Abstracts

RATIONALE: The mesial temporal lobe is composed of the hippocampus, amygdala and parahippocampal region. In temporal lobe epilepsy (TLE), the term [dsquote]mesial temporal sclerosis[dsquote] was introduced to describe widespread pathological changes of mesial temporal structures. Using high resolution MRI, we previously showed a reduction in volume of the entorhinal cortex (EC) in patients with TLE (Bernasconi N. et al. Neurology 1999;52:1870-1876). The purpose of this study was to examine if changes in the volume of the other two components of the parahippocampal region, namely the perirhinal cortex (PC) and posterior parahippocampal cortex (PPC), were apparent on MRI. Furthermore we were interested to study the degree of atrophy in different mesial temporal structures.
METHODS: MRI volumetric analysis of the hippocampal head, body and tail, amygdala, EC, PC and PPC was performed using a T1-weighted 3D gradient echo sequence in 20 healthy subjects and 25 TLE patients with unilateral TLE. Images were registered into a stereotaxic space to adjust for differences in total brain volume and brain orientation. The EEG focus was defined as right or left if more than 70% of seizures were recorded from one side. Thirteen patients had left TLE and twelve had right TLE. Group analysis was done using a multivariate analysis of variance (MANOVA) followed by Tuckey HSD post-hoc comparisons.
RESULTS: In controls, the mean volume of right EC (1,376[plusminus]170 mm3) was slightly greater than that of the left (1,305[plusminus]138 mm3; p=0.04); the mean volume of right PC was 2,771[plusminus]486 mm3 and that of the left PC was 2,900[plusminus]554 mm3; the mean volume of the left PPC (2,499[plusminus]583 mm3) was greater than that of the right (2,234[plusminus]404 mm3; p[lt]0.01). In TLE patients the hippocampus, amygdala, EC and PC were atrophic ipsilateral to the seizure focus. The mean volume of PPC was not different from normal controls. Within the parahippocampal region, the EC was more severely affected than the PC. Among all structures, hippocampal head and EC were the most atrophic.
CONCLUSIONS: Our MRI results confirm pathological findings of damage in the mesial temporal lobe, involving not only the hippocampus and the amygdala, but also the EC and the PC. Within mesial temporal lobe, hippocampal head and EC are the most severely atrophic structures. This pattern of atrophy may be explained by a disruption of entorhinal-hippocampal connections as a result of privileged electrical dialogue between these two structures. Studying the anatomical relationship between mesial temporal lobe structures may allow for a better understanding of limbic circuitry in TLE.
Support: Savoy Foundation and Canadian Institute of Health and Research.