Abstracts

Rationale: In the absence of structural defects on neuroimaging studies, diagnosis of the seizure onset zone (SOZ) is typically made through the analysis of electrophysiological data obtained during ictal activity. The use of interictal spikes to localize the SOZ would prove useful in the clinical setting due to the relative abundance of these events compared to the seizure activity. Additionally, noninvasive measurements, such as electroencephalography (EEG) could reduce or potentially eliminate the need for long-term invasive monitoring. In this study, we have examined the ability of source imaging of interictal spikes from high resolution EEG to identify epileptogenic foci in a cohort of patients with medically intractable epilepsy. Methods: High density EEG recordings utilizing a 76-electrode montage were obtained in a cohort of six patients undergoing surgical evaluation for the treatment of medically intractable epilepsy. The patients were studied under a protocol approved by the Institutional Review Boards at the University of Minnesota and the Mayo Clinic (Rochester, MN). Patient-specific boundary element models were constructed from MRI data in each of the six patients and the three-dimensional locations of the electrodes were digitized. The source locations of the interictal spike activity were obtained by means of the LORETA algorithm and the corresponding results were compared with the SOZs identified by conventional clinical methods. An average of eight spikes was analyzed in this manner for each patient. Results: In one patient, SPECT imaging identified a focus within the right mesial temporal lobe. LORETA analysis of eight interictal spikes in this patient revealed an average localization error from the site of SPECT activation of 4.08 mm (SD: 1.69 mm). A second patient with a right temporal focus had a MRI visible lesion which correlated with the location of the ictal activity. Analysis of nine interictal spikes in this patient revealed an average localization error of 2.20 mm (SD: 2.37 mm). In the remaining four patients, there were no abnormalities visualized on MRI and SPECT imaging was not available. In these patients, the location of all of the interictal spike source activity was within the surgically resected region as identified from operative records. Conclusions: In this study, we have found that the use of the LORETA algorithm on interictal spike activity obtained from high-density EEG recordings was able to identify sources of the interictal spikes which were highly co-localized with the clinically-identified SOZ. Such noninvasive source localization techniques could prove beneficial in the identification of the SOZ by reducing or eliminating the need for invasive intracranial recordings. Additionally, the use of interictal spikes as surrogate markers of the epileptogenic zone could have a profound impact upon the presurgical evaluation of patients with intractable epilepsy by dramatically shortening the length of the long-term EEG monitoring.