Abstracts

Rationale: Epilepsy surgery by removal of the epileptogenic zone (EZ) is effective to treat pharmacoresistant focal epilepsy patients. The localization of EZ is pivotal in patient selection for epilepsy surgery. Despite progress in imaging technologies, the use of invasive intracranial electroencephalogram (EEG) recording remains essential in the identification of EZ in half of epilepsy surgery candidates. Besides subdural electrodes, localization of EZ often includes the use of depth electrodes (DEs), especially for deep-seated EZ. In cases that evaluation with both DEs and subdural electrodes is indicated, DE is implanted via craniotomy to accommodate placement of its counterparts. Implantation of DEs was performed exclusively using frame-based stereotaxy until recently. However, the reference frames may hinder access to the surgical field and reduce the flexibility of head positioning when performed concomitantly with craniotomy. Here, we report our experience with frameless image-guided stereotactic implantation of DEs via craniotomy and the precision of DEs positioning of this approach. Methods: 11 consecutive patients with pharmacoresistant temporal lobe epilepsy (TLE) evaluated in our hospital were included in this study. Frameless image-guided stereotactic implantation of DEs was performed using the VarioGuide system (BrainLAB) via craniotomy for subdural grid electrodes placement (Figure 1). All the implantations were unilateral except in 2 patients with suspected bilateral TLE. DEs were orthogonally directed perpendicular to the longitudinal axis of the hippocampus. Prior to surgery, all patients underwent magnetic resonance imaging (MRI). Imaging data were transferred to iPlan® Cranial (BrainLAB) where targets and entrance points were planned (Figure 2). Post-implantation imaging data, acquired 10-13 days after implantation, were transferred to iPlan® Cranial and coregistered to the preoperative MRI of the surgical plan. We assessed the precision of implantation by comparing the planned target with the real electrode position, as determined by the post-implantation imaging. Target point deviation in the x, y, and z planes was assessed. Results: A total of 34 DEs were successfully placed without complication. The system provided ample working space for craniotomy design and great flexibility for choosing different entry points and trajectory angles. Precision assessment yielded a mean target point deviation of around 2 mm. The implanted DEs were utilized for acquisition of EEG recordings as well as for memory provocative test by electrical stimulation to predict postoperative memory impairment. EEG recordings and findings of memory provocative test were successfully acquired and were competent for surgical decision in all the patients. Conclusions: Our preliminary experience suggests that frameless image-guided stereotaxy is safe and accurate for DEs implantation performed via craniotomy. Further experience with a larger patient series would define the efficacy of this technique.