Flow diversion prostheses represent a new endovascular approach aimed at treating patients with large wide-neck aneurysms. Our objective is to present this new technology, to review the clinical studies on efficacy, and to emphasize its current limits. Flow diversion prostheses consist of a cylinder made of a large number of braided microfilaments providing a large metallic surface when deployed and inducing a blood flow diversion outside the aneurysm. Two different brands are currently available. Clinical data supporting their efficacy are currently limited to six non comparative cohort studies that included between 18 and 107 patients. Procedural implantation was shown to be feasible in more than 90% and safe with a thirty-day mortality between 2.8 and 5.5%. Complete occlusion rates at twelve months varied between 85.7 and 100%. Even though promising, the current status of flow diversion prostheses needs further evaluation with randomized, prospective, clinical trials with comparison to conventional strategies including endovascular coiling or surgical clipping.
According to a systematic review based on 23 studies, the incidence of unruptured intracranial aneurysm (UIA) is about 2% in the general population, greatly represented by saccular shapes [
Different approaches are then possible [
In this context, a new endovascular technique using flow diverter stents has emerged in the last four years. Our objectives are to present the potential interest and the current limits of this technique in the management of UIA.
Flow diversion prostheses represent a new approach aimed at treating patients with large wide-neck aneurysms. They consist of a cylinder made of a large number of braided microfilaments providing a total metallic surface of up to 30%, when deployed, as opposed to 10% for conventional intracranial stent. Once implanted in the parent artery across the neck, they induce a blood flow diversion outside the aneurismal sac. Since blood flow inside the aneurysm decreases, a progressive thrombosis is observed over time [
Two different brands have been developed over the last four years. The first one is called “SILK” prosthesis (BALT Extrusion Inc.). It is made of 48 braided nitinol and platinium strands that form a high-coverage cylinder once deployed. It exists with different diameters (2 to 5 mm) and lengths (15 to 40 mm) (Figure
SILK stent (BALT Extrusion Inc.).
The second one is called pipeline embolization device or PED (EV3 Inc.). It is delivered through a standard 0.027-inch internal diameter microcatheter and enables a endovascular “stent-like” construct. The PED has 48 microfilaments made of platinum and nickel-cobalt chromium alloy. (Figure
Pipeline embolization device (EV3 Inc.).
The PED has received a European Community and a Food and Drug Administration (FDA) approvals.
The procedures are performed under general anaesthesia and use a transfemoral artery approach. Unexpanded flow diversion stents inserted in the loading device are positioned up the aneurysm and delivered under fluoroscopic guidance.
Similarly to conventional stenting, patients receive dual antiplatelet therapy (clopidogrel and aspirin) to prevent stent thrombosis prior to the procedure and for at least six months after the procedure.
This concept of flow diversion stents may be considered as a breakthrough technology since the prostheses enable an intravascular reconstruction in comparison to the intrasaccular approach.
PubMed electronic database was queried in order to search for articles on clinical efficacy and tolerance data. The following key words were used in combination: “
Fiorella et al. published the first two experiences reporting a successful use of pipeline embolization device in a total of three patients with either giant sacciform or fusiform symptomatic intracranial aneurysm [
The FDA approval of PED was recently obtained based on the results of the PUFS (
The proportion of subjects with major ipsilateral stroke or neurologic death at 180 days after treatment was 5.6%. The PUFS study was then able to show safety and efficacy of PED in large or giant wide-neck aneurysms [
The results of two single-centre experiences are also available. A study from Lylyk et al. reported the use of PED in 53 patients treated in the Department of Neuroradiology of Buenos Aires [
In those cohort studies, more than one stent placement was observed on average, depending on the size of the aneurysm or the level of perioperative blood flow diversion observed on angiography.
The clinical experiences related to the use of SILK stent are more limited.
An 18-centre worldwide experience was published on 70 patients presenting an aneurysm and considered as ineligible for conventional endovascular or neurosurgical treatment [
Another experience of Lubicz and colleagues is published on 29 patients considered not eligible to other endovascular approaches (47% of fusiform or circumferential shape aneurysm) [
The six cohort study experiences are summarized in Table
Summary of clinical trials conducted on flow diversion stents.
Device | SILK stent (BALT) | Pipeline embolization device (EV3) | ||||
Number of centres | 3 | 18 | 4 | 10 | 1 | 1 |
Number of patients | 29 | 70 | 31 | 107 | 53 | 18 |
Number of aneurysms | 34 | 70 | 31 | 107 | 63 | 19 |
Number of small/large/giant aneurysms | 18/12/4 | 18/37/15 | 20/9/2 | 0/85/22 | 33/22/8 | 5/10/4 |
Procedural success implantation | 90% | 96% | 97. 9% | 99% | 97% | |
Adjunctive coil placement | 0% | 14% | 51.6% | 1% | 6% | 47% |
Number of stent implanted per patient | 1.42 | / | 1.35 | 3.1 | 1.37 | 2.16 |
30 days mortality rate | 4% | 2.8% | 0 | 2.8% | 0 | 5.5% |
3- or 6-month complete occlusion rate | 69% | 50% | 93% | 81.8% | 93% | 94% |
12-month complete occlusion rate | / | 100% | NA | 85.7% | 95% | / |
Reference | [ | [ | [ | [ | [ | [ |
The theoretical interest of flow diversion stents is to enable endovascular reconstruction when conventional approaches, mainly represented by endovascular coiling or surgical clipping, are not technically feasible or not appropriate because of a major risk of morbidity or recurrence over time. It is expected to relieve neurological symptoms due to mass effect and to decrease aneurismal recurrences over time. Even though promising and representing a breakthrough technology, the current knowledge on flow diversion prostheses is still limited, and their respective positioning in routine practice must be further evaluated.
First, questions may be raised on the technical procedure by itself. More precisely, there is currently no guideline regarding the number of stents needed to be implanted and the interest of adjunctive coil placement. The number of stents to implant will first depend on the aneurismal neck. In very large or giant aneurysm, the length of one flow diverter stent may be not sufficient to cover all the aneurysm, and therefore, several stents may be required. For example, Fiorella and colleagues reported the use of seven serial telescoped PEDs to bridge a 29 mm diameter aneurysm [
The second debate is on whether any adjunctive coil placement should be necessary.
Recently, Turoswki and colleagues reported an early fatal haemorrhage due the aneurysm rupture after vascular reconstruction using the SILK stent without any coils associated [
The current positioning of flow diversion prostheses in the strategy is also to be evaluated.
Firstly, clinical experiences reporting the use of flow diverters are limited to noncomparative cohort studies including between 18 and 107 patients. Thus, there is currently no head-to-head comparison of flow diversion prostheses versus other conventional approach, including endovascular coiling or surgical clipping in large wide-neck aneurysm in terms of long-term angiographic and clinical outcome.
Furthermore, flow diversion prostheses are expected to improve quality of life of patients due to the suppression of mass effect-associated symptoms, but no comparative data is currently available versus endovascular coiling.
A long-term view on tolerance is needed to accurately determine the real incidence of secondary rupture or the risk of stent thrombosis. Whereas certainly rare, this latter complication is possible as illustrated by Fiorella et al. concerning a very late thrombosis in a patient treated with PED [
At last, taking into consideration their substantial cost (approximately €10,000 per stent in Europe, i.e., about US $14,000) and the number of stents required per patient, the economic impact of flow diversion stents should be further evaluated through cost-benefit analysis.
Several randomized clinical trials are now ongoing to improve knowledge on this technology.
Taking into consideration that most of patients with large wide-neck aneurysm are suitable for conventional approach, the randomized FIAT trial (
In patients for whom an endovascular treatment is decided, three clinical trials are ongoing or planned. A first one called COCOA (
Endovascular reconstruction using flow diversion prostheses is a highly innovative technique for the treatment of patients with UIA. Whereas promising, many questions still remain regarding this technique by itself and its current positioning in the strategy. Flow diversion prostheses are indicated in rare clinical situations such as giant or fusiform aneurysm in which no acceptable therapeutic approach exists, and their interest in the management of large aneurysm should be evaluated prospectively with comparison to conventional techniques, including endovascular coiling or surgical clipping.
The authors declare that there is no conflict of interests.
The authors would like to thank Gillian Bell, PharmD and Michtell Lingerfelt, PharmD for the linguistic revision of the paper.