Abstracts

Rationale: Temporal lobe epilepsy (TLE) surgery may require a pre-operative Wada test for memory lateralization. This is an invasive procedure with potential serious side effects such as stroke and infection. Noninvasive alternatives, such as fMRI, are needed to replace the Wada test for this purpose. We hypothesize that resting fMRI functional connectivity between the hippocampus and other brain regions may be correlated with Wada memory score.Methods: We acquired resting state fMRI scans and Wada scores from 13 individuals with left TLE as indicated by presurgical evaluation. The fMRI images were collected on a 3T MRI scanner (Philips Healthcare, Inc., Best, Netherlands) across 200 time points; TR=2s; TE=35ms; voxel size = (3.75 37.5 5.0) mm; volume size = (64 64 30) voxels. We performed conventional preprocessing and spatial normalization followed by low-pass filtering at 0.10 Hz. Functional connectivity measures to the hippocampus were computed via general linear modeling (GLM). The Wada memory score after right injection (reflecting left hippocampal function) was converted to a percentage. The connectivity measurements were then correlated with Wada memory scores across subjects using GLM analysis.Results: Our results show several brain regions with strong associations between the functional connectivity of the left hippocampus and the Wada scores. Three regions showed particularly strong correlations: the right temporal-occipital fusiform cortex; the left precuneus; and the right caudate. Figure 1 shows the mean functional connectivity of the left hippocampus to these three brain regions v. Wada memory score across all left TLE individuals. The right temporal-occipital fusiform cortex shows a positive association with Wada scores (t=7.67; R²=0.74), i.e., the functional connectivity between the left hippocampus and the fusiform cortex increases as the Wada score increases. The left precuneus also shows a positive association with Wada scores (t=6.31; R²=0.66). The right caudate has a negative association with Wada scores (t=-3.79; R²=0.67), i.e., the functional connectivity decreases as the Wada score increases. These regions have been implicated in other studies of TLE.^1,2,3Conclusions: The strong associations between the functional connectivity of the left hippocampus to three brain regions v. Wada memory scores suggest that functional connectivity along specific networks may be an indicator of functional integrity of the left hippocampus. Future work should assess the ability of fMRI analyses to predict individual Wada test results, and identify patients at risk of postoperative memory deficits. [1] Henry, TR et al. Neurology 1998;50(3):787-790. [2] Adcock, JE et al. Neuroimage 2003;18(2):423-438. [3] Vella, N et al. Exp Brain Res 1991;85:240-242.