Abstracts

Rationale: With Electric Source Imaging (ESI) the source of interictal epileptiform activity in EEG can be localized with high accuracy. For accurate localization both a sufficient number of recording electrodes, and adequate spatial coverage of electrodes over the recording surface is necessary. The purpose of this study was to evaluate the ESI localization accuracy of epileptic spikes in patients with a temporal lobe focus, both with 256 electrodes, with sparser electrode arrays, and with incomplete coverage of the temporal lobe. Methods: In thirteen patients with temporal lobe epilepsy interictal epileptiform activity was recorded using high density EEG (256 electrodes). All patient subsequently had a temporal lobe resection. Individual spikes were subject to a regular downsampling to 128, 64 and 32 electrodes and to two kinds of irregular downsampling omitting electrodes over and below the temporal lobes. For each electrode setup, individual spikes were localized in a standard realistic brain using a distributed source model (sLORETA). The location of the source maximum was compared to a point within the parahippocampal gyrus considered to well represent the true localization of the epileptic focus Results: In 8 patients MRI showed a mesiotemporal abnormality. In all these patients the localization accuracy was best with 256 electrodes, and gradually decreased with the number of electrodes (Figure). In the remaining 5 patients MRI indicated a neocortical temporal focus. In these patients the results were similar to above except that the results with the reduced setup RED1 (189 electrodes, some reduction of temporal lobe coverage) was not significantly worse than with 256 electrodes. In both groups the worst results were obtained wit RED2 (156 electrodes but none below the T3-T4 level). Conclusions: Both a dense sampling on the head and a sufficient coverage of the lower temporal areas is necessary for correct ESI results. A sufficient coverage of lower temporal areas seems to be particularly important for mesiotemporal foci. Acknowledgement: This project was supported by Electrical Geodesic Inc