The use of endovascular procedures as first line treatment for the management of VRAA is becoming increasingly popular (Belli et al., 2012). A minimally invasive approach may reduce treatment-related morbidity and mortality with studies demonstrating success comparable to conventional open repair (Kok et al., 2016). Technical success rates as high as 94% with visceral preservation rates up to 99% have been reported. Importantly, there is low treatment-related morbidity (< 4%) (Kok et al., 2016). While the natural history of unruptured VRAA is unclear, rupture is a significant complication and associated with high morbidity and mortality (Belli et al., 2012). Indications for treatment include rapid interval enlargement, size > 2 cm, systemic hypertension, portal hypertension (hepatic and splenic VAA), organ transplantation and pregnancy and true aneurysms of the gastroduodenal artery and pancreaticoduodenal arcade as these are at high risk of rupture (Loffroy et al., 2015).
Conventional endovascular approaches involve selective catheterization of the parent artery and subsequent unassisted coil embolization, allowing for rapid thrombosis of the aneurysm and exclusion from the circulation. Additional endovascular approaches include the use of vascular plugs, front and back door parent artery embolization (where possible), liquid embolic agents such as n-butyl cyanoacrylate or ethylene vinyl copolymer (Onyx) and precipitating hydrophobic injectable liquid (PHIL) (Kok et al., 2016). Parent artery sacrifice is also possible but is associated with the complications of end organ infarction and is not appropriate in many patients.
More recently, covered stent grafts have been used to treat VRAA with a technical success rate of up to 96% and low complications rates reported (Cappucci et al., 2017; Venturini et al., 2017). The ability to cover the aneurysm neck while maintaining flow within the parent artery makes these devices useful when the target aneurysm has a large neck or unfavorable morphology for coiling. They are particularly useful in the treatment of pseudoaneurysms. However, covered stent grafts are limited by their physical properties including higher radial opening force and increased rigidity. As a result, they often force the parent artery to adapt to their shape resulting in loss of the natural arterial curvilinear course with the potential for end organ thromboembolic complications. Additionally, covered stents obstruct side branch perfusion and have a large profile making their use in smaller vessels potentially more difficult.
Recently, neurointerventional approaches used commonly in the treatment of intracranial aneurysms have found application in the peripheral vasculature and for VRAA (Maingard et al., 2017). Balloon-assisted and stent-assisted approaches as well as novel scaffolding devices have made it possible to treat complex aneurysms with unfavourable morphology safely and effectively (Marotta et al., 2018; Spiotta et al., 2016; Labeyrie et al., 2017; Lubicz et al., 2016). Complete occlusion has been demonstrated at short and mid-term follow up (Pierot et al., 2018; Wakhloo et al., 2015; Maingard et al., 2017; Labeyrie et al., 2017; Lubicz et al., 2016). Ideal characteristics of a stent include low profile, flexibility and adaptability to varying parent vessel diameters and aneurysm lengths.
Flow diversion for intracranial aneurysms is a well-established approach particularly in those with complex or difficult morphology or location (Rajah et al., 2017). These devices allow parent artery remodeling while significantly reducing aneurysmal flow leading to eventual thrombosis. Computational flow dynamic models of commercially available flow diverters have demonstrated reductions in aneurysmal flow between 65% and 82%. Increased porosity and reduced pore density are associated with further reductions in aneurysm flow (Dholakia et al., 2017).
Several well-known and FDA approved endoluminal devices are available including the Pipeline Embolisation Device (PED, Medtronic, MN, US), Silk (Balt Extrusion, Montmorency, France), Flow Reduction Endoluminal Device (FRED, MicroVention Inc., CA, USA), p64 (Phenox, Bochum, Germany) and LEO Baby (Balt Extrusion, Montmorency, France). The unique braided design of these devices produces low radial opening forces to facilitate navigation while greater metal coverage and reduced porosity encourages aneurysm occlusion. Rapid flow reduction promotes thrombosis within hours to days, and sometimes months, and provides a scaffold for neointimal proliferation over the aneurysm neck (Dholakia et al., 2017; Kadirvel et al., 2014). The braided design and metallic coverage disrupts in- and outflow patterns at the aneurysm neck causing sluggish flow and increased blood viscosity, promoting thrombosis, while perforator and side branch perfusion is preserved. These devices have proven efficacy and safety for the treatment of a wide range of intracranial aneurysms (Madaelil et al., 2017; Bhogal et al., 2017; Zanaty et al., 2014; Zhou et al., 2017). These unique properties have been utilized when treating peripheral vasculature pathology (including VRAA) with good technical success (Sfyroeras et al., 2012).
More recently the Surpass Streamline flow diverting stent has achieved FDA approval for investigational use in the intracranial vasculature with preliminary results demonstrating high safety and efficacy. Large series demonstrate complete aneurysm occlusion comparable to stent-assisted coil embolization (up to 94%) with no reported mortalities (Wakhloo et al., 2015; De Vries et al., 2013). Large trials are ongoing (Kan et al., 2016).
The stent is a self-expanding tubular mesh composed of cobalt chromium. It has a significantly lower porosity than currently available flow diverting stents with a metal surface area coverage of 30%, a high mesh density and single-layer braided and tubular structure. The device comes in varying diameters including 3, 4 and 5 mm and lengths ranging from 12 to 50 mm and is deployed via a 3.7 French distal catheter with a pusher. While the increased filament density facilitates deployment and a braid angle prevents changes to mesh density as luminal diameter and vascular curvature changes, an increased metal surface area coverage and mesh density assists aneurysm occlusion with an added 18–22% reduction in aneurysm flow. Platinum wires with the braided mesh design also improves visualization during deployment. The delivery system tracks over any 0.014 in. wire allowing more distal and smaller vessel deployment.
In both cases, the wide necked morphology, parent vessel diameter and curvature (both aneurysm arising from the outer convexity) and the presence of important arterial branches precluded the use of conventional coil embolization, standard self-expanding or balloon-expandable covered stents. The Surpass flow diverter was ideally suited obviating the risk of distal end organ ischaemic complications. Recent publications demonstrate successful treatment of visceral aneurysms with stents with flow diverting properties, where similarly, parent and side branch patency is deemed important. The double-layer micromesh Roadsaver, similar to the Surpass, has increased mesh density and reduced poor size resulting in improved aneurysm occlusion (Akkan et al., 2017).
Importantly, after deployment of a flow diverting stent persistent flow within an aneurysm is expected immediately post procedure. In contradistinction to covered stent grafts, the Surpass flow diverter porosity and pore density reduces but does not completely occlude aneurysm flow. On the final angiogram, reduced but persistent flow was observed in patient 1 (OKM Grade B1) indicating successful treatment. Importantly, in patient 2 no change in flow was observed. However, there was stagnation of contrast within the aneurysm (OKM Grade A3). The reduction in flow and stagnation highlights the advantage of the Surpass flow diverter which allows for preservation of side branch and perforator vessels. It is important for operators using this device to be aware that it can take hours to days to achieve complete aneurysm occlusion. At 8 week and 3 weeks respectively both aneurysms were completely occluded on follow up imaging. Systemic anticoagulation and antiplatelet agents may interfere with this; however, a short course of dual antiplatelet therapy is required to maintain stent patency until endothelialization occurs.
We advocate for the use of dual antiplatelet therapy to prevent in-stent thrombosis when using flow diverting stents for peripheral aneurysms. Both patients were administered dual antiplatelet therapy for 1 week prior to the procedure (100 mg Aspirin and 75 mg clopidogrel) with the plan to continue this for 6 months postprocedure, at which point clopidogrel is ceased. Most studies evaluating flow diverting stents have used similar regimes and premature cessation of clopidogrel has been linked with a higher risk stent thrombosis, even at 3 months (De Vries et al., 2013). An alternative regime is to load the patients with higher doses of aspirin and clopidogrel in the hours prior to the procedure (Lv et al., 2016).
We encountered no complications in the treatment of our patients. Foreseeable complications include rapid in-stent thrombosis which can be attenuated with the use of systemic heparin and administration of intraarterial antiplatelet agents (e.g. tirofiban) immediately following stent deployment. Importantly, the Surpass flow diverter is limited to use in smaller vessels with a maximum vessel diameter of 5.3 mm with the 5 mm device. Consideration should also be given to material costs compared to conventional endovascular methods.
