[Cerebral imaging and physiopathology of Alzheimer's disease]

Alzheimer's disease (AD) is characterised by macroscopic cerebral damages which can be studied in vivo with neuroimaging techniques, even at the earliest stage. Studies were conducted in patients with amnestic Mild Cognitive Impairment (MCI) who best represent incipient AD. Right temporo-parietal hypometabolism, assessed by resting-state (18)FDG-PET, distinguishes patients who further develop AD from those who remain stable. From the pre-dementia stage of MCI, atrophy of the hippocampal region detected with structural MRI contrasts with functional alteration of the posterior cingulate gyrus measured with (18)FDG-PET and SPECT. Results from resting-state fMRI confirm this pattern of functional abnormalities and highlight changes in the hippocampal region functional connectivity, decreased with the posterior cingulate region, and increased with some frontal areas. Altogether with a structural connectivity impairment highlighted by DTI, those results support the hypothesis of a dysconnexion between the hippocampal and the posterior cingulate regions. Finally, activation fMRI data support the hypothesis of a functional compensation involving not only the frontal cortex but also, at the pre-dementia stage, the hippocampal region. Thus, this synthesis focuses on the hypotheses of dysconnexion and functional compensation, suggested to explain the discrepancies between the structural and functional alteration patterns, as well as on relevant results from resting-state fMRI, DTI and activation fMRI studies. Furthermore, this synthesis emphasizes the relevance of neuroimaging for the early detection of AD.