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.

Representative 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 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.

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 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 insertion^{10,11,12,13,14}. 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 surgery^15,16,17. 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 equipment^2,3. Postoperatively increased impedances have been shown to be associated with hearing loss or vertigo^4,5. 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 cochlea^3,19,20,21. 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.

Materials

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