The diagnosis of hippocampal sclerosis: Other techniques

Pathologically, hippocampal sclerosis (HS) is characterized by neuronal loss and gliosis affecting particularly the pyramidal neurons of CA1, CA3, and CA4 with relative sparing of the CA2 neurons. This can be identified in vivo with magnetic resonance (MR) imaging techniques that can reveal both morphological and signal abnormalities. The morphological changes are atrophy and loss of the normal internal architecture of the hippocampus as seen in coronal section. There is also T₁- and T₂-weighted signal abnormality in the hippocampus. Quantitative techniques are very good at measuring any single one of these features, but the spectrum of HS includes cases in which a single feature can occasionally be misleading. Also, quantitation focuses entirely on the hippocampus, and it is becoming clear that HS may exist in the presence of other brain pathology that may affect proper management of the patient. Therefore, quantitative measures should always be interpreted in the context of optimised imaging sequences and visual inspection. For routine clinical purposes, the relative reliance on quantitation (hippocampal volume or T₂ measurements) depends entirely on the yield of visual inspection in any institution. This, in turn, depends on whether optimised imaging is performed and on the familiarity of the reporting specialist with the MRI features of HS. A technique which approaches 95–100% compared with pathology is essential in any epilepsy centre, and optimised visual analysis can achieve this. There are some cases where quantitation of a single feature can be misleading, so visual analysis should always be performed, and complements any quantitative study.