Abstracts

Rationale: Subclinical seizures have been reported to be clinically significant with their characteristics influencing surgical outcome. There have been few studies done evaluating the significance of subclinical seizures and the vast majority of seizures studied have been temporal in origin. The purpose of this study was to evaluate the clinical significance, characteristics and prognostic value of subclinical seizures of temporal and extra temporal origin. Methods: We reviewed our epilepsy surgical database from 2003-2011 for patients that underwent intracranial EEG recording, surgical resection and had subclinical seizures during their intracranial EEG recording. After those patients were identified we then evaluated seizure onset zone, electrographic pattern at onset, seizure type, seizure spread, presence or absence of MRI lesion, and surgical outcome at least one year post-surgery. Patients were grouped into two groups. Group 1 where both the clinical and subclinical seizures had the same seizure onset region, and group 2 where some or all of the clinical and subclinical seizures originated from different regions. Results: A total of 27 patients were found to have had subclinical seizures during their intracranial EEG recording. Of those, 14 patients had temporal lobe epilepsy(12 mesial and 2 lateral) and 15 were extra temporal(5 frontal, 8 parietal/occipital). There were a total of 791 seizures with 310 of those seizures being subclinical in nature. Among the clinical and subclinical seizures that were co-localized, 33% had poor surgical outcome, and 67% had excellent surgical outcome. For those that were partially co-localized or not co-localized 91% had poor outcome while 9% had excellent outcome. Eleven patients were found to have subclinical seizures that propagated to a region beyond their seizure onset zone. Of those 11 patients, 6 patients had subclinical seizures propagate to a different anatomical region than the clinical seizures. All six of these patients had poor surgical outcome. Conclusions: Our study like others has found that subclinical seizures are clinically significant and if are included with the region of surgical resection will predict good surgical outcome(Fisher's exact test p=0.011). An observation from our study is that subclinical seizures may represent a distinct epileptic network from clinical seizures as they can propagate to different anatomic regions from clinical seizures. Propagation of subclinical seizures to a different region than clinical seizures can affect surgical outcome(Fisher's exact test p=0.06). Further studies with a larger sample size will need to be conducted to fully assess the effect on surgical outcome and determine its statistical significance.