Hemodynamic changes in arterial feeders and draining veins during embolotherapy of arteriovenous malformations: an experimental study in a swine model

Real-Time MRI Monitoring of Liquid Embolic Agent (Onyx) Injection in a Swine Arteriovenous Malformation Model

The paradigm is gradually shifting, with radiosurgery and endovascular embolization being increasingly chosen over surgical resection in the selected cases of brain arteriovenous malformations. Routinely used X-ray monitoring of liquid embolic infusion has very good spatial and temporal resolution but is not without significant drawbacks regarding poor visualization of the complex AVM angioarchitecture, especially after many embolizations in the past and therefore limiting the technical ability of the embocure-total occlusion of the feeding arteries, nidus, and draining veins. The purpose of this study was to evaluate the use of real-time MRI guidance in endovascular embolization with Onyx (instead of X-ray) in a single swine rete mirabile (RM) AVM model in order to provide the scaffolding for the real-time MRI guidance method. Onyx propagation was observed in real-time dynamic GE-EPI scan with initial ipsilateral RM filling followed by main cerebral arterial branch distribution. The relatively bright signal within RM and the brain prior to Onyx injection provided a good background for the dark, low signal of the embolic agent spreading in rete mirabile and brain arteries. X-ray picture confirmed Onyx cast distribution at the end of the procedure. In this initial experience, real-time MRI seems to be a promising method that may significantly improve liquid embolic agent infusion monitoring in the future, although requiring further development before clinical use.

Possible Association Between Rupture and Intranidal Microhemodynamics in Arteriovenous Malformations: Phase-Contrast Magnetic Resonance Angiography-Based Flow Quantification

OBJECTIVE

To examine a potential association between intranidal microhemodynamics and rupture using a phase-contrast magnetic resonance angiography (PCMRA)-based flow quantification technique in arteriovenous malformations (AVMs).

METHODS

We retrospectively collected data on 30 consecutive patients with AVMs (23 unruptured and 7 ruptured). Based on PCMRA data, maximal (V_max) and mean (V_mean) intranidal velocities were calculated. Logistic regression analysis was performed to assess factors associated with previous AVM rupture.

RESULTS

All ruptures occurred within 6 months before PCMRA. The mean nidus volume was 4.7 mL. Eleven patients (37%) had deep draining vein(s), and 6 patients (20%) had a deep-seated nidus. The mean ± standard deviation V_mean and V_max were 9.6 ± 2.8 cm/second and 66.7 ± 26.2 cm/second, respectively. The logistic regression analyses revealed that higher V_max (P = 0.075, unit odds ratio [OR] = 1.05, 95% confidence interval [95% CI] = 1.00-1.10) was significantly associated with prior hemorrhage. The receiver-operating curve analyses demonstrated that a V_mean of 10.8 cm/second (area under the curve = 0.671) and V_max of 90.2 cm/second (area under the curve = 0.764) maximized the Youden Index. A V_max > 90 cm/second was significantly associated with AVM rupture both in the univariate (P = 0.025, OR = 9.0, 95% CI = 1.3-61.1) and multivariate (P = 0.008, OR = 51.7, 95% CI = 2.8-968.3) analyses.

CONCLUSIONS

Presence of faster velocities in intranidal vessels may suggest aberrant microhemodynamics and thus be associated with AVM rupture. PCMRA-based velocimetry seems to be a promising tool to predict future AVM rupture.

Assessment of human placenta as an ex-vivo vascular model for testing of liquid embolic agent injections with adjunctive techniques

PURPOSE

This project sought to test the utility of post-delivery human placenta (HP) as a vascular model for liquid embolic agent (LEA) simulation, along with adjunctive techniques.

MATERIALS AND METHODS

Twelve LEA injections were performed under fluoroscopy in HP with two reflux control methods: dual lumen 'mini' balloon-catheter (n=9); and injection after proximal nBCA plug formation through a second microcatheter ('pressure cooker') (n=3). Measured outcomes included liquid embolic agent (LEA) advancement and reflux. Reflux was categorized into three grades: grade 0=no reflux; grade 1=occlusion of side branches without reflux beyond the balloon or plug; and grade 2=reflux beyond the balloon or plug.

RESULTS

Simulation success was greater when a balloon was used rather than with a nBCA plug (89% vs 33%, P=0.054). In eight successful balloon-assisted injections, the reflux grades were: 50% grade 0; 12.5% grade 1; and 37.5% grade 2. The one successful nBCA plug injection had grade 2 reflux. All grade 2 balloon injections occurred when the balloon was positioned across a vessel bifurcation.

CONCLUSIONS

HP provides excellent simulation for liquid embolic agents with a dual lumen balloon catheter.

Animal Models in Studying Cerebral Arteriovenous Malformation

Brain arteriovenous malformation (AVM) is an important cause of hemorrhagic stroke. The etiology is largely unknown and the therapeutics are controversial. A review of AVM-associated animal models may be helpful in order to understand the up-to-date knowledge and promote further research about the disease. We searched PubMed till December 31, 2014, with the term “arteriovenous malformation,” limiting results to animals and English language. Publications that described creations of AVM animal models or investigated AVM-related mechanisms and treatments using these models were reviewed. More than 100 articles fulfilling our inclusion criteria were identified, and from them eight different types of the original models were summarized. The backgrounds and procedures of these models, their applications, and research findings were demonstrated. Animal models are useful in studying the pathogenesis of AVM formation, growth, and rupture, as well as in developing and testing new treatments. Creations of preferable models are expected.

Transvenous Approach to Intracranial Arteriovenous Malformations

A compartmental conceptualization of intracranial arteriovenous malformations (AVMs) allows recognition of feeding arteries, an intervening plexiform nidus, and draining veins. AVM therapy involves eliminating the nidus, which is the source of hemorrhage, without compromising normal arterial and venous drainage of the brain. Traditional methods of AVM therapy through microsurgery and endovascular embolization involve arterial devascularization, with preservation of AVM venous drainage, until the nidus is excluded. The transvenous approach in treating vascular malformations was popularized by successful treatment models for dural arteriovenous fistulas. More recently, high-flow intracranial AVMs are being managed with transvenous endovascular approaches, although this novel technique has its challenges and perils. We review the current literature on transvenous AVM therapy and highlight its role for AVM therapy in the present day.

Normal perfusion pressure breakthrough phenomenon: experimental models

One of the most life-threatening complications after the obliteration of intracranial arteriovenous malformations is the development of oedema and/or multifocal haemorrhage. Two main theories have been postulated so far in order to explain this situation. On one hand, "normal perfusion pressure breakthrough phenomenon" is based on the loss of cerebral vessel autoregulation due to the chronic vasodilation of perinidal microcirculation. On the other hand, the "occlusive hyperaemia" deals with thrombotic and venous obstruction phenomena that may also generate such manifestations. The aim of this study is to resume the main concepts of the "normal perfusion pressure breakthrough phenomenon" theory as well as the related animal models described up to date, their advantages and disadvantages, and the main conclusions obtained as a result of the experimental research.

Brain arteriovenous malformation modeling, pathogenesis, and novel therapeutic targets

Patients harboring brain arteriovenous malformation (bAVM) are at life-threatening risk of rupture and intracranial hemorrhage (ICH). The pathogenesis of bAVM has not been completely understood. Current treatment options are invasive, and ≈ 20 % of patients are not offered interventional therapy because of excessive treatment risk. There are no specific medical therapies to treat bAVMs. The lack of validated animal models has been an obstacle for testing hypotheses of bAVM pathogenesis and testing new therapies. In this review, we summarize bAVM model development and bAVM pathogenesis and potential therapeutic targets that have been identified during model development.

Direct continuous measurement of draining vein pressure during Onyx embolization in a swine arteriovenous malformation model

OBJECTIVE

Periprocedural intracranial hemorrhage secondary to intranidal flow redirection may develop after arteriovenous malformation (AVM) embolization. We hypothesized that continuous draining vein pressure monitoring may identify clinically relevant hemodynamic changes during devascularization. Our goal was to characterize the draining vein pressures in a swine rete mirabile AVM model during embolization with Onyx.

METHODS

An acute swine AVM model was constructed in six animals. Baseline, transoperative and final AVM area measurements were used to determine the degree of AVM embolization. Continuous video recordings were captured at 10 s intervals of active embolization. Draining vein pressure, arterial feeder pressure and heart rate were continuously monitored.

RESULTS

The baseline and post-embolization mean draining vein pressures were 49.8±17.2 and 33.0±11.7 mm Hg (p=0.01), mean arterial pressures were 79.8±19.4 and 79.6±25.2 mm Hg (p=0.94), mean transnidal pressures were 35.8±19.7 and 45.4±33.7 mm Hg (p=0.37) and mean heart rates were 81.1±11.9 and 83.1±12.8 bpm (p=0.38), respectively. The draining vein pressure was averaged according to the degree of AVM embolization and represented as a relative change compared with the baseline draining vein pressure, and the slopes were found to decrease in all cases (p=0.02). In half of the animals the draining vein pressure decreased progressively as the AVM was embolized. In the remaining animals the venous pressure only started to decline after the AVM had been devascularized by > 50%.

CONCLUSIONS

The draining vein pressure response during Onyx embolization in the swine AVM model is heterogeneous. Continuous draining vein pressure monitoring is feasible and may potentially identify clinically relevant hemodynamic changes during AVM embolization.

Is a swine model of arteriovenous malformation suitable for human extracranial arteriovenous malformation? A preliminary study

OBJECTIVE

A chronic arteriovenous malformation (AVM) model using the swine retia mirabilia (RMB) was developed and compared with the human extracranial AVM (EAVM) both in hemodynamics and pathology, to see if this brain AVM model can be used as an EAVM model.

METHODS

We created an arteriovenous fistula between the common carotid artery and the external jugular vein in eight animals by using end-to-end anastomosis. All animals were sacrificed 1 month after surgery, and the bilateral retia were obtained at autopsy and performed hematoxylin and eosin staining and immunohistochemistry. Pre- and postsurgical hemodynamic evaluations also were conducted. Then, the blood flow and histological changes of the animal model were compared with human EAVM.

RESULTS

The angiography after operation showed that the blood flow, like human EAVM, flowed from the feeding artery, via the nidus, drained to the draining vein. Microscopic examination showed dilated lumina and disrupted internal elastic lamina in both RMB of model and nidus of human EAVM, but the thickness of vessel wall had significant difference. Immunohistochemical reactivity for smooth muscle actin, angiopoietin 1, and angiopoietin 2 were similar in chronic model nidus microvessels and human EAVM, whereas vascular endothelial growth factor was significant difference between human EAVM and RMB of model.

CONCLUSIONS

The AVM model described here is similar to human EAVM in hemodynamics and immunohistochemical features, but there are still some differences in anatomy and pathogenetic mechanism. Further study is needed to evaluate the applicability and efficacy of this model.

Development of a cerebral microvascular dysplasia model in rodents

Normal vasculature development of the central nervous system is extremely important because patients with vascular malformations are at life-threatening risk for intracranial hemorrhage or cerebral ischemia. The etiology and pathogenesis of abnormal vasculature development in the central nervous system are unknown, and progress is hampered by the lack of animal models for human cerebrovascular diseases. Here, we report our current study on cerebral microvascular dysplasia (CMVD) development. Using vascular endothelial growth factor hyper-stimulation, we demonstrated that aberrant microvessels could be developed in the rodent brain under certain conditions (such as genetic deficient background, local cytokine and chemokine release, or exogenous vessel dilating stimulation) that may speed up focal angiogenesis and lead to cerebral vascular dysplasia.

An Arteriovenous Malformation Model for Stereotactic Radiosurgery Research

OBJECTIVE

To introduce the utilization of a swine arteriovenous malformation (AVM) model for stereotactic radiosurgery research and to describe the morphological changes in the vessels after radiation.

METHODS

The model was created in six animals by creation of a right-sided carotid-jugular fistula. Pre- and postsurgical hemodynamic evaluation was performed. The left rete was radiated in four animals; two animals were not radiated. All animals were sacrificed 4 months after surgery, and the bilateral retia were obtained at autopsy.

RESULTS

There were no procedure-related complications. A pressure gradient of 20 mmHg across the nidus was obtained after surgery. The peak velocity in the arterial feeder increased from 18.5 to 83 cm/s. Microscopic examination of the control animals showed intimal hyperplasia and disrupted internal elastic lamina, similar to human AVMs. The radiated retia showed more prominent intimal hyperplasia. This was confirmed by histometric studies showing greater luminal occlusion in radiated specimens. Adventitial fibrosis was prominent in the radiated retia and was absent in the control animals. Immunohistochemical studies showed proliferating smooth muscle cells in the intima. The adventitial fibrosis consisted of smooth muscle cells surrounded by collagen Type IV extracellular matrix.

CONCLUSION

The nidus component and high-flow vasculopathy make this an attractive model for stereotactic radiosurgery research. Histology of the radiated models is similar to those described in radiated human AVMs. Further studies of the model are warranted to gain a better understanding of the cellular and molecular events in AVM vessels after stereotactic radiosurgery.

Brain aneurysms and arteriovenous malformations: advancements and emerging treatments in endovascular embolization

BACKGROUND AND PURPOSE

Brain aneurysms and vascular malformations can cause cerebral hemorrhages, with devastating consequences for the patients and their families. Since the development of microcatheters and materials used for endovascular embolization, we have witnessed a rapid advancement in the technology and in the number or patients treated with this approach. The aim of this review is to survey recent data relevant to new technologies and emerging treatment strategies in these areas.

SUMMARY OF REVIEW

Clinical trials assessing the safety and efficacy of coil embolization for cerebral aneurysms were based on the use of bare platinum, helical coils. Since then, endovascular operators have been testing and using new materials such as bioactive coils, expandable coils, and complex-shaped coils. Based on the data so far obtained, third and fourth generation coil designs are rapidly emerging and will be ready for clinical application in the near future. Balloon- and stent-assisted coil embolization is enabling the treatment of complex, large-neck aneurysms and the vascular reconstruction of lesions previously considered not treatable. New open- and closed-cell designs allow the navigation and deployment of stents in extremely tortuous vessels. With regards to the embolization of vascular malformations, it is possible to safely navigate microcatheters and microwires through very small arteries previously considered not accessible. In addition, embolization materials such as n-butyl cyanoacrylate and ethylene-vinyl alcohol copolymer are now routinely injected to safely reduce or obliterate large and complex arteriovenous malformations and fistulae.

CONCLUSIONS

Advancements in technology are rapidly improving the endovascular approach to the treatment of cerebral aneurysms and arteriovenous malformations.

Calcium alginate gel as a biocompatible material for endovascular arteriovenous malformation embolization: six-month results in an animal model

OBJECTIVE

We sought to expand our assessment of calcium alginate as an embolic agent in an animal model of a cerebral arteriovenous malformation (AVM). The objective of this study was to assess the long-term biocompatibility and stability of calcium alginate in AVM swine models that survived from 1 to 6 months.

METHODS

The swine model included a carotid-jugular anastomosis to redirect flow to the rete mirabile (RM), thereby simulating flow to an AVM. Alginate and the reactive component, calcium chloride, were injected from double-lumen or concentric-tube microcatheters to form an occlusion of the RM feeding vessel and the inferior portion of the RM.

RESULTS

Angiography and histology verified complete occlusion of the RM feeding vessel for up to 6 months in eight of nine swine. Blood flow remained open to the superior portion of the RM and the circle of Willis. No evidence of downstream calcium alginate gel was seen in the follow-up angiograms or the histological preparations of the circle of Willis. A minor bioactive response to the alginate gel was noted at 1 month, yet no degenerative or inflammatory response was seen. At 6 months, there was moderate fibrous tissue around the alginate, which further sealed off flow to the embolized areas of the RM.

CONCLUSION

Over a period of 6 months, calcium alginate was an effective endovascular occlusion material that blocked blood flow to the inferior portion of the RM. The chronic AVM model verified the long-term stability and biocompatibility of calcium alginate.

Visualization and quantification of flow and velocity fields in intracranial arteriovenous malformations using phase-contrast MR angiography

OBJECTIVE

The objective of this study was to describe postprocessing tools for MR phase-contrast flow quantification images and apply those tools to cerebral arteriovenous malformations (AVMs) to visualize blood flow dynamics noninvasively.

CONCLUSION

Inflow and outflow zones were clearly depicted at different regions in the AVM. The processed images showed flow patterns including vortical flow and variations in velocity over the cardiac cycle. Particle tracking gave an impression of the overall flow state and of the venous drainage system in particular.

Stereotactic Radiosurgery of the Rete Mirabile in Swine: A Longitudinal Study of Histopathological Changes

OBJECTIVE:

Stereotactic radiosurgery is an established, effective treatment for brain arteriovenous malformations. The mechanisms of vessel occlusion in arteriovenous malformations has not been extensively evaluated. To better understand these mechanisms, we report histopathological changes in the swine rete mirabile after stereotactic radiosurgery.

METHODS:

Thirty-five swine were used, 15 as nonradiated controls and 20 as radiated. Two in the control group and five in the radiated group were sacrificed before the study endpoint. Tissue was obtained from 13 nonradiated (4 at 3 mo, 5 at 6 mo, 4 at 9 mo) and 15 radiated swine (2 at 3 mo, 3 at 6 mo, 10 at 9 mo) for histological, immunohistochemical, and morphometric analysis.

RESULTS:

Radiated vessels showed increasing intimal hyperplasia over the follow-up period. Histometrical analysis confirmed this with evidence of progressive luminal narrowing over the follow-up period. Immunohistochemical analysis showed intimal cells to be proliferating smooth muscle cells with surrounding extracellular collagen Type IV. Adventitial fibrosis composed of collagen Type IV was also seen with smooth muscle cells interspersed within the collagen matrix. The nonradiated animals showed no intimal hyperplasia or change in the appearance or size of the vessels over the same follow-up period. Adventitial fibrosis was minimal in the nonradiated animals.

CONCLUSION:

The vessels show an intimal response to radiation with progressive occlusion caused by migrating, proliferating smooth muscle cells, a likely source of the extracellular collagen in the intima. Cytokine mediated pathways likely produce these morphological changes. Future studies will be directed toward elucidating these underlying molecular mechanisms.

Experimental cerebral arteriovenous fistulas

OBJECTIVE

The aim of this paper is to analyse and summarize the main advances in experimental research on cerebral arteriovenous fistulas.

METHODS

A detailed analysis of the literature and my own research experience were employed to outline the methodology whereby experimental cerebral arteriovenous fistulas are created and further studied.

RESULTS

The analysis and quantification of the anatomical and functional variables in different experimental cerebral arteriovenous fistula models make it possible to develop more appropriate and individual ways of treatment in affected patients.

CONCLUSION

Experimental research on cerebral arteriovenous fistulas helps physicians to understand and predict more accurately the future evolution of arteriovenous malformations in humans.

Therapeutic management of cerebral arteriovenous malformations. Present role of interventional neuroradiology

This chapter summarizes the authors' experience in the endovascular therapy of cerebral arteriovenous malformations (AVMs). This clinical series includes 660 patients treated from 1980 to 2005. The first 148 patients were treated at University Hospital, in London, Ontario Canada, in association with Drs. Allan Fox, Dave Pelz, John Girvin and Charles Drake. The next 512 patients were treated at UCLA Medical Center, Los Angeles, California in association with Drs. Gary Duckwiler, Reza Jahan, Jacques Dion, Pierre Gobin, Neil Martin and John Frazee. Only patients treated with superselective endovascular/intraoperative catheterization and embolization of avm arterial feeders were included. Cerebral arteriovenous malformations treated by non-selective injection of beads in ICA or vertebral arteries were excluded. Modern neuroimaging modalities associated to the anatomical, topographic and functional evaluations of cerebral avms such as brain CT and CTA, MRI, MRA and functional MRI are all utilized at UCLA Medical Center. They have become essential in the therapeutic management of avms closely related to cerebral eloquent areas (figure 1).

Angiogenesis in the brain during development: the effects of vascular endothelial growth factor and angiopoietin-2 in an animal model

OBJECT

The goal of this study was to examine the roles of vascular endothelial growth factor (VEGF) and angiopoietin-2 (Ang-2) in the formation of blood vessels in the brain in a developmental animal model not routinely used for such a study.

METHODS

Either VEGF, Ang-2, or a combination of the two factors were injected into the optic tectum of 4-day-old quail embryos. Immunohistochemical analysis and laser confocal microscopy were used to observe the effects on endothelial cells in the brain. Vascular endothelial growth factor and Ang-2 had very different effects on the development of blood vessels; the former caused expansion and the latter retraction of these vessels. Treatment with a combination of VEGF and Ang-2 caused retroorbital or intraventricular hemorrhage, and brain blood vessels appeared enlarged and dysmorphic, with dramatically extended filopodia.

CONCLUSIONS

Some of these observations may provide insight into how one may develop a better model of brain arteriovenous malformations.

Endovascular Pressure Measurements: Validation with a Pulsatile Flow Model and Haemodynamic Assessment of Brain AVMs

SUMMARY

Intravascular pressure measurements for several types of endovascular catheters were obtained in an in vitro model to validate the pressure readings obtained during the interventional procedures of brain AVM embolization. An experimental model was used where the beat rate, flow and pressures were as close as possible to the average human values of interest. It is shown that the corrections increase with the decreasing inner diameter of the catheter used and with increasing vascular pressure. We have also shown that there were no differences between measurements made with the catheter in the direction of flow or against it. An average pressure reading corrections for the various microcatheters to compensate the readings obtained during in vivo monitoring is presented. The haemodynamic assessment of 81 brain AVMs was performed using the endovascular measurement of arterial pressure in 389 feeding arteries during embolization. Mostly, the feeders' arterial median pressure was half the systemic arterial pressure but there was a wide variability of AV shunts in brain AVMs not only from one brain AVM to another but also within the same brain AVM. Measurement of arterial feeder pressure is an inexpensive, quick and accurate tool to evaluate the type of AV shunts within brain AVM.

THE TRANSGENIC ARTERIOVENOUS FISTULA IN THE RAT: AN EXPERIMENTAL MODEL OF GENE THERAPY FOR BRAIN ARTERIOVENOUS MALFORMATIONS

OBJECTIVE

To introduce the transgenic arteriovenous fistula model in the rat, constructed by interposing mouse aorta in a fistula between the common carotid artery and external jugular vein in a nude rat, and to describe the model's technical feasibility, long-term patency, and expression of reporter genes.

METHODS

Carotid-jugular fistulae were surgically created in 112 rats. In 25 immunodeficient nude rats, wild-type mouse thoracic aorta (TAo) was interposed in the fistula; in 10 immunocompetent rats, TAo was interposed; in 19 nude rats, transgenic TAo with reporter genes for beta-galactosidase or green fluorescent protein was interposed; in 18 nude rats, wild-type mouse ascending aorta was interposed; and in 40 rats, a simple fistula was constructed without an interpositional graft. Host tolerance and graft viability were analyzed by histopathology and immunohistochemistry for CD31 (mouse endothelial cell marker), endothelial nitric oxide synthase, smooth muscle actin, fibronectin, beta-galactosidase, and green fluorescent protein.

RESULTS

The transgenic arteriovenous fistula was technically feasible and immunologically tolerated in nude rats but not in immunocompetent rats. The overall angiographic patency rate was 41% with TAo grafts and 56% with ascending aorta grafts, both lower than the 98% patency rate in fistulae with a single anastomosis and no interpositional graft. Mouse endothelium survived on the graft for 3 months according to CD31 staining, but longer survival by transgenic smooth muscle cells resulted in continued expression of beta-galactosidase for 6 months and green fluorescent protein for 4 months. Endothelium and smooth muscle in the fistula were functional, with normal expression of endothelial nitric oxide synthase as well as smooth muscle actin and fibronectin, respectively.

CONCLUSION

The transgenic arteriovenous fistula model enhances other carotid-jugular fistula models by integrating transgenic tissue, thereby creating an experimental system for investigating the molecular biology of and gene therapies for arteriovenous malformations.