4.1 Introduction

The development of the Guglielmi detachable coil in the early 1990s ¹ represented a major paradigm shift in the treatment of intracranial aneurysms, the first alternative to open surgical reconstruction. Further development of adjunct endovascular devices such as stents and balloons allowed for successful treatment of a wider range of aneurysms by stent-assisted coiling and balloon remodeling. These early stents were primarily utilized as a scaffold to contain coils, allowing for increased packing density within the aneurysm and preventing encroachment on the lumen of the parent vessel. However, even with these significant advancements in endovascular management of aneurysms, there remains a subset of lesions particularly difficult to treat. Specifically, large (> 10 mm), giant (> 25 mm), fusiform, and wide-neck (dome-to-neck ratio < 2) aneurysms still pose challenges to practitioners, as aneurysm remnant or recurrence rates remain high after treatment for these lesions. ¹ Additionally, these types of aneurysms portend a much poorer natural history than small, saccular aneurysms. Giant aneurysms have a 5-year cumulative rupture risk of 40% in the anterior circulation and 50% in the posterior circulation. Additionally, morbidity and mortality with surgical treatment may be as high as 30%. Endovascular treatment of these lesions remains a challenge as well with occlusion rates of 57% and mortality rates of 7 to 11%. ¹

Flow diverting stents with lower porosity (higher metallic coverage) were recently developed with the intention of diverting blood flow away from the aneurysm, resulting in aneurysmal stasis and thrombosis, endothelialization across the aneurysm neck, and aneurysm regression. ¹ , ² This mechanistic strategy allows for remodeling of the diseased segment of the parent artery itself. ¹ Theoretically, parent vessel reconstruction versus aneurysm occlusion and passive vessel healing may provide a more durable treatment.

There are multiple flow diverting stents in practice, including Surpass (Stryker), Silk (Balt), and FRED (Microvention). While all these devices have CE mark and are in various stages of U.S. Food and Drug Administration (FDA) approval, the Pipeline Embolization Device (Medtronic) was FDA approved (2011) for use in the United States. Currently, its FDA approval is for adults with large or giant, wide-neck intracranial aneurysms between the petrous and supraclinoid segment of the internal carotid artery (ICA) proximal to the anterior choroidal artery. ¹ A brief comparison between coil embolization and flow diversion for aneurysm treatment is outline in Table 4.1.

4.2 Flow Diversion and Coil Embolization Technique: Benefits and Risks

To understand the benefits and risks of flow diverting stents, a brief understanding of the deployment procedure is necessary. Deployment of flow diverting stents entails passing a microcatheter distal to an aneurysm, deployment of the stent, and subsequent removal of the deployment device. As compared to coil embolization alone, deployment of flow diverting stents presents some technical challenges, including navigating a larger device through the intracranial circulation, achieving adequate device apposition against the parent vessel, and appropriate stent placement. ² This entails choosing a suitable stent diameter and ensuring the stent covers both proximal and distal segments of the parent vessel, given that the stent foreshortens once deployed. ¹ Additionally, care must be exercised to ensure that the distal catheter tip and deployment wire remain in the parent vessel and do not deviate into smaller perforators that may lead to complications. Balloon angioplasty may be required to fully appose the stent against the vessel wall, given that an endoleak will prevent complete aneurysm occlusion and obstruct endothelialization. ¹ Multiple flow diverting stents may be placed. However, after a flow diverting stent is placed, the interventionalist permanently surrenders endovascular access to the aneurysm. Catheter-based treatment will now be restricted to further flow-diverter placement and commitment of an even greater hardware burden to the parent artery. This represents a significant limitation of flow diversion technology. Although most practitioners are well versed in stent deployment, this novel device does present new challenges and technical nuances that must be mastered for optimal patient outcome.

As discussed further, immediate aneurysm occlusion after treatment is rarely observed after flow diversion, with rates reported around 8 to 21%. ² These flow diverting stents are utilized with the knowledge that aneurysm occlusion may take 6 to 12 months. For this reason, flow diversion is primarily utilized for unruptured aneurysms, ² given that the delayed occlusion may still allow for re-rupture after initial subarachnoid hemorrhage (SAH).

In contrast to flow diversion techniques, coil embolization involves catheterization of the aneurysm directly for placement of coils within the aneurysm. The coils induce thrombus formation and aneurysm occlusion in a more immediate manner as compared to flow diversion. As discussed, stent-assisted coiling (higher porosity stent) and balloon remodeling are techniques to enhance coil embolization methods. Because coil embolization is thrombogenic, coiling remains the first-line method for endovascular treatment of ruptured aneurysm.

The major procedural advantage in the placement of flow diverting stents is elimination of need for direct catheterization of the aneurysm. The advantage of not needing to catheterize the aneurysm becomes particularly apparent in non–flow-related aneurysms projecting inferiorly or laterally from the petrous or cavernous carotid.