The Importance of Impedance: A Case of Responsive Neurostimulation Failure Characterized by Concurrent Stereo-electroencephalography

Introduction: The RNS system delivers stimulation from a single-current-source, and the relative flow of electrical current through each stimulating contact is inversely related to the relative impedance of each contact. Current shunting through low-impedance contacts (i.e. intraventricular contacts) may divert therapy away from intended targets. Here we present the case of a patient with temporal lobe epilepsy, who completed sEEG monitoring with active mesial temporal RNS system and demonstrate current shunting through low-impedance contacts using concurrent sEEG recordings.

Methods: Stereotactic EEG (sEEG) was implanted successfully without damaging the RNS system.

Results: sEEG recorded independent bitemporal interictal epileptiform discharges and seizures. RNS stimulation artifact was reduced in the left mesial temporal region relative to the right during bilateral RNS stimulation, consistent with current shunting through low-impedance right-lead contacts. Impedance measurements confirmed several low-impedance contacts from the right leads, with intraventricular position on imaging.

Conclusion : Therapy delivery from single-current-source pulse generators is critically dependent on the relative impedance of electrode contacts. This underappreciated fact is a potential failure point for the RNS system, and stimulation programs should be tailored to avoid unwanted current shunting. Concurrent sEEG monitoring and active RNS is feasible and can help characterize stimulation effects.