Regional grey matter loss and brain disconnection across Alzheimer disease evolution

It is becoming increasingly clearer that the clinical manifestations of Alzheimer's disease (AD) are not only associated with regional grey matter (GM) damage, but also with abnormal integration between cortical brain regions by disconnection mechanism. This concept comes from the evidence that white matter (WM) damage (as assessed by diffusion MR imaging) can be observed in patients with AD since the early clinical stages, and it correlates with clinical measures of cognitive disability. In this perspective, several functional imaging studies, based on PET and resting state fMRI, have provided evidence that brain hypometabolism/disconnection may precede the occurrence of GM atrophy in certain regions of AD brains, such as the cingulate cortex. The cingulum represents the most prominent WM tract of the limbic system, being directly connected to the medial temporal lobe structures. Therefore, this structure likely contributes to changes in functional connectivity observed within the so called default-mode network of AD patients, and its damage is likely to play a remarkable role in the conversion from mild cognitive impairment (MCI) to dementia. Nowadays, the combination of several neuroimaging techniques that provide both, measures of regional GM loss and measures of functional and structural connectivity offer the opportunity to investigate in vivo the pathophysiological changes of brain tissue modifications across the clinical evolution of AD. This paper reviews the main MR based methods of investigation of brain tissue involvement in patients with AD and MCI, and the role they have played in clarifying the differential contribution of GM damage and brain disconnection to AD pathophysiology. This subject seems to be relevant for both, speculative aspects of neurology and application to clinical trials.