Auditory Brainstem Implants

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Auditory Brainstem Implants


Jose N. Fayad, William M. Luxford, Derald E. Brackmann, and William E. Hitselberger


 


image Overview


In 1979, William F. House and William E. Hitselberger implanted a pair of electrodes into the cochlear nucleus of a human volunteer with neurofibromatosis type 2 (NF2) during a surgical procedure to remove a second side acoustic neuroma. Since then, more than 200 patients at our institution have undergone implantation with auditory brainstem implants (ABIs). Stimulation of the electrodes produces auditory sensation in most patients (90%), with results similar to those for a single-channel cochlear implant. More recently, a combination of a surface electrode and a penetrating electrode has been used, with the goal of taking advantage of cochlear nucleus complex tonotopic organization.


We developed the ABI for patients with NF2 to electrically stimulate the cochlear nucleus complex. Patients with NF2 usually have bilateral vestibular schwannomas (VSs), necessitating tumor removal, which often results in profound deafness. The ABI is introduced into the lateral recess of the fourth ventricle and placed over the area of the ventral and dorsal cochlear nuclei after tumor removal. The ABI is similar in design and function to multichannel cochlear implants (CIs), except for differences in the design of the stimulating electrode arrays. The programming of ABI devices, however, differs in several important aspects from CI programming. Multichannel CIs and ABIs were developed to capitalize on the frequency tuning of neurons in the human cochlea and cochlear nucleus complex, respectively. Cochlear implants, which electrically activate peripheral neural processes within the cochlea, are not an option for patients with NF2 because of their loss of integrity of the auditory nerve.


In multichannel CIs, the electrode is placed into the cochlea. Consistent placement of the electrode carrier and its depth of insertion are assured in normal cochleas. However, in ABI recipients, anatomic landmarks that are used in electrode array placement may be altered or obscured due to the presence of tumors, making electrode array placement more challenging. This chapter describes the surgical anatomy of the cochlear nucleus complex, and our experience and results with ABI placement in individuals with NF2.


image Patient Selection/Device and Results


With two exceptions, only patients with NF2 and bilateral acoustic neuromas have received the ABI at the House Clinic. In these patients, the goal is to restore some auditory function so that these individuals can continue to be a part of the hearing world and to improve their quality of life. The ABI is placed during removal of their first tumor even if they have hearing on the other side, which is usually the case. This approach allows patients to become familiar with the use of the device and prepares them for when all hearing is lost.



Other possible indications include bilateral transverse skull fractures and avulsion of both cochlear nerves. More recently, in Europe, the indications for the ABI have included cochlear nerve aplasia and severe cochlear malformations in children, and complete ossification of the cochlea or cochlear nerve disruption due to cochlear trauma in adults (Table 16.1).


The current surface electrode ABI consists of 21 electrodes embedded in a silicone carrier that is fixed to a fabric mesh, connected to an implantable internal receiver/stimulator (Cochlear Corp., Englewood, CO). It received Food and Drug Administration (FDA) approval for commercial use in 2000. The current investigational penetrating ABI (PABI) consists of two arrays, a 12-electrode surface array, plus a 10-electrode array with needle microelectrodes. The external equipment for both devices consists of an external transmitter coil held in place by magnetic tape placed on the scalp over the receiver/stimulator coil and connected to a sound processor and microphone, which contains the battery-operated power source. All of this is similar to a cochlear implant. As long as the magnet is removed from the implanted receiver/stimulator, follow-up serial magnetic resonance imaging (MRI) scans can be obtained, as the rest of the implanted hardware is nonferromagnetic.


The sound processing unit (speech processor) requires appropriate programming and must be fitted to the individual user. Programming speech processors involves psychophysical assessment of electrically induced auditory (and nonauditory) percepts including threshold, comfort level, and pitch. These measures are programmed into the processor and used to control the amplitude and the sequential patterns of stimulation. ABI recipients have variations in brainstem anatomy, electrode array placement, and tumor effects that require the use of more individualized stimulus patterns to code frequency cues, and manage any nonauditory sensations than is typical for cochlear implants. Special techniques and additional time is usually required to program ABI sound processors appropriately.


Initial stimulation is performed 1 to 2 months after the surgery. Any nonauditory sensations are reduced or, if possible, eliminated by altering the electrical parameters of stimulation (particularly pulse duration and reference ground electrode). Non-auditory sensations have included dizziness, sensation of vibration in the eye, throat sensations, and ipsilateral tingling sensations in the head or body. Stimulation for the surface ABI and the PABI differs in some respects. Because the PABI has two arrays (a surface and a penetrating array), the sound processor is configured with programs using each electrode array separately, and an additional program using a combination of electrodes from both arrays. Performance is assessed with each of these configurations, adding to the time necessary to manage these research patients.


To date, more than 500 patients have been implanted worldwide using the ABI. The safety of this device has been comparable to the safety of cochlear implants. About 92% of our patients have received auditory sensations from their ABIs, and 80% of those implanted are device users. Approximately 16% of patients have achieved limited open-set speech discrimination (at least 20% correct in sound only on the City University of New York [CUNY] Sentences Test). A few ABI recipients have scored in the vicinity of 50% or better in a sound-only condition on this test. The majority of the patients recognize some environmental sounds, and speech understanding ability is enhanced an average of 30% when ABI sound is added to lipreading cues. This enhancement has ranged up to as high as 70% improvement for some patients.


The PABI was developed in an effort to improve the precision of stimulation of brainstem auditory neurons, and is in the clinical trials phase under auspices of the FDA. Three PABI recipients implanted during phase I at House Research Institute (HRI) have been extensively tested over the past 2 years. The patients use their PABI devices daily, with benefit. Their performance has been stable, and speech perception as measured on the CUNY Sentences Test is improved an average of 30% in the sound plus lipreading condition (as compared with the lipreading only condition). PABI recipients report a wide range of pitch sensations on both penetrating and surface electrodes, which we believe enhances speech perception performance. The study will continue with implantation of up to 10 patients with the second-generation PABI.


image Surgical Anatomy


The cochlear nucleus complex (dorsal and ventral cochlear nuclei) lies in the lateral recess of the fourth ventricle. It is partially obscured by the cerebellar peduncles. A surface electrode introduced in the lateral recess crossing the tinea choroidea will stimulate viable cochlear nuclei.


Recently Abe and Rhoton described in detail the microsurgical anatomy of the cochlear nuclei. The vestibulocochlear nerve enters the brainstem slightly rostral and ventral to the foramen of Luschka near the flocculus at the lateral end of the pontomedullary sulcus. The facial nerve arises ventromedial to the vestibulocochlear nerve in the lateral part of the pontomedullary sulcus. The average distance between the center of the facial and cochlear nerves at their junction with the brainstem is 3.8 mm. The glossopharyngeal and vagus nerves arise caudal to the facial nerve. The average distance between the centers of the junction of the cochlear and glossopharyngeal nerves with the brainstem is 6.4 mm and between the facial and glossopharyngeal nerves is 6.3 mm.


The foramen of Luschka, the open end of the lateral recess of the fourth ventricle, is located slightly dorsal and caudal to the junction of the vestibulocochlear nerve with the brainstem, dorsal to the junction of the glossopharyngeal nerve with the brainstem, and ventral and caudal to the flocculus. The lateral recess is a narrow, curved pouch formed by the union of the roof and the floor of the fourth ventricle. The caudal wall of the recess is formed by the tela choroidea that stretches upward from the narrow ridge, called the taenia, along the lower edge of the fourth ventricle and lateral recess. The choroid plexus arises in and attaches to the inner surface of the tela. The lateral recess and the foramen of Luschka open into the medial part of the inferior limb of the cerebellopontine angle. The flocculus projects into the cerebellopontine angle at the rostral edge of the foramen of Luschka at the junction of the cerebellopontine and cerebellomedullary fissures. The choroid plexus extends through the lateral recess and the foramen of Luschka into the cerebellopontine angle and often sits on the posterior margin of the glossopharyngeal nerve.


The dorsal and ventral cochlear nuclei are positioned in the lateral recess near the foramen of Luschka. The dorsal cochlear nucleus produces a smooth prominence, the auditory tubercle, on the dorsal surface of the inferior cerebellar peduncle in the upper part of the floor of the lateral recess. The medial edge of the dorsal cochlea nucleus is located just lateral to the vestibular area. The ventral cochlear nucleus is positioned between the junction of the vestibulocochlear nerve with the brainstem and the lateral edge of the dorsal cochlear nucleus. The ventral cochlear nucleus does not produce a discrete prominence on the surface of the brainstem as does the dorsal cochlear nucleus, whose position is marked by the auditory tubercle. The ventral cochlear nucleus is often partially hidden by the rhomboid lip, choroid plexus, and flocculus protruding from the foramen of Luschka. The ventral cochlear nucleus has two parts, cisternal and ventricular, which are separated by the edge of the small ridge or taenia. The ventral cochlear nucleus commonly lies partially inside the foramen of Luschka within the lateral recess and partially outside the foramen within the cerebellopontine angle cistern with the taenia of the rhomboid lip crossing its lateral surface. Terr and Edgerton also noted that the attachment of the taenia along the rhomboid lip often crosses the lateral surface of the ventral cochlear nucleus. The vestibulocochlear nerve pursues a medial, posterior, and caudal course from the internal acoustic meatus to the brainstem. The cochlear nuclei are oriented in a more posterior direction than the vestibulocochlear nerve, thus creating an angle at the junction of the dorsal surface of the nerve and long axis of the cochlear nucleus that averages 138 degrees.


image Surgical Technique


We administer preoperative antibiotics and continue them for 24 hours postoperatively. Intraoperative furosemide and mannitol are given to allow gentle easier cerebellar retraction, if needed. We administer dexamethasone intravenously during the procedure and continue this for 24 hours postoperatively. Long-acting muscle relaxants are avoided during surgery so as not to interfere with facial nerve monitoring.


At the House Clinic we have exclusively used the translabyrinthine approach for placement of the ABI. More recently, we have been using a C-shaped incision that extends just 1 cm above the pinna (Fig. 16.1). It allows the placement of the internal receiver and magnet under the scalp. It is important that the incision not directly cross the area of the receiver/stimulator.


The translabyrinthine approach provides direct access to the cochlear nuclei. The jugular bulb is skeletonized to provide the widest access to this area. Anatomic landmarks used for placement include the stump of cranial nerve VIII, the origin of the glossopharyngeal nerve from the brainstem, the facial nerve, and the tinea choroidea, as well as the mouth of the lateral recess where all of these structures converge (Figs. 16.2, 16.3, and 16.4). The two features used by the neurosurgeon in identifying the lateral recess are its relationship to cranial nerve IX and the position of the jugular bulb. The entrance to the lateral recess is directly above the origin of the glossopharyngeal nerve at the brainstem. In the surgical setting, where there is almost always distortion of the brainstem from the tumor, the lateral recess is superior to cranial nerve IX, which is generally in a fixed anatomic position. Going from there, we can get to the lateral recess in almost every case. The jugular bulb is important because its position may vary. Indeed, with a contracted mastoid and a high jugular bulb, the exposure may be more difficult, although it should not be an impediment to placement of the ABI electrode. The key is to have good exposure of the jugular bulb, which will augment the exposure of the lateral recess (see Technical Pearls, below).



image


Fig. 16.1 C-shaped incision currently used for placement of an auditory brainstem implant (ABI).

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Apr 14, 2018 | Posted by in NEUROSURGERY | Comments Off on Auditory Brainstem Implants
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