Utilization of impedance measurements in pallidotomy using a monopolar electrode.

OBJECT This study was performed to evaluate the reliability and utility of using monopolar impedance monitoring for estimation of the location of the thermocouple electrode during a pallidotomy procedure. METHODS In 26 patients, impedance profiles were measured at 2-mm intervals and anatomical boundaries were marked on postoperative MRI studies in 19 patients. The pattern of change in impedance as the electrode was towards the target coordinates in the globus pallidus was studied empirically in relation to MRI-defined anatomy and by electrical stimulations. The pattern of impedance change was of greater reliability than absolute impedance values. A drop in impedance was noted in 10 cases where the electrode entered the putamen or globus pallidus upon exiting the internal capsule, and in 9 of these this drop was measured before or at the distal boundary of the internal capsule. A variable pattern in the region of the base of the globus pallidus was observed, with 10 of 19 cases demonstrating a decrease and 8 cases an increase, consistent with the proximity of a CSF cistern in the former and the ansa lenticularis and optic tract in the latter. Simulations showed that the monopolar electrode was more sensitive to the approach of a boundary than the bipolar electrode. CONCLUSION Our measurements and simulations suggest that a monopolar electrode could predict a nearby region of high or low impedance before penetrating it and a trend of decreasing impedance as the distal edge of the globus pallidus is approached.