Cerebral Haemodynamic Disturbances in Patients with Moderate Carotid Artery Stenosis

Neurocognitive Changes After Carotid Revascularization According to Perfusion Parameters: A Meta-analysis of Current Literature

BACKGROUND

Although the clinical outcomes continue to be scrutinized, there are a few data summarizing the changes in perfusion parameters in postoperative patients. The objective was to undertake a systematic literature review and perform a meta-analysis to assess the effects of cerebral perfusion changes in cognitive and functional status.

METHODS

A systematic search was conducted in July 2018 identifying articles comparing perfusion parameter changes before and after carotid revascularization in patients with carotid artery stenosis. Combined overall effect sizes were calculated using random-effects models.

RESULTS

The literature search identified 1031 unique articles eligible for analysis. Sixteen studies including 755 patients were identified. The studies were different for many methodological factors, for example, sample size, type of patients, statistical measure, type of test, timing of assessment, and so on. There were no differences in cerebral blood volume (CBV), cerebral metabolic rate of oxygen (CMRO₂), and relative cerebral blood volume (rCBV) between preintervention and postintervention, but there was a significant increase of cerebral blood flow (CBF) (95% confidence interval [CI] standardized mean difference [Std. MD] : -0.83 [-1.27, -0.40]; P = 0.0002; I² = 68%) and relative cerebral blood flow (rCBF) (95% CI Std. MD: -0.72 [-1.61, -0.27]; P < 0.0001; I² = 48%) after operation. In addition, the perfusion of mean transit time (MTT) (95% CI Std. MD: 1.26 [0.62, 1.90]; P = 0.0001; I² = 84%), oxygen extraction fraction (OEF) (95% CI Std. MD: 0.78 [0.24, 1.33]; P = 0.005; I² = 0%), time to peak (TTP) (95% CI Std. MD: 0.46 [0.16, 0.77]; P = 0.003; I² = 47%), and relative mean transit time (rMTT) (95% CI Std. MD: 0.41 [0.33, 0.50]; P < 0.00001; I² = 67%) was higher before than after operation.

CONCLUSIONS

The increase in changes in CBF and rCBF and the decrease in MTT, OEF, TTP, and rMTT after operation may indicate the improvement of cognition in the short term. Intraoperative perfusion parameters could be an important adjuvant monitoring method in neurocognitive changes after carotid revascularization.

Multimodality Imaging of Carotid Stenosis

Four diagnostic modalities are used to image the following internal carotid artery: digital subtraction angiography (DSA), duplex ultrasound (DUS), computed tomography angiography (CTA), and magnetic resonance angiography (MRA). The aim of this article is to describe the potentials of these techniques and to discuss their advantages and disadvantages. Invasive DSA is still considered the gold standard and is an indivisible part of the carotid stenting procedure. DUS is an inexpensive but operator-dependent tool with limited visibility of the carotid artery course. Conversely, CTA and MRA allow assessment of the carotid artery from the aortic arch to intracranial parts. The disadvantages of CTA are radiation and iodine contrast medium administration. MRA is without radiation but contrast-enhanced MRA is more accurate than noncontrast MRA. The choice of methods depends on the clinical indications and the availability of methods in individual centers. However, the general approach to patient with suspected carotid artery stenosis is to first perform DUS and then other noninvasive methods such as CTA, MRA, or transcranial Doppler US.

MRI multiparametric hemodynamic characterization of the normal brain

Characterization of the brain's vascular system is of major clinical importance in the assessment of patients with cerebrovascular disease. The aim of this study was to characterize brain hemodynamics using multiparametric methods and to obtain reference values from the healthy brain. A multimodal magnetic resonance imaging (MRI) study was performed in twenty healthy subjects, including dynamic susceptibility contrast imaging and blood oxygen level dependence (BOLD) during hypercapnia and carbogen challenges. Brain tissues were defined using unsupervised cluster analysis based on these three methods, and several hemodynamic parameters were calculated for gray matter (GM), white matter (WM), blood vessels and dura (BVD); the three main vascular territories within the GM; and arteries and veins defined within the BVD cluster. The carbogen challenge produced a BOLD signal twice as high as the hypercapnia challenge, in all brain tissues. The three brain tissues differed significantly from each other in their hemodynamic characteristics, supporting the graded vascularity of the tissues, with BVD>GM>WM. Within the GM cluster, a significant delay of ∼1.2 s of the bolus arrival time was detected within the posterior cerebral artery territory relative to the middle and anterior cerebral artery territories. No differences were detected between right and left middle cerebral artery territories for all hemodynamic parameters. Within the BVD cluster, a significant delay of ∼1.9 s of the bolus arrival time was detected within the veins relative to the arteries. This parameter enabled to differentiate between the various blood vessels, including arteries, veins and choroid plexus. This study provides reference values for several hemodynamic parameters, obtained from healthy brains, and may be clinically important in the assessment of patients with various vascular pathologies.

Carotid velocities determine cerebral blood flow deficits in elderly men with carotid stenosis <50%

To examine if mild carotid stenosis correlates with silent vascular brain changes, we studied a prospective population-based cohort “Men born in 1914.” Data from followups at ages 68 and 81, have been used. Carotid ultrasound was performed at age 81, and cerebral blood flow (CBF) was measured with SPECT at age 82. Out of 123 stroke-free patients, carotid stenosis <50% was observed in 94% in the right and 89% in the left internal carotid arteries (ICAs). In these subjects, Peak Systolic Velocities in ICA correlated negatively with CBF in a majority of several brain areas, especially in mesial temporal area. Results were limited to normotensive until their seventies, who developed late-onset hypertension with a subsequent blood pressure, pulse pressure, and ankle-brachial index growth. Elderly with asymptomatic carotid stenosis <50% and peak systolic velocities in ICA 0.7–1.3 m/s, should be offered an intensified pharmacotherapy to prevent stroke or silent cerebrovascular events.

Occult cerebrovascular disease and late-onset epilepsy: could loss of neurovascular unit integrity be a viable model?

Late-onset epilepsy (LOE) first occurs after 60 years of age and may be due to occult cerebrovascular disease (CVD) which confers an increased risk of stroke. However, patients with late-onset epilepsy are not currently consistently investigated or treated for cerebrovascular risk factors. We discuss how abnormalities of neurovascular unit function, namely, changes in regional cerebral blood flow and blood brain barrier disruption, may be caused by occult cerebrovascular disease but present clinically as late-onset epilepsy. We describe novel magnetic resonance imaging methods to detect abnormal neurovascular unit function in subjects with LOE and controls. We hypothesise that occult CVD may cause LOE as a result of neurovascular unit dysfunction.

Intraprocedural changes in angiographic cerebral circulation time predict cerebral blood flow after carotid artery stenting

Changes in the cerebral blood flow (CBF) are important for planning postoperative care in patients treated by carotid artery stenting (CAS). The relationship between intraprocedural changes in the angiographic cerebral circulation time (CCT) and perioperative CBF changes were retrospectively studied in 49 CAS procedures performed in 46 patients with carotid artery stenosis. The CCT, defined as the interval between the timing of maximal opacification at the terminal portion of the internal carotid artery and at the cortical vein, was determined by referring to time-density curves of data obtained from routine intraprocedural digital subtraction angiography. The intraoperative change in CCT (Delta CCT) was calculated for each of the 49 procedures. CBF studies, using dynamic perfusion computed tomography, were performed 10-2 days before and 2-4 days after CAS. Perioperative changes in the ratio of the CBF in the territory of the middle cerebral artery on the affected side to CBF on the contralateral side (%CBF) were calculated by subtracting pre- from postoperative %CBF (Delta%CBF) and the correlation between Delta CCT and Delta%CBF was evaluated. Mean CCT was shortened by 1.1 seconds from 5.3 to 4.2 seconds after CAS. Mean %CBF increased by 11.9% from 91.8% to 103.7% after the procedure. Delta CCT and Delta%CBF showed a significant positive correlation (r = 0.61, p = 0.008). Intraprocedural changes in angiographic CCT are predictive of postoperative CBF in patients with CAS.

Simulation of the interaction between blood flow and atherosclerotic plaque

It has been well accepted that over 50% of cerebral ischemic events are the result of rupture of vulnerable carotid atheroma and subsequent thrombosis. Such strokes are potentially preventable by carotid interventions. Selection of patients for intervention is currently based on the severity of carotid luminal stenosis. It has been, however, widely accepted that luminal stenosis alone may not be an adequate predictor of risk. To evaluate the effects of degree of luminal stenosis and plaque morphology on plaque stability, we used a coupled nonlinear time-dependent model with flow-plaque interaction simulation to perform flow and stress/strain analysis for stenotic artery with a plaque. The Navier-Stokes equations in the Arbitrary Lagrangian-Eulerian (ALE) formulation were used as the governing equations for the fluid. The Ogden strain energy function was used for both the fibrous cap and the lipid pool. The plaque Principal stresses and flow conditions were calculated for every case when varying the fibrous cap thickness from 0.1 to 2 mm and the degree of luminal stenosis from 10% to 90%. Severe stenosis led to high flow velocities and high shear stresses, but a low or even negative pressure at the throat of the stenosis. Higher degree of stenosis and thinner fibrous cap led to larger plaque stresses, and a 50% decrease of fibrous cap thickness resulted in a 200% increase of maximum stress. This model suggests that fibrous cap thickness is critically related to plaque vulnerability and that, even within presence of moderate stenosis, may play an important role in the future risk stratification of those patients when identified in vivo using high resolution MR imaging.

Changes in cerebral blood volume and amyloid pathology in aged Alzheimer APP/PS1 mice on a docosahexaenoic acid (DHA) diet or cholesterol enriched Typical Western Diet (TWD)

High dietary cholesterol and low dietary docosahexaenoic acid (DHA) intake are risk factors for Alzheimer's disease (AD). However, it is unclear how these components influence the course of the disease. We investigated the effects of dietary lipids on beta-amyloid deposition and blood circulation in the brains of 18-month-old APP/PS1 mice. Starting at 6 months of age, mice were fed a regular rodent chow, a Typical Western Diet (TWD) containing 1% cholesterol, or a diet with a high (0.5%) level of DHA for 12 months. Relative cerebral blood volume (rCBV) and flow (CBF) were determined with (2)H MR spectroscopy and gradient echo contrast enhanced MRI. Deposition of beta-amyloid was visualized in fixed brain tissue with immunohistochemistry. The TWD diet increased plaque burden in the dentate gyrus of the hippocampus, but did not significantly reduce rCBV. In contrast, the DHA-enriched diet increased rCBV without changing blood flow indicating a larger circulation in the brain probably due to vasodilatation and decreased the amount of vascular beta-amyloid deposition. Together, our results indicate that the long-term intake of dietary lipids can impact both brain circulation and beta-amyloid deposition, and support the involvement of hemodynamic changes in the development of AD.

Total cerebral blood flow estimated by color velocity imaging quantification ultrasound: a predictor for recurrent stroke?

Although the Virchow's triad on thrombosis includes reduced blood flow as a factor, there has been relatively little data on the importance of total cerebral blood flow on the risk of subsequent stroke. In the current study, we investigate whether total cerebral blood flow helps predict stroke recurrence. Extracranial arterial blood flow volume estimated by color velocity imaging quantification ultrasound (CVIQ) is an index of cerebral blood flow measurement. We performed a cohort study of 210 consecutive acute stroke patients. Patients were studied with transcranial Doppler and duplex ultrasound for intra- and extracranial large artery disease within 3 days of symptom onset. The association between the risk of recurrent stroke and CVIQ was analyzed with Cox proportional hazards model. Thirty-nine patients (17.7%) developed an ischemic stroke during a mean follow-up of 47.5 months. The mean extracranial blood flow volume was significantly lower for patients who had a recurrent stroke than those without (594.4+/-130.3 versus 683.8+/-176.9 mL/min; P=0.003). In a Cox proportional hazards model adjusting for potential confounding variables, extracranial blood flow volume (hazard ratio (HR) for lowest tertile, 4.1; 95% confidence interval (CI), 1.5 to 11.0) along with male sex (HR, 2.5; 95% CI, 1.3 to 5.1), diabetes (HR, 2.5; 95% CI, 1.2 to 5.0) and large artery stenosis (HR, 2.2; 95% CI, 1.1 to 4.4) were independent predictors for stroke recurrence. Our data indicated that patient with low amount of blood flow to the brain is at risk of recurrent stroke.