Considerable advances have been made since Hitselberger and associates1
implanted the first single-channel ABI in 1979 following the resection of a vestibular schwannoma. The first ABI device consisted of a pair of ball electrodes which were inserted into the substance of the cochlear nucleus. The electrodes were driven by a modified Bosch hearing aid. However, this electrode pair migrated and produced nonauditory side effects. At a revision operation in 1981 a paddle-shaped electrode array with two electrodes was implanted into the lateral recess of the fourth ventricle at the surface of the cochlear nucleus. Up until 1991 this type of electrode array was used and all newly developed arrays have followed this prototype and been implanted in the lateral recess.
The number of electrodes has increased and now the available systems boast anything from 8 to 21 channels.7c,7d,7e,7f
It might be argued that a large number of electrodes should increase auditory performance but this is not the case. In a comparison of two ABI-implanted patients, Otto and Staller8
in 1994 described comparable perceptual performance between a patient with six active electrodes and a much longer ABI experience with a patient with three electrodes and only minimal ABI training. In our experience, we have been unable to detect a correlation between functional performance and the number of active electrodes. However, from experience with cochlear implants we know that in general a minimum number of electrodes, four or more, is necessary to achieve adequate auditory perception. The number of active electrodes for useful hearing sensation in our series ranged from 4 to 12. Of more importance than the absolute number of electrodes is the ability of the patient to discriminate different tone pitches and rank them in order reproducibly. This ability was the main difference in the patients investigated by Otto and Staller.8
Improvements in speech processors are another important aspect. Technical developments during the past 20 years were inspired by the success of cochlear implants. A large number of speech coding strategies have been developed and tested in both laboratory and acoustic free field conditions. The high rate CIS strategy elaborated by Wilson and coworkers6
has been one of the most effective. Sixty patients were evaluated in a European multicenter clinical study4
and achieved mean monosyllabic word discrimination scores of 48% 6 months after the first fitting and 54% after 1 year. These results were better than those achieved with the multipeak (24.6%), spectral peak (33.8%), and low rate CIS (28%) strategies.9,10,11
Of further importance is that patients with the high rate CIS strategy implants scored significantly better in sentence recognition tests in a noisy environment (10- to 15-dB signal/noise ratio) compared with multipeak and spectral peak strategies.12
On the basis of these results it was reasonable for us to implement the high rate CIS strategy in an ABI device.
At present, patients who are selected for an ABI are those with NF-2 who have bilateral vestibular schwannomas. In the future, the indication may also include individuals after trauma and patients suffering from neuropathy of the eighth nerve. It may also include those with structural abnormalities that preclude conventional cochlea implantation, for example, ossification of the cochlear duct.13,14
Recently, Colletti and coworkers15
published a report on six patients who were implanted with an ABI after head injury. All recovered well and regained open set speech understanding.
Selection criteria should consider the individual's motivation for repeated postoperative fitting procedures and his or her willingness to undergo daily training with the device. The patients must be completely aware of their likely outcome to avoid any disappointment that might affect their general motivation and cooperation. Candidates should have a normal level of intelligence. Signs and symptoms of the underlying disease should not interfere with the fitting process and rehabilitation. In our opinion, the Wishard type of NF-2, in which there are severe signs and symptoms and a reduced life expectancy, should not be considered an exclusion. The pros and cons of implantation in this NF-2 subtype should be discussed thoroughly on an individual basis. If a patient is physically and psychologically able and willing to go through all the necessary postoperative rehabilitation and fitting procedures, and has good family support, there is no reason to withhold the chance of improving his or her hearing.
The patient's family must be involved from the outset. In our series, there was one patient who had a history of paraplegia after removal of a spinal neuroma at another institution. He had developed gliomas in the lower brainstem and in the contralateral cerebellar peduncle. His paraplegia resolved and he wanted to have an ABI when the vestibular schwannoma was resected on the side of his only hearing ear. During surgery good E-ABRs were recorded but postoperatively he had only minor initial auditory sensations. He refused any further fitting procedures and was reluctant to go to any hospital because of previous experiences. In his case, the additional pathology and the psychological problems worked together and illustrate quite well the many facets which have to be considered prior to implantation.
The question of whether to implant the first or second side is still a matter of debate. For several reasons we recommend implanting the first tumor side. If there is any functional hearing and the indication for surgery is tumor growth and/or compression of the brainstem, every effort should be made to preserve hearing or at least the anatomical integrity of the eighth nerve. In this way there might be the chance to restore hearing with a conventional cochlear implant. According to the data collected in Würzburg, the incidence of persistent electrical stimulability despite postoperative deafness in non–NF-2 schwannomas was 12.5% (personal communication). An ABI should be implanted if the eighth nerve is destroyed, especially in those who have a large tumor on the contralateral side and/or have poor hearing, as the chance of hearing preservation in the contralateral ear is very low. If the contralateral ear has been deaf for a long period (more than 8 to 10 years), or has developed a recurrence, the ipsilateral side should be implanted. More problematic cases are those who have a small or middle-sized contralateral tumor with good functional hearing. In this situation the patient must be very well informed about the surgical and technical problems that might impair implantation in the future. If the first side is not implanted, these problems might prejudice implantation when surgery is necessary on the second side. In that situation, the first side would have to be reopened and scarring might then make implantation extremely difficult, notwithstanding all the additional psychological stress for the patient and increased costs. Our patients who have serviceable hearing in the second ear use their implant for training purposes. This gives them confidence and reassurance for the future. This view is shared by others.16
The alternative argument that this policy might deprive the recipient of potential future improvements in implant technology does not hold as revision surgery is possible.
It is worth considering the functional problems that have been encountered when placing electrodes. In some cases we have experienced difficulties in deriving reproducible E-ABRs. If the patient was already deaf before surgery, it is impossible to know whether the cochlear nucleus can be stimulated at all or if the lack of response has been caused by a technical failure. Optimal placement of the array is also much more difficult and not possible without E-ABR feedback. The precise site of implantation is then determined by anatomical landmarks only. Fortunately, a reproducible E-ABR was derived every time in our series of patients. The position of the array did have to be modified to obtain the best responses by alternating bipolar stimulation of the four electrodes on the array. We found that in some individuals the lateral recess may be partially or completely occluded. In a study by Lang and Schäfer,17
complete occlusion was found in 3%, partial occlusion in 45%, and in 52% the recess was entirely open. Indeed this has been known for a very long time as Alexander18
(in 1926) reported occlusion of the recess in 20%. Other investigators have reported similar rates.17,19,20,21
In one of our cases, we observed another obstacle for implantation. The recess itself was open, but a large vein was running in a craniocaudal direction inside the recess. Some small veins coming from the floor of the recess drained into this vein. To implant, the vein would have had to be coagulated. This could have had a serious effect on the underlying brainstem and cochlear nucleus and therefore implantation was aborted.
We also have views about the three main approaches that are used for the resection of vestibular schwannomas. It is our opinion that the middle fossa approach is suitable only for small intracanalicular tumors and plays no role in ABI implantation. Both the translabyrinthine and retrosigmoid approach are employed for larger schwannomas and can be used for ABI placement. Rarely, some tumors invade the vestibule and resection of these by the retrosigmoid approach is more difficult. This situation can be predicted from preoperative magnetic resonance (MR) scans. It is still possible to achieve a complete resection through the retrosigmoid approach by extending the opening of the internal auditory canal down to the fundus. An additional important argument in favor of the retrosigmoid approach is that it preserves the cochlea and allows the subsequent use of a conventional hearing aid or cochlear implant.
Placement of the electrode array is feasible by both approaches. The route to the lateral recess is a little bit more straightforward in the translabyrinthine approach because the opening of the skull is more lateral than with the retrosigmoid approach. However, the lateral recess may be very deep and caudal in respect to the surgical opening. It may also be obscured by blood or CSF, which may interfere with safe placement without damage to the caudal cranial nerves by suction or manipulation. Fixation of the array by fibrin glue is more difficult in a wet environment, better achieved in almost dry surroundings. The retrosigmoid approach in the semi-sitting position achieves this albeit that the skull opening is more medial the resultant angle of access to the recess is more acute. This anatomical disadvantage can be reduced easily by rotating the head by 30 degrees toward the tumor side.
The tonotopy of the auditory nucleus is organized in a three-dimensional structure. High frequencies are represented in a deeper layer of the cochlear nucleus complex than lower ones.22,23,24
With surface electrodes it seems to be more difficult to stimulate more deeply located neurons of the ventral cochlear nucleus and thus it is more problematic to generate pitch differences by surface stimulation. This may interfere with speech recognition performance. To overcome this problem, penetrating ABI electrodes (PABI)25,26,27,28
have been developed and implanted in humans. Potential problems with this type of electrode are that the location and dimension of the cochlear nucleus has considerable variability. The initial results with this new type of electrode are comparable with those achieved by surface stimulation.26
Long-term results have yet to be published. Most patients achieve pitch discrimination with surface stimulation and this would suggest that a tonotopic pattern of stimulation is achievable.
Although CT scans are sufficient for follow-up, most patients require MR to assess their underlying disease process. Teissl and coworkers29
showed no adverse effects on implants by MR in 0.2 and 1.5 T machines. This has also been our experience with the Med-El system using a 1.5 T machine.