Autoregulation of cerebral blood flow to changes in arterial pressure in mild Alzheimer's disease

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

Dynamic cerebral autoregulation (dCA) is a key mechanism that regulates cerebral blood flow (CBF) in response to transient changes in blood pressure (BP). Impairment of dCA could increase vulnerability to hypertensive vascular damage, but also to BP lowering effects of antihypertensive treatment. The literature remains conflicted on whether dCA is altered in Alzheimer's disease (AD) and mild cognitive impairment (MCI). We summarized available data on dCA in AD and MCI, by searching PubMed, Embase, PsycINFO and Web of Science databases (inception-January 2022). Eight studies (total n = 443) were included in the qualitative synthesis of which seven were eligible for meta-analysis. All studies used Transcranial Doppler (TCD) ultrasonography and transfer function analysis or the autoregulatory index to assess dCA during spontaneous or induced BP fluctuations. Meta-analysis indicated no significant difference between AD, MCI and healthy controls in dCA parameters for spontaneous fluctuations. For induced fluctuations, the available data were limited, but indicative of at least preserved and possibly better autoregulatory functioning in AD and MCI compared to controls. In summary, current evidence does not suggest poorer dCA efficiency in AD or MCI. Further work is needed to investigate dCA in dementia with induced fluctuations controlling for changes in end-tidal carbon dioxide.

Effects of Hypertension on Alzheimer's Disease and Related Disorders

PURPOSE OF REVIEW

To review the pathophysiology of hypertension in Alzheimer's disease and related dementias and explore the current landscape of clinical trials involving treatment of hypertension to improve cognition.

RECENT FINDINGS

Hypertension is increasingly recognized as a contributor to cognitive impairment. Clinical trials that explore blood pressure reductions with cognitive outcomes have been promising. Various antihypertensives have been evaluated in clinical trials, with growing interest in those agents that impact the renin-angiotensin-aldosterone system due to its own association with cognitive impairment. No antihypertensive agent has been found to be superior to others in reducing cognitive impairment risk or conferring neuroprotective benefits. In this review, the pathophysiology of and clinical trial data involving hypertension and dementia will be explored. Hypertension is a significant risk factor for the development of neurodegenerative dementias, and clinical trials have been overall favorable in improving cognition by reductions in blood pressure using antihypertensive agents.

Effect of Antihypertensive Treatment on Cerebral Blood Flow in Older Adults: a Systematic Review and Meta-Analysis

BACKGROUND

In older age, the benefits of antihypertensive treatment (AHT) become less evident, with greater associated risk. Of particular concern is compromising cerebral blood flow (CBF), especially in those with cognitive impairment.

METHODS

We created a synthesis of the published evidence by searching multiple electronic databases from 1970 to May 2021. Included studies had participants with mean age ≥50 years, hypertension or cognitive impairment, and assessed CBF before and after initiating AHT. Two authors independently determined eligibility and extracted data. Study quality was assessed using The Risk of Bias in Nonrandomized Studies of Interventions tool. We summarized study characteristics (qualitative synthesis) and performed random-effects meta-analyses (quantitative synthesis).

RESULTS

Thirty-two studies (total n=1306) were included, of which 23 were eligible for meta-analysis. In line with the qualitative synthesis, the meta-analysis indicated no effect of AHT initiation on CBF (standardized mean difference, 0.08 [95% CI, -0.07 to 0.22]; P=0.31, I2=42%). This was consistent across subgroups of acute versus chronic AHT, drug class, study design, and CBF measurement. Subgroups by age demonstrated an increase in CBF after AHT in those aged >70 years (standardized mean difference, 4.15 [95% CI, 0.16-8.15]; P=0.04, I2=42%), but not in those aged 50 to 65 and 65 to 70 years (standardized mean difference, 0.18 [95% CI,-2.02 to 2.38]; P=0.87, I2=49%; standardized mean difference, 1.22 [95% CI, -0.45 to 2.88]; P=0.15, I2=68%). Overall, risk of bias was moderate-to-high and quality of evidence (Grading of Recommendations Assessment, Development and Evaluation) was very low, reflecting the observational nature of the data.

CONCLUSIONS

Accepting the observed limitations, current evidence does not suggest a harmful effect of AHT on CBF. Concerns over CBF should not preclude treatment of hypertension.

Neuroimaging of Mouse Models of Alzheimer’s Disease

Magnetic resonance imaging (MRI) and positron emission tomography (PET) have made great strides in the diagnosis and our understanding of Alzheimer’s Disease (AD). Despite the knowledge gained from human studies, mouse models have and continue to play an important role in deciphering the cellular and molecular evolution of AD. MRI and PET are now being increasingly used to investigate neuroimaging features in mouse models and provide the basis for rapid translation to the clinical setting. Here, we provide an overview of the human MRI and PET imaging landscape as a prelude to an in-depth review of preclinical imaging in mice. A broad range of mouse models recapitulate certain aspects of the human AD, but no single model simulates the human disease spectrum. We focused on the two of the most popular mouse models, the 3xTg-AD and the 5xFAD models, and we summarized all known published MRI and PET imaging data, including contrasting findings. The goal of this review is to provide the reader with broad framework to guide future studies in existing and future mouse models of AD. We also highlight aspects of MRI and PET imaging that could be improved to increase rigor and reproducibility in future imaging studies.

Autoregulation of White Matter Cerebral Blood Flow to Arterial Pressure Changes in Normal Subjects

Endothelial dysfunction causing impaired cerebrovascular vasodilatory capacity in response to reduced blood pressure has been proposed as a mechanism of white matter (WM) disease development. This study investigated autoregulation of CBF to blood pressure reduction in WM and gray matter (GM) in normal subjects recruited as controls for a study of cerebrovascular function in human immunodeficiency virus positive subjects. They underwent baseline CBF and oxygen extraction fraction measurement by MRI before and after mean arterial pressure (MAP) reduction. Autoregulatory Index (AI) was computed as CBF AI = -%CBF change/% MAP change. Thirty of 44 subjects achieved target MAP reduction. MAP was reduced -13.65 ± 2.35 (range 10 to 20) %. WM AI of -0.61 ± 1.23 was significantly more negative than GM AI of 0.02 ± 0.44 (paired t test, p= 0.016). WM CBF fell (paired Wilcoxon, p= 0.03) whereas GM CBF did not change (paired Wilcoxon, p=0.92). WM AI was different from 0 (p=0.011, one-sample t-test vs 0), whereas GM AI was not (p=0.913, one-sample t-test vs 0). These data demonstrate that maintenance of CBF to 10-20% reductions in MAP is less effective in WM than in GM. This may put WM at higher risk for ischemic damage.

Cerebral Small Vessel Disease in Sporadic and Familial Alzheimer Disease

Alzheimer disease (AD) is the most common cause of dementia. Biological definitions of AD are limited to the cerebral burden of amyloid β plaques, neurofibrillary pathology, and neurodegeneration. However, current evidence suggests that various features of small vessel disease (SVD) are part of and covertly modify both sporadic and familial AD. Neuroimaging studies suggest that white matter hyperintensities explained by vascular mechanisms occurs frequently in the AD spectrum. Recent advances have further emphasized that frontal periventricular and posterior white matter hyperintensities are associated with cerebral amyloid angiopathy in familial AD. Although whether SVD markers precede the classically recognized biomarkers of disease is debatable, post-mortem studies show that SVD pathology incorporating small cortical and subcortical infarcts, microinfarcts, microbleeds, perivascular spacing, and white matter attenuation is commonly found in sporadic as well as in mutation carriers with confirmed familial AD. Age-related cerebral vessel pathologies such as arteriolosclerosis and cerebral amyloid angiopathy modify progression or worsen risk by shifting the threshold for cognitive impairment and AD dementia. The incorporation of SVD as a biomarker is warranted in the biological definition of AD. Therapeutic interventions directly reducing the burden of brain amyloid β have had no major impact on the disease or delaying cognitive deterioration, but lowering the risk of vascular disease seems the only rational approach to tackle both early- and late-onset AD dementia.

Regulation of cerebral blood flow in humans: physiology and clinical implications of autoregulation

Brain function critically depends on a close matching between metabolic demands, appropriate delivery of oxygen and nutrients, and removal of cellular waste. This matching requires continuous regulation of cerebral blood flow (CBF), which can be categorized into four broad topics: 1) autoregulation, which describes the response of the cerebrovasculature to changes in perfusion pressure; 2) vascular reactivity to vasoactive stimuli [including carbon dioxide (CO2)]; 3) neurovascular coupling (NVC), i.e., the CBF response to local changes in neural activity (often standardized cognitive stimuli in humans); and 4) endothelium-dependent responses. This review focuses primarily on autoregulation and its clinical implications. To place autoregulation in a more precise context, and to better understand integrated approaches in the cerebral circulation, we also briefly address reactivity to CO2 and NVC. In addition to our focus on effects of perfusion pressure (or blood pressure), we describe the impact of select stimuli on regulation of CBF (i.e., arterial blood gases, cerebral metabolism, neural mechanisms, and specific vascular cells), the interrelationships between these stimuli, and implications for regulation of CBF at the level of large arteries and the microcirculation. We review clinical implications of autoregulation in aging, hypertension, stroke, mild cognitive impairment, anesthesia, and dementias. Finally, we discuss autoregulation in the context of common daily physiological challenges, including changes in posture (e.g., orthostatic hypotension, syncope) and physical activity.

Longitudinal changes in the control mechanisms for blood pressure and cerebral blood flow in Alzheimer's disease: Secondary results of a randomized controlled trial

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Cerebral autoregulation and baroreflex sensitivity are two key mechanisms that regulate the homeostasis of blood pressure and brain perfusion.

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For the first time, we performed repeated measurements of these mechanisms in a sample of Alzheimer's disease patients .

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In this sample, we found no evidence that these mechanisms become impaired over time.

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These patients did not demonstrate increased vulnerability towards starting mild blood pressure lowering therapy.

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This paves the way for further studies that investigate the safety and benefits of antihypertensive treatment in Alzheimer's disease

Objective

Dynamic cerebral autoregulation (dCA) and baroreflex sensitivity (BRS) are key mechanisms involved in the homeostasis of blood pressure (BP) and cerebral blood flow. We assessed changes in these mechanisms in Alzheimer's disease (AD) during a 1.5 year follow-up.

Methods

In this secondary analysis of a randomized controlled trial we measured beat-to-beat BP, heart rate, and cerebral blood flow velocity at baseline, 0.5 and 1.5 years, during: rest (spontaneous oscillations), repeated sit-stand maneuvers (induced oscillations), an orthostatic challenge, and hypo- and hypercapnia. dCA was estimated using transfer function analysis and the autoregulatory index on spontaneous and induced oscillations. BRS was estimated by calculating the heart rate response to BP changes during induced oscillations. Linear mixed models were used to assess changes over time.

Results

56 patients were included (mean age:73 ± 6 years, 57% female). BRS did not change over time. dCA parameters showed small changes after 0.5 years, suggestive of a reduction in efficiency (e.g. higher gain [linear mixed effect model: B = 0.09, SE = 0.03, P = 0.008] and lower phase [B = -9.7, SE= 3.2, P = 0.004] in the very low frequency domain, and lower autoregulatory index during induced oscillations [B = -0.69, SE = 0.26, P = 0.010]). These changes did not show further progression after 1.5 years of follow-up.

Discussion

In this sample of patients with dementia due to AD we found no evidence that dCA or BRS become impaired during AD progression. This paves the way for further studies that investigate the safety and benefits of antihypertensive treatment in patients with AD.

Steady-state cerebral autoregulation in older adults with amnestic mild cognitive impairment: linear mixed model analysis

We examined whether the efficacy of steady-state cerebral autoregulation (CA) is reduced in older adults with amnestic mild cognitive impairment (aMCI), a prodromal stage of clinical Alzheimer disease (AD). Forty-two patients with aMCI and 24 cognitively normal older adults (NC) of similar age, sex, and education underwent stepwise decreases and increases in mean arterial pressure (MAP) induced by intravenous infusion of sodium nitroprusside and phenylephrine, respectively. Changes in cerebral blood flow (CBF) were measured repeatedly in the internal carotid and vertebral artery. Linear mixed modeling, including random effects of both individual intercept and regression slope, was used to quantify the MAP-CBF relationship accounting for nonindependent, repeated CBF measures. Changes in end-tidal CO₂ (EtCO₂) associated with changes in MAP were also included in the model to account for their effects on CBF. Marginal mean values of MAP were reduced by 13-14 mmHg during sodium nitroprusside and increased by 20-24 mmHg during phenylephrine infusion in both groups with similar doses of drug infusion. A steeper slope of changes in CBF in response to changes in MAP was observed in aMCI relative to NC, indicating reduced efficacy of CA (MAP × Group, P = 0.040). These findings suggest that cerebrovascular dysfunction may occur early in the development of AD.NEW & NOTEWORTHY Cerebral autoregulation is a fundamental regulatory mechanism to protect brain perfusion against changes in blood pressure that, if impaired, may contribute to the development of Alzheimer's disease. Using a linear mixed model, we demonstrated that the efficacy of cerebral autoregulation, assessed during stepwise changes in arterial pressure, was reduced in individuals with amnestic mild cognitive impairment, a prodromal stage of Alzheimer's disease. These findings support the hypothesis that cerebrovascular dysfunction may be an important underlying pathophysiological mechanism for the development of clinical Alzheimer's disease.

CBF regulation in hypertension and Alzheimer's disease

PURPOSE

To review the recent developments on the effect of chronic high mean arterial blood pressure (MAP) on cerebral blood flow (CBF) autoregulation and supporting the notion that CBF autoregulation impairment has connection with chronic cerebral diseases. Method: A narrative review of all the relevant papers known to the authors was conducted. Results: Our understanding of the connection between cerebral perfusion impairment and chronic high MAP and cerebral disease is rapidly evolving, from cerebral perfusion impairment being the result of cerebral diseases to being the cause of cerebral diseases. We now better understand the intertwined impact of hypertension and Alzheimer's disease (AD) on cerebrovascular sensory elements and recognize cerebrovascular elements that are more vulnerable to these diseases. Conclusion: We conclude with the suggestion that the sensory elements pathology plays important roles in intertwined mechanisms of chronic high MAP and AD that impact cerebral perfusion.

Alzheimer's Disease: The Link Between Amyloid-β and Neurovascular Dysfunction

While prevailing evidence supports that the amyloid cascade hypothesis is a key component of Alzheimer's disease (AD) pathology, many recent studies indicate that the vascular system is also a major contributor to disease progression. Vascular dysfunction and reduced cerebral blood flow (CBF) occur prior to the accumulation and aggregation of amyloid-β (Aβ) plaques and hyperphosphorylated tau tangles. Although research has predominantly focused on the cellular processes involved with Aβ-mediated neurodegeneration, effects of Aβ on CBF and neurovascular coupling are becoming more evident. This review will describe AD vascular disturbances as they relate to Aβ, including chronic cerebral hypoperfusion, hypertension, altered neurovascular coupling, and deterioration of the blood-brain barrier. In addition, we will describe recent findings about the relationship between these vascular defects and Aβ accumulation with emphasis on in vivo studies utilizing rodent AD models.

Effects of Nilvadipine on Cerebral Blood Flow in Patients With Alzheimer Disease

Cerebrovascular changes, including reduced cerebral blood flow (CBF), occur early in the development of Alzheimer disease and may accelerate disease progression. This randomized, double-blind, placebo-controlled study investigated how 6 months of treatment with the calcium antagonist nilvadipine would affect CBF in patients with mild-to-moderate Alzheimer disease. CBF was measured with magnetic resonance arterial spin labeling in whole-brain gray matter and in a priori defined regions of interest including the hippocampus. Fifty-eight patients were randomly assigned (29 in each group), of whom 22 in both groups had no magnetic resonance exclusion criteria and were medication compliant over 6 months. Mean age was 72.8±6.2 years, mean mini-mental state examination was 20.4±3.4. Nilvadipine treatment lowered systolic blood pressure (Δ=-11.5 [95% CI, -19.7 to -3.2] mm Hg; P<0.01), while whole-brain gray-matter CBF remained stable (Δ=5.4 [95% CI, -6.4 to 17.2] mL/100 g per minute; P=0.36). CBF in the hippocampus increased (left: Δ=24.4 [95% CI, 4.3-44.5] mL/100 g per minute; P=0.02; right: Δ=20.1 [95% CI, -0.6 to 40.8] mL/100 g per minute; P=0.06). There was no significant change in CBF in the posterior cingulate cortex (Δ=5.2 [95% CI, -16.5 to 27.0] mL/100 g per minute; P=0.63) or other regions of interest. In conclusion, nilvadipine reduced blood pressure and increased CBF in the hippocampus, whereas other regions showed stable or small nonsignificant increases in CBF. These findings not only indicate preserved cerebral autoregulation in Alzheimer disease but also point toward beneficial cerebrovascular effects of antihypertensive treatment. Clinical Trial Registration- URL: http://www.clinicaltrials.gov . Unique identifier: NCT02017340.

Relative cerebral hyperperfusion during cardiopulmonary bypass is associated with risk for postoperative delirium: a cross-sectional cohort study

Background

Our objective was to evaluate if changes in on-pump cerebral blood flow, relative to the pre-bypass baseline, are associated with the risk for postoperative delirium (POD) following cardiac surgery.

Methods

In 47 consecutive adult patients, right middle cerebral artery blood flow velocity (MCAV) was assessed using transcranial Doppler sonography. Individual values, measured during cardiopulmonary bypass (CPB), were normalized to the pre-bypass baseline value and termed MCAV_rel. An MCAV_rel > 100% was defined as cerebral hyperperfusion. Prevalence of POD was assessed using the Confusion Assessment Method for the Intensive Care Unit.

Results

Overall prevalence of POD was 27%. In the subgroup without POD, 32% of patients had experienced relative cerebral hyperperfusion during CPB, compared to 67% in the subgroup with POD (p < 0.05). The mean averaged MCAV_rel was 90 (±21) % in the no-POD group vs. 112 (±32) % in the POD group (p < 0.05), and patients developing delirium experienced cerebral hyperperfusion during CPB for about 39 (±35) min, compared to 6 (±11) min in the group without POD (p < 0.001). In a subcohort with pre-bypass baseline MCAV (MCAV_bas) below the median MCAV_bas of the whole cohort, prevalence of POD was 17% when MCAV_rel during CPB was kept below 100%, but increased to 53% when these patients actually experienced relative cerebral hyperperfusion.

Conclusions

Our results suggest a critical role for cerebral hyperperfusion in the pathogenesis of POD following on-pump open-heart surgery, recommending a more individualized hemodynamic management, especially in the population at risk.

Electronic supplementary material

The online version of this article (10.1186/s12871-019-0705-y) contains supplementary material, which is available to authorized users.

Dynamic Regulation of Cerebral Blood Flow in Patients With Alzheimer Disease

Cerebral autoregulation and baroreflex sensitivity are key mechanisms that maintain cerebral blood flow. This study assessed whether these control mechanisms are affected in patients with dementia and mild cognitive impairment due to Alzheimer disease, as this would increase the risks of antihypertensive treatment. We studied 53 patients with dementia (73.1 years [95% confidence interval (CI), 71.4–74.8]), 37 patients with mild cognitive impairment (69.2 years [95% CI, 66.4–72.0]), and 47 controls (69.4 years [95% CI, 68.3–70.5]). Beat-to-beat blood pressure (photoplethysmography), heart rate, and cerebral blood flow velocity (transcranial Doppler) were measured during 5-minute rest (sitting) and 5 minutes of orthostatic challenges, using repeated sit-to-stand maneuvers. Cerebral autoregulation was assessed using transfer function analysis and the autoregulatory index. Baroreflex sensitivity was estimated with transfer function analysis and by calculating the heart rate response to blood pressure changes during the orthostatic challenges. Dementia patients had the lowest cerebral blood flow velocity ( P =0.004). During rest, neither transfer function analysis nor the autoregulatory index indicated impairments in cerebral autoregulation. During the orthostatic challenges, higher autoregulatory index ( P =0.011) and lower transfer function gain ( P =0.017), indicating better cerebral autoregulation, were found in dementia (4.56 arb. unit [95% CI, 4.14–4.97]; 0.59 cm/s per mm Hg [95% CI, 0.51–0.66]) and mild cognitive impairment (4.59 arb. unit [95% CI, 4.04–5.13]; 0.51 cm/s per mm Hg [95% CI, 0.44–0.59]) compared with controls (3.71 arb. unit [95% CI, 3.35–4.07]; 0.67 cm/s per mm Hg [95% CI, 0.59–0.74]). Baroreflex sensitivity measures did not differ between groups. In conclusion, the key mechanisms to control blood pressure and cerebral blood flow are not reduced in 2 stages of Alzheimer disease compared with controls, both in rest and during orthostatic changes that reflect daily life challenges.

Functional MRI of brain physiology in aging and neurodegenerative diseases

Brain aging and associated neurodegeneration constitute a major societal challenge as well as one for the neuroimaging community. A full understanding of the physiological mechanisms underlying neurodegeneration still eludes medical researchers, fuelling the development of in vivo neuroimaging markers. Hence it is increasingly recognized that our understanding of neurodegenerative processes likely will depend upon the available information provided by imaging techniques. At the same time, the imaging techniques are often developed in response to the desire to observe certain physiological processes. In this context, functional MRI (fMRI), which has for decades provided information on neuronal activity, has evolved into a large family of techniques well suited for in vivo observations of brain physiology. Given the rapid technical advances in fMRI in recent years, this review aims to summarize the physiological basis of fMRI observations in healthy aging as well as in age-related neurodegeneration. This review focuses on in-vivo human brain imaging studies in this review and on disease features that can be imaged using fMRI methods. In addition to providing detailed literature summaries, this review also discusses future directions in the study of brain physiology using fMRI in the clinical setting.

Visit-to-Visit Blood Pressure Variability and Alzheimer's Disease: Links and Risks

While hypertension has been shown to be a risk factor for vascular dementia, several studies have also demonstrated that hypertension also increases the risk of Alzheimer's disease (AD). Although the relationship between visit-to-visit blood pressure variability (VVV) and cognitive impairment, including AD, have been provided, the mechanisms remain poorly understood. This review paper focuses on the relationship of VVV with AD and summarizes the pathophysiology underlying that relationship, which appears to be mediated by arterial stiffness.

High-Sodium Diet Has Opposing Effects on Mean Arterial Blood Pressure and Cerebral Perfusion in a Transgenic Mouse Model of Alzheimer's Disease

BACKGROUND

Cerebral ionic homeostasis impairment, especially Ca2+, has been observed in Alzheimer's disease (AD) and also with hypertension. Hypertension and AD both have been implicated in impaired cerebral autoregulation. However, the relationship between the ionic homeostasis impairment in AD and hypertension and cerebral blood flow (CBF) autoregulation is not clear.

OBJECTIVE

To test the hypothesis that a high-salt diet regimen influences the accumulation of amyloid-β (Aβand CBF) and CBF, exacerbates cognitive decline, and increases the propensity to AD.

METHODS

Double transgenic mice harboring the amyloid-β protein precursor (APPswe), and presenilin-1 (PSEN1) along with control littermates, 2 months of age at initiation of special diet, were divided into 4 groups: Group A, APP/PS1 and Group B, controls fed a high-sodium (4.00%) chow diet for 3 months; Group C, APP/PS1 and Group D, controls fed a low-sodium (0.08%) regular chow diet for 3 months. Mean arterial blood pressure (MAP) and CBF were measured noninvasively using the tail MAP measurement device and magnetic resonance imaging, respectively. Aβ plaques numbers in the cortex and hippocampus of APP/PS1 were quantified.

RESULTS

In contrary to controls, APP/PS1 mice fed a high-salt diet did not show markedly elevated mean systolic and diastolic blood pressure (134±4.8 compared with 162±2.8 mmHg, and 114±5.0 compared with 137±20 mmHg, p< 0.0001). However, a high-salt diet increased CBF in both APP/PS1 and controls and did not alter the cerebral tissue integrity. Aβ plaques were significantly reduced in the cortex and hippocampus of mice fed a high-salt diet.

CONCLUSION

These data suggest that a high-salt diet differently affects MAP and CBF in APP/PS1 mice and controls.

Associations between amyloid β and white matter hyperintensities: A systematic review

INTRODUCTION

This systematic review synthesizes current evidence for associations between cortical amyloid β, visualized on amyloid positron emission tomography imaging, and white matter hyperintensity (WMH) burden on magnetic resonance imaging in healthy elderly adults and individuals with cognitive impairment and dementia.

METHODS

Following Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) systematic review guidelines, we systematically searched MEDLINE, Embase, Cochrane, and PsycINFO databases from January 2000 to September 2015.

RESULTS

Our search returned 492 articles, 34 of which met criteria for inclusion in the final selection. Most studies reported no significant relationships between amyloid β and WMH burden across diagnostic groups.

DISCUSSION

Findings of this systematic review suggest that amyloid accumulation and WMH are independent but additive processes. The limited number of independent cohorts, lack of longitudinal data, and exclusion of individuals with mixed dementia limit the generalizability of these findings. Further studies are required to elucidate the putative contributions of vascular processes to neurodegenerative pathology.

Cerebral blood flow and autoregulation: current measurement techniques and prospects for noninvasive optical methods

Cerebral blood flow (CBF) and cerebral autoregulation (CA) are critically important to maintain proper brain perfusion and supply the brain with the necessary oxygen and energy substrates. Adequate brain perfusion is required to support normal brain function, to achieve successful aging, and to navigate acute and chronic medical conditions. We review the general principles of CBF measurements and the current techniques to measure CBF based on direct intravascular measurements, nuclear medicine, X-ray imaging, magnetic resonance imaging, ultrasound techniques, thermal diffusion, and optical methods. We also review techniques for arterial blood pressure measurements as well as theoretical and experimental methods for the assessment of CA, including recent approaches based on optical techniques. The assessment of cerebral perfusion in the clinical practice is also presented. The comprehensive description of principles, methods, and clinical requirements of CBF and CA measurements highlights the potentially important role that noninvasive optical methods can play in the assessment of neurovascular health. In fact, optical techniques have the ability to provide a noninvasive, quantitative, and continuous monitor of CBF and autoregulation.

Reduced perfusion in normal-appearing white matter in mild to moderate hypertension as revealed by 3D pseudocontinuous arterial spin labeling

PURPOSE

To investigate the hemodynamic changes of normal-appearing white matter (NAWM) in hypertension using the 3D pseudocontinuous arterial spin labeling (pCASL) technique.

MATERIALS AND METHODS

Seventy-three subjects, including a patient group (n = 41; 30 males; age = 47.7 ± 8.3 years; test-time blood pressure [BP] = 155 ± 23/98 ± 11 mmHg) and an age-matched control group (n = 32; 14 males; age = 46 ± 8.3 years; test-time BP = 117 ± 8/76 ± 10 mmHg), were recruited and scanned on a 3.0T magnetic resonance imaging (MRI) system using routine MRI sequences and 3D pCASL sequence. The routine MRI sequences were used to further define the NAWM. The cerebral blood flow (CBF) values in various regions of interest (ROIs) were extracted. One-way analysis of variance (ANOVA) and unpaired t-test were performed to evaluate the significance of the intergroup difference in CBF modifications.

RESULTS

Compared to healthy volunteers, CBF values in global gray matter (GM) and various NAWM regions were found to be lower (P < 0.05) in hypertensive patients, except for genu of corpus callosum (CC), cingulate gyrus, amygdala, pallidum, putamen, and thalamus (P > 0.05). Furthermore, compared to the control group, mild hypertension showed significantly reduced CBF in various ROIs (P < 0.05), but no intergroup differences in GM, R anterior horn of periventricular WM, and genu of CC (P > 0.05), while moderate hypertension showed reduced CBF in all ROIs (P < 0.05). However, it was observed that, between mild and moderate hypertensive patients, there were no statistically significant difference in CBF values except for genu of CC (P < 0.05).

CONCLUSION

3D pCASL has the ability to detect subtle hemodynamic abnormalities in NAWM regions at relatively early stages of hypertension. The observed decreases in CBF in these regions may suggest an increased risk of cerebral small vessel diseases.

Cerebral autoregulation, beta amyloid, and white matter hyperintensities are interrelated

Emerging studies link vascular risk factors and cerebrovascular health to the prevalence and rates of progression in Alzheimer's disease (AD). The brain's ability to maintain constant blood flow across a range of cerebral perfusion pressures, or autoregulation, may both promote and result from small vessel cerebrovascular disease and AD-related amyloid pathology. Here, we examined the relationship among cerebral autoregulation, small vessel cerebrovascular disease, and amyloid deposition in 14 non-demented older adults. Reduced cerebral autoregulation, was associated with increased amyloid deposition and increased white matter hyperintensity volume, which, in turn were positively associated with each other. For the first time in humans, we demonstrate an interrelationship among AD pathology, small vessel cerebrovascular disease, and cerebral autoregulation. Vascular factors and AD pathology are not independent but rather appear to interact.

Elevated cerebrovascular resistance index is associated with cognitive dysfunction in the very-old

INTRODUCTION

Age-related vascular changes, including blood pressure elevation and cerebral blood flow (CBF) reduction, are associated with cognitive decline and Alzheimer's disease (AD). Evidence suggests that the relationship between blood pressure and dementia risk varies between younger and older samples within the elderly population.

METHODS

We examined the relationship between mean arterial pressure (MAP), CBF, and cognition in young-old (60 to 75 years of age) versus very-old (80+ years of age) adults. Fifty-eight non-demented older adults completed an arterial spin labeling MRI scan, and an index of cerebrovascular resistance (CVRi) was estimated for each participant by calculating the ratio of MAP and CBF.

RESULTS

Results demonstrated a similar negative relationship between MAP and CBF across both age groups. However, very-old participants exhibited elevated CVRi and reduced CBF compared to young-old participants in regions implicated in AD and cerebral small vessel disease. Furthermore, significant age by CVRi interactions revealed that elevated CVRi in the thalamus was inversely related to verbal fluency performance in the very-old group.

CONCLUSIONS

Findings support CVRi as a potential vascular biomarker and suggest that regionally-specific vascular changes may contribute to cognitive decline, particularly in the very-old.

Cerebral arterial inflow assessment with 18F-FDG PET: methodology and feasibility

Positron emission tomography (PET) with 18fluorodeoxyglucose (18F-FDG) is increasingly used in neurology. The measurement of cerebral arterial inflow (QA) using 18F-FDG complements the information provided by standard brain PET imaging. Here, injections were performed after the beginning of dynamic acquisitions and the time to arrival (t0) of activity in the gantry's field of view was computed. We performed a phantom study using a branched tube (internal diameter: 4mm) and a 18F-FDG solution injected at 240 mL/min. Data processing consisted of (i) reconstruction of the first 3s after t0, (ii) vascular signal enhancement and (iii) clustering. This method was then applied in four subjects. We measured the volumes of the tubes or vascular trees and calculated the corresponding flows. In the phantom, the flow was calculated to be 244.2 mL/min. In each subject, our QA value was compared with that obtained by quantitative cine-phase contrast magnetic resonance imaging; the mean QA value of 581.4±217.5 mL/min calculated with 18F-FDG PET was consistent with the mean value of 593.3±205.8 mL/min calculated with quantitative cine-phase contrast magnetic resonance imaging. Our 18F-FDG PET method constitutes a novel, fully automatic means of measuring QA.

Cerebral hemodynamics of the aging brain: risk of Alzheimer disease and benefit of aerobic exercise

Alzheimer disease (AD) and cerebrovascular disease often coexist with advanced age. Mounting evidence indicates that the presence of vascular disease and its risk factors increase the risk of AD, suggesting a potential overlap of the underlying pathophysiological mechanisms. In particular, atherosclerosis, endothelial dysfunction, and stiffening of central elastic arteries have been shown to associate with AD. Currently, there are no effective treatments for the cure and prevention of AD. Vascular risk factors are modifiable via either pharmacological or lifestyle intervention. In this regard, habitual aerobic exercise is increasingly recognized for its benefits on brain structure and cognitive function. Considering the well-established benefits of regular aerobic exercise on vascular health, exercise-related improvements in brain structure and cognitive function may be mediated by vascular adaptations. In this review, we will present the current evidence for the physiological mechanisms by which vascular health alters the structural and functional integrity of the aging brain and how improvements in vascular health, via regular aerobic exercise, potentially benefits cognitive function.

The cerebral vasomotor response in varying CO(2) concentrations, as evaluated using cine phase contrast MRI: Flow, volume, and cerebrovascular resistance indices

PURPOSE

Previous studies have identified that impaired cerebral vasomotor reactivity (VMR) is associated with a higher risk of stroke and transient ischemic attack. This study aims to evaluate VMR by measuring the blood flow waveforms of the supplying arteries and dural sinuses using cine phase contrast MRI (PC MRI) and hypercapnic challenge.

METHODS

PC MRI flow quantification was performed on an oblique slice approximately perpendicular to the target vessels to include the left (LICA) and right internal carotid artery (RICA), basilar artery (BA), sinus rectus (SR), and superior sagittal sinus (SSS). A total of four PC MRI scans were performed at different CO(2) concentrations (room air and 3%, 5%, and 7% CO(2)).

RESULTS

The analyses obtained the flow parameters and cerebrovascular resistance parameters for all five vessels. Results indicated that the vascular resistance indices decreased with increasing CO(2) concentration in four vessels (LICA, RICA, BA, and SR). The obtained VMR parameters demonstrated exponential increases with increasing CO(2) concentration.

CONCLUSIONS

Using entire blood flow waveforms, this study applied separate flow dynamics during systolic and diastolic periods to obtain cerebrovascular resistance parameters and extensive flow-related information. It is the first to investigate the cerebrovascular resistance parameters under hypercapnic challenge using cine MRI. This technique could provide a useful tool for clinical application in cerebrovascular disease.

Low Residual CBF Variability in Alzheimer's Disease after Correction for CO(2) Effect

We tested the claim that inter-individual CBF variability in Alzheimer's disease (AD) is substantially reduced after correction for arterial carbon dioxide tension (PaCO(2)). Specifically, we tested whether the variability of CBF in brain of patients with AD differed significantly from brain of age-matched healthy control subjects (HC). To eliminate the CO(2)-induced variability, we developed a novel and generally applicable approach to the correction of CBF for changes of PaCO(2) and applied the method to positron emission tomographic (PET) measures of CBF in AD and HC groups of subjects. After correction for the differences of CO(2) tension, the patients with AD lost the inter-individual CBF variability that continued to characterize the HC subjects. The difference (ΔK(1)) between the blood-brain clearances (K(1)) of water (the current measure of CBF) and oxygen (the current measure of oxygen clearance) was reduced globally in AD and particularly in the parietal, occipital, and temporal lobes. We then showed that oxygen gradients calculated for brain tissue were similar in AD and HC, indicating that the low residual variability of CBF in AD may be due to low functional demands for oxidative metabolism of brain tissue rather than impaired delivery of oxygen.

Cerebral autoregulation in Alzheimer's disease

Cerebral autoregulation aims to stabilize blood flow to the brain during variations in perfusion pressure, thus protecting the brain against the risks of low or high systemic blood pressure. This vital mechanism is severely impaired in the transgenic mouse model of Alzheimer's disease (AD) that abundantly produces amyloid-β peptide β(1-42). These observations have been extrapolated to human AD, wherein impairment of autoregulation could have important implications for the clinical management and prevention of AD. Research on cerebral autoregulation in human AD, however, has only recently become available. Contrary to the animal models, preliminary studies suggest that cerebral autoregulation is preserved in patients with AD. Further research is urgently needed to elucidate this discrepancy in the current literature, given the accumulating evidence that implicates cerebrovascular pathology in AD.