A State-of-the-Art Method for Preserving Residual Hearing During Cochlear Implant Surgery

The preservation of cochlear structures and possible residual hearing is one important factor to consider during cochlear implant surgery. Here, we present a state-of-the-art method for preserving residual hearing during cochlear implant surgery under local anesthesia.

The developments in surgical techniques and cochlear implant (CI) electrode design have expanded the indications for CI treatment. Currently, patients with high-frequency hearing loss may benefit from CIs when low-frequency residual hearing can be preserved, as this enables combined electric-acoustic stimulation (EAS). The possible benefits of EAS include, for example, improved sound quality, music perception, and speech intelligibility in noise.

The risks of inner ear trauma and a deterioration or even complete loss of residual hearing vary according to the surgical technique and the type of electrode array used. Short, lateral-wall electrodes with shallower angular insertion depths have demonstrated higher rates of hearing preservation than longer electrodes. The very slow insertion of the electrode array through the round window of the cochlea contributes to insertion atraumaticity and, thus, may lead to favorable hearing preservation results. However, residual hearing can be lost even after an atraumatic insertion.

Electrocochleography (ECochG) can be used to monitor inner ear hair cell function during the insertion of the electrode. Several investigators have demonstrated that the ECochG responses during surgery may predict postoperative hearing preservation results.

In a recent study, we correlated the patients' subjective hearing perception with simultaneously recorded intracochlear ECochG responses during the insertion. This is the first report evaluating the association between intraoperative ECochG responses and hearing perception in a subject undergoing cochlear implantation under local anesthesia without sedation. The combination of intraoperative ECochG responses with the patient's real-time feedback to sound stimuli has excellent sensitivity for the intraoperative monitoring of cochlear function. This paper presents a state-of-the-art method for the preservation of residual hearing during CI surgery. We describe this treatment procedure with the special consideration of performing the surgery under local anesthesia, which makes it feasible for monitoring the patient's hearing during the insertion of the electrode array.

Cochlear implantation is the only treatment in clinical use that restores the function of a sensory organ. Currently, cochlear implantation is applied to treat severe-to-profound sensorineural hearing loss in children and adults. The cochlear implant (CI) system consists of an implantable internal device that is combined with an external sound processor. The internal device is inserted during surgery through the mastoid cavity. The facial recess is opened to gain access to the middle ear and cochlea. The electrode array is inserted into the cochlea through the posterior tympanotomy and round window of the cochlea. The electrode array provides the electric stimulation by bypassing defective outer and inner hair cells and directly stimulating the auditory nerve's ganglion cells.

It has been demonstrated that the preservation of the integrity of the inner ear structures after CI surgery may contribute to more favorable hearing outcomes in comparison to electrode insertions, which can cause structural damage to the inner ear^1,2. This has led to the development of more delicate surgical techniques and thinner, more flexible, and thus, less traumatic electrode arrays. These developments have contributed to improved hearing outcomes, leading to an expansion of the indications for CI rehabilitation. Patients with high-frequency (>1.5 kHz) severe hearing loss may also benefit from cochlear implantation, especially when their natural hearing in the low frequencies can be preserved (<1 kHz). Low-frequency hearing after surgery makes it possible for the patient to benefit from electro-acoustic stimulation (EAS)^3,4. The EAS is a combination of electric stimulation to high-frequency areas of the cochlea, which are located in the basal parts of the cochlea, and acoustic amplification for the preserved low-frequency areas (typically 125-500 Hz) in the apical parts of the cochlea.

The best results in hearing preservation (HP) surgery have been achieved with shorter electrode arrays, specifically designed for EAS candidates^4,5,6. There may be one major drawback associated with the short electrode arrays-the loss of residual hearing (either after surgery or subsequently). The patient may ultimately benefit from a longer electrode array that covers a broader range of frequencies, especially in cases where there has been an unfortunate loss of residual hearing^5,7. Recently, the introduction of the partial insertion of the longer lateral-wall electrode in patients with moderate hearing loss at higher frequencies and normal or mild hearing loss at lower frequencies has created the option for initial shallow insertion in EAS candidates, but it also facilitates deeper insertion with the same array if the residual hearing is lost⁸.

Electrocochleography (ECochG) is a method that can be used to monitor cochlear function during cochlear implantation, and it has been increasingly exploited in clinical use. Currently, the three leading CI manufacturers provide a clinically approved tool, or at least a research tool, for intracochlear intraoperative ECochG measurements. One provides an all-in-one solution with the stimulation and response recording integrated into one tool, while the tools provided by the other manufacturers require a separate stimulator. ECochG measures the electrophysiological response from the inner ear and the auditory nerve to an acoustic stimulus. It seems that the amplitudes of the ECochG responses measured during the electrode insertion may predict postoperative hearing preservation. Thus, intraoperative ECochG seems to represent a promising tool to provide information that enables the prevention of trauma^9,10,11,12. Active research and the increased clinical interest in assessing intraoperative ECochG measurements have raised the question of how to analyze the ECochG responses objectively and how to react to the changes in the amplitude of the response during insertion.

The feasibility and benefits of CI surgery under local anesthesia have been reported in several studies^13,14,15,16. CI candidates with significant comorbidities can be operated on under local anesthesia, thus avoiding the risks related to general anesthesia. General anesthesia is associated with an increased risk of cognitive deterioration, especially in elderly patients, and it has also been associated with greater mortality in these groups¹⁷. When CI surgery is performed under local anesthesia, the recovery from the operation is faster, and, thus, the patient spends less time in the hospital as compared to with a similar operation performed under general anesthesia^14,15,18,19. It has also been proposed that patients with residual hearing could be able to monitor their hearing during the insertion, which may guide the surgeon during the insertion of the CI electrode and avoid intracochlear trauma¹⁴.

We recently compared ECochG and subjective sound perception as indicators of hearing preservation in CI surgery under local anesthesia²⁰. The results indicated a good prediction of hearing preservation with both methods, and similar patterns were observed between subjective reporting and ECochG monitoring. The patients' subjective feedback on sound perception, which was possible since only local anesthesia was in use, seemed to help minimize the risk of trauma to the inner ear during the insertion, even in cases when the ECochG responses could not be measured during the insertion.

Here, we present a state-of-the-art method for preserving residual hearing during CI surgery. We describe the treatment procedures, including the special considerations associated with performing the surgery under local anesthesia (e.g., ECochG) and subjective hearing monitoring.

The protocol was approved by the Institutional Review Board (5551877) and the Research Ethics Committee of the Northern Savo Hospital District (1690/2019) and was carried out according to the guidelines of the Declaration of Helsinki. Informed consent was taken from all of the patients who volunteered for the study.

1. Preoperative considerations

1. Assessment of the residual hearing threshold
   1. Ensure that the preoperative pure-tone threshold averages (PTA) are at 0.250-1 kHz at ≤75 dB and that the patient can clearly hear the stimulus at 500 Hz or 1,000 Hz at ≤100 dB (Figure 1).
2. Patient evaluation-inclusion and exclusion criteria
   1. Ensure that the patient is suitable for CI surgery to be performed under local anesthesia. Judge the suitability based on the patient's ability to be still and understand the communication during the surgery and the limitations of the surgery. Have the operating surgeon conduct a meticulous interview and examination in the outpatient clinic to assess whether the patient will be able to respond appropriately during the surgery.
   2. Check that the patient has normal anatomy in the temporal region based on preoperative imaging (magnetic resonance imaging [MRI] and computed tomography [CT]) and that there are no contraindications (e.g., missing cochlear nerve or obstructed cochlea) for the CI surgery.
      1. Open the software for the image viewer for the CT and MRI images, and from the dropdown menu, choose multi-planar reconstruction (MPR).
      2. Identify the key anatomical landmarks (mastoid cells, middle fossa dural plate, sinus sigmoideus, facial nerve, external ear canal, cochlea, internal ear canal) from the CT image stack.
      3. Open the preoperative MRI scan, and ensure the patient's suitability for cochlear implantation: the cochlea is open without any occlusions, and an intact cochlear nerve is found from the MRI scan.
3. Evaluation of the insertion depth and electrode selection according to the size of the cochlea
   1. Open the image viewer and preoperative CT scans. Then, choose MPR from the dropdown bar.
   2. Obtain the cochlea view from the CT images by aligning the images in the MPR in parallel with the cochlea.
   3. Assess the A-measures and B-measures: the cochlear size, the length from the round window through the modiolus, and the height perpendicular to the A line through the modiolus, according to Escudé et al.²¹, from the obtained cochlea view and at an insertion depth to 270° from the round window of the cochlea. Measure the distance to 270° by following the outer bony border of the cochlea.
   4. Choose an electrode length such that there are nine intracochlear electrode contacts with a 270°-300° insertion depth angle (IDA).

Figure 1: Preoperative audiogram. Indication thresholds are presented as a green zone. Please click here to view a larger version of this figure.

2. Perioperative preparations in the operating room (OR)

1. Ensure that there is sufficient space between the patient's face and the draping. In addition, check that communication with the patient is possible with their contralateral hearing aid (HA). Connect the HA wirelessly with a remote microphone given to the nurse. For the HA, use a long sound tube to prevent unpleasant feedback.
2. Alternatively, if the patient does not have the option for a contralateral HA, use a tablet computer to provide them with written communication. Test the communication with the patient.
3. Administer antibiotics and dexamethasone (e.g., 1.5 g of cefuroxime and 0.1 mg/kg dexamethasone) intravenously before the skin incision.
4. Insert the ECochG stimulator earphone into the patient's external ear canal prior to the incision. Seal the external ear canal in the earphone with bone wax (Figure 2).
   NOTE: Some manufacturers provide foam earphones in sterile packaging. Alternatively, the earphone can be inserted prior to the draping, and the tube can be fed under the draping to the ECochG computer.
5. Place an iodoform sheet over the operation area, and align the earphone and the sound tube away from the incision area. Take care that the iodoform draping does not bend the sound tube during the preparation.

3. Partial mastoidectomy, posterior tympanostomy, and drilling the implant bed with a surgical drill

1. Drill the partial mastoidectomy and posterior tympanostomy, and prepare the implant bed.

4. Insertion

1. For the ECochG preparation, ensure the presence of a medical physicist/clinical engineer in the OR, referred to here as the ECochG operator.
   1. Get the equipment ready for conducting the ECochG sound stimulation (see the Table of Materials) and response measurement through the CI; the setup varies slightly depending on the CI manufacturer.
      1. Connect the sound tube to the stimulator.
      2. Connect the implant coil and cable to the measuring device.
      3. Thread the measuring coil and cable into the sterile wrapping.
      4. Depending on the manufacturer, launch the measurement program from a laptop, or use a tablet-based system.
      5. The stimulus is a tone burst, and the burst length depends on the stimulus frequency. Select the stimulus frequency (usually, stimuli at the highest preservable frequency of 500 Hz or 1,000 Hz during insertion), the sound pressure to be used, and the stimulus amplitude (e.g., 80-100 dB nHL so that stimulus is well-heard and tolerable) from the ECochG software. Depending on the manufacturer, keep the other measurement parameters at the default values, or change them to the following: a measurement window of 6.5-9.7 ms with a delay up to 2 ms after stimulus onset, and an average of 40-150 recordings per data point. Ideally, repeat the stimulus with alternated polarity and responses with condensation, and subtract the rarefaction polarity to extract the cochlear microphonics component (Figure 3).
   2. Use the first or the second contact of the electrode for the ECochG measurement. Check the impedance of the measurement channel soon after the contact is in the cochlea and before the first measurement.
   3. Ask the OR assistant to go to the patient and repeat the instructions to them about the subjective reporting of the changes in the sound stimuli.
   4. Test the sound stimuli, and confirm from the patient that they can clearly hear the stimuli.
   5. Advise the patient to inform about any deviations, such as diminishing or disappearances of the perceived stimuli.
2. Surgeon preparations before insertion
   1. Take the CI device, and place the receiver/stimulator into the implant bed.
   2. Open the round window membrane of the cochlea. Make a slit with a hypodermic needle in the lower boundary of the round window, and carefully lift the membrane backward with a microsurgical hook. Flush the round window area with dexamethasone solution.
   3. Check the alignment of the CI lead wire within the mastoid cavity to avoid any rotational forces on the electrode and intracochlear structures from the lead wire after insertion.
   4. Obtain a good ergonomic position with the best support possible for the hands to facilitate a slow and controlled insertion.
3. Start of the insertion
   1. Have the surgeon inform the team that the insertion is starting, and make sure that everything is ready.
   2. Take the electrode with the insertion forceps, and start the insertion of the electrode into the cochlea through the round window.
   3. At the beginning of the insertion, first insert the electrode just inside the cochlea (one or two contacts inside the cochlea). Have the ECochG operator measure the impedance after the insertion of the first and/or second contact. If the impedance is very high, change the measuring channel.
   4. Have the ECochG operator start the measurement after the impedance measurements. Ensure the electrode is progressed very slowly with constant feedback from the person observing the ECochG responses (Figure 4).
      NOTE: Depending on the manufacturer, there might also be an option that the recording device provides auditory feedback-a tone that becomes louder with an increasing response. However, this tone along with other noise in the OR may disturb the patient's subjective evaluation of the perceived stimulus.
   5. Have the surgeon continue the insertion with 1-2 mm advancements (one electrode). Between these advancements, obtain feedback from the patient about the loudness of the perceived sound stimuli.
   6. If the ECochG responses become weaker or the patient reports a decrease in the perceived loudness of the sound stimuli, discontinue the insertion.
   7. Wait for 30-60 s, and then inquire about the stimuli from the patient. Simultaneously communicate with the ECochG operator about the current state of the ECochG monitoring (CM response).
   8. If either of the responses (the ECochG or subjective responses) do not recover when the electrode is motionless, draw the electrode back gradually by one to two contacts at a time (the value of ~2 mm is dependent on which brand of electrode is being used), and wait for the response each time. Draw the electrode back just enough until the responses have recovered.
   9. If the ECochG responses or the perceived loudness do not increase again after adjusting the insertion, but the desired IDA (270°-300°) (based on preoperative measurements and the number of electrodes inside the cochlea) has been achieved, discontinue the insertion. The situation is interpreted as a completed partial insertion.
   10. If the responses increase again, continue the insertion as described above (steps 4.4.5-4.4.7).
   11. Seal the round window with fat tissue gathered from the postauricular incision.
   12. In addition, use fibrin glue and bone dust to seal the groove for the lead wire between the receiver/stimulator and the mastoid cavity.
   13. Fix the lead wire inside the mastoid cavity with bone dust and fibrin glue.
4. Close the wound with surgical sutures. Monitor the patient after the operation during an overnight stay in the ward.

5. Postoperative care

1. Refer the patient to cone-beam computed tomography (CBCT) for postoperative imaging of the implantation.
   1. Verify the electrode's placement from the CBCT images.
      1. Open the CBCT images, and choose MPR from the dropdown list.
      2. Obtain the cochlear view in the MPR.
      3. Verify the IDA and the location of the electrode by scrolling through the images in the MPR. Check the number of intracochlear electrodes from the cochlear view to guide the fitting of the CI, especially in cases where there is a partial insertion, and identify the stimulating electrodes.
2. Refer the patient for a postoperative audiogram to measure the residual hearing on the first postoperative day after the surgery and 1 month later. Measure the residual hearing routinely during the fitting follow-ups.
   NOTE: The 1 month follow-up is more reliable than the first postoperative day in terms of the hearing preservation results.

Both subjective monitoring and ECochG may help prevent the occurrence of insertion trauma and, thus, provide better hearing preservation results postoperatively. In the audiogram, a decrease in the PTA_{(125-500 Hz)} within 15 dB of the preoperative hearing levels is considered to represent preserved residual hearing and, thus, a positive result after surgery. A negative result is the loss of residual hearing: a PTA_{(125-500 Hz)} change of over 30 dB from the preoperative hearing. All the data regarding the feasibility of preserving the residual hearing during cochlear implantation with this protocol were reported previously by Linder et al.²⁰. The role of ECochG in the prediction of insertion trauma has not yet been confirmed. On the one hand, the patient's subjective hearing seems to provide an additional tool with some advantages over ECochG, as it has no technical challenges and is generally always successful; however, on the other hand, it is highly subjective. The combination of both methods provides the opportunity to interpret the ECochG with respect to patient feedback, which can help the surgeon when making decisions during the insertion procedure. Based on the feasibility data for 10 patients, we believe that subjective feedback can predict the preservation of residual hearing. The hearing results with this protocol are presented in Table 1.

Figure 2: Insertion of the ECochG stimulator earphone into the patient's external ear canal prior to the incision and sealing of the external ear canal. (A) Earphone insertion and (B) sealing of the external ear canal and earphone with bone wax before draping the ear with an iodoform sheet. Abbreviation: ECochG = electrocochleography. Please click here to view a larger version of this figure.

Figure 3: ECochG settings for both commercially available programs. (A) Settings view for the Advanced Bionics software. (B) Settings view for the Medel software. Please click here to view a larger version of this figure.

Figure 4: Beginning of the insertions and example images from two different insertions with both commercial EcochG software types. (A) The view in the OR prior to the insertion, with the EcochG laptop in front. (B) Good ECochG responses with Advanced Bionics software (C) and with the Medel software. Please click here to view a larger version of this figure.

Table 1: Patients' ages, intraoperative monitoring status at the end of insertion, and hearing results as pure tone averages at 125-500 Hz pre-operatively and post-operatively. This table was modified from Linder et al.²⁰. The hearing preservation results are reported according to the classification used by Suhling et al.²³. Please click here to download this Table.

The optimal utilization of cochlear implantation is important for the effective management of hearing loss. The expansion of the indications for CI rehabilitation has created a grey zone where individualized decisions must be made regarding the rehabilitation modalities used. Nowadays, for patients with high-frequency hearing loss, there is a trend toward the provision of CIs. However, residual hearing in the low frequencies cannot be preserved in every patient. Cochlear monitoring via ECochG has been proposed as a way to improve hearing preservation. This paper presents a state-of-the-art method to achieve the reliable preservation of residual hearing during surgery by applying cochlear monitoring based on both electrophysiological and patient feedback responses.

Good, constantly increasing ECochG responses during the surgery have been reported to be related to better hearing preservation rates^{9,10,11,12,19}. A decline in the ECochG response seems to predict trauma and the loss of residual hearing, and this alerts the surgeon to adjust the insertion procedure. ECochG can be highly recommended for HP surgery under general anesthesia. The main drawback with ECochG is its occasional failure to record the responses, even though the preoperatively measured audiogram predicts good responses¹⁹. Even though ECochG is a valuable tool, the role of ECochG in preservation has not been fully explored, and more research is required.

The interpretation of ECochG is not always a straightforward procedure. The amplitude of the ECochG signal seems to be quite patient- and/or measurement-dependent. Therefore, the exact threshold for the accepted amplitude fluctuation has not been determined, and this warrants further studies. Measurement noise and artifacts from electrode array movement cause short-term decreases in the amplitude. In our experience, amplitude decreases resulting from pressure on the basilar membrane or cochlear damage are usually sustained at least until the array is moved to reduce the pressure.

Subjective hearing monitoring during electrode insertion is, to date, the most reliable method for avoiding cochlear trauma during the surgery. Of course, subjective hearing monitoring can only be done under local anesthesia without drug-induced sedation. As the insertion is carried out very slowly to avoid possible negative effects, and in order to be able to react to these negative effects if they occur, the patient has to be motionless for 7-10 min. Another important factor to take into consideration is the use of sound stimuli during the insertion, which should not exceed the comfort level of the patient's hearing.

The short lateral wall electrodes designed for hearing preservation are generally from 16 mm to 20 mm long. For the 16 mm electrodes, the typical IDA is 280° to 300°²², and these electrodes generally have good hearing preservation results and provide good hearing results with the EAS protocol. The results in terms of hearing preservation with standard-length LWs tend to be somewhat inferior compared to the results with short electrodes⁵. Thus, achieving at least the same IDA as would be achieved with short electrodes is considered sufficient for electric stimulation with EAS patients. A lower volume of the scala tympani in the deeper parts increases the risk of trauma during the insertion. The use of ECochG and subjective monitoring enables deeper insertion and enhances the possibility of hearing preservation.

The two most important factors for ensuring success in HP surgery under local anesthesia are the selection of the correct patient and the cooperation of the surgical team in the OR. It is crucial that the insertion should be performed in a very slow and stable manner, which requires that the patient should not move. A slow insertion is necessary so that the feedback from ECochG and subjective hearing can be verified and alterations to the insertion procedure can be performed before irreversible damage has occurred. There also needs to be seamless communication throughout the OR (between the surgeon, physician, nurses, and patient). The awareness of the current situation of the insertion helps everyone involved to orientate themselves and take the correct actions to ensure successful recordings and insertion. In addition, since the patient is conscious, it means that they are an active participant in the surgery and should be highly motivated to cooperate with the surgical team.

In the future, novel technical solutions to allow the patient to report changes in stimuli without having to speak would provide the surgical team with more convenient access to their subjective hearing state, and this could be automatically linked to the ECochG results. For example, this could be done by having an electronic switch that the patient could press to respond to changes in the loudness of the stimulus.

This is the first method utilizing subjective hearing feedback during cochlear implantation. Good subjective feedback during the operation seems to predict hearing preservation, and in addition, it creates possibilities for understanding the insertion dynamics and how this can be best linked with the information provided by ECochG. Nonetheless, it is evident that more research is needed to evaluate the possibilities of subjective hearing monitoring during the cochlear implantation procedure.

Overall, the simultaneous use of ECochG and subjective hearing monitoring under local anesthesia without drug-induced sedation and partial electrode insertion seem to be associated with reliable HP results postoperatively.

The authors report no conflicts of interest.

Aarno Dietz has received grants from the Academy of Finland (Grant number 333525) and from the North Savo Regional Grant. Pia Linder has received a grant from the Finnish Government research funding (Grant number 5551877). Matti Iso-Mustajärvi has received grants from the Finnish Government research funding (Grant number 5551876), the Instrumentarium Science Foundation, the North Savo Regional Grant, and The Finnish Society of Ear Surgery.