Calculation of the Circularvection Times in a Population

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CLINICAL INVESTIGATION
Calculation of the Circularvection Times
in a Population With Vestibular Pathology.
Influence of Visual Stimulus
Julio Rama-López,a Francisco Guillén-Grima,b and Nicolás Pérez a
a
b
Departameno de ORL, Clínica Universitaria de Navarra, Pamplona, Navarra, Spain
Unidad de Medicina Preventiva, Clínica Universitaria de Navarra, Pamplona, Navarra, Spain
Objective: To describe the results obtained for circularvection times (tCV) in a study of the phenomenon of visualvestibular interaction for a population with vestibular pathology and to analyze differences in its calculation among
patients reporting a worsening of their symptoms with visual stimuli.
Material and methods: A detailed case history was taken
for all patients, followed by a sensory organization test
using computerized dynamic posturography and the calculation of their tCV.
Results: The mean tCV results were: tCV2= 6.32±3.17 s;
tCV3=6.57±3.68 s; tCVr=6.27±6.02 s. Significant differences
were obtained in tCV2 (P=.046) and tCVr (P=.023).
Conclusions: tCV is a diagnostic test using simple tools that
can help differentiate patients in whom the visual stimulus
is influenced.
Cálculo de los tiempos de circularvección
en una población con patología vestibular.
Influencia del estímulo visual
Key words: Circularvection times. Visual vertigo. Posturography.
Palabras clave: Tiempos de circularvección. Vértigo. Visual. Posturografía.
INTRODUCTION
The relation between subjects and their surroundings may
change in many different ways: the object may move with
respect to the subject, or vice versa or both may be moving
simultaneously. The visual and vestibular information must
act synergically to keep the gaze stable in all these situations,
although when there is a conflict, visual information
dominates.1 In short, we can say that an efficient and normal
vestibulo-oculomotor reflex (VOR) contributes to maintaining
correct visual function and, at the same time, sight optimizes
the function of the VOR. However, VOR is not enough to
ensure satisfactory and accurate vision; we must take into
account other elements and systems (ocular tracking,
optokinetic nystagmus, movement prediction and planning,
cervico-oculomotor reflex) that are capable of improving
The combination of visual and vestibular signals to
maintain balance is known as visuovestibular interaction.
This is a specific aspect of a complex phenomenon of sensorial
integration that occurs at multiple levels.
Correspondence: Dr. J. Rama-López.
Dpto. de ORL. Clínica Universitaria de Navarra.
Avda. Pío XII, s/n. 31008 Pamplona. Navarra. España.
E-mail: [email protected]
Received December 15, 2006.
Accepted for publication December, 2006.
56 Acta Otorrinolaringol Esp. 2007;58(2):56-60
Objetivo: Describir los resultados obtenidos en una población con enfermedad vestibular en la realización de los
tiempos de circularvección (tCV), que estudian el fenómeno de interacción visuovestibular, y analizar las diferencias
en su cálculo en los pacientes que refieren un agravamiento de sus síntomas por estímulos visuales.
Material y métodos: En una población de 200 pacientes
con patología vestibular se realizó una anamnesis detallada de los pacientes, un test de organización sensorial mediante posturografía dinámica computarizada y el cálculo
de los tCV.
Resultados: Los valores medios fueron: tCV2 = 6,32 ± 3,17 s;
tCV3 = 6,57 ± 3,68 s; tCVr = 6,27 ± 6,02 s. Se obtuvieron diferencias estadísticamente significativas al analizar los resultados del tCV2 (p = 0,046) y el tCVr (p = 0,023).
Conclusiones: El cálculo de los tCV es una prueba diagnóstica que puede realizarse mediante un sencillo instrumental y permite diferenciar a pacientes en los que se produce una influencia del estímulo visual.
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Rama-López J et al. Calculation of the Circularvection Times in a Population with Vestibular Pathology. Influence of Visual Stimulus
the VOR’s operation, together making up the so-called visuovestibulo-ocular reflex (VVOR).2
The phenomenon of visuovestibular interaction has been
studied and widely described in many studies, and its
expression in the brain has currently been shown thanks to
new imaging tests such as positron emission tomography
(PET) and functional magnetic resonance. It implies the
correct operation of many structures at both the peripheral
and central levels and allows an adequate response by the
body to different stimuli affecting it and requiring changes
of posture. Its “action and flexibility” is of fundamental
importance in the performance of the activities of daily life.
According to Jeka et al,3 the information obtained from
the speed of the perceived stimulus is the most important
variable for maintaining stable posture at rest and the
information from the visual system is therefore a key element.
Once movement begins, the vestibular information detecting
acceleration has to interact correctly with the visual system.
This interaction between both systems is fundamental for
correct balance and, if it is absent, it is impossible to maintain
correct corporal orientation when the movement to be
performed occurs at high speed or its frequency component
is >2 Hz.4 By means of visuovestibular interaction, it is
possible to examine the different types of information about
the movement to produce a more appropriate response.5
The functionality of this interaction is affected by many
different factors, such as attention, the performance of other
activities,6 situations of weightlessness,7 neurotoxic drugs,8
medication affecting GABA receptors, age,9 sex,10 or trauma
injuries, as in the case of patients suffering from whiplash.11
This necessary functional interaction is maintained in
patients when the vestibular function is insufficient, thus
demonstrating the plasticity of the phenomenon of maximum
visuovestibular interaction. In these pathological situations,
this phenomenon continues to adapt to the information
received. There is an increase in the thresholds for response
to stimuli in movement, thus avoiding oscillopsia,12 or a
lower degree of activation or deactivation depending on the
need for postural control.13
The goal of this study is to describe the results obtained
in a wide population of patients with vestibular involvement
in the performance of a test studying the phenomenon of
visuovestibular interaction, namely circular vection times
(CVt), and to analyze the possible existence of differences
in their calculation in patients reporting a worsening of their
symptoms due to visual stimuli.
MATERIAL AND METHOD
The present study was performed on a sample of 200
consecutive patients seen at the ORL department for
presenting some type of peripheral vestibular disease
manifested by vertigo, dizziness or instability.
The diagnoses for this series of patients are shown in
Table I.
A detailed history was taken from all of the patients
including a specific reference to the possible influence of
visual stimuli on their clinical condition, a sensorial
Table I. Clinical diagnoses of the study population
Clinical Diagnosis
No. (%)
Menière’s disease
60 (30)
Positional vertigo
45 (22,5)
Instability
36 (18)
Spontaneous recurrent vertigo
29 (14.5)
Vestibular schwannoma
7 (3.5)
Perilymphatic fistula
Post-traumatic vertigo
Otosclerosis
6 (3)
6 (3)
5 (2.5)
Vestibular neuritis
6 (3)
Ototoxicity
2 (1)
organization test (SOT) using computerized dynamic
posturography (CDP) with the Smart Equitest System version
7.0 (Neurocom International, Inc., Clackamas, Oregon, United
States) and calculation of the CVt.
The CVt are determined by the different movement
sensations experienced by a subject when subjected to a
visual stimulus in movement. Subjects looking at a moving
object may have a perception of remaining still in space
(egocentric perception of movement) or they may feel that
the surroundings are stable while they are moving (rotating
or circular vection). A uniform movement completely
covering all of the visual field generates a sensation that the
subjects themselves are moving, and this sensation is
indistinguishable from true movement. Once the stimulus
starts, rotating vection begins with a latency of a few seconds,
slowly increases in apparent velocity up to saturation point,
may last for longer than the visual stimulus and depends
on the density of randomly distributed movement contrasts
within the field of vision and its total area.14
For the assessment of and measurement of these variables,
patients were placed inside an acoustically insulated room
in which, in darkness, they were stimulated by an optokinetic
projection (OKN) in clockwise or anti-clockwise direction
capable of covering the entire field of vision. Patients kept
their gaze fixed to the front attempting not to follow the lines.
In this way it is possible to measure 4 different stages of
circular vection depending on the sequential perception of
movement (Figure 1).
– Circular vection time 1 (CVt1): fraction of time in which
the patient, after being subjected to optokinetic visual
stimulation, begins to perceive exocentric movement
(perception of movement only of the surroundings).
Since this is a sensation that is triggered almost instantly
and is therefore difficult to measure, it was not included
in subsequent analyses.
– Circular vection time 2 (CVt2): fraction of time after which
the perception of exocentric and egocentric movement
begins (perception of self-movement), this is the patients’
sensation that they are moving in the opposite direction
from that of the optokinetic stimulation.
Acta Otorrinolaringol Esp. 2007;58(2):56-60
57
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Rama-López J et al. Calculation of the Circularvection Times in a Population with Vestibular Pathology. Influence of Visual Stimulus
Vision
Perception
Time
Figure 1. Drawing representing the measurement of the circularvection times (CVt). It displays the sense of the movement in the visual
surroundings, the sense perceived by he patient and the moment at which the CVt are measured.
– Circular vection time 3 (CVt3): fraction of time when
the subjects report only a perception of egocentric
movement or, in other words, that they are moving but
not the visual stimulus. The sensation of movement is
in the opposite direction from that of the optokinetic
stimulation (OKN).
– Residual circular vection time (rCVt): fraction of time
when the subjects continue to have a perception of selfmovement (egocentric) after withdrawal of the visual
stimulus. To calculate rCVt, the lights were switched
off 20 seconds after CVt3 and the duration of the
perception of self-movement was measured.
The entire experiment was conducted 3 times to calculate
the mean value.
Creation of Groups
Two groups of patients were created depending on the
influence of various visual stimuli on their clinical condition.
Thus, patients were placed in the groups “VIS+“ and “VIS–.”
The patients included in the VIS+ group complied with
at least one of the following criteria, allowing detection of
an influence of the visual stimulus: a) influence of the visual
information on the patients’ symptoms by acting as a trigger
or aggravant; b) visual preference pattern in PDC,15 and c)
sole or combined visual deficit in the performance of the
SOT in PDC.16
The rest of the patients, who did not comply with any of
the three criteria, were included in the VIS– group.
be classified by the influence of the visual stimulus on their
clinical condition. An unconstrained, asymptotic, population
age-, and gender-adjusted multivariable logarithmic
regression analysis was also performed.
RESULTS
The mean values for the study population were: CVt2=
6.32±3.17 s; CVt3=6.57±3.68 s; rCVt=6.27±6.02 s.
In the comparative analysis of the groups, statistically
significant differences were obtained when the results of
the CVt2 (P=.046) and rCVt (P=.023) were analyzed, as these
were longer than those in the VIS– group. The differences
were not significant on analysis of the CVt3 (Figure 2).
In the ROC curve analysis (Figure 3) of the sensitivity and
specificity for rCVt, the area under the curve was 0.4121.
BY establishing the cut-off point at 7 seconds, the rCVt
presents a sensitivity of 49% and a specificity of 90%.
Table II shows the result of the unconstrained, asymptotic,
multivariable logarithmic regression analysis of the
population analyzing the variable rCVt adjusted by gender.
It can be seen that the adjusted rCVt variable presents an
OR of 0.914 (P=.027). Thus, for each second of reduction in
the rCVt, the possibility of being VIS+ increases by
approximately 8%. The multivariable model obtained is not
modified by the introduction of the rest of the CVt.
DISCUSSION
Statistical Analysis
This was conducted using the SPSS statistical programme
(version 11.5) and we then proceeded to conduct a descriptive
study of the population’s CVt and a comparative analysis
of the 2 groups created using Student’s t test. In addition,
the rCVt’s sensitivity and specificity were studied using
ROC curves in order to obtain a cut-off to allow patients to
58 Acta Otorrinolaringol Esp. 2007;58(2):56-60
The methodology used for this study was an innovation
in terms of the logistics, but its physiological bases have
been widely described14 for some time. All of the authors
who have studied the phenomenon of circular vection agree
that there is an evident visuovestibular interaction in which
the visual stimulus provokes a series of postural variations
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1.00
10
Sensitivity
0.75
5
0.50
0.25
0
tCV2
tCV3
tCVr
VIS+
5.6
6.9
4.9
VIS–
6.6
6.6
6.8
0.00
0.00
0.25
0.50
0.75
1.00
Specificity
Area Under the ROC Curve =0.4121
Figure 2. Bar chart with the mean values of the circular vection times
(CVt) in each group.
Figure 3. ROC curve for the residual circular vection time (rCVt).
through a complex interaction with the vestibular system,17
which is really in charge of detecting accelerations. More
recent papers have highlighted the important role of the
cerebral cortex for interpreting this complex illusion of
movement.18,19
We must point out that this is a methodology that, despite
requiring special attention and dedication from the personnel
performing the vestibular tests, is perfectly capable of being
added to those currently performed in a laboratory to study
patients with balance disorders.
The times measured in our population, as they are patients
with vestibular involvement, are longer than those found
in a normal population such as that described by Brandt et
al.14 This is due to the fact that the interaction of the signals
received at the vestibular nuclei is defective because of the
vestibular damage sustained by our study population.
The times in the VIS+ group differ significantly from those
in the VIS– group. This indicates that this methodology is
effective in the identification of subjects with a visual
dependency phenomenon, an aspect manifested in the
analysis of the test using an ROC curve. This analysis shows
that the CVt that differ significantly from the remainder of
the patients present a high degree of specificity for the
detection of the influence of a visual stimulus. This has also
been shown in the multivariable analysis detecting (after
an appropriate gender adjustment) that the variations in
rCVt represent an evident risk of visual dependency.
The analysis of the CVt absolute values also provides us
with other interesting findings supporting the theory of the
phenomenon of visual dependency20 as a compensating
strategy. As has already been mentioned, the vestibular
deficit, due to the presence of a dysfunction, entails higher
times reflecting the defective system. According to this,
patients experiencing a visual dependency phenomenon
should objectively have even higher CVt. However this is
not the case as shown by our results. Quite the contrary,
what we have seen is that the CVt in VIS+ patients are
Table II. Results of the unconstrained, asymptotic, multivariable
logarithmic regression analysis of the rCVt adjusted by gender
␤
Gender
1.598
df
1
p
0.000
OR
95% CI
4.944
2.305-10.604
0.844-0.990
rCVt
–0.090
1
0.02
0.914
Constant
–1.022
1
0.005
0.360
df: degree of freedom; CI: confidence interval; OR: odds ratio; rCVt: residual
circular vection time.
significantly shorter than in the VIS– group. This description
of “visual dependency” as a compensatory phenomenon
derived from the vection study coincides with the
considerations made by Thurrel and Kuno. Thus, the
phenomenon of circular vection, that is to say, the perception
of self-movement generated by a visual stimulus, induces
greater corporal oscillations than the perception of movement
in objects. In this way, VIS+ subjects, as they have a greater
sensorial prevalence due to the greater visual stimulus that
those in the VIS– group, will start with the sensation of
vection sooner and so have greater corporal oscillations.
Here the compensating strategy favours the cessation of this
sensation and the reduction of the oscillations. This is shown
in their logically shorter time values. For this reason, CVt3
(determined when the subjects detect movement in the visual
environment and cease to perceive their own movement) is
not significantly different and is even greater in the VIS+
group.
This complex process cannot be explained other than from
the standpoint of a series of compensation mechanisms that
are well known to take place in the central nervous system,
the only element capable of inter-relating sensations, deficit
and the need for compensating mechanisms.21
From the results of this study we can conclude that the
calculation of CVt is a diagnostic test that can be performed
in a vestibular test laboratory using simple instruments and
Acta Otorrinolaringol Esp. 2007;58(2):56-60
59
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Rama-López J et al. Calculation of the Circularvection Times in a Population with Vestibular Pathology. Influence of Visual Stimulus
allows differentiation of patients in whom visual stimuli
influence clinical condition, thus constraining future
therapeutic decisions.
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