Abstract
Objectives
The aim of the present study was to investigate the effect of the electrode insertion depth in vestibular function after cochlear implantation.
Material and methods
In a retrospective observational study design, 41 adult patients who had undergone cochlear implantation between 2006 and 2012 at a tertiary referral university hospital were included. The postoperative performed radiograph images of the petrous bone were acquired according to the Stenvers method. These were analyzed to determine electrode insertion depth. Pre- and postoperative subjective vertigo symptoms were assessed by a questionnaire. The function of the horizontal semi-circular canal was evaluated by caloric irrigation and the function of the sacculus was tested by using cervical vestibular evoked myogenic potentials pre- and postoperatively.
Results
The average electrode insertion depth was 464°. A certain variability of insertion depth existed among the different electrodes according to their designs. No statistical difference of the insertion depth was found between patients with or without vertigo. There was also no correlation between electrode insertion depth and alterations of the measurable vestibular function.
Conclusion
In our study the variability of insertion depth didn’t have a significant influence on subjective vertigo, horizontal semi-circular canal function or saccular function. Plain radiography is a rapid, simple and cost-effective method to determine electrode insertion depth after implantation. However the scalar position of the electrode cannot be analyzed in plain radiography, so that an interscalar dislocation as a possible influence in vestibular function remains undetected.
1
Introduction
Over the last years, the number of cochlear implantations (CI) has increased steadily. Not only patients with complete deafness but also patients with severe hearing loss benefit from cochlear implants. With new operative techniques and implant technology, severe postoperative complications such as meningitis, facial palsy, and cholesteatoma are very rare . Nevertheless, many studies have shown that CI has a negative effect on vestibular function and may cause vertigo symptoms . Possible risk factors such as age, sex, and cause of deafness have shown to be no significant predictors for the postoperative development of vertigo . Regarding the surgical approach, new findings by Todt et al. have shown, that the use of the round window approach may cause less damage to the vestibular receptors. Another study of 21 cases by Coordes et al. showed that the round window membrane approach with location of the electrode in the scala tympani caused no damage to the vestibular receptors. However, there are still unsolved questions about vestibular dysfunction and development of vertigo after CI.
In the present study we aim to analyze, with the help of conventional radiography, the role of cochlear electrode design with its different insertion depth as a possible risk factor for postoperative vertigo and loss of vestibular function.
2
Material and methods
2.1
Subjects
In this retrospective observational study, 41 adult patients were included. All patients underwent CI between 2006 and 2012 at a tertiary referral university hospital.
Inclusion criteria were first CI surgery and no vestibular complaints before surgery. Exclusion criteria were age less than 14 years, bilateral implantation and re-implantation. 26 (63%) were male and 15 (37%) were female. Average age at the time of surgery was 56 years (median 56 years, SD ± 19).
The cochlear implants used were from the manufacturers Cochlear® GmbH (Hannover, Germany) and MED-EL® GmbH (Innsbruck, Austria). The following Cochlear® electrodes were implanted: CI24RECA and CI24REST. The following MED-EL® electrodes were implanted: Standard, Flex28 and Flexsoft.
The operation was performed by five different surgeons. The choice of the surgeon was independent of this study and the same standard surgical technique with round window approach was used by all five surgeons.
2.2
Determination of the electrode insertion depth
The electrode insertion depth was determined using the plain radiographs performed postoperatively to control the position of the cochlear implant .
The radiographs of the petrous bone were acquired according to Stenvers. The patient sits in front of a vertical x-ray plate and his forehead, nose and zygoma of the implanted side are oriented at a 45° angle to the film. The analysis of the radiographs was performed according to the criteria of Xu et al. .
To determine the electrode insertion depth the following steps were performed: The anterior semicircular canal was identified on the radiograph (see Fig. 1 ). A vertical line was drawn from the apex of the anterior semicircular canal towards the vestibule (line 1). This vertical line crosses the electrode at the region of the round window, where the electrode array is inserted into the cochlea. Perpendicular to the vertical line, an additional line through the center of the electrode spiral was drawn (marked as line 2). The center of the electrode spiral is considered the center of the cochlea. Originating from this point, two arrows (marked as 3 and 3’) are set to determine the angle between the entry point at the round window and the end of the electrode array. The first arrow (3) crosses the point where line 1 crosses the electrode, as the entrance of the electrode into the round window. The second arrow (3’) crosses the most apical electrode band. The angle between the arrows (3 and 3’) is measured, here 150.43°.
The number of completed turns of the electrode inside the cochlea is counted and the measured angle is logically added or subtracted to the full turns. In Fig. 1 , the resulting insertion depth of the electrode is 569.75° (2 completed turns minus 150.43°).
2.3
Vertigo symptoms
The subjective vertigo complaints of the patients after the implantation were assessed by means of a questionnaire. The patients completed the questionnaire before and 4 to 6 weeks after the operation.
The questionnaire was developed in our institution, specifically for the assessment of vertigo symptoms in CI candidates and has been described previously .
2.4
Vestibular function tests
The patients were examined before and 4 to 6 weeks after the implantation in our vestibular laboratory:
- –
Horizontal semicircular canal (hSCC) function test
hSCC function was evaluated with videooculography (SensoMotoricInstruments, Teltow-Berlin, Germany) by using the caloric vestibulo-ocular reflex (VOR). First, the horizontal spontaneous nystagmus (in sitting position, 30° head anteflexion, eyes open in darkness) was measured. Then, the caloric response to bithermal stimulation according to Hallpike with 100 ml of 30 °C and with 100 ml of 44 °C water over 30 s was measured. During caloric testing, patients were supine with 30° elevation of the upper body (resulting in vertical position of horizontal canals). Nystagmus was recorded over 80 s. From a 20 s interval, the mean maximal slow phase velocity (mSPV) was determined, and the degree of canal paresis was evaluated according to Jongkees et al. .
- –
Sacculus function test
The saccular function was tested by using cervical vestibular evoked myogenic potentials (cVEMPs) , based on the method described by Basta et al. . The acoustic stimulus was an air conducted 500 Hz tone burst (115 dB SPL, 7 ms duration, 5/s) applied monaurally via headphones. The electromyographic response of the ipsilateral sternocleidomastoid muscle was recorded by means of surface electrodes (impedance < 3 kΩ). The active electrode was placed over the middle of the muscle, the reference electrode on the upper sternum. The ground electrode was placed on the forehead. The patient was sitting and turning his head by 45°, which created a steady tonic activity in the sternocleidomastoid muscle (according to 50–200 μV in the EMG). At least 130 measurements were averaged (gain 5000 ×; filter 20–1500 Hz). The amplitude was determined as the average from two measurements of the difference between the first positive peak and the first negative peak (P1 − N1) in μV.
2.5
Data analysis
The postoperative results of the caloric VOR and cVEMP examinations in the implanted patients were analyzed with regard to the insertion depth of the electrode.
The caloric responses were classified as normal, canal paresis (difference > 20%) and caloric loss (SPV < 5°/s after warm and cold irrigation).
An absolute value of cVEMP amplitude of < 0.5 was considered a postoperative loss of the sacculus function.
For statistical analysis, the t-test and Mann–Whitney test were used (p < 0.05; IBM® SPSS® Statistics, Version 22).
2
Material and methods
2.1
Subjects
In this retrospective observational study, 41 adult patients were included. All patients underwent CI between 2006 and 2012 at a tertiary referral university hospital.
Inclusion criteria were first CI surgery and no vestibular complaints before surgery. Exclusion criteria were age less than 14 years, bilateral implantation and re-implantation. 26 (63%) were male and 15 (37%) were female. Average age at the time of surgery was 56 years (median 56 years, SD ± 19).
The cochlear implants used were from the manufacturers Cochlear® GmbH (Hannover, Germany) and MED-EL® GmbH (Innsbruck, Austria). The following Cochlear® electrodes were implanted: CI24RECA and CI24REST. The following MED-EL® electrodes were implanted: Standard, Flex28 and Flexsoft.
The operation was performed by five different surgeons. The choice of the surgeon was independent of this study and the same standard surgical technique with round window approach was used by all five surgeons.
2.2
Determination of the electrode insertion depth
The electrode insertion depth was determined using the plain radiographs performed postoperatively to control the position of the cochlear implant .
The radiographs of the petrous bone were acquired according to Stenvers. The patient sits in front of a vertical x-ray plate and his forehead, nose and zygoma of the implanted side are oriented at a 45° angle to the film. The analysis of the radiographs was performed according to the criteria of Xu et al. .
To determine the electrode insertion depth the following steps were performed: The anterior semicircular canal was identified on the radiograph (see Fig. 1 ). A vertical line was drawn from the apex of the anterior semicircular canal towards the vestibule (line 1). This vertical line crosses the electrode at the region of the round window, where the electrode array is inserted into the cochlea. Perpendicular to the vertical line, an additional line through the center of the electrode spiral was drawn (marked as line 2). The center of the electrode spiral is considered the center of the cochlea. Originating from this point, two arrows (marked as 3 and 3’) are set to determine the angle between the entry point at the round window and the end of the electrode array. The first arrow (3) crosses the point where line 1 crosses the electrode, as the entrance of the electrode into the round window. The second arrow (3’) crosses the most apical electrode band. The angle between the arrows (3 and 3’) is measured, here 150.43°.
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