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Performing Intracochlear Electrocochleography During Cochlear Implantation
Chapters
Summary March 8th, 2022
Electrocochleography (ECochG) measures inner ear potentials generated in response to acoustic stimulation. In cochlear implant (CI) candidates, such inner ear potentials can be measured directly with the implant electrodes. In this video, we systematically explain how to perform ECochG recordings during CI surgery.
Transcript
Our standardized guideline increases the reliability of intraoperative electrocochleography. Electrocochleography potentials reflect the residual function of the cochlear and can be used to monitor the implementation process. We have developed a standardized guideline to conduct intraoperative electrocochleography measurements.
This increases the reliability of these measurements and provides virtual insight into the state of the inner ear in a larger amount of patients. Performing intraoperative electrocochleography recordings is not trivial. Good communication during the entire integration is very important.
In addition, the system setup must ensure unimpeded sound transmission and good coupling between the receiving and stimulating coils. Mark the position of the processor, the implant and the skin incision. Inject the local anesthesia.
Check and clean the ear canal. Then, inspect the eardrum. Next, insert the sterile ear tip, which is connected to a sterile sound tube, deep into the external canal.
Place a large swab into the concha of the operated ear and tilt the ear forward. Fix the ear lobe, including the ear tip, sound tube and swab with a transparent adhesive foil. Check the functioning of the acoustic output before connecting the sound tube to the non-sterile sound transducer.
Cover the non-sterile part with a sterile blanket, ensuring that the sound transmission parts are tension-free. Incise the skin up to the temporalis fascia. Make an offset incision of the periosteal flap.
Dissect the mastoid plane and display the bony ear canal and hennoli spine for orientation. Dissect a tight subperiosteal pocket, accommodating the implant housing later on. Harvest a piece of dermal fat to seal the posterior tympanotomy and two to three small pieces of periosteum to seal the entrance point of the electrode into the inner ear later on.
Place the wound retractors and drill the mastoid bone with an overhang posteriorly after displaying the lateral skull base, cranially. Drill out the mastoid bone evenly with the deepest point of dissection above the antrum. Display the antrum with the lateral semicircular canal and then out the bony ear canal evenly until the short process of the incus is seen.
Drill the bone caudle to the lateral semicircular canal toward the mastoid tip, parallel to the expected facial nerve. Display the nerve and if possible, the chorda tympani. Access the middle ear via a posterior tympanotomy by drilling near the buttress, between the facial nerve and the chorda until the middle ear space is reached.
Enlarge the posterior tympanotomy caudally until the round window niche is visualized. Reduce the bony lip of the round window niche until the round window is seen completely. Drill an anterior step in the area of the planned implant housing position.
Drill a bony overhang of the mastoid cavity to accommodate the implant electrode array later on. Rinse the surgical site thoroughly and perform meticulous hemostasis. Then, pack the coil into a sterile sleeve.
Rinse the implant and insert it in the previously dissected subperiosteal pocket ensuring a stable implant position against the drilled bony step. Check that the ground and reference electrodes of the implant are well covered with soft tissue. Place the external coil above the magnet of the receiving coil.
Rotate the transmitting coil back and forth at an angle of 180 degrees to align the MR-compatible magnets. Start the EcochG software in intraoperative mode. Measure the wireless connection and when the connection is 100%fix the transmitting coil with an adhesive foil.
Inspect the middle ear again and ensure that the middle ear space is air-filled. Place a piece of gel foam to prevent blood entering the cochlea. Open the round window membrane.
After inserting the first electrode into the round window, perform an impedance check. Instruct the technician to record and clearly communicate the electrocochleography potentials, if there is a signal, how the signal evolves and if there are abrupt signal changes. Start the software in intraoperative mode and ensure that the settings are correct.
Use condensation polarity with a recording window of 9.6 milliseconds and set the measurement delay to one millisecond. Communicate each surgical step while the surgeon slowly inserts the electrode. And simultaneously, the technician measures EcochG potentials continuously.
As soon as the electrode is fully inserted, drape the electrode within the mastoid cavity and seal the round window with small pieces of the previously harvested periosteum. Stabilize the electrode within the posterior tympanotomy with a piece of dermal fat. Embed the electrode in the bony channel with some bone pate.
Stop the continuous EcochG recording and switch to post-insertion. Continue to record electrocochleography. After closing the wound in layers, remove the sound tube and ear tip.
Check for possible kinking or dislodgement. Check the ear canal and ear drum. Electrocochleography measurements were recorded during as well as after the electrode insertion.
During insertion, the maximum amplitude was recorded at the ninth inserted electrodes. After insertion, the maximum amplitude was measured at electrode seven. The authors performed intraoperative EcochG measurements in 12 patients.
An EcochG signal could be obtained in all 12 cases. The system setup must ensure unimpeded sound transmission of the acoustic stimulus and a good coupling of the transmitting and receiving coils. After the first electrode is inserted, an impedance measurement is necessary.
Measurements cannot be performed with high impedances. Electrocochleography is a promising tool to monitor inner ear function during cochlear implantation. These electrophysiological potentials complement the surgeon's assessment and haptic perception during hearing preservation surgery.
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