Abstracts

Rationale: The interpretation of ictal intracranial EEG (ICEEG) recordings for resection planning is a complex balance of the significance of specific rhythms (e.g. low voltage fast activity vs. rhythmic slowing) and their relative timing to seizure onset (initial ictal activity vs. later spread). Various interictal findings are also felt to be indicative of the epileptogenic zone. Ictal and interictal findings are then evaluated in light of findings from cortical stimulation of eloquent cortex to determine the area of resection. We describe factors involved in resection planning by statistically analyzing features that led to inclusion of an electrode in the resection map. Methods: Patients from April 2009 through May 2010 with ICEEG grid electrodes and subsequent surgical resection were retrospectively identified. The epileptologist who managed each case reviewed the ICEEG record noting ictal and interictal findings specific to each electrode. Only the first 15 seconds of ictal activity, which was divided into five 3-second epochs, was considered. The epoch in which each electrode became active and the pattern of activity observed at that time were recorded for each electrode. The descriptors used to codify the ictal and interictal EEG features are listed in Table 1. If multiple seizure types were seen in a single patient, a representative seizure was described for each type and weighted based on the proportion of that patient's seizures it represented. The presence of eloquent cortex or a known lesion under each electrode was noted as well. Every electrode in each patient was considered a separate observation in a logistic regression model to predict whether the cortex under a given electrode was included in the planned resection. Results: The 19 patients had a total of 37 unique seizures. Recordings from a total of 1306 electrodes were analyzed with results of the logistic regression analysis noted in Table 1. The strongest predictors of resection of cortex underlying a given electrode was presence of low-voltage fast activity in Epoch 1 (first three seconds of ictal activity), rhythmic spikes in Epoch 1, interictal paroxysmal fast activity, and low-voltage fast activity in Epoch 2. High-amplitude beta spikes and rhythmic slow waves were also significant predictors in Epoch 1, but were not significant in later epochs. Interictal spikes had a higher odds ratio of affecting the planned resection if described as continuous or very frequent , but less frequent spikes were also significant predictors. The presence of motor or language cortex were the strongest negative predictors of resecting underlying cortex, however eloquent sensory cortex was not found to be significant. Conclusions: Early low voltage fast activity and rhythmic spikes were the ictal rhythms that most strongly indicated the need for resection, but interictal paroxysmal fast activity was an equally strong predictor. Motor and language cortex were the strongest negative predictors for inclusion in the planned resection. The presence of sensory cortex was not a significant predictor.