Electron microscopy changes of cochlear implant electrodes with permanently high impedances

Impedance measurements by means of telemetry have provided us with an objective test of cochlear implant function. Unmeasurable impedance suggests a possible defect on the corresponding channel of the electrode. However, increasing impedances could suggest inflammatory and/or proliferating processes along the electrode. In this report we examined correlations between high impedance, measured in vivo and in vitro findings on the electrode surfaces.

We describe recurrent increases in cochlear implant electrode impedance in the same channels, which could initially be reversed by reprogramming the implant, and with anti-inflammatory treatment using cortisone and antibiotics. Subsequently, however, the impedances remained consistently high, indicating a cochlear implant defect. After the removal of the cochlear implant, the electrodes were examined using light and scanning electron microscopy.

Examination of the electrodes with normal impedance values revealed, in the great majority of the electrodes, silver shining surfaces. On the other hand, electrodes with high impedances were found coated with tissue. Further analysis of these electrodes using scanning electron microscopy also demonstrated surface changes.

Persistently high impedances were related to changes on the electrodes' surface and to coating with tissue in this case. These changes can result in adhesive processes and technical malfunction, leading to explantation and difficult re-implantation. Therefore, close and professional supervision of cochlear implant patients is needed.