27 Endovascular Management of Aneurysms of the Posterior Circulation
Abstract
The endovascular management of posterior circulation aneurysms represents one of the most challenging and rapidly evolving aspects of endovascular neurosurgery. Although the vascular anatomy of these lesions may seem relatively straightforward at first glance, this perception belies the truth that the territory supplied by these vessels is extremely eloquent. A detailed understanding of the anatomy is crucial to the appropriate management of vascular disease in this region. Recent advances in the devices available to the endovascular neurosurgeon are allowing ever more challenging pathologies to be treated. The use of devices such as flow diverters, neck-bridging devices, and intrasaccular flow diverters now affords a variety of different treatment strategies for aneurysms that were hitherto difficult, if not impossible, to treat. In this chapter, we discuss the nuances of aneurysmal disease of the posterior fossa with particular emphasis on treatment strategies at different anatomical locations, as well as the results of the most recent trials and case series regarding treatment of aneurysms of the posterior circulation.
Introduction
The endovascular management of intracranial vascular pathology has progressed at an exponential rate with new devices coming to market at a rapid pace. This advancing technology, together with an ever-increasing bank of knowledge and experience, means that the successful treatment of patients with aneurysmal pathologies, who even 10 years ago had no real hope of cure, is now within reach. These advances in skill and technology will likely continue and allow even more patients to be managed using minimally invasive endovascular approaches. In this chapter, we discuss the endovascular management of aneurysmal disease in the posterior circulation and highlight new technologies and current evidence.
Basilar Tip and Basilar Bifurcation Aneurysms
Clinical Aspects
The basilar apex is the most common site for aneurysm formation in the posterior circulation. Basilar tip and basilar bifurcation aneurysms comprise approximately 50% of all posterior circulation aneurysms and about 5% of all intracranial aneurysms. These aneurysms point superiorly but can be angulated either ventrally or dorsally, and they may point toward the left or the right, depending upon individual anatomy. The International Subarachnoid Aneurysm Trial demonstrated that, in addition to size, aneurysm location at the basilar apex is the strongest predictor of aneurysm rupture. 1 The Natural Course of Unruptured Cerebral Aneurysms in a Japanese Cohort study showed that the presence of a daughter sac also increases the risk for rupture. 2 For these reasons, basilar apex aneurysms are treated aggressively.
Treatment Strategies
Narrow-necked aneurysms can be treated with coiling alone ( Fig. 27.1 ); however, up to 60% of aneurysms arising from the basilar apex are wide necked and may incorporate the posterior cerebral arteries (PCAs) into the aneurysm neck. Therefore, a detailed analysis of each patient’s anatomy is required to determine an appropriate individualized treatment strategy. A variety of techniques and equipment has been developed to deal with bifurcation aneurysms.
Balloon Remodeling
One of the widely used methods is balloon remodeling. 3 , 4 This technique negates the need for antiplatelet medication and is particularly useful in treating acutely ruptured aneurysms. Several different techniques are available:
A super-compliant balloon is tracked into one branch and inflated to “bulge” into the aneurysm neck.
Two balloons are used, with one placed in each branch of the bifurcation (kissing balloon technique).
A balloon is tracked via a posterior communicating artery so that it is placed in both PCAs (horizontal), with coiling used subsequently.
A double-lumen balloon is used, with the tip of the balloon placed inside the aneurysm, and it is used to insert a coil while the balloon is inflated. In this technique, the balloon points straight up at the aneurysm.
Balloon-remodeling techniques have been shown to be safe and may allow for higher packing density of the aneurysm and an improved angiographic result. The development of newer dual-lumen balloons (e.g., Scepter; MicroVention) also offers neurosurgeons the ability to pass a low-profile stent through the central balloon lumen if, at the end of the procedure, this is believed to be necessary. This new feature may be particularly useful in treating acutely ruptured aneurysms, when the primary placement of stents may not be desirable but may become necessary at the end of the procedure if the coil mass begins to intrude into the lumen of the parent artery or adjacent branches after balloon deflation.
Stents
Stents are an alternative to balloons and can be used in similar configurations. The exact configuration will depend on the anatomical disposition of the aneurysm and the branches. For example, with a wide-necked aneurysm centered directly at the bifurcation, it would be difficult to protect both branches with only a single stent; therefore, either a Y-stenting technique or a horizontal-stenting technique should be considered. 5 With Y stenting, the second stent crosses the first stent and the distal ends of both stents are placed in the two PCAs that require protection ( Fig. 27.2 ). Lozen et al 6 reported six cases of aneurysms treated using the Y-stenting technique with good outcomes in all cases. One advantage of Y stenting is the change in angulation of both PCAs to the basilar artery (BA) that favors blood flow away from the aneurysm. 7 A variant of the Y-stenting technique, the modified Y-stenting or T-stenting method, is very similar; however, the second stent does not cross the first stent, and the second stent lands in the BA. Instead, its proximal end abuts the first stent and covers the neck of the aneurysm.
The Barrel device (Medtronic) is a new stent that aims to avoid the need to catheterize both efferent branches. The Barrel is a cone-shaped stent with a wide barrel-shaped structure at the mid-portion of the stent that should be placed at the level of the aneurysm neck. This device then braces itself against the walls of the parent artery and aneurysm neck to allow coiling of the aneurysm while protecting the neck, similar to the use of normally shaped stents or balloon placement across the aneurysm neck. This device has been evaluated in animal models and in a limited number of patients. Early results are promising with high rates of technical success and evidence of neo-endothelialization at the neck of the aneurysm. 8 A French prospective registry has investigated the device in treating wide-necked bifurcation aneurysms. 9 Seven patients were enrolled in the study, and complete or near-complete occlusion was achieved in 85% of aneurysms. The authors report the device to be easy to position and resheath if necessary. Coiling catheters could be used either in a “jailing” method or by crossing the struts of the Barrel, with the device itself providing good neck bridging and support for the coils.
On occasion, access to both branches may be extremely difficult. One option is to loop one or more guidewires and micro-catheters within the aneurysm. This is associated with inherent risks of aneurysm hemorrhage and should be avoided in ruptured aneurysms. However, when treating large unruptured aneurysms, this technique can be performed carefully. Once the distal end of the stent is deployed in the PCA, the stentdelivery catheter can be gently straightened and the stent can be deployed appropriately. Alternatively, a “waffle-cone” technique can be used. This technique involves the deployment of a stent straight up and into the neck of the aneurysm. Coiling is then performed and the distal end of the stent acts as a brace to prevent the coils from prolapsing into the parent artery. Sychra et al 10 reported on the use of this technique in four patients, all of whom had a good outcome. It has been suggested that having a stent directed straight into an aneurysm may direct flow into the aneurysm; however, this does not appear to be a problem. 11
Advancing on the idea of the waffle-cone technique is the pCONus stent (phenox GmbH). This stent-like device is implanted both inside and outside the aneurysm. At the distal end of the stent is a crown with four or six petals that is deployed at the level of the base of the aneurysm. These petals then prevent coils from protruding into the parent artery ( Fig. 27.3 ).
Several authors have published clinical series describing use of this device for treating both ruptured and unruptured aneurysms with good results and low complication rates. 12 , 13 , 14 , 15 An iteration of this device, the pCANvas (phenox GmbH), which has a membrane at the level of the crown, has been shown to immediately and significantly reduce blood flow into the aneurysm. 11 This device can be used alone or in combination with coils because a guidewire and a microcatheter can pierce the membrane and coils can be deployed within the aneurysm.
Other Devices
Other extra-aneurysmal devices have recently entered the market, including the PulseRider (Pulsar Vascular) and eCLIPS (Evasc Neurovascular Enterprises). The PulseRider is a self-expanding implant with distal struts that are designed to be deployed at the level of the aneurysm neck, either within the aneurysm itself or in the parent vessel. The PulseRider, as with the pCONus and pCANvas, is fully retrievable and comes in a variety of different sizes as well as in two different shape configurations: Y and T. These features, in combination with the ability to torque the device, mean that the device should accommodate most aneurysm morphologies. Published data on this device are limited; however, Spiotta et al 16 recently published their initial experience in three patients and reported excellent radiologic outcomes, with no complications. Recently, the results of the Adjunctive Neurovascular Support of Wide-necked Aneurysm Embolization and Reconstruction (ANSWER) trial have been published. 17 This was a prospective, nonrandomized, single-arm, multicenter study conducted at 10 neurovascular centers in the United States. The primary safety end points of the study were death or stroke in the downstream territory up to 180 days postprocedure. The technical success end points included device placement rates and ability of the device to retain coils within the aneurysm and the rate of aneurysm occlusion at day zero and at day 180, as adjudicated by the Raymond-Roy classification with angiographic evaluation conducted by a core laboratory. Thirty-four patients (29 female) were enrolled; their mean age was 60.9 years. Nine patients had previous aneurysm treatment with coils, and five had prior remote subarachnoid hemorrhage (SAH). The mean aneurysm neck size was 5.2 mm (range 2.3–11.6 mm), aneurysm height ranged from 2.4 to 15.9 mm (mean 7.2 mm), and dome size ranged from 2.8 to 16.3 mm (mean 7.0 mm). The mean dome/neck ratio was 1.4 (range 0.53–1.93) and the mean aspect ratio was 1.46 (range 0.3–2.76). Most of the treated aneurysms were located at the basilar apex (n = 27); the rest (n = 7) were located at the carotid apex. A T-configuration device was used in 23 aneurysms, and a Y-configuration device was used in the remaining 11 aneurysms. The device was placed in the branch vessels in 23 patients, in the aneurysm in 2 patients, and in a hybrid fashion in 9 patients (limbs of the device placed in both the aneurysm sac and the branch vessels). No deaths were attributed to the device or procedure, and 32 patients (94%) had a modified Rankin Scale score of 2 or less at 180-day follow-up. Three patients had intraprocedural complications, but none resulted in clinical sequelae or included rupture of an aneurysm. These complications included thrombus formation during coiling that was treated with abciximab and a femoral artery dissection. From a technical standpoint, the device was delivered and placed in all aneurysms. There were no cases of stent migration or coil herniation and only a single case of instent stenosis (3%) in a daughter vessel, which was both asymptomatic and less than 50%. Satisfactory aneurysm occlusion was observed in 79% of patients at day zero and in 88% of patients at 180-day follow-up.
The endovascular clip system (eCLIPs) is a hybrid device that shares similarities with a flow diverter. This device has an appearance similar to the skeletal structure of a snake; it consists of two parts, the “anchor” portion and a “leaf” portion. The anchor section is designed to be placed in an arterial branch to stabilize the device while the leaf portion covers the aneurysm neck and acts not only as a flow diverter but also as a brace if coils are placed in the aneurysm. Preclinical studies have shown that the device is easy to navigate, and porcine studies have shown complete or near-complete occlusion of aneurysms with preservation of the parent vessel on immediate postimplantation angiography. Neo-endothelialization across the aneurysm neck has also been shown on histopathologic examination. 18 Clinical evidence is limited regarding the use of this device. In 2018, Chiu et al 19 published their experience with the device in 33 patients from 13 centers in Canada and Europe. Of these 33 patients, 23 had 6-month follow-up data with 7 patients having Raymond-Roy classification I occlusion and 10 having Raymond-Roy classification II occlusion, resulting in 74% adequate occlusion. Two patients had acute periprocedural transient ischemic attacks; two patients had asymptomatic thrombotic events (morbidity of 9% at 6-month follow-up); and two patients had late aneurysm-related deaths unrelated to the device itself. The authors reported that, in eight patients, the device was not deployed for various reasons, including failure of the device to orient appropriately, a spasm in the branch vessel, and deciding not to deploy the device. This overall failure rate of approximately 24% may be explained in part by the learning curve. The eCLIPs device, unlike other available neck-bridging devices, requires the interventionist to catheterize both efferent branches consecutively. This, however, represents the most complicated part of the procedure when treating complex wide-necked bifurcation aneurysms. Additionally, although long-term follow-up data are not available, the device does not offer the potential advantage of altering the bifurcation angle of the daughter branches that must be catheterized if either Y-stenting or the eCLIPs device is used. This straightening and narrowing of the bifurcation angles shifts the high shear-stress gradients at the neck of the aneurysm and may assist in aneurysm occlusion and prevent recanalization. 20 Although this remodeling may not be required given the flow-diverting effect of the device, it remains to be seen whether the long-term outcome with this device is superior to standard Y-stenting.
Despite standard coiling of basilar apex aneurysms being safe, several studies have shown relatively low rates of complete occlusion (32–85%) 3 , 21 , 22 , 23 , 24 and high rates of recurrence and retreatment (17–23%). 3 , 21 , 22 Similarly, coil compaction is frequently seen with occurrence, with rates of 24% to 35%. 3 , 22 Newer intra-aneurysmal devices have been developed to address these deficiencies in coils. These intrasaccular flow diverters, namely the Woven EndoBridge (WEB; MicroVention) and Medina Embolization Device (MED; Medtronic) aim to disrupt intra-aneurysmal blood flow and promote thrombosis as well as neo-endothelialization at the aneurysm neck ( Fig. 27.4 ).
The MED is a three-dimensional layered structure made from a radiopaque shape-set core wire and shape-memory alloy filaments, which form a self-expanding mesh of multiple leaflets that enable flow diversion. These leaflets lie along the long axis of the core wire and, when they are deployed, the device assumes a spherical shape. The MED comes in two types: framing and filler. The filler variant is softer and designed to fill the internal space after a framing MED has been deployed to provide a suitable basket. 25 , 26 To date, the MED has not been assessed in a randomized controlled trial; however, small case series have been published and suggest a good aneurysm occlusion and safety profile. 25 , 27 Aguilar Pérez et al 27 point out that, since the MED conforms to a spherical shape, it may not be suitable for all aneurysms and careful patient selection is required when using this new device. Additionally, it is possible to use adjunctive devices with the MED, such as coils, a pCONus, or an intraluminal flow diverter.
The WEB has been more extensively investigated than the MED because it entered the marketplace earlier. In the WEB Clinical Assessment of Intrasaccular Aneurysm Therapy study, 56% of patients (23 of 41) had complete aneurysm occlusion at 6 months and 29% (12 of 41) had a neck remnant. 28 However, conflicting results regarding the long-term efficacy of this device have recently been published. Cognard and Januel 29 published the outcomes for 15 aneurysms treated with the WEB. In this series, radiologic deterioration in the appearance of the aneurysm was noted in 10 of 14 patients at initial follow-up (3–6 months) and in 4 of 7 patients with longer-term follow-up. This group also showed that compression of the WEB is possible and is likely involved in aneurysm recurrence. Sivan-Hoffmann et al 30 found radiologic deterioration in only 2 of 8 patients at 12 months. These disparities highlight the need for further long-term evaluation of this device.
Posterior Cerebral Artery Aneurysms
Clinical Aspects
Aneurysms of the PCA are rare, with an incidence of 0.5 to 2.3%. 31 , 32 PCA aneurysms are frequently fusiform, dissecting, and giant in comparison with aneurysms seen elsewhere. 33 Aneurysms of the P1 segment are often associated with the mesencephalic branches, whereas those arising from the more distal segment may represent dissecting aneurysms and often arise from the junction of the P2 and the posterior communicating artery. Patients with these aneurysms may present with SAH and with symptoms of mass effect, typically on the oculomotor nerve (cranial nerve [CN] III), especially if the aneurysm is located in the proximal segment of the vessel. Symptoms of visual disturbance, seizures, and temporal pain have all been described secondary to mass effect caused by PCA aneurysms. 33
Treatment Strategies
As with aneurysms elsewhere, coil occlusion of aneurysms of the P1 segment is a well-tested option with the aim to preserve the parent vessel. 31 Balloon-assisted coiling is an adjunctive technique for wide-necked aneurysms, as is stentassisted coiling. 34 Wherever possible, the P1 segment must be preserved because of the end-artery perforators and, in some circumstances, the artery of Percheron. For small P1 aneurysms, flow diversion is a viable option ( Fig. 27.5 ). For treatment of aneurysms located more distally in the vessel, beyond the P1/P2 junction, as well as dissecting aneurysms, parent artery occlusion (PAO) may be required, and this can be done with either detachable coils, detachable balloons, or polymer glue ( Fig. 27.6 ). 31 , 32 , 35 , 36 If the parent vessel is occluded, most patients are able to compensate via pial collaterals with the middle cerebral artery, communication between the anterior and posterior choroidal arteries, and the pericallosal arteries; however, ischemic complications have been reported with PAO. 31 , 36
Superior Cerebellar Artery Aneurysms
Clinical Aspects
Aneurysms arising from the superior cerebellar artery (SCA) are rare. Peluso et al 36 reported an incidence of 1.7% in their large series (36 of 2,112 aneurysms). The SCA is intimately related to the oculomotor, trochlear (CN IV), and trigeminal (CN V) nerves; thus, patients with aneurysms arising from this vessel can present with palsies of these nerves in addition to SAH. These symptoms may regress after treatment. 37 The aneurysms often arise from the very proximal segment of the SCA.
In the series of Peluso et al, 37 the mean size of SCA aneurysms was 7.3 mm and 65% (22 of 36) of the patients presented with acute SAH. Similarly, in the series of Haw et al, 38 64% (7 of 11) of the patients presented with SAH. In the series of Kim et al, 39 approximately 70% (37 of 53) of the aneurysms were less than 5 mm in size and more than 95% (51 of 53) of the aneurysms were 10 mm or smaller in size.
Related posts:
25 Brainstem and Thalamic Intraparenchymal Hemorrhage
29 Endovascular Management of Brainstem and Thalamic Arteriovenous Malformations
30 Surgery for Thalamic and Brainstem Cavernous Malformations
23 Brainstem Ischemia, Stroke, and Endovascular Revascularization of the Posterior Circulation
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31 Revascularization of the Brainstem
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