Abstracts

Subtracted ictal SPECT coregistered to MRI (SISCOM) and dipole source analyses of the same ictal event, when possible, offer a unique combination of temporal resolution with transient blood flow changes to better localize deep neocortical epileptic sources. A comparison was performed between localization obtained by seeding dipole source analyses with starting coordinates obtained from SISCOM data versus independent analyses for each modality without seeding. Stereotypic frontal lobe seizures were captured for 3 pediatric patients (11-15 yrs). Electrode positions (n=27) were digitized using a Polhemus^R digitizer. Source analyses of the ictal recordings were performed using the boundary element method (Curry^R v4.6). SISCOM was concurrently acquired for the same seizures. Ceretec^R (Tc⁹⁹), 15-27mCi aliquots stabilized in methylene blue, was administered by peripheral IV bolus using an autoinjector (Medrad^R) during the ictal onsets. Injections were completed within 12-24 seconds following the earliest features of the ictus by semiology or electrographic changes, whichever came first. SISCOM analysis (Analyze^R) included ictal and interictal SPECT normalization, subtraction and co-registration to the subjects high resolution gapless MRI. Seed coordinates were taken from the center of the primary SISCOM lesion for each patient defined by 50-75% or greater of maximal pixel intensity. Multiple source classification analysis modeled 3 dipoles for the initial 500 milliseconds of the identifiable ictal onsets acquired from subjects 1 and 2. Seeds obtained from concurrently acquired SISCOM data for these patients contributed to only a minimal shift in the fitted dipole locations. The ictal onset of patient 3 was associated with a generalized attenuated high frequency rhythm that could not be localized by source analysis. Source localization of stereotypic interictal discharges, however, did not shift when seeded with the SISCOM data. Source analyses of interictal data identified a larger region than did SISCOM alone. Patient 3 received subdural contacts covering the regions of combined analyses involving the left mesial, basal polar and lateral frontal cortex. Intracranially acquired interictal data showed concordance with the presurgical data. The ictal onset, however, was nonlocalizable and beyond the borders of the intracranial electrode set. Surgical manipulation was limited to intracranial contact insertion and removal. Seeding source analyses with concurrently acquired SISCOM coordinates, where transient blood flow changes are measured, offers a powerful strategy toward localizing deep frontal epileptic sources. The ictal onset zone, however, may remain elusive suggesting a need for further innovative diagnostic modalities.