14.1 Rationale for Multiple Devices

As previously described in earlier chapters, treatment of intra-cranial aneurysms with flow diversion is accomplished through progressive thrombosis of the aneurysm, as bulk flow is directed through the parent vessel and away from the aneurysm dome. ¹ Additionally, this change in hemodynamics between the parent vessel and the aneurysm allows the flow diverter to act as a scaffold for neointimal growth over the stent itself, and as a result, over the neck of the aneurysm. ² , ³ Incorporation of the device into the parent artery allows a permanent, durable treatment by reconstructing the parent artery. ⁴ , ⁵ , ⁶ , ⁷ , ⁸ , ⁹

All commercially available flow-diverting stents act as metal sleeves through which native arterial flow preferentially flows. While in vivo studies have shown that there is no immediate change in intra-aneurysmal pressure between pre- and post-placement of a single device, ¹⁰ in vitro and computational models have shown that increasing the amount of metal surface area during flow diversion is related to diminished flow into the aneurysm. ¹¹ Because the surface area of metal in the construct is directly related to the number of devices used, placement of additional flow diverters in theory will (1) increase the amount of flow away from the dome of the aneurysm and (2) increase the scaffolding available to promote neointimization. In clinical practice, placement of multiple devices is achieved by placing the initial flow-diverting device, then tracking the microcatheter used to deliver the device back across the stent pusher wire to a position beyond the aneurysm to ready it for deployment of an additional device. Directly overlaying more than one flow diverter increases the coverage across the neck of the aneurysm and diminishes flow into the dome.

The other common reason for using multiple devices is to lengthen the area of vessel reconstructed in large aneurysms, where one device will not adequately span the anatomic neck. In this case, the deploying microcatheter is tracked back over the pusher wire as mentioned earlier, but only a portion of each device is overlapped so as to lengthen the entire construct. This technique of multiple “telescoping” stents for purposes of diversion in a long construct was first described with non–flow-diverting stents, ¹² , ¹³ and subsequently with flow-diverting devices. ¹⁴ In reality, even if the intent is to cover a larger area, the telescoping stent method both lengthens the construct and provides increased coverage.

The use of multiple devices initially met with trepidation in the flow diverter era due to concern for perforator occlusion with overlapping stent coverage. However, in vitro models have suggested that placement of multiple flow diverters does not result in small vessel occlusion. ¹⁵ Clinically, results have also been favorable and have shown that angiographically visible vessels usually remain open, with complete occlusion being rare, ¹⁶ even when multiple stents are used. ¹⁷ When internal carotid artery (ICA) branches do become occluded (i.e., ophthalmic artery), this occlusion is usually noted on routine follow-up angiography and is a clinically silent event in most cases ( Fig. 14.1). ¹⁸

Nevertheless, placement of multiple devices is not without its own risk. The telescoping stent method increases catheter time and vessel manipulation, both known risk factors for endovascular complications. ¹⁹ Furthermore, additional metal surface area achieved with telescoping devices theoretically may lead to increased area for platelet aggregation for thromboembolic events. Multi-institutional series have shown ischemic stroke rates in flow diversion of 4.7%, ²⁰ although a recent meta-analysis of 29 studies showed stroke rate of up to 6% for all cases with the use of the Pipeline Embolization Device (Covidien, Irvine, CA). ²¹ Similarly, initial reports from the Surpass Study Group also showed ischemic stroke rates of 6%. ⁷ Despite these relatively high rates of stroke, no series has directly linked number of flow diverters placed with increased thromboembolic burden. Rather, the rate of thromboembolism is related to size of the aneurysm treated; patients with larger aneurysms are at increased risk of thromboembolic complications. ²⁰ , ²¹ It is reasonable to surmise that the larger the aneurysm, the greater the risk that multiple flow diverters will be required for adequate treatment; however, this cause-and-effect relationship has not been proven, and the mechanism of thromboembolic complications in large aneurysms is not well understood.

We recommend the use of as few flow diverters as is required to properly reconstruct the parent artery. The natural history following flow diverter placement is one of progressive occlusion over time, which has been documented over the evolution of flow diverter development in multiple studies. ⁴ , ⁶ , ⁷ , ⁸ , ⁹ , ¹⁶ , ²² Therefore, in most cases, we recommend using the minimum number of flow diverters required to adequately achieve aneurysm neck coverage. The placement of additional flow diverters to achieve stasis during the initial implantation is not necessary in most circumstances, given that the expected treatment effect for the majority of aneurysms following flow diversion is progressive aneurysm thrombosis. In our practice, we perform follow-up angiography at 6 months and then at doubling times subsequently until angiographic thrombosis is documented. In some patients, magnetic resonance angiography may substitute for angiography. If progressive thrombosis is not observed over time, placement of additional devices may be necessary. However, by placing the minimum number of devices necessary to reconstruct the parent vessel, regardless of achieving stasis at the time of initial implantation, catheter time is kept to a minimum and metal surface area is reduced, which theoretically reduces the risk of thromboembolic events.

In our experience, should either the neck of the aneurysm extend beyond the length of a single device or the device does not achieve good wall apposition (as may occur when deploying a flow diverter around a tight turn), then additional flow diverters should be placed in a telescoping fashion to achieve full neck coverage and adequate wall apposition. When good wall apposition is not achieved, placing another flow-diverting stent may be necessary to prevent creation of an endoleak. This may be encountered when the proximal or distal end of the device ends on an acute turn of the parent vessel. In such a case, the portion of the stent on the inside of the turn may project away from the vessel wall and into the lumen of the parent vessel. Telescoping an additional device to fully appose the stent to the vessel wall around the turn can reconstruct the vessel successfully and prevent an endoleak. If wall apposition is not confirmed adequately with two-dimensional angiography, we use cone beam computed tomography to ensure adequate wall apposition and assess the need for placement of additional devices.

14.2 Rationale for Adjuvant Coil Embolization

A rare, but potentially devastating complication following placement of an intracranial flow diverter is delayed rupture of the treated aneurysm causing subarachnoid hemorrhage (SAH). ⁷ , ²⁰ , ²³ , ²⁴ , ²⁵ , ²⁶ , ²⁷ Rates of rupture causing SAH vary between studies; however, a recent meta-analysis of pooled data suggests that the event occurs in 3% of cases. ²¹ This complication seems to be directly related to the size of the aneurysm, occurring in 4.5% of giant aneurysm cases, 0.6% of large aneurysm cases (> 10 mm), and 0% of small aneurysm cases (< 10 mm) in one study. ²⁰

The mechanism for delayed rupture of the treated aneurysm is unknown. Proposed mechanisms include creation of a ball-valve inflow between the stent and the aneurysm that decreases overall blood flow across the stent, but allows some blood into the dome that is unable to exit. ²⁸ This, in theory, creates a slow and steady increase in aneurysmal pressure that could potentiate rupture. Another proposed mechanism is that immediate formation of unstable, erythrocyte-predominant thrombus (red clot) in the dome of the aneurysm after flow diversion leads to an inflammatory reaction that weakens the wall of the aneurysm, potentiating rupture. ²³ , ²⁷ Finally, the change in intra-aneurysmal hemodynamics after flow diversion may lead to increased wall shear stress in areas of the dome that previously had reduced shear stress. This could expose weak portions of the aneurysm to higher hemodynamic stress than prediversion, which could increase the risk of rupture. ²³

Regardless of mechanism by which delayed rupture occurs, many interventionalists feel that placing coils into the aneurysm in addition to placing a flow-diverting device helps promote thrombosis of the aneurysm. In theory, this may lead to reduced risk of delayed rupture and SAH. ²⁹ , ³⁰ It is hypothesized that coiling in conjunction with flow diversion will lead to formation of a stable platelet-fibrin clot within the aneurysm, instead of the previously described red clot seen after flow diversion alone. This more organized thrombus may, in turn, limit the inflammatory reaction described earlier and thereby decrease delayed rupture. ²⁷

On a practical level, coiling in conjunction with flow diversion has been shown to lead to a higher rate of occlusion of the aneurysm than when using flow diversion alone. These higher rates of occlusion are seen not only in the immediate periprocedural period but also in the long-term follow-up (6 months). ³⁰ This provides the patient with not only more immediate dome protection (to prevent rupture during the latent period until thrombosis occurs) but also a more likely chance of treatment success with one intervention. Thus, treatment with coiling and flow diversion may lead to lower retreatment rates and potentially decreased amount of time required for the patient to continue dual-antiplatelet therapy. ²⁹ , ³⁰

Another potential benefit of concomitant coiling with flow diversion is that when immediate thrombosis is achieved intraoperatively, the successful angiographic result may prompt the interventionalist to use less flow-diverting stents to achieve stasis from flow diversion alone. As previously discussed, using a single flow-diverting stent reduces the metal surface area placed into the vessel, which may decrease the likelihood of potentiating thromboembolic events. ²⁹ Additionally, when using flow diversion to reconstruct giant aneurysms, concomitant coiling using the jailed technique may also offer the benefit of acting as an intra-aneurysmal scaffold to prevent herniation or prolapse of the flow diverter into the aneurysm itself during deployment. ³⁰

As a result of the theoretical benefits of coiling concurrently with flow diversion, many interventional surgeons have adopted this practice as first line. For virgin, elective aneurysm treatment with flow diversion at our institution, we routinely place coils loosely for large or giant aneurysms via a jailed microcatheter technique. Care is taken not to densely pack the aneurysm with coils in large aneurysms, given that this has not been shown to be necessary, ²⁹ , ³⁰ and may even produce mass effect that could lead to in-stent thrombosis. ³¹

14.2.1 Technique for Jailed Microcatheter

The most common method for coiling concurrently with placement of a flow diverter is the jailed microcatheter technique. This technique involves placing the flow diverter delivery microcatheter into the distal intracranial vasculature, and subsequently placing a second microcatheter within the aneurysm itself to deliver coils. The flow diverter is then deployed, “jailing” the coiling microcatheter, and the aneurysm subsequently coiled. We describe our jailing technique in detail below.

All patients are treated with aspirin 325 mg and clopidogrel 75 mg daily for 1 week prior to the procedure. VerifyNow (Accriva Diagnostics, San Diego, CA) P2Y12 Reaction Units (PRU) is checked on the morning of the procedure with a goal PRU between 50 and 180. Recently we have been augmenting PRU with the use of platelet mapping thromboelastography (TEG) because our preliminary data indicates this may provide a more accurate picture of platelet inhibition. Femoral access is then achieved in the usual fashion, and systemic heparinization is administered. Point-of-care activated clotting time testing is performed to ensure adequate heparinization. A diagnostic catheter is used to select the vessel to be treated, and then the entire system is exchanged for a 6F Cook Shuttle Sheath (Cook Medical, Bloomington, IN), which is navigated into the distal cervical carotid ( Fig. 14.2a).

A 0.72-inch Navien (Covidien, Irvine, CA) guide catheter is then inserted through the shuttle to the cavernous ICA. A Marksman (Covidien) microcatheter is then navigated into the largest distal M2 segment seen. Following this, a smaller microcatheter, such as a SL-10 (Stryker, Kalamazoo, MI) or Prowler-14 (Codman & Shurtleff, Raynham, MA) is chosen for coiling, and is navigated two-thirds of the way into the dome of the aneurysm. A slightly undersized framing coil is chosen, and the first three to four initial loops are deployed into the dome of the aneurysm. It is important to begin coiling the dome of the aneurysm before jailing the microcatheter with the stent so that soft coil loops are present at the end of the microcatheter to prevent rupture, should inadvertent manipulation of the microcatheter occur when deploying the flow diverter itself ( Fig. 14.2b).

Following this, the flow diverter is deployed as has been previously described, leaving the coiling microcatheter jailed into the aneurysm itself. When treating giant aneurysms, it is often helpful to partially track the Navien guide catheter over the Marksman (approximately one-fourth to one-third) across the neck of the giant aneurysm before beginning to deploy the flow diverter. Then, after initially deploying the flow diverter in the middle cerebral artery (MCA), the stent may be pulled back into the distal ICA landing zone with the support of the Navien to prevent prolapse into the aneurysm dome. This also allows for a more controlled deployment without contending with the smaller coiling microcatheter, which if left far from the guide catheter can often be hyper-mobile in the pulsatile flow of the giant aneurysm and interfere with the Marksman delivery catheter. After deployment of the flow diverter, the delivery wire is recaptured, and the delivery catheter (Marksman, in this case) is left in the distal M1 or proximal M2 segment to provide distal access if needed. Further coiling of the aneurysm is then performed until the coil mass loosely packs the aneurysm. The coiling microcatheter is then removed either over a coil pusher wire or over a microwire to provide enough support not to disturb the stent ( Fig. 14.2c).

It is imperative to maintain distal access until the coiling microcatheter is removed and adequate neck coverage is confirmed on check angiogram. Removing the jailed microcatheter can manipulate and potentially move the flow diverter, and additional stents may be necessary if the neck coverage becomes suboptimal during this process. Once satisfactory results are achieved, the system is removed. The patient is maintained on dual-antiplatelet therapy for 6 months.

Related posts:

9 OVERVIEW OF CURRENT FLOW-DIVERTING DEVICES 8 PHARMACOLOGY FOR FLOW DIVERSION 3 REVIEW OF CURRENT LITERATURE AND OCCLUSION RESULTS 16 POSTPROCEDURAL COMPLICATIONS 17 FLOW DIVERSION GRADING SCALES 1 THE BEGINNINGS OF FLOW DIVERSION: A HISTORICAL REVIEW

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