By Gabor Toth
Due to their unique characteristics, flow diverters were originally intended to provide a reconstructive treatment option for atypical cerebral aneurysms which previously had no feasible management alternatives, failed prior therapy, or conventional endovascular methods (coiling, stent-assisted coiling, balloon-assisted coiling, liquid embolic agents) showed suboptimal results or increased recurrence. These devices are metal mesh tubes, which on first glance may resemble conventional stents. However, significant differences are noted in that they have higher surface metal coverage and lower porosity than previously available vascular reconstruction devices.
As the name suggests, the primary goal of flow diverters is to redirect blood flow toward the normal route in the parent vessel, leading to thrombosis and obliteration of the aneurysm lumen. They also provide a scaffolding for later neoendothelial proliferation, and vessel wall healing. Flow diverters practically create a new, artificial lumen inside the parent vessel, bridging the diseased aneurysmal segment. This is considered a paradigm shift compared to prior endovascular methods, which predominantly aimed at providing treatment from inside the aneurysm sac. Another important consideration is that the porosity of these stents is designed to allow small perforators to be preserved due to flow demand. Early studies demonstrated involution or regression of the aneurysm sac with decreased mass effect on the surrounding parenchyma in many flow diverter cases, which would be difficult to achieve with previous endovascular techniques.
The first two available devices were the Pipeline (ev3/Covidien/Medtronic), FDA approved in the USA in 2011, and the Silk (SFD, Balt Extrusion), approved in Europe around 2008. The Pipeline device is a 25% platinum and 75% nickel-cobalt-chromium alloy with approximately 30–35% metal surface area coverage. Silk is a nickel-titanium (nitinol) alloy with platinum microfilaments providing a 35–55% metal surface area coverage with a pore size of 110–250µm. There are other devices currently in clinical trials. The Surpass (Stryker Neurovascular) is made of cobalt-chromium strands with interwoven platinum/tungsten wires, 30% metal surface area coverage, and 20–32 pores/mm2. The FRED device (Microvention) has a dual-layer design with woven inner and outer stents; the inner low-porosity stent has 48 braided nitinol wires, the outer high-porosity stent has 16 wires and serves as a scaffold.
The early results with flow diverters have been promising. Short-term, six-month occlusion rates ranged from 82–94% in the initial Pipeline studies (Lylyk et al, Neurosurg 2009; Szikora et al, AJNR 2010; Nelson PK et al, AJNR 2011), and one year occlusion rates were as high as 86.8% in the PUFS trial (Becske et al, Radiology 2013). There were no significant aneurysm recurrences in any of these studies. Major complication (stroke or death) rate with Pipeline was in the 5–6% range. Initial case series with the Silk device reported up to 5% mortality and 15% morbidity rates (Wagner et al, Neuroradiology 2012; Velioglu et al, Neuroradiology 2012; Lubicz et al, Stroke 2010). Complete occlusion rates with Silk were ranging 68-87% at 6-17 months of mean follow-up. Recent meta-analyses of multiple flow diverter studies including Pipeline and Silk devices (Brijinki et al, Stroke 2013; Arrese et al, Neurosurg 2013) also demonstrated over 75% overall occlusion rate of treated aneurysms, but mean follow-up times were less than one year. The overall early mortality rates appeared 4% or less, and morbidity rates were 7.3% or less in the studied timeframe. Potential complications mostly included early or delayed haemorrhagic (ICH and SAH) and ischaemic strokes. Not unexpectedly, more complications were noted in posterior circulation aneurysms compared to anterior circulation. Among others, our recently published paper (Toth et al, J Neurointerv Surg. 2014 Jul 1. pii: neurintsurg-2014-011281. doi: 10.1136/neurintsurg-2014-011281) also discusses the difficulties and increased flow diverter treatment risks in posterior fossa aneurysms. Particular challenges are fusiform basilar aneurysms, and the abundance of small perforators.
There are several trials ongoing to compare the efficacy and safety of flow diversion with surgical, conventional or other endovascular management options (FIAT, EVIDENCE, LARGE, MARCO POLO, COCOA). Some of the studies are looking at the possibility to extend the indications of flow diverter therapy to smaller and more aneurysm types in different locations.
Based on currently available evidence, flow diverter therapy appears relatively safe and feasible for large, dysplastic, wide neck anterior circulation aneurysms. Results with smaller aneurysms are promising, but will require additional confirmation in randomised trials. Posterior circulation, bifurcation and previously stented aneurysms have had less optimal outcomes. New flow diverter stents, and second and third generation versions of the currently available devices are being developed to improve flexibility, durability, deliverability and performance. Further data and extended follow-up are needed to better understand the long-term risks and beneﬁts of this rapidly evolving technology.
Gabor Toth is an assistant professor at the Cleveland Clinic Lerner College of Medicine, and staff neurointerventionalist at the Cleveland Clinic Cerebrovascular Center in Cleveland, USA