Key points
n in the setting of VS should completely change the treatment strategy for this disease(10). However, patient outcomes with CIs show wide variation—with some patients having no auditory perception, while others exhibit open-set speech understanding. The intraoperative testing of cochlear nerve function using electrically evoked brainstem response audiometry (eABR) provides a means of objectively assessing cochlear nerve conduction, and has been increasingly applied in this setting(11-14). In the present study, we aimed to assess outcomes of patients undergoing vestibular schwannoma resection and cochlear implantation, and to develop a new scoring system to preoperatively identify suitable patients for this treatment course. Appropriate preoperative classification of patients can have important impacts on patient counseling and expectations.

Design

Each study participant gave their informed consent to participate. This prospective study was conducted in accordance with the principles stated in the Declaration of Helsinki. The trial was approved by the local ethics committee 1111/2017, and is registered at www.clinicaltrials.gov (NCT03745560). All 17 patients underwent translabyrinthine vestibular schwannoma resection, during which eABR were recorded with an intracochlear test electrode (ITE) before and after tumor resection. In cases with positive responses after complete tumor resection, a CI was placed. In cases with positive responses before tumor removal and negative responses after tumor removal with the ITE, a placeholder electrode was inserted to avoid ossification of the cochlea, as recovery of the cochlear nerve and the option of a staged procedure has been described previously(15). In cases with negative responses both before and after tumor removal, no CI or placeholder electrode was placed. Overall, ten patients received a CI, three patients a placeholder, and four patients neither. All patients received a cochlear implant with a Flex 28 or Flex Standard electrode (MED-EL, Innsbruck, Austria) depending on cochlear duct length. All patients were followed-up for at least six months after surgery. MRI was conducted six months after vestibular schwannoma resection to evaluate for residual or recurrent schwannomas. Due to CI-related artefacts, special sequences were used for those patients(16).

Data availability statement 

The data that support the findings of this study are available from the corresponding author upon request.

Setting

The study was conducted at a tertiary care center by the department of Otorhinolaryngology, Head and Neck Surgery and the department of Neurosurgery.

Participants

Seventeen patients with unilateral sporadic VS were screened between January 2017 and January 2020 (see Table 1). Inclusion criteria were as follows: sporadic vestibular schwannoma, no ipsilateral functional hearing, desire to undergo cochlear implantation, and general good health. Exclusion criteria were prior treatment for vestibular schwannoma, neurofibromatosis type 2, history of pathology requiring multiple head magnetic resonance imaging (MRI), or history of malignant disease of the head and neck. All patients underwent preoperative imaging (MRI) and computed tomography (CT), pure-tone audiometry, a word recognition test (WRS), caloric testing, and a video head impulse test. WRS were evaluated using the Freiburg number, and monosyllable test. A lack of functional hearing was defined as ≤50% monosyllable recognition at 80 dB HL. Sixteen patients underwent preoperative eABR with a promontory stimulation electrode. Patient number one did not undergo this measurement since the electrode was not available at that time.

Main outcome measures

The patients’ preoperative findings and results were re-evaluated to develop a new scoring system for patient selection. All patients underwent MRI of the brain performed using a 3.0 Tesla magnetic resonance (MR) unit (Philips Achieva; Philips Medical Systems, Best, the Netherlands). To visualize the cerebellopontine angle, the MR protocol included a 3D balanced fast field echo sequence that was used for further assessment. Using a picture archiving and communication system (IMPAX; AGFA HealthCare, Bonn, Germany), all MR examinations were anonymized and randomly presented to a neuroradiologist who was not aware of any clinical data. In each case, the four-grade Koos classification system was used to assess the tumor based on size, extension, and brainstem compression.(17) Additionally, a four level grading system was used to evaluate intrameatal extension towards the modiolus (Table 2). EABR with promontory stimulation is performed in local anesthesia. A gold coated rigid electrode with a rounded-bent (hockey stick) tip stimulation probe manufactured by MED-EL (Innsbruck, Austria) is inserted and placed on the promontory. The  ground electrodes are placed on the zygomatic bone and the angle of the mandible. Promontory stimulation eABR was performed with the Neuropack, Nihon Kohden, Tokyo; evoked potential recording device. Number of sweeps varied from 1,000 to 1,500. A recording window of 10 ms was used. One of the main technical difficulties, which is encountered using promontory stimulation eABR are artifacts. In order to minimize the artefacts an adopted approach was used (18). For the stimulation, alternating biphasic pulses with the stimulating rate of 34 pulses per second is used. Phase duration was set to 100 µs and increased in a step of 50 cu until a response was detected. Stimulations are performed at various intensities, depending on the clarity of the response and tolerance of the patient. A positive response is defined as a clear wave V reproducible in latency and amplitude. In cases with a wave V that can only be seen at high stimulation intensities or that is not reproducible an unclear response is documented. No identifiable wave V is defined as a negative response. The pure-tone average (4-PTA) was calculated as the mean for each patient, using the results at frequencies of 0.5, 1, 2, and 4 kHz. Monosyllables were measured at 65 and 80 dB HL. Based on these results, patients were categorized in four groups as seen in Table 3. STARD guidelines were followed when reporting the data of this study.

Results

Seventeen patients underwent translabyrinthine vestibular schwannoma resection. The final decision regarding cochlear implantation was made intraoperatively, after tumor removal, based on the results of eABR with the ITE. A total of ten patients had positive eABR results and consequently received a CI. After six months of follow-up, ten patients were daily users of the CI. Their mean aided pure-tone average was 38 dB HL, and their mean WRS was 28% at 65 dB, and 52% at 80 dB. Nine of the ten patients had open-set speech understanding. The patient without open-set speech understanding has significant hearing loss on the contralateral side and a language barrier. Facial nerve outcomes were measured using the House-Brackmann (HB) score. On the first postoperative day, four of seventeen patients had a facial nerve palsy, with HB scores of 2, 3, 5, and 5. Two of these patients exhibited complete resolution at the one-month follow-up appointment. One patient showed marginal facial weakness (HB 2) at the six-month follow-up appointment. One patient exhibited permanent facial nerve weakness with a synkinesis. Preoperative imaging of this individual showed that the vestibular schwannoma seemed to follow the facial nerve up to the geniculate ganglion, which might explain the persistent palsy. Table 4 presents the detailed hearing results and facial nerve outcomes for each patient. In all cases, the six-month follow-up MRI showed no sign of residual or recurrent tumor. The internal auditory canal was sufficiently visible in all cases despite CI placement.

The scoring system

Table 3 shows the scoring system that was developed based on the results of our previous study(14) and the experiences gained in the present study. One included factor was Koos scoring, which is a commonly used system for grading the size and general extension of a vestibular schwannoma. Our system also included points related to residual hearing, which is an important factor reflecting the state of the nerve and the cochlea before an intervention. We identified extension towards the modiolus as an important predictive factor. In seven cases, the vestibular schwannoma made no contact with the modiolus, and a CI was placed in all seven of these cases. In five cases, the tumor was in contact with the modiolus, of which three received a CI. Individuals in whom CT showed that the vestibular schwannoma seemed to be in contact with the modiolus, and who received a CI, exhibited similar outcomes to the other patients in this study. Five patients exhibited either modiolus infiltration or transmodiolar extension of the schwannoma, none of whom could be implanted. Table 5 presents the results of the patients presented in the study and according to the point system.

Discussion

In the present study, we summarize the outcomes of 17 patients who underwent translabyrinthine sporadic vestibular schwannoma resection. After final intraoperative eABR measurements with an intracochlear stimulation electrode, ten patients were implanted with a CI. At the six-month follow-up, ten patients were daily users, of whom 90% had open-set speech understanding. Various advancements over recent years have made CI a good option for restoring binaural hearing in patients with VS. Importantly, CIs have become MRI compatible. Imaging sequences have been improved and CI magnets have been developed to reduce metal artifacts(19) and allow postoperative evaluation of the internal auditory canal and cerebellopontine angle for tumor follow-up(16). Several studies have focused on implant positioning to further improve the visibility of these anatomical structures(20, 21). Another recent development is the possible use of intraoperative eABR with an ITE, enabling objective assessment of cochlear nerve functionality. In the past, the outcome of CIs in the context of VS was considered unpredictable, and many patients did not benefit from this technique. However, the advent of testing systems, such as intraoperative eABR, permits more precise evaluation of possible outcomes and rationale for CI during surgery. Although simultaneous VS resection and cochlear implantation is possible with only visual assessment of the cochlear nerve(22), eABR measurements are a useful tool for objectifying the surgeon’s assessment. An alternative approach is to place a recording electrode on the nerve, as for recording cochlear nerve action potentials (CNAP)(23) and dorsal cochlear nucleus action potentials (DNAP);(24) however, this approach is far more invasive. Various centers have used eABRs to evaluate auditory brain stem implant position(25) in cases of suspected auditory nerve hypoplasia,(26) auditory neuropathy,(27) or labyrinth malformations.(28) Only two prior studies have described the use of eABR during translabyrinthine VS resection(11, 14). One study included eight patients(11). The other was a pilot study by our research group(14). In this previous manuscript, we demonstrate the intraoperative objective evaluation of the cochlear nerve with eABR, using an ITE, during translabyrinthine VS resection and cochlear implantation(14). Our preliminary results indicated that positive eABR results (clear wave V) seem to be reliable, and to correlate well with CI-aided auditory perception. This thesis was further supported by the results of our present study, in which all patients with positive eABR results and who received a CI showed auditory perception with their CI, and most (90%) even have open-set speech understanding. As mentioned above, cochlear implantation has become a good option in cases of VS. A remaining major challenge is to identify patients with a high chance of cochlear nerve preservation. Here we reviewed the outcomes of the 17 included patients, along with their preoperative audiometric data (including eABR with a hockey stick-shaped promontory stimulation electrode) and preoperative imaging results, to identify factors associated with the most suitable patients for simultaneous cochlear implantation. As a logical next step, this knowledge can be used to create a grading system with the aim of preoperatively determining candidacy, as was performed herein. Individuals with residual speech understanding were more likely to have positive eABR results after tumor resection, and to receive a CI consequently. These results are in line with the findings of Sanna et al(22). In their study, individuals with good functional hearing underwent simultaneous translabyrinthine VS resection and cochlear implantation. The decision regarding cochlear implantation was made intraoperatively. After tumor removal, surgeons evaluated the cochlear nerve and visually determined whether it was intact(22). Of the nineteen included patients, thirteen were able to receive a CI, of whom 84% use their CI daily or almost daily(22). Another identified predictive factor was the VS extension towards the cochlear modiolus. This anatomic structure also reportedly plays an important role when assessing CI candidacy in cases of malformation,(29) as well as the need of its preservation in intracochlear VSs(30). Seven of our patients exhibited no contact between the VS and the modiolus, and all seven received a CI. In five patients, the tumor was in contact with the modiolus, of whom three had positive eABR results after schwannoma resection. The remaining five patients exhibited infiltration of the modiolus or transmodiolar extension of the schwannoma, and none of these patients could receive an implant. Further, to categorize size and tumor extension, the Koos classification was used. Obviously, tumor resection is increasingly challenging with greater VS size, reducing the possibility of nerve protection. The last category of our scoring system was promontory stimulation, which is especially usefull in patients with no residual hearing. One issue with promontory stimulation is that it produces a substantial number of artefacts in awake patients. Another problem is that some patients do not tolerate stimulation well, leading to a wide variety of stimulations intensities between different individuals. Artefacts and said stimulation range diminish the interpretation possibilities and reliability of the results. One limitation of the present study was the relatively small sample size on which the scoring system was based. Nevertheless, this study represents the largest group of patients with vestibular schwannomas, in whom eABR was intraoperatively performed. As well as the first study assessing predictive factors for cochlear implantation which can be assessed preoperatively. Overall, our results indicated that patients with a Class I scoring had a very high chance of receiving a CI. Among patients categorized as Class II (6–8 points according to our system), the majority could be implanted, but they had a distinctly worse chance of receiving an implant, and thus careful counseling is essential in these cases. Patients categorized as Class III and IV were not able to be implanted with a CI.

Conclusion

Simultaneous translabyrinthine VS excision and cochlear implantation using intraoperative eABR measurements is a good option for hearing rehabilitation and provides binaural hearing. Preoperative accurate assessment of the size and extent of VS, audiometric testing, and promontory stimulation eABR improves preoperative patient selection, help manage patient expectations, and predict the possibility of cochlear implantation.