Auditory brainstem implants, first intended for auditory rehabilitation in deaf patients with neurofibromatosis-2, have been more recently applied in the pediatric populations who are not candidates for cochlear implantation. Herein we review the history of these devices and surgical techniques in auditory brainstem implantation.
We include a review of recently published literature including outcomes data in this unique population.
34 Brain Stem Implantation
Hearing rehabilitation options are dictated by the nature and etiology of the hearing loss. Patients who do not have a functional auditory nerve connecting the cochlea to the cochlear nucleus in the brainstem cannot benefit from a cochlear implant because they lack a means of sending auditory information centrally. In order to bypass the peripheral auditory system, auditory brainstem implants (ABIs) were developed. This chapter will review ABI technology and potential applications in the pediatric population.
The first successful ABI for long-term auditory rehabilitation was performed in 1979 at the House Ear Institute in California. 1 , 2 The patient was a 51-year-old woman with neurofibromatosis type 2 (NF2) who was undergoing vestibular schwannoma removal in her only hearing ear secondary to tumor growth and development of mild hydrocephalus. She was initially implanted with a platinum electrode pair with 0.5 mm balls separated by 1.5 mm, which was placed within the brainstem in the region that was determined to be the cochlear nucleus. She reported benefit from the implant, with enhanced lip reading and environmental sound awareness, for approximately two months and then unfortunately auditory benefit deteriorated. A new electrode system was designed and constructed and the patient was re-implanted with a new system composed of two 1.7 × 2.0 mm platinum pads mounted on a Dacron mesh, separated by a distance of 3.0 mm. The new design was developed to reduce the problem of electrode migration thought to be the cause of the initial implant failure. Re-implantation was performed in 1981 with the electrode pads positioned on the surface of the cochlear nucleus. Postoperatively, the patient again received similar benefit to the initial implant and per last published report continues to use the implant in her daily life. 2
Based on further experience with ABI, and documenting safety and potential efficacy, in 2000, the Food and Drug Administration (FDA) approved the use of a multichannel ABI in patients 12 years of age and older who have been diagnosed with NF2. In 2013, the FDA approved an investigational device exemption (IDE) for the use of ABIs in children who are born without an auditory nerve or without a cochlea or who are otherwise not candidates for cochlear implantation. Over 1,000 ABI procedures have been performed worldwide. 3 Results have been mixed, but the ABI continues to offer hope for a population of patients who otherwise do not have access to the auditory world. This can be important from a quality-of-life standpoint, and added safety and security that comes from environmental sound awareness. 4 , 5 It may also serve as an adjunct to speech reading and enhance total communication approaches. ABI design, placement, and speech processing strategies continue to be a topic of interest in attempt to more consistently maximize results from these devices.
All auditory nerve afferent fibers carry information from the cochlea to the cochlear nucleus (CN) located near the surface of the brainstem. 6 The CN is located at the lateral aspect of the pontomedullary junction, and is divided into a ventral cochlear nucleus (VCN) and a dorsal cochlear nucleus (DCN). 7 The VCN is further subdivided into an anteroventral cochlear nucleus (AVCN) and a posteroventral cochlear nucleus (PVCN). 8 These nuclei are identified next to the lateral recess of the fourth ventricle. 7 There is noticeable variation in the shape of the CN in individuals; however, each CN extends about 8 mm dorsoventrally, 10 mm mediolaterally, and the rostrocaudal extent is less than 3 mm. 9 Each division of the nucleus is composed of different sets of neurons and each cell type encodes specific sound information. 8 The shape of the VCN and its proximity to the surface can be variable in humans. 9 Sound waves of low frequency are relayed to the VCN and sound waves of high frequency are relayed to the DCN. 8 , 10 , 11 However, the DCN has another level of complexity as it is also responsible for integrating auditory and multisensory inputs from distinct pathways, and studies have shown neuromodulators are responsible for enhancing inputs from one pathway while inhibiting inputs from another. 12 These excitatory and inhibitory pathways need to be further parsed to determine the optimal stimulation paradigm that an ABI would provide.
A study looking at the morphological and functional maturation of the CN showed that development occurs during mid-gestation and continues up to term. 8 The foramen of Luschka is the pathway to the CN intraoperatively. The foramen of Luschka projects into the cerebellopontine angle (CPA) at the lateral border of the pontomedullary sulcus. 13 The junction of the glossopharyngeal and vagus nerves with the brainstem is just ventral to the foramen and the junction of the facial and vestibulocochlear nerves with the brainstem is anterior inferior to it. The junction of the accessory and hypoglossal nerves with the brainstem is antero-inferior to the foramen as well. The cerebellar flocculus is directly superior to the foramen. The anterior inferior cerebellar artery (AICA) and the posterior inferior cerebellar artery (PICA) can have close and variable courses with the foramen. The vein of the cerebellomedullary fissure can course between the flocculus and the foramen of Luschka. 14 , 15
34.1.3 Auditory Brain Stem Implant Device
Just like a cochlear implant, the components inherent in all ABIs are a microphone, a speech processor, a receiver/stimulator, and an electrode array. The electrode array configuration is the main difference from a cochlear implant since the electrode array is designed to be placed on the surface of the cochlear nucleus. Cochlear Corporation (Sydney, Aus) is the only manufacturer of an ABI with current Food and Drug Administration (FDA) approval in the United States. In 2016, its newest model of the ABI, the Cochlear Nucleus Profile Auditory Brainstem Implant (ABI541) was introduced. 16 It has a paddle that consists of 21 platinum electrodes that is 8.5 × 3.0 mm in area and it’s designed to work with the Nucleus 6 sound processor.
MED-EL Corporation (Innsbruck, Au) also manufactures an ABI used elsewhere. The external audio processor is either an OPUS 2 or OPUS 1. The implant itself consists of the receiver/stimulator and an implantable soft silicone matrix with a 12-contact electrode array. The dimensions of the matrix are 5.5 × 3.0 mm. An additional reference electrode is also present to facilitate different stimulation modes. 17
Oticon medical is the third manufacturer of ABIs. The Digisonic SP ABI brainstem implant is designed with an array of 15 surface electrodes that is positioned on the cochlear nucleus. The Digisonic SP ABI is used with the Saphyr neo collection sound processor. 18
34.2 Preoperative Evaluation and Anesthesia
ABIs are meant to provide auditory stimulation directly to the cochlear nucleus; therefore, they can be considered in any patient who has an intact cochlear nucleus but does not have functional auditory nerves or in an individual with bilateral cochlear agenesis who cannot undergo cochlear implantation.
Traditionally, the main patients considered for ABIs were NF2 patients who have poor hearing bilaterally and large tumors that do not allow for preservation of the cochlear nerve. In these patients an ABI can be placed at the time of tumor removal from one side. More pediatric non-NF2 patients who are congenitally deaf are now being considered for implantation, such as patients with bilateral cochlear agenesis or absent cochlear nerves on imaging.
There are cases where a cochlea is present and there is a semblance of a cochlear nerve innervating the cochlea on high-resolution MRI. Even in these cases of suspected cochlear nerve deficiency, our current protocol is to perform cochlear implantation on one side as some of these children may receive more auditory benefit than expected with an ABI. The children who do not appear to receive any benefit, or minimal benefit, postcochlear implantation can then be considered for an ABI.
This raises an important clinical consideration relating to where to implant the ABI when other hearing devices are present. Considerations may include the degree of malformation or suspected presence of an adequate cochlear nerve. The side with a more developed lateral recess should be preferred. A more developed lateral recess entails a favorable entrance to the lateral recess or where cerebellar retraction could be minimized. Also the side with more developed neuronal structures should be targeted as this may have implications for the degree of development of other structures inherent to the central auditory pathway.
Among our cohort of pediatric ABI recipients, some continue to use a cochlear implant (CI) contralateral to the ABI while others have their CI device removed at the time of their ABI placement based on the above considerations.
Evaluations must take into account other deficits that the patients have, especially visual and neurocognitive deficits. Evaluation for candidacy should be performed in a multi-disciplinary approach involving pediatric neurotology, neurosurgery, audiology, speech therapy, and neuropsychology. Candidates should be able to undergo postimplant speech and auditory rehabilitation in order to gain the maximum benefit from their implant. Patients and parents of patients undergoing ABI should be counseled with regard to expectations after implantation.
34.3 Surgical Technique
An ABI can be placed using either a translabyrinthine approach or a retrosigmoid approach. In the NF2 population in which tumor removal is being performed simultaneously, a translabyrinthine approach is often preferred. Our protocol in the non-NF2 pediatric population involves a retrosigmoid approach as no tumor removal is involved. General anesthesia without long-term paralytics should be administered to allow for nerve monitoring. As for all skull base surgery, an experienced neuro-anesthesia team and electrophysiologic monitoring team is critical. Pediatric ABI should only be undertaken at centers with prior experience with adult ABI and with pediatric neurosurgery expertise.
Perioperative antibiotics are administered as well as decadron, 0.5 mg/kg up to a max of 10 mg. Mannitol is also given once the craniotomy has begun for brain relaxation. The patient is positioned in the supine position with the head turned to the contralateral side. Continuous electromyograph facial nerve monitor electrodes are applied to the orbicularis oris and oculi on the surgical side and a Prass probe for active stimulation is made available. Subdermal electrodes are also placed for measuring an electrical auditory brainstem response (EABR) once the implant is in place. These electrodes are placed at the vertex of the head, over the seventh cervical vertebrae and the hairline of the occiput. An endotracheal tube with recurrent laryngeal nerve monitoring electrodes is also used for intubation in order to monitor cranial nerve X (▶ Fig. 34.1).
The patient is prepped and draped for a standard retrosigmoid incision. If a tumor is present, an appropriate approach is planned with regard to tumor size and location. The abdomen is prepped and draped for abdominal fat graft harvest as well.
The skin is incised down to the level of the temporalis fascia and reflected anteriorly and posteriorly. The incision is then carried through to bone and periosteum is elevated off the bone posteriorly and superiorly to the planned craniotomy site and the lateral sinus. Using a silicone replica of the receiver/stimulator a bony seat is drilled for the device as well as a trough for the electrode lead wire before it enters the dura through the craniotomy site (▶ Fig. 34.2). In order to secure the device, a permanent suture is used to close the mouth of the soft tissue pocket in which the receiver-stimulator will reside. The approach to the CPA is performed in the standard fashion and the lower cranial nerves as well as CN VII and VIII (if present) are identified (▶ Fig. 34.3). Nerve stimulation can be used to verify cranial nerve positions of VII and X. The foramen of Luschka is found between the roots of CN VII and CN IX, where the choroid plexus is identified. Alternatively, if a remnant of CN VIII is present, this can be followed back to the brainstem where the choroid plexus can then be identified. The choroid can then gently be spread to enlarge the opening into the foramen of Luschka. Often the tinea, a soft tissue arachnoid band is opened to allow access to the foramen. In addition, a vein over the foramen can be dissected away from the opening. To verify correct identification of the lateral recess of the fourth ventricle, a valsalva can be performed and CSF outflow should be noted.
At this point the device can be brought into the field and placed into the well previously made and sutured into place with a permanent suture (▶ Fig. 34.4). Both the suture and the bony seat prevent migration of the receiver/stimulator. The lead wires are placed into the trough and the free ground wire is placed medial to the temporalis muscle periosteum similar to a cochlear implant. We found that proper positioning of the Nucleus 24 Auditory Brainstem Implant required trimming the wings of the mesh on the back of the electrode paddle without damaging the electrodes. We then insert the paddle with the electrodes facing superiorly and anteriorly into the lateral recess of the fourth ventricle (▶ Fig. 34.5). To obtain optimal placement and the least amount of non-auditory stimulation, our audiology colleagues test the electrodes on the device by sending electrical stimulation to bipolar pairs of electrodes and determine whether an (EABR) is evoked or whether non-auditory stimuli patterns are noted such as facial nerve stimulation, myogenic responses, or changes in pulse rate or hemodynamics. The electrode paddle can be shifted if intraoperative EABR implant testing indicates a row or column of electrodes does not evoke any electrophysiologic responses consistent with auditory stimulation. With this intraoperative feedback, an optimal position of the paddle over the cochlear nucleus is obtained. A paddle-o-gram, which details auditory stimuli and non-auditory stimuli, is made by the audiology team (▶ Fig. 34.6). This assists the team during initial stimulation of the device postoperatively.
A piece of Teflon felt posterior to the electrode paddle can be used to stabilize the paddle in the lateral recess. The matrix of dacron mesh on the electrode paddle provides a scaffold for ingrowth of fibrous tissue that will further stabilize the electrode position.
The electrode lead is brought out through the inferior aspect of the dural flap if a retrosigmoid approach is performed and the dura is closed with 4–0 Neurolon suture with standard technique. The portion of the electrode that spans the dura is wrapped with a piece of muscle to prevent CSF leak from around the electrode insertion through the dura. Tisseal tissue glue is placed over the electrode entry into the dura and over the dural suture line. The bone plate is fixed with miniplates and screws and abdominal fat can be used as needed to pack craniectomy sites and to seal off mastoid air cells. Standard closure of the surgical site is then performed with standard mastoid dressing (▶ Fig. 34.7).