Efficacy and Safety of Flow Diversion for Paraclinoid Aneurysms: A Matched-Pair Analysis Compared with Standard Endovascular Approaches

Discussion

In this retrospective matched-pair analysis, we found a significantly higher incidence of complete occlusion at follow-up in patients treated with the PED compared with similar aneurysms matched for size and location that were treated with other “conventional” endovascular techniques. In the PED group, by using very strict angiographic criteria, complete occlusion on DSA was achieved in three-quarters of patients, while such occlusion was achieved in less than one-half of patients treated by conventional means. Notably, rates of adverse events were similar between groups. These results suggest strongly that flow diversion might improve long-term angiographic outcomes in patients with “difficult” aneurysms in the periophthalmic region and may offer substantial benefit over other endovascular options.

The incomplete degree of angiographic obliteration and the risk of aneurysm recurrence have been the main limitations of endovascular treatment of intracranial aneurysms. These limitations have been the impetus behind the modifications of the coil (such as bioactive coils, coating with expandable materials like hydrogel, and so forth) and the development of microcatheter-delivered stents to be used in stent-assisted coiling procedures. Despite these advances, the recurrence rate and the rate of incomplete angiographic obliteration after aneurysm treatment with “modified” or “biologically” active coils and stent-assisted coiling continue to be discouragingly high.^14,15 Flow diverters may fill the need for a more effective and definitive endovascular treatment, especially as it applies to paraclinoid aneurysms. Flow diverters seem to have multiple theoretic and some demonstrated properties that may increase the degree of angiographic obliteration and, with time, promote aneurysm shrinkage.^16,17 By redirecting flow into the parent artery and away from the aneurysm sac, flow diverters induce progressive aneurysm thrombosis leading to angiographic obliteration with time. As the clot organizes and retracts, aneurysm shrinkage ensues.^{18⇓⇓–21} Moreover, the presence of the flow diverters in the aneurysm neck provides structural support of the “diseased” portion of the vessel, and with time as the device is incorporated into the vessel wall, it is covered by an endothelial layer that “seals” the neck.^22,23

The high rate of complete angiographic occlusion at follow-up that we have observed is in line with that in other studies, though other studies did not have a control group. The PUFS study is the study that eventually led to FDA approval of the PED.¹⁰ In this multicenter prospective study, only giant and large aneurysms of the proximal ICA with wide necks were included. In PUFS, the rate of complete angiographic obliteration was 82% at 6 months and 86% at 1 year (T. Becske for the PUFS investigators, presented in Colorado Springs, July 2011, unpublished data). Lylyk et al,⁵ in a large single-center consecutive series of 54 patients, reported rates of complete angiographic obliteration of 56%, 93%, and 95% of aneurysms at 3, 6, and 12 months, respectively. Our data are similar to those in these prior reports, with high rates of complete occlusion; we have added important additional information in that we included similar types and sizes of aneurysm treated by other endovascular means so that practitioners can better assess the relative merits of flow diversion compared with coil embolization or parent artery sacrifice.

Treatment with flow diverters is particularly effective in aneurysms involving the proximal internal carotid artery. These aneurysms have a “sidewall” geometry, which makes them more suitable for these treatment paradigms. Proximal ICA aneurysms often have a fusiform shape with involvement of the entire circumference of the vessel, which makes them less suitable for effective treatment with other conventional endovascular techniques. In our study, 36.4% of the controls had been treated with parent artery sacrifice. While parent artery sacrifice is a well-established and effective treatment technique for aneurysms of the proximal internal carotid artery, several limitations of this approach exist. These are related to the risk of flow-related aneurysm formation along the collateral pathways and to the decrease in cerebrovascular reserve after 1 carotid artery has been sacrificed. Moreover, because proximal carotid aneurysms are often related to an “intrinsic” weakness of the vessel wall in this segment, patients with these aneurysms often have contralateral aneurysms or are at high risk of developing contralateral mirror aneurysms, which makes carotid artery sacrifice a less appealing option.

The effectiveness of flow diverters has to be weighed against their safety. In our study, complications with flow diverters were not significantly different from those in controls, and in the PED group, no patient had a permanent neurologic deterioration. Other studies have also confirmed the relative safety of these devices, despite the complexity of aneurysms treated. In the PUFS study, the rate of major ipsilateral stroke or neurologic death was 5.6%, while the intracranial hemorrhage rate was 4.7%.¹⁰ Midterm safety is also confirmed by our and other studies.^3,5,8,9 We did not observe new permanent neurologic deficits after a mean follow-up of 13.4 ± 6.9 months, and only 1 patient had episodes possibly consistent with amaurosis fugax 22 months after treatment. These episodes resolved after reinstitution of antiplatelet therapy. The durability of the device is also confirmed in our study because only 1 patient had an asymptomatic internal carotid artery occlusion and no hemodynamically significant (>50%) stenosis was observed at 6 months in any of the patients or at 12 months in 13 patients who reached this follow-up milestone. In the PUFS study, stenosis of >50% occurred in 2 patients (1.9%) and 1 patient (1%) was found to have an asymptomatic ICA occlusion at the 6-month follow-up. Delayed aneurysm rupture (usually within the first 2 weeks of treatment) and distal hemorrhages have been reported after treatment with flow diverters and are one of the major concerns of this technology. No such events were observed in this series, and the causes leading to these feared and often devastating complications are still mostly unknown.

Our study has limitations. While the data for patients in the PED group were collected prospectively, data for the controls were retrospectively collected. Moreover, patients in the PED group were treated during the past 2 years, while controls had been treated across a longer and earlier interval and increased experience may have accounted for some of the better results seen with the PED. However, this effect would have been partially counteracted by the steep learning curve for the PED. While every patient in the PED group had at least 1 control DSA at follow-up, radiologic follow-up consisted of MRA in 14 (66.7%) patients in the control group, but this, if anything, should skew the results against the PED because MRA is not as sensitive as DSA in depicting persistent flow within the aneurysm. These limitations notwithstanding, our study is the only study comparing results of flow diversion with results of treating similar aneurysms with other endovascular techniques, to our knowledge.