Auditory Brainstem and Midbrain Implants

12.1.1

Surface Electrodes

,c reported the first long-time results of an auditory brainstem implantation performed in 1979. They had placed a single electrode on the surface of the CN via a translabyrinthine craniotomy at the time of the vestibular schwannoma removal. Their patient had benefit from this direct stimulation: “She reported being able to hear her dog bark, a knock on the door, an airplane overhead, and the sound of the sink garbage disposal. She had considerable relief of tinnitus. She noted increasing discrimination, and the unit was of considerable help in lipreading.” explored the psychoacoustics of CN stimulation using an ABI, consisting of two electrodes, each a 0.75×2.5 mm rectangular pad of platinum foil fixed to a Dacron mesh pad and 4.25 mm apart. They found that: “The usable range of electrical amplitudes above threshold was comparable with that of cochlear implants, typically 10–15 dB. Little temporal integration occurred over a range of stimulus durations from 2–1000 ms and patients’ ability to detect amplitude modulation as a function of modulation frequency was similar to that of cochlear implant patients and normal listeners.” reported on results with a three-channel device. The ABI was positioned in the lateral recess of the fourth ventricle, adjacent to the cochlear nuclei, and consisted of three platinum plates mounted on a Dacron mesh backing. The electrodes remained stable for over 10 years. These ABIs had comparable psychophysical and speech performance to single-channel CIs. ABI patients had significant enhancement of speech understanding when the sound from the ABI was combined with lip reading. A detailed history of ABI application can be found in the study of .

Modern ABIs use the same electrode stimulator and processing strategies as a standard CI (see chapter: Cochlear Implants ), except that the electrode array is placed on the CN. Each electrode potentially activates a variety of neuron types, excitatory as well as inhibitory, possibly with different characteristic frequencies (see chapter: Hearing Basics ). The first company-based multichannel electrode array was developed by Cochlear Ltd. and tested in Europe and the United States. At the same time devised an array consisting of 20 platinum electrodes that were assembled inside a silastic carrier 7 mm in diameter. This allowed them to use the Nucleus mini-22 device CI stimulator. Each electrode was a tiny round plate with a diameter of 0.6–0.7 mm. found that in the nine implanted NF2 patients the speech perception results and patient satisfaction were encouraging, and the data showed even limited open speech recognition.

The House group ( ) then developed an eight-electrode multichannel ABI that was evaluated in 20 patients who had at least 3 months’ experience with the device. Perceptual performance indicated benefit from the device for communication purposes, including sound-only sentence recognition scores in three patients ranging from 49% to 58% and ability to converse on the telephone. In a later evaluation of this ABI type ( ) reported that the multichannel ABI was effective and safe and gave useful auditory sensations in most patients with NF2. They found that: “The ABI improved patients’ ability to communicate compared with the lipreading-only condition, it allowed the detection and recognition of many environmental sounds, and in some cases it provided significant ability to understand speech by using just the sound from the ABI (with no lipreading cues). Its performance in most patients has continued to improve for up to 8 years after implantation.” Another follow-up of the same device ( ) concluded that at least three spectral channels, programmed in the appropriate individual tonotopic order, were required for satisfactory speech recognition in most patients with ABI. However, patients with ABIs did not receive more frequency information with more than five stimulated surface electrodes. assumed that this was largely due to limited access to the tonotopic axis of the CN.

Current ABIs comprise an external part similar to a CI device and an implanted portion designed for surface plate electrodes. There are differences in the number of electrode channels and shapes of the pads between the different companies ( Fig. 12.1 ). Each pad has a mesh to help secure the pad to the brainstem surface and in which tissue can grow over time.

Existing ABI devices use the same processing strategies as CIs. The electrodes of the matrix array are assumed to stimulate different tonotopic layers of the CN, and the variations of electrical level on each electrode encode envelope variations of the signal in a narrow frequency band associated with that electrode. However, the simple place and amplitude codes that were originally developed for stimulating the AN in CI devices may not adequately convey the important information in the speech signal with an ABI. The CN is located higher in the auditory processing hierarchy than the AN, and its direct stimulation may bypass many important processes required for transmitting speech information ( ). Furthermore, the electrodes are placed on the surface of the CN and may not sufficiently access its tonotopic organization that exist within deeper regions.

reported that between May 1996 and April 2000, 14 patients with NF2 underwent implantation with a multichannel ABI. Three different types of multichannel ABIs, composed of eight Nucleus M22, one Nucleus M24 (Cochlear Ltd., Sydney, Australia), and five Clarion-1.2 (magnet-free with 16 contact electrodes; Advanced Bionics Corp., Sylmar, CA), were implanted in their patients. found that “lip-reading enhancement improved within the first 6 months and then entered a plateau phase. In the auditory alone mode, showed that more than half of the patients showed their first positive result in the vowel test 3 months after device activation, but it took about 6 months until half of the patients revealed a result above the chance level in the consonant and Freiburger numbers tests. Open set speech recognition in the auditory alone mode (in the speech-tracking test) was not common and happened relatively late (within 1 year or later).” For the same patients, “concluded that the auditory brainstem implant is an effective support for receiving and, to some degree, differentiating environmental sounds, and that as an adjuvant to lip-reading, it enhances speech perception, especially in quiet surroundings.”

In a multicenter study, described findings in 27 subjects who received a Nucleus 20- or 21-channel ABI. All subjects involved in the trial had NF2 bilateral tumors removed. The majority of users had environmental sound awareness and used stress and rhythm cues in speech that assisted with lip reading. Only two subjects from this series did receive sufficient benefit from the ABI in conversation without lip reading.

More promising results came from who compared ABI performance in 10 NF2 patients and 10 nontumor patients (e.g., those with cochlear nerve aplasia/avulsion or cochlear ossification). They concluded: “The difference in modulation detection between the two groups suggests a difference in the survival of specific cells in the cochlear nucleus that support modulation. The pattern of results indicates a separate pathway of auditory processing that is specialized for modulated sounds, and that pathway is critical for speech understanding. In NF2 patients, the tumor and surgery may selectively damage this pathway , resulting in poor speech recognition with prosthetic stimulation.” In a follow-up paper on ABIs implanted in 112 patients (83 adults and 29 children) with tumor (T) and nontumor disorders (NT). Of the 112 patients, 15 have previously had a CI elsewhere with no sound detection ( ). They found that “at the most recent follow-up, NT adults scored from 10 to 100% in open-set speech perception tests (average, 59%), and T patients scored from 5 to 31% (average, 10%). The differences between these results are statistically significant. The best performance was observed in patients who lost their nerve VIII from head trauma or severe ossification.” However, in a later review of ABI use, stated: “It is now clear that ABIs can produce excellent speech recognition in some patients with NF2, allowing even conversational telephone use. Although the factors leading to this improved performance are not completely clear, these new results show that excellent hearing is possible for NF2 patients with the ABI.”

12.1.2

A Note on Electrode Placement

showed that central regions of the posteroventral CN in the cat are suitable for penetrating auditory brainstem implantation. found that the rostrolateral and rostromedial region of the ventral cochlear nucleus (VCN) when stimulated electrically and chronically elicited significantly lower degrees of frequency specificity in receiving ICC neurons compared with the caudolateral and caudomedial VCN regions. This suggested that the electrodes had to access the tonotopic organization of the VCN to achieve good performance. used “multielectrode recordings [in the rat] to assess the frequency specificity of activation in the central nucleus of the inferior colliculus (ICC) produced by electrical stimulation of localized regions within the ventral cochlear nucleus. In 26% of the 193 ICC sites, they found a high correlation between acoustic tuning and electrical tuning obtained through VCN stimulation. A high degree of frequency specificity was found in 58% of the 118 lowest threshold VCN–ICC pairs. Frequency specific stimulation was obtained for medial, central, and posterolateral VCN regions rather than more anterolateral regions.” concluded that “if a surface array is placed on the posterolateral surface of the VCN in a dorsoventral direction using the present commercial ABI, some frequency-specific stimulation may be achievable but a penetrating array in the PVCN would result in an increased likelihood of frequency-specific ICC activation.”