Effect of cerebrovascular risk factors on regional cerebral blood flow

The Role of Methionine-Rich Diet in Unhealthy Cerebrovascular and Brain Aging: Mechanisms and Implications for Cognitive Impairment

As aging societies in the western world face a growing prevalence of vascular cognitive impairment and Alzheimer's disease (AD), understanding their underlying causes and associated risk factors becomes increasingly critical. A salient concern in the western dietary context is the high consumption of methionine-rich foods such as red meat. The present review delves into the impact of this methionine-heavy diet and the resultant hyperhomocysteinemia on accelerated cerebrovascular and brain aging, emphasizing their potential roles in cognitive impairment. Through a comprehensive exploration of existing evidence, a link between high methionine intake and hyperhomocysteinemia and oxidative stress, mitochondrial dysfunction, inflammation, and accelerated epigenetic aging is drawn. Moreover, the microvascular determinants of cognitive deterioration, including endothelial dysfunction, reduced cerebral blood flow, microvascular rarefaction, impaired neurovascular coupling, and blood-brain barrier (BBB) disruption, are explored. The mechanisms by which excessive methionine consumption and hyperhomocysteinemia might drive cerebromicrovascular and brain aging processes are elucidated. By presenting an intricate understanding of the relationships among methionine-rich diets, hyperhomocysteinemia, cerebrovascular and brain aging, and cognitive impairment, avenues for future research and potential therapeutic interventions are suggested.

Impact of sex and APOE ε4 on age-related cerebral perfusion trajectories in cognitively asymptomatic middle-aged and older adults: A longitudinal study

Cerebral hypoperfusion is thought to contribute to cognitive decline in Alzheimer's disease, but the natural trajectory of cerebral perfusion in cognitively healthy adults has not been well-studied. This longitudinal study is consisted of 950 participants (40-89 years), who were cognitively unimpaired at their first visit. We investigated the age-related changes in cerebral perfusion, and their associations with APOE-genotype, biological sex, and cardiometabolic measurements. During the follow-up period (range 0.13-8.24 years), increasing age was significantly associated with decreasing cerebral perfusion, in total gray-matter (β=-1.43), hippocampus (-1.25), superior frontal gyrus (-1.70), middle frontal gyrus (-1.99), posterior cingulate (-2.46), and precuneus (-2.14), with all P-values < 0.01. Compared with male-ɛ4 carriers, female-ɛ4 carriers showed a faster decline in global and regional cerebral perfusion with increasing age, whereas the age-related decline in cerebral perfusion was similar between male- and female-ɛ4 non-carriers. Worse cardiometabolic profile (i.e., increased blood pressure, body mass index, total cholesterol, and blood glucose) was associated with lower cerebral perfusion at all the visits. When time-varying cardiometabolic measurements were adjusted in the model, the synergistic effect of sex and APOE-ɛ4 on age-related cerebral perfusion-trajectories became largely attenuated. Our findings demonstrate that APOE-genotype and sex interactively impact cerebral perfusion-trajectories in mid- to late-life. This effect may be partially explained by cardiometabolic alterations.

Detrimental effect of systemic vascular risk factors on brain hemodynamic function assessed with MRI

Background and purpose Vascular risk factors have been associated with decreased cerebral blood flow (CBF) but this is etiologically nonspecific and may result from vascular insufficiency or a response to decreased brain metabolic activity. We apply new MRI techniques to measure oxygen extraction fraction (OEF) and cerebral metabolic rate of oxygen consumption (CMRO₂), hypothesizing that decreased CBF related to these vascular risk factors will be associated with increased OEF, confirming a primary vascular insufficiency. Methods 3T MRI was obtained on 70 community-based participants in this IRB-approved study with informed consent, with previous assessment of systolic blood pressure, hypertension medication, elevated serum triglycerides, low serum HDL, and diabetes mellitus. CBF was measured using phase contrast adjusted for brain volume (ml/100 g/min), OEF (%) was obtained from T2-Relaxation-Under-Spin-Tagging (TRUST), and CMRO₂ (μmol/100 g/min) was derived using the Fick principle. Stepwise linear regression identified optimal predictors of CBF with age, sex, and hematocrit included for adjustment. This predictive model was then evaluated against OEF and CMRO₂. Results Hypertriglyceridemia was associated with low CBF and high OEF. High systolic blood pressure was associated with high CBF and low OEF, which was primarily attributable to those with pressures above 160 mmHg. Neither risk factor was associated with significant differences in cerebral metabolic rate. Conclusion Low CBF related to hypertriglyceridemia was accompanied by high OEF with no significant difference in CMRO₂, confirming subclinical vascular insufficiency. High CBF related to high systolic blood pressure likely reflected limitations of autoregulation at higher blood pressures.

Cerebral Hemodynamic and White Matter Changes of Type 2 Diabetes Revealed by Multi-TI Arterial Spin Labeling and Double Inversion Recovery Sequence

Diabetes has been reported to affect the microvasculature and lead to cerebral small vessel disease (SVD). Past studies using arterial spin labeling (ASL) at single post-labeling delay reported reduced cerebral blood flow (CBF) in patients with type 2 diabetes. The purpose of this study was to characterize cerebral hemodynamic changes of type 2 diabetes using a multi-inversion-time 3D GRASE pulsed ASL (PASL) sequence to simultaneously measure CBF and bolus arrival time (BAT). Thirty-six patients with type 2 diabetes (43-71 years, 17 male) and 36 gender- and age-matched control subjects underwent MRI scans at 3 T. Mean CBF/BAT values were computed for gray and white matter (GM and WM) of each subject, while a voxel-wise analysis was performed for comparison of regional CBF and BAT between the two groups. In addition, white matter hyperintensities (WMHs) were detected by a double inversion recovery (DIR) sequence with relatively high sensitivity and spatial resolution. Mean CBF of the WM, but not GM, of the diabetes group was significantly lower than that of the control group (p < 0.0001). Regional CBF decreases were detected in the left middle occipital gyrus (p = 0.0075), but failed to reach significance after correction of partial volume effects. BAT increases were observed in the right calcarine fissure (p < 0.0001), left middle occipital gyrus (p < 0.0001), and right middle occipital gyrus (p = 0.0011). Within the group of diabetic patients, BAT in the right middle occipital gyrus was positively correlated with the disease duration (r = 0.501, p = 0.002), BAT in the left middle occipital gyrus was negatively correlated with the binocular visual acuity (r = -0.408, p = 0.014). Diabetic patients also had more WMHs than the control group (p = 0.0039). Significant differences in CBF, BAT, and more WMHs were observed in patients with diabetes, which may be related to impaired vision and risk of SVD of type 2 diabetes.

Cerebral blood flow, blood supply, and cognition in Type 2 Diabetes Mellitus

We investigated whether type 2 diabetes (T2DM) and the presence of cognitive impairment are associated with altered cerebral blood flow (CBF). Forty-one participants with and thirty-nine without T2DM underwent 3-Tesla MRI, including a quantitative technique measuring (macrovascular) blood flow in the internal carotid artery and an arterial spin labeling technique measuring (microvascular) perfusion in the grey matter (GM). Three analysis methods were used to quantify the CBF: a region of interest analysis, a voxel-based statistical parametric mapping technique, and a 'distributed deviating voxels' method. Participants with T2DM exhibited significantly more tissue with low CBF values in the cerebral cortex and the subcortical GM (3.8-fold increase). The latter was the only region where the hypoperfusion remained after correcting for atrophy, indicating that the effect of T2DM on CBF, independent of atrophy, is small. Subcortical CBF was associated with depression. No associations were observed for CBF in other regions with diabetes status, for carotid blood flow with diabetes status, or for CBF or flow in relation with cognitive function. To conclude, a novel method that tallies total 'distributed deviating voxels' demonstrates T2DM-associated hypoperfusion in the subcortical GM, not associated with cognitive performance. Whether a vascular mechanism underlies cognitive decrements remains inconclusive.

Aging is associated with changes to the biomechanical properties of the posterior cerebral artery and parenchymal arterioles

Artery remodeling, described as a change in artery structure, may be responsible for the increased risk of cardiovascular disease with aging. Although the risk for stroke is known to increase with age, relatively young animals have been used in most stroke studies. Therefore, more information is needed on how aging alters the biomechanical properties of cerebral arteries. Posterior cerebral arteries (PCAs) and parenchymal arterioles (PAs) are important in controlling brain perfusion. We hypothesized that aged (22-24 mo old) C57bl/6 mice would have stiffer PCAs and PAs than young (3-5 mo old) mice. The biomechanical properties of the PCAs and PAs were assessed by pressure myography. Data are presented as means ± SE of young vs. old. In the PCA, older mice had increased outer (155.6 ± 3.2 vs. 169.9 ± 3.2 μm) and lumen (116.4 ± 3.6 vs. 137.1 ± 4.7 μm) diameters. Wall stress (375.6 ± 35.4 vs. 504.7 ± 60.0 dyn/cm(2)) and artery stiffness (β-coefficient: 5.2 ± 0.3 vs. 7.6 ± 0.9) were also increased. However, wall strain (0.8 ± 0.1 vs. 0.6 ± 0.1) was reduced with age. In the PAs from old mice, wall thickness (3.9 ± 0.3 vs. 5.1 ± 0.2 μm) and area (591.1 ± 95.4 vs. 852.8 ± 100 μm(2)) were increased while stress (758.1 ± 100.0 vs. 587.2 ± 35.1 dyn/cm(2)) was reduced. Aging also increased mean arterial and pulse pressures. We conclude that age-associated remodeling occurs in large cerebral arteries and arterioles and may increase the risk of cerebrovascular disease.

Hyperperfusion in progressive multifocal leukoencephalopathy is associated with disease progression and absence of immune reconstitution inflammatory syndrome

We sought to characterize perfusion patterns of progressive multifocal leukoencephalopathy lesions by arterial spin labelling perfusion magnetic resonance imaging and to analyse their association with immune reconstitution inflammatory syndrome, and survival. A total of 22 patients with progressive multifocal leukoencephalopathy underwent a clinical evaluation and magnetic resonance imaging of the brain within 190 days of symptom onset. The presence of immune reconstitution inflammatory syndrome was determined based on clinical and laboratory criteria. Perfusion within progressive multifocal leukoencephalopathy lesions was determined by arterial spin labelling magnetic resonance imaging. We observed intense hyperperfusion within and at the edge of progressive multifocal leukoencephalopathy lesions in a subset of subjects. This hyperperfusion was quantified by measuring the fraction of lesion volume showing perfusion in excess of twice normal appearing grey matter. Hyperperfused lesion fraction was significantly greater in progressive multifocal leukoencephalopathy progressors than in survivors (12.8% versus 3.4% P = 0.02) corresponding to a relative risk of progression for individuals with a hyperperfused lesion fraction ≥ 4.0% of 9.1 (95% confidence interval of 1.4-59.5). The presence of hyperperfusion was inversely related to the occurrence of immune reconstitution inflammatory syndrome at the time of scan (P = 0.03). Indeed, within 3 months after symptom onset, hyperperfusion had a positive predictive value of 88% for absence of immune reconstitution inflammatory syndrome. Arterial spin labelling magnetic resonance imaging recognized regions of elevated perfusion within lesions of progressive multifocal leukoencephalopathy. These regions might represent virologically active areas operating in the absence of an effective adaptive immune response and correspond with a worse prognosis.

Hyperlipidemia disrupts cerebrovascular reflexes and worsens ischemic perfusion defect

Hyperlipidemia is a highly prevalent risk factor for coronary and cervical atherosclerosis and stroke. However, even in the absence of overt atherosclerosis, hyperlipidemia disrupts endothelial and smooth muscle function. We investigated the impact of hyperlipidemia on resting-brain perfusion, fundamental cerebrovascular reflexes, and dynamic perfusion defect during acute focal ischemia in hyperlipidemic apolipoprotein E knockout mice before the development of flow-limiting atherosclerotic stenoses. Despite elevated blood pressures, absolute resting cerebral blood flow was reduced by 20% in apolipoprotein E knockout compared with wild type when measured by [(14)C]-iodoamphetamine technique. Noninvasive, high spatiotemporal resolution laser speckle flow imaging revealed that the lower autoregulatory limit was elevated in apolipoprotein E knockout mice (60 vs. 40 mm Hg), and cortical hyperemic responses to hypercapnia and functional activation were attenuated by 30% and 64%, respectively. Distal middle cerebral artery occlusion caused significantly larger perfusion defects and infarct volumes in apolipoprotein E knockout compared with wild type. Cerebrovascular dysfunction showed a direct relationship to the duration of high-fat diet. These data suggest that hyperlipidemia disrupts cerebral blood flow regulation and diminishes collateral perfusion in acute stroke in the absence of hemodynamically significant atherosclerosis.

Novel MRI approaches for assessing cerebral hemodynamics in ischemic cerebrovascular disease

Changes in cerebral hemodynamics underlie a broad spectrum of ischemic cerebrovascular disorders. An ability to accurately and quantitatively measure hemodynamic (cerebral blood flow and cerebral blood volume) and related metabolic (cerebral metabolic rate of oxygen) parameters is important for understanding healthy brain function and comparative dysfunction in ischemia. Although positron emission tomography, single-photon emission tomography, and gadolinium-MRI approaches are common, more recently MRI approaches that do not require exogenous contrast have been introduced with variable sensitivity for hemodynamic parameters. The ability to obtain hemodynamic measurements with these new approaches is particularly appealing in clinical and research scenarios in which follow-up and longitudinal studies are necessary. The purpose of this review is to outline current state-of-the-art MRI methods for measuring cerebral blood flow, cerebral blood volume, and cerebral metabolic rate of oxygen and provide practical tips to avoid imaging pitfalls. MRI studies of cerebrovascular disease performed without exogenous contrast are synopsized in the context of clinical relevance and methodological strengths and limitations.

Magnetic resonance imaging quantification of regional cerebral blood flow and cerebrovascular reactivity to carbon dioxide in normotensive and hypertensive rats

Hypertension afflicts 25% of the general population and over 50% of the elderly. In the present work, arterial spin labeling MRI was used to non-invasively quantify regional cerebral blood flow (CBF), cerebrovascular resistance and CO(2) reactivity in spontaneously hypertensive rats (SHR) and in normotensive Wistar Kyoto rats (WKY), at two different ages (3 months and 10 months) and under the effects of two anesthetics, α-chloralose and 2% isoflurane (1.5 MAC). Repeated CBF measurements were highly consistent, differing by less than 10% and 18% within and across animals, respectively. Under α-chloralose, whole brain CBF at normocapnia did not differ between groups (young WKY: 61 ± 3ml/100g/min; adult WKY: 62 ± 4ml/100g/min; young SHR: 70 ± 9ml/100g/min; adult SHR: 69 ± 8ml/100g/min), indicating normal cerebral autoregulation in SHR. At hypercapnia, CBF values increased significantly, and a linear relationship between CBF and PaCO(2) levels was observed. In contrast, 2% isoflurane impaired cerebral autoregulation. Whole brain CBF in SHR was significantly higher than in WKY rats at normocapnia (young SHR: 139 ± 25ml/100g/min; adult SHR: 104 ± 23ml/100g/min; young WKY: 55± 9ml/100g/min; adult WKY: 71 ± 19ml/100g/min). CBF values increased significantly with increasing CO(2); however, there was a clear saturation of CBF at PaCO(2) levels greater than 70mmHg in both young and adult rats, regardless of absolute CBF values, suggesting that isoflurane interferes with the vasodilatory mechanisms of CO(2). This behavior was observed for both cortical and subcortical structures. Under either anesthetic, CO(2) reactivity values in adult SHR were decreased, confirming that hypertension, when combined with age, increases cerebrovascular resistance and reduces cerebrovascular compliance.

SURF-1 gene mutation associated with leukoencephalopathy in a 2-year-old

Mutations in the nuclear SURF-1 gene lead directly to cytochrome-c oxidase deficiency, the most common respiratory chain defect in Leigh syndrome, a neurodegenerative mitochondrial disease involving the deep gray matter and brain stem. We describe the second documented case in the literature to have a SURF-1 mutation presenting with diffuse leukodystrophy, adding to the growing number of cases of mitochondrial syndromes presenting with white matter disease. We examine magnetic resonance imaging (MRI) findings, which suggest that high-grade cytotoxic edema on diffusion-weighted imaging may be a helpful diagnostic feature in differentiating mitochondrial leukodystrophy from other, more common leukodystrophies. We show how MRI white matter findings may progress to include the brain stem, suggesting that a leukodystrophy due to respiratory chain defects can precede more classic Leigh syndrome deep gray matter radiographic findings.

Preclinical atherosclerosis covaries with individual differences in reactivity and functional connectivity of the amygdala

BACKGROUND

Cardiovascular disease (CVD) is a major source of medical comorbidity for patients with mood and anxiety disorders, and it remains the leading public health burden for the general population in industrialized nations. Indirect neurobiological evidence suggests that preclinical risk for atherosclerosis, the main contributor to CVD, may be conferred by interindividual variation in the functionality of the amygdala, a brain system jointly involved in processing behaviorally salient stimuli and regulating the cardiovascular system.

METHODS

In a neuroimaging study of 36 middle-aged adults (18 women) who were screened for confounding clinical cardiovascular and psychiatric disorders, we examined the direct covariation between a marker of preclinical atherosclerosis, carotid artery intima-media thickness (IMT), and interindividual variation in amygdala reactivity and functional connectivity assessed during the processing of behaviorally salient stimuli (angry and fearful facial expressions).

RESULTS

After accounting for traditional CVD risk factors, a thickening of carotid IMT across individuals covaried with greater amygdala reactivity and a more positive functional connectivity between the amygdala and perigenual anterior cingulate cortex, a corticolimbic area also implicated in behavioral salience processing and cardiovascular regulation.

CONCLUSIONS

Individual differences in amygdala reactivity and functional connectivity may reflect facets of a novel, systems-level neural phenotype conferring risk for atherosclerosis and CVD.

Brain imaging findings predict blood pressure response to pharmacological treatment

Hypertension appears to alter brain morphology, as well as the cerebrovascular support for information processing. Because these effects might reflect progressive effects of essential hypertension on the brain, we asked whether structural and functional brain indices would predict the success of pharmacological treatment of hypertension among 45 previously unmedicated individuals. After initial structural MRI and functional positron emission tomography imaging, subjects were randomly assigned in a double-blind fashion for treatment for 1 year with either lisinopril or atenolol. Systolic and diastolic blood pressure decreases after treatment stabilization were correlated to a pretreatment index of brain aging (combined ratings of ventricle and sulcal size and white matter hyperintensities) and the pretreatment change in regional cerebral blood flow during working memory in the thalamus and posterior parietal regions of interest. In multiple regression analyses, the structural brain index and the blood flow response in the thalamus predicted 20% of the variance in the systolic blood pressure response to treatment controlling for pretreatment blood pressure, age, gender, and type and dose of medication. Alcohol use influenced the thalamic response measure, but covariates did not alter the relation between greater indices of brain aging and less successful blood pressure response to treatment. The state of the brain may be an important factor in the remediation of blood pressure.

Cerebral perfusion in relation to cognitive function and type 2 diabetes

AIM/HYPOTHESIS

Underlying mechanisms for decreased cognitive functioning in patients with type 2 diabetes are unclear. In the general population, cerebral hypoperfusion is a risk factor for cognitive dysfunction and dementia. Reduced cerebral perfusion may account for cognitive impairments in diabetic patients relative to controls.

METHODS

A total of 98 patients with type 2 diabetes and 47 control participants underwent neuropsychological evaluation. Total cerebral blood flow (CBF) was assessed non-invasively by measuring the volume flow in the internal carotid arteries and basilar artery with two-dimensional phase-contrast magnetic resonance angiography. Relative total CBF, a measure of mean total cerebral perfusion, was obtained by expressing total CBF per 100 ml brain parenchyma volume.

RESULTS

Patients with type 2 diabetes performed worse on neuropsychological tests (p < 0.05). Total CBF per 100 ml brain parenchyma volume did not differ between participants with and without diabetes (difference -2.3 ml min(-1) 100 ml(-1); 95% CI -6.0, 1.3). In the entire group, total CBF per 100 ml brain parenchyma volume was positively associated with cognitive functioning (0.09 SD increase in composite z score per 10 ml min(-1) 100 ml(-1) increase in relative total CBF). This association was not affected by type 2 diabetes.

CONCLUSIONS/INTERPRETATION

Although total CBF per 100 ml brain parenchyma volume was associated with cognitive functioning, it did not explain cognitive impairments in patients with type 2 diabetes relative to controls.