Which Hippocampal Sclerosis is Imaged With 7-T MRI?

Commentary The hippocampus is the most frequent target of surgical treatment for epilepsy. Approximately 30% of patients with mesial temporal sclerosis (MTS) on surgical pathology, however, have normal findings on conventional MRI prior to surgery (1). Epilepsy specialists have hoped that ultra-high–field 7-T MRI might improve detection and characterization of MTS for pre-surgical screening (2). Henry et al. have shown that ultra-high– field strength 7-T MRI might be helpful in defining structural changes in hippocampal sclerosis (HS) that may not be visible on conventional MRI imaging. This study did not, however, evaluate whether 7-T MRI has a higher sensitivity for identifying MTS than does conventional MRI—all eight of the patients had MTS on 1.5-and 3-T MRI. The authors did show that ultra-high–field MRI might be helpful in characterizing different subtypes of MTS in patients with temporal lobe epilepsy. Recently, Thom et al. (3) and others (4) have shown that several subtypes of MTS identified on histologic analysis may differ in their etiologies, associated clinical findings, and surgical prognoses. In 165 patients receiving medial temporal lobe resections for drug-resistant epilepsy , Thom et al. (3) identified six different pathologic patterns: 1) 36% had classic HS with neuronal loss in CA1, hilus, and CA3 regions of hippocampus, with sparing of CA2/subiculum. This is similar to Ammon's horn sclerosis, an older definition that includes mossy fiber sprouting from dentate granule cells; some patients with this type have granule cell dispersion; 2) 24% of patients had total HS with neuronal loss in all hippocampal subfields, including CA2/subiculum; 3) 3% had endfolium sclerosis (EFS), with neuronal loss in the hilus and portions of CA3 regions of hippocampus (i.e., the endfolium) but with sparing of CA1 and CA2 regions; 4) 6% of patients had primary CA1 atrophy (termed CA1p); 5) 21% had indeterminate HS with neuronal loss in CA3 and CA2 areas and with preserved CA4; and 6) and 10% of the patients had electroclinical syndromes of medial temporal lobe epilepsy but did not have HS. In this current study, Henry et al. evaluated hippocampal sclerosis using 7-T MRI in a small group of eight patients, but they included controls and were able to identify classic HS (i.e., Ammon's horn sclerosis) in several patients. They also identified total HS in one patient and probable EFS in another. Henry et al. also demonstrated a common morphologic pattern in all the patients with HS: The head of the …