Hippocampal Disconnection in Early Alzheimer's Disease: A 7 Tesla MRI Study

Background: In patients with Alzheimer's disease (AD), atrophy of the entorhinal cortex (ERC) and hippocampal formation may induce degeneration of connecting white matter tracts.

Objective: We examined the association of hippocampal subfield and ERC atrophy at 7 tesla MRI with fornix and parahippocampal cingulum (PHC) microstructure in patients with early AD.

Methods: Twenty-five patients with amnestic mild cognitive impairment (aMCI) (n = 15) or early AD (n = 10) and 17 controls underwent 3 tesla diffusion MRI to obtain fractional anisotropy (FA) of the fornix and PHC and 7 tesla MRI to obtain ERC and hippocampal subfield volumes. Linear regression analyses were performed, adjusted for age, gender, and intracranial volume.

Results: Fornix FA was significantly lower and subiculum, cornu ammonis (CA) 1, and dentate gyrus & CA4 volume were significantly smaller in patients with MCI or AD as compared to controls. In patients with MCI or AD, fornix FA was positively associated with subiculum volume (beta = 0.53, 95% CI 0.10; 0.96), but not with ERC/other subfield volumes. PHC FA was not associated with ERC/subfield volumes.

Conclusion: These findings indicate that in early AD subiculum atrophy is associated with lower FA of the fornix, which primarily consists of axons originating in the subiculum. This suggests that degeneration of subicular cell bodies and their axons are related processes in early AD.