The influence of hemodynamic stress factors on intracranial aneurysm formation

Influence of circle of Willis modeling on hemodynamic parameters in anterior communicating artery aneurysms and recommendations for model selection

Computational fluid dynamics (CFD) has been utilized to calculate hemodynamic parameters in anterior communicating artery aneurysm (AComA), which is located at a junction between left and right A1 and A2 segments. However, complete or half circle of Willis (CoW) models are used indiscriminately. This study aims to suggest recommendations for determining suitable CoW model. Five patient-specific CoW models with AComA were used, and each model was divided into complete, left-half, and right-half models. After validating the CFD using a flow experiment, the hemodynamic parameters and flow patterns in five AComAs were compared. In four out of five cases, inflow from one A1 side had a dominant influence on the AComA, while both left and right A1 sides affected the AComA in the remaining case. Also, the average difference in time-averaged wall shear stress between the complete and half models for four cases was 4.6%, but it was 62% in the other case. The differences in the vascular resistances of left and right A1 and A2 segments greatly influenced the flow patterns in the AComA. These results may help to enhance clinicians' understanding of blood flow in the brain, leading to improvements in diagnosis and treatment of cerebral aneurysms.

Pathogenic Factors and Prognosis of De Novo Aneurysms vAfter Aneurysm Clipping

OBJECTIVE

To confirm this hypothesis, this study aimed to explore the pathogenic factors, prognosis, and their relationship in de novo aneurysms and to reach a consensus on their management.

METHODS

First, the clinical data of 5 patients with de novo aneurysms from April 1998 to October 2021 were analyzed retrospectively. Then, the English literature on de novo aneurysms reported in Pubmed from 1985 to 2021 was systematically reviewed, and 18 case reports from 17 articles and 16 case series were identified. Univariate and multivariate analyses and modified Fisher test were used to analyze the relationship between pathogenic factors and prognosis.

RESULTS

Hypertension was noted in 60% of our clinical cases, 50% of the case series identified in the literature review, and 66.7% of the case reports in the literature review. In the case reports identified from our literature review, the proportion of original aneurysms in the anterior circulation was 96.3%. Moreover, in our 5 cases, all original aneurysms occurred in the anterior circulation. The rupture rate of original aneurysms in our 5 cases was 100%, and that of the cases reported in the literature review was 88.9%. Univariate logistic analysis showed that the time interval was related to the prognosis of de novo aneurysms with a P value of 0.048 and an odds ratio of 0.968 (95% confidence interval 0.938-1.000). Modified Fisher exact tests showed that patient age at the occurrence of de novo aneurysm P = 0.029) was related to the prognosis of de novo aneurysms.

CONCLUSIONS

Hypertension, an original aneurysms located in the anterior circulation and rupture represent the pathogenic factors associated with de novo aneurysms. The time interval to de novo aneurysm and patient age at the occurrence of de novo aneurysm are predictive of prognosis. Based on the above information, we can prevent and improve the prognosis of de novo aneurysms.

Imaging markers of intracranial aneurysm development: A systematic review

BACKGROUND

Imaging markers of intracranial aneurysm (IA) development are not well established.

PURPOSE

To provide an overview of imaging markers of IA development.

METHODS

A systematic search of PubMed and Embase up to December 1st 2020 using predefined criteria. Thirty-six studies met our inclusion criteria. We performed a quantitative summary of the included studies.

RESULTS

We found converging evidence for A1 segment asymmetry as an anatomical marker of anterior communicating artery (Acom) aneurysm development, and moderate evidence for several other markers. No hemodynamic markers yielded converging or moderate evidence. There was large heterogeneity across studies, especially in the definitions of imaging markers and study outcomes used. Due to the poor methodological quality of many studies and unavailability of effect sizes or crude data to calculate effect sizes, a formal meta-analysis was not possible. Many studies had poor methodological quality and varied inmarkerdefinitions and outcome measuresused, which prevented us from performing a formal meta-analysis.

CONCLUSIONS

We only identified A1 segment asymmetry as an imaging marker of Acom aneurysm development with converging evidence. A meta-analysis was not possible due to the heterogeneity of marker definitions and outcomes used, and poor methodological quality of many studies. Future studies should use robust study designs and uniformly defined imaging markers and outcome measures.

Factors affecting formation and rupture of intracranial saccular aneurysms

Unruptured intracranial aneurysms represent a decisional challenge. Treatment risks have to be balanced against an unknown probability of rupture. A better understanding of the physiopathology is the basis for a better prediction of the natural history of an individual patient. Knowledge about the possible determining factors arises from a careful comparison between ruptured versus unruptured aneurysms and from the prospective observation and analysis of unbiased series with untreated, unruptured aneurysms. The key point is the correct identification of the determining variables for the fate of a specific aneurysm in a given individual. Thus, the increased knowledge of mechanisms of formation and eventual rupture of aneurysms should provide significant clues to the identification of rupture-prone aneurysms. Factors like structural vessel wall defects, local hemodynamic stress determined also by peculiar geometric configurations, and inflammation as trigger of a wall remodeling are crucial. In this sense the study of genetic modifiers of inflammatory responses together with the computational study of the vessel tree might contribute to identify aneurysms prone to rupture. The aim of this article is to underline the value of a unifying hypothesis that merges the role of geometry, with that of hemodynamics and of genetics as concerns vessel wall structure and inflammatory pathways.

Physical factors effecting cerebral aneurysm pathophysiology

Many factors that are either blood-, wall-, or hemodynamics-borne have been associated with the initiation, growth, and rupture of intracranial aneurysms. The distribution of cerebral aneurysms around the bifurcations of the circle of Willis has provided the impetus for numerous studies trying to link hemodynamic factors (flow impingement, pressure, and/or wall shear stress) to aneurysm pathophysiology. The focus of this review is to provide a broad overview of such hemodynamic associations as well as the subsumed aspects of vascular anatomy and wall structure. Hemodynamic factors seem to be correlated to the distribution of aneurysms on the intracranial arterial tree and complex, slow flow patterns seem to be associated with aneurysm growth and rupture. However, both the prevalence of aneurysms in the general population and the incidence of ruptures in the aneurysm population are extremely low. This suggests that hemodynamic factors and purely mechanical explanations by themselves may serve as necessary, but never as necessary and sufficient conditions of this disease's causation. The ultimate cause is not yet known, but it is likely an additive or multiplicative effect of a handful of biochemical and biomechanical factors.

"Sit back, observe, and wait." Or is there a pharmacologic preventive treatment for cerebral aneurysms?

Intracranial aneurysms (IA) are a relatively frequent vascular abnormality. The prevailing opinion is that cerebral aneurysmal disease is related to hemodynamic and genetic factors, associated with structural weakness in the arterial wall which was acquired by a specific, often unknown, event. Possibly the trigger moment of aneurysm formation may depend on the dynamic arterial growth, which is closely related to aging/atherosclerosis. In most individuals, an endovascular/microsurgical approach is possible in order to obliterate the IA. However, in a number of patients with an unruptured IA (UIA), the neurosurgeon's decision is to just "sit back, observe, and wait", based on the favorable natural history of some of the UIAs. Furthermore, some individuals need to be kept under close observation since they have a higher chance of developing IA, especially those with at least two affected first-degree relatives with an IA, subjects with polycystic kidney disease, and patients who have undergone an aneurysm intervention. In these examples prophylactic strategies should be adopted, if it is at all possible. The main question is deciding the best option of clinical treatment for these cases, when surgical approach is contraindicated, or for those subjects who are more prone to develop an IA. In the present article, we hypothetically suggest a pharmacologic form of treatment with statins, beta-adrenergic blocker agents, and/or angiotensin-converting-enzyme inhibitor/angiotensin II receptor blockers to inhibit or slow down IA formation, taking into consideration some pathophysiological aspects related to aneurysmal development, such as: hemodynamic stress, arterial wall inflammation, nitric oxide formation, and atheromatous disease.

Numerical simulations of flow in cerebral aneurysms: comparison of CFD results and in vivo MRI measurements

Computational fluid dynamics (CFD) methods can be used to compute the velocity field in patient-specific vascular geometries for pulsatile physiological flow. Those simulations require geometric and hemodynamic boundary values. The purpose of this study is to demonstrate that CFD models constructed from patient-specific magnetic resonance (MR) angiography and velocimetry data predict flow fields that are in good agreement with in vivo measurements and therefore can provide valuable information for clinicians. The effect of the inlet flow rate conditions on calculated velocity fields was investigated. We assessed the internal consistency of our approach by comparing CFD predictions of the in-plane velocity field to the corresponding in vivo MR velocimetry measurements. Patient-specific surface models of four basilar artery aneurysms were constructed from contrast-enhanced MR angiography data. CFD simulations were carried out in those models using patient-specific flow conditions extracted from MR velocity measurements of flow in the inlet vessels. The simulation results computed for slices through the vasculature of interest were compared with in-plane velocity measurements acquired with phase-contrast MR imaging in vivo. The sensitivity of the flow fields to inlet flow ratio variations was assessed by simulating five different inlet flow scenarios for each of the basilar aneurysm models. In the majority of cases, altering the inlet flow ratio caused major changes in the flow fields predicted in the aneurysm. A good agreement was found between the flow fields measured in vivo using the in-plane MR velocimetry technique and those predicted with CFD simulations. The study serves to demonstrate the consistency and reliability of both MR imaging and numerical modeling methods. The results demonstrate the clinical relevance of computational models and suggest that realistic patient-specific flow conditions are required for numerical simulations of the flow in aneurysmal blood vessels.

Sub-piexl methods for improving vector quality in echo PIV flow, imaging technology

Developments of many cardiovascular problems have been shown to have a close relationship with arterial flow conditions. An ultrasound-based particle image velocimetry technique(Echo PIV) was recently developed to measure multi-component velocity vectors and local shear rates in arteries and opaque fluid flows by identifying and tracking flow tracers (ultrasound contrast microbubbles) within these flow fields. To improve the measurement accuracy, sub-pixel calculation method was adopted in this paper to maximize the ultrasound RF signal and B mode image correlation accuracy and increase the image spatial resolution. This algorithm is employed in processing both computer-generated particle image patterns and the B-mode images of microbubbles in rotating flows obtained by a high frame rate (up to 1000 frames per second) ultrasound imaging system. The results show the correlation of particle patterns and individual flow vector quality are improved and the overall flow mappings are also improved significantly. This would help the Echo PIV system to provide better multi-component velocity accuracy.

Use of antihypertension agents for the suppression of arterial pulse pressure waveforms in patients with intracranial aneurysms

Object

Patients with intracranial aneurysms tend toward raised blood pressure and abnormal pulse pressure profiles. The authors have investigated the influence of three antihypertension agents on blood pressure and pulse pressure waveforms in patients with known intracranial aneurysms, with a view to assessing the potential benefits of longterm antihypertension therapy on the progression of unruptured intracranial aneurysms.

Methods

Nineteen patients with a mean age of 56 years (range 38–76 years) were recruited for this study. All patients had confirmed intracranial aneurysms. A double-blind, randomized, crossover study was performed using perindopril, irbesartan, isosorbide mononitrate, and a placebo. Blood pressure and pulse pressure waveforms were assessed at the end of each 4-week treatment period.

Perindopril and irbesartan were well tolerated. For all measured parameters except heart rate (p = 0.03), no significant difference between baseline and placebo was identified. Each drug when compared with placebo reduced peripheral arterial blood pressure. Perindopril significantly decreased mean blood pressure by 10 mm Hg (p = 0.004), irbesartan by 9 mm Hg (p = 0.004), and isosorbide mononitrate by 13 mm Hg (p = 0.005).

The administration of each drug effected a significant reduction in the carotid artery augmentation index (AIX) compared with baseline values (perindopril p = 0.01, irbesartan p = 0.0002, and isosorbide mononitrate p = 0.03). There was also a significant difference in the AIX between irbesartan and the placebo (p = 0.05). Compared with the placebo, there was a significant difference in AIX (adjusted for heart rate) following the administration of irbesartan (p = 0.003) and isosorbide mononitrate (p = 0.01), but not with perindopril (p = 0.17).

Conclusions

Irbesartan appears to be the most effective treatment for the combined suppression of blood pressure and AIX in patients with intracranial aneurysms and has a high degree of patient tolerance.

Possible participation of clip rotation in the formation of de novo aneurysm

During long term follow-up after successful treatment of ruptured intracranial aneurysms, a few patients develop newly formed (de novo) aneurysms, which account for 0.9-1.1% of all cerebral aneurysms. As for the etiology of de novo cerebral saccular aneurysms, hemodynamic stress is considered to be the most important factor. The woman whose case we present here had developed a ruptured anterior communicating aneurysm at the age of 52, and the aneurysm with its anterior projection was completely clipped. Eight years later, she suffered intraventricular hemorrhage. Angiogram showed a newly formed anterior communicating artery aneurysm, which projected to the side opposite the clip. Surgical exploration found the head of the clip had rotated and become trapped between optic nerves. We speculated the rotation of the clip used for the aneurysm at the first operation was one of the possible mechanisms, which caused a change in the direction of hemodynamics to the anterior communicating artery, leading to the development of de novo aneurysm. Every neurosurgeon should pay attention to the clip position until dural closure has been completed.

Augmentation index as a measure of peripheral vascular disease state

Pulse pressure, especially in central arteries, is an independent predictor of adverse cardiovascular events in patients with increased elastic artery stiffness (or elastance). The central arterial pressure wave is composed of a forward traveling wave generated by left ventricular ejection and a later arriving reflected wave from the periphery. Increased stiffness of elastic arteries is the primary cause of increased pulse pressure in subjects with degeneration and hyperplasia of the arterial wall. As stiffness increases, transmission velocity of both forward and reflected waves increase, which causes the reflected wave to arrive earlier in the central aorta and augments pressure in late systole [ie, augmentation index = (augmented pressure/pulse pressure) increases]. These changes in wave reflection properties are associated with vascular disease and aging and cause an increase in left ventricular afterload, myocardial mass, and oxygen consumption. Vasoactive drugs have little direct effect on large elastic arteries but can markedly change wave reflection amplitude and augmentation index by altering stiffness of the muscular arteries and modifying transmission velocity of the reflected wave from the periphery to the heart. This change in amplitude and timing of the reflected wave causes a generalized change in central arterial systolic and pulse pressure that is not detected by cuff pressure measurements in the brachial artery.