8 Auditory Brainstem Implantation Auditory brainstem implantation was originally designed for patients deafened by neurofibromatosis type 2 (NF2) to regain meaningful hearing. However, it can also be indicated in other conditions and has been shown to be useful.1,2 The auditory brain implant (ABI) device is similar in design and function to a cochlear implant (CI) but its implantation requires a more extended approach gaining access to the brainstem to position the ABI in contact with the cochlear nucleus.3 Current multichannel ABI technology affords a level of performance in aided speech comprehension but may also just provide a signal function. After programming and training, ~85% of patients experience auditory perception with their ABIs and between 44 and 97% of the ABI-implanted patients do actually use their implants.4 The results are unpredictable, however, and overall are worse than those with a cochlear implant.4 Nonetheless, the majority will develop the ability to recognize environmental sounds and the ABI facilitates lip-reading skills.3 As well as NF2, several other indications have been reported over the years and seem plausible in cases were no cochlear implant or other means of auditory rehabilitation can be used. Proper diagnostic evaluation needs to be performed before a decision to implant an ABI can be made. Given the current outcomes of auditory brainstem implantation compared with the good and reliable outcomes of cochlear implantation, placement of a CI as the choice for hearing restoration always has to be contemplated first. In many cases a stepwise strategy is appropriate; sometimes a stepwise surgical strategy may have to be completed before a decision about auditory brainstem implantation can be considered. NF2 is the most common indication for auditory brainstem implantation,3,4 but careful counseling is necessary as in some cases of NF2 cochlear implantation might still be possible, depending on the integrity and functionality of the cochlear nerve. Sometimes the decision for ABI or CI has to be taken during surgery.5–11 See Chapter 16. In case of bilateral ossified cochleae ABI implantation is only indicated when both cochleae are fully ossified, confirmed by scanning and by surgery; a cochlear drill-out procedure is needed to assess whether cochlear implantation is not an option. Since 2003, several papers have discussed this indication for ABI implantation in cases of postmeningitic ossified cochleae.2,12–14 All point out that a drill-out procedure with an attempt at CI insertion should precede ABI placement. In acute meningitis the hearing should be evaluated early and frequently (see Chapter 12) and MRI should be performed if hearing loss is confirmed. With these precautions, cochlear implantation in the postmeningitic deafened patients can be safeguarded before total obliteration of the cochlea has occurred.15 In cases presented months to years after having had meningitis, MRI will provide the most reliable information on the patency of the cochlea.16,17 When partial or total ossification is present, cochlear implantation surgery should be scheduled and various strategies, such as scala vestibuli insertion,18,19 partial insertion,20 basal turn drill-out, or double array insertion,21 should be considered (see Chapters 11 and 12). If no cochlear lumen can be found during the drill-out procedure, one may convert to translabyrinthine approach for ABI implantation or schedule a separate ABI implantation. In the most common inner ear malformations, such as incomplete partition type 2, common cavity, and other conditions, a cochlear lumen is present.22 In these cases cochlear implantation is the preferred type of rehabilitation—with the proviso that the cochlear nerve is present—as it has shown to be beneficial in these patients.23–25 When no cochlear lumen is present or it is insufficient in both ears (i.e., cochlear aplasia, cochlear hypoplasia), a primary indication for an ABI emerges.1,26 These cases sometimes are associated with a small internal auditory canal and with hypoplasia or aplasia of the cochlear nerve on imaging. Before a final decision to implant (CI or ABI) the existence of a (vestibulo) cochlear nerve should be assessed.27–29 See Chapter 15. Vestibulocochlear malformations are often associated with a small internal auditory canal and coupled with hypoplasia or aplasia of the cochlear nerve as seen on imaging.1 Hypoplasia or even aplasia of the nerve can also be found in solitary cases.30 Nevertheless, recent publications point out that the absence of a visible cochlear nerve on imaging does not preclude auditory innervation of the cochlea in some cases.27–29 Cochlear implantation can be a valuable option for patients with apparent cochlear nerve aplasia as long as they have undergone appropriate audiometric testing.27,29 Use of electrically evoked ABRs is critical in the evaluation of this group of patients.27,31,32 If there is proof of a bilateral absence of the cochlear nerve (on scanning), and possible cochlear nerve fibers (not clear on scanning) cannot be stimulated, or if there is a complete cochlear aplasia or a bilateral Michel deformity, ABI can be the only solution to provide some chance of hearing development.1 It seems that in the majority of inner ear malformations there is no primary indication for an ABI33 (see Chapter 15). Two possible causes of total bilateral deafness after blunt head trauma are mentioned in the literature in relation to ABI: a bilateral posttraumatic fracture of the cochlea/labyrinth or cochlear nerve avulsion. Several patients with posttraumatic deafness, diagnosed as either bilateral fracture or cochlear nerve avulsion, have been implanted with an ABI. These indications are true controversial ABI indications as discussed in literature33 and in the following two sections. A fracture of the petrosal bone can be otic capsule–sparing or otic capsule–involving. When the otic capsule is involved in the fracture, total deafness usually occurs. In very rare cases both otic capsules could be fractured, resulting in total bilateral deafness. As soon as the patient is recovering from this huge trauma, occurrence of inflammation and finally ossification of the cochlea should be assessed with CT and especially with MRI to evaluate the patency of the cochlea, as in patients with meningitis. In our opinion, there is an indication for an ABI in cases with complete ossification of both cochleae and after unsuccessful cochlear drill-out attempts (bilateral). In the literature not a single case has been demonstrated with bilateral complete ossification due to head trauma, so this indication for an ABI remains hypothetical. Cochlear implantation is the means of auditory rehabilitation in cases of bilateral otic capsule fractures.34–37 The other theoretically valid indication reported for an ABI in deafness due to trauma is bilateral avulsion of the cochlear nerve.38 This indication has also been mentioned in the consensus paper on the indications of ABI.26 Surprisingly, in a literature review of all neuroradiologic papers or books concerning this topic there has been reported only a single case presenting a unilateral nerve rupture.39 Cochlear nerve avulsion in both ears has never been reported and seems practically impossible without fatal damage to the head and brain. Accordingly, the indication for ABI for this specific entity seems very unlikely (see Chapter 15). Auditory brainstem implantation is not primarily indicated in severe to profoundly deaf patients with an intact nerve and an open cochlea; for these patients cochlear implantation is the first option. Even if the result with a CI is not as good as hoped for or as predicted, the option of refitting/reprogramming the processor, replacing the ipsilateral, or evaluating the contralateral ear for cochlear implantation should be contemplated, before even considering an ABI because a CI in general gives better outcomes in hearing restoration than an ABI33. In patients with a vestibular schwannoma (VS) in the only hearing ear there seems to be few indications for an ABI. The contralateral deaf ear is almost always compatible for a CI, although the results with a CI will depend on the duration of deafness and more specifically on the development of the auditory pathways serviced by that cochlear nerve. Many options are possible before auditory brainstem implantation enters the picture.40 The options before surgery of the VS in these cases are: a hearing aid in that ear if surgery can wait, or a cochlear implant in the contralateral ear.41 The result of cochlear implantation can sometimes be poor, which suggests subsequent auditory brainstem implantation during the tumor removal procedure.42 However, it does not seem proper to implant an ABI before a contralateral CI has been tried. Two other options should be considered prior to surgery of the tumor: radiotherapy or cochlear implantation on the VS side without tumor removal. Also several options during surgical removal of the VS are possible: the hearing may be preserved during the removal of a small VS11; or the cochlear nerve may be preserved and a CI implanted in the same ear43; or the tumor may be debulked and a CI inserted. A primary ABI indication seems less favorable in regard to all the above-mentioned options in patients with a VS in the only hearing ear (see Chapters 16 and 17). Ossification of the cochlea can occur in autoimmune inner ear diseases similarly to that in postmeningitis cases, but bilateral fully ossified cochleae in these cases have not been reported.44 See Chapter 11. In general, Von Hippel-Lindau disease is associated with endolymphatic sac tumors that can destroy the labyrinth and the temporal bone. Bilateral complete destruction of the labyrinth including the cochlea appears to be very exceptional but would represent an ABI indication. However, successful CI placement in similar cases with Von Hippel-Lindau disease have been published.45 Auditory neuropathy spectrum disease (ANSD) is also mentioned as an ABI indication in the literature.38 However, this little-known diagnostic entity presents predominantly with a normal cochlea and an intact cochlear nerve. The precise location of the pathology in ANSD is variable, making cochlear implantation an option only for some patients. In patients, mostly children, with ANSD the stepwise rehabilitation should be: hearing aids; cochlear implant; and if the young patient still does not show adequate improvement or when speech and language development has reached a standstill, ABI could then be an option. Thorough and careful analysis and counseling of these patients should be performed as in this variable disease decisions are very individually based. The literature shows average to good results in patients with ANSD and a CI.46,47 In severe retrofenestral otosclerosis, the temporal bone shows otospongiotic hypodense lesions around the otic capsule and at the same time often intracochlear ossification, most prominently present in the scala tympani.48 These lesions can hamper the insertion of a cochlear implant and create potential risks, as explained in Chapter 13.49–51 Despite these potential complications, cochlear implantation still remains the first choice in hearing rehabilitation in patients with otosclerosis and severe to profound sensorineural hearing loss.33 Auditory brainstem implantation presents a solution for hearing revalidation in a very limited group of patients. The outcomes in hearing with an ABI at present are unpredictable and in general worse than with a CI, which limits the value an ABI. In general, if a patent cochlea and a cochlear nerve are present the first choice for hearing restoring would be cochlear implantation. ABI is indicated in • NF2 patients when the cochlear nerve is not preserved and the contralateral ear has no functional hearing. • NF2 patients when the cochlear nerve is preserved but stimulation via a cochlear implant is not possible. • Completely ossified cochleae; but a cochlear implantation with drill-out procedure should be attempted first. • Absence of the cochlear nerve proven on scanning and in audiometric testing. • Bilateral complete cochlear aplasia (Michel deformity). CI is first choice of treatment in • Normal cochlea and cochlear nerve present. • Posttraumatic deafness. • Otosclerosis. • Most of the malformations of the cochlea with some sort of cochlear lumen, in presence of a cochlear nerve. • Solitary vestibular schwannoma in the only hearing ear (several options of alternative treatment possible). • Autoimmune inner ear disease, Von Hippel–Lindau disease, auditory neuropathy spectrum disorders (ANSD). The surgical anatomy of the lateral skull base, brainstem, and cochlear nucleus is addressed in detail in Chapter 2. Since the ABI relies on the electrical stimulation of the cochlear nucleus complex for the perception and recognition of speech, the surgical goal of auditory brainstem implantation is to position the electrode array correctly against the surface of the cochlear nuclei in the lateral recess of the fourth ventricle. Usually, the intraventricular part of the ventral cochlear nucleus and the dorsal cochlear nucleus are both stimulated, although the ventral cochlear nucleus has the best connections with the auditory pathway. The extraventricular ventral cochlear nucleus represents the continuation of the root of the eighth cranial nerve. Access to the lateral recess of the fourth ventricle is gained through the foramen of Luschka (see Fig. 8.1–Fig. 8.3). Landmarks of the Lateral Recess • Origin of the vestibulocochlear and glossopharyngeal nerve • The flocculus of the cerebellum • The choroid plexus exiting the foramen of Luschka (this is the most reliable landmark according to our experience).
8.1 Rationale
8.2 Indications
8.2.1 Neurofibromatosis Type 2
8.2.2 Postmeningitis Ossification
8.2.3 Inner Ear Malformation
8.2.4 Cochlear Nerve Aplasia/Cochlear Nerve Deficiency
8.3 Controversial Indications
8.3.1 Otic Capsule Fracture and Cochlear Nerve Avulsion
Otic Capsule Fracture
Cochlear Nerve Avulsion
8.3.2 Bilateral or Unilateral Patent Cochlea
8.3.3 Vestibular Schwannoma in the Only Hearing Ear
8.3.4 Autoimmune Inner Ear Disease
8.3.5 Von Hippel–Lindau Disease
8.3.6 Auditory Neuropathy
8.3.7 Otosclerosis
8.4 Indications for ABI/CI: Summary
8.5 Surgical Steps of Auditory Brainstem Implantation
8.5.1 Surgical Anatomy
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