Impedance variations over time for a closed-loop neurostimulation device: early experience with chronically implanted electrodes.

OBJECTIVE: Responsive neurostimulation (RNS®) is an investigational treatment modality for intractable focal epilepsy. We analyzed impedance values over time to gain a better understanding of RNS device stability, potential differences between depth and strip electrodes, and general implications of long-term electrode implantation.

MATERIALS AND METHODS: Impedance measurements were retrospectively reviewed from seven patients over a 3-year period. Statistical analysis was performed to investigate trends in impedance values over time and to determine if any differences existed between the means and variances of impedance values for the two different electrodes.

RESULTS: Intraparenchymal depth electrodes demonstrated lower mean impedances and had less variation over time when compared to subdural strip electrodes. There was no significant change in mean impedance over time for depth electrodes, while that of subdural electrodes increased over time to peak at 16-20 months and returned to baseline by 2 years.

CONCLUSION: The RNS device provides unique long-term data for both depth and subdural electrodes in human subjects. Although changes in impedance were observed with respect to time after implantation and electrode type, these variations were of a clinically insignificant magnitude. As a constant-current system, the device is ultimately able to administer stable therapeutic doses in the setting such minor variability. While further animal research and post mortem investigations in human subjects are necessary to broaden our understanding of the mechanisms occurring at the electrode-neural interface and the biocompatibility of chronically implanted electrodes, the relative stability of impedances seen in this cohort of chronic implants brings promise to future implementations of chronic intracranial neural implants.