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. Basal temporal lobe encephaloceles are rare congenital malformations which are prone to causing medically refractory focal epilepsy. In this study, we examined the role that electrical dipole source analysis may have in the surgical evaluation of patients with these malformations. Methods: A database of epilepsy patients seen at Texas Children's Hospital was reviewed to identify patients with medically refractory epilepsy from basal temporal encephaloceles. 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, using the Nicolet digital video/EEG system. A neurophysiologist performed electrical source analysis on both the interictal and ictal waveforms. The dipole source models were co-registered to the patient's pre-surgical MRI using the BrainVoyager software. The dipole sources were compared to the location of the encephaloceles. MRI findings were assessed in each patient. Surgical outcome was noted in the cases, when appropriate, and assessed in comparison to the dipole sources. Results: Four patients were identified with basal temporal encephaloceles. In three cases, MRI abnormalities were restricted to the encephaloceles. One case had increased T2 signal in the hippocampus without architectural abnormalities. Electrical source imaging in all cases revealed sources with a lateral orientation in the lower anterior temporal lobe, suggesting involvement of the inferior antero-lateral temporal cortex. Two eventually underwent an anterior temporal lobe resection with hippocampal sparing. One patient had a recurrence of seizures after surgery, underwent a resection of the hippocampus and became seizure free afterwards. Interestingly, this patient had a normal appearing hippocampus on MRI. The remaining surgical patient had increased T2 signal abnormalities in the ipsilateral left hippocampus, as well as a very active hippocampal spike focus on electrocorticography. This patient remained seizure free 2 years after hippocampal-sparing resection. Conclusions: Electrical source analysis of this series of patients suggests that basal temporal encephaloceles cause discharges involving the inferior antero-lateral temporal cortex. This study suggests that electrical source analysis may predict seizure freedom with anterior temporal lobectomies, but may not predict whether hippocampal resection is required. This study also suggests that absence of hippocampal MRI abnormalities may not predict whether hippocampal resection is needed in these cases.