It is well established that wide-necked intracranial aneurysms are difficult to treat endovascularly due to the risk of coil protrusion in the parent artery and aneurysm recurrence. Stent-assisted coiling (SAC) has been established as one of the treatment options for these endovascularly challenging aneurysms, thereby improving the results. Intracranial stents allow the reconstruction of the aneurysm neck and help maintain the coil mass within the aneurysm sac. Recently, the Neuroform Atlas stent (Stryker Neurovascular, Kalamazoo, MI, USA), a microcatheter-delivered, self-expanding, mixed open-cell/closed-cell stent, has been used widely. With low-profiled, easy handling deliveries, the Neuroform Atlas stent is being widely applied for wide-necked intracranial aneurysms in various ways [9,12].

Although SAC using the Neuroform Atlas stent has been reported to be safe and effective in treating wide-necked aneurysms [1,6,7,13,14], the technique has procedure-related complications that interventionists need to understand completely. However, none of those studies reported the incidence of and risk factors for procedure-related complications. Thus, this study aimed to report our experience with SAC using the Neuroform Atlas stent in treating unruptured wide-necked aneurysms and to evaluate the incidence of and risk factors for procedure-related complications.

Baseline demographics and aneurysm characteristics are summarized in Table 1. Of the 123 patients included in this study, there were 96 women, and the median age was 60.2 years (range from 29 to 79). Patient baseline characteristics included hypertension in 63 patients (51.2%), diabetes in 29 (23.6%), smoking history in 19 (15.4%), dyslipidemia in 16 (13.0%), and coronary heart diseases in 11 (8.9%). Aneurysms were located in the internal carotid artery in 65 cases (50.0%), posterior communicating artery in nine (6.9%), middle cerebral artery in 18 (13.8%), anterior communicating artery in 17 (13.1%), anterior cerebral artery in 10 (7.7%), basilar artery in nine (6.9%), and vertebral artery in two (1.5%). The mean aneurysm size was 5.3±1.7 mm, and the mean neck size was 3.9±0.8 mm.

Table 2 shows the clinical and radiographic outcomes of the aneurysms. In terms of technical considerations, there was one case (0.8%) of failure of stent deployment and one (0.8%) of suboptimal positioning of the stent. These two events happened because the delivery microcatheters suddenly dropped to the proximal part of very tortuous internal carotid arteries. We performed Y-stenting (Atlas-Atlas; Stryker Neurovascular, Fremont, CA, USA) in three patients (2.3%) and stent salvage in six (4.6%). Immediate postembolization angiographic results showed complete obliteration in 61 aneurysms (46.9%), residual neck in 44 (33.8%), and residual aneurysm in 25 (19.2%). Follow-up angiographic results were obtained in all aneurysms and showed complete obliteration in 103 (79.2%), residual neck in 16 (12.3%), and residual aneurysm in 11 (8.5%). There was no recurrence of the aneurysm during the follow-up period. With respect to clinical outcomes, there were no morbidity or mortality cases even though there were eight cases (6.2%) of procedure-related complications (two dissections, five thromboembolisms, and one hemorrhage) and two (1.5%) of delayed complications (one ischemia and one hemorrhage). All five patients with thromboembolic complications underwent intra-arterial thrombolysis in which a glycoprotein IIb/IIIa antagonist (tirofiban) was used and showed successful recanalization without any symptoms. Two patients experienced vertebral artery dissection due to guiding catheter placement and were treated conservatively.

Logistic regression analysis was used to determine independent associations of procedure-related complications with other factors. On univariate analysis, aneurysm locations in the middle cerebral artery (odds ratio [OR], 2.211; 95% CI, 1.196–3.993; p=0.046) and the anterior communicating artery (OR, 2.850; 95% CI, 1.298–5.143; p=0.039) was associated with procedure-related complications (Table 3). However, no independent risk factor for procedure-related complications was noted in the multivariate logistic regression analysis adjusted for age and sex.

In the present study, the procedure-related complication rate was 6.2% among 123 patients with 130 unruptured wide-necked aneurysms. Further, no case of morbidity/mortality was noted with SAC for unruptured wide-necked intracranial aneurysms with the Neuroform Atlas. Thromboembolic complications were the most common at 3.8% followed by dissection (1.5%) and hemorrhage (0.8%). The technical success rate of delivering and deploying the stent was 98.5% among the 130 cases. Delayed ischemia due to transient ischemic attack and a small amount of delayed hemorrhage in the left frontal lobe occurred in one case each during the follow-up period, with both patients being asymptomatic. We found that stent deployment in the middle cerebral artery and anterior communicating artery was associated with procedure-related complications in the univariate analysis.

An expected benefit of the Neuroform Atlas stent is its increased navigability, which allows easy delivery [12]. The Neuroform Atlas stent is the latest version of the Neuroform series with reinforced navigability within a smaller microcatheter that confers stability to the vessel wall (high radial force), enables accurate placement (ease of use with very low foreshortening), and allows wall apposition, conformability, and coil protection. It can be delivered via a 0.0165-inch microcatheter, the smallest profile, allowing neurointerventionists to perform SAC without difficulty. The cell size of the Neuroform Atlas stent is smaller compared to that of its predecessor, the Neuroform stent. This enables better coil protection in aneurysms and allows the use of smaller coils. Additionally, the width of the struts has been reduced to improve flexibility and conformability of the stent. Furthermore, the segmental opening of the struts allows stable positioning and direct vessel wall apposition by anchoring the stent after the first rows of the struts exit the microcatheter. These characteristics of the Neuroform Atlas make SAC safer and more effective.

There have been a few reports on SAC with Neuroform Atlas stents, of which most have reported that SAC using Neuroform Atlas stents is safe and effective in treating wide-necked aneurysms [1,6,7,13,14]. From their experience with 27 consecutive patients, the authors of one study concluded that Neuroform Atlas SAC is feasible for treating ruptured and unruptured wide-necked aneurysms that are not amenable to conventional coiling or clipping [13]. The aneurysm occlusion and favorable clinical outcomes were consistent with those previously reported for SAC of wide-neck aneurysms using other devices [13]. These previous authors reported favorable and complete occlusion rates of 69.2% and 53.8% at 6 months, respectively. Another report concluded that deployment of the Neuroform Atlas stent was a safe and effective method for the treatment of wide-necked aneurysms from their experiences with 37 aneurysms in 36 patients [14]. They reported satisfactory immediate angiographic outcomes : complete aneurysm obliteration was observed in 84% cases, while neck remnants were observed in the remaining 16%. Furthermore, high-resolution time-of-flight magnetic resonance angiography revealed an occlusion rate of 100% at the 5-month follow-up. Additionally, other reports showed a complete occlusion rate of 86.7% with the Neuroform Atlas system [6]. The complete occlusion rate of 79.2% in the present study is in accordance with the results published previously, highlighting the efficacy of the Neuroform Atlas.

Compared to the Low-profile Visualized Intraluminal Support Junior device (LVIS Jr; MicroVention, Tustin, CA, USA), the Neuroform Atlas is less visible because only the stent markers at each end of the stent are radiopaque, although the struts themselves are invisible. Nevertheless, the visibility of the Neuroform Atlas was sufficient to allow accurate placement and deployment of the device in the present study. Additionally, the high outward radial force of the Neuroform Atlas ensures opening and wall apposition of the stent. If the complete opening of the stent is doubtful, the struts can be visualized by unenhanced flat-panel detector CT [13]. This approach can be employed to overcome the disadvantage of the open-cell design of the Neuroform Atlas stent, which causes resheathing after its deployment difficult. However, neurointerventionists should be aware of the possibility of delivery microcatheters unexpectedly falling down in tortuous parent arteries for the optimal deployment of the stent. According to the thromboembolic complications, it seems that there is no difference in the rate of thromboembolic complications between Neuroform Atlas and LVIS stents. We reported 6.5% of thromboembolic complications using Neuroform Atlas stents, whereas LVIS family (LVIS and LVIS Jr) had 3.6–5.8% [2,5,15]. However, intra-procedural thromboembolic complications occurred in 8% among 25 patients in SAC of anterior communicating artery aneurysms using the LVIS Jr stent [10], supporting our result of possible higher rate of procedure-related complications in the parent arteries with small diameter, such as middle cerebral artery or anterior communicating artery. Another report showed that a series of 100 patients treated with the LVIS Jr stent with thromboembolic complications occurring in 14% of the cases and concluded that the potentially inadequate antiplatelet response and use of the stent itself are possible reasons for thromboembolic phenomena in patients with acutely ruptured aneurysms [11].

With increasing clinical experience and development of new devices, SAC of aneurysms with a small parent artery has been known to be feasible in selected cases and show good long-term patency rates of parent arteries [4,8,16,17]. Regardless of the stent type (open-cell or closed-cell), the deployment of stents in small arteries, such as anterior cerebral arteries, middle cerebral arteries, posterior cerebral arteries, or posterior inferior cerebellar arteries, was safe and resulted in good patency. Nevertheless, the possibility of thromboembolic complications and parent artery occlusion should not be neglected when performing SAC of aneurysms with a small parent artery. In the present study, no independent risk factors for procedure-related complications could be determined through a multivariate logistic regression analysis adjusted for age and sex. However, SAC of aneurysms located in the middle cerebral artery or anterior communicating artery showed an association with procedure-related complications in univariate analysis. Thus, using the Neuroform Atlas in distal small parent arteries might be feasible, although its possible procedure-related complications should be considered.

Stents with an open-cell design show better wall apposition in tortuous vessels compared with closed-cell stents. However, incomplete stent apposition is more likely to be found in small arteries with tortuous anatomy. In addition, the low radial force of stents could be one cause of poor wall apposition. Although poor wall apposition may improve by open-cell stents, it might not work properly in a small-sized tortuous parent artery. The stent might not fully deploy in small arteries with acute angles because of kinking, twisting, or ovalization at the angles. This is especially concerning in arteries with a small diameter. Furthermore, increased metal surface coverage might increase the risk of thromboembolism when stents are deployed in small arteries. Thus, it can’t be too careful of performing SAC in the parent artery with small diameter, such as anterior cerebral artery or middle cerebral artery.

The present study had several limitations including the retrospective data collection and the small number of patients with an unruptured aneurysm enrolled, and thus, the effects of possible selection bias cannot be excluded. Moreover, as the follow-up period was short (less than 2 years), longer follow-up is needed to assess the stability of adequate obliteration. Furthermore, there was no true control group for the evaluation of the efficacy and safety of the device. Thus, comparisons with different devices and laboratory experiments to improve the technical defects of the Neuroform Atlas should be performed in additional clinical trials.

The Neuroform Atlas stent showed a high rate of technical success with good navigability. Good clinical and radiographic outcomes in early follow-up suggest that the device is safe and reliable with minimal risks for SAC of unruptured intracranial aneurysms in both anterior and posterior circulation. In cases of SAC of unruptured intracranial aneurysms in the middle cerebral artery or anterior communicating artery, more careful use of the Neuroform Atlas may be required to prevent possible procedure-related complications. Large cohort studies with a longer follow-up are necessary to determine the long-term durability and safety of the device as well as the risk factors for procedure-related complications.