Here we present a protocol to conduct repeated impedance telemetry measurements during cochlear implantation (CI). They may allow conclusions on the electrode’s and implant’s function. Repeated impedance measurements enable objective feedback on whether the electrode is positioned inside the perilymph or outside the inner ear.
Impedance measurements are routinely performed during cochlear implantation (CI) after finalized electrode insertion. They may allow conclusions on the electrode’s and implant’s function. In the postoperative setting, the analysis of impedance changes enables the identification of scarring or inflammation processes around the electrode. Recent studies report associations between impedance telemetry and the site of stimulation. Consequently, repeated impedance measurements during cochlear implant electrode insertion may enable objective feedback on whether the electrode is positioned inside the perilymph or outside the inner ear. With the presented novel method, impedances can be measured in real-time during cochlear implantation. This protocol systematically explains how to perform repeated impedance recordings during CI surgery. These repeated measurements are challenging since they depend on multiple intraoperative methodological factors starting with the draping of the patient. Thus, for successful recordings, a standardized procedure is mandatory. In this article, we comprehensively illustrate the system setup and procedure of performing intraoperative measurements during CI surgery.
Preserving residual hearing is an increasing topic of interest in cochlear implantation (CI) surgery, and the indication has changed towards candidates with functional residual hearing. Thus, measurements that may objectify the position of the electrode array and the resulting potential intracochlear damage during surgery are becoming increasingly important. CI-users with successfully preserved hearing have been shown to have superior hearing performance with the implant after surgery, even when stimulated electrically-only1. Some of them may additionally benefit from acoustic stimulation (electro-acoustic stimulation; EAS). Perioperative hearing loss is assumed to result from traumatic insertion. To improve insights into these intraoperative changes and to establish monitoring algorithms, objective measures, and biomarkers are needed. In this context, impedance telemetry may be of interest2,3. Increased impedances have been shown to be associated with hearing loss or vertigo4,5. Further evidence associates blood inclusions during the insertion of the electrode array6. Nevertheless, further investigation is necessary to explore to what extent impedances may be associated with surgical trauma and postoperative performance. For this purpose, repeated intraoperative impedance measurements are a promising approach. On the other hand, impedances deliver additional information about the electrode's position. High impedances indicate poor conductivity and thus indicate a contact position outside the cochlea, whereas low impedances (impedance drops) may indicate already inserted contacts. Thus, impedances may be used as an objective feedback mechanism for the status of the electrode array insertion. In this video, we present our setup and first experiences with this novel approach of repeated impedance measurements using flexible lateral wall electrodes from the cochlear implant manufacturer MED-EL (Innsbruck, Austria)7.
A study software designed for research purposes is used to perform repeated impedance measurements. In this study, the software is verified according to the MED-EL internal procedures for research-use-only devices. During surgery, only the most recent impedance telemetry data is shown. Figure 1 shows the electronic measurement setup. The Insertion Monitoring (IM) software has buttons to mark the number of electrode contacts currently inserted (red/green highlighting). After starting, the software measures impedances repeatedly in cycles. The IM software shows a table of the measured impedance results and impedances across time in 12 plots. Furthermore, it shows warnings in case of connection problems. A video recording Software (Open Broadcaster Software [OBS]) is used to record (i) the video of electrode insertion (microscope attached, e.g., via HDMI), (ii) a video of the IM software user interface, including all user interactions and (iii) sound. An audio editor software (Audacity) is used to regularly play a sound during the insertion of the electrode array to facilitate a slow insertion.
This protocol was approved by the local Ethics Committee in accordance with the Helsinki Declaration (Ruhr-University Bochum: Reg.-No.: 21-7373; Medical University Innsbruck Reg.-No.: 1060/2021). Informed consent was obtained from all participants.
1. Preparation for the surgery
2. Surgery
For repeated impedance measurements during cochlear implantation, a standardized procedure is mandatory to achieve the highest possible reproducibility. The major aspects that have been considered to play an important role are the video quality as well as the insertion angle. Both may impede the visualization of the electrode contacts entering the round window and thus, the interpretation of the video for future analyses. Further, the placement of the receiver coil is crucial to prevent interruptions during insertion and thus, data loss. Consequently, the area of the receiving coil needs to be shaved, and the drapes need to be as thin as possible. Additionally, the implant body needs to be covered by soft tissue because it contains the ground electrode.
Using this measurement protocol, we performed measurements with 33 patients. Impedances were measured at the end of the second phase of a biphasic stimulation pulse in monopolar mode. Phase duration was 27 µs, stimulation amplitude was 150 µA when measuring at the stimulating contact, and 600 µA when measuring at nonstimulating contacts. All used electrodes contain 12 stimulation contacts that can be used for stimulation, but also for voltage measurements. Therefore, in total 144 single measurements were performed for retrieving a 12 x 12 voltage matrix. Every 2s the whole matrix measurements was repeated. We found a good visual alignment between the surgeon's annotations, the video, and the drop of impedances (Table 1, and Figure 2). Thus, we hypothesize that the method may be used as an objective insertion marker and feedback mechanism for monitoring the status of electrode insertion. The audio signal is considered an additional helpful approach to guide the insertion speed that was chosen to be approximately 10 s per electrode contact in this protocol (corresponding to approximately 4 mm/s). It thus is comparable to medium insertion speed in robotic surgery9.
In one case using this method, electrode extrusion during the closing of the wound was suspected by increased impedance values and verified by the clinical software. As a consequence, the wound was re-opened, and an extrusion of the electrode was verified. A fixation clip was used to ensure the electrode array's position inside the cochlea and the posterior tympanotomy. In the postoperative CT scan, the electrode array was completely recessed inside the cochlea. Additionally, the impedances dropped to a lower level postoperatively.
Figure 1: Measurement setup. The sketch shows the two computers used for the measurements. The study computer uses the IM software to send the stimulation and recording commands via the MAX Box to the implant and to receive the results. The results are displayed on the study computer screen. The Video recording software, which is running on the same computer, records both the IM graphical user interface as well as the microscope image, which is connected using an appropriate adaptor cable. Since the same computer is needed, both are saved synchronized using the study computer timestamp. In order to separate the study data from the clinically necessary data, a second computer (the regular OR computer) runs the MAESTRO fitting software. The MAX coil is shown with a dashed line since the exact same coil (equipped with a sterile sleeve) is used subsequently to connect to both computers via the MAX Boxes. Please click here to view a larger version of this figure.
Figure 2: Insertion monitoring tool. (A) Screenshot of the insertion monitoring tool. The contact row indicates the number of inserted contacts. Green indicates an already inserted electrode contact, and red indicates an electrode outside the cochlea. Buttons with arrowheads enable marking the next electrode contact as inserted when pushing forwards or marking an extrusion by pushing backward. The impedances of the individual electrode contacts are indicated separately and are plotted against the time starting at the insertion of the first electrode contact. (B) Insertion video as displayed from the surgeon's view. The example shows an impedance drop of electrode contact number 1 at 10 s. Further, the impedances of electrode contacts number 2 and 3 have already dropped. Electrode contacts number 4 is just inserted. Please click here to view a larger version of this figure.
Patients (n = 33) | ||
Mean age – years (SD) | 53.2 (19.6) | |
Sex – n (%) | Female | 20 (60.6) |
Male | 13 (39.4) | |
Full insertion – n (%) | Yes | 33 (100.0) |
No | 0 (0) | |
Electrode array type | Standard (31.5 mm) | 19.0 (57.6) |
FlexSoft (31.5 mm) | 4.0 (12.1) | |
Flex28 (28.0 mm) | 10.0 (30.3) |
Table 1: Patients' demographics and outcome.
Repeated impedance measurements are a promising approach to gain real-time feedback from the cochlea during the insertion process. They indicate which electrode contacts are positioned inside the perilymph or not. With the here presented novel method for flexible lateral wall electrodes (MED-EL, Innsbruck, Austria)7, impedances may be measured in real-time during cochlear implantation utilizing the inserting electrode array. However, for reliable measurements, a standardized procedure is crucial.
This protocol describes an adaption of existing impedance measurements that are usually performed after electrode insertion. The modification mainly focuses on the repeated measurements during the insertion process giving second feedback on the insertion progress together with visual feedback from the surgeon. Constant communication between the surgeon and the clinician, engineer, or audiologist during the insertion progress is of particular interest to reporting the progress of insertion and the ongoing measurements. Concerning the system setup, a proper connection between the implant and the telemetry coil needs to be guaranteed.
The successful application of the method depends on a proper connection between the implant and the telemetry coil and the ability to measure impedances. Thus, the draping is of particular importance. Further, changes in intracochlear impedances, e.g., due to air bubbles, may impede these measurements and lead to high impedances even when the electrode is already inserted. Thus, the visual feedback from the surgeon and the video file is mandatory for later annotations and insertion analysis, and the use of suction has to be avoided. However, from our preliminary results, in all of our cases, the high impedances immediately dropped during surgery. Thus, we hypothesize that the fast resolution may indicate the occurrence of air bubbles during insertion.
The visual feedback by the surgeon can be limited by the exposure of the round window niche. In order to perfectly control the number of inserted contacts, a perpendicular view of the round window niche is necessary. However, this is not always possible, depending on the size of the posterior tympanotomy. Thus, an inaccuracy of the reported electrodes, especially of the middle part, may result. This inaccuracy can be limited with a wider posterior tympanotomy.
Patients with successfully preserved hearing during CI surgery have been shown to have better hearing performance with the implant. Perioperative hearing loss is assumed to result from traumatic insertion10,11,12,13,14. To improve insights into these intraoperative changes and to establish monitoring algorithms, objective measures are needed. Currently, electrocochleography is the method of choice to monitor hearing preservation during surgery15,16,17. However, this method depends on functional residual hearing as well as an additional setup for acoustic stimulation during surgery. In this context, impedance telemetry may be of interest since impedance measures rely on the CI system only without any additional equipment2,3. Postoperatively increased impedances have been shown to be associated with hearing loss or vertigo4,5. Further, there is evidence that they may be associated with electrode translocations18 or blood inclusions that can occur during implantation and are considered deleterious for hearing preservation6. Consequently, it has to be further investigated to what extent impedances may be associated with surgical trauma and postoperative results.
Secondly, there is evidence that impedances are associated with the electrode position inside the cochlea3,19,20,21. Consequently, they are a promising approach to gaining information about the insertion depth. Repeated measurements may serve individually adjusted insertion depths that have a clinical consequence in anatomy-based fitting approaches.
Repeated impedance measurements are able to provide objective feedback to the surgeon during the implantation process. One example is that the method is able to detect impedance increases during the closing of the wound that may indicate an electrode extrusion. Thus, the wound may be re-opened during surgery without the need for additional surgery and narcosis. Further, the method may reduce the need for postsurgical radiation exposure for position control. Since the method currently presents a preliminary approach, conclusions on the adjustments concerning intracochlear trauma may not be drawn so far. However, an adjustment that may be reasonable to assess in the future is the reduction of insertion speed in cases of high impedances to distinguish from air bubbles and bleeding or scalar dislocation.
The protocol does not lead to severe changes in the common surgical method. The only additional effort that has to be taken is the positioning of the receiver coil and the consequent communication with the clinician, engineer, or audiologist. However, we consider the advantages of the repeated measurements highly superior to the potential extra effort and even consider increased communication also to increase the safety of the procedure.
The authors have nothing to disclose.
None.
Audacity | Open source | https://www.audacityteam.org/ | Audio editor software |
Coil cable | Any appropriate brand | Implant interface | |
Computer | Any appropriate brand | ||
Electrode array | MED-EL | https://s3.medel.com/pdf/21617.pdf | Standard, FlexSoft, Flex28 |
IM Software | MED-EL | https://www.medel.com/ | |
Maestro | MED-EL | https://www.medel.com/ | |
MAX Interface USB | Any appropriate brand | Interface connection | |
Octenisept | SCHÜLKE & MAYR GmbH | N/A | |
Open Broadcaster Software | Open source | https://obsproject.com/ | Video recording software |
Spongostan | Ethicon | N/A | Resorbable sponge |
Ultracain 1% | Suprarenin, Sanofi | N/A | Local anesthesia |