Performing Repeated Intraoperative Impedance Telemetry Measurements during Cochlear Implantation

We are working with impedances since they provide an objective feedback on whether the cochlear implant electrode array is located outside the cochlear or inside the cochlear. High impedances indicate or suggest a poor conductivity, and thus, an electrode contact outside the cochlear. Various lower impedances suggest that the conductivity is better, and thus, the electrode contact already inserted.

And this information may be used to reduce radiation exposure in the aftercare of cochlear implantation aftercare. Hearing preservation is an increasing topic of interest in the cochlear implantation field, and with these methods that objectify the position of the electrode array inside the cochlear, and with this, the potentially occurring intracochlear damage are increasingly becoming very important. The current methods for the intraoperative monitoring of the electrode array insertion and the hearing preservation are the electrocardiography and the here presented impedance telemetry.

The major task to overcome are unexpected high impedances. These high impedances resolve with time and electrostimulation. Nevertheless, they negatively influence the real-time measurements.

We established a method to perform repeated and continuous impedance telemetry measurements, and they open up a whole new perspective on gaining data already during the insertion of a cochlear implant electrode array. To start surgical implant preparation, recline the patient's head slightly to the side, ensuring the facial nerve mastoid segment runs horizontally. Next, shave the hair in the retroauricular region to prevent it from penetrating the situs.

Cover both the microscope and the patient with self-sticking, single-layered sterile foil draping. Now prepare for the implantation by marking the position of the processor and an eight-centimeter long skin incision. Place the wound retractor opposite to the planned implant to avoid telemetric coil interaction.

After performing antrotomy, mastoidectomy, and posterior tympanotomy, proceed with the cochlear implantation. Now using a large cutting burr, expose the dura to the middle cranial fossa as a first landmark. Then thin out the posterior canal wall of the outer ear canal.

Next, expose the short incus process in the antrum, and identify the lateral semicircular canal. Use a large diamond burr to drill the bone parallel to the expected facial nerve path. This exposes the chorda-facial angle where the chorda tympani exits the facial nerve.

Next, open the facial recess with a small diamond burr, ensuring to leave the facial nerve with thin, bony coverage. After performing a posterior tympanotomy, enlarge it caudally until the round window niche comes into view. Maximize the posterior tympanotomy to ensure great exposure of the middle ear.

Then remove the bony overhang of the round window niche until the round window membrane is completely visible. To perform implant positioning, start by drilling a bed into the bone at the planned position of the receiving coil. Use the manufacturer-provided implant housing dummy to mark the borders of the implant bed.

Create an edge with the Rosen burr to prevent implant slippage. Use a diamond burr to smoothen the surface. Next, pack the coil into a sterile sleeve.

Position the implant in the drilled implant bed. Secure it with sutures, and cover it with the muscle flap. Finally, replace the retractors to avoid any magnetic interaction with the stimulating coil.

Engage two computers, one for operating the MAESTRO and one for impedance measurement. Proceed to launch the impedance measurement software on its dedicated computer. Next, position the telemetry coil directly above the magnet of the receiving coil.

Ensure to secure it with a clamp to prevent any movement or disconnection. Secure a reliable connection by adjusting the position of the receiver coil when the engineer provides coupling feedback. Report the visibility status of the surgical field visible on the screens.

Next, use a needle to open the round window membrane. Communicate the insertion's commencement to the engineer so that they may adjust the status buttons accordingly. Ensure the engineer marks the status buttons in succession as the electrode is inserted.

Insert the initial electrode contact into the cochlea, pausing for feedback and the audio signal. Every 10 seconds, apply a constant pulsing sine wave audio signal of 500 hertz, maintaining a steady insertion pace with an overall two-minute insertion time. Now insert the electrode array slowly to avoid cochlear damage.

Advance one electrode contact every 10 seconds. Ensure that accurate information about the number of contacts is input into the impedance measurement software. Instruct the engineer to note the number of inserted electrode contacts using the surgeon's feedback as a guide.

Continue measurements until the electrode is fully inserted in order to observe any impedance drops. At the end of the surgery, store the electrode lead safely within the mastoid cavity. Continue repeated telemetry measurements until the wound is fully closed, avoiding the stimulating coil dislocation.

A good visual alignment between the surgeon's annotations, the videos, and the drop in impedances was observed. Thus, it is hypothesized that the method may be used as an objective insertion marker and feedback mechanism for monitoring the electrode insertion status, especially in cases where visual feedback is limited.