Cochlear implant recipients often complain of postoperative symptoms of dizziness [
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 [
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 [
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 possible effect of electrical-acoustic stimulation on vestibular function in CI patients by using the EquiTest. For this aim a large test population was used, allowing statistical analysis to be made.
A test population
The majority of the patients
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 nystagmus test, Roberg test, Unterberger stepping test, and caloric test. Postoperatively was the EquiTest performed.
A nystagmus test, the Romberg test, the Unterberger stepping test and caloric testing were carried out 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.
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. [
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 move (Figure
Test conditions SOT 1 to 6 under which equilibrium score 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.
In sensory analysis, use of an algorithm enables the balance functions (visual, vestibular and somatosensory) to be considered separately (Table
Algorithms for calculating the individual components of balance (visual, somatosensory, vestibular, and visual preference) and short explanation.
Comparison | Short explanation for functional relevance | |
---|---|---|
Somatosensory system (SOM) | Quotient | Patient's ability to use input from the |
SOT2/SOT1 | ||
Visual system (VIS) | Quotient | Patient's ability to use input from the |
SOT4/SOT1 | ||
Vestibular system (VEST) | Quotient | Patient's ability to use input from the |
SOT5/SOT1 | ||
Visual preference (PREF) | Quotient | The degree to which a patient relies on visual information to maintain balance, even when the information is incorrect. |
SOT3+SOT6/SOT2+SOT5 |
The MCT assesses automatic motor responses. A sequence of unexpected forward and backward translational movements and slight tilting movements of the platform (also forward and backward) elicits automatic, evaluate muscular postural responses. Measurements enable to evaluate the symmetry of weight distribution, the response speed (i.e., latency) and the intensity and symmetry. The main purpose of weight symmetry testing, which detects a shift in the center 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 of the support surface and the patient’s active response to this 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 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.
The patients were familiarized 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 (“Comité Consultativ International Télégraphique et Téléphonique” = 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”.
The values for weight symmetry and latency were taken for the various backward and forward translational movements. The composite values for both equilibrium score and latency were incorporated into the analysis. Means were also determined from the six adaptation scores. Both weight symmetry and adaptation scores that fell outside the normal range were also 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%
In
Provocation nystagmus was found in 15 patients (30%). Nystagmus abnormality was classifiable as pronounced (i.e., nystagmus of 6–15 beats/minute was inducible in one of the six defined positions) in six patients and as extreme (nystagmus of more than 30 beats/minute in several positions) 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.
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 symptoms prior to surgery. Three patients (5%) complained of sensations such as parasthesia and irritation of the facial nerve while the cochlear implant was activated.
Significantly poorer equilibrium scores were obtained under conditions 1 and 2 (Figure
Equilibrium score.
Strategy analysis.
Taking the test population as a whole, the latency periods for forward and backward translational movements of the posturography force-plate showed no significant differences (Figure
Sensory analysis of the overall test population.
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.
Weight symmetry results for all patients (small, medium, and large movements of the posturography platform).
Postoperative balance disorders are widely reported in cochlear implant recipients. Differnt etiologies are postulated for these postoperative symptoms of dizziness. Van den Broek et al. describes in his study perilymphatic fistula induced by cochlear fenestration or a disruption of endolymphatic flow caused by the electrode itself, which could lead to an endolymphatic hydrops, similar to Ménière`s disease [
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 [
In the present study under the dynamic/sensory conditions 4 and 5, in which primarily the vestibular organ itself is tested (both including and excluding the visual system), 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 vestibular component of the balance system is not as sensitive 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 visual scores are also achieved when, with the support surface 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 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. 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 equilibrium scores and strategy scores under conditions 1 and 2, and the function of the somatosensory system (as revealed by sensory analysis), are detrimentally affected. In principle the first two conditions represent an objectification of conventional vestibulospinal tests such as the Romberg and Unterberger tests. It can therefore be assumed that vestibulospinal functions 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 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 posturography [
When the results are broken down by preoperative caloric response, strategy analysis reveals a significant improvement under condition 3 in those patients with a normal preoperative caloric response. In the attempt to maintain balance under stimulation, the patients make greater use of ankle movements than hip movements, as is the case in unimpaired 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.
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 preoperative impairment of the vestibular system, attributable to the primary condition that led to deafness.