To Use the Back-up Cochlear Implant or Not? Using Intra-operative Impedance to Guide Your Decisions

Intra-operative electrophysiological testing is being increasingly used to determine device functionality. Impedance abnormalities (open or short circuits) measured at time of surgery pose a dilemma: is it likely to resolve or is it a permanent fault? There is little in the literature on how to manage these intraoperative finding and if, at time of surgery, the back-up device should be used.

We routinely undertake impedance testing twice intraoperatively, as well as at switch on, 1 and 3 months postoperatively. Retrospective impedance thresholds were analysed for one surgeon's cases between January 2018 and December 2019.

There were 235 cochlear implants performed for 217 patients (5,020 electrode contacts) analysed. Thirty-three electrodes had abnormal impedance thresholds on first intraoperative cycle of testing, 76% resolving with the second testing cycle electrode contacts that demonstrated abnormal impedance during both intraoperative test cycles were 16.54 times (95%CI 2.55-107.13, p = 0.003) more likely to be abnormal at three months. Fifty percent resolved by switch on. The intraoperative abnormalities made up 26% of electrode abnormalities seen at 3 months postoperatively.

This study demonstrates the utility of 2 cycles of intraoperative impedance testing, with persistently abnormal electrodes having 16 times the likelihood of persistent abnormalities of impedance, and 50% resolution. These persistent intra-operative abnormal electrodes are responsibly for 26% of electrode abnormalities at 3 months. This information is useful for the surgeon when considering use of the backup cochlear implant device.