Abstracts

Rationale:
Temporal plus epilepsy is a significant cause of ongoing seizures following surgery for drug-resistant temporal lobe epilepsy (TLE), in which several distinct patterns of semiology and seizure propagation have been described. Structural connectivity (SC) studies have also found temporal lobe subregions with unique whole-brain SC profiles that may represent potential seizure propagation pathways. Optimal intracranial electroencephalography (iEEG) sampling strategies ideally sample such pathways. Here, we constructed subnetworks of normal temporal SC and assessed how well our standard iEEG implantation strategy covered the brain regions involved in each subnetwork in a group of TLE patients.
Method:
15 healthy adults (8 females, ages 34.5 ± 14.4) underwent diffusion tensor imaging (DTI). 45 images with max b-value of 1100 were preprocessed with TORTOISE to correct for distortions. AFNI’s FATCAT was used to compute diffusion tensors and perform probabilistic tractography with thresholds of 0.2 min FA, 60° max angle, and 20mm min tract length. Average weighted whole-brain SC matrices were constructed across subjects using 419 cortical and subcortical regions-of-interest (ROI) as in Schaefer et al [1] and FreeSurfer subcortical segmentations. Spectral clustering, with eta-squared as the distance metric, was used to group left temporal lobe ROIs into 6 clusters with distinct whole-brain connectivity profiles. For each cluster, we assessed how thoroughly its connections were covered in our standard temporal lobe iEEG subdural implantation using electrode coordinates of 17 TLE patients (6 right TLE).
Results:
Temporal lobe clusters were spatially contiguous, with similar surface areas (Fig 1a) and variable connectivity profiles (Fig 1b-g). Relatively unique connections were noted in 1) temporal tip to orbitofrontal, 2) mesial temporal to mesial temporo-parieto-occipital, and 3) posterior temporal to posterior parietal and occipital cortices. No contralateral hemisphere connections were observed. When compared to the heatmap of iEEG electrode placements (Fig 2), the anterior superior and inferior temporal clusters (b,e) had the greatest mean electrode coverage (47%, 43%), significantly more than the mesial temporal cluster (f) which had the least coverage (28%) (p< 0.025).
Conclusion:
Our standard iEEG implantation provided the best spatial coverage for the anterior superior and inferior temporal neocortex subnetworks, and the least coverage for the mesial temporal subnetwork. These findings suggest regions in which seizure foci may have been undersampled, potentially leading to worsened seizure outcomes following surgery in patients with temporal plus epilepsy.  References: [1] Schaefer A, Kong R, et al. Local-Global Parcellation of the Human Cerebral Cortex from Intrinsic Functional Connectivity MRI. Cereb Cortex 2018;28(9);3095-3114.
Funding:
:This research was funded through the NIH Intramural Research Program in the National Institute of Neurological Disorders and Stroke.