84.2.1 Auditory Brainstem Implant

Drs. William Hitselberger and William House performed the first ABI in 1979 by placing a ball-type electrode onto the surface of the cochlear nucleus.s. Literatur ^,​ s. Literatur U.S. Food and Drug Administration (FDA) approval was obtained in 2000 for the multichannel ABI manufactured by Cochlear Limited (Sydney, Australia). ABIs have also been produced by other implant manufacturers including Med-El, which are currently not approved in the United States.

A penetrating electrode ABI was subsequently developed in an effort to improve the precision and selectivity of brainstem auditory neuron stimulation with the goal of improving speech recognition (Fig. 84‑1 A). A hybrid ABI array consisting of a version of the present surface electrode and a 10-electrode penetrating array was studied in an FDA clinical trial of 10 patients. While this device resulted in increased selectivity and lowered charge requirements in the usable penetrating electrodes, the device did not provide improved speech discrimination.s. Literatur As of 2017, all commercially available devices do not utilize penetrating electrodes.

The production of the Cochlear Nucleus 24 ABI was discontinued in 2015, and the replacement Cochlear Device, ABI 541, was not approved by the FDA until 2016 (Fig. 84‑1 B). During this time, both Cochlear and Med-El devices were implanted under the auspices of “Compassionate Use” approval by the FDA in the United States on an individual, case-by-case basis.s. Literatur FDA approval was granted in June 2016 for the ABI 541. The Med-El device continues to be implanted in the United States through an open clinical trial (ClinicalTrials.gov ID: NCT01736267). Approximately 1,300 patients have been implanted worldwide to date (Fig. 84‑1 c).

84.2.2 Auditory Midbrain Implant

The inferior colliculus (ICC) is a desirable target for an AMI auditory prosthesis because the central nucleus of the ICC is a loci of convergence for ascending auditory pathways and is readily accessible during surgery.s. Literatur ^,​ s. Literatur Currently, two AMI options include the use of a surface electrode using the Med-El ABI array and a penetrating array utilizing a Cochlear Corporation deep brain stimulation (DBS) array (Fig. 84‑1 D).s. Literatur ^,​ s. Literatur

84.2.3 Cochlear Implantation

The prospect of performing a CI in NF2 patients was first proposed by Cueva et al after successfully performing promontory stimulation in patients with deafness after VS resection.s. Literatur The benefits of this approach were first realized in a single NF2 patient who received a CI after resection of VS where the cochlear nerve was spared.s. Literatur

84.3.1 Auditory Brainstem Implant

This device was originally designed for NF2 patients with bilateral VSs. FDA criteria includes bilateral eight-nerve tumors, age 12 years or older, psychological suitability, willingness to comply with the follow-up protocol, and realistic expectations. Several reports suggest positive outcomes in adults and children for the treatment of deafness resulting from traumatic transection or avulsion of the cochlear nerve, cochlear ossification after meningitis, and congenital cochlear aplasia or cochlear nerve deficiency. These topics will not be reviewed further in this chapter, but the following references are available for the interested reader.s. Literatur ^,​ s. Literatur ^,​ s. Literatur ^,​ s. Literatur

At the House Clinic, our NF2 protocol allows implantation at the time of first- or second-side VS removal or in patients whose tumors have previously been removed. Implantation during removal of the first tumor allows the patient to gain experience with the device with the goal of possible enhanced performance if the patient loses all hearing. Suitable candidates are patients undergoing translabyrinthine or retrosigmoid VS removal who have (1) nonaidable hearing or an only hearing ear with a symptomatic tumor or (2) serviceable hearing in the contralateral ear but a contralateral tumor of sufficient size to indicate that hearing will potentially be lost. Implantation on the first side gives the patient two chances at obtaining an optimally functioning system should the procedure in the first side not be successful, which may occur in up to 8% of cases.

84.3.2 Auditory Midbrain Implant

Damage to the brainstem region, either due to the tumor or during tumor removal, may make placement of an ABI impossible or ineffective. In these rare cases, AMI may be advantageous to bypass this damaged region to provide the best opportunity for auditory rehabilitation. AMI placement remains an area of active investigation and is only being performed in a research setting.s. Literatur

84.3.3 Cochlear Implantation

Cochlear implantation is traditionally used in patients without retrocochlear disease because a lesion involving of the cochlear nerve or proximal could render the procedure ineffective. However, recent data suggest benefit for properly selected patients with NF2. CIs are advocated for cases of nonserviceable hearing with intracochlear and intralabyrinthine schwannomas.s. Literatur Further indications include stable VSs with an anatomically intact cochlear nerve after surgical resection, radiosurgery, or during the course of conservative observation. The cochlear nerve is presumed to be intact in all patients with conservatively managed tumors or those who have received radiation treatment. Patient candidacy for CI after microsurgical resection may be more difficult, particularly if details regarding the original surgery are not well described. In such cases, electrical promontory stimulation testing may be beneficial.s. Literatur Ideal postoperative candidates are those with smaller tumors who underwent attempted hearing preservation surgery, but lost serviceable hearing as a result of presumed vascular compromise. However, even patients who underwent translabyrinthine surgery may benefit from CI if the cochlear nerve was preserved. In these cases, use of a spacer or immediate implantation is recommended given the concern for cochlear ossification following translabyrinthine drilling. In an English consensus statement protocol evaluating the candidacy for ABI or CI in NF2, CI is considered to be the most efficacious modality for hearing rehabilitation in patients who have been treated with radiation or who have two stable VSs.s. Literatur

84.4.1 Auditory Brainstem Implant

During tumor dissection, care is taken to avoid trauma or injury to the area of the brainstem adjacent to the root entry zone of the vestibulocochlear nerve. The anatomical target for the ABI is the ventral cochlear nucleus, which is the main relay for cochlear nerve input and the ascending auditory pathway. The ventral cochlear nucleus lies within the lateral recess of the fourth ventricle. The surgical approach for tumor removal in ABI cases at the House Clinic has been exclusively via translabyrinthine craniotomy; however, other centers offer the retrosigmoid approach with success.s. Literatur ^,​ s. Literatur The retrosigmoid approach is particularly beneficial in cases where the sigmoid sinus is located extremely anteriorly or in pediatric cases. Electrodes are placed for recording electrically evoked auditory brainstem responses (EABRs) and for monitoring cranial nerves VII and IX/X. The choroid plexus marks the entrance to the lateral recess (foramen of Luschka), and the taenia choroidea obliquely traverses the roof of the lateral recess, marking the surface of the ventral cochlear nucleus. The ninth cranial nerve can also be used as a reference point for the lateral recess. After identifying the foramen of Luschka, microinstruments are used to insert the electrode array into the lateral recess with the electrodes facing superiorly (see Chapter 30).

The technique of electrode placement is dependent on the device that is utilized. The Med-El Synchrony device first uses a four-channel ABI placing electrode as a test electrode that is inserted into the lateral recess of the fourth ventricle to generate EABRs from the cochlear nucleus. Measurements are repeated until adequate EABR tracings are obtained for all electrode combinations. The placing electrode is then removed, and the active implant is then placed using EABR data to assist with positioning. A final EABR recording using the active electrode array is then performed confirming correct placement. For the Cochlear ABI 541 device, EABR is performed after placement of the active ABI, confirming placement at the cochlear nucleus. Repositioning is performed as needed with the active ABI device. Both electrode arrays are secured by a small piece of Teflon felt packed into the meatus of the lateral recess.

The cortical bone posterior to the mastoid is flattened, and a trough is drilled to accommodate the receiver/stimulator electrode similar to CI. The receiver/stimulator is placed into a circular area of bony cortex posterosuperior to the mastoid defect created by an otologic drill. A tie-down technique or tight periosteal pocket is used to secure the device. The magnet from the receiver-stimulator is removed to allow high-quality surveillance magnetic resonance imaging (MRI). Abdominal fat is used to obliterate the mastoid defect followed by a three-layered closure.

84.4.2 Auditory Midbrain Implant

Resection of VS followed by AMI placement utilizes a suboccipital craniotomy. Following resection of the VS, the cerebellum is dissected inferiorly to provide exposure to the ICC.s. Literatur Arachnoid is resected to expose the midbrain and the ICC is visualized. The AMI is inserted into the ICC with the tonotopic orientation of the ICC. Landmarks for insertion include the rostral border of the ICC with the superior colliculus, caudal ICC edge corresponding to the exit point of the trochlear nerve, and the midline between both ICCs.s. Literatur Insertion requires an angle of 40 degrees relative to the sagittal plane.

84.4.3 Cochlear Implantation

Standard cochlear implant arrays and techniques are utilized for cases of implantation in patients with NF2. Carlson et al advocated consideration of an electrode with a stylet in cases of intracochlear schwannomas with late deployment to overcome possible resistance that may be encountered due to the presence of intracochlear tumor.s. Literatur

84.5.1 Auditory Brainstem Implant

The goal of ABI is to provide the patient with some degree of environmental sound awareness, word recognition, and also improved communication in conjunction with lipreading. A minority of patients may also obtain some open-set speech understanding. Preoperatively, specific effort should be made to inform patients of these limitations and to help form realistic hearing expectations.

Over 310 patients with NF2 have been implanted with the Nucleus multichannel ABI system at the House Clinic between 1992 and 2017. With notable exceptions, ABI performance generally has not reached the high levels typically seen with CIs. Eighty percent of the patients are device users, and 92% have received auditory sensations from their ABIs. The majority of patients recognize a high percentage of environmental sounds, and the ability of speech understanding is enhanced an average of 35% when ABI sound is combined with lipreading. This enhancement has reached as high as 75% in some individuals. Although improvements are generally greatest during the first year, many patients have continued to improve even after 10 years of use. All patients have used the SPEAK (spectral maxima) speech processing strategy.s. Literatur

At our center, approximately 25% of ABI users have achieved some open-set speech discrimination defined as at least 20% correct without lipreading cues on the CUNY Sentence Test.s. Literatur ^,​ s. Literatur Ten patients at the House Clinic have scored 65% or better, and three patients have scored 82% or better on this test. Table 84‑1 shows outcomes from the House Ear Institute and from other centers.

Recent reports notably illustrate the successful ABI outcomes among NF2 patients with as high as a 37% rate of open speech perception at 1 year and 41% at 2 years after activation.s. Literatur ^,​ s. Literatur An inverse relationship between deafness duration and open speech perception was noted among these patients. These outcomes have reportedly improved upon data from the United States and factors accounting for these improved outcomes are not well understood and an area of active investigation.s. Literatur Hypothetically, differences in surgical technique may account for improved outcomes. The retrosigmoid approach in the semi-sitting position with a Med-EL ABI device (not FDA approved in the United States) is primarily utilized by European centers. The semi-sitting position may facilitate brain relaxation and bloodless dissection. Positioning may improve hemostasis and health of the neural tissue in the auditory brainstem nucleus. The semi-sitting position does increase the risk of air embolism. A second untested hypothesis is that device differences may also account for differential outcomes. The Med-El device has enhanced cable flexibility and a smaller surface array profile, possibly aiding placement. The Med-El device also has a four-channel test electrode that allows EABR confirmation prior to opening the ABI device packaging and subsequent implantation. Other possible ABI hearing outcome prognostic factors include a history of Gamma Knife radiosurgery (GKRS) for VS which is associated with a higher rate of no hearing benefit (~30 vs. 9%). However, there are many examples of patients with large VS at our center, or a history of GKRS, who have experienced great benefit from their ABIs.