Traditional indications for auditory brainstem implant (ABI) in patients with neurofibromatosis type 2 (NF2) have been broadened to include other applications of the device in patients without an adequate cochlear nerve (CN) for implantation. Studies of auditory perception in many non-NF2 patients demonstrate superior outcomes to average NF2 patient function. As further clinical studies are completed in the US, ABI may become standard treatment for patient groups with limited hearing rehabilitation options, with ABI approval for CN aplasia likely to be the most imminent expanded indication.
4 Clinical Indications for ABI: Patient Selection and Alternatives
The auditory brainstem implant (ABI) was developed for patients with neurofibromatosis type 2 (NF2) because removal of bilateral acoustic tumors results in loss of the auditory nerve. 1 , 2 Since its first use in an NF2 patient by House and Hitselberger in 1979, the indications for ABI have expanded to include other patient populations in which stimulation of the auditory nerve by cochlear implantation is not possible. Acquired indications include complete cochlear ossification and skull base trauma resulting in severed cochlear nerves. ABI has also been found to be useful in pediatric patients with cochlear anomalies in which cochlear implants (CIs) are not beneficial; ABI is also the only treatment that can offer some hearing benefit for patients with congenital cochlear nerve (CN) aplasia. 3
ABIs are FDA approved only for NF2 patients older than 12 years; however, expanded indications in non-NF2 patient groups, particularly in pediatric patients implanted before the age of 2, have demonstrated superior outcomes. Expanded ABI indications have been more widely implemented in Europe, and clinical trials are currently underway in the US for investigation of ABI use in patients with CN atresia.
Preservation of the CN during translabyrinthine and retrosigmoid resections of acoustic tumors has allowed for simultaneous CI in some cases; this is a viable alternative to ABI for NF2 patients, although CI function is variable and unpredictable, probably due to trauma to the CN from the tumor and from resection. Others have advocated early treatment of NF2 tumors with hearing preservation approaches when possible; 4 although maintaining auditory function after treatment is ideal, this alternative to ABI is not always achievable.
As groups in Europe, especially that of Vittorio Colletti in Verona, Italy, push the boundaries of traditional ABI indications with clinical studies demonstrating its efficacy in a variety of conditions, it becomes apparent that ABI is the gold standard for treatment of disorders in which the CN is absent or deficient. Although still a very important tool in the management of patients with NF2, ABI may become a more accepted treatment for hearing rehabilitation when the cochlear nucleus is the most distal relay in the auditory pathway that is available to stimulate.
4.2 Clinical Indications
4.2.1 Regulatory Status of the Device
As previously mentioned, the first ABI was performed at the House Clinic in 1979. Subsequently, Cochlear Corporation helped design/manufacture the multichannel ABI in 1993. In March 2000, the Nucleus 24 ABI was submitted to the FDA, and the ENT Advisory Panel recommended Nucleus 24 ABI for use by individuals with NF2. The Nucleus ABI received approval for Premarket Approval Application (PMA) in October 2000 (PMA No. P000015). The Cochlear Nucleus Profile Auditory Brainstem Implant (ABI541) was granted PMA approval in 2016.
4.2.2 Current FDA Approval
In the original PMA, the ABI was approved for patients with NF2 who are at least 12 years old. 5 The FDA specified that implantation may occur during first-side or second-side tumor removal, or in patients with previously removed bilateral tumors. Patients must have appropriate expectations regarding the ABI and need to be motivated to participate in the rehabilitation process postoperatively. These same indications hold for the newer ABI541.
4.2.3 Current European CE Mark Approval
Although the MED-EL ABI device is not FDA approved, it is more widely used in Europe. In 2011, MED-EL introduced the Concerto ABI. ABI candidacy was originally approved in Europe for patients with NF2 who are at least 15 years old. MED-EL specifies that device implantation and tumor removal should take place in the same surgery. 6
More recently, Ingeborg Hochmair, the CEO of MED-EL, informed us that CE mark approval has been obtained for ABI in children 12 months of age and older.
4.3 Patient Selection in Adults
4.3.1 Patient Selection in NF2
As discussed earlier, NF2 patients of 12 years or older must have realistic expectations and should be able to comply with auditory rehabilitation postoperatively in order to be selected for ABI. Although many patients are deaf at implantation or are undergoing surgical excision of a tumor in an only-hearing ear at the time of implantation, candidates may also be selected who have contralateral hearing but with reasonable expectation of hearing loss in the future. Although many patients do not use these “sleeper” ABIs for as long as they have useable hearing in the contralateral ear, sleeper ABI function has not been found to be significantly different from nonsleeper ABI function in NF2 patients. 7
Patients implanted with ABI may undergo either a translabyrinthine or retrosigmoid approach for tumor resection and access to the lateral recess of the fourth ventricle and cochlear nucleus. Although the translabyrinthine approach has been traditionally used for ABI placement, proponents of the retrosigmoid approach cite possible advantages of decreased operative time and decreased risk of wound contamination with tympanomastoid flora. 8
4.3.2 Patient Selection in Non-NF2 Indications for ABI
Although the most common indication for ABI today is still NF2, the ABI has more recently been used in other conditions in which either the cochlea or CN is not adequate for cochlear implantation. Colletti describes ABI use in 49 patients after trauma (with CN avulsion), with cochlear malformations, with auditory neuropathy, or with altered cochlear patency. 9 With the exception of patients with auditory neuropathy, these groups have superior ABI performance to NF2 patients with ABI (Fig. 4.1 and Table 4.1)
Colletti et al explain that post-meningitic cochlear ossification results in poor CI outcomes, 10 , 11 with about half of the patients lacking open-set speech even with partial insertions or double-array electrodes. Neuronal degeneration associated with cochlear ossification may be the cause of this poor function. Cochlear ossification can also continue after implantation in these patients, resulting in declining CI function. These authors suggest that an ABI might provide better neural access to the auditory system in these cases.
Similarly, advanced otosclerosis may entail neoossification of the cochlea that poses similar challenges to cochlear implantation and may also result in facial nerve stimulation. Although most patients with otosclerosis do well with CI, those with advanced otosclerosis and poor CI function who require reimplantation have poor outcomes. 12 , 13 In those cases, ABI provided open-set speech in some patients.
Skull base trauma can result in labyrinthine fracture, inner ear concussion, perilymphatic fistula, and CN avulsion. Colletti points out that CN avulsion can result from acceleration/deceleration injuries and that the VIIIth nerve is particularly vulnerable due to its long central portion (8–10 mm). 14 Cochlear fracture or hemorrhage can also lead to ossification of the cochlear lumen that makes CI after hearing loss from trauma less effective than nontrauma CI. 15 Prior to implantation with ABI, trauma patients should have round window electrical stimulation that fails to elicit electric auditory brainstem responses (EABRs) and may also undergo a hearing aid trial. 16 Colleti et al proposed selection criteria for ABI after trauma (Box 4.1).
4.4 Patient Selection in Children
4.4.1 Pediatric Patient Selection for NF2
ABI has been shown to be similarly effective in 12 to 18 years old teenagers as it is when used to rehabilitate hearing in adult patients with NF2. 16 In this group, family support, expectations, and motivation to participate in rehabilitation were important factors that separated users from non-users. In 2013, the FDA approved ABI use in clinical trials for children younger than 12 years old. Small pediatric studies have been done in the US and recent larger studies have been recently published on ABI in young pediatric patients. 17
4.4.2 Cochlear Malformations
As suggested by Colletti et al, 9 ABI may also be useful in patients with cochlear malformations when CI does not provide them with good auditory function. It may be difficult or impossible to determine the location of the spiral ganglion in relation to the cochlear cavity at cochlear implantation in these patients, and anomalies of the facial nerve and increased risk of cerebrospinal fluid (CSF) leak with implantation pose specific surgical risks that may lead to failed CI. As long as cochlear implantation is feasible (e.g., not in Michel aplasia in which there is no cochlea to implant), we suggest CI, and consider ABI in cochlear malformations only when CI is ineffective.
4.4.3 Cochlear Nerve Aplasia
Lack of auditory nerve is a clinical indication for ABI currently in clinical trials in the United States. CN aplasia is indicated by a narrow internal auditory canal on computed tomography (CT) and lack of CN on magnetic resonance imaging (MRI) (Fig. 4.2). Preliminary results of a Phase I clinical trial indicate that four children who completed 1 year follow-up demonstrated speech detection thresholds of 30 to 35 dB and had pattern perception when presented with closed-set words. 18 Patients with CN aplasia have been implanted for some time in Europe. In 2008, Eisenberg et al reported on auditory testing at House Ear Institute of a 3-year-old patient implanted in Verona, Italy. 19 After 6 weeks of consistent stimulation, he had sound awareness and increased vocalization. Later testing showed him reaching closed-set speech at 6 months, and at 1 year he reached similar auditory function to CI patients implanted at a similar age.
In 2008, Colleti and Zoccante reported on 19 pediatric patients with CN aplasia implanted with ABI, 5 of whom had received a previous CI resulting in no auditory perception. 20 Although these authors did not separate CN aplasia patients from tumor patients, they did demonstrate that all pediatric patients receiving an ABI showed improved lipreading and environmental awareness of sound; they also demonstrated significant improvement in cognitive tests over those without ABI. Colletti, Wilkinson, and Colletti reported that 21 pediatric patients with failed CI and surgically confirmed absence of the CN who were reimplanted with ABI showed statistically significant improvement on auditory testing. 21
In 2011, a consensus statement of several European ABI centers included congenital indications for ABI that are particularly relevant for this discussion. 22 In addition to CN aplasia, complete labyrinthine aplasia (Michel aplasia), cochlear aplasia, and cochlear aperture aplasia were included in well-defined congenital indications. It is clear in these diagnoses that a competent CN is not available for cochlear implantation. Both well-defined and possible congenital indications can be seen in Box 4.2. Review of these indications suggests that in cases where it is unclear if there is a competent CN, such as CN hypoplasia, CI can be attempted and improved responses to auditory stimuli have been achieved in some cases. In those where minimal progress is made, ABI can be performed secondarily.
Patients with CN aplasia should have a likelihood of a normal cochlear nucleus when being evaluated for ABI placement. This can be assumed from normal anatomy of the fourth ventricle and nearby brainstem on MRI. Similar to CI, ABI is likely to have better auditory outcomes and possibly result in speech understanding when patients are implanted at a younger age (i.e., before age of 2). 23
Counseling of families with children with CN aplasia is particularly imperative and is also challenging. They need to understand that CI cannot function without the presence of a CN. At the same time, the level of function after ABI for these patients is variable and the families need to have realistic expectations about postoperative rehabilitation and function. Families should also understand that patients with developmental delays tend to perform more poorly on subjective measures of hearing performance, although they may enjoy their ABI and find it useful. 22