34.1 Introduction

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.

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).