Abstracts

Source localization of EEG potentials allows one to locate the cortical generators of scalp-recorded epileptic activity. The value of relatively newer techniques such as Low Resolution Brain Electromagnetic Tomography (LORETA) for seizure localization still remains to be determined. We evaluated the seizure localizing value of applying LORETA to early ictal discharges by comparing the results to invasive intracranial recordings.
We selected 14 patients who had undergone invasive intracranial EEG monitoring at our center. Of these, 7 had bilateral subdural strip recordings, and the remaining had unilateral subdural grid and strip recordings prior to resection of their epileptic foci. All had undergone prior scalp-EEG studies which captured one or more of their typical seizures. We applied LORETA to a total of 25 scalp-recorded seizures, with at least one seizure per patient. From each recording, we selected one-second-long epochs of EEG data from within the first 5 seconds of ictal EEG changes. Source localization of the ictal discharges was then performed using LORETA, and the results rendered in Talairaich space on a 3-D brain model. We then compared the results of conventional EEG analysis, LORETA, and intracranial recordings with separate scores for the hemispheric and lobar locations of the identified foci.
Lateralization of seizure foci based on LORETA was concordant with the results of intracranial recordings in 13 (92%) of the14 patients while localization to a lobe was concordant in 12 (85%). The corresponding figures for visual EEG analysis were 12 (85%) and 10 (71%). Among the 7 patients with bilateral strip recordings, LORETA lateralization and localization compared well with intracranial results in 6 (85%) patients. Visual analysis of the EEG was concordant with intracranial data in 6 (85%) patients for lateralization, and 5 (71%) for localization. In two patients whose ictal scalp EEG[apos]s could not be lateralized visually, LORETA lateralization and localization were both concordant with intracranial recordings. Four of the 14 patients had undergone prior ictal SPECT studies. SPECT lateralization and localization was concordant with intracranial data only in one patient, while visual EEG analysis was concordant in 3, and LORETA in all 4 patients.
Our data suggest that the application of LORETA to early scalp-recorded ictal potentials can be a valuable tool for localizing epileptic foci. Although LORETA was more accurate than conventional visual EEG analysis in our patient group, the difference was not statistically significant given its relatively small size. Seizure localization based on LORETA surpassed visual EEG analysis in two instances where seizures could not be lateralized by visual analysis. Considering the low added cost of LORETA analysis when ictal EEG data is available, further evaluation of its clinical utility appears warranted.