Dynamic cerebral autoregulation in Alzheimer's disease and mild cognitive impairment: A systematic review

Cerebrovascular CO2 reactivity and dynamic cerebral autoregulation through the eighth decade of life and their implications for cognitive decline

Aging is accompanied by a decrease in cerebral blood flow (CBF), especially in the presence of preclinical cognitive decline. The role of cerebrovascular physiology including regulatory mechanisms of CBF in processes underlying aging and subclinical cognitive decline is, however, not fully understood. We explored changes in cerebrovascular CO2 reactivity and dynamic cerebral autoregulation (dCA) through the eighth decade of life, and their relation with early cognitive decline. After 10.9 years, twenty-eight (age, 80.0 ± 3.5 years; 46% female) out of forty-eight healthy older adults who had participated in a previous study (age at baseline, 70 ± 4 years; 42% female), underwent repeated transcranial Doppler assessments. Linear mixed-model analyses revealed small reductions in cerebrovascular CO2 reactivity with aging (-0.37%/mmHg, P = 0.041), whereas dCA was modestly enhanced (gain: -0.009 cm/s/mmHg, P = 0.038; phase: +8.9 degrees, P = 0.004). These changes were more pronounced in participants who had developed subjective memory complaints at follow-up. Our observations confirm that dCA is not impaired in aging, despite lower cerebral perfusion and cerebrovascular reactivity. Altogether, this unique longitudinal study highlights the involvement of cerebrovascular health in preclinical cognitive decline, which is of clinical relevance in the development of dementia management strategies.

Orthostatic Blood Pressure Change, Dizziness, and Risk of Dementia in the ARIC Study

BACKGROUND

Abnormal orthostatic blood pressure (BP) regulation may result in cerebral hypoperfusion and brain ischemia and contribute to dementia. It may also manifest as early symptoms of the neurodegenerative process associated with dementia. The relationship between the magnitude and timing of orthostatic BP responses and dementia risk is not fully understood.

METHODS

We conducted a prospective cohort analysis of the associations of orthostatic BP changes and self-reported orthostatic dizziness with the risk of dementia in the Atherosclerosis Risk in Communities study (ARIC). We calculated changes in BP from the supine to the standing position at 5 measurements taken within 2 minutes after standing during the baseline visit (1987-1989). The primary outcome was adjudicated dementia ascertained through 2019.

RESULTS

Among 11 644 participants (mean [SD] age, 54.5 [5.7] years; 54.1% women; 25.9% Black), 2303 dementia cases were identified during a median follow-up of 25.9 years. Large decreases in systolic BP from the supine to standing position measured at the first 2 measurements ≈30 and 50 seconds after standing, but not afterward, were associated with orthostatic dizziness and a higher risk of dementia. Comparing a decrease in systolic BP of ≤-20 or >-20 to -10 mm Hg to stable systolic BP (>-10 to 10 mm Hg) at the first measurement, the adjusted hazard ratios were 1.22 (95% CI, 1.01-1.47) and 1.10 (95% CI, 0.97-1.25), respectively.

CONCLUSIONS

Abnormal orthostatic BP regulation, especially abrupt drops in BP within the first minute, might be early risk markers for the development of dementia. Transient early orthostatic hypotension warrants more attention in clinical settings.

Cerebrovascular Dysfunction in Alzheimer's Disease and Transgenic Rodent Models

Alzheimer's Disease (AD) and Alzheimer's Disease-Related Dementia (ADRD) are the primary causes of dementia that has a devastating effect on the quality of life and is a tremendous economic burden on the healthcare system. The accumulation of extracellular beta-amyloid (Aβ) plaques and intracellular hyperphosphorylated tau-containing neurofibrillary tangles (NFTs) in the brain are the hallmarks of AD. They are also thought to be the underlying cause of inflammation, neurodegeneration, brain atrophy, and cognitive impairments that accompany AD. The discovery of APP, PS1, and PS2 mutations that increase Aβ production in families with early onset familial AD led to the development of numerous transgenic rodent models of AD. These models have provided new insight into the role of Aβ in AD; however, they do not fully replicate AD pathology in patients. Familial AD patients with mutations that elevate the production of Aβ represent only a small fraction of dementia patients. In contrast, those with late-onset sporadic AD constitute the majority of cases. This observation, along with the failure of previous clinical trials targeting Aβ or Tau and the modest success of recent trials using Aβ monoclonal antibodies, has led to a reappraisal of the view that Aβ accumulation is the sole factor in the pathogenesis of AD. More recent studies have established that cerebral vascular dysfunction is one of the earliest changes seen in AD, and 67% of the candidate genes linked to AD are expressed in the cerebral vasculature. Thus, there is an increasing appreciation of the vascular contribution to AD, and the National Institute on Aging (NIA) and the Alzheimer's Disease Foundation recently prioritized it as a focused research area. This review summarizes the strengths and limitations of the most commonly used transgenic AD animal models and current views about the contribution of Aβ accumulation versus cerebrovascular dysfunction in the pathogenesis of AD.

Cerebrovascular Hemodynamics in Cognitive Impairment and Dementia: A Systematic Review and Meta-Analysis of Transcranial Doppler Studies

INTRODUCTION

Transcranial Doppler (TCD) sonography is a noninvasive tool for measuring cerebrovascular hemodynamics. Studies have reported alterations in cerebrovascular hemodynamics in normal aging, mild cognitive impairment (MCI), and dementia, as well as in different etiologies of dementia. This systematic review and meta-analysis was designed to investigate the relationship between cerebral blood velocity (CBv) and pulsatility index (PI) in the middle cerebral artery (MCA) in persons with MCI and dementia.

METHODS

A systematic literature search was conducted in Pubmed, Embase, Cochrane Library, Epistemonikos, PsychINFO, and CINAHL. The search was performed according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines. After screening of 33,439 articles, 86 were reviewed in full-text, and 35 fulfilled the inclusion criteria.

RESULTS

CBv was significantly lower and PI significantly higher in MCA in vascular dementia (VaD) and Alzheimer's disease (AD) compared to cognitively normal (CN) older persons. Also, CBv was lower in MCI compared to CN. There were no significant differences in CBv in MCA in AD compared with VaD, although PI was higher in VaD compared to AD.

CONCLUSION

Alterations in cerebrovascular hemodynamics are seen in AD, VaD, and MCI. While PI was slightly higher in VaD compared to AD, the reduction in CBv appears to be equally pronounced across neurodegenerative and vascular etiologies of dementia.