Postoperative monitoring of the electrical properties of tissue and electrodes in deep brain stimulation.

The aim of this study was to investigate the causation of an insufficient clinical response to acute external stimulation after implantation of brain stimulating electrodes in patients with idiopathic Parkinson's disease (IPD). This study represents to our knowledge the first consecutive postoperative monitoring of the electrical properties of electrodes and tissue in this patient cohort. We hypothesized that changes in brain tissue resistance would be etiologic for this clinically observed phenomenon. Eight patients with IPD were monitored over a fixed time course (day 1-7). The voltage drop on the electrodes and in the tissue was measured. Our measurements indicate a significant inhomogeneity of the surrounding tissue and a marked interpatient difference of the electrode voltage drop. The changes of the voltage drop on the electrodes and the tissue were insignificant over the time course. We conclude that the decline of the clinical benefit is not due to an alteration of tissue resistivity. As an explanation, we postulate that the reactive formation/resolution of edema around the electrode has the same resistivity as the target and is therefore not detectable by our measurements or modulated by a still unknown mechanism.