Incidence of VRAAs is increasing due to widespread use of cross-sectional imaging, as well as a rise in invasive procedures resulting in iatrogenic aneurysms. Prophylactic treatment for incidental aneurysms is favourable in situations where there is a high-risk of rupture given the potential for high morbidity and mortality. Excellent technical success rates with 90–95% of VRAAs able to be excluded with preservation of afferent arteries using an endovascular approach (Loffroy et al. 2015), also means that the risk-benefit ratio of treatment has shifted allowing treatment in more complex morphologies including those with wide necks or significant parent artery tortuosity.
Increasingly, endovascular therapy has been used as first-line treatment for VRAAs. Benefits include potential avoidance of general anaesthesia, shorter hospital stay, lower complication rates, improved visceral organ preservation and reported high technical success rates (Belli et al. 2012; Kok et al. 2016).
Possible complications of endovascular treatment include end-organ infarction (which may be clinically insignificant), arterial dissection, thrombosis, aneurysm or parent artery rupture, access complications such as groin haematoma or pseudoaneurysm, and major cardiac or cerebrovascular thromboembolic events. Published re-intervention rates due to delayed recurrence or aneurysm enlargement are low (4.4%) (Kok et al. 2016).
Conventional approaches with stent graft repair are complicated by the potential for kinking and injury of the parent artery with stiffer delivery systems, difficulty with accessibility of parent vessels and difficulties with adequate stent wall apposition and endoleak in tortuous anatomy (Sachdev-Ost 2010). In the past, repair utilising stents was hindered by tortuosity of the arteries, especially the splenic artery. However, reductions in available device diameters and improvements in flexibility make stenting an increasingly viable option.
Flow-diversion is a newer technique which can be utilised for VRAAs. Flow diversion is commonly utilized in neurointervention for intracranial aneurysms to reduce aneurysm inflow and outflow resulting in progressive stasis and thrombosis of the aneurysm sac. In VRAA, flow diverter stents are potentially useful for aneurysms with wide necks or that cannot be managed with covered stent placement because ofinsufficient landing zones within tortuous anatomy. This is especially important in distal renal artery aneurysms which occur in proximity to the bifurcation where flow-diversion is particularly valuable in preserving side-branch patency (Loffroy et al. 2015). Recent reports of flow diversion for VRAA and peripheral aneurysms demonstrate technical feasibility and early safety with aneurysm thrombosis reported in up to 90.6% to 98.5% of cases with aneurysm volume reduction in up to 82.7% of cases (Colombi et al. 2018; Maingard et al. 2019; Sfyroeras et al. 2012). Reported complication rates are low with 8.3% rate of acute stent thrombosis (Colombi et al. 2018; Maingard et al. 2019; Sfyroeras et al. 2012).
The CASPER stent provides a suitable alternative to flow diversion (Fig. 5). While not certified as a flow diverter the stent acts to reduce intra-aneurysmal flow in a similar mannerIt is designed as a braided, dual layer micromesh nitinol stent designed to prevent embolization of plaque in the treatment of carotid artery stenosis). The inner mesh has a smaller cell size of 375-500 μm to prevent embolic release from mural atheroma, while the braided nitinol design aims to minimize kinking and improve conformability to arterial anatomy. This is optimal for preventing embolization to distal organs in VRAAs but also provides flow diverting properties not dissimilar to certified flow diverters used to treat intracranial aneurysms where increased metal coverage reduces inflow jets and causes significant aneurysm sac thrombosis. A recent report by Akkan et al. reports successful treatment in 3 visceral aneurysms with the Roadsaver stent with successful obliteration on follow up imaging without reported complications (Akkan et al. 2017). Additional benefits for its use in VRAA include availability of larger stent sizes compared to conventional flow diverters which may be more suitable in larger splenic arteries with increased flexibility compared to covered stents, a potential advantage in tortuous or loop arteries, improved stent delivery and accuracy of deployment and improved wall apposition with the ability to use smaller 5Fr access depending on parent vessel tortuosityreducing post-procedure groin complications. In addition to this, the stent can be repositioned until up to 50% has been unsheathed. Importantly, this stent allows for side-branches to be preserved.
Its properties are offset by the reduced rates of early aneurysm occlusion compared to covered stents and increased risk of early thrombosis compared to uncovered stents used during stent assisted coiling due to increased metal coverage in its dual micromesh design.
Our preliminary experience with the CASPER stent for treatment of VRAAs shows that this is a technically feasible and relatively safe technique, with successful deployment and patency of the parent artery preserved during the procedure in all cases and immediate reduction of flow in all treated aneurysms. Aside from a single case, follow-up imaging demonstrated durable preservation of parent arterial patency and either reduction in size, partial or complete thrombosis of the aneurysms. Unfortunately, a single patient suffered a delayed 1 month large infected splenic infarct necessitating surgical splenectomy which was felt to most likely relate to thromboembolic complications due to multifocal stent narrowing which was initially managed with post deployment angioplasty with an initial good angiographic result.
The expectation when deploying a flow diverter is to see either slight flow reduction on the post deployment angiogram or no discernible difference in aneurysm flow. This was observed in all of the treated aneurysms in this series. Completion angiography is mostly useful to demonstrate patency and flow in the parent vessel and its branches, stent position and stent apposition to the vessel wall. A CTA at 6 weeks post procedure should show partial thrombosis and a reduction in aneurysm size and a 6-month post procedure CTA should show occlusion of the aneurysm in most cases although complete aneurysm occlusion can take upwards of 12 to 18 months depending on parent vessel morphology, aneurysm size and the utilisation of antiplatelet medications or anticoagulation. If there is persistent high flow and no reduction in aneurysm size at 6 to 12-month follow up, consideration should be given to deploying a second stent to further reduce inflow and promote stasis.
Deployment of the CASPER stent can be combined with advanced access techniques to access tortuous peripheral visceral arteries. The balloon anchoring technique was utilised in case 1 in order to advance the 6Fr NeuronMAX 088 into the mid to distal tortuous splenic artery. This is a commonly used technique in neurointervention. For example, the Flowgate balloon guide catheter (Stryker, MI, USA) can be used in order to anchor into distal tortuous vessels to provide a scaffold to either straighten the proximal catheter itself through proximal arterial tortuosity or through a difficult aortic arch or to advance more flexible distal access catheters without losing position. Similarly, within the intracranial vasculature, this can be performed with dual lumen balloon microcatheters to anchor distally while advancing distal access catheters to a more distal and stable position for intracranial aneurysm coil embolization or during endovascular thrombectomy during stroke. Significant tortuosity in case 1 was overcome using this technique using a 8x40mm Armada balloon to provide a scaffold to advance the 6Fr Neuronmax 088 guide sheath (Fig. 2). This technique can be used for difficult tortuous access in many peripheral interventions.
We routinely use dual antiplatelet therapy in patients in whom CASPER stents will be deployed to reduce the risk of acute stent thrombosis. Our practice is to utilise 7 days of 100 mg aspirin and 75 mg clopidogrel prior to the procedure. Intra-arterial glycoprotein IIb/IIIa inhibitors can be used as a rescue therapy for acute stent thrombosis intra-procedurally. Dual antiplatelet therapy is routinely continued following stent deployment for at least 3 months to reduce the risk of early in stent stenosis.
There were some limitations associated with using the CASPER stent. In case 2, the stent would not re-sheath for repositioning due to excess splenic artery tortuosity resulting in poor wall apposition and persistent flow at 12 months requiring a repeat procedure. Following a second CASPER deployment the aneurysm was completely occluded at 1 month. Use of the CASPER stent can be restricted due to limited size ranges, where the diameter is too small for some splenic arteries and the 40 mm length limit too short. Additionally, the cost associated with parent vessel reconstruction due to the cost of the CASPER stent itself and the need for additional guide sheaths and intermediate catheters should be weighed against parent artery sacrifice using conventional coiling which can be technically less challenging and significantly cheaper.
This series is limited by the variability in imaging follow up modalities with 2 patients followed up at external institutions with duplex Doppler ultrasound rather than CTA.