Relative Thrombus Burden Ratio Reveals Overproportioned Intraluminal Thrombus Growth-Potential Implications for Abdominal Aortic Aneurysm

Background: An intraluminal, non-occlusive thrombus (ILT) is a common feature in an abdominal aortic aneurysm (AAA). This study investigated the relative progression of ILT vs. AAA volume using a novel parameter, the so-called thrombus burden ratio (TBR), in non-treated AAAs. Parameters potentially associated with TBR progression were analyzed and TBR progression in large vs. small and fast- vs. slow-growing AAAs was assessed. Methods: This retrospective, single-center study analyzed sequential contrast-enhanced computed tomography angiography (CTA) scans between 2009 and 2018 from patients with an AAA before surgical treatment. Patients' medical data and CTA scans were analyzed at two given time points. The TBR was calculated as a ratio of ILT and AAA volume, and relative TBR progression was calculated by normalization for time between sequential CTA scans. Spearman's correlation was applied to identify morphologic parameters correlating with TBR progression, and multivariate linear regression analysis was used to evaluate the association of clinical and morphological parameters with TBR progression. Results: A total of 35 patients were included. The mean time between CT scans was 16 ± 15.9 months. AAA volume progression was 12 ± 3% and ILT volume progression was 36 ± 13%, resulting in a TBR progression of 11 ± 4%, suggesting overproportioned ILT growth. TBR progression was 0.8 ± 0.8% per month. Spearman's correlation verified ILT growth as the most relevant parameter contributing to TBR progression (R = 0.51). Relative TBR progression did not differ significantly in large vs. small and fast- vs. slow-growing AAAs. In the multivariate regression analysis, none of the studied factors were associated with TBR progression. Conclusion: TBR increases during AAA development, indicating an overproportioned ILT vs. AAA volume growth. The TBR may serve as a useful parameter, as it incorporates the ILT volume growth relative to the AAA volume, therefore combining two important parameters that are usually reported separately. Yet, the clinical relevance in helping to identify potential corresponding risk factors and the evaluation of patients at risk needs to be further validated in a larger study cohort.

Computational Study of Abdominal Aortic Aneurysm Walls Accounting for Patient-Specific Non-Uniform Intraluminal Thrombus Thickness and Distinct Material Models: A Pre- and Post-Rupture Case

An intraluminal thrombus (ILT) is present in the majority of abdominal aortic aneurysms, playing a crucial role in their growth and rupture. Although most computational studies do not include the ILT, in the present study, this is taken into account, laying out the whole simulation procedure, namely, from computed tomography scans to medical image segmentation, geometry reconstruction, mesh generation, biomaterial modeling, finite element analysis, and post-processing, all carried out in open software. By processing the tomography scans of a patient's aneurysm before and after rupture, digital twins are reconstructed assuming a uniform aortic wall thickness. The ILT and the aortic wall are assigned different biomaterial models; namely, the first is modeled as an isotropic linear elastic material, and the second is modeled as the Mooney-Rivlin hyperelastic material as well as the transversely isotropic hyperelastic Holzapfel-Gasser-Ogden nonlinear material. The implementation of the latter requires the designation of local Cartesian coordinate systems in the aortic wall, suitably oriented in space, for the proper orientation of the collagen fibers. The composite aneurysm geometries (ILT and aortic wall structures) are loaded with normal and hypertensive static intraluminal pressure. Based on the calculated stress and strain distributions, ILT seems to be protecting the aneurysm from a structural point of view, as the highest stresses appear in the thrombus-free areas of the aneurysmal wall.

Intraluminal Thrombus of the Extracranial Cerebral Arteries in Acute Ischemic Stroke: Manifestations, Treatment Strategies, and Outcome

Background

Intraluminal thrombus (ILT) of the cervical arteries is an uncommon finding that can lead to acute or recurrent ischemic stroke. Currently, antithrombotic therapy in the form of antiplatelet and/or anticoagulation is considered the mainstay of treatment, but evidence of which one has a better outcome is lacking.

Methods

A retrospective cohort study included 28 patients diagnosed with acute stroke or transient ischemic attack with ILT of the extracranial arteries from 2013 to 2022. The primary efficacy outcome was assessed as recurrent stroke, and the primary safety outcome was assessed as hemorrhagic complications. Secondary outcomes were assessed as the resolution of thrombi by CT angiography (CTA) and clinical improvement by the Modified Rankin Scale (mRS) and NIH Stroke Scale (NIHSS).

Results

Out of 28 patients, more than half (57.1%; n = 16) were males with a mean age of 57.8 ± 9.5 years and an average BMI of 26.9 ± 4.5 kg/m2. As initial treatment, twenty-four patients received anticoagulation and four received antiplatelet agents. Recurrent strokes were found in four patients (14.29%), and all were initially treated with anticoagulation. One patient in the anticoagulation group had a significant retroperitoneal hemorrhage. None of the patients in the antiplatelets group had a recurrent stroke or bleeding event. Initial treatment with antiplatelet agents significantly improved the NIHSS on day 7 (P = 0.017). A significant improvement in NIHSS on day 90 was observed in the anticoagulant group (P = 0.011). In the follow-up CTA performed on 24 patients, 18 (75%) showed complete resolution (3 out of 3 (100%) in the antiplatelet group and 15 out of 21 (71.43%) in the anticoagulant group).

Conclusion

Initial treatment with anticoagulants improves neurologic outcomes in patients with ILT-induced acute ischemic stroke but carries the risk of recurrent stroke and bleeding. However, initial treatment with dual antiplatelet agents appears to have comparable efficacy without sequelae, particularly in atherosclerosis-induced ILT.

Partially thrombosed giant basilar artery aneurysm with attenuated contrast enhancement of the intraluminal thrombus on vessel wall MRI after flow diversion treatment: illustrative case

BACKGROUND

The effect of vessel wall magnetic resonance imaging (VW-MRI) enhancement in partially thrombosed aneurysms has previously indicated aneurysmal instability and a rupture risk. However, whether the contrast effect of the wall changes before or after flow diversion treatment is still under investigation.

OBSERVATIONS

The authors report a case of a partially thrombosed basilar artery aneurysm that increased in size over a short period, worsened brainstem compression symptoms, and was treated with a flow diverter stent with good results. In this case, VW-MRI after surgery showed a reduced contrast effect on the intraluminal thrombus within the aneurysm. The aneurysm thrombosed and markedly regressed over the next 5 months, with remarkable improvement in the brainstem compression symptoms.

LESSONS

This finding on VW-MRI may indicate an attenuation of neovascularization in the thrombus wall and be a sign of aneurysm stabilization.

Effect of Intraluminal Thrombus Burden on the Risk of Abdominal Aortic Aneurysm Rupture

Abdominal aortic aneurysm (AAA) is a critical health disorder, where the abdominal aorta dilates more than 50% of its normal diameter. Enlargement in abdominal aorta alters the hemodynamics and flow-induced forces on the AAA wall. Depending on the flow conditions, the hemodynamic forces on the wall may result in excessive mechanical stresses that lead to AAA rupture. The risk of rupture can be predicted using advanced computational techniques such as computational fluid dynamics (CFD) and fluid-structure interaction (FSI). For a reliable rupture risk assessment, formation of intraluminal thrombus (ILT) and uncertainty in arterial material properties should be taken into account, mainly due to the patient-specific differences and unknowns in AAAs. In this study, AAA models are computationally investigated by performing CFD simulations combined with FSI analysis. Various levels of ILT burdens are artificially generated in a realistic AAA geometry, and the peak effective stresses are evaluated to elucidate the effect of material models and ILT formation. The results indicate that increasing the ILT burden leads to lowered effective stresses on the AAA wall. The material properties of the artery and ILT are also effective on the stresses; however, these effects are limited compared to the effect of ILT volume in the AAA sac.

Features and risk factors of early intraluminal thrombus formation within the frozen elephant trunk stent graft

OBJECTIVE

The frozen elephant trunk is a standard treatment method for aortic arch pathologies extending into the descending aorta. We previously described the phenomenon of early postoperative intraluminal thrombosis within the frozen elephant trunk. We investigated the features and predictors of intraluminal thrombosis.

METHODS

A total of 281 patients (66% male, mean age 60 ± 12 years) underwent frozen elephant trunk implantation between May 2010 and November 2019. In 268 patients (95%), early postoperative computed tomography angiography was available to assess intraluminal thrombosis.

RESULTS

The incidence of intraluminal thrombosis after frozen elephant trunk implantation was 8.2%. Intraluminal thrombosis was diagnosed early after the procedure (4.6 ± 2.9 days) and could be successfully treated with anticoagulation in 55% of patients. A total of 27% developed embolic complications. Mortality (27% vs 11%, P = .044) and morbidity were significantly higher in patients with intraluminal thrombosis. Our data showed a significant association of intraluminal thrombosis with prothrombotic medical conditions and anatomic slow flow features. The incidence of heparin-induced thrombopenia was higher in patients with intraluminal thrombosis (18% vs 3.3%, P = .011). Stent-graft diameter index, anticipated endoleak Ib, and degenerative aneurysm were significant independent predictors of intraluminal thrombosis. Therapeutic anticoagulation was a protective factor. Glomerular filtration rate, extracorporeal circulation time, postoperative rethoracotomy, and intraluminal thrombosis (odds ratio, 3.19, P = .047) were independent predictors of perioperative mortality.

CONCLUSIONS

Intraluminal thrombosis is an underrecognized complication after frozen elephant trunk implantation. In patients with risk factors of intraluminal thrombosis indication for frozen elephant trunk should be carefully evaluated and postoperative anticoagulation considered. Early thoracic endovascular aortic repair extension should be considered in patients with intraluminal thrombosis to prevent embolic complications. Stent-graft designs should be improved to prevent intraluminal thrombosis after frozen elephant trunk implantation.

A Ticking Time Bomb: A Case of Floating Distal Aortic Arch Intraluminal Thrombus

Aortic arch thrombus is a rare entity that can result in catastrophic sequelae. This is a case report of a 65-year-old female patient who presented with chest pain that started one day prior to arrival at the emergency department. Acute coronary syndrome (ACS) and pulmonary embolism (PE) were ruled out. A filling defect at the distal aortic arch evident on chest computed tomography angiography (CTA) was confirmed to be a floating distal aortic arch thrombus on transesophageal echocardiogram (TEE). There was no evidence of an underlying aneurysm, dissection, or significant atherosclerosis. The patient was considered to be at high risk for surgical intervention, hence, a decision was made to start the patient on chronic anticoagulation with direct oral anticoagulants (DOACs). A follow-up CTA three months later showed total resolution of the thrombus. The report highlights this treacherous pathology and provides an overview of the predisposing factors, radiologic findings, as well as management strategies for floating aortic arch thrombi.

Towards Precritical Medical Therapy of the Abdominal Aortic Aneurysm

Pharmacotherapy for abdominal aortic aneurysm (AAA) can be useful for prevention, especially in people at higher risk, for slowing down AAA progression, as well as for post-surgery adjuvant treatment. Our review focuses on novel pharmacotherapy approaches targeted towards slowing down progression of AAA, known also as secondary prevention therapy. Guidelines for AAA are not specific to slow down the expansion rate of an abdominal aortic aneurysm, and therefore no medical therapy is recommended. New ideas are urgently needed to develop a novel medical therapy. We are hopeful that in the future, pharmacologic treatment will play a key role in the prevention and treatment of AAA.

The Detrimental Role of Intraluminal Thrombus Outweighs Protective Advantage in Abdominal Aortic Aneurysm Pathogenesis: The Implications for the Anti-Platelet Therapy

Abdominal aortic aneurysm (AAA) is a common cardiovascular disease resulting in morbidity and mortality in older adults due to rupture. Currently, AAA treatment relies entirely on invasive surgical treatments, including open repair and endovascular, which carry risks for small aneurysms (diameter < 55 mm). There is an increasing need for the development of pharmacological intervention for early AAA. Over the last decade, it has been increasingly recognized that intraluminal thrombus (ILT) is involved in the growth, remodeling, and rupture of AAA. ILT has been described as having both biomechanically protective and biochemically destructive properties. Platelets are the second most abundant cells in blood circulation and play an integral role in the formation, expansion, and proteolytic activity of ILT. However, the role of platelets in the ILT-potentiated AAA progression/rupture remains unclear. Researchers are seeking pharmaceutical treatment strategies (e.g., anti-thrombotic/anti-platelet therapies) to prevent ILT formation or expansion in early AAA. In this review, we mainly focus on the following: (a) the formation/deposition of ILT in the progression of AAA; (b) the dual role of ILT in the progression of AAA (protective or detrimental); (c) the function of platelet activity in ILT formation; (d) the application of anti-platelet drugs in AAA. Herein, we present challenges and future work, which may motivate researchers to better explain the potential role of ILT in the pathogenesis of AAA and develop anti-platelet drugs for early AAA.

Mechanisms of abdominal aortic aneurysm progression: A review

Abdominal aortic aneurysm (AAA) is a common disease associated with significant cardiovascular morbidity and mortality. Up to now, there is still controversy on the choice of treatment method of AAA. Even so, the mechanisms of AAA progression are poorly defined, making targeting new therapies problematic. Current evidence favors an interaction of the hemodynamic microenvironment with local and systemic immune responses. In this review, we aim to provide an update of mechanisms in AAA progression, involving hemodynamics, perivascular adipose tissue, adventitial fibroblasts, vasa vasorum remodeling, intraluminal thrombus, and distribution of macrophage subtypes.

Intraluminal thrombus effect on the progression of abdominal aortic aneurysms by using a multistate continuous-time Markov chain model

OBJECTIVE

To investigate the relationship between the characteristics of intraluminal thrombus (ILT) with abdominal aortic aneurysm (AAA) expansion.

METHODS

This retrospective clinical study applied homogeneous multistate continuous-time Markov chain models to longitudinal computed tomography (CT) data from Korean patients with AAA. Four AAA states were considered (early, mild, severe, fatal) and the maximal thickness of the ILT (max_ILT), the fraction of the wall area covered by the ILT (area_frac) and the fraction of ILT volume (vol_frac) were used as covariates.

RESULTS

The analysis reviewed longitudinal CT images from 26 patients. Based on likelihood-ratio statistics, the area_frac was the most significant biomarker and max_ILT was the second most significant. In addition, within AAAs that developed an ILT layer, the analysis found that the AAA expands relatively quickly during the early stage but the rate of expansion reduces once the AAA has reached a larger size.

CONCLUSION

The results recommend surgical intervention when a patient has an area_frac more than 60%. Although this recommendation should be considered with caution given the limited sample size, physicians can use the proposed model as a tool to find such recommendations with their own data.

Oxygen transport in a permeable model of abdominal aortic aneurysm

This paper reports on modelling and simulation of the coupled mass and momentum transport through the arterial lumen and the porous arterial wall of an abdominal aortic aneurysm (AAA). The effect of porous structure and intramural flow, which is usually neglected, is essential to quantify the oxygen concentration in the aneurysmal wall and will be addressed in this work via a novel coupled fluid-porous structure-mass transport model for AAA. The computational solver provides a prediction of the filtration velocity and oxygen concentration in the artery and further simulations are used to investigate the relation between the wall oxygen concentration and permeability. The results demonstrate the essential coupling between blood and filtration flow and specifically how their interactions affect oxygen transport.

Influence of preoperative statins and aspirin administration on biological and magnetic resonance imaging properties in patients with abdominal aortic aneurysm

Background: Main objective of this study was to evaluate the influence of statins and/or acetylsalicylic acid on biochemical characteristics of abdominal aortic aneurysm (AAA) wall and intraluminal thrombus (ILT). Patients and methods: Fifty patients with asymptomatic infrarenal AAA were analyzed using magnetic resonance imaging on T1w sequence. Relative ILT signal intensity (SI) was determined as a ratio between ILT and psoas muscle SI. Samples containing the full ILT thickness and aneurysm wall were harvested from the anterior surface at the level of the maximal diameter. The concentration of enzymes such as matrix metalloproteinase (MMP) 9, MMP2 and neutrophil elastase (NE/ELA) were analyzed in ILT and AAA wall; while collagen type III, elastin and proteoglycan 4 were analyzed in harvested AAA wall. Oxidative stress in the AAA wall was assessed by catalase and malondialdehyde activity in tissue samples. Results: Relative ILT signal intensity (1.09 ± 0.41 vs 0.89 ± 0.21, p = 0.013) were higher in non-statin than in statin group. Patients who were taking aspirin had lower relative ILT area (0.89 ± 0.19 vs 1.13. ± 0.44, p = 0.016), and lower relative ILT signal intensity (0.85 [0.73-1.07] vs 1.01 [0.84-1.19], p = 0.021) compared to non-aspirin group. There were higher concentrations of elastin in AAA wall among patients taking both of aspirin and statins (1.21 [0.77-3.02] vs 0.78 (0.49-1.05) ng/ml, p = 0.044) than in patients who did not take both of these drugs. Conclusions: Relative ILT SI was lower in patients taking statin and aspirin. Combination of antiplatelet therapy and statins was associated with higher elastin concentrations in AAA wall.

Aortic sac enlargement after endovascular aneurysm repair: volume-related changes and the impact of intraluminal thrombus

Purpose

Abdominal aortic aneurysm (AAA) growth after endovascular aneurysm repair (EVAR) is still unpredictable. The issue of optimal frequency of computed tomography angiography for surveillance and its measurement method accuracy remain unclear. We aimed to assess the value of abdominal aneurysm sac volume measurement for detecting expansions and the association of preprocedural intraluminal thrombus (ILT) volume with aneurysm sac growth following EVAR.

Material and methods

A total of 107 patients underwent elective EVAR. Inclusion criteria provided a cohort of 39 patients. Changes of postoperative maximum aneurysm sac diameter and AAA volume were calculated. Volumetric AAA changes and demographic data of the cases with clinically irrelevant AAA diameter enlargement were evaluated. Preoperative ILT volumes were collected. ILT and AAA sac volume ratio was calculated. Statistical data analysis was performed using standard methods.

Results

The mean changes of maximum AAA diameter and volume in percentage after EVAR were –5.08 ± 8.20 mm and –13.39 ± 23.32%, respectively. A moderate positive linear correlation between those changes was found (R² = 0.731; p < 0.0001). The mean relative AAA volume increase in cases without clinically relevant diameter enlargement was 11.50 ± 8.27%. The means of ILT and AAA sac ratios were 0.59 ± 0.17 and 0.52 ± 1.8 in growing AAA sac and in stable or shrinking AAA sac groups, respectively (p = 0.308).

Conclusions

Volumetric AAA measurement may be useful as an additional method to diameter measurement after EVAR to identify clinically relevant sac growth. Preoperative volume of ILT may not significantly affect the growth rate of AAA after EVAR.

Anatomical Differences Between Intact and Ruptured Large Abdominal Aortic Aneurysms

Purpose: To compare different anatomical characteristics between intact and ruptured large abdominal aortic aneurysms (rAAA >80 mm) with the goal of refining the process of estimating rupture risk. Materials and Methods: A retrospective study involving 62 male patients with large (>80 mm) aneurysms matched for age and smoking produced a 31-patient elective group with a mean maximum aneurysm diameter of 92±9.7 mm and a 31-patient rAAA group (mean maximum aneurysm diameter 95.7±12 mm). Preoperative computed tomography angiography scans were analyzed with a dedicated workstation, and anatomical characteristics of the aortic neck, iliac arteries, and aneurysm were compared in multivariable regression analyses; the outcomes are given as the odds ratio (OR) with 95% confidence interval (CI). The prognostic utility of several characteristics as predictors of rupture occurrence was examined with receiver operating characteristic (ROC) curves. Results: Anatomical characteristics differing significantly between elective and ruptured aneurysms were the infrarenal aortic neck diameters at 5 mm, 10 mm and 15 mm; the neck length and calcification; the common iliac artery (CIA) lengths; the iliac artery indexes; the left CIA and external iliac artery diameters; and the total and true lumen aneurysm volumes. Intraluminal thrombus (ILT) volume did not differ (p=0.76), although its distribution in elective vs ruptured cases did [absent: 0% vs 19%, respectively (p=0.025); circumferential: 61% vs 35%, respectively (p=0.04)]. Total aneurysm volume was higher in rAAA (442±140 mL) vs intact AAA (331±143 mL, p=0.014), while the ILT/total aneurysm volume rate was lower in rAAA (55%) vs intact AAA (70%, p=0.02). Multivariate analysis determined that a shorter left CIA (OR 1.07, 95% CI 1.01 to 1.1, p=0.016) and a smaller total aneurysm volume (OR 1.007, CI. 1.001 to 1.014, p=0.016) were associated with intact AAA. After a ROC curve analysis, left CIA length <50 mm demonstrated a lower incidence of rupture (sensitivity 60% and specificity 78%), while total aneurysm volume <380 mL had 60% sensitivity and specificity. Conclusion: Large rAAAs seem to have different anatomical characteristics than similarly sized intact AAAs. Large intact AAAs have lower total aneurysm volumes and shorter left CIAs, with higher ILT/aneurysm volume rates.

Role of intraluminal thrombus in abdominal aortic aneurysm ruptures: A hemodynamic point of view

PURPOSE

Intraluminal thrombus (ILTs) are found in most abdominal aortic aneurysms (AAAs) of clinically relevant size; however, the role of ILTs in AAA ruptures remains unclear. This study investigated the role of the presence and thickness of ILTs in AAA ruptures by analyzing the hemodynamic environment in ruptured AAAs (RAAAs).

METHODS

Three-dimensional reconstructions from computed tomography scans were performed, and 13 RAAA cases were categorized into a no-ILT group, a thin-layered ILT group (thickness < 3 mm), and a thick-layered ILT group. The hemodynamic features of the RAAAs were assessed using computational fluid dynamics simulation.

RESULTS

The thin- and thick-layered ILT groups showed significant differences in aneurysm diameters (P < 0.05). The three types of AAAs ruptured at different flow regions, with different hemodynamic features: (a) the no-ILT AAAs ruptured at regions of flow recirculation where velocity and wall shear stresses (WSSs) were close to zero; (b) the thin-layered ILT AAAs ruptured at sites at which the dominant flow impinged the wall; and (c) the thick-layered ILT AAAs ruptured at the border of the dominant flow channel and recirculation zone where the flow velocity and pressure changed dramatically.

CONCLUSIONS

Hemodynamic characteristics influence the rupture mechanisms of particular AAAs differently on the basis of the presence and thickness of ILTs. Recirculation flows and low WSSs may have negative effects by inducing local rupture or positive effects by promoting the formation of thin-layered ILTs. However, eccentrically located thick-layered ILTs may increase the rupture risk of small AAAs because of their location in the sac lumen, which results in chaotic flow patterns and rapid increases in flow resistance.

Evaluation of the distribution and progression of intraluminal thrombus in abdominal aortic aneurysms using high-resolution MRI

BACKGROUND

Intraluminal thrombus (ILT) signal intensity on MRI has been studied as a potential marker of abdominal aortic aneurysm (AAA) progression.

PURPOSE

1) To characterize the relationship between ILT signal intensity and AAA diameter; 2) to evaluate ILT change over time; and 3) to assess the relationship between ILT features and AAA growth.

STUDY TYPE

Prospective.

SUBJECTS

Eighty AAA patients were imaged, and a subset (n = 41) were followed with repeated MRI for 16 ± 9 months.

FIELD STRENGTH/SEQUENCE

3D black-blood fast-spin-echo sequence at 3 T.

ASSESSMENT

ILT was designated as "bright" if the signal was greater than 1.2 times that of adjacent psoas muscle. AAAs were divided into three groups based on ILT: Type 1: bright ILT; Type 2: isointense ILT; Type 3: no ILT. During follow-up, an active ILT change was defined as new ILT formation or an increase in ILT signal intensity to bright; stable ILT was defined as no change in ILT type or ILT became isointense from bright previously.

STATISTICAL TESTS

Shapiro-Wilk test; Mann-Whitney U-test; Fisher's exact test; Kruskal-Wallis test; Spearman's r; intraclass correlation coefficient (ICC), Cohen's kappa.

RESULTS

AAAs with Type 1 ILT were larger than those with Types 2 and 3 ILT (5.1 ± 1.1 cm, 4.4 ± 0.9 cm, 4.2 ± 0.8 cm, P = 0.008). The growth rate of AAAs with Type 1 ILT was significantly greater than that of AAAs with Types 2 and 3 ILT (2.6 ± 2.5, 0.6 ± 1.3, 1.5 ± 0.6 mm/year, P = 0.01). During follow-up, AAAs with active ILT changes had a 3-fold increased growth rate compared with AAAs with stable ILT (3.6 ± 3.0 mm/year vs. 1.2 ± 1.5 mm/year, P = 0.008).

DATA CONCLUSION

AAAs with bright ILT are larger in diameter and grow faster. Active ILT change is associated with faster AAA growth. Black-blood MRI can characterize ILT features and monitor their change over time, which may provide new insights into AAA risk assessment.

LEVEL OF EVIDENCE

2 Technical Efficacy Stage: 5 J. Magn. Reson. Imaging 2019;50:994-1001.

Aortic Lumen Area Is Increased in Ruptured Abdominal Aortic Aneurysms and Correlates to Biomechanical Rupture Risk

PURPOSE

To investigate how 2-dimensional geometric parameters differ between ruptured and asymptomatic abdominal aortic aneurysms (AAAs) and provide a biomechanical explanation for the findings.

METHODS

The computed tomography angiography (CTA) scans of 30 patients (mean age 77±10 years; 23 men) with ruptured AAAs and 60 patients (mean age 76±8 years; 46 men) with asymptomatic AAAs were used to measure maximum sac diameter along the center lumen line, the cross-sectional lumen area, the total vessel area, the intraluminal thrombus (ILT) area, and corresponding volumes. The CTA data were segmented to create 3-dimensional patient-specific models for finite element analysis to compute peak wall stress (PWS) and the peak wall rupture index (PWRI). To reduce confounding from the maximum diameter, 2 diameter-matched groups were selected from the initial patient cohorts: 28 ruptured AAAs and another with 15 intact AAAs (diameters 74±12 vs 73±11, p=0.67). A multivariate model including the maximum diameter, the lumen area, and the ILT area of the 60 intact aneurysms was employed to predict biomechanical rupture risk parameters.

RESULTS

In the diameter-matched subgroup comparison, ruptured AAAs had a significantly larger cross-sectional lumen area (1954±1254 vs 1120±623 mm², p=0.023) and lower ILT area ratio (55±24 vs 68±24, p=0.037). The ILT area (2836±1462 vs 2385±1364 mm², p=0.282) and the total vessel area (3956±1170 vs 4338±1388 mm², p=0.384) did not differ statistically between ruptured and intact aneurysms. The PWRI was increased in ruptured AAAs (0.80 vs 0.48, p<0.001), but the PWS was similar (249 vs 284 kPa, p=0.194). In multivariate regression analysis, lumen area was significantly positively associated with both PWS (p<0.001) and PWRI (p<0.01). The ILT area was also significantly positively associated with PWS (p<0.001) but only weakly with PWRI (p<0.01). The lumen area conferred a higher risk increase in both PWS and PWRI when compared with the ILT area.

CONCLUSION

The lumen area is increased in ruptured AAAs compared to diameter-matched asymptomatic AAAs. Furthermore, this finding may in part be explained by a relationship with biomechanical rupture risk parameters, in which lumen area, irrespective of maximum diameter, increases PWS and PWRI. These observations thus suggest a possible method to improve prediction of rupture risk in AAAs by measuring the lumen area without the use of computational modeling.

Temporal Changes in Intraluminal Thrombus Volume Within Abdominal Aortic Aneurysms: Implications for Planning Endovascular Aneurysm Sealing

PURPOSE

To explore whether or not there are temporal changes in the abdominal aortic aneurysm (AAA) and intraluminal thrombus (ILT) volumes between planning and implantation of the endovascular aneurysm sealing (EVAS) device and how these changes influence lumen volume.

METHODS

A retrospective review was conducted of 51 AAA patients (mean age 76±7.1 years; 36 men) in whom 2 serial preoperative computed tomography angiograms (CTAs) had been performed within 1 to 18 months before fenestrated endovascular repair. The 2 preoperative CTAs were analyzed to identify changes in total sac, ILT, and lumen volumes.

RESULTS

Over a median 7.0 months (interquartile range 4, 10), 46 (90%) of 51 AAAs increased in volume between the 2 CTAs. ILT volume increased in 44 aneurysms. In contrast, lumen volume increased in 31 and decreased in 20 AAAs. There was a strong correlation between changes in AAA volume and ILT volume (r_s=0.859, p<0.001), which remained significant after adjustment for initial volumes (r_s=0.815; p<0.001). There was no correlation between the time interval separating the 2 CTAs and changes in AAA volume (r_s=0.115; p=0.421), changes in ILT volume (r_s=0.084; p=0.599), or changes in lumen volume (r_s=0.060; p=0.676). The AAA growth rate (defined as the change in AAA size/days between CTAs) showed a weak correlation with ILT volume (r_s=0.272, p=0.054), which disappeared after adjustment for initial AAA size (r_s=-0.002, p=0.991). Between the 2 CTAs, 12 aneurysms crossed the new <1.4 Nellix maximum aorta/lumen diameter ratio.

CONCLUSION

As AAAs grow, the increase in aortic volume is largely occupied by additional ILT formation, with minimal change in lumen volume. These changes may alter the suitability of the aneurysm for the Nellix device and could have implications for EVAS planning and device deployment.

A comparison of hemodynamic metrics and intraluminal thrombus burden in a common iliac artery aneurysm

Aneurysms of the common iliac artery (CIAA) are typically found in association with an abdominal aortic aneurysm (AAA). Isolated CIAAs, in the absence of an AAA, are uncommon. Similar to AAAs, CIAA may develop intraluminal thrombus (ILT). As isolated CIAAs have a contralateral common iliac artery for comparison, they provide an opportunity to study the hemodynamic mechanisms behind ILT formation. In this study, we compared a large isolated CIAA and the contralateral iliac artery using computational fluid dynamics to determine if hemodynamic metrics correlate with the location of ILT. We performed a comprehensive computational fluid dynamics study and investigated the residence time of platelets and monocytes, velocity fields, time-averaged wall shear stress, oscillatory shear index, and endothelial cell activation potential. We then correlated these data to ILT burden determined with computed tomography. We found that high cell residence times, low time-averaged wall shear stress, high oscillatory shear index, and high endothelial cell activation potential all correlate with regions of ILT development. Our results show agreement with previous hypotheses of thrombus formation in AAA and provide insights into the computational hemodynamics of iliac artery aneurysms.

Fluid displacement from intraluminal thrombus of abdominal aortic aneurysm as a result of uniform compression

Objectives The results after aneurysm repair with an endovascular aneurysm sealing (EVAS) system are dependent on the stability of the aneurysm sac and particularly the intraluminal abdominal aortic thrombus (ILT). The postprocedural ILT volume is decreased compared with preprocedural ILT volume in aortic aneurysm patients treated with EVAS. We hypothesize that ILT is not stable in all patients and pressurization of the ILT may result in displacement of fluids from the ILT, no differently than serum is displaced from whole blood when it settles. To date, the mechanism and quantification of fluid displacement from ILT are unknown. Methods The study included 21 patients who underwent elective open abdominal aortic aneurysm repair. The ILT was harvested as a routine procedure during the operation. After excision of a histologic sample of the ILT specimen in four patients, ILT volume was measured and the ILT was compressed in a dedicated compression setup designed to apply uniform compression of 200 mmHg for 5 min. After compression, the volumes of the remaining thrombus and the displaced fluid were measured. Results The median (interquartile-range) of ILT volume before compression was 60 (66) mL, and a median of 5.7 (8.4) mL of fluid was displaced from the ILT after compression, resulting in a median thrombus volume decrease of 11% (10%). Fluid components can be up to 31% of the entire ILT volume. Histologic examination of four ILT specimens showed a reduction of the medial layer of the ILT after compression, which was the result of compression of fluid-containing canaliculi. Conclusions Applying pressure of 200 mmHg to abdominal aortic aneurysm ILT resulted in the displacement of fluid, with a large variation among patients. Fluid displacement may result in decrease of ILT volume during and after EVAS, which might have implications on pre-EVAS volume planning and on stability of the endobags during follow-up which may lead to migration, endoleak or both.

von Willebrand Factor and Prekallikrein in Plasma Are Associated With Thrombus Volume in Abdominal Aortic Aneurysms

OBJECTIVES

Disruption of the endothelial lining may be one of the events linking intraluminal thrombus and abdominal aortic aneurysm growth. In the present study, we examined whether von Willebrand factor activity in plasma, contact proteins of blood coagulation, and inflammatory biomarkers may be associated with intraluminal thrombus volume in search of a biochemical marker of endothelial damage and thrombus size.

DESIGN

Prospective study, correlating potential endothelial biomarkers and intraluminal thrombus volume acquired by computed tomography angiography.

MATERIALS AND METHODS

Plasma was consecutively obtained from 38 patients with asymptomatic infrarenal abdominal aortic aneurysm. von Willebrand factor activity, thrombin generation time, factor XII, and prekallikrein concentration were measured in plasma on automated and in-house platforms. In total, 8 patients were excluded due to ongoing anticoagulant therapy, renal impairment, or nonappearance, thus leaving 30 patients for further analysis. All patients had computed tomography angiography, and intraluminal volume was quantified off-line by OsiriX 6.5.

RESULTS

Median intraluminal thrombus volume was 42.7 mL. Spearman correlation analysis revealed a positive correlation between thrombus volume, von Willebrand factor activity (ρ = 0.56, P = .0013), and prekallikrein concentration in plasma (ρ = 0.54, P = .002).

CONCLUSION

von Willebrand factor activity and concentration of prekallikrein may both be of importance regarding the evolution of thrombus in abdominal aortic aneurysm and possible biomarkers for aneurysm growth.

Recanalization and Remarkable Outcome after Subocclusive Thrombus: A Case Report

Minimal literature exists regarding subocclusive thrombi and their management in acute ischemic stroke, and no randomized trials or guidelines are currently available. We present the case of an 83-year-old man with multiple vascular risk factors and recent cardiac surgery who presented with a severe left middle cerebral artery (MCA) syndrome due to a subocclusive thrombus. Intravenous (IV) tissue plasminogen activator was contraindicated given the patient's recent surgery, and endovascular therapy was deferred given improvement in symptoms. He was placed on IV heparin. His symptoms improved with recanalization of the MCA. Optimal management of subocclusive thrombi, which appear to have better outcomes than occlusive thrombi, has yet to be elucidated. Acute anticoagulation and endovascular therapy may both be reasonable; however, there are minimal data supporting the use of either and further studies are needed to develop the most favorable treatment algorithm. In the meantime, management decisions should be made on a case-by-case basis by multidisciplinary care teams.

Proteolytically active ADAM10 and ADAM17 carried on membrane microvesicles in human abdominal aortic aneurysms

The intraluminal thrombus (ILT) of human abdominal aortic aneurysm (AAA) has been suggested to damage the underlying aortic wall, but previous work found scant activity of soluble proteases in the abluminal layer of the ILT, adjacent to the aneurysm. We hypothesised that transmembrane proteases carried by membrane microvesicles (MV) from dying cells remain active in the abluminal ILT. ILTs and AAA segments collected from 21 patients during surgical repair were assayed for two major transmembrane proteases, ADAM10 (a disintegrin and metalloprotease-10) and ADAM17. We also exposed cultured cells to tobacco smoke and assessed ADAM10 and ADAM17 expression and release on MVs. Immunohistochemistry showed abundant ADAM10 and ADAM17 protein in the ILT and underlying aneurysmal aorta. Domain-specific antibodies indicated both transmembrane and shed ADAM17. Importantly, ADAM10 and ADAM 17 in the abluminal ILT were enzymatically active. Electron microscopy of abluminal ILT and aortic wall showed MVs with ADAM10 and ADAM17. By flow cytometry, ADAM-positive microvesicles from abluminal ILT carried the neutrophil marker CD66, but not the platelet marker CD61. Cultured HL60 neutrophils exposed to tobacco smoke extract showed increased ADAM10 and ADAM17 content, cleavage of these molecules into active forms, and release of MVs carrying mature ADAM10 and detectable ADAM17. In conclusion, our results implicate persistent, enzymatically active ADAMs on MVs in the abluminal ILT, adjacent to the aneurysmal wall. The production of ADAM10- and ADAM17-positive MVs from smoke-exposed neutrophils provides a novel molecular mechanism for the vastly accelerated risk of AAA in smokers.

Prediction of Rupture Sites in Abdominal Aortic Aneurysms After Finite Element Analysis

PURPOSE

To associate regions of highest local rupture risk from finite element analysis (FEA) to subsequent rupture sites in abdominal aortic aneurysms (AAA).

METHODS

This retrospective multicenter study analyzed computed tomography angiography (CTA) data from 13 asymptomatic AAA patients (mean age 76 years; 8 men) experiencing rupture at a later point in time between 2005 and 2011. All patients had CTA scans before and during the rupture event. FEA was performed to calculate peak wall stress (PWS), peak wall rupture risk (PWRR), rupture risk equivalent diameters (RRED), and the intraluminal thrombus volume (ILTV). PWS and PWRR locations in the prerupture state were compared with subsequent CTA rupture findings. Visible contrast extravasation was considered a definite (n=5) rupture sign, while a periaortic hematoma was an indefinite (n=8) sign. A statistical comparison was performed between the 13-patient asymptomatic AAA group before and during rupture and a 23-patient diameter-matched asymptomatic AAA control group that underwent elective surgery.

RESULTS

The asymptomatic AAAs before rupture showed significantly higher PWRR and RRED values compared to the matched asymptomatic AAA control group (median values 0.74 vs 0.52 and 77 vs 59 mm, respectively; p<0.0001 for both). No statistical differences could be found for PWS and ILTV. Ruptured AAAs showed the highest maximum diameters, PWRR, and RRED values. In 7 of the ruptured AAAs (2 definite and 5 indefinite rupture signs), CTA rupture sites correlated with prerupture PWRR locations.

CONCLUSION

The location of the PWRR in unruptured AAAs predicted future rupture sites in several cases. Asymptomatic AAA patients with high PWRR and RRED values have an increased rupture risk.

Wall stress in media layer of stented three-layered aortic aneurysm at different intraluminal thrombus locations with pulsatile heart cycle

At the point when the aorta ruptures suddenly, as opposed to as the after-effect of injury, it is for the most part in aortic aneurysm. Aortic aneurysm rupture happens when the wall stress surpasses the strength of the vascular tissue. Intraluminal thrombus (ILT) may have advantages as it can absorb tension and decrease aortic aneurysm wall stress. This study aims to investigate the presence and growth effects of ILT on the wall stress in a stented aneurysm in one heart cycle. A virtual stented aneurysm model with ILT was made to study the flow and wall dynamics using fluid-structure interaction (FSI) analysis. Wall stresses at the center line of media layer of aorta thickness were calculated by two-dimensional axisymmetric finite element analysis. Calculations were executed as thrombus elastic modulus increased from 0.1 to 2 MPa and calculations were repeated as thrombus depth was increased in 10% increment until thrombus filled the whole aneurysm cavity. The von Mises stresses were compared in three sections, namely proximal, aneurysm and distal sections in the abdominal aorta. The wall stress showed its maximum value during a peak flow and pressure and gradually decreased as the pressure and velocity of blood reduced in all three aforementioned sections. As the intraluminal thrombus depth increased from 10% to 100%, the wall stress in distal, proximal and centre of aneurysm during one heart cycle was decreased. Furthermore, increasing the elastic modulus of thrombus from 10% to 100% triggered a reduction in wall stress in proximal, centre of intraluminal thrombus and distal regions during one heart cycle. The achievements of this study may have implications not only for understanding the wall stress in ILT, but also for providing more detailed information about aortic aneurysm with intraluminal thrombus and can help surgeons to do their best.

Novel aspects of the pathogenesis of aneurysms of the abdominal aorta in humans

Aneurysm of the abdominal aorta (AAA) is a particular, specifically localized form of atherothrombosis, providing a unique human model of this disease. The pathogenesis of AAA is characterized by a breakdown of the extracellular matrix due to an excessive proteolytic activity, leading to potential arterial wall rupture. The roles of matrix metalloproteinases and plasmin generation in progression of AAA have been demonstrated both in animal models and in clinical studies. In the present review, we highlight recent studies addressing the role of the haemoglobin-rich, intraluminal thrombus and the adventitial response in the development of human AAA. The intraluminal thrombus exerts its pathogenic effect through platelet activation, fibrin formation, binding of plasminogen and its activators, and trapping of erythrocytes and neutrophils, leading to oxidative and proteolytic injury of the arterial wall. These events occur mainly at the intraluminal thrombus-circulating blood interface, and pathological mediators are conveyed outwards, where they promote matrix degradation of the arterial wall. In response, neo-angiogenesis, phagocytosis by mononuclear cells, and a shift from innate to adaptive immunity in the adventitia are observed. Abdominal aortic aneurysm thus represents an accessible spatiotemporal model of human atherothrombotic progression towards clinical events, the study of which should allow further understanding of its pathogenesis and the translation of pathogenic biological activities into diagnostic and therapeutic applications.