Diffusion tensor imaging in medial temporal lobe epilepsy with hippocampal sclerosis
Introduction
Diffusion tensor imaging (DTI) is an imaging technique which permits visualization of the architecture of cerebral tissue. The magnitude (diffusivity) and the directionality (anisotropy) of molecular displacement by diffusion can be quantified using DTI (Pierpaoli et al., 1996). In epilepsy patients, a decreased diffusivity was first reported in cases of both focal (Wieshmann et al., 1997, Lansberg et al., 1999) and non-convulsive status epilepticus (Flacke et al., 2000, Chu et al., 2001). Decreased diffusivity has also been observed inconsistently in the early post-ictal period following focal short-lasting seizures (Diehl et al., 2001, Hufnagel et al., 2003).
In medial temporal lobe epilepsy (MTLE) patients with hippocampal sclerosis (HS), interictal diffusion imaging studies (Hugg et al., 1999, Wieshmann et al., 1999, Kantarci et al., 2002, Assaf et al., 2003, Londono et al., 2003, Lee et al., 2004, Duzel et al., 2004) found an increased diffusivity in the hippocampal region ipsilateral to the side of seizure focus. The majority of these studies have focused on the sole hippocampus using regions of interest (ROIs) methods. Anyway, there is a growing evidence from electroclinical data that in MTLE, the epileptogenic onset zone, i.e., the medial temporal lobe, is expanded to incorporate a larger epileptic network, i.e., larger cortical or subcortical regions involved in seizure propagation or in seizures control (Nair et al., 2004, Deransart et al., 2000, Chauvel, 2001). Numerous neuroimaging studies have demonstrated that structural or functional abnormalities (metabolic changes, subtle structural lesions) may extend beyond the seizure onset zone in unilateral MTLE associated with HS (Duncan, 1997, Bernasconi et al., 2003, Moran et al., 2001, Woermann et al., 1999, Tasch et al., 1999) even though the exact physiopathology of these abnormalities is still unknown.
In this study, DTI combined with statistical parametric mapping allowed us to explore the entire brain and to gain further insight into the organization and the structure of cerebral tissue in well-defined MTLE. This method provided a way to characterize diffusion abnormalities in a large network. To better determine clinical factors that might influence diffusion abnormalities to be associated with MTLE, we specifically studied the influence of epilepsy duration, age at onset, frequency of generalized tonic–clonic seizures (GTCS), frequency of complex partial seizures (CPS), or history of febrile seizures (FS).
Section snippets
Patients and controls
The study population included 35 patients (24 women and 11 men, mean age 33.9 years ± 8.8, range 19 to 53 years) with MTLE who were undergoing presurgical evaluation for anterior temporal lobectomy at the epilepsy unit of the Salpêtrière hospital. The control group included 36 healthy volunteers (14 women and 22 men, mean age: 32.8 years ± 9.3, range 18 to 57 years) with no history of neurological disorders and normal standard MRI.
The proportion of women was larger in the group of patients than
Normality of residuals for anisotropy data
We found a proportion of 14.13% voxels with non-gaussian residuals, mainly in the cerebral peduncles and orbito-frontal region. The overlap between regions of significant patient–control differences (at P < 0.001 cluster corrected) and regions in which residuals were deemed to be non-normally distributed was 7.33%. We therefore assumed that it was an acceptable proportion of voxels with non-normally distributed residuals.
Maps of mean diffusivity
Patients exhibited an increased MD in the temporal lobe ipsilateral to the
Discussion
This study led to the findings that diffusion abnormalities are not restricted to the seizure onset zone, i.e., the pathologic hippocampus but extend in the adjacent temporal lobe, in the contralateral hippocampus, and in distant extratemporal structures. Furthermore, correlations were found between diffusion abnormalities and two of the clinical factors studied.
Acknowledgments
We wish to thank the physicians and the technical staff of the department of neuroradiology for their collaboration and assistance. We are grateful to Richard Miles for comments on the manuscript. This work was supported by the IFR 49 and by a grant from the Collège des Enseignants de Neurologie.
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