Abstracts

Rationale: Because approximately 60% of the cortex is below the surface, depth electrodes provide a better view of sources that are deep in sulci or whose orientation is tangential to the plane of subdural electrodes. To avoid injury to large draining veins or dural venous lakes and consequent risk of hemorrhage or venous infarction, depth electrodes would be implanted, especially at inferior temporal, opercular and interhemispheric regions. We have used the frameless stereotactic navigation to direct the placement of orthogonal depth electrodes in conjunction with extensive subdural grid monitoring. Methods: Twelve consecutive patients (Mean age, 10.9±4.4 years) who had epilepsy surgery due to medically intractable neocortical epilepsy between 2008 and 2009 were reviewed. Resective surgery (9/12) or functional hemispherectomy (3/12) were done after intracranial monitoring with extensive electrode arrays (117.6±28.6) including grids (94.8±27), strips (9.7±6.1), and depth electrodes (13.2±5.1). Grid maps were generated using coregistration software by incorporating preoperative MR imaging and post-implantation high resolution CT scan. Resective surgery was done in one or more regions; Frontal (2), Temporal (2), Frontotemporal (1), Frontoparietal (2), temporooccipital (1), and Frontoparietotemporal (1). The indications for functional hemispherectomy were Rasmussen encephalitis, unilateral perisylvian schizencephaly and unilateral opecular malformation of cortical development (MCD). The most common pathology was focal cortical dysplasia (type I (1), IIA (3), IIB (5)) and tuber (2). Results: Depth electrode-monitoring represented either ictal onset zone (4/12) or early propagating zone (9/12), which provided additional information on the resection margin in either extent (7/12) or depth (3/12). The ratio of electrodes involved in resection to implanted electrodes was low (0.26±0.09). Eleven patients became seizure free after surgery (follow-up duration, 8.7±4.7 months). One patient who presented with epilepsia partialis continua (EPC) secondary to MCD no longer had EPC, but remained with catamenial seizures. No surgical complications occurred related to intracranial monitoring. Conclusions: Most patients (8/9) became seizure-free when we could monitor extensively but resect relatively small seizure-onset regions. The extensive monitoring also gave us the rationale for functional hemispherectomy. Supplemental depth electrodes provided useful data for decision-making on the resection margin in either extent or depth. Depth electrodes can be safely placed utilizing frameless stereotactic navigation and contribute significantly to localization of the epileptogenic zone. The placement of intracranial electrodes should be tailored with enough coverage by the firm hypothesis to answer a specific question in an individual patient.