Loss of integrity of the auditory nerve after surgical removal of vestibular schwannomas (VS) in neuroﬁbromatosis type 2 (NF2) is a frequent occurrence. In cases where patients completely deafened either by the natural history of NF2 or by surgery, the auditory brainstem implant (ABI) may be used to bypass the auditory nerve and directly stimulate the cochlear nucleus complex. Patients have significantly benefited from ABI and useful auditory sensations have resulted in the majority of patients. 1 , 2 The multichannel version of the ABI (Nucleus®, Cochlear Corporation, Englewood, Colorado) successfully completed US Food and Drug Administration (FDA) clinical trials in July 2000 and received approval for commercial release. ABIs have also been produced by other implant manufacturers including MED-EL, which are not approved for use in the US. This chapter summarizes the performance of patients with NF2 implanted with an ABI and focuses on the programming and auditory rehabilitation.
12 Programming, Rehabilitation, and Outcome Assessment for Adults: I
Establishing reasonable expectations for patients is paramount from counseling perspective in auditory brainstem implant (ABI) recipients. It should be clearly defined that ABI provides most patients with some degree of environmental sound awareness and in conjunction with lipreading is typically is a beneficial rehabilitation strategy. 3 , 4 , 5 , 6 Notably, a minority of patients obtain open-set speech understanding. 3 , 7 , 8
Over 350 patients with NF2 have been implanted with the multichannel ABI system at House Clinic between 1992 and the present (Fig. 12.1) and approximately 1,300 patients have been implanted worldwide. 2 With few exceptions at our center, the performance of ABI does not typically reach the results of cochlear implantation. Approximately, 80% of our implant recipients are device users and higher percentage (92%) received auditory sensations. Most patients recognize environmental sounds and speech understanding is enhanced by an average of 35% when ABI sound is combined with lipreading. In our clinic, approximately 25% of ABI users have achieved open-set speech discrimination (at least 20% correct without lipreading cues on the City University of New York [CUNY] Sentence Test). 9 , 10
Reports from some European centers suggest notably better audiometric outcomes compared to data from the United States. Among NF2 recipients, open-set speech perception was noted in 37 and 41% of patients within 2 years of activation. 8 , 11 These outcomes have significantly improved upon data from the United States and factors accounting for these improved outcomes are an area of active investigation. One interesting consideration is whether phonetic differences between languages play a role in open-set outcome results 7 , 12 , 13 , 14 (Table 12.1).
A comprehensive analysis of the available literature suggests preponderance of patients’ benefit from a rehabilitation standpoint. We performed a comprehensive analysis of all the world literature. Key words for our literature search included auditory brainstem implant, neuroprosthetic device, auditory neuropathy, deafness, deafness treatment, acoustic stimulation, auditory cortex, brain mapping, evoked potentials, recovery of function, psychoacoustics, and humans. Our search generated 117 articles of which 26 were included in our literature review (Table 12.1). Articles were included in our analysis if they were original prospective or retrospective studies which included a primarily adult population. The majority of patients experienced some component of sound perception. Associated with lipreading, ABI is a beneficial strategy. A minority of patients achieved open-set speech recognition (Table 12.1).
Factors leading to high performance of the ABI remain an area of active investigation and the specific factors that may lead to enhanced performance remain to be fully characterized. 31 Large case series are not available for analysis and multivariate analysis has not been performed to date. Several variables are suggested to be important factors for performance. These include the number of electrodes utilized during programming, tumor characteristics such as tumor size at the time of resection or prior history of radiation, the influence of time after implantation to maximal performance, differences in device design and processor function, and differences in surgical technique. It is also likely that some of the factors that are reported to be important for cochlear implantation performance, such as duration of deafness, are also important factors for ABI. Differences in patient selection may also be an important variable which may differ between implantation centers.
12.1.1 Electrode Number—Evolution of the ABI
William House and William Hitselberger developed the first ABI and implanted the first patient in 1979 1 with a modified cochlear implant fitted with two ball electrodes placed on the cochlear nucleus. The device used in the early 1990s included three platinum plates mounted on Akron mesh backing. Stimulation of electrodes produced auditory sensations in most patients, which were comparable to the results with the original ABI. 9 Over time, the device was modified to include eight electrodes through a collaboration with Cochlear Corporation and the eight-electrode device was implanted between 1993 and 1999, leading to FDA approval in 2000. 2 Early in the experience with the eight electrode multichannel ABI, it was demonstrated that high performance with the ABI was possible. In a series of 20 patients, 3 patients achieved sound-only sentence recognition scores between 49 and 58% with the notable ability to speak on the telephone. 32 These results were further supported with long-term data, where the 8-electrode multichannel device proved capable of providing some patients with an ability to understand speech by using the sound from the ABI without the assistance of lipreading cues. 6
The device that has been implanted at our center from 2000 to present includes 21-electrode contacts in the array. With this retrospective historical data, an important question is whether or not the increase in electrode contacts is important. Data from the House Ear Institute indicates that performance improved after 1999, when the number of electrodes was increased to 21 electrodes from 8 electrodes (NUCHIPS testing, and univariate analysis, unpublished data) (Fig. 12.2). Clearly, other variables such as improvements in surgical techniques or other factors could also account for these findings.
Among high performers, analysis was performed with the goal of determining the number of active electrodes that are needed for open-set performance. In an analysis of 22 patients, all patients had seven or more active electrodes. The correlation of performance to a greater number of active electrodes was a weak correlation (R 2 = 0.2554) (Fig. 12.3). Similarly, in a multicenter retrospective study of 26 patients who performed highly with 30% or better on correct identification of sentences testing, there was no correlation between performance and electrodes utilized. 21
12.1.2 Tumor Characteristics—Size and History of Radiation
Tumor characteristics could also account for differences in outcomes. Specifically, tumor size where larger tumors equate to poor performance is hypothesized. A secondary consideration would be a prior history of stereotactic radiosurgery prior to implantation. In an early study of outcomes at our institution of 17 ABI implants, tumor size was not correlated to patient outcomes. 33 A similar finding was also demonstrated among high-performing ABI recipients. Tumor size was also not correlated to audiometric performance. 21 We have analyzed patients who received stereotactic radiosurgery prior to tumor resection. Patients with a history of stereotactic radiosurgery had a higher incidence of failure to achieve auditory sensations from their ABIs (30%) than those who did not (8%). When active ABI users were compared to historical controls, there was no difference between audiometric outcomes (NUCHIPS testing). Patients who had previously received ipsilateral stereotactic radiosurgery (N = 8) achieved a NUCHIPS score of 80% + /–6.9 Standard Error of the Mean (SEM) versus 71% + /–1.39 (N = 130) for historical controls (unpublished data). Follow-up times between both groups were comparable (radiosurgery 38 months + /–14.4 SEM; historical controls 42 months + /–3.7 SEM). Based on these data, it is likely that previous ipsilateral stereotactic radiosurgery does not influence audiometric outcomes if auditory sensations may be initially achieved after implantation.
12.1.3 Time to Optimal Performance
While the majority of the improvements were seen during the first year, many patients continue to improve over a decade. In the recently unpublished analysis of the results at the House Ear Institute, the average duration of best performance using NUCHIPS testing was 36 months. Fig. 12.4 illustrates the overall trend of longer follow-up times to better ABI performance. While clearly this data is possibly skewed toward successful implant recipients who seek further programming modifications, the overall trend suggest that sustained utilization of the device leads to better outcomes. Similar improvements with sustained use have also been reported in other studies. 8 , 12 , 24