Investigation of Balance Function Using Dynamic Posturography under Electrical-Acoustic Stimulation in Cochlear Implant Recipients

Abstract

Introduction. The purpose of the present study is to investigate the effect of electrical-acoustic stimulation on vestibular function in CI patients by using the EquiTest and to help answer the question of whether electrically stimulating the inner ear using a cochlear implant influences the balance system in any way. Material and Methods. A test population (n=50) was selected at random from among the cochlear implant recipients. Dynamic posturography (using the EquiTest) was performed with the device switched off an switched on. Results. In summary, it can be said that an activated cochlear implant affects the function of the vestibular system and may, to an extent, even lead to a stabilization of balance function under the static conditions of dynamic posturography, but nevertheless also to a significant destabilization. Significant improvements in vestibular function were seen mainly in equilibrium scores under conditions 4 and 5, the composite equilibrium score, and the vestibular components as revealed by sensory analysis. Conclusions. Only under the static conditions are significantly poorer scores achieved when stimulation is applied. It may be that the explanation for any symptoms of dizziness lies precisely in the fact that they occur in supposedly noncritical situations, since, when the cochlear implant makes increased demands on the balance system, induced disturbances can be centrally suppressed.

Full text

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Hindawi Publishing Corporation
International Journal of Otolaryngology
Volume 2010, Article ID 978594, 7 pages
doi:10.1155/2010/978594
Clinical Study
Investigation of Balance FunctionUsing
DynamicPosturography underElectrical-Acoustic
StimulationinCochlearImplant Recipients
B. Schwab,M. Durisin,andG.Kontorinis
Department of Otolaryngology, Medical University of Hannover, 30625 Hannover, Germany
Correspondence should be addressed to B. Schwab, schwab.burkard@mh-hannover.de
Received 9 February 2010; Revised 4 May 2010; Accepted 11 May 2010
Academic Editor: Peter Sargent Roland
Copyright © 2010 B. Schwab et al. This is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Introduction.Thepurposeofthepresentstudyistoinvestigate theeffectofelectrical-acoustic stimulationonvestibular functionin
CI patients by using the EquiTest and to help answer the question of whether electrically stimulating the inner ear using a cochlear
implant influences the balance system in any way. Material and Methods. A test population (n = 50) was selected at random from
among the cochlear implant recipients. Dynamic posturography (using the EquiTest) was performed with the device switched off
an switched on. Results. In summary, it can be said that an activated cochlear implant affects the function of the vestibular system
and may, to an extent, even lead to a stabilization of balance function under the static conditions of dynamic posturography, but
nevertheless also to a significant destabilization. Significant improvements in vestibular function were seen mainly in equilibrium
scores under conditions 4 and 5, the composite equilibrium score, and the vestibular components as revealed by sensory analysis.
Conclusions. Only under the static conditions are significantly poorer scores achieved when stimulation is applied. It may be that
the explanation for any symptoms of dizziness lies precisely in the fact that they occur in supposedly noncritical situations, since,
when the cochlear implant makes increased demands on the balance system, induced disturbances can be centrally suppressed.
1.Introduction
Cochlear implant recipients often complain of postoperative
symptoms of dizziness [1–4]. Although the auditory and
vestibular systems are clearly distinct from one another, the
mechanisms of neural transmission are identical. For this
reason the electrical stimulation through the agency of the
cochlearimplantmayhaveaneffectbothontheauditoryand
vestibular systems.
Several studies on the effect of electrical stimulation by
the cochlear implant were carried out. Ito in 1998 described
in his study that 18% of 55 cochlear implant recipients saw
connection between dizziness and the activation of the CI.
This gave grounds for supposing that the electricity spreads
diffusely and could therefore stimulate the nerve endings of
the vestibular nerve [5].
Bance et al. tested 17 patients for spontaneous nystagmus
using video nystagmography. Only one patient produced
eye movements under electrical stimulation by the cochlear
implant, although no discomfort was reported [6]. In this
study, the proportion of cases in which the cochlear implant
had a detrimental effect on the vestibular organ was 6%,
which closely corresponds to the findings of the investigation
by Shea into the stimulation of the facial nerve by the
cochlear implant.
In order to verify this possible effect on a larger patient
population, we used the EquiTest to assess 50 postlingually
deafened patients under acoustic stimulation markedly
above the threshold, and compared the results with those in
the stimulation-free situation.
The purpose of the present study is to investigate the
possibleeffectofelectrical-acousticstimulationonvestibular
function in CI patients by using the EquiTest. For this aim a
large test population was used, allowing statistical analysis to
be made.
2.MaterialandMethods
2.1. Patients. A test population (n = 50) was selected at ran-
dom from among the cochlear implant recipients implanted

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2 International Journal of Otolaryngology
at the Medical University of Hannover’s Department of
Otolaryngology. The test population comprised 27 female
and 23 male postlingually deafened adult patients with an
averageageof47years.Theaverageageatonsetofdeafnessin
28 patients with acute hearing deterioration was 39.5 years.
In 22 patients the time of deafness could not be precisely
pinpointed, as the clinical course had been characterized
by continuous progression. The average age at implantation
was 46.3 years. The cause of deafness was unknown in
most cases (n = 26). Further causes were sudden hearing
loss (n = 7), meningitis and otosclerosis (both n = 3),
asphyxia and hereditary causes (both n = 2), destructive
choleastoma, mumps, diphtheria, fibroinflammatory pseu-
dotumors, chronic otitis media, hypoglycemic coma, and
rhesus incompatibility (all n = 1). None of the patients
show any physical and mental disabilities or inadequate
compliance which could negatively affect results of testing
using dynamic posturography.
Themajorityofthepatients(n = 43)weretestedbetween
the sixth and eighth weeks following implantation. Six of
the patients had been using a cochlear implant for a fairly
longperiodoftime(onaverage4.75years).Detailedpatient’s
medical history was taken in relation to any symptoms of
dizziness experienced before and after the surgery.
Three models of CI were represented: 42 patients were
Clarion (Advanced Bionics Corporation, Sylmar, California,
USA) recipients, four had the Nucleus 22 and four the
Nucleus 24 (Cochlear Limited, Lane Cove, Australia).
All patients were tested preoperatively by using nystag-
mus test, Roberg test, Unterberger stepping test, and caloric
test. Postoperatively was the EquiTest performed.
2.2. Methods
2.2.1. Nystagmus Test, Roberg Test, Unterberger Stepping Test,
and Caloric Test. A nystagmus test, the Romberg test, the
Unterbergersteppingtestandcalorictestingwerecarriedout
as part of the routine preoperative procedure; reference was
therefore made to the results of these tests as the basis for
assessing preoperative vestibular function.
2.2.2. The EquiTest. The EquiTest assesses both the balance
system as a whole and its individual components—that is,
the vestibular, visual and somatosensory systems—in their
own right. The EquiTest protocol was developed by Nashner
et al. [7–9] and has been in commercial use since 1986.
It comprises the Sensory Organisation Test (SOT) and the
Motor Control Test (MCT).
The SOT involves six test conditions of increasing
difficulty. In condition 1 (SOT 1) is patient in the starting
position with open eyes, in condition 2 (SOT 2) with
closed eyes. For SOT 1 and 2 both the platform and the
surround remain immobilized. In condition 3 (SOT 3) is
patient in starting position, however the surround moves,
in condition 4 (SOT 4) the platform moves, however the
surround remains fixed. In condition 5 (SOT 5) the paltform
moves while the subject keeps his/her eyes closed, and in
condition 6 (SOT 6) both the surround and the platform
SOT 1 SOT 2SOT 3
SOT 4 SOT 5SOT 6
Figure1:TestconditionsSOT1to6underwhichequilibriumscore
is determined: under condition 1 (eyes open), and condition 2 (eyes
closed), both the platform and the surround remain immobilized.
Under condition 3, the surround moves. Under condition 4, the
platform moves and the surround remains fixed. Under condition
5, the platform moves while the subject keeps his/her eyes closed.
Under condition 6, both the surround and the platform move.
1
Device off
Device on
23
Conditions
456 Comp.
30
40
50
60
70
80
90
100
Figure 2: Equilibrium score.
move (Figure 1). Adaption scores of 6 conditions as well as
the composite equilibrium were evaluated. The composite
equilibrium score is a mathematical-analytic indicator of
balance.Itiscalculatedbyindependentlyaveragingthescores
achieved under conditions 1 and 2, adding these two values
to the sum of all three scores under sensory conditions 3, 4,
5, and 6, and then dividing this sum by the total number of
trials. The highest possible score is 100. It is the best means
of providing an overall impression of how an individual
organizes sensory information.
In sensory analysis, use of an algorithm enables the bal-
ance functions (visual, vestibular and somatosensory) to be
considered separately (Table 1). The term “visualpreference”
isalsointroducedinthisconnection;thisdescribestheability
to suppress visual information perceived as incorrect.
The MCT assesses automatic motor responses. A
sequence of unexpected forward and backward translational
movements and slight tilting movements of the platform

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International Journal of Otolaryngology3
Table 1: Algorithms for calculating the individual components of balance (visual, somatosensory, vestibular, and visual preference) and
short explanation.
Comparison
Quotient
SOT2/SOT1
Quotient
SOT4/SOT1
Quotient
SOT5/SOT1
Quotient
SOT3+SOT6/SOT2+SOT5
Short explanation for functional relevance
Patient’s ability to use input from the somatosensory system to
maintain balance
Somatosensory system (SOM)
Visual system (VIS)
Patient’s ability to use input from the visual system to maintain
balance
Vestibular system (VEST)
Patient’s ability to use input from the vestibular system to maintain
balance
Visual preference (PREF)
The degree to which a patient relies on visual information to
maintain balance, even when the information is incorrect.
1
Device off
Device on
23
Conditions
456
30
40
50
60
70
80
90
100
Figure 3: Strategy analysis.
(alsoforwardandbackward)elicitsautomatic,evaluatemus-
cular postural responses. Measurements enable to evaluate
thesymmetryofweightdistribution,theresponsespeed(i.e.,
latency) and the intensity and symmetry. The main purpose
ofweightsymmetrytesting,whichdetectsashiftinthecenter
of gravity in both the static and dynamic situations, is to
aid the correct analysis of the latency measurements. If the
center of gravity is displaced to one side this may indicate
either false adaptation or a musculoskeletal deficiency, such
as muscular weakness or an orthopedic problem. If the
center of gravity is shifted, this compounds the patient’s
difficulty in maintaining stability: larger movements are only
poorly tolerated. Unilateral latency abnormalities, that is,
extended period, indicate a localized disorder in the spinal
cord, brainstem or subcortex. Latency is defined as the time
(milliseconds) between the onset of a horizontal movement
ofthesupportsurfaceandthepatient’sactiveresponsetothis
movement. If such delays are observed bilaterally, then the
disorder is global and central. Greater latencies in only one
direction point to a disorder in the efferent limb. A score of
100 indicates that the weight is evenly distributed over both
legs. Theoretically possible scores range from 0 to 200, with
the extreme value indicating that weight is placed on only
one leg.
The strategy analysis was performed in oder to evaluate
the relative contributions of movement about the ankles and
Som
Device off
Device on
VisVes Pref.
40
50
60
70
80
90
100
Figure 4: Sensory analysis of the overall test population.
the hips that are required to maintain balance during the
test. The sole use of the ankles to keep balance generates a
score of 100%, whereas if only the hips are used the resulting
score is 0%. Normally only ankle movements are required
to maintain balance. It is not until the limits of the capacity
for compensation are reached that hip movements are also
employed to achieve compensation.
2.3. Procedure for the EquiTest. The patients were famil-
iarized with the test procedure and secured using a safety
harness. The EquiTest assessment was first carried out
with the cochlear implant switched off. The room was not
sound-isolated, but there were no sources of noise. Before
each section of the trial, the patients were given written
instructions detailing what would follow. After a single test
run there was a break of 10–15 minutes’ duration before
the second test run was carried out with the cochlear
implant switched on. With a direct cable connection to the
speech processor established, a CD player was used to play
white noise as per the CCITT G.227 standard (“Comit´ e
Consultativ International T´ el´ egraphique et T´ el´ ephonique”
= Consultative Committee for International Telegraph and
Telephone), for which the patients were able to adjust
the volume to the subjective setting “very loud, but not
unpleasant”.

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4 International Journal of Otolaryngology
Small
Backward
Device off
Device on
Large Medium
Forward
Small
Backward
Large MediumComposite
0
20
msec.
40
80
60
100
120
140
160
LeftRight Forward
Figure 5: Latency periods exhibited by the overall test population for forward and backward translational movements (separate data for
each foot) and for small, medium, and large movements of the force-plate.
Small
Device off
Device on
Large Medium
Backward
Small LargeMedium
Forward
100
100.5
101
101.5
102
102.5
103.5
103
104
|
Figure 6: Weight symmetry results for all patients (small, medium,
and large movements of the posturography platform).
2.4. Data Collection and Statistical Analysis. The values for
weight symmetry and latency were taken for the vari-
ous backward and forward translational movements. The
compositevaluesforbothequilibriumscoreandlatencywere
incorporated into the analysis. Means were also determined
from the six adaptation scores. Both weight symmetry and
adaptationscoresthatfelloutsidethenormalrangewerealso
included.
The results from the EquiTest under acoustic stimulation
were compared with the results without stimulation and
transferred to the Excel spreadsheet program, taking into
account the requirements of data protection. The statistical
analysis of the data was carried out using the Statistical
Program for Social Science (SPSS). The Wilcoxon test was
used in all cases to determine significance. The significance
level was set at 5% (P < .05); P < .01 is deemed highly
significant.
3.Results
3.1. Preoperative Status of Vestibular Organ. In 58% (n =
29) of the patients studied, the vestibular organ had a
normal level of excitability and no pathological nystagmus
was present.
Provocation nystagmus was found in 15 patients (30%).
Nystagmus abnormality was classifiable as pronounced (i.e.,
nystagmus of 6–15beats/minute was inducible in one of
the six defined positions) in six patients and as extreme
(nystagmus of more than 30beats/minute in several posi-
tions) in nine. In caloric testing, the response to excitation
was either reduced or absent in eight patients (16%). Thus,
the peripheral vestibular organ was nonexcitable in three
patients, possibly excitable in three, and hypoexcitable in
two. In most of these individuals, however, a pathological
response was seen in either caloric or nystagmus testing,
but not both; only two patients showed both abnormal
nystagmus and a pathologically abnormal caloric response.
In anamnesis, 36 patients (72%) reported that they had
no vestibular symptoms and 14 (28%) affirmed that they
did, although most described these as only temporary. Of
the latter, nine correlated with a pathologically abnormal
nystagmic or caloric response.
3.2. Postoperative Status of Vestibular Organ. Postoperatively,
vestibular abnormality was observed in only one patient;
here the status of the preoperatively normal vestibular organ
was classified as “possibly excitable”, but no pathological
nystagmus was detectable. Postoperatively 33 patients (66%)
said they had no vestibular symptoms, whereas 17 (34%)
reported temporary symptoms of dizziness; of these, only
eight patients (16%) also showed corresponding symp-
toms prior to surgery. Three patients (5%) complained of

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International Journal of Otolaryngology5
sensations such as parasthesia and irritation of the facial
nerve while the cochlear implant was activated.
3.3. The Sensory Organisation Test (SOT). Significantly
poorer equilibrium scores were obtained under conditions
1 and 2 (Figure 1) with, accordingly, lower strategy scores
under the same two conditions when electrical stimulation
was applied (Figure 2). Under condition 3 of the Sensory
Organization Test electrical stimulation produced no signifi-
cant changes compared to the same test without stimulation.
There was a remarked improvement in results under electri-
cal stumulation in condition 4 and 5. The strategy analysis
results obtained under conditions 4 to 6 were also signif-
icantly better. The functions of the somatosensory system
and the visual preference, as revealed by sensory analysis,
remain approximately the same under electrical stimulation
(Figure 3). Poor visual scores are also achieved when, with
the support surface in motion, the vestibular system—as
opposedtothevisualsystem—assumesadominantfunction.
3.4. The Motor Control Test (MOT). Taking the test popula-
tionasawhole,thelatencyperiodsforforwardandbackward
translational movements of the posturography force-plate
showed no significant differences (Figure 4). The values for
weight symmetry (two-scale test) reveal significant differ-
ences in terms of a better average distribution of body weight
between the two posturography force-plates (Figure 5) with
the CI activated. The dominating dextroposition of the
body’scenterofgravitywiththeCIswitchedoffisstriking,an
effect still observed—albeit less markedly—under acoustic-
electrical stimulation.
4.Discussion
Postoperative balance disorders are widely reported in
cochlearimplantrecipients.Differntetiologiesarepostulated
for these postoperative symptoms of dizziness. Van den
Broek et al. describes in his study perilymphatic fistula
inducedbycochlearfenestrationoradisruptionofendolym-
phatic flow caused by the electrode itself, which could lead
to an endolymphatic hydrops, similar to M´ eni` ere‘s disease
[3]. A mechanical irritation of the membranous labyrinth or
the labyrinthitis triggered by a foreign body in the cochlea
could also caused vestibular disoder in patients with cochlear
implant [4]. Thus, hyporeflexia of the vestibular organ may
be caused either by intraoperative damage [1, 4, 10–12] or by
a disorder that existed preoperatively [13, 14].
The effect of the electrical stimulation on the vestibular
system could be shown in different studies. As early as
the beginning of the 19th century Ritter and Augustin
independently published reports of dizziness symptoms that
were triggered by electrical stimulation of the moistened
outer ear [15, 16]. At higher currents eye movements can be
induced, the cause presumably being electrical stimulation
of the vestibular structures in the semicircular canal system
[17–19]. In experiments on cats (involving stimulation of
the round window) a lower threshold was required for
stimulating vestibular fibers than auditory nerve fibers [20].
These trials were facilitated by the fact that it is possible
in animal experiments to directly stimulate vestibular struc-
tures electrically. In guinea pigs vestibular potentials were
producedbyelectricalstimulationoftheroundwindow[21].
It is also known that, with the cochlear implant activated,
current may also spread beyond the cochlea. This was shown
by cochlear implant-induced stimulation of the facial and
glossopharyngeal nerves, during which patients complained
of pain [22].
In the present study under the dynamic/sensory condi-
tions 4 and 5, in which primarily the vestibular organ itself is
tested(bothincludingandexcludingthevisualsystem),there
was a marked improvement in results. Here it is specifically
the somatosensory system that is targeted for irritation by
the movement of the platform, this must then be centrally
compensated for.
Visual preference describes the extent to which the
patient relies on visual input to maintain balance, even
when this information is incorrect. Therefore, even poor
scores were obtained, the cause would not be disruption
to the individual components; rather, an adaptive problem
would be the likely explanation. In this study, however, the
visual and vestibular functions improved significantly. The
vestibularcomponentofthebalancesystemisnotassensitive
as the visual or somatosensory aspects. Nevertheless, the
EquiTest does not distinguish between a peripheral and a
central component. Moreover, the test is unable to reveal
vestibular disorders that are well compensated for. Poor
visualscoresarealsoachievedwhen,withthesupportsurface
in motion, the vestibular system—as opposed to the visual
system—assumes a dominant function.
In addition, enhanced attentiveness on the part of the
patients may have led to an improvement in the dynamic test
results. An alternative hypothesis is that acoustic orientation
during stimulation by the cochlear implant brings about
stabilization in vestibular performance. Significant improve-
mentsinvestibularfunctionwereseenmainlyinequilibrium
scores under conditions 4 and 5, the composite equilibrium
score, and the vestibular components as revealed by sensory
analysis. This effect could be caused by stimulation of
the inhibitory parts of the vestibular organ, explainable in
terms of their having greater sensitivity than the excitatory
portions.
Only under the static conditions are significantly poorer
scores achieved when stimulation is applied: both equilib-
riumscoresandstrategyscoresunderconditions1and2,and
thefunctionofthesomatosensorysystem(asrevealedbysen-
soryanalysis),aredetrimentallyaffected.Inprinciplethefirst
two conditions represent an objectification of conventional
vestibulospinal tests such as the Romberg and Unterberger
tests. It can therefore be assumed that vestibulospinal func-
tions are influenced. Another conceivable cause is irritation
during the initial phase, since conditions 1 and 2 of the
SOT are applied at the start of the test sequence. However,
none of the patients report substimulatory sensations when
switching on the cochlear implant or during the fitting
phase. It may be that the explanation for any symptoms
of dizziness lies precisely in the fact that they occur in
supposedly noncritical situations, since, when the cochlear

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6 International Journal of Otolaryngology
implant makes increased demands on the balance system,
induced disturbances can be centrally suppressed.
This indicates that the suppression of automatic
responses to disruptive environmental influences is largely
attributable to the somatosensory system. Similar results
could be shown in a study byEisenberg, 22 patients were
tested using ENG (spontaneous nystagmus, postural testing,
eye-tracking, thermal testing), coordination tests, and pos-
turography[23].Itwasshownthatasingle-electrodeimplant
did not significantly disrupt the balance system. Indeed, the
patients studied actually showed a subjective improvement
in postural stability through electrical stimulation, as was
corroborated in the present study with a representative
number of subjects.
Whentheresultsarebrokendownbypreoperativecaloric
response, strategy analysis reveals a significant improvement
under condition 3 in those patients with a normal preoper-
ative caloric response. In the attempt to maintain balance
under stimulation, the patients make greater use of ankle
movementsthanhipmovements,asisthecaseinunimpaired
individuals. No significant differences became apparent in
the preoperative nystagmus test and in the questioning of
patients as to their symptoms prior to the operation.
It is clear that dynamic posturography is a technique
which allows the effects of electrical stimulation by the
cochlear implant on the balance-maintaining system to
be demonstrated. In the present study could be shown,
that an activated cochlear implant may, to an extent, even
lead to a stabilization of balance function under the static
conditions of dynamic posturography, but nevertheless also
to a significant destabilization.
5.Conclusions
In summary, it can be affirmed that in our study electrical
stimulation affect the function of the vestibular system
and especially under the challenging test conditions of the
EquiTest—actually even led to vestibular stabilization. To an
extent, a learning effect may be a contributory factor here,
since testing under stimulation was carried out subsequent
to testing without stimulation.
This provides at least a partial explanation for the
occasionally reported vestibular symptoms experienced by
cochlear implant recipients. It also appears that, in many
cases, these problems are additionally caused by a preoper-
ative impairment of the vestibular system, attributable to the
primary condition that led to deafness.
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