Abstract
Mesial temporal lobe epilepsy (MTLE), one of the most common types of refractory focal epilepsy, has shown white matter abnormalities both within and beyond the temporal lobe. In particular, the white matter abnormalities in the ipsilateral hemisphere are more obvious than those in the contralateral hemisphere in MTLE, that is, the abnormalities present asymmetrical characteristics. However, very few studies have characterized the white matter microstructure asymmetry in MTLE patients specifically. Thus, we performed diffusion tensor imaging (DTI) to investigate the white matter microstructure asymmetries of patients with MTLE with unilateral hippocampal sclerosis (MTLE-HS). We enrolled 25 MTLE-HS (left MTLE-HS group, n = 13; right MTLE-HS group, n = 12) and 26 healthy controls (HC). DTI data were analyzed by tract-based spatial statistics (TBSS) to test the hemispheric differences across the entire white matter skeleton. We also conducted a two-sample paired t-test for 21 paired region of interests (ROIs) parceled on the basis of the ICBM-DTI-81 white-matter label atlas of bilateral hemispheres to test the hemispheric differences. An asymmetry index (AI) was calculated to further quantify the differences between the left and right paired-ROIs. It was found that the asymmetries of white matter skeletons were significantly lower in the MTLE-HS groups than in the HC group. In particular, the asymmetry traits were moderately reduced in the RMTLE-HS group and obviously reduced in the LMTLE-HS group. In addition, AI was significantly different in the RMTLE-HS group from the LMTLE-HS or HC group in the limbic system and superior longitudinal fasciculus (SLF). The current study found that the interhemispheric white matter asymmetries were significantly reduced in the MTLE-HS groups than in the HC group. The interhemispheric white matter asymmetries are distinctly affected in left and right MTLE-HS groups. The differences in AI among RMTLE-HS, LMTLE-HS, and HC involved the limbic system and SLF, which may have some pragmatic implications for the diagnosis of MTLE and differentiating LMTLE-HS from RMTLE-HS.
Highlights
Both structural and functional asymmetries have long been observed in the human brain and thought to be a central principle of central nervous system architecture and to relate to differences in most cognitive and neurobehavioral functioning
The leftward asymmetry reductions in left MTLE (LMTLE)-hippocampal sclerosis (HS) included the cingulum, superior corona radiata, external capsule, posterior limb of the internal capsule, and uncinate fasciculus (UF), and those in right MTLE (RMTLE)-HS included the body of the corpus callosum, superior corona radiata, external capsule, and posterior limb of the internal capsule
The Mean diffusivity (MD) map presented a relatively small area of asymmetry in the healthy controls (HC) group compared to the Fractional anisotropy (FA) map and a smaller area in the LMTLE-HS and RMTLEHS group (Figure 1B)
Summary
Both structural and functional asymmetries have long been observed in the human brain and thought to be a central principle of central nervous system architecture and to relate to differences in most cognitive and neurobehavioral functioning. Numerous studies have shown that the structural and/or functional changes of MTLE involve the temporal lobe and extend beyond the temporal regions [13,14,15,16], and these structural and/or functional abnormalities present asymmetric characteristics. A review of the voxel-based morphometry (VBM) of TLE found significant volume reductions in 26 brain regions and a strong preference for asymmetric abnormalities to be observed ipsilateral to the epileptogenic zone [15]. Left MTLE (LMTLE) manifests as more widespread abnormalities and involves the contralateral hemisphere more widely than right MTLE (RMTLE) [13, 18,19,20]. LMTLE often exhibits more intense cognitive impairment than RMTLE [21, 22]. These data imply that LMTLE and RMTLE may be two different diseases with different pathogenesis [13, 23, 24]
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