIT system, VertiGoggles? ZT-VNG-I (VG) performed as good as
the long-tested Otometrics?ICS impulse (Oto) and EyeSeeCam? (ESC).
Conclusion: We proposed a new protocol to set the targets 25 degrees
from right ahead after tilt head 45 degrees to evaluate vertical canals
during vHIT. Furthermore, the new VG system is good for clinical
practices.%
\end{abstract}%



\sloppy


\textbf{Analysis on the optimal horizontal eye position as well as the
effectiveness of}

\textbf{a new system for video head impulse test}

\textbf{Abstract}

\textbf{Objective:} To identify an optimal gaze angle for precisely
evaluating the function of vertical semicircular canals in video head
impulse test (vHIT) and draw a comparison on the performances among
three different systems for vHIT.

\textbf{Design:} Two-center prospective study.

\textbf{Setting:} Participants were sit 1.2 meters away from the wall in
a noise-proved room that dedicated for vHIT experiments. For the
gaze-angle projects, targets were placed on the wall sequentially at the
pre-marked lines for different angles. Duringthe comparison experiments,
similar settings were ensured in both hospitals, like distance to wall
and angle of staring. Same examiner performed the comparison between two
systems in one location.

\textbf{Participants:} 26 healthy participants were recruited in the
gaze-angle experiments, 16 of which were further involved in
inter-examiner tests. For the comparison projects, 9 and 13 participants
were recruited, respectively. Any participant with otologic or
vestibular disorders was excluded.

\textbf{Results:} Our research showed that instead of right ahead at 0
degree, when torsion applied at vertical semicircular canal planes,
25-degree is a better place to set the targets. At this angle, a more
accurate VOR gain was easier to be obtained. Besides, our evaluations of
three different systems showed that a new vHIT system, VertiGoggles?
ZT-VNG-I (VG) performed as good as the long-tested Otometrics?ICS
impulse (Oto) and EyeSeeCam? (ESC).

\textbf{Conclusion:} We proposed a new protocol to set the targets 25
degrees from right ahead after tilt head 45 degrees to evaluate vertical
canals during vHIT. Furthermore, the new VG system is good for clinical
practices.

\textbf{Key Points:}

\begin{enumerate}
\tightlist
\item
  Increasing attention paid on vertigo complaint in otolaryngology
  clinics.
\item
  Effective assessments and quick diagnosis of vestibular disorder are
  essential for clinicians.
\item
  Video head impulse test (vHIT) is a standard and repetitively test to
  evaluate functions of the semicircular canals.
\item
  25 degrees rather than conventional 0 degree is a better gaze angle
  while performing vHIT on vertical canals.
\item
  Cross-centers comparison of three vHIT systems indicated a new system
  with a new kind of goggle is able to perform as good.
\end{enumerate}

\textbf{Introduction}

Vertigo is a common complain of patients that visiting
otolaryngologist's office. With a lifetime prevalence of 7.8\%, patients
experienced vertiginous symptoms, such as dizziness, imbalance, sense of
spinning, and loss of coordination, which have large impact on their
daily life (1, 2). Not only to individuals, these imbalance problems
also bring considerable cost to healthcare system and society. As an
apparatus of inner ear, the vestibular system is composed by two otolith
organs and three semicircular canals (3). Semicircular canals are
perpendicular oriented to each other to sense angular acceleration in
three dimensions (Figure 1). In the clinics, since the beginning of the
20\textsuperscript{th} century, efforts were made to evaluate the
function of semicircular canals. Caloric test (4, 5),
electronystagmography (ENG) (6), and head-impulse test (HIT) (7) are
introduced during the years that still widely used in today's practices
to locate the defected vestibular organ.

Among the different tests, recently, the vHIT has been considered as the
standard even the initial test since it is applicable repetitively in a
short interval to all six semicircular canals (1). A passive,
unpredictable, fast angular head movement (\textgreater{}150\selectlanguage{ngerman}°/s) is
applied to activate a specific pair of semicircular canals (8) (Figure
1). Coordinately, eyes are able to move against head through the
vestibular-ocular reflex (VOR), to stabilize the image on the retina.
During the tests, the relative of eye movements to head movements is
recorded as the gain of VOR to assess the vestibular system. Damaged
semicircular canal will not be able to move eyes against to head during
the test, then the patient has to make a corrective saccade at the end
of head impulse. During the practices, the results of VOR gains for
vertical canals usually arise controversy due to the technical
complications. First of all, the recording is binarization of a 3-D
movement that need carefully align the testing parameters for vertical
and torsional components to avoid noises (Figure 1A and C) (9). The
pupil was driven toward the eyelids while performing the vertical
impulses, in which case, eyelid obscures part of the pupil that
diminished the vertical gain recorded. Efforts were made to find an
optimal staringposition to minimize the adverse effects. Previous paper
found that with the increased of gaze angle on the horizontal plane away
from right-ahead, the measured vertical VOR gain decreased (10).
However, this is not we found in daily practices that with one eye open
and head tilted 45 degrees to ensure impulses at vertical canal planes,
patients are hard to stare at the targetduring head torsions, which
attributes to mixed outcomes that obscure the real defects (Figure 1D).

In addition to the gaze angle, another concern about the frequently-used
vHIT systems was raised. The goggle that was used in some vHIT systems
could only record VOR from the right eye. The inconvenience raised from
some patients hard to see the straight targets when head tilted
especially to the right. Besides, it is even harder to record when any
patient suffered a right eye damaged or defected. In this case, a goggle
that can record VOR gain from either eye is needed. The new system,
VertiGoggles\selectlanguage{english}? ZT-VNG-I (VG) provides this kind of google to eliminate
the inconveniences. To improve the protocol of vHIT, we first tried to
find a more suitable gaze angle on horizontal plane to obtain a better
VOR gain to perform the vertical impulses. Furthermore, to make sure the
new goggle and vHIT system has as good effectiveness as the existed
popular systems under various conditions, we started a two-center
project to compare between VG system to the long-standing vHIT systems.
Our results demonstrated that staring at 25 degree has a higher VOR gain
in vertical canal vHIT. Besides, the new kind of goggle offered
comparable or even better vHIT results, especially when head tilted to
right.

\textbf{Materials and methods}

\textbf{Subjects}

Healthy participants excluded any otology disease nor vertigo complaints
were recruited in our experiments. When compared the three systems at
two different clinical center, 9 and 13 participants were recruited,
respectively. For the gaze angle experiments, we had 26 participants, 16
of whom we performed the inter-examiner evaluation. The informed
consents were obtained from the participants. The experimental
procedures in our study were in accordance with the ethical standards of
the institutional and/or national research committee and with the 1964
Helsinki declaration and its later amendments or comparable ethical
standards.

\textbf{System settings}

For the gaze angle experiments, comparisonbetween VG and Otowere
performed in Shanghai Changzheng Hospital, while the tests between VG
and ESC were performed at Fudan University affiliated Eye and ENT
hospital. In either experiment, we were inclined to record from right
eyes. Recording from the left eye only if the patient could not see the
target from the right eye. All the experiments were applied by the
trained examiners.

\textbf{Experimental procedures}

Participants in different centers were tested in identical surrounding
settings. The room was lit and noise-proved. A non-rotatable wooden
chair was fixed at a certain spot for participants to sit 1.2 meters (m)
away from the wall. All goggles are tightened firmly to avoid any kind
of slippage. Each kind of goggle carries a high-speed digital camera and
a gyroscope to catch the movements of pupils and sense the angular
acceleration. Each equipment was calibrated according to the
manufacture's guidelines. We defined the straight-ahead when facing the
wall is 0 degree. From this point on, the 15, 25, 35, and 45 degrees was
marked. We pre-calculated and draw a perpendicular line of each angle on
the wall (Figure 1F). The targets were placed on the perpendicular lines
according to the participant's height. When performing the vertical
impulses, participants tilted their heads 45 degrees away while keeping
their eyes on the targets at 0, 15, 25, 35, and 45 degrees,
respectively.

Searching the best gaze angle, only the vertical semicircular canals
were tested. All directional degrees were collected on the two pairs of
vertical planes. After distracted the participants' head 45 degree to
the right, we asked them to stare at the targets that placed at 0, 15,
25, 35, and 45 degrees sequentially. At each angle, at least 10
successful impulses were recorded after outliers were excluded. After
the right-side experiments finished, we turned their heads to the left,
to perform another round of tests.

In contrast, all six semicircular canals were tested with eithervHIT
system in the comparison project. We first asked the participants to
look straight forward at the target and move the head at ground-parallel
plane to test the horizontal semicircular canals. Then we followed the
manufactures' instructions when performing vHIT on LARP and RALP planes.
In the VG and Oto systems, we turned the participants' head 45 degree
left or right with the anterior-posterior axis fixed, while their eyes
stared at the same target. For the ESC system, we kept the participants'
head at straight-ahead but rotated side- upwards and downwards. To
perform the inter-examiner tests on the gaze-angle experiments, we have
two independent examiners to performed on the same participants.

\textbf{Data analysis}

The VOR gain was obtained based on different system settings. To
analysis the gain from gaze-angle experiments, since we repeated
performed the vHIT on the same participant, we performed the one-way
repeated measure ANOVA on the data from different angles, with the angle
as the variable of interest. We further check the assumptions with the R
function. The level of statistical significance was set at\emph{p}
\textless{}0.05, which means the angle contributed to the significant
difference between groups. Then we performed Bonferroni test between
data of each angle to 0deg to determine which group(s) was significantly
better than 0deg.

To evaluate the inter-examiner differences, we performed limits of
agreement (LOAs) and Bland-altman plots to compare the distributions of
values and statistical differences.

For the comparison experiments, we aimed to compare VG respectively to
Oto and ESC, rather than in between all three of these systems. Thus, we
applied t-test analysis on either VG to Oto and VG to ESC with SPSS
software.

\textbf{Results}

\textbf{Experiment 1. Different from theoretical model and existing
data, the VOR gain manifested as a bell curve with angle increased}

Among the daily tests, certain number of patients complain that they
can't stare at the target right ahead when head tilted 45 degrees away.
In this case, we first investigated which angle is better to stare when
their head position was insured. To carry out this experiment, we
applied the head impulse of each vertical semicircular canal while
asking the participants to gaze at targets at 0deg, 15deg, 25deg, 35deg, and 45deg
(Figure 1F). As claimed by the existing data and protocols (10), VOR
gain should be decreased with angle increased that 0 degree has the
highest gain. However, different from expectations, during our tests,
the VOR gains displayed rather a bell curve with angle increased. The
gain was increased with the gaze angle increased that reach the peak
around to 25deg. For the left anterior (LA) and left posterior (LP) canal,
highest gain was obtained at 25deg. The best angle for right anterior (RA)
and right posterior (RP) canal was 15deg and 35deg, respectively.

To understand if the angle caused a significant difference of VOR gain,
we performed one-way repeated measure ANOVA analysis. Our data
demonstrated the gaze angle do lead to statistical significance on the
VOR gain (Figure 2). We further analysis which angle was significant
better. The results revealed that 25deg has the lowest p-value for LARP
and LP plane. For the RA plane, the 15deg was significant better. Given
the uniformity requirement during clinical practice, we proposed a new
gaze angle at 25deg during vHIT on vertical semicircular canals. Placing
the target at 25 degrees away from right-ahead after tilted the head of
patients to 45 degrees will give the examiners a more accurate result.

To exclude any system bias, we conducted an inter-examiner test. Two
independent examiners were recruited to performed the vHIT on the same
participant, respectively. Our results indicated that there were no
differences in VOR gain of either semicircular canals between examiners
(Figure S1).

\textbf{Experiment 2. As good performance of VG on vHIT compared to Oto
and ESC systems.}

To test the efficiency of the new goggle that could record from either
eye and further evaluate performances of the new vHIT system,
VertiGoggles\selectlanguage{english}? ZT-VNG-I (VG), we conducted a two-center clinical project
in comparison to the longstanding systems, Otometrics\selectlanguage{english}?ICS impulse (Oto)
and EyeSeeCam\selectlanguage{english}? (ESC). Given that the experiments were carried out
separately, we applied the t-test to analyzed our data (Figure 3 and
Table S1). First of all, there was no significant difference found in
between the systems on the lateral semicircular canals, which indicated
the stability and reliability of this plane across systems. However,
differences showed up on the vertical canals. For the RALP plane, the
VOR gain were similar between VG and Oto. But VG showed significant
difference in comparison to ESC that the gain was closer to 1 with
smaller variations. Furthermore, for the LARP plane, three systems
performed similarly on the LA. But the results of VG were different from
either Oto or ESC on the RP canal. The VOR gain we obtained from VG was
closer to 1 with less standard deviations in either comparison. Our data
indicated three systems has comparable performance in vHIT. Besides,
with the convenience provided by the new goggle, the VOR gain from RP
canal is recorded even better.

\textbf{Discussion}

Previous paper claimed that relationship between the VOR gain and
horizontal eye position was inversed (10). This was similar to the
theoretical model that the VOR gain is calculated
as\(\text{Cos}\left(\text{gaze\ angle}\right)*eye\ ball\ radius\)indicating the vertical movement of eye during the
head impulses on vertical canals (Figure 4). However, during clinical
practices, especially when performing vHIT with Oto system, patients
hard to follow the targets during the vertical impulses when head turned
45 degrees away.

In our investigations, we combined the VOR gains from all different
vertical plane at different angles together, before compared it to the
theoretical model (Figure 4A). This comparison revealed that the 25deg has
the best gain that most significant different from 0deg. One explanation
we had was the range of pupil movement when staring at the 0deg was
shorten by the canthus and eyelids (Figure 4B-F). Even instructed the
participants to open their eyes up, the relative distance of pupil shift
was cut. Moreover, most of the time, patients can't open their eyes
large enough for systems to detect the real eye position, which usually
were underestimated the eye movement (Figure 4E). Even though pupil
movement would not be blocked at 45deg, the VOR gain was decreased, which
agreed with previous data that suggested decreased oculomotor kinematics
at 45deg affected (10). Thus, we proposed that different from ``ideal''
situation and previous papers, 25deg is a better gaze angle when
practicing vertical vHIT.

Similar to formerdata, ESC performed less well on vertical semicircular
canals (1). In contrast, Oto was considered as a gold standard system by
former investigations (1, 11). We compared the new device to either of
these systems. Our results showed that first of all, the pupil detection
system with the newly reformed goggle worked just as fine. Secondly,
different from Oto system, the new goggle allows the patients at LARP
plane, especially, to stare at the targets easier. This convenience not
only leads to a better result on the RP plane, but offers examiners the
freedom to pick whichever eye for VOR tests. In a word, this newly build
VG system performed as good as Oto, even better than ESC system.

With the limitations of our experiments, there are still several
directions that we would like to investigate in the future. First of
all, our hypothesis was performed with limited participants recruited.
It would be better to test this modified protocol with broader subjects.
Furthermore, we would like to test the VG system out in vertical
semicircular canals to verify the universality of our newly proposed 25deg
gaze angle. Besides, in the previous publications, inverse relationships
between the VOR gain and distance was claimed (12, 13). This increased
gain was much lower than predicted calculation that indicated the
limitation of VOR in vHIT. In clinics, the chair usually was set at 1m
to 1.5m away from the target in a consideration of the vergence reflex
and accommodations. However, a unified standard of this distance is
still missing. In general, these experiments may help a new practice
protocol to be introduced in the future.

\textbf{Conclusion}

Our experiments demonstrated the VOR gain reached peak around 25deg, which
was contradict to the theoritical model and existing data. We proposed
to set the target at the deflected angle while applied torsion at
vertical semicircular canal planes. Besides, we test the new system that
was able to perform vHIT that as good as long-standing vHIT systems,
which indicated a vast potential for future developments and practices.

\textbf{Data availability statements}

The authors confirm that the data supporting the findings of this study
are available within the article and its supplementary materials.

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\textbf{Hosted file}

\verb`Figure.docx`  available at \url{https://authorea.com/users/567380/articles/709498-analysis-on-the-optimal-horizontal-eye-position-as-well-as-the-effectiveness-of-a-new-system-for-video-head-impulse-test}

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