17.2.1 NF2 Patients

Most deaf patients with neurofibromatosis type 2 (NF2) who have been implanted with ABIs utilize them on a daily basis. ⁴ This signifies that these patients obtain benefit from their devices, at least in forming an auditory connection to their environments. It is important to understand, however, that, at least with NF2, auditory results are generally modest and are highly variable. ABIs do not restore normal hearing, and only a small percentage of patients obtain any significant understanding of conversational speech.

The auditory benefit obtained by NF2 patients with ABIs was initially described in detail by Otto, et al, in 2002. ⁵ A battery of tests were conducted on 61 patients who had been implanted with the 21-electrode Nucleus ABI24 designed to measure the comprehension of environmental sounds, consonant and vowels sounds, words, and sentences. The battery included both closed-set and open-set testing as well as testing of both sound only and sound in combination with vision (lipreading). On closed-set tests of environmental sounds, consonant and vowel sounds, and words, patients with ABIs scored significantly higher than random chance. However, on open-set sentence tests, only a small percentage of patients achieved significant comprehension. In terms of speech understanding, the major benefit of ABIs for most patients is significant improvement when tested together with lipreading when compared to lipreading alone.

More recently, improved audiological results with ABIs in NF2 patients have been reported by two centers in Europe. ^{7 , 8} These centers use the MED-EL ABI and perform implantation using the retrosigmoid approach in the sitting position. The MED-EL device is not approved by the FDA for use in the United States. In these centers, results continue to vary among patients, but up to 30% of patients achieve significant open-set speech comprehension with sound alone. It should be noted that a PubMed search using the terms “auditory brainstem implant” and “neurofibromatosis type 2” yields 14 reports, with none of the other 12 reporting significant open-set speech comprehension in a similar percentage of patients. ^{5 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19}

17.2.2 Adult Non-NF2 Patients

More recently, ABIs have been implanted in adult patients without NF2. These patients undergo craniotomy for the express purpose of ABI implantation. A small number of adults who were deafened postlingually by causes not amenable to cochlear implantation, such as traumatic transection of the cochlear nerves and post-infectious ossification of the cochlea, have received ABIs with results reported to be better than for patients with NF2. ²⁰ The indications for the implantation of ABIs in non-NF2 adults are very rare, and the small numbers of such patients make clear determination of the audiological benefits difficult.

17.2.3 Pediatric Patients

ABIs have also been used in congenitally deaf children unable to benefit from cochlear implantation due to cochlear malformations or absence of the cochlear nerve. ^{21 , 22} Again, reports are guardedly encouraging that results are considerably better than those for patients with NF2. It is important to understand, however, that the evaluation of pediatric patients is considerably more complicated than that of adults. ^{23 , 24} Although there is clear evidence of the development of speech understanding in a subset of these patients, it remains unclear what percentage or how many pediatric patients will benefit significantly. There are currently four ongoing studies of ABI implantation in the pediatric population in the United States. One of these is supported by the NIH.

17.3.1 Pediatric ABI Surgery

The use of ABI in the pediatric population also raises additional ethical issues. Due to an inherently emotionally charged situation, parents of young children who cannot benefit from cochlear implantation may be particularly credulous in terms of unrealistic expectations, and it is exactly these pediatric patients that may be exposed to the highest risk with the most uncertain benefit.

17.3.2 The Learning Curve

There is no direct data regarding the surgeon’s learning curve for ABI placement. The presence of a learning curve has been demonstrated with other technically challenging operations. ^{26 , 27 , 28} There is no reason to think that a learning curve does not also exist for other such procedures. Certainly, acoustic tumor resection experience would be expected to translate to ABI placement. ABI placement, however, requires full exposure of the lateral recess and the foramen of Luschka in situations in which the normal features of the brainstem are typically distorted by tumors. In addition, there is significant variability of the anatomy of the lateral recess in children with congenital deafness due to abnormalities of embryonic and fetal development. ²⁹ Furthermore, ABI electrode array and cable handling can be cumbersome due to their construction and inherent torque.

The most common indication for ABI is NF2, which has an estimated prevalence of 1/60,000. ³⁰ Only a portion of these patients ever become candidates for ABI. Other indications, such as congenital absence of the cochlear nerves or cochlear malformations of the cochlea, appear to be yet less common. The rarity of potential ABI recipients would be expected to limit progress along the learning curve.

Review of the literature does suggest that there are differences in ABI outcomes among centers. For instance, the only centers that report any patients with significant open-set speech comprehension are those with the highest volumes. This is true both for patients with NF2 and for those without.

17.4.1 NF2 Patients

As discussed previously, in the group of patients with NF2 undergoing craniotomy for tumor removal, ABIs can be placed with little marginal risk since the major risk of the operation can be attributed to the need for tumor resection. For that reason, it is reasonable and wholly appropriate for centers with significant NF2 volume to develop programs and to engage in ABI placement.

We would recommend that training of ABI surgeons should involve observation of surgeons at existing higher volume centers. While some training should be in person in order to allow for real-time interaction, video demonstration is also useful. The development of high-definition intraoperative video along with the ability to disseminate such video on the internet has significantly elevated the resources available to prospective ABI surgeons.

Likewise, successful ABI centers require a high level of audiological expertise. ABI programs should not be initiated without a very high level of audiological commitment, such as might only be available at a large cochlear implant center. ABI programming is significantly more complicated that cochlear implant programming and must be recognized as such. The availability of video conferencing and internet communication may further facilitate the development of requisite programming skills.

It is important for prospective ABI centers to be realistic about patient volume. Even with a large number of NF2 patients, the percentage of patients with NF2 who are good ABI candidates at any one time is small. It is probably not realistic to expect sufficient volume if fewer than several dozen NF2 patients are being seen annually. The development of new therapies, such as bevacizumab, as well as an understanding that some NF2 patients benefit from cochlear implantation, has also served to delay ABI placement in many patients and to obviate the benefit of ABI placement entirely in some. ³¹

The first few prospective patients of any ABI program should not be counseled to expect good audiological benefit. At least initially, ABI patients should undergo surgery for the primary purpose of tumor resection, and this should be stressed. A poor audiological outcome would be expected more acceptable to a patient in whom surgery was done for the primary purpose of tumor resection than to a patient undergoing craniotomy primarily for the ABI.

17.4.2 Adult Non-NF2 Patients

Due to the rarity of non-NF2 pathology that causes postlingual deafness in adults it is unlikely that any center will be able to specialize in this area. Thus, we would find it problematic if these operations were to be carried out by anyone other than experienced NF2 ABI surgeons in centers that provide the necessary audiological and programming expertise. In addition, these operations should probably only be undertaken at all if the expectations of audiological results are higher than with NF2.

17.4.3 Pediatric Patients

For a number of reasons, the standards must be “higher” when considering ABI surgery in pediatric patients. These reasons are both practical and ethical.

In pediatric, congenitally deaf patients, the entire craniotomy is being done for the sole purpose of ABI implantation. All of the risk of surgery is assigned to the ABI; ABI placement is not simply an incremental risk. Because of this, it is critical to both minimize risk and maximize benefit. Arguably, the best source of new pediatric ABI surgeons is adult ABI centers. Certainly, the surgeon taking responsibility for ABI implantation should be intimately familiar with and experienced in cerebellopontine angle surgery. In addition to experience, preparation for the nuances of brainstem surgical anatomy and device implantation is critical. Training should consist of multiple site visits, familiarization with the ABI receiver, cable, and electrode array, as well as, perhaps, simulation.

The lead ABI surgeon may, however, be the only one member of the necessary pediatric ABI team. If the lead ABI surgeon is a neurosurgeon, an experienced and interested neurotologist will be needed for patient selection and medical auditory management. In addition, if the lead ABI surgeon does not have a pediatric-focused practice, which could be expected to be the case due to rarity of lateral skull base pathology in pediatric neurosurgery practice, then a pediatric neurosurgeon must be involved as well.

It should also be recognized that ABI implantation is only the first step in optimizing ABI outcome. Proper device programming, audiological assessment, and speech pathology input are also critical. Enthusiastic participation of an entire such team, presumably constituted by specialists from a large pediatric cochlear implant program, is required in order to have any hope of achieving benefit that would make the surgery worthwhile in the first place. It should also be understood that an ABI program must also be considered a long-term proposition. Attention must be focused on these patients for 5, 10, or more years. A pediatric ABI program should not be initiated unless it is clear that there is institutional support for a long-term endeavor.

Pediatric ABI implantation also carries ethical concerns. It should be made clear to families that pediatric implantation is an off-label use. More importantly, it should be made clear to families that long-term benefit from ABI in this population remains uncertain. Parents of young, deaf children who either cannot undergo cochlear implantation or who failed to achieve benefit from cochlear implantation may make decisions on an emotional basis without understanding the risks or potential benefit. It is important for surgeons and teams both to be realistic about the state of the field as well as about expected results in their own hands rather than in the hands of a different surgeon who has done perhaps many dozens of cases.