Impedance for different electrode types
Reviewed by Kerri Millward & Morag Lockley

Measuring cochlear implant (CI) electrode impedances is common in CI programming appointments to measure the integrity of the implant e.g. whether there are any open or short electrodes. This is because impedance measures the flow of current between intra and extra cochlea electrodes. One of the influences on impedance measurements is how far away the array is from the modiolar wall, with an array positioned closer to the modiolus (perimodiolar) having lower impedances and, thus, having lower power consumption. Whereas straight electrodes tend to sit against the lateral wall, the mid-scala array sits in between the two. Impedances normally settle once the speech processor has been programmed and worn over a period of time. However, impedances can change over time. These changes can be attributed to biochemical changes within the body, use of the device, or sometimes for unknown reasons. The subcomponents of electrode impedances can be looked at separately and each one can give an indication of why changes have been observed in the impedances recorded from a patient’s CI. Within this study, the components of electrode impedances were measured to see how they varied between three different types of Advanced Bionics electrode arrays. Generally, there were minimal significant differences found between the arrays. However, measurements at the basal end tended to be higher for all arrays, possibly because of fibrous tissue growth. As the mid-scala electrode has larger contacts, this showed significantly lower measurements for some components. Therefore, this study demonstrated that it was not only electrode distance from the modiolus which was influential, but also electrode contact size. This is clinically useful when developing future electrode arrays and when reviewing fluctuations in impedance measures.

Analysis of electrode impedance and its subcomponents for lateral wall, mid-scala, and perimodiolar electrodes in cochlear implants.
Saoji AA, Graham M, Stien A, Koka K.
COCHLEAR IMPLANTS INT
2022;23(2):87-94.