Abstracts

This study was undertaken to examine the spatiotemporal dynamics of interictal epileptiform discharges (IEDs) in temporal lobe epilepsy by applying methods that improve both spatial localization and temporal precision in mapping the rapidly evolving discharges. Eight patients with temporal lobe epilepsy underwent dense array 256 channel scalp EEG recordings. Localization of IEDs at 10 msec intervals in relation to a standard MRI model was accomplished with dipole fits and with two distributed linear inverse methods of source analysis, using probabilistic gray matter voxels as source constraints. The results were compared to conventional EEG waveform interpretations and the limited source analysis possible with conventional EEG. The three methods of source localization yielded convergent results. The initial onset of the averaged IED localized to one basal-mesial temporal lobe. Adjacent temporal lobe regions become involved within 10-20 msec. In several patients, there was rapid spread to the opposite temporal lobe and, in some cases, to extratemporal regions before returning to the original temporal lobe. Both temporal lobes often were sources for IEDs. Each IED showed unique propagation patterns. Initial IED temporal involvement was concordant with the site of surgery in most subjects. Restriction of mapping to a conventional EEG montage resulted in fewer IED detections and frequently misleading results from both visual inspection and source localization. Discerning the rapid propagation of IEDs in temporal lobe epilepsy can be achieved by applying source analysis to dense array recordings. Dynamic brain mapping improves discharge detection, reduces margin of error in source analysis calculations, enhances reliability of anatomic localization of discharges with msec resolution, and yields insight into cortical network involved in epileptic discharges.