This stents are placed in the parent artery at the level of the aneurysm neck to disrupt the intra-aneurysmal flow thus favoring intra-aneurysmal thrombosis.
The use of flow diverting stents (FDS) has increased in popularity since their introduction into clinical practice.
Flow diversion is now an established technique to treat unruptured intracranial aneurysms not readily amenable to endovascular coil embolization or open microsurgical occlusion.
The endovascular treatment of intracranial aneurysms with unfavorable anatomy (large aneurysms, wide-neck) is frequently challenging and is also associated with a high incidence of significant recurrences.
The use of these stents is advisable mainly for unruptured aneurysms, particularly those located at the internal carotid artery or vertebral and basilar arteries, for fusiform and dissecting aneurysms and for saccular aneurysms with large necks and low dome-to-neck ratio. The rate of aneurysm occlusion progressively increases during follow-up (81.5% overall rate in this review). The non-negligible rate of ischemic (mean 4.1%) and hemorrhagic (mean 2.9%) complications, the neurological morbidity (mean 3.5%) and the reported mortality (mean 3.4%) are the main limits of this technique 1).
The mechanism of action that results in aneurysm exclusion from the circulation initially involves flow redirection with the subsequent development of a neointimal covering on the surface of the FDS that reconstructs the parent vessel and excludes the aneurysm from the circulation 2).
They take advantage of altering hemodynamics at the aneurysm/parent vessel interface, resulting in gradual thrombosis of the aneurysm occurring over time. Subsequent inflammatory response, healing, and endothelial growth shrink the aneurysm and reconstruct the parent artery lumen while preserving perforators and side branches in most cases. Flow diverters have already allowed treatment of previously untreatable wide necked aneurysm and giant aneurysms.
The emerging strategy of maximum FD compaction can double aneurysmal flow reduction, thereby accelerating aneurysm occlusion. Moreover, ultrahigh blood shear stress was observed through FD pores, which could potentially activate platelets as an additional aneurysmal thrombosis mechanism 3).
Flow-diversion technique is well-suited for the treatment of large, giant, wide-necked, and fusiform intracranial aneurysms because it does not rely on endosaccular packing with coils but rather on the strategy of placing a stent across the aneurysm “neck” or across the diseased segment of a vessel in case of a fusiform aneurysm. Over time, neointimal endothelium covers the flow diverter such that it becomes incorporated into the parent vessel wall and occludes the aneurysm from the circulation, effectively repairing the diseased parent vessel segment 4).
The use of FD has recently expanded to cover many types of IAs in various locations. Some institutions even attempt FD as first line treatment for unruptured IAs.
The woven endobridge aneurysm embolization device (WEB) is the first intrasaccular flow-diverter device dedicated to IA treatment.
This treatment was feasible and mostly used in bifurcation aneurysms (MCA, BA, ICA) with unfavorable anatomy. Further studies are needed to precisely evaluate the indications, safety, and efficacy of this new technique 5).
Surpass flow diverter
The most widely used devices are the pipeline embolization device (PED), the SILK flow diverter (SFD), the flow redirection endoluminal device (FRED), and Surpass. Many questions were raised regarding the long-term complications (i.e., delayed bleeding and device migration), the optimal regimen of dual antiplatelet therapy (APT), and the durability of treatment effect 6).
The FD technique relies on a concept of endoluminal reconstruction of the parent artery and the aneurysm neck by excluding the aneurysm from the circulation. The stasis of blood flow in the aneurysm leads to an inflammatory response followed by thrombosis and “healing” of the aneurysm while the stent acts as a scaffold for neointimal proliferation and remodeling of the parent vessel. Therefore, the FD approach is considered physiologic as it restores the normal homeostasis. A recent study showed that flow-diverter device (FDD) reduces the velocity in the aneurysm sac significantly more than multiple “non-flow diverter” stents, even though both dramatically reduce the aneurysmal fluid movement 7).
To break the communication between the parent artery and the aneurysm while maintaining a patency of sidewall branches, the device must fulfill two requirements: a low porosity (metal-free to metal-covered area) and a high pore density (number of pores per square millimeters for a given porosity) 8) 9). However, sidewall branch occlusions do not always lead to ischemia since collaterals may maintain flow to the dependent area. Even more, when collaterals are not present, the increased demand for tissue perfusion may, in some cases, generate a pressure gradient sufficient to maintain an anterograde flow through the device 10). The technique involves navigating an FDD through the arterial system and deploying it across the aneurysm neck. Proper deployment is essential as inadequate wall apposition may decrease the flow with consequent thrombus formation at the interface followed by thromboembolic events 11). Proper deployment and adequate wall apposition can be achieved by balloon (Boston angioplasty) 12) , though not always needed. More so, the increased turbulence along with the lytic enzymes released from platelet aggregation predisposes to a possible lysis of the aneurysmal wall that can usually occur in the following days post-op 13).
Kiselev et al., from the Centre of Angioneurology and Neurosurgery of E.N. Meshalkin National Medical Research Center in Novosibirsk, Russia, performed a prospective randomized comparison of clinical and surgical outcomes of flow diversion versus PVO and bypass in patients with complex anterior circulation aneurysms.
39 out of 40 patients (97.5%) from matched flow diverter (FD) group reached good clinical outcome. In the matched bypass group acceptable outcome was achieved in 32 (80%) out of 40 patients (difference between groups p = 0.029). The morbidity and mortality rates were 15% and 5%, respectively. Difference in the rates of favorable outcomes, compared by χ2 met statistical significance (p = 0.014). The rate of complete aneurysm occlusion at 6 months was 42.5% in the FD group and 95% in surgical group (p < 0.0001). The rate of complete occlusion at 12 months was 65% in the FD group and 97.5% in surgical group. The difference between groups was still significant (p = 0.001). There were no significant differences between groups by occurrence of ischemic (p = 0.108) and hemorrhagic (p = 0.615) complications.
The study demonstrated superior clinical outcomes for endovascular flow diversion in comparison with bypass surgery in treatment of complex aneurysms. Though, both techniques grant similar percentage of major neurologic complications and comparable cure rate for cranial neuropathy. Nevertheless, flow diversion is associated with significantly lower early obliteration rate, thus possesses patient for risks of prolonged dual antiplatelet regimen and delayed rupture. Hence, it's important to stratify patient by the natural risk of aneurysm rupture prior to treatment selection 14).
Eleven patients with 12 aneurysms were treated with flow diverters. Two patients had ruptured dissecting aneurysms. One patient with a basilar trunk aneurysm died of acute in stent thrombosis and another patient died of brain stem ischaemia at 32 months follow-up. One patient had ischaemia with permanent neurological deficit. Two aneurysms are still open at up to 30 months follow-up. Flow diversion was used in 2% of all endovascular treatments. Both our own poor results and the high complication rates reported in the literature have converted our initial enthusiasm to apprehension and hesitancy. The safety and efficacy profile of flow diversion should discourage the use of these devices in aneurysms that can be treated with other techniques 15).
Although initially considered safe when covering bifurcation sites, flow-diverting stents may provoke thrombosis of side branches that are covered during aneurysm treatment.
There are risks with flow diverters including in-stent thrombosis, perianeurysmal edema, distant and delayed hemorrhages, and perforator occlusions. Comparative efficacy and safety against other therapies are being studied in ongoing trials. Antiplatelet therapy is mandatory with flow diverters, which has highlighted the need for better evidence for monitoring and tailoring antiplatelet therapy 16).
Symptomatic modifications of side branches after flow diverter treatment depend on the extent and type of collateral supply 17).
A systematic electronic database search was conducted using MEDLINE, PubMed, Springer, and EBSCO for all accessible articles on FDDs published until December 2014. Abstracts, full-text manuscripts, and the reference lists of retrieved articles were analyzed. Random effects meta-analysis was used to pool the occlusion rate outcomes across studies.
Fifty-nine studies containing efficacy data on 2263 patients with more than 2493 treated aneurysms were included in the analysis. The overall complete occlusion rate was 82.5% (95% CI, 78.8%-86%) across studies. The success rate of FDD implantation was 97.4% (95% CI, 95.4%-99.4%). The occlusion rate for anterior circulation aneurysms was 83.3% (95% CI, 71.2%-95.4%); with regard to complete occlusion, the odds ratio for anterior circulation aneurysms was significantly higher than that of posterior circulation IAs (odds ratio, 1.93; 95% CI, 1.00-3.73).
FDDs have high technical success rates in the management of IAs. Additional studies on well-designed multicenter randomized controlled trials will be required to validate the findings of the present study and to identify the best therapeutic strategy for IAs depending on their size, location, and characteristics 18).