Accuracy of Frame-Based Stereotactic Depth Electrode Implantation during Craniotomy for Subdural Grid Placement

Abstract

Background: Frame-based stereotaxy and open craniotomy may seem mutually exclusive, but invasive electrophysiological monitoring can require broad sampling of the cortex and precise targeting of deeper structures. Objectives: The purpose of this study is to describe simultaneous frame-based insertion of depth electrodes and craniotomy for placement of subdural grids through a single surgical field and to determine the accuracy of depth electrodes placed using this technique. Methods: A total of 6 patients with intractable epilepsy underwent placement of a stereotactic frame with the center of the planned cranial flap equidistant from the fixation posts. After volumetric imaging, craniotomy for placement of subdural grids was performed. Depth electrodes were placed using frame-based stereotaxy. Postoperative CT determined the accuracy of electrode placement. Results: A total of 31 depth electrodes were placed. Mean distance of distal electrode contact from the target was 1.0 ± 0.15 mm. Error was correlated to distance to target, with an additional 0.35 mm error for each centimeter (r = 0.635, p < 0.001); when corrected, there was no difference in accuracy based on target structure or method of placement (prior to craniotomy vs. through grid, p = 0.23). Conclusion: The described technique for craniotomy through a stereotactic frame allows placement of subdural grids and depth electrodes without sacrificing the accuracy of a frame or requiring staged procedures.