Thrombosis heralding aneurysmal rupture: an exploration of potential mechanisms in a novel giant swine aneurysm model

The surgical strategy and technical nuances of in situ side-to-side bypass for the management of complex intracranial aneurysms

Background

Despite continuous advances in microsurgical and endovascular techniques, the treatment of complex aneurysms remains challenging. Aneurysms that are dilemmatic for conventional clipping or endovascular coiling often require bypass as part of a strategy to reduce the risk of ischemic complications. In anatomically favorable sites, the intracranial-intracranial in situ bypass may be an appealing choice. This article details the surgical strategies, operative nuances, and clinical outcomes of this technique with a consecutive series in our department.

Methods

A retrospective review of a prospectively maintained neurosurgical patient database was performed to identify all patients treated with side-to-side in situ bypass from January 2016 to June 2022. In total, 12 consecutive patients, including 12 aneurysms, were identified and included in the series. The medical records, surgical videos, neuroimaging studies, and follow-up clinic notes were reviewed for every patient.

Results

Of the 12 aneurysms, there were 5 middle cerebral artery aneurysms, 4 anterior cerebral artery aneurysms, and 3 posterior inferior cerebellar artery aneurysms. The morphology of the aneurysms was fusiform in 8 patients and saccular in the remaining 4 patients. There were 3 patients presented with subarachnoid hemorrhage. The treatment modality was simple in situ bypass in 8 cases and in situ bypass combined with other modalities in 4 cases. Bypass patency was confirmed in all cases by intraoperative micro-doppler probe and (or) infrared indocyanine green (ICG) video angiography intraoperatively and with digital subtraction angiography (DSA) or computed tomography angiography (CTA) postoperatively. None of the patients developed a clinically manifested stroke due to the procedure though a callosomarginal artery was intentionally removed in one patient. The median follow-up period was 16.2 months (6-36). All patients had achieved improved or unchanged modified Rankin scale scores at the final follow-ups.

Conclusion

Cerebral revascularization technique remains an essential skill for the treatment of complex aneurysms. The in situ bypass is one of the most effective techniques to revascularize efferent territory when vital artery sacrifice or occlusion is unavoidable. The configuration of in situ bypass should be carefully tailored to each case, with consideration of variations in anatomy and pathology of the complex aneurysms.

Imaging of intracranial aneurysms in animals: a systematic review of modalities

Intracranial aneurysm (IA) animal models are paramount to study IA pathophysiology and to test new endovascular treatments. A number of in vivo imaging modalities are available to characterize IAs at different stages of development in these animal models. This review describes existing in vivo imaging techniques used so far to visualize IAs in animal models. We systematically searched for studies containing in vivo imaging of induced IAs in animal models in PubMed and SPIE Digital library databases between 1 January 1945 and 13 July 2022. A total of 170 studies were retrieved and reviewed in detail, and information on the IA animal model, the objective of the study, and the imaging modality used was collected. A variety of methods to surgically construct or endogenously induce IAs in animals were identified, and 88% of the reviewed studies used surgical methods. The large majority of IA imaging in animals was performed for 4 reasons: basic research for IA models, testing of new IA treatment modalities, research on IA in vivo imaging of IAs, and research on IA pathophysiology. Six different imaging techniques were identified: conventional catheter angiography, computed tomography angiography, magnetic resonance angiography, hemodynamic imaging, optical coherence tomography, and fluorescence imaging. This review presents and discusses the advantages and disadvantages of all in vivo IA imaging techniques used in animal models to help future IA studies finding the most appropriate IA imaging modality and animal model to answer their research question.

Activated Platelet-Homing Nanoplatform for Targeting Magnetic Resonance Imaging of Aneurysm-Related Thrombus in Rabbits

Thrombosis is closely related to the instability of intracranial aneurysm (IA), whose rupture is associated with high morbidity and mortality. It is difficult to detect an IA-related thrombus because traditional magnetic resonance imaging (MRI) and even contrast-enhanced MRI cannot clearly distinguish a thrombus from the surrounding tissues. Herein, a nanoplatform [(MFe₂O₄-ZnDPA nanoparticles (NPs)], consisting of Zn_0.4Co_0.6Fe₂O₄@Zn_0.4Mn_0.6Fe₂O₄ NPs for imaging and Zn(II)-bis(dipicolylamine) (ZnDPA) for thrombus targeting, is constructed to target an experimental aneurysm-related thrombus in rabbits via MRI. In vitro experiments including platelet safety evaluation primarily prove that MFe₂O₄-ZnDPA NPs with a high MRI transverse relaxation time (T₂) have good biocompatibility. MFe₂O₄-ZnDPA NPs could target a thrombus via the special interaction between ZnDPA and phosphatidylserine of activated platelets in the thrombus through MRI and Fe quantification assays. Moreover, after MFe₂O₄-ZnDPA NPs are injected into the ear vein of common carotid artery aneurysm model rabbits, MRI shows that MFe₂O₄-ZnDPA NPs could accumulate in the aneurysm-related thrombus from 0 to 15 min after injection and decrease in the next 45 min. Meanwhile, MFe₂O₄-ZnDPA NPs could decrease the MRI T₂ signal of the aneurysm-related thrombus to enhance the outline of the aneurysm. This study demonstrates that a nanoplatform can enhance the detection of an aneurysm-related thrombus as well as aneurysm itself to assist further treatment of IA.

In situ decellularization of a large animal saccular aneurysm model: sustained inflammation and active aneurysm wall remodeling

OBJECTIVE

To investigate in situ decellularization of a large animal model of saccular aneurysm as a strategy for achieving aneurysmal growth and lasting inflammation.

METHODS

18 New Zealand White rabbits were randomized 2:1 to receive endoluminal sodium dodecyl sulfate infusion (SDS, 1% solution, 45 min) following elastase or elastase-only treatment (control). All aneurysms were measured by digital subtraction angiography every 2 weeks. Every 2 weeks, three of the rabbits (two elastase + SDS, one control) underwent MRI, followed by contrast injection with myeloperoxidase (MPO)-sensing contrast agent. MRI was repeated 3 hours after contrast injection and the enhancement ratio (ER) was calculated. Following MRI, aneurysms were explanted and subjected to immunohistopathology.

RESULTS

During follow-up MRI, the average ER for SDS-treated animals was 1.63±0.20, compared with 1.01±0.06 for controls (p<0.001). The width of SDS-treated aneurysms increased significantly in comparison with the elastase aneurysms (47% vs 20%, p<0.001). Image analysis of thin sections showed infiltration of MPO-positive cells in decellularized aneurysms and surroundings through the 12-week observation period while control tissue had 5-6 times fewer cells present 2 weeks after aneurysm creation. Immunohistochemistry demonstrated the presence of MPO-positive cells surrounding decellularized lesions at early time points. MPO-positive cells were found in the adventitia and in the thrombi adherent to the aneurysm wall at later time points.

CONCLUSIONS

In situ decellularization of a large animal model of saccular aneurysms reproduces features of unstable aneurysms, such as chronic inflammation (up to 12 weeks) and active aneurysm wall remodeling, leading to continued growth over 8 weeks.

Flow-diverting stent and delayed intracranial bleeding: the case for discussing acquired von Willebrand disease

A unique feature of the flow-diverting stent (FDS) has rendered it useful in the endovascular treatment of selected intracranial aneurysms for the last decade. Delayed aneurysmal rupture and intracranial parenchymal bleeding are two leading hemorrhagic complications after FDS. It has recently been shown for the first time that there is a relationship between FDS and reduced level of vWF activity in patients undergoing endovascular cerebral aneurysm treatment. Here, the current literature is reviewed in the context of this novel finding to propose an illustrative scenario that conceptually links implantation of FDS to delayed intracranial bleeding, through the mechanism of shear-induced activation of vWF. In this scenario, after FDS implantation, sustained release of activated vWF in association with platelets plays a pivotal role in the mechanisms of delayed intracranial hemorrhages.

Saccular Aneurysm Models Featuring Growth and Rupture: A Systematic Review

BACKGROUND

Most available large animal extracranial aneurysm models feature healthy non-degenerated aneurysm pouches with stable long-term follow-ups and extensive healing reactions after endovascular treatment. This review focuses on a small subgroup of extracranial aneurysm models that demonstrated growth and potential rupture during follow-up.

METHODS

The literature was searched in Medline/Pubmed to identify extracranial in vivo saccular aneurysm models featuring growth and rupture, using a predefined search strategy in accordance with the PRISMA guidelines. From eligible studies we extracted the following details: technique and location of aneurysm creation, aneurysm pouch characteristics, time for model creation, growth and rupture rate, time course, patency rate, histological findings, and associated morbidity and mortality.

RESULTS

A total of 20 articles were found to describe growth and/or rupture of an experimentally created extracranial saccular aneurysm during follow-up. Most frequent growth was reported in rats (n = 6), followed by rabbits (n = 4), dogs (n = 4), swine (n = 5), and sheep (n = 1). Except for two studies reporting growth and rupture within the abdominal cavity (abdominal aortic artery; n = 2) all other aneurysms were located at the neck of the animal. The largest growth rate, with an up to 10-fold size increase, was found in a rat abdominal aortic sidewall aneurysm model.

CONCLUSIONS

Extracranial saccular aneurysm models with growth and rupture are rare. Degradation of the created aneurysmal outpouch seems to be a prerequisite to allow growth, which may ultimately lead to rupture. Since it has been shown that the aneurysm wall is important for healing after endovascular therapy, it is likely that models featuring growth and rupture will gain in interest for preclinical testing of novel endovascular therapies.

Reduced Activity of von Willebrand Factor after Flow-Diverting Stent Implantation for Intracranial Aneurysms: A Link to Acquired von Willebrand Disease?

BACKGROUND AND PURPOSE

Vascular devices generating high shear stress can cause type 2A acquired von Willebrand disease, which is characterized by low von Willebrand factor activity accompanied by hemorrhagic complications. The braided mesh structure of flow-diverting stents with a relatively small strut size can create abnormally high shear stress while arterial blood flows through the stent struts into the aneurysm, and flow-diverting stent may be associated with reduced von Willebrand factor activity.

MATERIALS AND METHODS

Aneurysmal morphologic parameters and patient data were examined retrospectively among patients who had an unruptured intracranial aneurysm treated with a flow-diverting stent. The RISTOtest (test for whole blood ristocetin-induced platelet aggregation) for von Willebrand factor activity, as well as tests for aspirin and clopidogrel/prasugrel effectiveness, were performed immediately before the endovascular procedure and 24 hours later by multiple electrode aggregometry.

RESULTS

A total of 39 patients with 56 aneurysms were recruited, and statistical analyses were performed in 32 patents with 49 aneurysms. Compared with the baseline values, von Willebrand factor activity was reduced in 16 patients but increased in 23 patients. Aneurysmal variables (eg, neck area, volume, volume-to-neck area ratio, size ratio, and morphologic index) clearly distinguished patients with reduced von Willebrand factor activity from those with nonreduced von Willebrand factor activity. The receiver operating characteristic curve showed that the morphologic index and volume had the highest discriminative power, with an area under the curve of 0.99.

CONCLUSIONS

In high-volume/large-neck aneurysms, flow-diverting stent implantation can cause reduced von Willebrand factor activity, which may be linked causally to acquired von Willebrand disease.

In vivo cerebral aneurysm models

Cerebral aneurysm rupture is a devastating event resulting in subarachnoid hemorrhage and is associated with significant morbidity and death. Up to 50% of individuals do not survive aneurysm rupture, with the majority of survivors suffering some degree of neurological deficit. Therefore, prior to aneurysm rupture, a large number of diagnosed patients are treated either microsurgically via clipping or endovascularly to prevent aneurysm filling. With the advancement of endovascular surgical techniques and devices, endovascular treatment of cerebral aneurysms is becoming the first-line therapy at many hospitals. Despite this fact, a large number of endovascularly treated patients will have aneurysm recanalization and progression and will require retreatment. The lack of approved pharmacological interventions for cerebral aneurysms and the need for retreatment have led to a growing interest in understanding the molecular, cellular, and physiological determinants of cerebral aneurysm pathogenesis, maturation, and rupture. To this end, the use of animal cerebral aneurysm models has contributed significantly to our current understanding of cerebral aneurysm biology and to the development of and training in endovascular devices. This review summarizes the small and large animal models of cerebral aneurysm that are being used to explore the pathophysiology of cerebral aneurysms, as well as the development of novel endovascular devices for aneurysm treatment.

Usefulness of the Pipeline Embolic Device for Large and Giant Carotid Cavernous Aneurysms

PURPOSE

Conventional coil embolization for large carotid cavernous aneurysms (CCAs) has limited utility due to its inability to prevent recurrences and reduce mass effect. Trapping of the parent artery may have a risk of ischemic complications due to intracranial perfusion disorders. We successfully treated 24 patients with large CCAs using a flow diverter (Pipeline™ embolic device: PED), and this report discusses the safety and efficacy of this method.

MATERIALS AND METHODS

Twenty four patients (23 females, mean age 71.5 years old) with large CCAs, including 6 giant CCAs, were treated with a PED over three years. Under sufficient dual anti-platelet management, the PED was deployed over the orifice of the aneurysm. Two patients required multiple telescoping stents. Clinical and radiological states were checked with MRI at 1, 3 and 6 months post-surgically. Angiographic follow-up was performed at 6 months.

RESULTS

In all patients, PED was appropriately deployed. Stagnation of contrast with eclipse signs was observed post-angiogram in 21 cases. One patient requiring 5 telescoping stents experienced temporary ischemic symptoms. Fourteen patients experienced improvement of ocular motor impairment deficiency, including 6 patients who recovered. Angiograms at 6 months follow-up showed complete occlusion in 63% (12/19) of patients, and MRI showed reduction of aneurysm volume in 89% (17/19) of patients.

CONCLUSION

Flow diverters for large CCAs showed promising clinical and radiological efficacy. They can shrink the aneurysm and improve symptoms without sacrificing the parent artery. It will be necessary to summarize the cases and to verify the long-term results.

Very large and giant microsurgical bifurcation aneurysms in rabbits: Proof of feasibility and comparability using computational fluid dynamics and biomechanical testing

BACKGROUND

Giant aneurysms are challenging lesions with unacceptable high rates of aneurysm recanalization and rerupture following embolization. Reliable in vivo models are urgently needed to test the performance of new more efficient endovascular devices.

MATERIALS AND METHODS

Aneurysms were created in 11 New Zealand white rabbits (4.5-5.5kg): A long venous pouch (length 25-30mm) mimicking the aneurysm sac was derived from the external jugular vein and sutured into a microsurgically created bifurcation between both common carotid arteries. After 4 weeks the rabbits underwent 3T Magnetic resonance angiography (3T-MRA). Exemplary computational fluid dynamics (CFD) simulations were performed to compare the flow conditions of giant rabbit and human aneurysms. We used species-related boundary conditions, in particular, we measured blood viscosity values. Biaxial mechanical tests were performed for the mechanical characterization and comparison.

COMPARISON WITH EXISITING METHOD(S)

None.

RESULTS

No peri- or postoperative mortality was observed. 3T-MRA showed aneurysm patency in 10 out of 11 aneurysms (90.9%). Aneurysm lengths ranged from 21.5-25.6mm and aneurysm necks from 7.3-9.8mm. CFD showed complex flow profiles with multiple vortices in both, rabbit and human aneurysms. Lower blood viscosity values of the rabbit (3.92mPas vs. human 5.34mPas) resulted in considerable lower wall shear stress rates (rabbit 0.38Pa vs. human 1.66Pa). Mechanical tests showed lower stiffness of rabbit aneurysms compared to unruptured human aneurysms.

CONCLUSIONS

The proposed model showed favorable aneurysm patency rates, low morbidity and good hemodynamic comparability with complex flow patterns. Biomechanical testing suggests that experimental aneurysms might be even more fragile compared to human aneurysms.

Bypass Surgeries in the Treatment of Cerebral Aneurysms

BACKGROUND

During surgery for cerebral aneurysm, revascularization techniques are occasionally needed to (1) treat an aneurysm (trapping or flow alteration); (2) preserve blood flow during temporary parent artery occlusion (insurance); and (3) repair accidentally injured vessels (troubleshooting). Herein we present our surgical case experiences.

METHODS

Revascularization modalities were employed in 33 (7.6 %) of 452 cases of surgically treated aneurysms. The aneurysm locations and associated required bypass procedures were: (1) 7 middle cerebral artery (MCA) aneurysms with 7 superficial temporal artery (STA)-MCA bypass procedures; (2) 10 internal carotid artery (ICA) aneurysms with 9 high-flow and 1 STA-MCA procedures; (3) 10 vertebro-basilar artery aneurysms with 2 high-flow, 6 occipital artery (OA)-posterior ICA, and 1 STA-superior cerebellar artery (SCA) procedures; (4) 1 posterior cerebral artery (PCA) aneurysm with OA-PCA bypass; and (5) 5 anterior cerebral artery aneurysms with 4 A3-A3 and 1 A3-STA-A3 procedure. Curative bypasses for aneurysmal treatment, temporary bypasses, and troubleshooting procedures were performed in 25, 3, and 5 cases, respectively.

RESULTS

Among the 26 aneurysms treated via curative bypass, 16 aneurysms that were trapped or clipped using revascularization techniques had better outcomes (no aneurysmal rupture and 1 perforator infarction), whereas among the 10 aneurysms that could not be trapped or clipped and were thereby treated via flow alteration (e.g., bypass plus proximal artery clipping), 2 developed symptomatic infarction and 2 exhibited aneurysmal rupture after partial thrombosis. Patients whose bypass procedures were used for temporary parent artery occlusion (insurance) or troubleshooting had no complications.

CONCLUSION

Complex aneurysm clipping or trapping using bypass techniques yielded good results. In particular, perforator vessel ischemia still requires resolution. Flow alteration techniques leading to aneurismal thrombosis carried the risks of ischemic and hemorrhagic complications when applied to intracranial aneurysms. Bypasses for temporary use or troubleshooting were quite effective.

MR imaging of myeloperoxidase activity in a model of the inflamed aneurysm wall

BACKGROUND AND PURPOSE

Although myeloperoxidase activity in vivo can be visualized by using noninvasive imaging, successful clinical translation requires further optimization of the imaging approach. We report a motion-sensitized driven-equilibrium MR imaging approach for the detection of a myeloperoxidase activity-specific gadolinium-containing imaging agent in experimental aneurysm models, which compensates for irregular blood flow, enabling vascular wall imaging in the aneurysm.

MATERIALS AND METHODS

A phantom was built from rotational angiography of a rabbit elastase aneurysm model and was connected to a cardiac pulse duplicator mimicking rabbit-specific flow conditions. A T1-weighted turbo spin-echo-based motion-sensitized driven-equilibrium pulse sequence was optimized in vitro, including the addition of fat suppression and the selection of the velocity-encoding gradient parameter. The optimized sequence was applied in vivo to rabbit aneurysm models with and without inflammation in the aneurysmal wall. Under each condition, the aneurysms were imaged before and after intravenous administration of the imaging agent. The signal-to-noise ratio of each MR imaging section through the aneurysm was calculated.

RESULTS

The motion-sensitized driven-equilibrium sequence was optimized to reduce flow signal, enabling detection of the myeloperoxidase imaging agent in the phantom. The optimized imaging protocol in the rabbit model of saccular aneurysms revealed a significant increase in the change of SNR from pre- to post-contrast MR imaging in the inflamed aneurysms compared with naïve aneurysms and the adjacent carotid artery (P < .0001).

CONCLUSIONS

A diagnostic MR imaging protocol was optimized for molecular imaging of a myeloperoxidase-specific molecular imaging agent in an animal model of inflamed brain aneurysms.

Delayed rebleeding of a spontaneously thrombosed aneurysm after subarachnoid hemorrhage

Background:

This report provides a rare documentation of spontaneous thrombosis of a ruptured aneurysm followed by delayed recanalization and subsequent rerupture.

Case Description:

A 47-year-old female presented with spontaneous subarachnoid hemorrhage (SAH). Four aneurysms were identified on CT angiogram including a basilar apex aneurysm, considered source of bleeding. Cerebral angiogram on postbleed day (PBD) #1 showed spontaneous thrombosis of basilar apex aneurysm. The patient was discharged to a nursing home on PBD #18 after two subsequent studies showed no recanalization of the basilar aneurysm. The patient returned on PBD #26 with a second episode of spontaneous SAH. The previously thrombosed basilar aneurysm had recanalized and reruptured, which was now treated with coil embolization.

Conclusion:

We are not aware of a previous report of saccular cerebral aneurysm documenting spontaneous thrombosis after SAH and recanalization with second hemorrhage. This occurrence presents a dilemma regarding the timing and frequency of subsequent cerebrovascular imaging and treatment.

Secondary histomorphological changes in cerebral arteries of normotensive and hypertensive rats following a carotid-jugular fistula induction

Haemodynamic changes in cerebral circulation are associated with the natural ageing process and associated pathology, leading to the development of incapacitating neurological and neurovascular diseases. Due to inherent biological limitations, current literatures mostly aimed at studying the correlation descriptively or quantifying the relationship in vitro or using computational models. In this paper, a model of a carotid-jugular fistula in the rat was used to create a haemodynamic insult to the intracranial arterial circulation and subsequent venous drainage. An arterial-venous (AV) fistula was created in 12 rats, 6 of which are normotensive Wistar-Kyoto strain (WKY) and the rest spontaneously hypertensive strain (SHR) with an additional 6 in each strains designed as controls without previous surgery. After 4 weeks of convalescence, all 24 rats were euthanised and their cerebral circulation was examined histomorphologically. We confirmed an intrinsic morphological difference between normotensive WKY and hypertensive SHR and found a modest but significant arterial shrinkage in both strains induced with AV fistula. We also reported that alterations in blood flow are also associated with marked extracellular matrix changes. We concluded that the model was suitable for studying the relative contributions of altering haemodynamic patterns and venous drainage on cerebrovascular changes. We also found that hypertension modulated cerebral vascular changes in addition to disrupted blood flow.

Aneurysm rupture after endovascular flow diversion: the possible role of persistent flows through the transition zone associated with device deformation

Haemorrhagic complications can occur following aneurysm treatment with flow diverters (FD), but the underlying mechanism remains unknown. We describe a case where deformation of the device may have contributed to the complication. A patient with a giant, previously unruptured cavernous aneurysm that extended intracranially to cause oedema of the internal capsule was treated with flow diversion. Treatment was followed by multiple episodes of peri-aneurysmal haemorrhages within eight days. A deformation of the device which occurred where it curved to cross the aneurysm neck created residual flows which, in the presence of a stent stenosis immediately beyond the neck, may have contributed to the observed ruptures. Following multiple haemorrhages the patient subsequently died. Autopsy demonstrated early red thrombus partially bridging the struts of the flow diverter, and intra-aneurysmal thrombus of various ages. Microscopic pathology showed an aneurysm wall consisting of collagen infiltrated with neutrophils, but the wall was absent near the cerebral peduncle, adjacent to the brain haemorrhage. Radiographs of the extracted specimen confirmed deformation of the FD construct, located at the transition zone of the stent, leading to increased pore size and porosity. The site of the deformation correlated with the angiographic presence of a continued blood inflow jet into the aneurysm. Stent deformation at the transition zone may promote persistent blood entry into the aneurysm, and in turn potentially contribute to haemorrhagic complications.