Large Basilar Apex Aneurysms Treated with Flow-Diverter Stents

Discussion

Of the few articles reporting the treatment of posterior circulation aneurysms with flow-diverter stents,^7,10,11 to the best of our knowledge, none have focused on the use of flow-diverter stents for the treatment of wide-neck, large BAAs. In this clinical report, we describe the remarkable midterm follow-up results we obtained with the use of flow-diverter stents in this infrequent location. We detailed the feasibility and efficacy of the technique, including its ability to determine aneurysmal growth arrest in all the cases of aneurysmal recurrence after previous endovascular treatments.

Use of flow diverters in the posterior circulation carries higher complication rates compared with use in the anterior circulation. However, in the literature, flow diverters in the posterior circulation were mostly used for the treatment of dissecting, fusiform, and/or partially thrombosed aneurysms¹⁸; this makes the comparison of our findings with previous series difficult.

In this study, flow-diverter stent deployment was feasible in all patients, and compared with surgical clipping, the flow-diverter stent deployment presented a less technical challenge.¹⁹ The fact that all treated patients in this series except 1 (Patient 2) had wide-neck, large BAA recurrences after previous endovascular treatments (ie, coiling or stent + coiling) was the determinant for us to adopt a different strategy concept: flow diversion. In all cases of BAA recurrence (Patients 1, 3, 4, and 5), the flow-diverter stent was intended as a “rescue” strategy where a coiling procedure had failed. The flow-diverter deployment determined progressive sac exclusion, with no need for further treatments during follow-up.

Although other treatment options were available, the use of a flow-diverter stent offered a more effective scaffolding to the diseased target artery compared with a conventional stent; moreover, other techniques such as Y-stent placement have demonstrated efficacy in previous studies. However, according to Bartolini et al,²⁰ Y-stent placement carries the risk of up to 10% procedure-related permanent neurologic deficits. Finally, the flow-diverter stent deployment was a less technically demanding procedure than the Y-stent placement technique.

Patient 2 was treated for an unruptured BAA with coiling and flow-diverter stent in a single session, with a “first intention-to-cure” strategy, and the patient experienced an early postprocedural hemorrhage.

The delayed aneurysm rupture risk after flow-diverter stent deployment has been previously described.¹⁴ The small population of this study determines the impossibility of relating the only hemorrhagic complication observed to a specific type of device rather than to the flow-diversion effect. However, the rapid intra-aneurysmal thrombus formation is a source of various proteases with high proteolytic activity, which could participate in the degradation of the arterial wall and lead to aneurysm rupture. Moreover, the larger aneurysms, like those presented in this series, are generally more likely to have intraluminal thrombus.^21⇓–23 Thus, this mechanism probably was the most involved in the delayed aneurysm rupture observed in Patient 2. In this case, the steroids were administered only after aneurysmal rupture, mainly to reduce the midbrain posthemorrhagic edema because, to the best of our knowledge, the efficiency of steroids to prevent the lytic activity has not been demonstrated yet.²⁴ Unfortunately, how to prevent this dramatic complication is still a matter of debate.

To protect against delayed aneurysm rupture,²⁵ our choice was the use of loose-packing coiling before flow-diverter stent. Retrospectively, we assume that the loose-packing coiling was not sufficient to act as a scaffold to organize thrombi into stable fibrous tissue²⁶ and that the time between the procedures was not long enough to allow a progressive thrombus formation after coiling. Our experience confirms that the optimum packing attenuation of coils and the timing between coiling and flow-diverter stent placement is still to be determined.

Posterior circulation flow-diverter stents carry a risk of ischemic lesions caused by the occlusion of covered branches (ie, superior cerebellar arteries and PCAs),²⁷ including a higher rate of injury to posterior perforators relative to their anterior counterparts.^3,28,29

In this series, we observed 1 case (Patient 5) of early ischemic lesion after flow-diverter stent coverage. In this case, the placement of a flow diverter in front of the superior cerebellar arteries led to their early occlusion, with a unilateral transient cerebellar syndrome observed after the procedure. At the subsequent follow-up, the superior cerebellar artery occlusion remained clinically silent. To the best of our knowledge, there is no previous literature specifically analyzing superior cerebellar artery occlusion after flow-diverter coverage. However, in the anterior circulation, slow flow of the side branches covered by the flow-diverter stent is observed in up to 19.1% of cases³⁰ and vessel occlusion is observed in up to 21%.³¹ In most of these cases, neither permanent deficit or death nor neurologic deficit were described within covered branches, perhaps because of the good collateral circulation.^11,27,30 As described by Alqadri et al,³² several collateral anastomoses also exist in the posterior circulation. These findings suggest that in Patient 5, the occlusion of the superior cerebellar arteries after flow-diverter stent deployment was responsible for an initial vascular and hemodynamic regional unbalance, which determined an early initial clinical manifestation. The subsequent early activation of the posterior collateral circulation allowed for the patient's rapid clinical improvement without neurologic manifestation during the follow-up. In case of uncertainty regarding the collateral circulation, it could be valuable to perform a balloon occlusion test to assess the vascular territory supply of the covered branches.

Finally, deploying flow-diverter stents in the basilar bifurcation determines flow modification at the level of the covered PCA.

In accordance with the anatomic knowledge of Brassier et al,³³ to protect as many perforators as possible, the flow diverter should reasonably be deployed in the most caudal PCA, where the number of perforators is less common. However, it also should be taken into account that most of the flow diverters are composed of microfilaments of 30–35 μm, and the pore size varies between 110–250 μm (much depending on the final FD morphology and selection of the proper size adapted to the vessel diameter).³⁴ These anatomic characteristics suggest that in the worst case, when 2 filaments cross in front of a 100 μm perforator vessel, this small artery will lose no more than 55% of its orifice area. This still provides sufficient blood flow to the distribution territory.³⁵

Thus, to choose which PCA to put the stent in, we mostly focused our attention on hemodynamic criteria. In particular, we simply decided to cover the smallest P1 segment with the largest posterior communicating artery with the stent. This approach was based on several reasons. First, in case of subsequent occlusion of the covered P1 segment, this would be supplied by a larger posterior communicating artery. Second, in this configuration, the larger uncovered posterior communicating artery is preferentially used to supply the distal P2 segment in an anterograde fashion (rather than the P1 segment in a retrograde fashion). As a consequence, the filling flow into the aneurysmal sac from the covered P1 is potentially reduced. Finally, a flow diverter deployed in the P1 segment with the smaller or absent posterior communicating artery determines a higher gradient pressure at that level because of the reduced flow competition between the P1 segment and posterior communicating artery. This theoretically favors the perforators' patency maintenance.

In line with these considerations, in Patients 1, 2, and 3, this approach induced flow changes in the smaller covered posterior communicating artery, with posterior communicating arteries still patent, but no more visible except after performing an occlusion test (Patient 3); in Patient 4, we decided to put the flow-diverter stent in the right PCA because the right posterior communicating artery was smaller compared with the left one; finally, in Patient 5, we decided to deploy the stent in the right PCA only because it was technically easier because neither anatomic nor hemodynamic differences were observed between the right and left side (with a symmetry of both PCAs and posterior communicating arteries and the aneurysmal neck centered between the 2 PCAs). This approach has proved to be efficient in preserving covered PCA patency without significant aneurysm sac supply at midterm follow-up and with no perforator infarction observed.