Results of Temporary Stent-assisted Coil Embolization (CATS) for the Treatment of Wide-neck Aneurysms

Coil embolization is the most established technique for endovascular aneurysm treatment. In wide-neck aneurysms, however, simple coil embolization is often not a feasible option. A variety of adjunct techniques have been introduced to overcome this limitation. The use of balloons helps to safely place coils in the aneurysm. However, blood flow to the brain is impaired during inflation of the balloon, and there is a risk of additional thrombus formation or vessel dissection [1]. Stent-assisted coil embolization does not temporarily interrupt blood flow and helps to stabilize the coils within the aneurysm [2]. However, permanent placement of a stent in an intracranial vessel requires long-term platelet inhibition. Especially in patients with acutely ruptured aneurysms, this may increase complication rates [3].

To overcome this limitation, temporary stent-assisted coil embolization (coiling assisted by temporary stenting, CATS) has been proposed as an alternative method, with the advantage that blood flow is not interrupted during treatment and no implants are left behind in the parent vessel [4]. Two microcatheters are needed for this technique: The first microcatheter is advanced into the aneurysm to perform coil embolization. The second microcatheter is used to deploy a stent or detachable stent-retriever and hereby cover the aneurysm neck without permanently releasing the stent. The microcatheter can be placed in the aneurysm either before or after stent deployment [5]. After completion of coil embolization, the stent is recovered (Fig. 1). If there is coil protrusion during stent recovery, the stent can be redeployed and released for permanent implantation. Variations of CATS using a third microcatheter are possible, such as the for double microcatheter embolization (Fig. 1), for additional microcatheter remodeling, or for double temporary stent remodeling.

The CATS is an established standard technique in our institution for the treatment of wide-neck aneurysms. To date, only case reports and small case series on this technique have been published [4, 6‐10]. To assess safety and effectiveness of CATS, we performed a retrospective intention-to-treat analysis of all consecutive patients, who were intended to be treated with CATS over a period of 10 years.

Temporary stenting has been suggested as an alternative to the well-established remodeling techniques of balloon-assisted coil embolization and stent-assisted coil embolization. To the best of our knowledge, however, since the first report on this technique in 2013, only 6 case reports or small case series have been published with a total number of reported cases of 65 [4, 6‐10]. This already included 33 cases from our own group. We thus performed a retrospective intention-to-treat analysis on 97 aneurysm treatments to expand on this and determine the safety and effectiveness of this technique.

The results of our study validate the technique of using a temporary stent to assist coil embolization of wide-neck aneurysms as practical and effective. The stents could be successfully retrieved in 93% of cases. In the remaining cases stent recovery was deemed unsafe due to impending coil migration into the parent artery, and the treatment was finished as permanent stent-assisted coil embolization without complications. Thus, the success rate of CATS in terms of whether it can avoid a permanent stent implantation in wide-necked aneurysms is 93%. However, there were no instances of unsuccessful attempted stent retrieval due to a technical inability to retrieve the stent in cases when interventionalists had considered it safe to try removing the stent. Thus, the technical success rate of stent recovery was 100%.

In our cohort, we achieved an adequate aneurysm occlusion, including a neck remnant (Raymond-Roy classes I + II), in 82% of cases. Looking at immediate effectiveness of treatment, our results appear to be comparable with previous studies, indicating stent-assisted coiling to be superior to balloon remodeling regarding aneurysm occlusion [16‐18].

Our study is the first to report follow-up data on patients treated with temporary stent-assisted coil embolization. With recurrence rates of 10% after 6 months and 17% after 1 year or later, our results appear both acceptable and comparable with results achieved using permanent stent-placement or balloon remodeling [16‐18]. It has been reported that initial aneurysm occlusion as characterized by the Raymond-Roy classification is a predictor for recurrence [19]. In our cohort, we did not find such a correlation between the grade of initial aneurysm occlusion and the risk of recurrences at both 6 and 12 months. However, this may be due to the limited size of our sample. In our patients the diameter of the aneurysm neck was the only significant predictor of aneurysm recurrence. The validity of our findings is, however, impaired by the suboptimal follow-up rates of our cohort, which were caused by the retrospective design of our study, as well as partly by the COVID pandemic.

We observed a complication rate of 12.6%. This appears to be comparable to the techniques of balloon remodeling and permanent stent-assisted coiling, for which also complication rates between 10% and 20% have been reported [20‐22]. However, we would like to draw attention to the fact that it was our intention to record all kinds of unintended events during the procedure as complications, even when they were not associated with the procedure of temporary stenting (e.g., stretching of coils). We did this to allow a thorough evaluation of our technique. Therefore, it should be noted that our complication rate of 12.6% does not correspond to the specific risks of temporary stenting. While several events were not clearly attributable to our technique (e.g., thrombus formation), we also observed an event of coil displacement during stent removal, which is definitely a complication associated with temporary stenting. Therefore, at least one of our complications was clearly associated with temporary stenting (1%). However, from a scientific perspective, we cannot state a valid complication rate specifically related to the use of temporary stenting. Still, it is safe to assume that this measure was considerably smaller than 12.6% in our study.

With regard to complication rates it also needs to be considered that our study is a real-world best-practice report. This means that we also included interventions performed by younger, less experienced interventionalists, even when it was the first time they used this method. Then, it needs to be considered that the majority of treated aneurysms in our study were acutely ruptured. It is reasonable to expect that complication rates in acutely ruptured cases will be higher than in elective cases. For one thing, acutely ruptured aneurysms carry a higher risk for periinterventional rupture than unruptured aneurysms. Consequently, all three periprocedural aneurysm ruptures occurred in acutely ruptured cases, whereas our periprocedural aneurysm rupture rate in elective cases was zero. In addition, there is evidence that the coagulation system is being activated following acute SAH. Although the evidence regarding this is not entirely clear, it has been speculated whether the risk for thrombus formation may be higher in acutely ruptured cases. Consequently, only one case of thromboembolism occurred in one of our elective cases. The two remaining complications of thromboembolism and all four cases of local thrombus formation occurred in acutely ruptured aneurysms. As expected, complications occurred significantly more frequent in acutely ruptured cases than in elective cases. In total, 11 of the 12 complications occurred in acutely ruptured cases, whereas only one complication occurred in the elective cases.

Although the majority of our aneurysms were acutely ruptured, the vast majority of treatments were performed with platelet inhibition during the procedure, with 88 patients receiving intravenous tirofiban, and 2 patients being pretreated with dual platelet inhibition. This strategy is in accordance with Almekhlafi and Goyal who have also recommended effective platelet inhibition during temporary stent-assisted coil embolization to prevent thromboembolic complications [23]. As compared to permanent stent placement, the majority of patients in our cohort did not require dual-antiplatelet therapy following embolization. This may be considered an advantage, since it has been shown that dual-antiplatelet therapy increases the risk of occurrence of hemorrhagic events after aneurysm embolization [3, 24]. Temporary stenting may thus be especially useful for patients with an acutely ruptured aneurysm. The nature of our study, however, does neither allow to confirm this hypothesis nor to determine the size of this effect in our cohort.

There are different opinions between interventionalists whether the microcatheter for coil embolization should be placed in the aneurysm first before the stent is expanded (jailing technique), or whether the microcatheter should be navigated into the aneurysm through the mesh of the expanded stent. In our institution usually the jailing technique is being used. However, our results show that the microcatheter can be pulled back and navigated through the mesh of the stent should this prove to be necessary during embolization.

Recently, the Comaneci device has been introduced. This stent-like device also allows temporary remodeling during coil embolization of aneurysms. Sirakov et al. reported the use of this device in 118 acutely ruptured aneurysms with rates for technical success and aneurysm recurrence comparable to our technique, and a device-related complication rate of 10% [25]. A drawback of this technique, however, is that the device cannot be detached if coil protrusion is observed at the end of the procedure. Lately, the Comaneci device has also been used in our institution in a few cases with good results. It seems to us, that while use of the Comaneci device and the technique of CATS as described in this paper are in essence quite similar, there are also some differences: The expansion of the Comaneci device can to some extent be controlled by the user. This may allow for better adaptation to the aneurysm neck in some cases, whereas on the other hand it could increase complication rates following too forceful expansion. On the other hand, when the interventionalist starts to close the remodeling device at the end of the procedure and realizes that the coil package is not stable, CATS has an advantage over the Comaneci device. In a CATS procedure, the stent can be instantly redeployed, thus preventing full protrusion of the coils into the parent vessel. The Comaneci device needs to be completely removed, and a new catheter be placed before deployment of a permanent stent. Admittedly, only in a small number of cases will this potentially make a difference in interventional outcome.

In the past years alternatives to coil embolization for wide-necked bifurcation aneurysms have been introduced, such as the WovenEndobridge (WEB) device, the Contour Neurovascular System, or Flow diverters. These techniques may be an alternative to treat ruptured aneurysms with low morbidity and mortality [22, 26, 27]. However, prospective randomized studies to compare the various treatment methods have yet to be performed.

In conclusion, our data show that temporary stent-assisted coil embolization may be a safe and effective alternative to other remodeling techniques. In comparison to permanent stent placement, patients do not require long-term platelet inhibition. In comparison to balloon remodeling, it has the benefit of avoiding temporary blood flow inhibition. Potentially, a disadvantage of CATS may be that there is the risk of coil loops intermingling with the stent. However, this risk appears to be rather small since we did not observe such a complication in our cohort. In addition, especially for younger interventionalists CATS may be considered a technically easier approach than balloon remodeling [6, 8].

However, although the literature on the results of the various strategies for aneurysm embolization is extensive, there is no prospective randomized study to compare the different methods with their respective advantages and disadvantages against each other.

The limitations of our study are a possible selection bias as the patients in our study were not randomized beforehand to the opposing treatment techniques for wide-neck aneurysms. To reduce this possible effect, however, we conducted our study as an intention-to-treat analysis. A second limitation of our study is that we were able to collect a 6-month FU in 71% of patients, and a 12-month FU in 44% of patients only. This is due to the retrospective design of our study and the fact that it covers a period of more than 10 years. Prospective studies with long-term follow-up data are required to compare embolization methods. Other limitations are that antiplatelet medication protocols varied within our cohort, and that our results were not evaluated by an independent core laboratory. To minimize this potential bias, we did not rely on the interventional reports but re-analyzed all images. Moreover, there is some potential inhomogeneity due to the use of three different stents.

In this retrospective intention-to-treat study, temporary stenting appeared to be an effective technique for the treatment of wide-neck aneurysms. Procedural safety appeared to be comparable to balloon remodeling and permanent stent-assisted coiling. The major advantage of our treatment may be a diminished need for posttreatment antiplatelet therapy, which might cause cerebral hemorrhage or complicate further treatment in the intensive care unit. We thus consider it possible that this technique may be particularly useful for patients with an acutely ruptured intracranial aneurysm. However, to define the true value and potential benefit of this technique, further prospective studies comparing it to other techniques for treatment of wide-necked aneurysms are required.