Abstracts

Rationale: Historically, the localization of interictal and ictal sources has primarily relied on visual inspection of scalp EEG traces. Identification of the source is based mainly on the assumption that the source underlies the most prominent negative activity. With the advent of modern computer-assisted techniques such as dipole source modeling, more quantitative methods are now available to localize EEG activities. In this study, we examined the role that electrical dipole source analysis may have in the surgical evaluation of tuberous sclerosis patients with medically refractory epilepsy. Methods: A database of tuberous sclerosis patients seen at TCH was examined to identify patients with medically refractory epilepsy who underwent epilepsy surgery after a Phase II evaluation involving invasive electrodes. Of the 129 patients in the database, six were identified meeting these criteria. Two patients were excluded due unavailability of pre-surgical scalp EEG data. The remaining four patients' presurgical scalp EEG's were retrospectively analyzed using the Brain Electrical Source Analysis (BESA) software, developed by MEGIS Software. The scalp EEG was recorded using 10-20 international system. The EEG activity was digitally recorded referentially to midline electrodes. A single neurophysiologist performed dipole source analysis on the ictal waveforms using BESA. The dipole source models were co-registered to the patient's presurgical MRI using the BrainVoyager software. The ictal dipole source models (co-registered to the patients' MRIs) were then provided independently to two neurophysiologists. The neurophysiologists were asked identify the structural lesion that they felt that the dipole source corresponded to. This information was then compared to the surgical resections that were eventually performed. Long term surgical outcomes were determined from the patients' medical record. Results: The number of cortical structural lesions for each patient ranged from 4 to 10, with a mean of 6.25. The structural lesions were judged from official MRI reports to be either cortical tubers or focal cortical malformations, based on appearance. The two neurophysiologists identified the same cortical lesion in all four patients (100%). Compared to the surgical resections eventually performed based on intracranial recordings, the resections involved all of the lesions identified by the two neurophysiologists.. Based on surgical pathology, three of the resections involved cortical tubers, the remaining case involved a cortical malformation. At five years after resection, all four patients are seizure free. Conclusions: The results of this study suggest that dipole source modeling may accurately identify the ictal onset zone in tuberous sclerosis patients. In all patients examined, dipole source modeling identified the regions that were eventually resected based on subdural electrode recordings. Overall, this study suggests that dipole source modeling may play a valuable role in the surgical evaluation of tuberous patients with medically refractory epilepsy.