MRI volumetry of medial temporal lobe structures in amnesia following herpes simplex encephalitis

Using a magnetic resonance imaging-based volumetry, we quantified the volumes of the hippocampal formation (HF), parahippocampal gyrus (PHG), amygdaloid body (AMB), and anterior temporal neocortex (ATN) in 5 postherpes simplex encephalitic (post-HSE) patients with temporal lobe damage and memory impairment at 12-52 months after the onset, and in 10 age- matched control subjects. In the post-HSE patients, the HF (p < 0.001) and PHG (p < 0.005) were significantly atrophic, while the AMB (p = 0.155) and ATN (p = 0.102) were smaller but not significantly. Performances on the verbal learning memory tests correlated highly with the HF volume, and length of dense retrograde amnesia with the PHG atrophy. Two patients with severe lasting amnesia had a marked atrophy of the HF and PHG, while 3 patients with good recovery from initial amnesia had temporal lobe structures larger than 50% of the control mean volumes. These results suggest that anterograde and retrograde memory functions involve different neural structures; the former is related to the HF and the latter to the PHG. For producing lasting amnesia, either severe HF damage or a combined damage of the HF and PHG might be necessary.