Endovascular approaches to arteriovenous malformations (AVMs) are often necessary to define and help treat these often complex lesions. Angiography provides important information to help plan surgical or radiosurgical approaches. Modern embolization techniques allow AVMs to be treated with the goals of making surgery safer and easier, eliminating high-risk features in patients with AVMs who are otherwise not candidates for treatment, and even potentially curing the patient of the lesion. Liquid embolic agents have significantly advanced what is possible with endovascular treatment of AVMs.
Key points
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Endovascular treatment of arteriovenous malformations requires a multidisciplinary team approach.
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Physicians must select patients carefully and consider each patient’s Spetzler-Martin grade, individual lesion angioarchitecture, and high-risk features before attempting treatment.
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The goals of therapy should be established before treatment and may include preoperative, targeted, palliative, or curative embolization.
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Technical advances in endovascular techniques and embolic materials have resulted in safer, more complete embolization.
Introduction
Cerebral arteriovenous malformations (AVMs) are high-flow lesions consisting of an abnormal tangle of vessels that connect the arterial and venous systems. Although they seem to be primarily congenital in origin, these clearly dynamic lesions are capable of spontaneous growth and regression, and have often been reported arising de novo. Although hemorrhage is the most common presentation, AVMs can also cause seizures, focal neurologic deficits, and headaches. The goals of therapy are to eliminate or reduce the risk of bleeding and to ameliorate symptoms. Cerebral AVMs lie at a therapeutic nexus and may require treatment with open surgery, radiosurgery, or endovascular embolization, or with a multimodality approach using a combination of these techniques.
Introduction
Cerebral arteriovenous malformations (AVMs) are high-flow lesions consisting of an abnormal tangle of vessels that connect the arterial and venous systems. Although they seem to be primarily congenital in origin, these clearly dynamic lesions are capable of spontaneous growth and regression, and have often been reported arising de novo. Although hemorrhage is the most common presentation, AVMs can also cause seizures, focal neurologic deficits, and headaches. The goals of therapy are to eliminate or reduce the risk of bleeding and to ameliorate symptoms. Cerebral AVMs lie at a therapeutic nexus and may require treatment with open surgery, radiosurgery, or endovascular embolization, or with a multimodality approach using a combination of these techniques.
Natural history
The first step in any treatment algorithm for an AVM is deciding whether treatment is necessary, which requires weighing the risks of the natural history of the disease against the risks of treatment. The ubiquitously quoted annual rupture risk for cerebral AVMs of 2% to 4% is from a Finnish study in which unruptured AVMs were significantly underrepresented, making up only 29% of the sample. More recent data suggest that the annual hemorrhage rate of unruptured AVMs without deep venous drainage or deep location may be as low as 0.9%. However, the optimal approach to unruptured AVMs remains an area of controversy.
One recent study, A Randomized Trial of Unruptured Brain AVMs (ARUBA), compared the risks of treating unruptured AVMs with the risks of observation. Enrollment was terminated early, after it was observed that the end point of death or stroke occurred in 10% of those randomized to medical management and in 29% of those randomized to intervention. Short follow-up and sample heterogeneity preclude any definitive recommendations from this study, although enrolled patients will continue to be followed.
In the absence of further data on the best treatment method for unruptured AVMs, the decision to offer intervention must be based on an analysis of the high-risk features of the lesion and patient symptoms. Deep location and exclusively deep venous drainage are associated with an increased risk of hemorrhage, as are anatomic features such as intranidal aneurysms, venous stenosis, and venous ectasia. In addition, an attempt should be made to verify whether the AVM is the source the patient’s symptoms.
There is more certainty regarding the role of intervention in ruptured AVMs. Once an AVM has ruptured, the risk of rerupture increases to 7% to 17% in the first year. In this setting, prompt treatment is indicated.
Risks of endovascular treatment
Endovascular therapy has associated risks. Aside from the normal complications of diagnostic angiography, which include groin or retroperitoneal hematoma, contrast-induced allergy, contrast-induced nephrotoxicity, and vessel injury, embolization is associated with a risk of intracerebral hemorrhage and infarction. Estimates of the likelihood of these complications vary widely, with reports of permanent neurologic deficits ranging from 2% to 20% and mortality ranging from 1% to 3%. One large prospective study of 201 patients undergoing 339 embolizations showed a risk of 9% for permanent neurologic deficit per patient, with 2% mortality.
Goals of treatment
The role of endovascular treatment in patients with AVMs is evolving. Decisions regarding the optimal treatment strategy for any AVM are best made by incorporating input from a team of specialists with expertise in open, endovascular, and radiosurgical approaches. Particularly for endovascular treatment, the ultimate goal of the intervention defines the approach taken and techniques used. These goals include preoperative embolization to facilitate a planned surgery; attempted curative embolization; prestereotactic radiosurgery to reduce target size; targeted embolization of high-risk features; and palliative embolization in cases of symptomatic, large, complex AVMs that are otherwise not amenable to treatment.
Preoperative Embolization
Preoperative embolization should be targeted toward achieving a stepwise reduction in blood flow. Of highest priority for occlusion are deep-feeding arterial pedicles and other arteries that would be problematic or difficult to access at the time of surgery or those that supply deep portions of the AVM.
The appropriate timing of surgery after AVM embolization is controversial. Although some clinicians advocate waiting for up to 3 weeks after embolization to allow for progressive thrombosis and stabilization of local and regional hemodynamic changes, others perform surgery within a few days because of the perceived increased risk of hemorrhage associated with embolization.
Surgical treatment of patients with Spetzler-Martin grade I and II lesions is associated with low morbidity and mortality. For these lesions, preoperative embolization may increase the risk for adverse events without significantly decreasing the risk of surgery, and therefore may be indicated only in select cases. In contrast, patients with Spetzler-Martin grade III lesions may often benefit from embolization before definitive treatment with resection or radiosurgery. Those with Spetzler-Martin grade IV and V lesions who need treatment often require multimodality therapy with embolization as one part of a larger treatment strategy.
Curative Embolization
Patients with certain types of AVMs may be amenable to curative endovascular treatment with embolization. Before the introduction of ethyl vinyl alcohol copolymer (EVOH), a small series showed that embolization using n-butyl cyanoacrylate (n-BCA) achieved complete angiographic occlusion in approximately 20% of patients. More recent series using EVOH (Onyx, ev3, Irvine, CA) have achieved angiographic obliteration rates as high as 51% in patients undergoing embolization and as high as 96% in carefully selected patient cohorts with the stated goal of AVM cure. AVM characteristics that are favorable for curative embolization are small or medium size, superficial location, a simple nidus with a single or few arterial pedicles from a single vascular territory, and an easily identifiable draining vein. The feeding pedicles must be amenable to catheterization with a microcatheter and, ideally, allow for reflux of Onyx for 2 to 3 cm before encountering a branch point supplying normal brain. However, some lesions that were thought to be angiographically cured after embolization showed residual filling at the time of surgery. There is little long-term follow-up evidence regarding the durability of Onyx or its long-term effects in vivo; thus serious consideration should be given to surgical resection of the AVM nidus when feasible.
Embolization Before Radiosurgery
The role of embolization to facilitate radiosurgery is becoming limited to select cases. Embolization has been used to shrink the AVM nidus to facilitate radiosurgery, allowing a decreased integral dose to be delivered, with a corresponding reduced risk to surrounding normal brain tissue. However, many studies have now shown a reduced rate of AVM obliteration after embolization with no differences in rates of hemorrhage during the latency period. Embolization before radiosurgery may adversely affect obliteration rates by fragmenting the nidus into discrete compartments, which makes treatment planning more difficult. There is also the potential for recanalization of previously embolized areas of nidus and difficulty during the targeting stage of radiosurgery because embolic material may introduce image distortion. Studies have found, however, that Onyx does not significantly reduce the radiation dose delivered by linear accelerator stereotactic radiosurgery.
For large, high-grade AVMs in eloquent areas, therapeutic options may be limited. Staged endovascular embolization to reduce the volume and allow stereotactic radiosurgery in large AVMs compares favorably with other treatment options in these difficult-to-treat lesions. For these large and otherwise untreatable AVMs, embolization before radiosurgery also allows the treatment of high-risk features, such as intranidal aneurysms or direct fistulous connections.
Targeted Embolization
Certain angioarchitectural features are associated with an increased risk of hemorrhagic presentation. These features may include associated aneurysms, venous abnormalities such as ectasia and stenosis, and the presence of a single draining vein. There is no evidence that partially treating an AVM, without specifically targeting high-risk features, reduces the risk of hemorrhage, whereas some evidence suggests that it may adversely affect the natural history. In carefully selected patients, it may be reasonable to target high-risk features, such as aneurysms and venous ectasias, for embolization ( Fig. 1 ), particularly in patients who are not surgical candidates already hemorrhaged. Partial targeted embolization of intranidal aneurysms and venous ectasias may provide some protection against hemorrhage.
Palliative Embolization
Some investigators have attributed progressive neurologic deficits and intellectual decline in patients with AVMs to vascular steal. There are limited case reports, but no large series or randomized studies, suggesting that partial embolization to reduce arteriovenous shunting may improve both perfusion and symptoms in brain adjacent to the AVM. It is important to consider this option in the context of the multiple studies showing an increased risk of hemorrhage from partially treated AVMs and, if attempted, to avoid compromising venous outflow ( Fig. 2 ).
Technical considerations
Successful endovascular treatment of AVMs begins with appropriate, careful patient selection along with clearly defined goals of embolization. Before beginning any embolization, careful evaluation of the arterial supply, nidal component, and venous drainage of the AVM is mandatory.
A small number of large feeding vessels creates a favorable environment for embolization. Multiple, small, and diffuse feeding vessels may make embolization impossible or prohibitively risky. Feeding arteries may directly supply the AVM or may supply predominantly normal cortex with small branching en-passage vessels feeding the AVM. The former are safer for embolization. When embolizing en-passage vessels there is often little leeway for tolerating reflux of embolization material; any reflux along the catheter could result in stroke. The nidus should be evaluated to delineate any distinct hemodynamic compartments.
The nature of the connection between the arterial and venous compartments must also be appreciated. These connections can be nidal or fistulous. Fistulous connections require close attention to ensure that rapid progression of embolisate through the fistula to the venous side does not occlude a major draining vein.
In addition, the nature of venous drainage must be considered. Embolic material that progresses into a draining vein may result in sudden pressure changes within the AVM and precipitate rupture. This result is more likely in AVMs with a single draining vein, stenosis, or obstruction of a draining vein compared with an AVM that has multiple draining veins.
Strong consideration should be given to routine neuromonitoring of somatosensory evoked potentials and electroencephalography for AVM embolization. In select cases the use of pharmacologic provocation with amobarbital (Amytal, Marathon Pharmaceuticals, Northbrook, IL, USA) can be a useful adjunct.
Embolic Agents
Although a host of agents have been used to block blood flow to AVMs, including calibrated particles, silk, and absolute alcohol, current strategies rely primarily on liquid embolic agents. The chosen embolic agent must penetrate into the nidus of the AVM; blocking arterial vessels proximal to the nidus allows vessel recruitment and persistence of the AVM. For this reason, detachable coils are minimally effective, although, in rare instances, they are a useful adjunct to liquid embolic agents to treat high-flow lesions.
EVOH
EVOH, sold under the trade name Onyx, is a liquid embolic agent approved by the US Food and Drug Administration (FDA) in 2005 as a humanitarian use device for AVM embolization. The EVOH is mixed with tantalum powder to make it radiopaque and dissolved in dimethyl sulfoxide (DMSO). It requires catheters that are compatible with DMSO. When injected into an aqueous solution (eg, blood) the DMSO diffuses into the bloodstream, allowing the Onyx to precipitate. After filling the dead space of the microcatheter with DMSO, Onyx is injected slowly (typically over as long as 90 seconds). Rapid injection of DMSO is associated with angionecrosis. EVOH is less adhesive than n-BCA, allowing slower injections over a longer period of time. Although long-term follow-up data are lacking, there is some evidence that recanalization can occur after embolization with EVOH. Given the prolonged working time of EVOH it may be prudent to have a strategy for dealing with nidal or vessel rupture and hemorrhage, such as ready access to n-BCA.
The handling characteristics of EVOH may be altered by varying its concentration. It is currently available in 2 viscosities for AVM embolization: Onyx 18 (6% EVOH) and Onyx 34 (8% EVOH). Onyx 18 is less viscous and is used to achieve distal nidal penetration. Onyx 34 is more viscous and may be used in high-flow lesions with a fistulous component or to create a plug around the catheter. A common technique for AVM embolization with Onyx is a continuous injection to form a plug around the catheter tip with frequent pauses when reflux is observed. Once this plug is formed, Onyx 18 can be used to achieve distal nidal penetration.
There is an increasing number of reports of using a dual lumen–compliant balloon catheter for AVM embolization. This technique may prevent unwanted proximal reflux and decrease procedure time and fluoroscopy exposure, but it is associated with the risk of vessel perforation from balloon inflation. More proximal occlusion using a separate balloon catheter can also be used to decrease flow and minimize the risk of uncontrolled embolization into draining veins ( Fig. 3 ). An alternative technique to improve efficiency, and possibly occlusion rates, in AVM embolization is positioning 2 microcatheters within separate feeding arteries and concurrently injecting Onyx.
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