INTRODUCTION
Flow diverting stent (FDS) is one of the main devices to treat unruptured intracranial aneurysm (UIA). From the first use of FDS, numerous kinds of FDS have been developed over the past two decades. The Surpass Evolve (SE) stent is new-generation consisting of 48 to 64 wires, which was approved in March 2019 [1,24]. Many studies demonstrated the efficacy and safety of previous FDS for UIA [3,13,19,23,25,31]. However, there were few studies about risk factors for poor occlusion rate using SE stent. In this study, we aimed to investigate which factors were associated with poor occlusion after flow diversion for UIA using SE stent.
MATERIAL AND METHODS
Study population and clinical characteristics
All patients who underwent treatment of unruptured intracranial aneurysms with the Surpass Evolve flow diverter stent between December 2019 and July 2024 were included in this study. Cases in which concurrent procedures, such as coil embolization or double stent deployment, were also included in the analysis. A total of 68 consecutive unruptured intracranial aneurysms in 52 patients were treated with SE stent (Stryker Neurovascular) in our institution from December 2019 to July 2024. We analyzed the baseline characteristics of all patients with intracranial aneurysms treated at our hospital, using data from a prospectively collected clinical database. Age, sex, diabetes mellitus, hypertension, dyslipidemia, presence of previous subarachnoid hemorrhage (SAH), infarction, a history of previous aneurysm treatment, and the concurrent use of coils were included. The patients were maintained on dual antiplatelet therapy with aspirin (100 mg daily) and clopidogrel (75 mg daily) for the first 6 months following the procedure. After 6 months, the single antiplatelet therapy with aspirin alone was continued for up to 2 years. Major complications were defined as permanent neurological deficits with a modified Rankin Scale (mRS) score ≥2, confirmed in cases where acute infarction or acute hemorrhage was detected on magnetic resonance imaging (MRI) or computed tomography scan. Minor complications were defined as transient neurological deficits with complete recovery, corresponding to an mRS score of 0 or 1 during 1-year follow-up. The silent infarction was defined as patient with positive infarct sign on diffusion MRI without definite matched symptoms.
Radiological characteristics
All 68 aneurysms before treatment were assessed using digital subtraction angiography (DSA)
. Size, location, and incorporated branch were included as radiological characteristics. Immediately after flow diversion, postoperative angiographic results were analyzed by DSA. The first follow-up imaging was conducted by magnetic resonance angiography (MRA) at 3 months. Six to twelve months follow-up DSA was performed. Additional follow-up MRA was conducted according to the previous result. If follow-up MRA scan showed still subtotal filling, additional follow-up DSA was recommended. O’Kelly-Marotta (OKM) grading scale was used to evaluate occlusion. We defined favorable radiological outcome as OKM scale C (entry remnant, <5%) and D (no filling, 0%) in this study. Among the 68 aneurysms, DSA follow-up was completed in 64 cases (94.1%). In-stent stenosis (>50% of stenosis) and stent migration were also examined based on the follow-up DSA. Two independent investigators (J.K and W.C.C) retrospectively reviewed the radiological results, blinded to the data.
Statistical analysis
All statistical analyses were performed using R software (version 4.5.1; R Foundation for Statistical Computing, Vienna, Austria). Continuous variables were presented as mean ± standard deviation (SD), and categorical variables as number and percentage. Normality of continuous variables was assessed using the Shapiro-Wilk test and visual inspection of histograms. Between-group comparisons were performed using the independent sample t-test for normally distributed continuous variables and the Wilcoxon rank-sum test for non-normally distributed data. Categorical variables were analyzed using Fisher’s exact test when expected cell counts were <5, and the Chi-squared test was applied otherwise.
Variables with a p-value <0.05 in univariate analysis were included in a multivariate logistic regression model to identify independent predictors of favorable radiological outcome. Variables entered the multivariate logistic regression model were selected based on both statistical significance in univariate analyses and clinical plausibility. Because of the relatively small sample size, the number of covariates was restricted according to the general rule of at least 10 events per predictor to minimize the risk of model overfitting. Odds ratios (ORs) and 95% confidence intervals (CIs) were calculated. A two-tailed p-value <0.05 was considered statistically significant.
RESULTS
Baseline characteristics
The baseline characteristics of the patients and aneurysms are presented in (Table 1). A total of 52 procedures were performed on 68 aneurysms. The mean age of patients was 57.3±13.4 years, and 38 patients (73.1%) were female. Fourteen patients (20.6%) had multiple aneurysms. A history of previous SAH was present in 3 patients (5.8%), and prior ischemic infarction was noted in 2 patients (3.8%). Previous endovascular or surgical treatment had been performed in 9 cases (17.3%). Of these, seven aneurysms were previously treated with any stent. Fifty-seven aneurysms (83.8%) were in the anterior circulation. Cavernous to ophthalmic segment of internal carotid artery (ICA) aneurysms were the most common (n=52, 76.5%), and vertebral fusiform or dissecting aneurysms were 8 cases (11.8%). The mean maximum aneurysm diameter was 12.2±8.0 mm. Among them, giant aneurysm (≥25 mm) and large aneurysm (≥10 mm) were 5 (7.4%) and 35 cases (51.5%), respectively. There were 6 cases (8.8%) which used concurrent coil embolization.
Clinical and radiological outcome
Clinical outcomes were evaluated in 52 procedures, with a mean clinical follow-up duration of 17.9±9.8 months (Table 2). One major complication (1.9%) occurred, consisting of the progression of brainstem compression following treatment. No cases of major hemorrhage or major ischemic stroke were reported. Minor complications occurred in 5 procedures (9.6%), all of which were ischemic strokes. Among these, 4 cases (7.7%) were silent infarction, and 1 case (1.9%) was symptomatic. No minor hemorrhagic events were observed. There was no delayed rupture or delayed remote intraparenchymal hemorrhage. A procedure-related mortality was 0%. Aneurysm recurrence occurred in 2 cases (3.8%), and 1 case (1.9%) underwent retreatment during the follow-up period.
Table 3 showed radiological outcomes were assessed for 68 aneurysms, with a mean DSA follow-up duration of 12.0±7.8 months. At final imaging follow-up, no aneurysms remained in total filling (OKM A, 0.0%), while 14 aneurysms (20.6%) showed subtotal filling (OKM B), 18 (26.5%) in entry remnant (<5%) (OKM C), and 36 (52.9%) in complete occlusion (OKM D). A favorable radiological outcome, defined as OKM grade C or D, was observed in 54 of 68 aneurysms (79.4%). One case (1.9%) of in-stent stenosis (>50% stenosis) and one case (1.9%) of distal stent migration with ICA occlusion had occurred (Figs. 1 and 2).
Univariate and multivariate analyses for favorable occlusion
Table 4 summarizes the results of both univariate and multivariate analyses evaluating factors associated with favorable radiological outcome. In univariate analysis, aneurysms without an incorporated branch showed significantly higher rates of favorable occlusion compared to those with a branch (84.6% vs. 50.0%, p=0.008). A history of previous treatment was more likely to have poor occlusion (31.3% vs. 9.6%, p=0.047). The favorable occlusion group was younger on average (54.5±12.9 vs. 62.2±12.3 years, p=0.039), and aneurysm size was smaller (10.4±6.4 vs. 18.2±10.1 mm, p=0.002). In multivariate logistic regression, smaller aneurysm size remained significantly associated with favorable occlusion, with an odds ratio (OR) of 0.89 (95% CI, 0.81-0.98; p=0.011). The absence of an incorporated branch was also independently associated with favorable occlusion (OR, 8.26; 95% CI, 1.78-38.28; p=0.007). Age and previous treatment history did not reach statistical significance in the adjusted model (age: OR 0.95; p=0.123; previous treatment: OR 2.55; p=0.293, respectively).
DISCUSSION
Our study demonstrates treatment outcomes for UIAs with Surpass Evolve stent. Within our cohort, the larger size of an aneurysm and a branch-incorporated aneurysm were independent risk factors for poor occlusion following flow-diversion using the SE stent.
Several papers have established that the placement of flow-diverting stent (FDS) for UIA is effective [2,10,28]. Hanel et al. and Meyers et al. also described the treatment outcome using the Surpass Streamline (SS) stent [10,21]. The SS stent was designed with 64 to 96 braided wires, which is higher than previous other FDS, to achieve more effective occlusion. However, the increased number of braided wires was associated with greater stiffness, making deployment of the SS stent technically more difficult. Furthermore, the SS stent was associated with a higher incidence of thromboembolic (TE) complications. Consequently, the Surpass Evolve stent was introduced to the market as a next-generation stent consisting of fewer wires (48-64) to reduce both stiffness and device-related TE complications [6,26,29]. A recent meta-analysis study demonstrated the efficacy of SE stent, with complete and adequate occlusion (OKM C-D or Raymond-Roy Occlusion Classification 1-2) rates of 72.4% and 86.6%, respectively [12]. In the present study, the complete and adequate occlusion rates were 52.9% and 79.4%. In the recently published SEASE multicenter registry, the complete occlusion rate at a mean follow-up of 10.2 months was 73%, with only 18.4% of large or giant aneurysms. In comparison, our study demonstrated a lower complete occlusion rate, likely attributable to the relatively high proportion of large and giant aneurysms (58.9%) in our cohort. Larger aneurysm size has consistently been identified as a negative predictor of complete occlusion after flow diversion, and thus, the size distribution in our population may have contributed to the reduced overall success rate observed.
On the other hand, prior studies demonstrated the safety of FDS for aneurysm treatment [4,5,15,16,18]. According to these studies, ischemic complications occurred in 4.1-6.1% of cases, intraparenchymal hemorrhage in 2.4-3.0%, and SAH in 0.6-4.0%. The neurological morbidity and mortality were 3.5-7.4 and 1.6-4.0%, respectively. In the present study, there was one case of delayed symptomatic ischemic infarction (1.9%). No hemorrhagic complication or mortality was observed. One patient had delayed distal stent migration which led to distal ICA occlusion (Fig. 1). In this case, collateral flows from the anterior and posterior communicating arteries compensated for the ipsilateral ICA occlusion without causing any neurological symptom. Additionally, one case of in-stent stenosis (>50% of stenosis) was observed (Fig. 2).
Previous studies suggested that diverse factors were associated with aneurysm occlusion following flow-diversion, such as larger aneurysm size, incorporated branch, younger age, location, previous treatment, and follow-up duration [9,20,22,24]. Meyers et al. suggested that aneurysm size highly affects the aneurysm occlusion after flow-diversion [20]. Other studies showed that larger aneurysm size has been identified as a risk factor for residual aneurysm following FDS treatment [9,24]. Hemodynamically, large aneurysms typically develop slow but persistent vortical flow patterns [17] which delay thrombus stabilization and endothelialization across the aneurysm neck [14]. Thereby prolonging the time to complete occlusion and increasing the likelihood of residual filling. On the other hand, although the SEASE (Safety and Effectiveness Assessment of the Surpass Evolve) trial did not reach statistical significance (p=0.06) between larger aneurysm (≥10 mm) and aneurysm occlusion, they demonstrated a negative trend toward lower complete occlusion rates [31]. Nevertheless, an increase in aneurysm size was associated with a poor occlusion rate following FDS using SE stent in our study. In SEASE trial, only 18.4% of aneurysms were ≥10 mm. In contrast, the aneurysms in our study were larger, which may have influenced the different results.
Other studies showed that the absence of a branch from an aneurysm was associated with higher complete occlusion [8,20,30]. A recent study on the SE stent, subgroup analyses from the SEASE registry, suggested that treatment with FDS for bifurcation aneurysm or dome-originated branch incorporated aneurysm resulted in poor occlusion [7]. The persistent flow demand and wall shear stress were known factors that delay thrombosis and neointimal coverage [8,11,22,26,27]. In our study, the presence of any incorporated branch, regardless of anatomical origin, was significantly associated with lower occlusion rates. Fig. 3 shows that an unruptured vertebrobasilar junction aneurysm was treated with SE stent and concurrent use of coils. One year later, an additional overlapping SE stent was deployed due to a remnant sac. Nevertheless, follow-up DSA revealed aneurysm growth with coil compaction, which was accompanied by persistent flow to the anterior inferior cerebellar artery.
The management of large aneurysms or those incorporating branch vessels has challenges in achieving a favorable occlusion. In our practice, these cases are followed more closely in the outpatient setting. During a certain period, if the follow-up image demonstrates limited occlusion, the additional interventions such as deployment of a second flow-diverter stent or adjunctive coil embolization are considered to augment treatment efficacy. In certain cases, additional destructive surgery such as parent artery occlusion with bypass surgery was considered. Nevertheless, these approaches are individualized and applied selectively. Larger, prospective, multicenter studies are needed to validate whether such stepwise strategies can reliably improve long-term occlusion rates in aneurysms with large size or branch incorporation.
There are several limitations to this study. First, while almost all aneurysms underwent follow-up catheter-based angiography (94.1%, n=64/68), the follow-up duration was heterogeneous. Consequently, we did not perform a time-stratified analysis. The limited sample size and the small number of patients with long-term radiological follow-up would have led to unstable statistical estimates with excessive censoring, precluding meaningful interpretation. Gupta et al. have reported that complete occlusion rates increase with longer follow-up after flow diversion [9]. One illustrative case in our series showed a similar pattern. A fusiform aneurysm involving cavernous segment of left ICA treated with SE stent (Fig. 4a, 4b). Six months follow-up DSA showed subtotal filling (OKM B, Fig. 4c). At 5-year follow-up, angiography confirmed complete occlusion (OKM D, Fig. 4d). This case reflects the potential for progressive aneurysm occlusion over extended follow-up. Therefore, a homogenous long-term study should be necessary. Second, although our study suggests that a larger size of aneurysm and branch-incorporated aneurysm are independent risk factors for poor occlusion after flow-diversion, no additional risk factors were identified compared to previous studies with other flow-diverters. However, to our best knowledge, there were a few studies about risk factors associated with poor occlusion using SE stent. At last, our study was a retrospective, single-center study with a relatively small cohort, which may have introduced bias. We acknowledge the potential risk of overfitting in the multivariate model due to the relatively small sample size. To minimize this concern, we limited the number of covariates and selected factors primarily based on those consistently reported in previous studies. Nevertheless, the limited cohort size could not fully eliminate this issue, and therefore, the estimates should be interpreted with caution, as exploratory findings rather than definitive conclusions.
CONCLUSIONS
The Surpass Evolve stent might be feasible for the treatment of unruptured intracranial aneurysm with adequate occlusion and low complications. The presence of any incorporated branch and a larger size of aneurysm were identified as independent risk factors for poor occlusion following flow diversion using the Surpass Evolve stent.
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