[MRI of hippocampal malformations in patients with intractable temporal lobe epilepsy]

Abstract

Objective: In some patients with temporal lobe epilepsy, recent MRI studies have revealed several morphological features indicative of discrete hippocampal malformation (HM). Its prevalence is unknown and the relationship between the HM and the origin of seizures has never been investigated. Our purpose is to define the MRI findings of this new entity and to determine its incidence in a group of patients and in a control group in order to evaluate its clinical significance.

Materials and methods: MR imaging findings in 97 patients suffering from medically intraceable temporal epilepsy were prospectively evaluated during the preoperative evaluation of surgical candidates. The MR-imaging protocol included oblique coronal slices perpendicular to the temporal lobes using high resolution T2 weighted (HR TSE T2), Fluid attenuated inversion recovery (FLAIR) and inversion-images. This protocol has been completed by axial FLAIR images and axial and sagittal IR images of the whole brain. Coronal HR TSE T2 images were performed in 50 healthy control subjects. Cerebral lesion and hippocampal morphology were evaluated in both groups.

Results: Fourteen patients (14%) showed hippocampal morphological modification. The most frequent and specific findings were lack of visualization of the internal hippocampal (lack of linear T2 hypointensity within the hippocampus) and the abnormal shape (pyramidal, vertically oriented or globular-shaped). Other signs were: abnormal position of the hippocampus (medically located hippocampus) and vertical collateral sulcus. Cases without visualization of the internal structure of the hippocampus were considered as a complete form of HM and were correlated with temporal epilepsy. A vertical collateral sulcus was observed in some control group subjects.

Conclusion: Complete forms of HM could be considered as epileptogenic lesions. Nevertheless, interpretation of the incomplete form is delicate: the abnormal angle of the collateral sulcus can be encountered in healthy subjects and could therefore be considered a normal variant.