Stimulating the cochlear apex without longer electrodes

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PROJECT SUMMARY Cochlear implant (CI) electrode arrays are only partially inserted into the cochlea, in most cases leaving more than half of the cochlea unstimulated. In the normal hearing ear, the cochlear region left unstimulated by most CIs represents frequencies below approximately 800Hz. Providing electrical stimulation to a broader region of the cochlea with a CI has the potential to enhance performance. For example, increasing apical coverage has been shown to improve speech perception and accelerate CI patients’ adaptation to their device. These benefits may be due to representation of low-frequency information closer to the normal cochlear place. Furthermore, stimulation deeper than one cochlear turn may provide better perception of temporal information and better sound quality. However, using longer electrode arrays to access deep apical regions has several disadvantages. First, because the scala tympani diameter decreases with increasing cochlear depth, the likelihood of an incomplete insertion increases with electrode array length. Second, if deeper insertion is achieved, the probability of damage to cochlear structures increases as the walls of the cochlear duct become closer to the electrode. Third, even the longest electrodes only stimulate ~70% of the cochlear length. To address these shortcomings, we developed a novel approach to stimulate the cochlear apex without increasing the electrode array length. Moreover, our approach uses existing FDA-approved CIs, speech processors, and commercial fitting software without modification. In Cochlear CI devices, two extra-cochlear electrodes (ECEs) are used for grounds: ECE1 (usually placed under the temporalis muscle) and ECE2 (located on the implant case). In the novel approach, ECE1 is placed into the cochlear helicotrema via an apical cochleostomy and the electrode array is inserted from the basal end of the cochlea through a traditional cochleostomy. When an electrode from the array is grounded to ECE1 in the cochlear helicotrema, the electric field is driven towards the cochlear apex, stimulating residual neural tissue at sites deeper than available with the standard configuration of electrode arrays and ground electrodes. Using ECE2 as the ground provides monopolar stimulation, which is the clinical standard. Thus far, we have successfully implanted three patients using this novel surgical approach and implemented novel signal processing using the ECE1 electrode. All reported a lower pitch when using ECE1 instead of ECE2 as a ground. This new approach provides a unique opportunity to answer important scientific questions and to evaluate a new clinical intervention. It provides the first opportunity to directly stimulate the cochlear helicotrema (apex) in humans. We can now study if such stimulation improves temporal coding (Aim 1) and extends the tonotopic pitch range (Aim 2). Additionally, we can study if the intervention improves clinical outcomes (Aim 3). We propose to implant 15 patients with this new approach to address the three aims.