Cerebrovascular resistance: effects on cognitive decline, cortical atrophy, and progression to dementia

Abstract

Evidence for vascular contributions to Alzheimer's disease has been increasingly identified, with increased blood pressure and decreased cerebral blood flow both linked to in vivo biomarkers and clinical progression of Alzheimer's disease. We therefore hypothesized that an elevated ratio of blood pressure to cerebral blood flow, indicative of cerebrovascular resistance, would exhibit earlier and more widespread associations with Alzheimer's disease than cerebral blood flow alone. Further, we predicted that increased cerebrovascular resistance and amyloid retention would synergistically influence cognitive performance trajectories, independent of neuronal metabolism. Lastly, we anticipated associations between cerebrovascular resistance and later brain atrophy, prior to amyloid accumulation. To evaluate these hypotheses, we investigated associations between cerebrovascular resistance and amyloid retention, cognitive decline, and brain atrophy, controlling for neuronal metabolism. North American older adults (n = 232) underwent arterial spin labelling magnetic resonance imaging to measure regional cerebral blood flow in brain regions susceptible to ageing and Alzheimer's disease. An estimated cerebrovascular resistance index was then calculated as the ratio of mean arterial pressure to regional cerebral blood flow. Positron emission tomography with ¹⁸F-florbetapir and fludeoxyglucose was used to quantify amyloid retention and neuronal metabolism, respectively. Cognitive performance was evaluated via annual assessments of global cognition, memory, and executive function. Results indicated diminished inferior parietal and temporal cerebral blood flow for patients with Alzheimer's disease (n = 33) relative to both non-demented groups, but no cerebral blood flow differences between non-demented amyloid-positive (n = 87) and amyloid-negative (n = 112) cases. In contrast, the cerebrovascular resistance index was significantly elevated in amyloid-positive versus amyloid-negative cases, with additional elevation in patients with Alzheimer's disease. Furthermore, cerebrovascular resistance index group differences were of greater statistical effect size and encompassed a greater number of brain regions than those for cerebral blood flow alone. Cognitive decline over 2-year follow-up was accelerated by elevated baseline cerebrovascular resistance index, particularly for amyloid-positive individuals. Increased baseline cerebrovascular resistance index also predicted greater progression to dementia, beyond that attributable to amyloid-positivity. Finally, increased cerebrovascular resistance index predicted greater regional atrophy among non-demented older adults who were amyloid-negative. Findings suggest that increased cerebrovascular resistance may represent a previously unrecognized contributor to Alzheimer's disease that is independent of neuronal hypometabolism, predates changes in brain perfusion, exacerbates and works synergistically with amyloidosis to produce cognitive decline, and drives amyloid-independent brain atrophy during the earliest stage of disease.

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