Fundamentals of Endovascular Treatment for Arteriovenous Malformations and Arteriovenous Fistulas

19 Fundamentals of Endovascular Treatment for Arteriovenous Malformations and Arteriovenous Fistulas

Badih Daou, Pascal Jabbour, Stavropoula I. Tjoumakaris, and Robert H. Rosenwasser

Abstract

Endovascular management of cerebral arteriovenous malformations (AVMs) and arteriovenous fistulas (AVFs) has evolved tremendously over the past few decades and has become one of the primary treatment modalities to manage these lesions. Endovascular embolization of AVFs can be accomplished through a transarterial or transvenous approach depending on the location and complexity of the AVF and its vascular characteristics. The potential roles of embolization in AVF management include complete elimination of the AVF, palliation of disabling neurological symptoms, or a decrease in the flow through the AVF before other surgical or radiosurgical interventions are performed. Endovascular management of AVMs can be used for presurgical embolization of large AVMs, embolization of AVMs before radiosurgical intervention and for treating residual lesions that persist after radiosurgery, palliative embolization in patients with progressive or refractory neurological deficits, and finally as a primary treatment for curative embolization of small AVMs and AVMs that are not candidates for other surgical or radiosurgical approaches. N-butyl cyanoacrylate (NBCA) and Onyx liquid embolic agent are the most widely used agents for the embolization of AVFs and AVMs. Management of AVMs and AVFs requires a multidisciplinary team with expertise in the different treatment strategies available. Endovascular therapy has become a highly utilized, safe, and effective tool in the management of cerebral AVMs and AVFs either alone as a curative intervention or as an adjunct to other interventions.

Keywords: arteriovenous fistula, arteriovenous malformation, embolization, endovascular treatment, NBCA, Onyx

Key Points

There has been a tremendous evolution in the endovascular management of arteriovenous malformations (AVMs) and arteriovenous fistulas (AVFs) over the past few decades and represents a safe and effective primary treatment option for many lesions.
Potential roles for embolization of AVFs include complete elimination, palliation of symptoms, or as an adjunct prior to surgery or radiosurgery.
Endovascular embolization of AVMs may be performed as a preoperative adjunct prior to resection or radiosurgery, to treat residual lesions after prior intervention, and as palliative treatment to reduce neurological symptoms (such as from steal) and less commonly to achieve complete nidus obliteration in select lesions.
Management of AVMs and AVFs is best accomplished in the setting of a multidisciplinary team.

19.1 Introduction

The management of cerebrovascular malformations including arteriovenous malformations (AVMs) and arteriovenous fistulas (AVFs) has evolved tremendously over the past few decades. With the development of catheter and guidewire technology and novel embolic materials, endovascular management of these malformations has gained significant popularity and has become common practice. Intracranial AVMs are vessel abnormalities that constitute a connection between the arterial and venous systems with a lack of an intervening capillary bed.¹ The intervening network of vessels between the distal aspects of the arterial feeders and the proximal aspects of the draining veins is called the nidus and is the primary target of embolization.² In contrast, AVFs are direct fistulous connections between cerebral arteries and veins in the absence of an intervening nidus.³ They can be stratified based on arterial supply into dural AVF and pial AVF. Dural arteriovenous fistulas (DAVFs) are arteriovenous shunts from a dural arterial supply to a dural venous channel, typically supplied by pachymeningeal arteries and located near a major venous sinus.⁴ Pial arteriovenous fistulas (PAVFs) are rare vascular abnormalities that account for only 1.6% of all intracranial vascular malformations.⁵ They are composed of one or more pial and cortical arterial feeders with a single venous channel and they are not located within the leaflets of the dura.⁶ Intracranial AVMs and AVFs are relatively uncommon lesions, but the occurrence of these lesions can result in severe neurological symptoms, including seizures, headaches, focal neurological deficits, neuro-ophthalmologic symptoms, and more importantly intracerebral hemorrhage and death.³^,⁷^,⁸^,⁹^,¹⁰ There are several treatment modalities available to manage AVMs and AVFs and these include expectant management, surgical, radiosurgical, and endovascular techniques, or a combination of these approaches. The appropriate treatment strategy depends on a multitude of factors and should be tailored according to patient characteristics (age, comorbidities, and clinical presentation) and characteristics related to the malformation including the location, classification, natural history of the lesion, and angiographic features. The objective of this chapter is to review and highlight the role of endovascular treatment in the management of AVMs and AVFs.

19.2 Materials and Methods

The materials included in this chapter represent an analysis of published articles, including published institutional results along with the authors’ experience in the endovascular management of cerebral AVMs and AVFs.

19.3 Results

19.3.1 Arteriovenous Fistulas

Endovascular management has become the primary treatment modality of AVFs. Endovascular embolization of the fistula can be accomplished through a transarterial or transvenous route. Both approaches are usually performed through transfemoral access by catheterization of the femoral artery or femoral vein, respectively. The choice between the two access routes and the aim of the endovascular intervention depend on the location and complexity of the AVF, its vascular characteristics, and the potential complications inherent to each technique. The potential roles of embolization include complete elimination of the AVF, palliation of disabling neurological symptoms, or a decrease in the flow through the AVF before other surgical or radiosurgical interventions are undertaken.

There are several embolic materials that have been used for endovascular management of AVFs, including particles, liquid silicone, ethyl alcohol, polyvinyl alcohol, platinum microcoils, and cyanoacrylates. Liquid embolic agents are the most commonly used materials to manage AVFs, including N-butyl cyanoacrylate (NBCA; Trufill; Codman, Raynham, MA) (► Fig. 19.1) and Onyx liquid embolic agent (eV3 Neurovascular; Covidien, Irvine, CA) which is an ethylene vinyl alcohol polymer dissolved in dimethyl sulfoxide (DMSO).

Fig. 19.1 (a) A 56-year-old male patient who presented with an occipital hemorrhage. Angiography with right external carotid artery injection showed a dural arteriovenous fistula fed by feeders from the middle meningeal artery, draining into the superior sagittal sinus with cortical venous drainage. (b) Left external carotid artery injection showing the middle meningeal feeders of the fistula. (c) Embolization with N-butyl cyanoacrylate was performed. Follow-up angiography showed complete occlusion of the fistula and associated cortical venous drainage after a single embolization session.

19.3.2 Arteriovenous Malformations

Similarly, embolic materials used in the management of intracranial AVMs are divided into solid or liquid agents. Solid materials include polyvinyl alcohol particles, fibers, microballoons, and microcoils. Liquid agents include cyanoacrylate monomers such as NBCA and IBCA (I-butyl cyanoacrylate) as well as polymer solutions such as ethylene vinyl alcohol polymer. Other liquid agents include absolute ethanol which is not commonly used. The principal agents currently in use for treating brain AVMs are NBCA and Onyx.

The aim of the endovascular intervention differs based on the characteristics of the lesion. Endovascular management can be used for presurgical embolization of large or giant cortical AVMs (► Fig. 19.2), embolization of AVMs before radiosurgical intervention and for treating residual lesions that persist after radiosurgery, palliative embolization in patients with progressive or refractory neurological deficits, and in patients with large AVMs and AVMs that may be difficult to manage surgically or with radiosurgery, as well as finally as a primary treatment for curative embolization of small AVMs and AVMs that are not candidates for other surgical or radiosurgical approaches. Endovascular treatment can also be used to eliminate AVM-associated aneurysms, especially in acutely ruptured cases where the aneurysm is the source of hemorrhage.

Fig. 19.2 (a,b) A 58-year-old male patient who was found to have an arteriovenous malformation (AVM) during evaluation for headaches. Angiography with right vertebral artery injection showed a right-sided temporal lobe Spetzler-Martin grade II AVM, fed by multiple feeders of the distal posterior cerebral artery on the right and posterior temporal arteries and draining deeply through two draining veins. (c,d) Preoperative embolization with Onyx was performed. Control angiography with selective right vertebral artery injection was performed showing 50% decrease in size of the AVM. (e) Surgical resection was performed. Control angiography showed 100% occlusion of the AVM.

19.4 Discussion

Most of the data available regarding endovascular treatment of AVMs and AVFs stem from level III, IV, and V evidences with level I and level II studies significantly lacking from the literature.

19.4.1 Endovascular Management of AVFs

Although endovascular management has become the mainstay of DAVF therapy, the best approach to manage each lesion should be individualized with involvement of a multidisciplinary team to evaluate each case. A meticulous evaluation of the patient’s characteristics and clinical presentation and location, classification, and type of the lesion should be performed prior to treatment. The decision to manage AVFs through an endovascular approach should be based on an assessment of the risk of the intervention against the natural history of the lesion.

Transvenous Access for AVF Management

Transvenous embolization aims at causing thrombosis of the venous side of the lesion and disconnection of leptomeningeal or cortical reflux with preservation of normal venous drainage.⁴^,¹¹ Transvenous embolization often includes the obliteration of the adjacent dural venous sinus.¹¹ The transvenous approach has been primarily employed in the management of DAVFs, involving the cavernous, transverse, and sigmoid sinuses by allowing access to the affected venous sinus after which coils, balloons, or liquid embolic agents can be deployed.¹²^,¹³^,¹⁴ However, the transvenous route may not be an option for many DAVFs, including tentorial incisura DAVFs and anterior cranial fossa lesions, which frequently behave aggressively.⁶ Transvenous embolization of DAVFs involving the superior sagittal sinus is less optimal as well.¹¹^,¹⁵ Patient selection for transvenous embolization depends on several important factors.¹⁶ First, the segment of the sinus to be occluded must be in proximity to the fistula and receive its entire venous drainage. Second, the diseased sinus should not be crucial to normal venous outflow and can be occluded. For this reason, cerebral venous drainage should be carefully assessed before transvenous embolization can be undertaken to determine the alternate pathways for cerebral venous drainage and avoid potential venous infarction or hemorrhage. Third, complete occlusion of the involved sinus segment is essential to avoid diversion of the flow into confluent cerebral veins and worsening of the cerebral venous drainage which may result in an acute venous infarct or hemorrhage.¹⁶ Transvenous embolization is used in the management of large, complex DAVFs that have an accessible venous drainage. It is especially useful when the AVF has multiple arterial feeders. Occlusion of the venous side of DAVFs is usually well tolerated if the involved sinus is arterialized and does not serve as a site of drainage of normal circulation.⁴ Usually, the pathologic segment is often associated with retrograde leptomeningeal venous drainage and these channels may be easier to obliterate using transvenous embolization. The main advantages of this approach are related to the ease of access to the fistulous site and the ability to obliterate the fistula in a single session.¹¹ Transvenous embolization is associated with a high efficacy and low complication rate.¹⁷^,¹⁸ The rates of complete angiographic obliteration of the AVF by the transvenous route have ranged mainly between 71 and 87.5%.¹¹^,¹⁵^,¹⁷^,¹⁹ Complications associated with this approach include venous sinus thrombosis and infarction, vessel injury or rupture, disruption of the venous drainage, and altered hemodynamic patterns resulting in hemorrhage and worsening neurological deficits. Permanent complications have been reported in 4 to 7% of cases.¹¹^,¹⁵^,¹⁷^,¹⁹

Transarterial Access for AVF Management

The success of transarterial embolization in completely eliminating the DAVF depends on the number of accessible arterial feeders. This approach is most successful in achieving complete elimination of the lesion when the fistula has a small number of feeders, whereas DAVFs with a large number of feeders are rarely successfully treated with transarterial embolization alone. In DAVFs with multiple feeders, transarterial embolization may result in obliteration of the filling of the AVF after one injection, but the DAVF might still continue to draw feeders from other sources which may lead to development of new collaterals that may be more difficult to treat.⁴^,¹⁶ These partially treated DAVFs may later recur and result in hemorrhage. If, however, transarterial embolization results in occlusion of the common receptacle for all arterial feeders and disconnects any associated venous outflow, the embolization can be curative. Transarterial embolization can be used for the purpose of symptomatic palliation of disabling neurological symptoms through occlusion of arterial feeders without achieving complete obliteration of the fistula.⁶ DAVFs that are followed expectantly or treated palliatively should be monitored closely with serial diagnostic imaging. Transarterial embolization can improve the safety and efficacy of other interventions as well by decreasing flow through the DAVF before other interventions are undertaken, such as surgical, radiosurgical, or transvenous management.²⁰^,²¹ Transarterial embolization might offer advantages over transvenous access in certain cases.¹⁶ Transvenous access to the fistula might be limited by venous sinus occlusion or high-grade stenosis in certain cases. Similarly, transvenous access to high-grade AVFs draining directly into remote small cerebral veins may result in significant complications. Complex fistulas may require management using a multistaged approach, combining transarterial and transvenous techniques to eliminate cortical venous drainage and occlude the fistula. Kirsch et al reported in a study of 150 patients with DAVFs that immediate occlusion occurred in 30% of patients after transarterial embolization versus 81% with transvenous treatment alone. After combined transarterial/transvenous treatment, the angiographic cure rate was 54%.¹⁴

Alternative Access for AVF Management

In some cases, transfemoral access might not be possible because of unfavorable anatomy, tortuous vasculature, or femoral vascular pathology. Alternative transarterial access routes include the transradial approach, transcarotid access either percutaneously or through carotid cutdown, or direct puncture of a cavernous or ophthalmic fistula through the orbit.²² Alternative transvenous routes include percutaneous cannulation of the facial vein, angular vein, superior ophthalmic vein, inferior ophthalmic vein, or direct transorbital puncture of the cavernous sinus.²² A hybrid approach using surgical exposure of the superior ophthalmic vein or cavernous sinus followed by endovascular catheterization is also another option.²³

Classification of AVFs and Endovascular Management

Angiography is essential for definitive diagnosis of AVFs and to provide a detailed evaluation of the arterial supply, anastomoses, and venous anatomy prior to embolization.⁶ Treatment of DAVFs is guided by the classification of these lesions. One of the most well-recognized classification systems was reported by Djindjian and colleagues.²⁴^,²⁵ According to this system, type I DAVFs are characterized by normal anterograde drainage into a venous sinus or meningeal vein; type II lesions drain into a sinus, with reflux into adjacent sinuses or cortical veins, type III DAVFs drain directly into cortical veins with retrograde flow into the cerebral venous compartment, and type IV DAVFs have drainage directly into a venous pouch (venous lake or venous ectasia).

Cognard and colleagues developed another classification system based on a modified version of the Djindjian’s classification.²⁶^,²⁷^,²⁸ They defined five types of DAVFs. Type I DAVFs were characterized by normal antegrade flow into the affected dural sinus. Type II lesions were associated with an abnormal direction of venous drainage within the affected dural sinus and were classified into three subtypes: type IIa, lesions with retrograde flow exclusively into a sinus or sinuses; type IIb, lesions with retrograde venous drainage into the cortical veins only; and type IIa + b, lesions with retrograde drainage into sinuses and cortical veins. Type III DAVFs had direct drainage into cortical veins without venous ectasia, whereas type IV DAVFs had direct drainage into cortical veins with venous ectasia greater than 5mm in diameter and three times larger than the diameter of the draining vein. Type V DAVFs had drainage into spinal perimedullary veins. Type I DAVFs are considered benign, and do not usually require treatment. There is no evidence demonstrating significant benefits to prophylactic treatment of unruptured DAVFs that are not associated with leptomeningeal cortical venous drainage.⁴ Expectant follow-up of these lesions using serial MRI and angiographic evaluation should be performed. Type IIa AVFs are best treated with transarterial embolization, whereas the best management strategy for type IIb and IIa + b lesions DAVFs is more challenging and these lesions usually require both transarterial and transvenous embolization to achieve complete cure.⁶ For types III to V, the efficacy of endovascular management alone decreases and often complete occlusion of the fistula requires combined transarterial and occasionally transvenous embolization and surgical techniques to successfully eliminate the cortical venous drainage.⁶

Borden et al based their classification system on the venous anatomy and identified three categories.²⁹ Type I DAVFs drain directly into venous sinuses or pachymeningeal veins. Type II DAVFs drain into dural sinuses or pachymeningeal veins with retrograde drainage into subarachnoid veins. Type III DAVFs drain only into subarachnoid veins without any dural sinus or meningeal venous drainage. Borden type I DAVFs are mostly benign as well. Regardless of the classification system, the main factor indicating an aggressive clinical course appears to be the presence of leptomeningeal cortical venous drainage. Treatment of these lesions should be highly encouraged in asymptomatic patients and/or in patients with incidentally discovered lesions with leptomeningeal venous drainage.

Agents Used in Endovascular Treatment of AVFs

NBCA has been one of the main liquid embolic agents used for transarterial embolization of DAVFs with fairly good results.³⁰^,³¹^,³² Kim et al evaluated 121 DAVFs treated with transarterial glue embolization and reported angiographic cure in 29.8% of patients (immediate cure in 14% and progressive complete thrombosis of the residual shunt in 15.7%).³² Surgical intervention or transvenous coil embolization was necessary for 45.2% of all cases. In a study that included 11 patients treated with transarterial injection of NBCA, 63.6% of patients were cured following the endovascular intervention.³² Guedin et al reported a rate of complete closure of the AVF in 34 of 38 (89.5%) patients with Borden type II or III DAVFs and occlusion of cerebral venous drainage in all other patients who were treated with transarterial embolization using NBCA with no permanent morbidities or mortalities.³⁰

The use of NBCA has some drawbacks. It is an adhesive agent that has a rapid rate of polymerization, which makes it somewhat difficult to use and may increase the risk of microcatheter retention or avulsion of the feeding artery upon removal of the microcatheter.³³ The injection must be performed fast and in a continuous fashion, which may decrease the precision of injection and result in suboptimal penetration into the target site. Both preparation and delivery of NBCA require an experienced user. Use of a wedged microcatheter technique with low-concentration glue may maximize glue penetration into the venous drainage route.²³

Onyx offers several advantages over NBCA which allow for safer and more efficient treatment of DAVFs. Owing to its lava-like flow pattern and its nonadhesive nature, Onyx facilitates longer, slower, and more controlled injections with better penetration of the fistula.²³ Onyx can penetrate the depths of the AVF with high efficiency, which facilitates the embolization of a large portion of the lesion from a single pedicle injection decreasing the need for multiple embolization attempts.²³ In addition, Onyx injection can be discontinued for angiographic assessment of the embolization and evaluation of collaterals that may become evident during the course of embolization. Furthermore, Onyx is less adherent to the microcatheter than NBCA with possibly a lower risk of catheter retention and arterial rupture.²² On the other hand, Onyx has some disadvantages in AVF management when compared to NBCA. An increase in fluoroscopy and procedure times and procedure costs has been reported with Onyx.²³^,³⁴ Cranial nerve injury, DMSO-induced angiotoxicity, and potential for distal embolization into the venous system and the pulmonary circulation are potential drawbacks as well.²³ Onyx comes in three different concentrations (6, 6.5, and 8%) with increasing viscosity. Although Onyx comes in readymade vials containing EVOH, DMSO, and tantalum powder, it must be shaken for 20 minutes before injection to maximize its radioopacity.³⁵

Various rates of successful treatment have been reported with the liquid embolic agent Onyx, with most results characterized by remarkably high cure rates with an important number of treatments being completed in one session.²⁸^,³⁶^,³⁷^,³⁸^,³⁹ Macdonald et al performed embolization of 52 DAVFs: transvenous embolization was successful in 11 of 15 patients, and transarterial embolization was successful in 27.3% in the non-Onyx group versus 72.7% in the Onyx group.³⁶ Abud et al reported their experience with transarterial Onyx embolization of 44 DAVFs and reported complete occlusion in all but nine patients, five of whom were successfully treated by transvenous embolization with coils and Onyx.³⁷ Furthermore, complete cure was achieved with a single procedure in 81% of patients. Stiefel et al reported angiographic cure in 72% of DAVFs (21/29 DAVFs) treated with Onyx embolization, mainly using a transarterial embolization with complications occurring in 9.7% with permanent morbidity in 2.4% of patients.³⁸ In a prospective study of 30 patients with DAVFs (10 grade II, 8 grade III, and 12 grade IV fistulas), Cognard et al reported that complete angiographic cure occurred in 24 of 30 patients (80%) with only two complications, including a temporary cranial nerve palsy and postprocedure hemorrhage.²⁸ Hu et al reported a complete angiographic cure rate of 79% in patients with DAVFs treated with transarterial embolization.³⁹ More specifically, when Onyx was used as the sole embolic agent, angiographic cure was observed in 87% of DAVFs.³⁹ Our published institutional results show that endovascular treatment was successful in achieving complete obliteration of the fistula in 28 of 39 patients (71%) after a mean number of 2.1 interventions, 21 patients by endovascular means alone, and 7 patients with combined endovascular and surgical approaches. The transarterial approach using Onyx embolization was the preferred treatment and most successful treatment (occlusion rate of 75%) with elimination of CVD in up to 85% of patients with Onyx embolization.⁴ In general, cure rates between 63 and 100% have been reported with Onyx embolization of DAVFs.²²

Onyx has also been used with transvenous access, especially in the management of carotid-cavernous fistulas (CCFs) with significant and rapid improvement in neuroophthalmologic symptoms (► Fig. 19.3). Elhammady et al performed transvenous embolization of eight CCFs and transarterial embolization of four CCFs and achieved complete obliteration of all lesions in a single session with resolution of symptoms in all patients by 2 months.⁴⁰ They observed three cranial nerve palsies.⁴⁰ Suzuki et al reported complete cure and resolution of symptoms in three cases with spontaneous CCFs treated with transvenous Onyx embolization.⁴¹ Our institutional experience using Onyx embolization of CCFs through surgical cannulation of the superior ophthalmic vein showed complete obliteration in 8 out of 10 patients and a significant reduction in fistulous flow in the other 2 patients, which later progressed to near-complete occlusion on angiographic follow-up.²³ All patients experienced a complete clinical recovery and did not have any complications or recurrence. In another study by Zaidat et al, Onyx was successfully used in combination with coils or stents in five cases with complete occlusion.⁴² Liquid embolic agents can be used alone or in combination with coils or balloons of the feeding artery or venous outflow. Balloon-assisted and coil-assisted embolization can help in achieving a controlled delivery of the embolic agent into the fistula and can be useful in protecting the patency of arterial collaterals and critical venous pathways by limiting distal flow of the embolic agent.

Fig. 19.3 (a) A 48-year-old female patient who presented with proptosis and chemosis. Right internal carotid artery catheterization showed a carotid-cavernous fistula (CCF) with small meningeal feeders. (b) Left internal carotid artery catheterization showed multiple meningeal feeders going into the cavernous sinus. (c) Surgical exposure for the left-sided superior ophthalmic vein was performed followed by catheterization of the superior ophthalmic vein and embolization with Onyx. Control angiography showed 100% occlusion of the CCF.

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