Auditory Midbrain Implant: Present Clinical Results and Future Directions
Introduction: Tumor-induced damage in the cochlear nucleus may be responsible for the limited success of the auditory brainstem implant (ABI) in neurofibromatosis type 2 (NF2) patients. Therefore, an alternative implantation site that bypasses the damaged pathways in the brainstem may provide better hearing performance in NF2 patients. The central nucleus of the inferior colliculus (ICC) provides access to almost all ascending auditory information and has a well-defined tonotopic organization, which is important for an auditory prosthesis. The auditory midbrain implant (AMI) is a single-shank multielectrode array designed according to the dimensions of the human ICC with the goal of stimulating the different frequency layers of its central nucleus.
Methods: The AMI clinical study was performed in 5 NF2 patients. In all of the patients, tumor removal and AMI implantation were performed in a single surgical setting through the lateral suboccipital, infratentorial-supracerebellar approach.
Results: None of the patients developed complications either due to tumor removal or insertion of the AMI. A follow-up of up to 2 years has shown no evidence of electrode migrations in these 5 patients. Paresthesia was the most common nonauditory side effect, which could be managed successfully by turning off the responsible sites at the first fitting. All the patients can receive environmental sounds and show lip-reading enhancement using their AMIs. Psychoacoustic evaluation of the AMI patients revealed 3 interesting phenomena in the auditory pathways of the midbrain. First, the hearing performance depends strongly on the location of the implant in the midbrain. Stimulation of the ICC can produce better results compared with the lateral lemniscus and dorsal cortex of the ICC. Second, adaptation is an issue for stimulation of certain areas of the auditory midbrain; to overcome the adaptation, 3D stimulation strategies may be required. Third, stimulation of the ICC can induce high levels of plasticity, which provides a higher chance of better hearing performance compared with the cochlear nucleus; however, it also requires more flexible speech processing strategies.
Conclusions: Based on our new insights into the properties of the auditory midbrain in humans, we have developed a new generation of double-shank AMIs for better 3D stimulation of the ICC. In addition, with proper placement of the implant into the ICC and using new flexible stimulation strategies to cope with adaptation and plasticity, better results compared with the ABI may be achieved in NF2 patients.