Longitudinal surface-based morphometry changes in the hippocampus in dementia.

The hippocampus is central to Alzheimer's disease (AD), characterized by atrophy and cognitive decline. While volume loss is well-documented, surface-based morphometric (SBM) features-curvature, gyrification, and thickness-remain less explored. Using T1-weighted MRI data from the Alzheimer's Disease Neuroimaging Initiative (3144 timepoints; CN: 450, MCI: 284, AD: 204), hippocampal subfields were analyzed with HippUnfold. Linear mixed effects models examined volume and SBM changes, tracking cognitive trajectories in stable (n = 710) and progressing (n = 228) individuals. Focusing on CA1 as a representative subfield, baseline volume differed significantly across diagnostic groups, with all clinical groups showing reduced volume relative to cognitively normal individuals, consistent with disease severity. In contrast, surface-based morphometry (SBM) measures showed minimal baseline group differences, with only the CN to MCI/AD group exhibiting a significant negative main effect across SBM metrics. Longitudinal analyses revealed that disease-related changes were primarily captured by time-dependent interactions. In particular, the MCI to AD group demonstrated the most robust longitudinal changes, showing the largest magnitude of volume loss and accompanying SBM alterations over follow-up. Individuals progressing from CN to MCI/AD exhibited similar, though slightly smaller, longitudinal SBM changes, indicating early surface remodeling prior to overt clinical progression. In contrast, Stable AD showed significant longitudinal effects only for volume loss, and Stable MCI displayed minimal or nonsignificant SBM and volumetric changes. Analyses linking longitudinal morphometric change to cognitive domains was differentially associated with rates of cognitive decline across domains and disease stages, with SBM changes linked to slower language decline in early stages and to accelerated memory and visuospatial decline during progression from MCI to AD. While hippocampal volume loss remains a robust marker of AD, it does not fully capture longitudinal morphometric changes associated with disease progression. In particular, SBM measures showed their strongest and most consistent longitudinal effects in individuals progressing from MCI to AD, where changes in surface geometry accompanied accelerated volume loss. These findings suggest that SBM features are most sensitive to morphometric reorganization during active disease progression and provide complementary information beyond volume alone.