Detection of Extracochlear Electrodes Using Stimulation-Current- Induced Non-Stimulating Electrode Voltage Recordings With Different Electrode Designs.

HYPOTHESIS: Stimulation-Current-Induced Non-Stimulating Electrode Voltage Recordings (SCINSEVs) can help detect extracochlear electrodes for a variety of Cochlear Implant (CI) devices. BACKGROUND: Extracochlear electrodes (EEs) occur in 9 to 13% of cochlear implantations and commonly go unnoticed without imaging. Electrodes on the electrode array located extracochlearly are associated with non-auditory stimulation and a decrease in speech outcomes. We have previously shown that SCINSEVs, with hardware and software from one manufacturer, could detect EEs. Here, we test the generalizability to other manufacturers. METHODS: Fresh-frozen human cadaveric heads were implanted with Cochlear Ltd. CI522 (CI-A) and MED-EL's FLEX24 (CI-B) electrodes. Contact impedances and SCIN- SEVs were measured, with Cochlear Ltd. research Custom Sound software (Transimpedance Matrix) and MED-EL's clinical MAESTRO (Impedance Field Telemetry), for full insertion and EEs in air, saline and soft tissue. An automated detection tool was optimized and tested for these implants. Intra-operative SCINSEVs with EEs were collected for clinical purposes for six patients. RESULTS: The pattern of SCINSEVs changed in the transition zone from intracochlear to extracochlear electrodes, even with low contact impedances on EEs. Automated detection in the cadaveric specimens, with two or more EEs in saline or soft tissue, showed a mean 91% sensitivity and specificity for CI-A and 100% sensitivity and specificity for CI-B. Quantification of EEs showed significant correlations of r  = 0.69 between estimated and actual EEs for CI-A and r = 0.76 for CI-B. CONCLUSION: The applicability of SCINSEVs to detect extra- cochlear electrodes could be expanded to other cochlear implant companies despite differences in electrode array design and measurement software.

Detection of Extracochlear Electrodes in Cochlear Implants with Electric Field Imaging/Transimpedance Measurements: A Human Cadaver Study.

OBJECTIVES: Extracochlear electrodes in cochlear implants (CI), defined as individual electrodes on the electrode array located outside of the cochlea, are not a rare phenomenon. The presence of extracochlear electrodes frequently goes unnoticed and could result in them being assigned stimulation frequencies that are either not delivered to, or stimulating neurons that overlap with intracochlear electrodes, potentially reducing performance. The current gold-standard for detection of extracochlear electrodes is computed tomography (CT), which is time-intensive, costly and involves radiation. It is hypothesized that a collection of Stimulation-Current-Induced Non-Stimulating Electrode Voltage recordings (SCINSEVs), commonly referred to as "transimpedance measurements (TIMs)" or electric field imaging (EFI), could be utilized to detect extracochlear electrodes even when contact impedances are low. An automated analysis tool is introduced for detection and quantification of extracochlear electrodes. DESIGN: Eight fresh-frozen human cadaveric heads were implanted with the Advanced Bionics HiRes90K with a HiFocus 1J lateral-wall electrode. The cochlea was flushed with 1.0% saline through the lateral semicircular canal. Contact impedances and SCINSEVs were recorded for complete insertion and for 1 to 5 extracochlear electrodes. Measured conditions included: air in the middle ear (to simulate electrodes situated in the middle ear), 1.0% saline in the middle ear (to simulate intraoperative conditions with saline or blood in the middle ear), and soft tissue (temporal muscle) wrapped around the extracochlear electrodes (to simulate postoperative soft-tissue encapsulation of the electrodes). Intraoperative SCINSEVs from patients were collected, for clinical purposes during slow insertion of the electrode array, as well as from a patient postoperatively with known extracochlear electrodes. RESULTS: Full insertion of the cochlear implant in the fresh-frozen human cadaveric heads with a flushed cochlea resulted in contact impedances in the range of 6.06 ± 2.99 kΩ (mean ± 2SD). Contact impedances were high when the extracochlear electrodes were located in air, but remained similar to intracochlear contact impedances when in saline or soft tissue. SCINSEVs showed a change in shape for the extracochlear electrodes in air, saline, and soft tissue. The automated analysis tool showed a specificity and sensitivity of 100% for detection of two or more extracochlear electrodes in saline and soft tissue. The quantification of two or more extracochlear electrodes was correct for 84% and 81% of the saline and soft tissue measurements, respectively. CONCLUSIONS: Our analysis of SCINSEVs (specifically the EFIs from this manufacturer) shows good potential as a detection tool for extracochlear electrodes, even when contact impedances remain similar to intracochlear values. SCINSEVs could potentially replace CT in the initial screening for extracochlear electrodes. Detecting migration of the electrode array during the final stages of surgery could potentially prevent re-insertion surgery for some CI users. The automated detection tool could assist in detection and quantification of two or more extracochlear electrodes.