Subjects with a nonfunctioning auditory nerve do not benefit from cochlear implantation. In such cases, if the subject is older than 12 months of age, an auditory brainstem implant (ABI) may be an option to treat the hearing impairment.
This chapter provides details of the state-of-the-art MED-EL ABI system, along with providing an overview of the tools developed to aid the surgeon in determining the functionality of the auditory nerve prior to implantation.
An ABI placing system based on electrophysiologic guidance was developed to determine the number of electrodes required to stimulate the auditory system and the optimum placement of the implanted electrode array.
This chapter also provides an overview of the use of intraoperative electrophysiology to aid electrode placement using both an ABI placing system and the ABI itself. This procedure involves recording electrically evoked auditory brainstem responses (eABR), which allows for the electrode array to be placed onto the cochlear nuclei (CN) by distinguishing between auditory and nonauditory responses.
11 ABI Engineering and Intraoperative Monitoring: MED-EL
The auditory brainstem implant (ABI) was originally developed at the House Ear Institute in 1979 for NF2 patients who lost their VIII nerve function bilaterally after surgery to remove vestibular schwannomas (VS). 1 , 2 , 3 , 4 The first implantation of the MED-EL Combi 40 + ABI was performed in 1997 by Prof. Behr at the Univesity of Wurzburg, Germany. In 2014, MED-EL introduced the fourth generation of ABI implant, the SYNCHRONY ABI system.
Technically, the ABI shares a common design concept with the cochlear implant (CI). As shown in Fig. 11.1, the ABI consists of three components: (1) audio processor, (2) receiver-stimulator, and (3) electrode array. An audio processor picks up sounds from the environment and digitizes them; a receiver-stimulator is placed under the skin and receives electrical signals along with obtaining power from the audio processor via a transmitting coil. The signals are decoded and delivered in a controlled manner through a surface electrode array to the cochlear nuclei (CN) in the brainstem, thereby bypassing the nonfunctioning auditory nerve. Such stimulation produces responses that the brain can interpret as sounds.
11.2 State-of-the-Art MED-EL ABI System
As shown in Fig. 11.2, the MED-EL ABI array comprises 13 platinum disc electrode contacts (12 stimulation electrodes and 1 reference electrode), with a diameter of 0.6 mm each, which are embedded in a silicone carrier (5.5 × 3.0 × 0.6 mm). The overall size of the array allows the pad to fit within the lateral recess of the fourth ventricle and to adapt to the surface of the CN. On the opposite side of the silicone carrier, there is a polymer mesh backing which promotes fibrous ingrowth and secures the electrode in the desired location, thereby increasing stability of the electrode array on the surface of the CN. The electrode array is pre-shaped by cross-running platinum wires. This allows for an element of individual shaping so that it adapts to the contour of the CN.
Besides the implant, the system consists of an externally worn audio processor, with a coil that is held over the implant via magnetic attraction. Additionally, the implant system comprises Maestro fitting software with MAX interface box as well as various tools and accessories.
The Mi1200 SYNCHRONY (PIN) implant received the CE mark in June 2014. The SYNCHRONY (PIN) receiver-stimulator uses the i100 electronics with a reduced thickness of 4.5 mm for the stimulator. The HDCIS coding strategy (modified Continous interleaved Sampling coding strategy with the Hilbert Transform envelope extraction) with a maximum stimulating rate of 50,704 pps is used. 5 Monopolar stimulation is used postoperatively. The speech outcomes in ABI adult patients by use of this coding strategy are discussed in a report by Behr et al. 6
The Mi1200 SYNCHRONY (PIN) consists of a hermetically sealed stimulator, a coil with a magnet at its center, a reference electrode, an electrically evoked action potential (EAP) reference electrode, and a variant of an active electrode. The stimulator consists of the implant circuitry and a microchip, encapsulated in hermetically sealed titanium housing, covered in silicone, with a reference electrode and an EAP electrode mounted on the housing. Two variants of the titanium housing exist:
With a flat bottom—the Mi1200 SYNCHRONY,
With two pins, protruding from the flat bottom, which are used to further ease the immobilization of the device—Mi1200 SYNCHRONY with suffix PIN. The two pins secure the implant against translational and rotational motion. These pins add 1.4 mm to the bottom of the device.
An important feature of the SYNCHRONY ABI is the possibility of receiving a magnetic resonance imaging (MRI) of up to 1.5 Tesla without having to remove the magnet. The magnet is diametrically magnetized and rotatable within its housing, and the whole magnet assembly is also removable (in this case, it is necessary to use a nonmagnetic spacer instead). The torque forces related to the performance of an MRI on the rotatable magnet are considered negligible. Therefore, the magnet does not become discharged and no magnetic discharge is possible during an MRI treatment, which increases patients’ comfort whenever an MRI is needed.
A typical MRI distortion of the SYNCHRONY ABI is approximately 3 cm around the implant case. Although the use of a nonmagnetic spacer instead of the magnet results in less MRI distortion, the option of keeping the magnet in place during an MRI is preferable. Importantly, this means that no additional surgery for magnet removal and placement is required, and with the use of an MRI and a high-resolution computed tomography (CT), it is often possible to obtain necessary information from the distorted area. Positioning the implant in a more horizontal and posterior way as well as a distance of at least 9 cm between the magnet and the external ear canal have shown to improve the visibility of the internal auditory canal. 7 , 8
11.3 ABI Indication
The initial indication was etiology related. ABI candidates needed to be 15 years of age or older with NF2 and both cochlear nerves nonfunctional, or anticipated to be rendered nonfunctional by the presence or removal of a tumor. In 2017, the indication criteria were expanded to include candidates that are aged 12 months or older who would not benefit from a CI due to a nonfunctioning auditory nerve. The cause of nonfunctioning auditory nerves may be congenital due to auditory nerve aplasia or auditory nerve hypoplasia or accrued during the life span due to cochlear nerve disruption caused by head trauma, non-NF2 tumors, or severe cochlear ossification.
11.4 Tools in Questionable Candidacy for CI or ABI
The goal of the developed tools is to minimize the time of hearing deprivation in questionable candidates, who normally would not be implanted or implanted with uncertainty, and to help the implant team to decide which implant is the best choice for each candidate. Both of the tools discussed below and the evoked auditory brainstem responses (eABR) stimulation were developed for the Maestro MED-EL clinical system, version 9.0 and higher, and require only a dedicated evoked potential measuring system (i.e., no other equipment is needed).
In some instances candidates show no response or a questionable response to sound while diagnostic imaging tests suggest normal or abnormal anatomy. This may include individuals with a narrow internal auditory canal, or individuals with either malformed or patent cochlea. For this purpose, the preoperative promontory stimulation system was developed. The benefits of this test were shown in initial studies with a success rate of 80 to 90% in children implanted with a CI. 9 , 10 In our case, a transtympanic electrode is placed on the round window niche. Biphasic electrical pulses are delivered to the transtympanic electrodes. At the time of stimuli, the MAX interface triggers the evoked potential device and the eABR response is obtained from the surface electrodes. If the eABR shows a positive response, the implant team may decide to proceed with cochlear implantation. If no responses are obtained, the candidate may be considered for an ABI, or further tests may be required. The transtympanic electrode is available in two different sizes in order to accommodate different middle ear space sizes.
In situations where an individual shows no response to the sound or the individual is expected to have no response to the sound, imaging tests show normal or abnormal anatomy, and the individual has already been selected for either a CI or an ABI, an intraoperative test of nerve functionality may be used. This test includes placement of the cochlear test electrode array into the scala tympani (Fig. 11.3).
The cochlear test electrode consists of four electrode contacts. It is intended to be inserted into the cochlea during surgery. The length of the electrode is 18 mm, as indicated by the marker ring. Three of the electrode contacts are placed directly into the cochlea, with the fourth electrode contact placed under the temporalis muscle. Biphasic pulses can be applied using all electrode combinations. The biphasic pulses are generated using the MAX interface and delivered to the cochlea. At the time of stimuli, the MAX interface triggers the evoked potential device and eABR response is obtained from the surface electrodes.
This tool is suitable for individuals with questionable functionality of the auditory nerve, individuals with a narrow internal auditory canal and patent or malformed cochlea, in tumor patients to monitor nerve functionality during the tumor removal, or in situations where any other tests/methods failed to show CI candidacy including the use of eABR with the promontory stimulation system. 11 , 12 , 13
To improve the reliability of the recordings from both of these tools, a new eABR artifact cancellation method was implemented.