Introduction: The success of resective surgery in patients with medically intractable epilepsy relies on adequate localization of the ictal focus. When non-invasive electrographic investigations are inconclusive, phase II invasive monitoring is pursued, with either grid/strip subdural electrodes or stereotactically placed depth electrodes. Conventional grid/strip electrodes provide excellent spatial coverage over the cortical surface. However, multiple depth electrodes may provide sufficient 2-dimensional spatial coverage alongside 3-dimensional data for epileptogenic zone localization, while offering advantages in ease of bilateral electrode placement, ability to optimize craniotomy for resection, and diminished surgical time. We investigated whether depth electrodes as a first phase II investigation would generate definitive recommendations on resective surgery.
Methods: The study retrospectively reviewed 63 patients who underwent a total of 68 phase II investigations with depth and/or grid/strip electrodes from 2006-2012. All patients were refractory to medical management and had inconclusive non-invasive investigations. 33 depth studies and 35 grid/strip studies were performed.
Results: 28/33 (84.8%) of depth electrode procedures resulted in definitive recommendations, with 16 patients referred for resection, 2 deemed high risk for post-operative deficit secondary to Wada testing, and 10 with bilateral or multifocal ictal onset. 35/35 (100%) of grid/strip investigations resulted in definitive recommendations, with 32 referred for resection and 3 deemed high risk for post-operative deficit. In patients who underwent resection, anesthesia time was reduced by an average of 16.1% (p<0.01) and surgical time reduced by 25.3% (p<0.01) amongst depth electrode patients. In patients who were not candidates for resection, anesthesia time was reduced by 58.0% (p<0.01) and surgical time reduced by 72.4% (p<0.01).
Conclusions: This study supports the use of stereotactic depth electrodes as a primary invasive monitoring technique, with a high rate of conclusive study and statistically significant decreases in procedure time, most notably in patients with multifocal ictal onset precluding focus resection.
Patient Care: This study supports the use of stereotactically placed depth electrodes in phase II epilepsy investigations. This modality is used more often in European centers, but not widely used in the US. The study demonstrates high rate of definitive investigation with statistically significant reductions in anesthesia and surgical times.
Learning Objectives: By the conclusion of this session, participants should be able to:
1) Describe the differences between grid/strip subdural electrodes and depth electrodes.
2) Explain the pros and cons of each modality in phase II epilepsy investigations.
3) Discuss patient-specific and surgeon-specific factors that may lead the neurosurgeon to favor one modality over another.