Treatment threshold for VAA depends on both the size and the location of the aneurysm: for splenic aneurysms the treatment is commonly indicated in case of lesions larger than 2 cm, while for renal artery aneurysm the threshold is slightly lower (1.5 cm) (Belli et al. 2012; Carroccio et al. 2007; English et al. 2004). However, according to some reports, there is no correlation between renal artery aneurysm diameter and rupture; therefore, since data from published series are scarce and heterogenous, and there is not a recommended standard for indication, a decision to treat should be made on a case by case basis (Yasumoto et al. 2013; Pitton et al. 2015). There are few data comparing surgery with endovascular treatment of VAA, mostly because of the uncommonness of the condition; however a reduction in complication rates, hospitalization time and overall cost has been observed with endovascular techniques (Hislop et al. 2009), which also have been shown to have excellent early and midterm outcomes (Etezadi et al. 2011). Among endovascular options, coil embolization and stent graft exclusion are the most frequently utilized techniques. Nevertheless, embolization with coils requires sac catheterization and is not feasible in the case of unfavorable large aneurysm neck or in the presence of sidebranches arising from the neck or the aneurysmal sac itself. Moreover, intrasaccular maneuvers are not safe in the case of pseudoaneurysms, and may lead to intraprocedural rupture of the aneurysm which is a life-threatening complication. Furthermore, aneurysmal exclusions with covered stents may affect sidebranches perfusion¸ potentially causing end-organ ischemia; also, stent-grafts tend to have a large and stiff profile making their use in smaller and more tortuous vessels potentially more difficult (Murray et al. 2019).
An ideal device for visceral aneurysms repair should have a low profile and be flexible enough to be deployed in difficult anatomies; furthermore, it should also achieve aneurysm exclusion avoiding the risk of sac catheterization and potential rupture, even in presence of large, unfavorable aneurysm necks, while preserving existing sidebranches patency. The option of flow-modulation as a treatment tool aimed at excluding aneurysms, while preserving the patency of the parent artery and sidebranches, was the strategy behind using Cardiatis Multilayer Stent (Cardiatis, Isnes, Belgium), which is a cobalt self-expandable stent specifically approved for visceral and peripheral artery aneurysm repair. Early reports showed encouraging results with stent patency rate of 89% and aneurysm exclusion rate of 84% at 1-year follow-up (Ruffino et al. 2012). However, mid-term results were judged unsatisfactory, since the data showed a drop in stent patency rates (60% at 2-years) and safety concerns were raised after a case of disconnection at 2 years was reported (Balderi et al. 2013; Ferrero et al. 2013). Cerebral FDS, on the other hand, have unique characteristics which make these device close to the definition of the “ideal tool”: in fact the greater metal coverage of these stents gives them a design that promotes slow and progressive thrombosis of the aneurysm by reducing the flow at the aneurysm neck, disrupting aneurysm influx and efflux and creating a turbulence which leads to an increased blood viscosity within the sac (Seshadhri et al. 2011). It has been shown that the endovascular mesh operates as a frame for endothelization, jailing the aneurysm neck and resulting in angiographic aneurysm exclusion (Kallmes et al. 2007). When an FDS is placed across a sidebranch or a perforator, the laminar flow into these vessels is preserved through the stent interstices, as long as a pressure gradient persists (Bhogal et al. 2017). Although the flow-modulation mechanism leads to earlier sac depressurization, the time required to achieve aneurysm exclusion, in comparison to conventional techniques, is longer, in terms of weeks to months (Sfyroeras et al. 2012; Dholakia et al. 2017; Leonardi et al. 2011). However, rather than the thrombosis, the most predictive effect of clinical success is the dimensional reduction of the sac, which is the result of aneurysm depressurization (Sfyroeras et al. 2012). Nonetheless, in the majority of published series, aneurysm sac thrombosis was evaluated primarily and the aneurysms were shown to undergo progressive exclusion over the following six to 12 months, but the rate and degree of thrombosis were inconstant (Lylyk et al. 2009). Recently published results from the Fred Italian Registry Follow-up have shown complete or nearly complete occlusion of the aneurysm in 94% of cases at 3–6 months, increasing to 96% at 12–24 months’ follow-up. The use of cerebral FDS for the treatment of hepatic, renal and other VAA and VAP has been reported since 2012, with good clinical outcomes (Hardman et al. 2015; Colombi et al. 2018; Abraham et al. 2012; Adrahtas et al. 2016; Maingard et al. 2019; Shlomovitz et al. 2011). Similarly to the results of those reports, in this series the stent primary and assisted primary patency at 1-year were 83.3% and 100%, and all aneurysms showed dimensional reduction varying from 12.5 to 100% (mean 55.8%). Eight out of nine sidebranches (88.9%) arising from the aneurysm and covered by the flow diverters were patent at the 1-year follow up. One segmental renal branch underwent ostial occlusion between 6 and 12 months, as a result of aneurysm sac thrombosis, and the patient developed a small asymptomatic ischemia of the upper lobe of the kidney (see Figs. 4 and 5). However, the extent of the organ ischemia was smaller with respect to the size of the occluded branch: this could be explained by the presence of a collateral circulation, which progressively lowered the pressure gradient necessary for maintaining a direct flow into the sidebranch.
The pseudoaneurysms treated in this series were a consequence of acute spontaneous renal artery dissection, which affected the renal arteries at the level of the segmental bifurcation. The clinical manifestation was the combination of severe flank pain and uncontrollable hypertension. Additionally, in these cases, baseline CTA showed the presence of circumscribed renal infarctions. The decision was made to use an FDS for the treatment in order to achieve flow remodulation without excluding the segmental sidebranches, and potentially worsening renal perfusion. For this reason, it was not considered appropriate to use either stent-grafts or coronary stents, which would have not allowed the exclusion of the pseudoaneurismatic sac from the flow, and would furthermore have had a stiffer profile than an FDS.
When using an FDS for treating a visceral aneurysm, the operator must take into account that this kind of stent requires a necessary learning curve for non-neurointerventionists; in fact the deployment mechanism of the device is not based on the classical pull-back stent movement; rather, a combination of push-forward and pull-back (“push and pull”) techniques may be required, because FDS have low radial opening forces (Dmytriw et al. 2019). It is also necessary that the device be correctly sized during preprocedural planning, because undersizing of the FDS may cause inadequate wall apposition of the device and incomplete coverage of the aneurysm neck, which may compromise aneurysm occlusion (Estrade et al. 2013; Mut and Cebral 2012). On the other hand, oversizing of the device could alter the hemodynamic properties of the FDS, possibly leading to in-stent stenosis (Kellermann et al. 2019). Since these stents are specifically designed for intracranial circulation, the maximum available diameter is 5.5 mm, thus the treatment can only be proposed for vessels with a maximum caliber of 5 mm. Furthermore, since FDS have greater metallic surface coverage, with higher porosity in comparison to traditional bare stents, they are burdened by a higher incidence of thrombosis, up to 8.3% at 30-days (Sfyroeras et al. 2012), therefore double antiaggregation is mandatory. Finally, the high cost of these devices must be taken into account, inasmuch as the use of the cerebral FDS in the peripheral system is off-label, thus healthcare reimbursement may not cover the full cost of the product.
However, there are several clear advantages to using flow diversion techniques: the aneurysmal artery is treated at the neck, which is the point most at risk of future recurrence, sidebranches patency is preserved and, finally, the risk of incidental rupture associated with aneurysm sac catheterization and intrasaccular maneuvers are avoided.