Abstract
Hippocampal atrophy is one of the anatomical hallmarks in Alzheimer’s disease (AD) patients and the substrate of their episodic memory deficits. However, its specificity has been questioned since semantic dementia (SD) patients also display an atrophy of the hippocampus, but the cognitive symptoms do not include episodic memory impairments, and these patients rather manifest semantic memory deficits due to a deterioration of the anterior temporal lobes. Based on our previous finding (Chapleau et al., 2016), our hypothesis is that the posterior portion of the hippocampus is atrophied in AD patients only, and therefore explains the absence of episodic memory deficits in SD subjects.We conducted a cross-sectional study to quantify sub-hippocampal volumes in 9 SD patients, 11 AD patients and 12 controls, using the recent version of the automated segmentation software Free Surfer 6.0. The results showed that SD and AD subjects presented significant and bilateral volume differences, when compared to CTRLs, regarding whole hippocampi volumes and most sub-regions. In terms of the antero-posterior axis, the posterior regions were more atrophied in AD vs SD subjects. The hippocampal tails were bilaterally atrophied in AD compared to CTRLs, but only the left in SD, and the right fimbria was significantly smaller in AD vs CTRLs, but no difference was found in SD. While the sub-cortical results seem to confirm the pattern that has been observed in previous structural imaging studies, the results from an antero-posterior perspective seem to provide important new information.