Introduction: Medial Temporal Lobe Epilepsy (mTLE) with
hippocampal sclerosis is the most frequent cause of
drug-resistant focal epilepsy in adults. These patients
suffer from widespread subtle white matter abnormalities
and abnormal functional connectivity extending beyond the
affected lobe, as revealed by Diffusion Tensor Imaging
and functional MRI studies. Diffusion spectrum imaging
(DSI) is a new diffusion imaging technique with high
angular resolution for improving the mapping of white
matter tracts [1]. In this study, we used DSI to
investigate the whole brain effect of TLE as reflected by
topological measures of the structural connectivity
network. Methods: Eight patients with right-sided mTLE
and hippocampal sclerosis and 25 controls underwent our
DSI protocol at 3T using a 32 channel coil: DSI (voxel
size 2.2x2.2x3mm; 44 slices, 257 diffusion directions;
max b-value = 6400 s/mm2), 3D T1-weighted MPRAGE and 3D
T2-weighted images. The analysis was performed using the
Connectome Mapper. The cortical and subcortical grey
matters were parcellated into 86 Regions of Interest
(ROI) with anatomical landmarks using Freesurfer 5.0. The
ROIs were coregistered to the diffusion data by using a
nonlinear registration between T1 and T2, then T2 to the
diffusion space. The Diffusion Tool Kit was used for the
reconstruction of the diffusion Orientation Distribution
Function in each voxel. Streamline fibre-tracking was
then performed from each voxel in the white matter areas
using an in-house streamline-based algorithm. Two scalar
maps, the Fractional Anisotropy and the Generalized
Fractional Anisotropy were calculated. For each patient,
the connectivity between every region pair was estimated
was used to construct a connectivity matrix (the
adjacency matrix of the structural network). Finally we
used topological measures in order to reduce the data and
obtain whole-brain characteristics of the network [2]:
characteristic Path Length (L), Clustering Coefficient
(C) and Small Worldness (S) where S=(C/Crand)/(L/ Lrand)
with Crand and Lrand are computed for a random network.
Significant differences between the mTLE and control
groups were assessed using a Wilcoxon rank-sum test.
Results: In patients, we found a higher characteristic
path length (0.0288), with a lower clustering coefficient
(0.0079) and reduced small-worldness (p=0.000058)
compared to controls. Network nodes that contributed
significantly to these alterations were located within as
well as outside the temporal lobe in both hemispheres,
although with a predominance for the ipsilateral
hemisphere and bilateral limbic structures. The pattern
of significant nodes showed concordance with brain
regions known to be involved in epileptic networks in
patients with mTLE. Conclusions: Our results show a
reduced efficiency of structural brain networks and
altered connectivity patterns that are concordant with
the mapping of functional epileptic networks [3] and
altered functional connectivity [4] in patients with
mTLE. Therefore, the network in mTLE is less segregated
than in controls with reduced global interactions between
the nodes and reduced specialised communities. Further
studies are needed to establish the relevance of these
findings with respect to the propagation of epileptic
activity, cognitive deficits in mTLE and outcome of
epilepsy surgery in individual patients.