Hindawi Publishing Corporation Stroke Research and Treatment Volume 2014 Article ID 415653 12 pages http dx doi org 10 1155 2014 415653 Review Article Flow Diverters for Intracranial Aneurysms Yazan J Alderazi Darshan Shastri Tareq Kass Hout Charles J Prestigiacomo and Chirag D Gandhi Division of Endovascular Neurosurgery Department of Neurological Surgery Rutgers University New Jersey Medical School 90 Bergen Street Suite 8100 Newark NJ 07101 USA Correspondence should be addressed to Chirag D Gandhi gandhich njms rutgers edu Received 1 December 2013 Accepted 29 April 2014 Published 20 March 2014 Academic Editor Moneeb Ehtesham Copyright 2014 Yazan J Alderazi et al This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited Flow diverters pipeline embolization device Silk flow diverter and Surpass flow diverter have been developed to treat intracranial aneurysms These endovascular devices are placed within the parent artery rather than the aneurysm sac 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 neck and giant aneurysms 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 In this paper we review the devices their uses associated complications evidence base and ongoing studies 1 Introduction During recent decades endovascular treatment of cerebrovascular aneurysms has evolved to include unassisted coil embolization techniques whose efficacy and safety are supported by class 1 evidence assisted coil embolization techniques and newly developed techniques using flow diverters 1 While the various coil embolization techniques including balloon assisted and stent assisted coiling are targeted towards the aneurysm sac flow diverters represent a paradigm shift with the intervention carried out in the parent artery 2 3 Flow diverter aneurysm embolization can be combined with coil embolization further expanding the options available to clinicians and patients 3 Flow diverters were first tested in untreatable aneurysms or those that had failed previous endovascular therapy 2 With the approval of these devices in the USA Europe and other countries experience with off label uses is evolving In this paper we review the use of flow diverters for treatment of intracranial cerebral aneurysms We review the putative mechanism of action the technical features of devices and their uses and the evidence for efficacy and safety of flow diverters for intracranial aneurysms 2 Flow Diversion and Mechanism of Action Flow diverters are stent like devices that are deployed endovascularly to treat aneurysms Conceptually flow diverters allow endoluminal reconstruction rather than endosaccular filling Flow diverters take advantage of changing the parent artery aneurysm sac interface for example altering in flow and out flow jets to induce aneurysm thrombosis Intrasaccular thrombosis ensues after device deployment Subsequent neointimal overgrowth covers the stent reconstructing the parent artery and eliminating the aneurysm parent vessel interface This process usually spares the origins of perforators 4 5 Furthermore when used for fusiform aneurysms 2 these processes allow reconstruction of a smooth endothelial covered channel in continuation with the parent artery 4 These features are thought to allow for durable reduction in rupture rates With time the aneurysm shrinks and collapses around the device construct relieving symptoms from mass effect 2 The thrombosis and associated inflammation of the aneurysm may be accompanied by temporary perianeurysmal edema in surrounding brain tissue 6 In summary flow diverters take advantage of hemodynamics thrombosis inflammation healing and endothelial regrowth to achieve endoluminal reconstruction and aneurysm obliteration As opposed to coil embolization techniques flow diverter techniques cause aneurysms to occlude over time rather than immediately at the end of the procedure This explains why aneurysm occlusion rates continue to increase between 6 and 12 months with flow diverters 3 7 Side branches such as the ophthalmic artery with internal carotid flow diverters may remain patent or be occluded after flow diverter implantation Figure 3 8 Similarly perforators such as those from the middle cerebral artery or those from the basilar artery usually remain patent however occlusions may occur 5 9 The incidence clinical relevance and risk factors for these occlusions are areas of ongoing research The terms porosity metal coverage and pore density are used to describe device and deployment features that are important for flow diverter efficacy The terms porosity and metal coverage are related Porosity is defined as the proportion of the open metal free area to the total stent area and metal coverage is the closed metal covered area divided by the total stent area Occasionally porosity or metal coverage is used to refer specifically to the area across the aneurysm neck Some authors have termed this part of the stent the free stent segment 10 Pore density is the number of pores per area pores mm2 Depending on the flow diverter pore density may change or remain constant as the size of the diverter is increased For example in larger diameter flow diverters additional wire struts within the flow diverter wall are needed to maintain constant pore density 11 Metal coverage across the aneurysm neck can be changed by vessel curvature and stent compaction during deployment 12 13 Experimental models have suggested that porosity is the most important factor in reducing intra aneurysmal flow with porosity of 60 76 being optimal Figure 1 14 15 3 Devices and Technique Currently there are three main flow diverters the pipeline embolization device
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