Abstracts

Rationale: Many years of experience at our institution and emerging evidence elsewhere support the presence of epileptogenic networks. While these networks tend to involve regions normally strongly interconnected (ie. temporal-occipital network) the epileptogenic network often extends beyond focal abnormalities and may be larger than predicted. Furthermore, a recent study involving the placement of several "microelectrodes" between typical macroelectrodes revealed seizure activity within microelectrodes that was not detected on the typical clinical macroelectrodes (Stead etal. 2010). Thus we have more to learn about the spatial and temporal scales of epilepsy and epileptic networks. Together, these observations suggest that intracranial EEG (icEEG) recordings that are focused only on areas of small structural or metabolic abnormalities are likely to under-sample a given patient's epileptogenic network. In order to facilitate increased data acquisition, high density recording systems that are widespread in the neurosciences are becoming more commonly used in epilepsy centers. Accordingly, our practice in the Yale Epilepsy Surgery Program (YESP) has been one of increasingly broad EEG coverage in the evaluation of most forms of LRE. As we increase the number, density and types of contacts used to improve icEEG monitoring, patient safety must remain paramount. Methods: The Yale Epilepsy Surgery Program has pro-spectively maintained a database of all patients undergoing icEEG monitoring. We are retrospectively reviewing this database from 1991 to 2011 and reviewing morbidity data when available. The number of icEEG contacts placed per patient as well as the types of electrodes used were analyzed. Surgical outcomes were reviewed with attention to morbidity during icEEG studies. Results: Previously, we have reported a steady increase in the number of icEEG contacts used during icEEG monitoring sessions, with average numbers of contacts rising from 80-100 per patient in the early 1990s to 200 per patient in the mid-2000s. This trend has continued in recent years as our technical capabilities have increased (p < .01, R2 = .81). Despite the significant increase in the number and types of electrodes, the morbidity of icEEG implantation and monitoring has not changed significantly (p > 0.20) and infection has remained the most common morbidity. Conclusions: The widescale ongoing review of the Yale Epilepsy Surgery Database has not revealed any significant change in patient morbidity, despite the significant increase in electrodes used for icEEG. However, the mass effect, bleeding and infection risks of intracranial electrodes are well known and new approaches to improve these devices should be an ongoing process. We believe that broad, multimodal icEEG monitoring is needed to fully understand the cases of LRE requiring icEEG recordings and to provide the best treatment strategies.