Performing Repeated Intraoperative Impedance Telemetry Measurements during Cochlear Implantation

Summary

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.

Abstract

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.

Introduction

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 interest^2,3. Increased impedances have been shown to be associated with hearing loss or vertigo^4,5. 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.

Protocol

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

1. Make sure to obtain audiological testing reports, including pure-tone audiometry, objective testing (e.g., brainstem evoked response audiometry, speech testing), and vestibular diagnostics (e.g., video head impulse test or caloric testing). Obtain radiological imaging reports, including high-resolution computed tomography (HRCT) and magnetic resonance imaging (MRI) of the skull base, as preparation and indication for surgery.
   NOTE: MRI is required to secure the presence of the vestibulocochlear nerve and the patency of the cochlea. In the HRCT, anatomic variances (e.g., of the facial nerve) or malformations may be identified.
2. Check the required hardware and software for impedance measurements (i.e., IM software, video recording software, MAESTRO).
3. Obtain written informed consent from the patient.
4. Perform general anesthesia considering the clinical history and risk factors of the patient as per the decision of the anesthesiologist.
5. Position the patient's head in such a way that the mastoid segment of the facial nerve runs approximately horizontally. Achieve this by reclining the head and slightly positioning it to the opposite side.
6. Shave the hair in the retro-auricular region (approximately 3 cm) to avoid the penetration of hair into the situs.
7. Install facial nerve monitoring. Insert the monitoring needles into the obicularis oris muscle and the orbicularis oculi muscle. Place the reference electrodes on the thorax or shoulder. Make sure to fix the cables in order not to extract them accidentally during surgery.
8. Disinfect the surgical site. Use disinfectant (e.g., Octenisept) according to the clinic's standard.
9. Use sterile foil draping to cover the microscope and the patient. Ensure that the cover is as thin as possible in the area of the planned receiver coil position to avoid connection problems between the transmitting and receiving coil. Do not use hair wraps made of cloth as used in many ORs. Utilize self-sticking foil draping and make sure to avoid multi-layering.

2. Surgery

1. Preparing for the implant
   1. Mark the position of the processor and the skin incision of approximately 8 cm. Ensure enough space between the auricle and the implant housing.
   2. Inject local anesthesia (e.g., Ultracain 1% -Suprarenin, Sanofi, Paris, France) in an s-shaped area behind the auricle before skin incision.
   3. Incise the skin up to the temporal fascia using a scalpel. Dissect the periosteum and display the outer ear canal and Henle's spine.
      NOTE: The surgical instruments are variable and not decisive for the presented method. Appropriate surgical instruments can be used to perform the surgical steps.
   4. Set the wound retractors on the opposite of the planned implant side to avoid later interaction (magnetic attraction/electric interference) with the telemetry coil.
   5. For cochlear implantation, perform antrotomy, mastoidectomy, and posterior tympanotomy⁸.
   6. Expose the dura to the middle cranial fossa as a first landmark using a large cutting burr.
      NOTE: The size of the burr may vary depending on the surgical site.
   7. Thin out the posterior canal wall of the outer ear canal with the same burr.
   8. Expose the short incus process in the antrum and identify the lateral semicircular canal.
      NOTE: Both antrum and semicircular canal are identified visually during the drilling procedure.
   9. As a next surgical step, expose the chorda facial angle where the chorda tympani leaves the facial nerve. In this step, use a large diamond burr and drill the bone parallel to the expected facial nerve.
      NOTE: The size of the diamond burr may vary depending on the surgical site.
   10. Open the facial recess with a small diamond burr. Leave the facial nerve with thin bony coverage.
   11. Access the middle ear through posterior tympanotomy.
   12. Enlarge the posterior tympanotomy caudally until the round window niche is visualized.
   13. Maximize the posterior tympanotomy for great exposure of the middle ear, optimizing the visualization of the insertion process. Remove the bony overhang of the round window niche using a small diamond burr until the round window membrane may be visualized completely.
       NOTE: When moving the ossicular chain, a movement of the round window membrane may be seen to clearly identify the round window membrane. Make sure not to touch the membrane with the burr or accidentally open it.
   14. Before opening the membrane, put some adrenaline (1 mg/mL) in the middle ear and protect it from bone dust with a resorbable sponge (e.g., Spongostan).
2. Positioning the Implant
   1. Prepare the implant housing by drilling a bed into the bone in the future position of the receiving coil. Use the implant housing dummy provided by the manufacturer to mark the borders of the implant bed. Avoid slipping of the implant by creating an edge with the rosen burr. Use a diamond burr to smooth the surface.
      NOTE: The edge of the implant housing prevents the movement of the implant toward the mastoidectomy defect.
   2. Pack the coil into a sterile sleeve since the coil is placed already at the beginning of the insertion process.
   3. Position the implant in the drilled implant bed, fix it with sutures, and cover it with the muscle flap.
   4. Replace the retractors to avoid any magnetic interaction with the stimulating coil.
3. Implantation and impedance measurement
   1. Use two computers - one for Maestro and one for IM recordings.
   2. Start the IM-software.
   3. Place the telemetry coil above the magnet of the receiving coil. Secure it with a clamp to avoid movements and disconnections.
   4. Wait for the clinician, engineer, or audiologist to start the software and give feedback about the coupling. Change the position of the receiver coil until a secure connection is achieved.
   5. Ensure that no bleeding impedes the visualization of the insertion process or enters the perilymph.
   6. Communicate the visibility of the surgical field on the screens. Make adjustments to the optical focus of the microscope so that the clinician, engineer, or audiologist can follow the insertion process.
   7. Ensure the facial recess is as dry as possible to avoid early false positive measures.
   8. Open the round window membrane with a needle.
   9. Avoid any suction nearby the round window.
   10. Do not touch the electrode array.
   11. Communicate the start of insertion to the clinician, engineer, or audiologist so that he/she may adjust the status buttons. Instruct the engineer to mark one status button after the other as the electrode is inserted.
   12. Insert the first electrode contact into the cochlea. Wait for the feedback of the clinician, engineer, or audiologist and the audio signal to proceed. Apply an audio signal every 10 s to enable a constant and slow insertion speed with an overall insertion time of approximately 2 min.
       1. For the auditory signal, play a sine wave signal of 500 Hz constant pulsing on and off and repeat it every 10 s using the software Audacity. Do not adjust the timing of the signal and pauses according to the insertion process but keep it constant.
   13. Insert the electrode array slowly to avoid cochlear damage. Advance one electrode contact every 10 s (as guided by the sound signal).
   14. Inform the clinician, engineer, or audiologist about the progress by clearly communicating the number of contacts inserted so that the clinician, engineer, or audiologist can mark the progress in the software as precisely as possible by pressing the status buttons of the IM tool.
   15. Instruct the clinician, engineer, or audiologist to mark the number of inserted electrode contacts according to the feedback of the surgeon (Figure 2).
   16. Continue the measurements until full insertion to observe impedance drops.
   17. Communicate the surgical steps.
   18. Store the electrode lead in the mastoid cavity. Make sure to avoid extrusion forces by finding a tension-free position.
4. End of surgery
   1. Seal the round window niche with fascia.
   2. Check the integrity of the implant (impedance telemetry, electrically evoked stapedius reflex thresholds, if applicable, and electrically evoked compound action potentials).
   3. Close the wound in layers.
   4. Continue repeated telemetry measurements until the wound is fully closed. Avoid a dislocation of the stimulating coil.
   5. Check the eardrum and the skin of the outer ear canal for any surgical damage before ending the surgery.

Results

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...

Discussion

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 cruc...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
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