Effects of aging and Alzheimer's disease along the longitudinal axis of the hippocampus

The hippocampus is often treated as a uniform structure, but possesses differential projections to surrounding cortex along its longitudinal axis. This heterogeneity could create varied susceptibility to pathological influences, potentially leading to non-uniform volumetric associations with advancing age and Alzheimer's disease (AD). Previous examinations of aging and AD effects on hippocampal subdivisions have produced highly discrepant findings. To clarify these inconsistencies, we examined the hippocampal head, body, and tail in a large sample of 292 cognitively normal, 37 very mildly demented, and 18 mildly demented individuals, divided into two independent samples. As often done in the literature, we characterized qualitative patterns across these regions, but extended these results by explicitly testing for quantitative differences. In each sample of cognitively normal individuals, the head and body demonstrated greater age effects than the tail. In each sample contrasting AD and cognitively normal individuals, all three regions showed significant volume reductions, with the greatest effect on the head. When examining increasing severity of dementia, the hippocampal head showed progressive volume loss, while the body and tail did not. The patterns of results examining both aging and AD were relatively consistent across the independent samples. These results indicate that there is an anterior-to-posterior gradient of loss within the hippocampus with both advancing age and AD.