By Pedro Lylyk
Giant aneurysms are defined as those whose maximum diameter exceeds 2.5cm. The natural history of these lesions is associated with high morbidity and mortality rates. The first report on giant aneurysms dates back to 1875, written by Hutchinson. In the large patient series based reports of Drake and Sundt, published between 1979 and 1991, they were considered as being operable with an acceptable morbidity-mortality rate. Giant aneurysms are more frequent in the female population, becoming symptomatic between ages of 50–60. A small minority (approximately 5%) of giant aneurysms are found in the population aged less than 20 years.
Endovascular treatment of intracranial aneurysms with detachable coils, which was introduced in 1996, allowed for an alternative therapeutic option to surgical clip ligation. Despite the introduction of improved coils as well as stent- and balloon-assisted coiling, it does suffer from some limitations; incomplete aneurysm occlusions in some aneurysms as well recurrence in complex large and giant aneuryms.
Due to the high porosity of the initially utilised endovascular material, stents were primarily used as a scaffold to support coils being inserted in the wide neck aneurysms. With the development of novel intracranial stents with improved porosity, the concept of “endovascular flow diversion” was proposed.
The main goal of traditional endovascular treatment for intracranial aneurysms has been endosaccular aneurysm embolisation with parent vessel preservation. The success of this strategy is limited if the aneurysm is fusiform, large/giant, or wide-necked. Therefore, these aneurysms have remained as “primary surgical” aneurysms, treated with either direct surgical reconstruction, or a bypass. In contrast to the traditional endovascular approach, flow diversion aims to cure aneurysms by endovascular reconstruction of the parent vessel, without even performing endosaccular embolisation. Thus, the flow diversion could be thought of almost as an “endovascular equivalent” to the surgical clip (“extravascular” parent vessel reconstruction).
We went in for early adoption of flow diverters and have used all generations of them so far. The first generation was basically a telescoped set of overlapping stents, produced by different vendors (Neuroform, Enterprise and Solitaire in particular). The second generation was characterised by utilisation of single stent with a monolayer configuration (Pipeline, Silk and Surpass). The third stent generation consisted of multilayer configured stents, used as an endoprosthesis (Cardiatis and Web). The most recent one is the fourth generation that is a multilayer braided stent with additional anti-platelet coating.
Our total patient cohort, treated with the flow divertor devices, includes 444 aneurysms, 55.5% of which were giant or large. The technical success was 95% with a procedure effectiveness of 90% at one year follow up and over 3–4 years without recurrences. The procedure-related combined morbidity and mortality was 5.6%, and includes thromboembolic and haemorrhagic complications.
One of the major concerns related to the use of flow diverters is the potential occlusion of perforating arteries or other side branches, with secondary ischaemic complications. A series of in vitro performed studies on technical phantoms and animal models reproduced conditions in which flow diverters are placed across aneurysmal neck and collateral branches.
The studies demonstrated that flow through perforators was largely preserved, where more than 90% of the perforator inlet area resulted in flow reduction of less than 10%. These results were not unexpected, presuming that the blood flow through a perforator is driven by a pressure gradient.
Flow diverter devices represent a major paradigm shift in the endovascular treatment of intracranial aneurysms. Placement of the flow-diverting device within the parent artery and across the orifice of the aneurysm creates a re-enforcement of blood flow stream through the parent artery and establishes a low-flow haemodynamic state within the aneurysm sack that would favour its thrombosis, with high rates of complete aneurysm occlusion. These devices allow the reconstruction of the diseased vessel segment by providing a scaffold for neointimal formation, while diverting flow away from the aneurysm into the parent vessel. This subsequently results in aneurysm thrombosis and complete exclusion of the aneurysm sack from the parent vessel.
Pedro Lylyk is associate professor of Neurosurgery at the University of Buenos Aires (UBA), Argentina. He is also a professor of Vascular Medicine, Universidad del Salvador (USAL), Buenos Aires, Argentina, and chairman of the Endovascular Neurosurgery and Interventional Radiology Institute “ENERI” (Equipo de Neurocirugía Endovascular Radiología Intervencionista).