Spreading of Amyloid, Tau, and Microvascular Pathology in Alzheimer's Disease: Findings from Neuropathological and Neuroimaging Studies

Primary pathologies including amyloid-beta (A beta) plaques and neurofibrillary tangles (NFT) develop many years before the onset of dementia symptoms in Alzheimer's disease (AD). Age-related small vessel disease (SVD) is common in elderly subjects and may contribute to the clinical syndrome of AD. Each type of pathology shows a specific spatio-temporal sequence of spreading in the brain. Here, we review neuropathological and neuroimaging findings (PET tracers of A beta and NFT, MRI markers of SVD) to assess whether staging of these primary pathologies is useful to predict clinical symptoms in AD. On the basis of neuropathological data, early stages of A beta plaque and NFT pathology distribution occur in preclinical AD, but advanced stages with spreading into further brain regions are associated with dementia symptoms. Amyloid PET presumably detects A beta in advanced neuropathological A beta stages, and increased global amyloid PET uptake is associated with clinical worsening in non-demented subjects. Tau PET may provide additional predictive value by detecting NFT in the allocortex. There is weak evidence that SVD is related to amyloid or NFT pathology. Global volume of MRI-assessed white matter hyperintensities (WMH) contribute in addition to biomarker levels of A beta to predict cognitive decline. Regional differences of the effect of WMH on cognition have been demonstrated but are not yet established as a biomarker in AD. In conclusion, biomarkers for amyloid and tau pathology allow a distinction between early and advanced stages of AD, but a subgroup of pathologically identified preclinical AD cases is not identified by the currently available biomarkers.