Abstracts

The generally accepted neuroimaging hallmarks of hippocampal sclerosis (HS) are hippcampal volume loss and increased T2-weighted signal intensity. These two findings are most often seen together, but either of these findings seen without the other still carries a strong positive predictive value for a pathologic diagnosis of HS. Some authors have suggested that a third neuroimaging hallmark of HS exists, namely, loss of definition of the internal architecture of the hippocampus. In the past, differentiation of the hippocampal internal architecture was only possible with special scanner arrangements, but now 3-Tesla (3T) scanners with phased array coils are becoming common making high resolution imaging more widely available. We examined a series of patients with proven temporal lobe epilepsy (TLE) and a 3T MRI according to our temporal lobe protocol. We report the incidence of asymmetry of hippocampal internal architecture among patients with and without other findings indicative of HS., Included patients had a temporal lobe protocol 3T MRI between 4/2004 and 6/2005 and a video-EEG (VEEG) monitoring study indicating unilateral TLE. Twenty-five patients were identified as meeting criteria. Three patients were excluded because of prior temporal lobectomy or unacceptable artifact. A single reviewer examined all cases while blinded to patient identity and VEEG results. Each hippocampus of each patient was rated as having clearly defined, somewhat defined, or undefined internal architecture as seen on a high-resolution T2-weighted sequence and a heavily T1-weighted inversion recovery sequence. Evidence of HS (signal abnormality and/or volume loss) was also recorded., Of the 20 studies suitable for analysis, nine (45%) had evidence of HS and 11 (55%) did not. Of the nine with HS, seven (78%) had an asymmetry in the clarity of the internal architecture but two (22%) had clearly visible layering on both sides.
Of the eleven patients without HS, three (27%) had asymmetries in the the clarity of the internal architecture of the hippocampus. In each of these three cases the side with no perceptible internal architecture correctly predicted the VEEG-based lateralization of the epileptogenic temporal lobe., These data confirm that poor or absent definition of hippocampal internal architecture usually accompanies classic MRI findings indicating HS. However, some patients will have well visualized internal architecture bilaterally despite evidence of unilateral hippocampal volume loss or signal abnormality. Therefore, loss of definition of internal architecture, like the other hallmarks of HS, is a frequent but not universal finding in HS patients.
Most TLE patients without MRI evidence for HS will have no asymmetry of visualization of hippocampal internal architecture. But, when a unilateral loss of hippocampal internal architecture is present in a TLE patient it may indicate the laterality of the epileptogenic temporal lobe.,