Review ArticleFlow Diverters for Intracranial AneurysmsYazan J. Alderazi, Darshan Shastri, Tareq Kass-Hout,Charles J. Prestigiacomo, and Chirag D. GandhiDivision of Endovascular Neurosurgery, Department of Neurological Surgery, Rutgers University, New Jersey Medical School,90 Bergen Street, Suite 8100, Newark, NJ 07101, USACorrespondence should be addressed to Chirag D. Gandhi; [email protected] 1 December 2013; Accepted 29 April 2014; Published 20 March 2014Academic Editor: Moneeb EhteshamCopyright © 2014 Yazan J. Alderazi et al. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.Flow diverters (pipeline embolization device, Silk flow diverter, and Surpass flow diverter) have been developed to treat intracranialaneurysms. These endovascular devices are placed within the parent artery rather than the aneurysm sac. They take advantage ofaltering hemodynamics at the aneurysm/parent vessel interface, resulting in gradual thrombosis of the aneurysm occurring overtime. Subsequent inflammatory response, healing, and endothelial growth shrink the aneurysm and reconstruct the parent arterylumen while preserving perforators and side branches in most cases. Flow diverters have already allowed treatment of previouslyuntreatable wide neck and giant aneurysms. There are risks with flow diverters including in-stent thrombosis, perianeurysmaledema, distant and delayed hemorrhages, and perforator occlusions. Comparative efficacy and safety against other therapies arebeing studied in ongoing trials. Antiplatelet therapy is mandatory with flow diverters, which has highlighted the need for betterevidence for monitoring and tailoring antiplatelet therapy. In this paper we review the devices, their uses, associated complications,evidence base, and ongoing studies.1. IntroductionDuring recent decades, endovascular treatment of cerebro-vascular aneurysms has evolved to include unassisted coilembolization techniques, whose efficacy and safety are sup-ported by class-1-evidence, assisted coil embolization tech-niques, and newly developed techniques using flow divert-ers [1]. While the various coil embolization techniques,including balloon assisted and stent assisted coiling, aretargeted towards the aneurysm sac, flow diverters representa paradigm shift with the intervention carried out in theparent artery [2, 3]. Flow diverter aneurysm embolization canbe combined with coil embolization, further expanding theoptions available to clinicians and patients [3].Flow diverters were first tested in untreatable aneurysmsor those that had failed previous endovascular therapy [2].With the approval of these devices in the USA, Europe, andother countries experience with “off-label” uses is evolving. Inthis paper we review the use of flow diverters for treatmentof intracranial cerebral aneurysms. We review the putativemechanism of action, the technical features of devices andtheir uses, and the evidence for efficacy and safety of flowdiverters for intracranial aneurysms.2. Flow Diversion and Mechanism of ActionFlow diverters are stent-like devices that are deployed endo-vascularly to treat aneurysms. Conceptually, flow divertersallow endoluminal reconstruction rather than endosaccularfilling. Flow diverters take advantage of changing the parentartery/aneurysm sac interface, for example, altering in-flowand out-flow jets, to induce aneurysm thrombosis. Intrasac-cular thrombosis ensues after device deployment. Subsequentneointimal overgrowth covers the stent reconstructing theparent artery and eliminating the aneurysm/parent vesselinterface. This process usually spares the origins of perfora-tors [4, 5]. Furthermore, when used for fusiform aneurysmsHindawi Publishing CorporationStroke Research and TreatmentVolume 2014, Article ID 415653, 12 pageshttp://dx.doi.org/10.1155/2014/4156532 Stroke Research and Treatmentthese processes allow reconstruction of a smooth endothelialcovered channel in continuation with the parent artery [4].These features are thought to allow for durable reduction inrupture rates. With time, the aneurysm shrinks and collapsesaround the device construct relieving symptoms from masseffect [2]. The thrombosis and associated inflammation of theaneurysm may be accompanied by temporary perianeurys-maledemainsurroundingbraintissue[6]. In summary,flow diverters take advantage of hemodynamics, thrombosis,inflammation, healing, and endothelial regrowth to achieveendoluminal reconstruction and aneurysm obliteration.As opposed to coil embolization techniques, flow divertertechniques cause aneurysms to occlude over time rather thanimmediately at the end of the procedure. This explains whyaneurysm occlusion rates continue to increase between 6 and12 months with flow diverters [3, 7]. Side branches, such as theophthalmic artery with internal carotid flow diverters, mayremain patent or be occluded after flow diverter implantation(Figure 3)[8]. Similarly, perforators such as those fromthemiddlecerebralarteryorthosefromthebasilararteryusually remain patent; however, occlusions may occur [5, 9].The incidence, clinical relevance, and risk factors for theseocclusions are areas of ongoing research.The terms porosity, metal coverage, and pore densityare used to describe device and deployment features thatare important for flow diverter efficacy. The terms porosityand metal coverage are related. Porosity is defined as theproportion of the open metal-free area to the total stentarea and metal coverage is the closed metal-covered areadivided by the total stent area. Occasionally porosity or metalcoverage is used to refer specifically to the area across theaneurysm neck. Some authors have termed this part of thestent the free stent segment [10]. Pore density is the number ofpores per area (pores/mm2). Depending on the flow diverter,pore density may change or remain constant as the size ofthe diverter is increased. For example, in larger diameterflow diverters additional wire struts within the flow diverterwall are needed to maintain constant pore density [11]. Metalcoverage across the aneurysm neck can be changed by vesselcurvature and stent compaction during deployment [12, 13].Experimental models have suggested that porosity is the mostimportant factor in reducing intra-aneurysmal flow, withporosity of 60–76% being optimal (Figure 1)[14,
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