CHAPTER 161 Central Neural Auditory Prosthesis
Multichannel cochlear implantation is now well established as an extremely effective method of hearing restoration for congenital and acquired sensorineural hearing loss. When conditions involving the cochlear nerve make peripheral intracochlear stimulation ineffective, some patients cannot benefit from cochlear implantation. Most commonly, this situation occurs in patients with neurofibromatosis type 2 (NF2), where bilateral vestibular schwannoma growth or surgical removal results in loss of cochlear nerve function. In this situation, direct electric stimulation of the cochlear nucleus at the brainstem using an auditory brainstem implant (ABI) is possible. Although the hearing outcomes with ABIs are limited compared with cochlear implants, the ABI is currently the most sophisticated and successful central neural prosthesis.
There is now a large experience with the use of ABIs in NF2 patients. Although some auditory benefit is usually achieved, it is generally limited to perception of environmental sound and an aid to lip reading. Rarely, open set speech understanding is achieved.
More recently, there has been increased interest in the use of ABIs for conditions where cochlear implants are ineffective, in particular, postmeningitic cochlear ossification or bilateral eighth cranial nerve injury from temporal bone fractures or avulsion. Better hearing outcomes have been observed in these patients without NF2, suggesting that NF2 is associated with intrinsic dysfunction of the cochlear nucleus. This suggestion has led investigators to attempt stimulation higher up the auditory pathway at the inferior colliculus with auditory midbrain implants (AMIs). The improved performance with ABIs in patients without NF2 has also provided hope that children with cochlear aplasia, who are unsuitable for cochlear implants, might benefit from ABIs.
Direct stimulation of the central nervous system entails unique safety concerns. The mechanisms of tissue damage resulting from the direct injection of current into brain parenchyma have been extensively studied.1–3 Charge transfer from an electrode to biologic tissue depends on two independent electrochemical mechanisms, as follows:
Safety thresholds of neuroprosthetic stimulation depend on several factors. Yuen and colleagues4 have shown that the extent of neural injury with contemporary electroprosthetic materials producing direct stimulation closely correlates with the charge density per phase. Charge density directly relates to stimulation intensity, stimulation duration, and the effective area of the electrode terminus. The effective area of an electroprosthesis can differ from its geometric area. Brummer and Turner5 measured the real surface area and suggested that surface roughness factors would range from 1.4 to 30.
The threshold for damage to the cerebral cortex found by Agnew and associates6 was 320 µA, with a corresponding charge density of 3200 µcoul/cm2/phase. Niparko and colleagues7 found that the threshold for tissue damage within the cochlear nucleus (with a penetrating electrode) was 150 µA, a current that corresponded with a charge density of 600 µcoul/cm2/phase. Stimulation at intensities of 150 µA and 200 µA (approximately 600 µcoul/cm2/phase and 800 µcoul/cm2/phase) produced significant tissue response at the site of the electrode terminus with neuronal loss, fiber necrosis, and reactive cells present. The injury threshold was found to exceed the threshold for functional activation, however, by a factor of at least five across all animals studied.
Differences between the damage thresholds determined across studies of central nervous system stimulators may relate to differences in implantation sites and variance in calculations that relate to effective versus geometric surface area. Differences also may be attributable to differences in microelectrode design (e.g., percentage of iridium) and fabrication that affect real charge densities at the electrode terminus for the same stimulation intensities.
The concept of auditory brainstem implantation was introduced by House and Hitselberger at the House Ear Institute (HEI, Los Angeles, CA). In 1979, they implanted a two-ball electrode in a woman with NF2 undergoing acoustic neuroma removal. This first ABI was stimulated with a modified hearing aid, and the patient received useful auditory sensations.8 Nonauditory stimulus occurred, which limited use of the electrode. Subsequently, this original electrode was replaced with a specially designed two-electrode surface array, which used a Dacron mesh carrier. This electrode was attached to a percutaneous pedestal connector and was stimulated by a modified 3M-House cochlear implant speech processor. This patient has continued to achieve acoustic percepts with electric stimulation, and has continued to use the device successfully 10 to 12 hours a day.9 Between 1984 and 1992, 24 NF2 patients were implanted at HEI, with the single-channel, initially two-electrode, and subsequently three-electrode, ABI under an investigational device exemption from the U.S. Food and Drug Administration (FDA).
The first fully implantable ABI was manufactured by Cochlear Pty Ltd, based on the CI 22 mini–cochlear implant. The electrode array was modified to an eight-electrode pad, developed by HEI in collaboration with Cochlear Ltd and Huntington Medical Research Institute (Pasadena, CA). After initial trials at HEI, a multicenter clinical trial, based in North America, was undertaken. In Europe, a 21-electrode array was also developed by Cochlear Ltd and used with the same receiver stimulator.10–13 Clinical experience from these trials led to the best features of both electrode arrays being combined to create the current Nucleus ABI electrode, a 21-electrode array connected to the CI 24M receiver stimulator (Fig. 161-1). The system functions similar to a cochlear implant with a fully implantable receiver stimulator package and an externally worn speech processor and transmitting coil. The receiver stimulator package has a removable magnet because many recipients require repeated postoperative cranial magnetic resonance imaging examinations.
Figure 161-1. The current auditory brainstem implant (ABI) is a 21-electrode array that is stimulated similar to a cochlear implant through a transcutaneous radiofrequency transmitter and receiver. A, Internal component of ABI. B, Enlarged view of electrode array.
(Courtesy of Cochlear Ltd.)
ABIs have also been produced by Med El in Europe and Advanced Bionics in the United States. The function of all these devices is the same as a cochlear implant with variation only in the size of the electrode pad and number of electrode contacts.
The Nucleus 24 ABI is the multichannel 21-electrode surface array, which remains commercially available with FDA and CME approval. Studies have been undertaken at HEI with a penetrating electrode array with limited success. The most recent development is the use of an AMI, in an attempt at inferior colliculus stimulation, either with a surface array or with a penetrating electrode.
With respect to biocompatibility, there is no clinical evidence for neuronal injury occurring as a result of chronic electric stimulation, as evidenced by the demonstrated gradual improved performance over time.
The most common indication for an ABI is in patients with NF2. In the United States, the ABI is approved only for use in NF2 patients. The current criteria for use in the United States are postlingual NF2 patients 12 years old or older, undergoing first or second side vestibular schwannoma removal. There are no specific audiologic criteria.
Auditory brainstem implantation should be considered in all NF2 patients undergoing vestibular schwannoma removal, in whom there is no expectation of ipsilateral residual hearing. ABIs are indicated particularly if contralateral hearing is absent or poor, or if there is a contralateral tumor that is likely to result in hearing loss in the foreseeable future.
Usually, the ABI is placed at the time of second side vestibular schwannoma removal, when the patient either has a preoperative profound-to-total hearing loss or will have one postoperatively. Some patients choose to have an ABI placed at the time of first side tumor removal (if hearing preservation is not attempted), as a so-called sleeper, in the expectation that the patients will not use the device while they have useful residual hearing in the contralateral ear.
An ABI can be successfully placed as a second-stage procedure after previous vestibular schwannoma surgery. There may be an advantage in delayed implantation if, at the first surgery, there is a large tumor deforming the brainstem and distorting the lateral recess anatomy and position. There is potential, however, for fibrosis and loss of brainstem landmarks occurring after initial tumor surgery that would make placement of the ABI difficult at a second procedure. Prior stereotactic radiotherapy, particularly by gamma knife, is a relative contraindication to ABI placement because of the potential for radiation necrosis of the cochlear nucleus region or fibrous tissue preventing proper device placement. There have been reports where this has been a problem, and where successful ABI use after radiotherapy was achieved.14,15 The use of ABIs in patients without NF2 is discussed later in this chapter.
Much of the early ABI research focused on gaining a better understanding of the three-dimensional anatomy of the cochlear nerve root, foramen of Luschka, and cochlear nucleus, and potential surgical approaches.16–23 The cochlear nucleus comprises ventral and dorsal components, situated in the pons where the eighth cranial nerve enters the brainstem at the pontomedullary junction, medial to the cerebellar peduncle. The surfaces of the ventral and dorsal cochlear nuclei are exposed in the anterosuperior aspect of the lateral recess of the fourth ventricle. The ABI electrode is placed into this recess. The foramen of Luschka is the lateral opening of the recess and contains choroid plexus.
The surface landmarks for the lateral recess are the adjacent lower cranial nerves. The foramen of Luschka is situated immediately posterior to the origin of the ninth glossopharyngeal and tenth vagal nerve rootlets, inferior to the eighth nerve root entry zone and posteroinferior to the seventh nerve origin. The flocculus of the cerebellum overlies the foramen. The recess runs in a posteromedial direction from the foramen of Luschka to enter the fourth ventricle.
The translabyrinthine approach has been favored by most surgeons for ABI placement in NF2 patients undergoing tumor removal.10 Some centers have used a retrosigmoid approach for recipients with NF2 and without NF2.24 The translabyrinthine approach provides a more lateral view of the brainstem and better view into the foramen of Luschka. Possibly this better view assists in atraumatic electrode array placement; however, translabyrinthine and retrosigmoid approaches can be used effectively. Cerebellar retraction is reduced or avoided by the translabyrinthine approach. If a retrosigmoid or lateral suboccipital approach is used, it is important that the exposure is made as far forward and inferiorly as possible, skeletonizing and retracting the sigmoid sinus anteriorly, to allow a low anterior approach to give the most direct access to the region of the foramen, with the least cerebellar retraction.
The cochlear nucleus is identified within the lateral recess of the fourth ventricle. The cochlear nucleus is identified by following the eighth and ninth cranial nerves medially. The flocculus is retracted posteriorly, and the foramen of Luschka is identified, immediately posterior to the origin of the ninth cranial nerve. The cochlear nerve is followed medially, as it enters the lateral recess of the fourth ventricle. Dissection is carried between the eighth cranial nerve and the choroid plexus, which may need to be removed or shrunk with bipolar cautery. Within the lateral recess, as the cochlear nerve is followed medially, two prominent structures can be visualized anterosuperiorly curving around the brainstem caudally to the pontobulbar sulcus. The cranial structure is the bulging of the cochlear nucleus, and the caudal structure is the pontobulbar body. Between them, a small straight vein is a constant finding and important landmark. The taenia of the choroid plexus is a fibrous band at the entrance of the fourth ventricle. This band usually needs to be divided to allow visualization within the lateral recess and create room for the electrode array insertion. The location of the lateral recess usually can be confirmed by observing flow of cerebrospinal fluid as the anesthesiologist induces a Valsalva maneuver in the patient.
Positioning the electrode array within the lateral recess is crucial to successful auditory stimulation. If the array is too lateral, stimulation of the glossopharyngeal and facial nerves or other lower cranial nerves and cerebellar flocculus can occur. If the array is too deep, the medial electrode contacts would lie within the fourth ventricle and be ineffective. Similarly, caudal or cranial rotation of the electrode array can significantly affect position over the cochlear nucleus. Figure 161-2 shows a schematic representation of the electrode array position.