The auditory brainstem implant (ABI), while closely related in many ways to a cochlear implant (CI), is beset with its own unique technical, practical, and clinical challenges as it is implanted within the brainstem and involves stimulating a structure that can barely be seen during surgery. Through over 20 years of experience, the electrode array design and the surrounding surgical and clinical procedures have been tailored to meet the constraints imposed by the brainstem anatomy, implantability, electrical safety, long-term stability, and clinical effectiveness. This chapter details the ABI design along with its main technical features. It also addresses the way in which electrophysiology performed during device insertion, with careful selection and stimulation of discrete regions of the array, can be used to help ensure correct placement of the electrode over as much of the stimulable cochlear nucleus as possible. A procedure along with the key parameters for electrophysiological testing is described. With careful attention to the electrode placement, this helps to ensure each patient has the best chance of a positive clinical outcome, including useful auditory awareness and improved quality of life in patients without any other hope of hearing.
Key wordsbrainstem implant – intraoperative monitoring – electrophysiology – EABR – cochlear nucleus – electrode array
10 ABI Engineering and Intraoperative Monitoring: Cochlear
Ever since the first aspiration to achieve auditory awareness through electrical stimulation of the cochlear nucleus, both engineering design and anatomical understanding have grown hand in hand. The cochlear nucleus, our target for electrical stimulation, is hidden from view surgically and often pushed to one side or its recess compressed by a growing brainstem tumor making location and access sometimes quite difficult. 1 Despite this adversity, a brainstem implant can be inserted safely, fixed securely, and work reliably for the majority of recipients. All these factors come together to present quite a unique challenge surgically, mechanically, and practically. This chapter explores the unique ABI design focusing on the latest Nucleus ABI541 device that is commercially available. It also explains the electrophysiological testing that can be performed to try to work around the adversity of a hidden nucleus or strangely unfamiliar brainstem distorted by a large tumor. Placed optimally and operating effectively, the ABI can provide significant assistance to speech understanding. 2 , 3 , 4 , 5 Suboptimal placement or an electrode displaced by a moving brainstem can twist the outcome to nothing more than a range of unhelpful side effects. Clearly, this is a device with which experts in their respective fields need to work together.
10.2 ABI Design Engineering
The ABI takes its inspiration from the design principles of a cochlear implant (CI) yet comprises a quite different electrode array design due to its position within the lateral recess of the brainstem. Here it aims to stimulate the 2nd neuron of the auditory pathway at the exposed surface of the cochlear nucleus complex (CNC)—the point where the auditory nerve normally joins the brainstem. Remarkably the first reported use of an ABI was in 1978 6 when even CIs were in their infancy. Yet those early results proved not only that the cochlear nucleus was accessible surgically, but also that auditory sensations were indeed possible when modulated electrical current was applied.
Cochlear’s involvement in the ABI development started in 1990. With a growing reputation for producing reliable CIs, Cochlear was approached by William Hitselberger and William House from the House Ear Institute in Los Angeles, California, to help create an implant based upon the commercial design of the Nucleus CI22 M CI that existed at that time. With the availability of many separate stimulation electrodes, this saw a design change from a three-electrode, single channel device to an eight-electrode, multichannel system comprising two staggered rows of four electrodes 1 mm in diameter mounted on a silicone elastomer carrier about 3 mm wide and 8 mm long. Almost simultaneously a slightly different design of ABI electrode was proposed by a team in Hannover, Germany lead by Roland Laszig. 7 The basic design mirrored developments at the House Ear Institute except that a 20-channel electrode array was proposed, necessitating much reduced electrode diameters to fit within the available paddle size. Two designs were accomplished, followed by two pilot studies and then two clinical trials between 1992 and 2000. 8 , 9
This chapter will not dwell upon all the device iterations that occurred—especially with the early Hannover design. By 2000, a combined design of ABI electrode array was established and since then, the ABI electrode has remained largely unchanged. It is this design that over 1100 patients globally have received.
10.2.1 Physical Design
The Nucleus ABI541 is electronically identical to the CI500-series CI but physically there are two main differences pertaining to the electrode array and the electrode leadwire design (see Fig. 10.1). The electrode array, in particular, needs to be placed within the lateral recess of the fourth ventricle at a structure known as the cochlear nucleus. The exposed surface of the cochlear nucleus within the lateral recess measures, on average, 3 mm in width and 10 mm in length, 10 and this presents the “target area” for stimulation. To make use of as much of the exposed surface of the cochlear nucleus as possible, the ABI electrode array consists of a silicone electrode carrier, often referred to as the “paddle,” 9.9 mm long by 3.5 mm wide and covered by a 7 × 3 staggered matrix of platinum electrode contacts measuring 0.7 mm in diameter. Each one of these contacts, which hopefully delivers some unique pitch sensation based upon the tonotopicity of the CNC, can be independently stimulated by a modulated electrical pulse train presented in a way so as to hopefully elicit an auditory sensation during the device “activation” or “switch-on”—a topic covered in Chapter 12.
The electrode array is augmented by soft weave of polyethylene-terephtalat (PET) mesh on its rear surface, formed in the shape of a “T.” This biocompatible material promotes adhesion of fibrous tissue. Since the lateral recess has no solid landmarks with which to safely stabilize the electrode once inserted, this fibrous tissue adherence acts as an important “glue” holding the device firmly in position, provided there is no movement of the electrode array within the first few days after surgery. The specific PET shape, that of a “T”-shaped wing, is designed so that as the electrode array is inserted, this wing can be looped back upon itself, not only providing a greater area for tissue growth, but also applying just a small amount of pressure to the lateral end of the electrode array within the entrance to the recess.
In the Nucleus ABI electrode array, the rear of the array also possesses a tiny positioning tube at its medial end. This tube allows the electrode array to be grasped by surgical instruments such as forceps or a claw, without damage, and then gently advanced into the opening of the recess.
The other notable difference of the ABI device compared to its CI cousin is the electrode leadwire which is both longer and more flexible. The increased length is simply to reach the target CNC in the brainstem, which is further away from the implant electronics package than the cochlea. The improved flexibility of the leadwire is achieved by using a smaller radius of wound wires within a narrower silicone leadwire. Not only does this promote improved handling, reducing the springiness of the electrode array, but it also minimizes fatigue stresses on the electrode wires themselves since this array will be sitting within a slightly moving brainstem for all of its working life.
Finally, a small revision was made to the implant design in around 2010 when a 10 mm square pad of PET mesh was attached to the leadwire more proximal to the electronics package. This mesh was added to address a few cases of cerebrospinal fluid (CSF) leakage occurring after surgery, the CSF wicking along the electrode leadwire. It was proposed by Derald Brackman from the House Ear Institute that adding a mesh pad where the electrode array comes through the dura would promote better fibrous tissue growth, form a better seal as well as improve options for stabilization of the array.
A final feature of the Nucleus ABI common to all generations of the product and not explicitly related to the electrode array is a removable magnet. This strong, rare-earth magnet, designed to hold the external sound processor’s coil to the head during use, may pose an unwanted disturbance in the event that subsequent magnetic resonance imaging (MRI) is required. While the ABI541 is approved for use in 1.5 Tesla MRI machines with a tight pressure dressing, imaging with any kind of magnet in situ becomes a problem due to the extensive artifact created. Removal of the magnet, either temporarily or permanently, can be undertaken as required. Once the magnet is removed, the ABI may also then be imaged at 3.0T if required.
10.2.2 Electrical Design and Safety
It has been detailed earlier that the electronic design of the ABI541 is identical to that of a CI, and indeed no specific changes were necessary due to the high degree of flexibility that the electronics provide. The electronic stimulator of the ABI is capable of delivering sequential biphasic current pulses where the pulse amplitude, the pulse width, the pulse rate, the active electrode, and the stimulation mode can all be set within wide parameters according to the needs of the individual patient.
Early patients receiving the ABI were stimulated using relatively modest stimulation rates of around 250 pulses per second (pps) in bipolar modes. It was quickly seen that while the electrode array might be in intimate contact with the neural substrate it was stimulating, electrical levels to reach audibility were quite large compared to a CI. These high stimulation levels necessitated careful assessment of the charge density around the electrode contacts. It was this consideration that lead to the current electrode diameters of 0.7 mm as this was the requirement to keep stimulation within the established safety margins as proved in experiments by McCreery et al 11 and summarized eloquently in a review paper by Shannon in 1992. 12 In fact the classic safety equation formula relating charge density ( D ) and charge ( Q ), namely log D = k – log Q , is now programmed into Cochlear’s fitting software to ensure that if stimulation parameters do need to rise for some patients, at all times established safety levels are respected.
10.3 Electrophysiology to Support Optimal Intraoperative Electrode Placement
Insertion of an ABI electrode array into the lateral recess in such a way that it is optimally positioned over the exposed surface of the cochlear nucleus is hampered by the simple fact that in most cases the target area is just out of sight (Fig. 10.2). While in some patients it may be possible to visualize the white bulge of the cochlear nucleus as it descends into the recess, in no case is it possible to assess its precise full size or indeed its exact orientation. This “half blind” approach can, however, potentially be aided by the use of electrophysiology, 13 which is now commonplace in ABI surgery.
The basic principle of the electrophysiological testing is quite simple. When the ABI electrode array has already been positioned in a location that looks anatomically correct with respect to the landmarks of the VIIth and IXth nerves, the choroid plexus, and any observed CSF flow from the lateral recess, the electrode array is stimulated while simultaneously recording the electrically evoked auditory brainstem response (EABR). In theory, if the electrode array is over the cochlear nucleus, then it should be possible to observe a number of small, positive, repeatable biological peaks on an evoked potential (EP) machine that characterize the firing of the cochlear nucleus and the ascending auditory pathway via the superior olive, lateral lemniscus, and inferior colliculus. Given that stimulation is occurring at the cochlear nucleus we might hope to record peaks corresponding to waves III, IV, and V from the classic five-peak ABR. Observation of these peaks, if in the right amplitude and latency, then acts as “proof positive” that the stimulated electrode, at least, is in close proximity to the cochlear nucleus. It logically follows that if a number of electrodes are stimulated across the array, then it should be possible to map-out the alignment of the array over the cochlear nucleus and then, if necessary, the device gently moved to optimize its location. Fig. 10.3 visualizes this alignment process.
10.3.1 Equipment Setup for EABR
In practice, measuring and interpreting the EABR reliably, and then giving appropriate direction to guide electrode placement requires a number of technical and practical details to be taken into consideration. The equipment setup for EABR is shown in Fig. 10.4.
From an equipment perspective, it is necessary to use a commercial averager (aka “EP machine”) to record the biological potentials from surface electrodes placed on the patient. It is also necessary to have an equipment capable of delivering a stimulus from the ABI device, which in the case of the Nucleus ABI consists simply of the standard programming hardware (a sound processor and coil which are placed over the implant during testing; plus a programming Pod interface) connected to a computer running Custom Sound EP software. Then, to ensure the EP machine records potentials linked with the stimulation, an all-important trigger cable connecting the stimulation and recording hardware is necessary to synchronize the two machines.
10.3.2 Recording Electrode Montage
The electrode montage frequently used for “traditional” EABR in a CI recipient would position the active recording electrode on the upper forehead, the indifferent electrode on the mastoid contralateral to the implant, and the ground electrode on the lower forehead. This montage has been adopted to minimize the amount of electrical stimulus artefact picked up by the EP machine—something that makes EABR considerably more challenging than an ABR. However, the above montage is based upon a CI electrode within the cochlea whereas we have an ABI electrode in the brainstem. This change of position and electrode array orientation have led to a more widely adopted montage using a true vertex positive electrode, a C7 negative electrode, and a ground at about the hairline, all on the midline. 14 This orientation not only helps to minimize electrical artifacts from the ABI during stimulation but is also sensitive to vertical electrical activity in the brainstem tracts that the biological potentials travel through as they ascend. In no way does this represent the only option for successful EABR. A montage that uses a high forehead positive, combined with an ipsilateral tragus negative and a low forehead ground, is increasingly being used. This has the slight advantage that its orientation is more favorable to the crossing depolarization of wave III of the EABR and, since it avoids the vertex, it lends itself to the use of disposable rather than needle electrodes for the recording. Many electrophysiologists have quite firm views on electrode montage and electrode types, but in the author’s experience, fresh disposable electrodes generally have lower noise and lower impedances than needles. This needs to be tempered against the need for electrode stability during a potentially long surgery. So in practice, if the surgery will be of long duration (e.g., over 5–6 hours) and the patient has a tumor to be removed, then needle electrodes are usually preferred. However, if the surgery might be shorter, disposable electrodes secured carefully with additional tape are adequate.