Key points
- Simultaneous translabyrinthine tumor resection and cochlear
implantation is a promising treatment method and hearing
rehabilitation for sporadic vestibular schwannomas.
- Size of vestibular schwannoma, distance to the modiolus, residual
hearing and promontory stimulation electrical brainstem response
audiometry are important preoperative predictive factors for chance of
cochlear implantation.
- Electrically evoked brain stem response audiometry with an
intracochlear test electrode is an important additional tool during
vestibular schwannoma resection to decide on cochlear implantation.
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.