Three-dimensional surface mapping of hippocampal atrophy progression from MCI to AD and over normal aging as assessed using voxel-based morphometry.

The hippocampus is the brain structure of highest and earliest structural alteration in Alzheimer's disease (AD). New developments in neuroimaging methods recently made it possible to assess the respective involvement of the different hippocampal subfields by mapping atrophy on a 3D hippocampal surface view. In this longitudinal study on patients with mild cognitive impairment (MCI), we used such an approach to map the profile of hippocampal atrophy and its progression over an 18-month follow-up period in rapid converters to AD and "non-converters" compared to age-matched controls. For the sake of comparison, we also assessed the profile of hippocampal atrophy associated with AD and with increasing age in a healthy control population ranging from young adult to elderly. We found major involvement of the lateral part of the superior hippocampus mainly corresponding to the CA1 subfield in MCI and AD while increasing age was mainly associated with subiculum atrophy in the healthy population. Moreover, the CA1 subfield also showed highest atrophy rates during follow-up, in both rapid converters and "non-converters" although increased effects were observed in the former group. This study emphasizes the differences between normal aging and AD processes leading to hippocampal atrophy, pointing to a specific AD-related CA1 involvement while subiculum atrophy would represent a normal aging process. Our findings also suggest that the degree of hippocampal atrophy, more than its spatial localization, predicts rapid conversion to AD in patients with MCI.