Figure 5.

Subcortical impedances measured with the RC + S system in a group of movement disorders patients (7 STN, 4 GP, 2 dystonia) represented as absolute unipolar and percentage difference mismatch between paired electrodes. (A) Distribution of single measurement points of each subcortical electrode contact referenced to the IPG case, with a range of variability from 500 to 2500 ohms. (B) Impedance mismatch of a subcortical electrode pair defined as the absolute difference of the impedance of each contact divided by the mean value between them. Electrode pairs are defined as those capable of providing a symmetric sense dipole around a monopolar stimulation contact. With a quadripolar subcortical lead, this leads to stimulation contacts C1 or C2 with subsequently symmetric sense pair electrodes C0–C2 or C1–C3. By pairing electrodes in this manner, calculated impedance mismatches were 20.2% (75th percentile 45.7%) for C0–C2 and 15.8% (75th percentile 26.9%) for C1–C3. Note that the outliers observed in C0-C2 (panel (B)) could be due the large range of variability between C0 and C2 (panel (A)). This large variability can be expected due to: (a) the different target electrode locations (STN or GP), (b) the use of different electrode lead models (geometry of STN electrode lead differs from GP lead), and (c) the impedance variability of intrinsic anatomical target regions due to different brain conductivity media (in the GP contact C2 is placed at the intersection of the GPe and GPi (gray and white matter), see supplementary material figure S1).