Flow alteration strategies for complex basilar apex aneurysms: multicenter experience, systematic review, and insights from computational fluid dynamics

Flow alteration strategies for complex basilar apex aneurysms: multicenter experience, systematic review, and insights from computational fluid dynamics | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Flow alteration strategies for complex basilar apex aneurysms: multicenter experience, systematic review, and insights from computational fluid dynamics June Ho Choi, Mahmoud Elhadidy, Minwoo Kim, Wonhyoung Park, Jung Cheol Park, and 8 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7248899/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Complex basilar apex aneurysms (CBAAs) present a significant challenge due to their unfavorable natural history and difficulty with conventional treatments. This study aimed to provide insights into flow alteration strategies by combining a systematic review using PRISMA methodology with a multicenter experience from South Korea. We analyzed 57 cases, finding that flow preservation with aneurysm obliteration was performed in 12.7%, while mild, moderate, and maximum flow reduction were applied in 77.2%, 7.0%, and 3.5% respectively. Outcomes showed that 75.8% of patients with available imaging achieved satisfactory aneurysm obliteration. A good clinical outcome (mRS 0–2) was observed in 49.1% of cases. However, poor outcomes (mRS 4–6) were reported in 31.6%, with a mortality rate of 17.5%. Beyond simply reducing intra-aneurysmal flow, computational fluid dynamics (CFD) simulations revealed that alterations in flow balance and direction significantly influenced hemodynamic stress. Given the severe prognosis of CBAAs, flow alteration strategies can serve as viable alternatives when conventional treatments are not feasible. Furthermore, CFD simulations might hold promise in identifying optimal strategies that can maximize aneurysm control while minimizing procedural risks. aneurysm basilar apex aneurysm complex intracranial aneurysm computational fluid dynamics flow alteration flow reduction Figures Figure 1 Figure 2 Figure 3 Introduction Basilar apex aneurysms (BAAs) pose significant treatment challenges. While many are referred for endovascular treatment, complex anatomy or morphology may preclude this option [30, 36]. Moreover, endovascular approaches are associated with high recurrence and rupture rates, and large BAAs carry procedural risks comparable to surgical clipping. New devices, such as endovascular flow disruptors, are being utilized for the treatment of BAAs. However, these devices have not significantly expanded the technical success and safety of treatment [30]. When endovascular treatment is not suitable, surgical clipping is often advocated [20]. However, due to the anatomy of the upper BA, direct clipping or trapping is frequently difficult [2]. Surgical clipping of large and complex BAAs (CBAAs) presents high procedural risks [3]. Consequently, various flow alteration methods have been attempted in the treatment of complex basilar apex aneurysms (CBAAs) [7, 13, 19, 28, 36]. The primary goal of flow alteration is to maintain perfusion to perforators or distal branches that are involved in the aneurysm process while altering the natural history of the lesion by reducing the risk of aneurysm growth and rupture [13]. Given that CBAAs are a relatively rare disease entity, large-scale studies of flow alteration are limited. Here, we report multicenter results, conduct a systematic review of prior studies, and investigate fluid dynamic changes associated with various flow alteration methods using an actual case. Methods This study was approved by the Institutional Review Boards at each participating center. Due to the retrospective nature of the study, the requirement for informed consent was waived. The data used in this study are available from the corresponding author upon reasonable request. Case series from study centers We retrospectively reviewed patients with BAAs treated between 2000 and 2023. Inclusion criteria were as follows: 1) BAAs unsuitable for microsurgical clipping or endovascular intervention, treated with flow alteration; 2) long-term clinical and radiological follow-up. Patients were excluded if they: 1) had no follow-up records; or 2) had BAAs amenable to alternative treatments based on multidisciplinary assessment. We collected data on patient demographics, clinical and radiological characteristics, flow alteration methods, and clinical and radiological outcomes. Flow alteration methods included flow preservation and flow redirection and reduction (Online Resource 1). Flow preservation included bypass surgery and subsequent surgical or endovascular aneurysm obliteration. Flow redirection and reduction was further classified, referring to Miyamoto et al., into mild, moderate, or maximum flow reduction [28]. Mild flow reduction included proximal occlusion alone or superficial temporal artery–middle cerebral artery (STA–MCA) bypass(es) aimed at reducing flow burden. Moderate and maximum flow reduction involved proximal occlusion combined with additional branch occlusion(s), with or without bypass(es). After treatment, changes in flow direction and their relationship to the aneurysm were classified into five categories: 1) obliteration with branch; 2) no change but flow reduction; 3) side wall type; 4) stagnant junction type; 5) blind alley type. Clinical outcomes were assessed using the modified Rankin Scale (mRS). Radiological outcomes on the last follow-up image were categorized as: 1) enlargement; 2) incomplete obliteration; 3) virtually complete obliteration (filling of the ectatic aneurysmal base); 4) complete obliteration [39]. Literature review Search strategy This study followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. In consideration of the timeline for intracranial stent approval in aneurysm treatment, we conducted a systematic review of PubMed and EMBASE from January 2000 to April 2024. The search terms used were: (“basilar apex” OR “basilar top” OR “basilar tip” OR “basilar bifurcation” OR “basilar quadrification”) AND “aneurysm”. The reference lists of all retrieved articles were also reviewed to identify additional relevant studies. Study selection and data extraction Duplicate records were removed, after which two authors (JHC and MK) screened titles and abstracts for studies on the surgical treatment of BAAs. Only English-language articles were included; conference papers and commentaries were excluded. Full-text review applied the following inclusion criteria: 1) CBAAs difficult to treat with microsurgical clipping or endovascular methods; 2) treatment with flow alteration, with or without bypass; and 3) at least one clinical and radiologic follow-up ≥ 6 months post-treatment. Studies lacking sufficient detail on aneurysm characteristics, treatment, or outcomes were excluded. In cases of duplicate reports, the most recent publication was used. Disagreements on study selection were resolved through author discussion. Only CBAAs treated with flow alteration were included in the review, and data were extracted to match variables collected in the case series. Computational fluid dynamics methodology Computational fluid dynamics (CFD) simulation was performed using the open-source SimVascular software. Blood flow was assumed to be Newtonian, and the incompressible Navier-Stokes equations were solved to model pulsatile flow. Blood density was set to 1.06 g/cm 3 , viscosity to 4 cP, and a rigid wall condition was assumed. The base case (Case 2 of this study, Fig. 1) used a simplified assumption by neglecting flow in the posterior communicating arteries (PCoAs) [ The pulsatile inlet flow rate was scaled to achieve the required outlet flow rates at the four outlets: two posterior cerebral arteries (PCAs) and two superior cerebral arteries (SCAs) [4, 43]. Subsequently, ten scenarios were analyzed in addition to the baseline (Online Resource 2). In these scenarios, the BA was clipped while keeping all four outlets open, or one/multiple outlets were occluded. Additionally, PCoAs were considered as inlets in some cases, and in certain cases, some of them were occluded. Outlet resistances were applied as boundary conditions and iteratively optimized to match the target flow rates. A mesh independence study using 0.5, 1.5, and 6 million tetrahedral elements showed that a 1.5-million-element mesh yielded acceptable outlet flow accuracy. Simulations ran for three cardiac cycles (0.8 s each) with a 0.00008 s time step (30,000 total steps). Flow patterns were analyzed during the final cycle. Results Systematic review A total of 906 articles were identified from the two databases after duplicate removal. An additional 14 articles were identified through citation searching and screened for eligibility. Following eligibility assessment, 25 articles were included (Fig. 2) [1, 8–11, 13, 15, 16, 18, 19, 21–25, 28, 31–36, 38, 42, 46]. Each of these articles contained at least one case meeting the inclusion criteria. A summary of the included cases is provided in Online Resource 3. When multiple publications from the same institution reported the same case at different time points, these were combined and treated as a single case. Clinical characteristics A total of 57 patients with CBAAs, including those from this case series, were enrolled. Clinical characteristics were summarized in Table 1 based on the data available from the articles. The median age was 55 years (interquartile range [IQR], 45–66), and 69.4% of the patients were female. Previous interventions were performed in 52.1% (25 of 48) of patients. The median maximum aneurysm diameter was 25.5 mm (IQR, 20–34.5). Flow preservation with aneurysm obliteration was performed in 12.3% (7 of 57) of cases. Mild, moderate, and maximum flow reduction was carried out in 77.2% (44 of 57), 7.0% (4 of 57), and 3.5% (2 of 57) of cases, respectively. Bypass surgery was performed in 59.6% (34 of 57) of patients. Table 1 Summary of included cases of complex basilar apex aneurysms treated with flow alteration strategies Characteristics No. (%, total number) a Total number of cases 57 Age, median (IQR, total number) 55 (45–66, 49) Sex Female 34 (69.4, 49) Presentations SAH 9 (20.5, 44) Infarction 3 (6.8, 44) Brainstem compressive symptoms 15 (34.1, 44) Hydrocephalus 6 (13.6, 44) Previous interventions 25 (52.1, 48) Maximum diameter, median (IQR, total number) 25.5 (20-34.5, 42) Flow alteration Flow preservation with aneurysm obliteration 7 (12.3, 57) Mild flow reduction 44 (77.2, 57) Moderate flow reduction 4 (7.0, 57) Maximum flow reduction 2 (3.5, 57) Bypass Yes 34 (59.6, 57) Changed aneurysm type Obliteration with branch 7 (26.9, 26) No change but flow reduction b 1 (3.8, 26) Side wall type 9 (34.6, 26) Stagnant junction type 7 (26.9, 26) Blind alley type 2 (7.7, 26) Postoperative complications c 27 (52.9, 51) Infarction 18 (35.3, 51) Cranial nerve palsy 4 (7.8, 51) Transient neurologic deterioration 8 (15.7, 51) Brainstem compression 4 (7.8, 51) Delayed rupture 2 (3.9, 51) Follow-up image d 33 (57.9, 57) Enlargement 4 (12.1, 33) Incomplete obliteration 4 (12.1, 33) Virtually complete obliteration 10 (30.3, 33) Complete obliteration 15 (45.5, 33) Follow-up period (months), median (IQR, total number) 12 (6-42.25, 48) a: Data are reported as No. (%, total number) unless otherwise indicated. Total number is the sum of cases with available data. b: To reduce the hemodynamic burden in a patient with bilateral internal carotid artery occlusion, superficial temporal artery-middle cerebral artery bypass surgery was performed bilaterally. c: Each complication was counted separately. d: Among the 24 cases without follow-up images, 14 were not described in the original article, and 10 were due to mortality, with 8 of those attributed to complications. No., number; IQR, interquartile range; SAH, subarachnoid hemorrhage Clinical and radiological outcomes Figure 3 shows changes in preoperative and last follow-up mRS scores. Preoperative mRS was undocumented in five cases; of these, two had a last mRS of 3 and three had an mRS of 6. Among the 52 patients with available preoperative scores, 42.3% (22) improved, 26.9% (14) were unchanged, and 30.8% (16) worsened. A good outcome (mRS 0–2) was seen in 49.1% (28 of 57), and a poor outcome (mRS 4–6) in 31.6% (18 of 57). Infarction was the most frequent complication, occurring in 35.3% (18 of 51) (Table 1 ). At final imaging follow-up, 75.8% (25 of 33) showed favorable obliteration. Median follow-up was 12 months (IQR, 6–42.25). CFD experiments for various flow alteration situations Based on the blood flow velocities and time-averaged wall shear stress (TAWSS) results for the 11 experimental configurations, each strategy had distinct effects on aneurysm hemodynamics (Online Resource 2). The maximum and mean TAWSS values in the aneurysm are summarized in Table 2 . In the baseline state, very high TAWSS values were observed in the stenosis region just proximal to the aneurysm, reaching 530 dynes/cm². The flow from the BA created an impingement jet at the apex of the aneurysm, producing a TAWSS of 290 dynes/cm². The “stagnant junction type with bifurcation”, which can be achieved by performing only proximal occlusion, lowered the maximum TAWSS to 64 dynes/cm² from 290 dynes/cm² in the original state, with the mean value reduced to just 10% of its original. However, it increased TAWSS in the PCoAs to 760 dynes/cm². The “stagnant junction type with a branch” achieved significantly lower maximum and mean TAWSS values in the aneurysm than the “stagnant junction type with bifurcation,” at 15 and 0.05 dynes/cm², respectively. Among the side wall types, the “side wall type (acute angled)” recorded the lowest maximum and mean TAWSS values, marginally lower than those of the “side wall type (obtuse angled).” TAWSS analysis revealed that the “side wall types with a branch or a bifurcation” and the “side wall type (straight)” exhibited relatively higher TAWSS values in the aneurysm region compared to the “side wall types (acute or obtuse angled).” In the “stagnant junction type” and “blind alley type,” CFD simulations showed extremely slow and stagnant blood flow in the aneurysm. Table 2 Summary of maximum and mean time-averaged wall shear stress values at the aneurysm region Experiments Max TAWSS at the aneurysm (dynes/cm 2 ) Mean TAWSS at the aneurysm (dynes/cm 2 ) Original state 290.29 2.68 Stagnant junction type with a bifurcation 64.66 0.21 Stagnant junction type with a branch 15 0.05 Side wall type with a bifurcation 67.83 0.3 Side wall type with a branch (acute) 46.44 0.52 Side wall type with a branch (obtuse) 24.26 0.21 Side wall type (straight) 18.69 0.08 Side wall type (obtuse angled) 9.19 0.06 Side wall type (acute angled) 8.1 0.02 Stagnant junction type 1.01 1.3 × 10⁻³ Blind alley type 5.5 × 10⁻³ 4.3 × 10⁻⁵ TAWSS, time-averaged wall shear stress. Discussion In this systematic review of flow alteration for the treatment of complex basilar apex aneurysms (CBAAs), a good outcome was observed in 49.1% of patients, while 69.2% experienced either improvement or no change in their mRS scores. Among those with available follow-up imaging, 75.8% achieved favorable obliteration. Postoperative complications occurred in over half of the cases, with infarction being the most common. Eight cases resulted in death due to complications. However, given that more than 80% of patients with untreated giant BAAs die and the remainder are severely disabled [39], flow alteration may be an alternative option when conventional treatments are deemed difficult or infeasible. Moreover, computational fluid dynamics (CFD) simulations suggest that, beyond simply reducing aneurysmal inflow, optimizing the balance and direction of inflow may mitigate hemodynamic shear stress and enhance aneurysm control. These insights might guide the selection of flow-alteration strategies; however, careful consideration of the requisite surgical techniques and associated risks remains crucial. Review of flow alteration methods for CBAAs In this study, we included all treatment approaches that modify the existing flow architecture, ranging from flow preservation to flow redirection and reduction. Even in cases of CBAAs, “flow preservation with aneurysm obliteration” may be achieved. Although the number of such cases was limited, our systematic review suggested a relatively lower complication rate (28.6%, 2 of 7) with this approach compared to other methods. Flow redirection is considered when the aneurysm cannot be completely excluded from the circulation, aiming to alter the lesion’s hemodynamics and impact its progression [13, 32]. Proximal occlusion, which has been employed for a long time [7], can be achieved surgically or endovascularly. Endovascular occlusion has targeted both bilateral vertebral arteries (VAs) and the BA, though outcomes are variable and may risk compromising brainstem perforators [12, 44]. Surgical occlusion allows direct visualization, potentially reducing this risk. Kellner et al. reported nine BAA cases treated with surgical BA occlusion after successful balloon test occlusion; all but one had favorable long-term outcomes [19]. When collateral flow is inadequate, bypass may be used to augment circulation [14, 40]. In unclippable vertebrobasilar system aneurysms, the outcomes of deliberate occlusion of the BA or VA have been linked to the size of the PCoAs [39]. This correlation likely reflects not only the adequacy of collateral flow but also the degree of flow passing through the aneurysm orifice. When both PCoAs have a diameter ratio of < 0.45, BAAs show no change or occlusion following proximal occlusion; completely thrombosed aneurysms and partially thrombosed aneurysms had a ratio above 0.6 and a ratio between 0.46 and 1, respectively [6]. These findings suggest that the greater the flow passing through the aneurysm orifice after proximal occlusion, the lower the likelihood of subsequent aneurysm occlusion. Similarly, Nagasawa et al. performed simulations in a BAA model and found that a high PCoA diameter ratio was associated with markedly increased intra-aneurysmal stagnation [29]. Other studies have reported that persistent blood flow within the aneurysm can result in a lack of treatment response or even rupture [41, 46]. Miyamoto et al. introduced moderate and maximum flow reduction techniques, hypothesizing that reducing and stagnating blood flow at the aneurysm orifice aids in aneurysm obliteration [28]. By diminishing both inflow and outflow, they reported a high rate of aneurysm obliteration and favorable clinical outcomes. Although the number of cases was small, the theoretical rationale appears sound, but these methods carry considerable risk due to the complexity of staged surgical procedures. Complication risk is an important consideration in flow alteration. In this study, 52.9% (27 of 51) experienced complications; however, 69.2% showed improved or stable mRS scores, suggesting not all complications had lasting effects. Early thromboembolic brainstem ischemia and aneurysm thrombosis with mass effect may occur [39]. One case developed a giant PCA aneurysm from long-term hemodynamic changes [24], highlighting the need for continued follow-up. Hemodynamic considerations in various flow alteration methods Previous studies have demonstrated that both high and low WSS can influence aneurysm formation, progression, and rupture at different stages [26, 27, 37, 45]. However, persistently high WSS can lead to wall degradation, delamination, and imbalances in collagen remodeling, increasing the risk of rupture [5, 17, 26, 45]. In large or giant aneurysms, the complex intra-aneurysmal flow can create localized regions of high WSS that exacerbate wall injury [17]. Our CFD simulations demonstrated that proximal occlusion effectively reduces both maximal and mean TAWSS within the aneurysm and mitigates impingement jets (resulting in “stagnant junction type with a bifurcation” or “side wall type with a bifurcation” configurations). Nevertheless, compared with other flow alteration techniques, proximal occlusion continued to exhibit relatively high hemodynamic stress. This mechanism might explain previous reports of poor prognosis following proximal occlusion when persistent blood flow remains within the aneurysm [41, 46]. In particular, when a PCoA diameter ratio is nearly 1, the “stagnant junction type with a bifurcation” is theoretically associated with a favorable outcome [6, 29, 39]. However, based on our simulation findings, additional flow reduction might be even more beneficial. By contrast, for the “side wall type with a bifurcation,” the inflow direction and hemodynamic burden may be unfavorable for aneurysm control. In one report of a large BAA treated with bilateral VA occlusion, jet flow from the P1 segment continued to enter the aneurysm, as it washed out the earlier thrombosed region [1]. Shojima et al. also described a CBAA that became a “side wall type with a bifurcation” after BA occlusion and ruptured 6 months later; based on CFD analysis, the authors concluded that flow dynamics could worsen in certain cases [38]. Moreover, our simulation revealed highly elevated TAWSS within the remaining PCoA, which could increase the risk of de novo aneurysm formation in the long term [24]. Thus, in side wall configurations, the angle between the inlet and the aneurysmal sac—as well as the associated flow burden—may play a pivotal role in treatment outcomes. Indeed, for “side wall type without a branch,” the angled configuration demonstrated more favorable results than the straight configuration. A likely explanation is that the outlet in the angled configuration, supplying the SCA, requires less flow than the straight configuration, which supplies the PCA. Finally, compared with the “stagnant junction type with a bifurcation,” the “stagnant junction type with a branch” configuration demonstrated approximately one-fourth the maximal and mean TAWSS. The impact of reducing the flow burden for a single SCA in the “stagnant junction type with a bifurcation” is greater than in the “side wall type with a bifurcation.” In addition, both the “blind alley type” and “stagnant junction type” nearly eliminate flow through the aneurysm. Consequently, not only reducing inflow but also balancing and redirecting flow may yield theoretically better outcomes. Yet achieving these configurations often entails complex surgical techniques and concomitant risks that must be carefully weighed. Limitations Although we aggregated prior data, CBAAs remain rare, limiting case numbers. Treatment methods varied, constraining generalizability. Due to time and resource constraints, only one case was used for CFD simulations, reducing representativeness. Boundary conditions were extrapolated from the literature (not directly measured) [4, 6, 38, 43], and rigid-wall assumptions omitted real viscoelastic effects. However, to our knowledge, this is the first systematic review of this condition. Our comprehensive simulations of various flow‑alteration strategies offer potentially valuable insights. Larger cohorts and more advanced CFD models are needed to validate and expand these findings. Conclusion Given the severe natural course of CBAAs, flow alteration strategies may serve as viable alternatives when conventional treatment options are not feasible. CFD simulations may assist in identifying optimal strategies that maximize aneurysm control while minimizing procedural risk. Abbreviations BAAs, basilar apex aneurysms CBAAs, complex basilar apex aneurysms CFD, computational fluid dynamics mRS, modified Rankin Scale PRISMA, Preferred Reporting Items for Systematic Reviews and Meta-Analyses TAWSS, time-averaged wall shear stress Declarations Competing Interests The authors have no relevant financial or non-financial interests to disclose. Authour Contributions Conceptualization: June Ho Choi, Amirhossein Arzani, Jae Sung Ahn; Data curation: June Ho Choi, Mahmoud Elhadidy, Minwoo Kim, Sung-Pil Joo, Sang Hyo Lee; Formal analysis and investigation: June Ho Choi, Mahmoud Elhadidy; Methodology: June Ho Choi, Mahmoud Elhadidy, Amirhossein Arzani; Project administration: Michael T. Lawton, Amirhossein Arzani, Jae Sung Ahn; Resources: June Ho Choi, Mahmoud Elhadidy, Sung-Pil Joo, Sang Hyo Lee, See Un Lee, Jae Seung Bang, Amirhossein Arzani; Supervision: Michael T. Lawton, Amirhossein Arzani, Jae Sung Ahn; Writing - original draft preparation: June Ho Choi, Mahmoud Elhadidy; Writing - review and editing: June Ho Choi, Mahmoud Elhadidy, Wonhyoung Park, Jung Cheol Park, Byung Duk Kwun, Jae Seung Bang, Michael T. Lawton, Amirhossein Arzani, Jae Sung Ahn. Ethics Approval All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards. This is a retrospective review study. Collection of retrospective data was approved by Institutional Review Board of each center. Informed consent T he need for patient consent was waived because of the retrospective nature of the study. Clinical trial number Not applicable. Funding No funding was received for this research. Author Contribution Conceptualization: June Ho Choi, Amirhossein Arzani, Jae Sung Ahn; Data curation: June Ho Choi, Mahmoud Elhadidy, Minwoo Kim, Sung-Pil Joo, Sang Hyo Lee; Formal analysis and investigation: June Ho Choi, Mahmoud Elhadidy; Methodology: June Ho Choi, Mahmoud Elhadidy, Amirhossein Arzani; Project administration: Michael T. Lawton, Amirhossein Arzani, Jae Sung Ahn; Resources: June Ho Choi, Mahmoud Elhadidy, Sung-Pil Joo, Sang Hyo Lee, See Un Lee, Jae Seung Bang, Amirhossein Arzani; Supervision: Michael T. Lawton, Amirhossein Arzani, Jae Sung Ahn; Writing - original draft preparation: June Ho Choi, Mahmoud Elhadidy; Writing - review and editing: June Ho Choi, Mahmoud Elhadidy, Wonhyoung Park, Jung Cheol Park, Byung Duk Kwun, Jae Seung Bang, Michael T. Lawton, Amirhossein Arzani, Jae Sung Ahn. Acknowledgement We would like to thank Editage (www.editage.co.kr) for English language editing. 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AJNR Am J Neuroradiol 35:1254–1262. doi: 10.3174/ajnr.A3558 Meng H, Wang Z, Hoi Y, Gao L, Metaxa E, Swartz DD, Kolega J (2007) Complex hemodynamics at the apex of an arterial bifurcation induces vascular remodeling resembling cerebral aneurysm initiation. Stroke 38:1924–1931. doi: 10.1161/STROKEAHA.106.481234 Miyamoto S, Funaki T, Iihara K, Takahashi JC (2011) Successful obliteration and shrinkage of giant partially thrombosed basilar artery aneurysms through a tailored flow reduction strategy with bypass surgery: Clinical article. J Neurosurg 114:1028–1036. doi: 10.3171/2010.9.JNS10448 Nagasawa S, Kawanishi M, Tada Y, Kawabata S, Ohta T (1999) Simulation of therapeutic parent artery occlusion for basilar head aneurysms. Hemodynamic effect of occlusion sites and diameters of collateral arteries. Neurol Res 21:180–184. doi: 10.1080/01616412.1999.11740915 Ozpeynirci Y, Hutschenreuter B, Forbrig R, Brückmann H, Liebig T, Dorn F (2021) Endovascular treatment of basilar tip aneurysms in the era of endosaccular flow disruption: a comparative study. Neuroradiology 63:619–626. doi: 10.1007/s00234-020-02555-0 Pahl FH, Oliveira MFD, Rotta JM (2017) Microsurgical treatment of basilar tip aneurysms: is it still acceptable? Arq Neuropsiquiatr 75:697–702. doi: 10.1590/0004-282x20170120 Ponce FA, Albuquerque FC, McDougall CG, Han PP, Zabramski JM, Spetzler RF (2004) Combined endovascular and microsurgical management of giant and complex unruptured aneurysms. Neurosurg Focus 17:1–7. doi: 10.3171/foc.2004.17.5.11 Ramanathan D, Ciporen J, Ghodke B, Ellenbogen RG, Sekhar LN (2010) Treatment of coil embolization failed recurrent giant basilar tip aneurysms with bypass and surgical occlusion. J Neurointerventional Surg 2:237–241. doi: 10.1136/jnis.2010.002519 Ravina K, Strickland BA, Buchanan IA, Rennert RC, Kim PE, Fredrickson VL, Russin JJ (2019) Postoperative Antiplatelet Therapy in the Treatment of Complex Basilar Apex Aneurysms Implementing Hunterian Ligation and Extracranial-to-Intracranial Bypass: Review of the Literature with an Illustrative Case Report. World Neurosurg 123:113–122. doi: 10.1016/j.wneu.2018.11.237 Russell SM, Nelson PK, Jafar JJ (2002) Neurological deterioration after coil embolization of a giant basilar apex aneurysm with resolution following parent artery clip ligation. Case report and review of the literature. J Neurosurg 97:705–708. doi: 10.3171/jns.2002.97.3.0705 Sekhar LN, Tariq F, Morton RP, Ghodke B, Hallam DK, Barber J, Kim LJ (2013) Basilar tip aneurysms: a microsurgical and endovascular contemporary series of 100 patients. Neurosurgery 72:284–298; discussion 298-299. doi: 10.1227/NEU.0b013e3182797952 Sforza DM, Putman CM, Cebral JR (2009) Hemodynamics of Cerebral Aneurysms. Annu Rev Fluid Mech 41:91–107. doi: 10.1146/annurev.fluid.40.111406.102126 Shojima M, Morita A, Kimura T, Oshima M, Kin T, Saito N (2014) Computational fluid dynamic simulation of a giant basilar tip aneurysm with eventual rupture after Hunterian ligation. World Neurosurg 82:535.e5–9. doi: 10.1016/j.wneu.2013.09.034 Steinberg GK, Drake CG, Peerless SJ (1993) Deliberate basilar or vertebral artery occlusion in the treatment of intracranial aneurysms: Immediate results and long-term outcome in 201 patients. J Neurosurg 79:161–173. doi: 10.3171/jns.1993.79.2.0161 Sundt TM, Piepgras DG, Houser OW, Campbell JK (1982) Interposition saphenous vein grafts for advanced occlusive disease and large aneurysms in the posterior circulation. J Neurosurg 56:205–215. doi: 10.3171/jns.1982.56.2.0205 Takahashi JC, Murao K, Iihara K, Nonaka Y, Taki J, Nagata I, Miyamoto S (2007) Successful “blind-alley” formation with bypass surgery for a partially thrombosed giant basilar artery tip aneurysm refractory to upper basilar artery obliteration: Case report. J Neurosurg 106:484–487. doi: 10.3171/jns.2007.106.3.484 Takeuchi S, Tanikawa R, Tsuboi T, Noda K, Oda J, Miyata S, Ota N, Yoshikane T, Kamiyama H (2015) Superficial temporal artery to proximal posterior cerebral artery bypass through the anterior temporal approach. Surg Neurol Int 6:95. doi: 10.4103/2152-7806.157949 Vali A, Abla AA, Lawton MT, Saloner D, Rayz VL (2017) Computational Fluid Dynamics modeling of contrast transport in basilar aneurysms following flow-altering surgeries. J Biomech 50:195–201. doi: 10.1016/j.jbiomech.2016.11.028 Wenderoth JD, Khangure MS, Phatouros CC, ApSimon HT (2003) Basilar Trunk Occlusion during Endovascular Treatment of Giant and Fusiform Aneurysms of the Basilar Artery. AJNR Am J Neuroradiol 24:1226 Xiang J, Natarajan SK, Tremmel M, Ma D, Mocco J, Hopkins LN, Siddiqui AH, Levy EI, Meng H (2011) Hemodynamic-morphologic discriminants for intracranial aneurysm rupture. Stroke 42:144–152. doi: 10.1161/STROKEAHA.110.592923 Yasui T, Komiyama M, Iwai Y, Yamanaka K, Matsusaka Y, Morikawa T, Ishiguro T (2004) Regrowth and fatal rerupture despite proximal occlusion after coil embolization of a ruptured large basilar bifurcation aneurysm--case report. Neurol Med Chir (Tokyo) 44:587–590. doi: 10.2176/nmc.44.587 Additional Declarations No competing interests reported. Supplementary Files ESM1.eps Online Resource 1, Figure. Schematic diagrams of possible flow alteration methods for complex basilar artery aneurysm. A, Flow preservation with aneurysm obliteration (“obliteration with a branch”). Bypass and clip occlusion of aneurysm with a branch or combination of bypass with branch occlusion and endovascular treatment (inset). B, Mild flow reduction (“no change but flow reduction”). Achieved by bilateral superficial temporal artery to middle cerebral artery bypasses in bilateral internal carotid artery occlusion (inset). C and E, Mild flow reduction by proximal occlusion. D, F-K, Moderate flow reduction by proximal occlusion and obliteration of 1 or 2 branches with or without bypass(es). LMaximum flow reduction by obliteration of all but one artery and proximal occlusion. In C, D, and K, the aneurysm changed into a “stagnant junction type” aneurysm. In E-J, the aneurysm changed into a “side wall type” aneurysm. In L, the aneurysm changed into a “blind alley type” aneurysm. ESM2.eps Online Resource 2, Figure. Computational fluid dynamics results (time-averaged wall shear stress (TAWSS) and velocity streamline) for 11 experimental configurations for an actual case in this case series. A, TAWSS result of the original state (assuming no flow from the posterior communicating arteries). B, Velocity streamline result of the original state. C, TAWSS result of the “stagnant junction type with a bifurcation”. D, Velocity streamline result of the “stagnant junction type with a bifurcation”. E, TAWSS result of the “stagnant junction type with a branch”. F, Velocity streamline result of the “stagnant junction type with a branch”. G, TAWSS result of the “side wall type with a bifurcation”. H, Velocity streamline result of the “side wall type with a bifurcation”. I, TAWSS result of the “side wall type with a branch (acute)”. J, Velocity streamline result of the “side wall type with a branch (acute)”. K, TAWSS result of the “side wall type with a branch (obtuse)”. L, Velocity streamline result of the “side wall type with a branch (obtuse)”. M, TAWSS result of the “side wall type (straight)”. N, Velocity streamline result of the “side wall type (straight)”. O, TAWSS result of the “side wall type (acute angled)”. P, Velocity streamline result of the “side wall type (acute angled)”. Q, TAWSS result of the “side wall type (obtuse angled)”. R, Velocity streamline result of the “side wall type (obtuse angled)”. S, TAWSS result of the “stagnant junction type”. T, Velocity streamline result of the “stagnant junction type”. U, TAWSS result of the “blind alley type”. V, Velocity streamline result of the “blind alley type”. ESM3.docx Online Resource 3, Table. Summary of included cases of complex basilar apex aneurysms treated with flow alteration strategies. 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15:12:12","extension":"html","order_by":13,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":96348,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-7248899/v1/1d63b2fcbe4d8321fbc04573.html"},{"id":93246208,"identity":"d43bff87-7e56-4206-b5dd-efa11ef36ea8","added_by":"auto","created_at":"2025-10-10 15:12:12","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":66420,"visible":true,"origin":"","legend":"\u003cp\u003e3D mesh reconstruction of a complex basilar apex aneurysm (case 2 of this study). The red arrow designates inflow from the basilar artery, and the blue arrows indicate outflow to the posterior cerebral arteries and superior cerebellar arteries. The purple arrow and the yellow line highlight the assumption that there is no blood flow through the posterior communicating artery.\u003c/p\u003e","description":"","filename":"fig1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7248899/v1/ee04cfd73e02614a01ece1f8.jpg"},{"id":93246212,"identity":"144a69e3-9461-4fe8-9c7e-05325ee1a75c","added_by":"auto","created_at":"2025-10-10 15:12:12","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":765100,"visible":true,"origin":"","legend":"\u003cp\u003ePRISMA flow diagram for the literature review.\u003c/p\u003e","description":"","filename":"fig2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7248899/v1/4f5df851b735c7a314a7b344.jpg"},{"id":93246210,"identity":"b4837693-f60c-4078-8931-4529b75b4097","added_by":"auto","created_at":"2025-10-10 15:12:12","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":58859,"visible":true,"origin":"","legend":"\u003cp\u003eSankey diagram illustrating the transition of modified Rankin Scale (mRS) scores from the preoperative period (left) to the last follow-up (right). Each bar on the left represents the preoperative mRS scores (0–5) and N/A (no data available), and each bar on the right represents the postoperative mRS scores (0–6). The thickness of each flow is proportional to the number of patients moving from one mRS category to another.\u003c/p\u003e","description":"","filename":"fig3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7248899/v1/a95e57f19f88dbb39595fe72.jpg"},{"id":95527730,"identity":"04753461-5b0d-454c-94d2-aca27ffdfd2a","added_by":"auto","created_at":"2025-11-10 10:14:35","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1695505,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7248899/v1/cd72325e-e350-4bfc-9959-9f537c7cb379.pdf"},{"id":93246866,"identity":"f477a597-d765-4c7b-be0b-c436600c0baa","added_by":"auto","created_at":"2025-10-10 15:20:12","extension":"eps","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":477699,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eOnline Resource 1, Figure. \u003c/strong\u003eSchematic diagrams of possible flow alteration methods for complex basilar artery aneurysm. \u003cstrong\u003eA\u003c/strong\u003e, Flow preservation with aneurysm obliteration (“obliteration with a branch”). Bypass and clip occlusion of aneurysm with a branch or combination of bypass with branch occlusion and endovascular treatment (inset). \u003cstrong\u003eB\u003c/strong\u003e,\u003cstrong\u003e \u003c/strong\u003eMild flow reduction (“no change but flow reduction”). Achieved by bilateral superficial temporal artery to middle cerebral artery bypasses in bilateral internal carotid artery occlusion (inset).\u003cstrong\u003e C \u003c/strong\u003eand\u003cstrong\u003e E\u003c/strong\u003e, Mild flow reduction by proximal occlusion. \u003cstrong\u003eD\u003c/strong\u003e,\u003cstrong\u003e F-K\u003c/strong\u003e, Moderate flow reduction by proximal occlusion and obliteration of 1 or 2 branches with or without bypass(es). \u003cstrong\u003eL\u003c/strong\u003eMaximum flow reduction by obliteration of all but one artery and proximal occlusion. In \u003cstrong\u003eC\u003c/strong\u003e, \u003cstrong\u003eD\u003c/strong\u003e, and \u003cstrong\u003eK\u003c/strong\u003e, the aneurysm changed into a “stagnant junction type” aneurysm. In \u003cstrong\u003eE-J\u003c/strong\u003e, the aneurysm changed into a “side wall type” aneurysm. In \u003cstrong\u003eL\u003c/strong\u003e, the aneurysm changed into a “blind alley type” aneurysm.\u003c/p\u003e","description":"","filename":"ESM1.eps","url":"https://assets-eu.researchsquare.com/files/rs-7248899/v1/53dd8b6b2d49bea86ac7104f.eps"},{"id":93246219,"identity":"19908dd8-d2d7-4086-98d9-880db66f76d4","added_by":"auto","created_at":"2025-10-10 15:12:12","extension":"eps","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":5686039,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eOnline Resource 2, Figure.\u003c/strong\u003e Computational fluid dynamics results (time-averaged wall shear stress (TAWSS) and velocity streamline) for 11 experimental configurations for an actual case in this case series. \u003cstrong\u003eA\u003c/strong\u003e, TAWSS result of the original state (assuming no flow from the posterior communicating arteries). \u003cstrong\u003eB\u003c/strong\u003e, Velocity streamline result of the original state. \u003cstrong\u003eC\u003c/strong\u003e, TAWSS result of the “stagnant junction type with a bifurcation”. \u003cstrong\u003eD\u003c/strong\u003e, Velocity streamline result of the “stagnant junction type with a bifurcation”. \u003cstrong\u003eE\u003c/strong\u003e, TAWSS result of the “stagnant junction type with a branch”. \u003cstrong\u003eF\u003c/strong\u003e, Velocity streamline result of the “stagnant junction type with a branch”. \u003cstrong\u003eG\u003c/strong\u003e, TAWSS result of the “side wall type with a bifurcation”. \u003cstrong\u003eH\u003c/strong\u003e, Velocity streamline result of the “side wall type with a bifurcation”. \u003cstrong\u003eI\u003c/strong\u003e, TAWSS result of the “side wall type with a branch (acute)”. \u003cstrong\u003eJ\u003c/strong\u003e, Velocity streamline result of the “side wall type with a branch (acute)”. \u003cstrong\u003eK\u003c/strong\u003e, TAWSS result of the “side wall type with a branch (obtuse)”. \u003cstrong\u003eL\u003c/strong\u003e, Velocity streamline result of the “side wall type with a branch (obtuse)”. \u003cstrong\u003eM\u003c/strong\u003e, TAWSS result of the “side wall type (straight)”. \u003cstrong\u003eN\u003c/strong\u003e, Velocity streamline result of the “side wall type (straight)”. \u003cstrong\u003eO\u003c/strong\u003e, TAWSS result of the “side wall type (acute angled)”. \u003cstrong\u003eP\u003c/strong\u003e, Velocity streamline result of the “side wall type (acute angled)”. \u003cstrong\u003eQ\u003c/strong\u003e, TAWSS result of the “side wall type (obtuse angled)”. \u003cstrong\u003eR\u003c/strong\u003e, Velocity streamline result of the “side wall type (obtuse angled)”. \u003cstrong\u003eS\u003c/strong\u003e, TAWSS result of the “stagnant junction type”. \u003cstrong\u003eT\u003c/strong\u003e, Velocity streamline result of the “stagnant junction type”. \u003cstrong\u003eU\u003c/strong\u003e, TAWSS result of the “blind alley type”. \u003cstrong\u003eV\u003c/strong\u003e, Velocity streamline result of the “blind alley type”.\u003c/p\u003e","description":"","filename":"ESM2.eps","url":"https://assets-eu.researchsquare.com/files/rs-7248899/v1/06d70512c35b5565cf214646.eps"},{"id":93246865,"identity":"256121ab-c5da-4ff2-9154-a8652a7d117a","added_by":"auto","created_at":"2025-10-10 15:20:12","extension":"docx","order_by":3,"title":"","display":"","copyAsset":false,"role":"supplement","size":59051,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eOnline Resource 3, Table. \u003c/strong\u003eSummary of included cases of complex basilar apex aneurysms treated with flow alteration strategies.\u003c/p\u003e","description":"","filename":"ESM3.docx","url":"https://assets-eu.researchsquare.com/files/rs-7248899/v1/748c6d7b4f11292804cb8471.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Flow alteration strategies for complex basilar apex aneurysms: multicenter experience, systematic review, and insights from computational fluid dynamics","fulltext":[{"header":"Introduction","content":"\u003cp\u003eBasilar apex aneurysms (BAAs) pose significant treatment challenges. While many are referred for endovascular treatment, complex anatomy or morphology may preclude this option [30, 36]. Moreover, endovascular approaches are associated with high recurrence and rupture rates, and large BAAs carry procedural risks comparable to surgical clipping.\u003c/p\u003e\u003cp\u003eNew devices, such as endovascular flow disruptors, are being utilized for the treatment of BAAs. However, these devices have not significantly expanded the technical success and safety of treatment [30]. When endovascular treatment is not suitable, surgical clipping is often advocated [20]. However, due to the anatomy of the upper BA, direct clipping or trapping is frequently difficult [2]. Surgical clipping of large and complex BAAs (CBAAs) presents high procedural risks [3].\u003c/p\u003e\u003cp\u003eConsequently, various flow alteration methods have been attempted in the treatment of complex basilar apex aneurysms (CBAAs) [7, 13, 19, 28, 36]. The primary goal of flow alteration is to maintain perfusion to perforators or distal branches that are involved in the aneurysm process while altering the natural history of the lesion by reducing the risk of aneurysm growth and rupture [13]. Given that CBAAs are a relatively rare disease entity, large-scale studies of flow alteration are limited. Here, we report multicenter results, conduct a systematic review of prior studies, and investigate fluid dynamic changes associated with various flow alteration methods using an actual case.\u003c/p\u003e"},{"header":"Methods","content":"\u003cp\u003e This study was approved by the Institutional Review Boards at each participating center. Due to the retrospective nature of the study, the requirement for informed consent was waived. The data used in this study are available from the corresponding author upon reasonable request.\u003c/p\u003e\u003cp\u003e\u003cb\u003eCase series from study centers\u003c/b\u003e\u003c/p\u003e\u003cp\u003e We retrospectively reviewed patients with BAAs treated between 2000 and 2023. Inclusion criteria were as follows: 1) BAAs unsuitable for microsurgical clipping or endovascular intervention, treated with flow alteration; 2) long-term clinical and radiological follow-up. Patients were excluded if they: 1) had no follow-up records; or 2) had BAAs amenable to alternative treatments based on multidisciplinary assessment.\u003c/p\u003e\u003cp\u003eWe collected data on patient demographics, clinical and radiological characteristics, flow alteration methods, and clinical and radiological outcomes. Flow alteration methods included flow preservation and flow redirection and reduction (Online Resource 1). Flow preservation included bypass surgery and subsequent surgical or endovascular aneurysm obliteration. Flow redirection and reduction was further classified, referring to Miyamoto et al., into mild, moderate, or maximum flow reduction [28]. Mild flow reduction included proximal occlusion alone or superficial temporal artery–middle cerebral artery (STA–MCA) bypass(es) aimed at reducing flow burden. Moderate and maximum flow reduction involved proximal occlusion combined with additional branch occlusion(s), with or without bypass(es).\u003c/p\u003e\u003cp\u003eAfter treatment, changes in flow direction and their relationship to the aneurysm were classified into five categories: 1) obliteration with branch; 2) no change but flow reduction; 3) side wall type; 4) stagnant junction type; 5) blind alley type. Clinical outcomes were assessed using the modified Rankin Scale (mRS). Radiological outcomes on the last follow-up image were categorized as: 1) enlargement; 2) incomplete obliteration; 3) virtually complete obliteration (filling of the ectatic aneurysmal base); 4) complete obliteration [39].\u003c/p\u003e\u003cp\u003e\u003cb\u003eLiterature review\u003c/b\u003e\u003c/p\u003e\u003cp\u003e\u003cb\u003eSearch strategy\u003c/b\u003e\u003c/p\u003e\u003cp\u003e This study followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. In consideration of the timeline for intracranial stent approval in aneurysm treatment, we conducted a systematic review of PubMed and EMBASE from January 2000 to April 2024. The search terms used were: (“basilar apex” OR “basilar top” OR “basilar tip” OR “basilar bifurcation” OR “basilar quadrification”) AND “aneurysm”. The reference lists of all retrieved articles were also reviewed to identify additional relevant studies.\u003c/p\u003e\u003cp\u003e\u003cb\u003eStudy selection and data extraction\u003c/b\u003e\u003c/p\u003e\u003cp\u003eDuplicate records were removed, after which two authors (JHC and MK) screened titles and abstracts for studies on the surgical treatment of BAAs. Only English-language articles were included; conference papers and commentaries were excluded. Full-text review applied the following inclusion criteria: 1) CBAAs difficult to treat with microsurgical clipping or endovascular methods; 2) treatment with flow alteration, with or without bypass; and 3) at least one clinical and radiologic follow-up ≥ 6 months post-treatment. Studies lacking sufficient detail on aneurysm characteristics, treatment, or outcomes were excluded. In cases of duplicate reports, the most recent publication was used. Disagreements on study selection were resolved through author discussion. Only CBAAs treated with flow alteration were included in the review, and data were extracted to match variables collected in the case series.\u003c/p\u003e\u003cp\u003e\u003cb\u003eComputational fluid dynamics methodology\u003c/b\u003e\u003c/p\u003e\u003cp\u003eComputational fluid dynamics (CFD) simulation was performed using the open-source SimVascular software. Blood flow was assumed to be Newtonian, and the incompressible Navier-Stokes equations were solved to model pulsatile flow. Blood density was set to 1.06 g/cm\u003csup\u003e3\u003c/sup\u003e, viscosity to 4 cP, and a rigid wall condition was assumed.\u003c/p\u003e\u003cp\u003eThe base case (Case 2 of this study, Fig.\u0026nbsp;1) used a simplified assumption by neglecting flow in the posterior communicating arteries (PCoAs) [ The pulsatile inlet flow rate was scaled to achieve the required outlet flow rates at the four outlets: two posterior cerebral arteries (PCAs) and two superior cerebral arteries (SCAs) [4, 43]. Subsequently, ten scenarios were analyzed in addition to the baseline (Online Resource 2). In these scenarios, the BA was clipped while keeping all four outlets open, or one/multiple outlets were occluded. Additionally, PCoAs were considered as inlets in some cases, and in certain cases, some of them were occluded. Outlet resistances were applied as boundary conditions and iteratively optimized to match the target flow rates.\u003c/p\u003e\u003cp\u003eA mesh independence study using 0.5, 1.5, and 6\u0026nbsp;million tetrahedral elements showed that a 1.5-million-element mesh yielded acceptable outlet flow accuracy. Simulations ran for three cardiac cycles (0.8 s each) with a 0.00008 s time step (30,000 total steps). Flow patterns were analyzed during the final cycle.\u003c/p\u003e"},{"header":"Results","content":"\u003cp\u003e\u003cb\u003eSystematic review\u003c/b\u003e\u003c/p\u003e\u003cp\u003eA total of 906 articles were identified from the two databases after duplicate removal. An additional 14 articles were identified through citation searching and screened for eligibility. Following eligibility assessment, 25 articles were included (Fig.\u0026nbsp;2) [1, 8\u0026ndash;11, 13, 15, 16, 18, 19, 21\u0026ndash;25, 28, 31\u0026ndash;36, 38, 42, 46]. Each of these articles contained at least one case meeting the inclusion criteria. A summary of the included cases is provided in Online Resource 3. When multiple publications from the same institution reported the same case at different time points, these were combined and treated as a single case.\u003c/p\u003e\u003cp\u003e\u003cb\u003eClinical characteristics\u003c/b\u003e\u003c/p\u003e\u003cp\u003eA total of 57 patients with CBAAs, including those from this case series, were enrolled. Clinical characteristics were summarized in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e based on the data available from the articles. The median age was 55 years (interquartile range [IQR], 45\u0026ndash;66), and 69.4% of the patients were female. Previous interventions were performed in 52.1% (25 of 48) of patients. The median maximum aneurysm diameter was 25.5 mm (IQR, 20\u0026ndash;34.5). Flow preservation with aneurysm obliteration was performed in 12.3% (7 of 57) of cases. Mild, moderate, and maximum flow reduction was carried out in 77.2% (44 of 57), 7.0% (4 of 57), and 3.5% (2 of 57) of cases, respectively. Bypass surgery was performed in 59.6% (34 of 57) of patients.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eSummary of included cases of complex basilar apex aneurysms treated with flow alteration strategies\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"2\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eCharacteristics\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eNo. (%, total number)\u003csup\u003ea\u003c/sup\u003e\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTotal number of cases\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e57\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eAge, median (IQR, total number)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e55 (45\u0026ndash;66, 49)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSex\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eFemale\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e34 (69.4, 49)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003ePresentations\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSAH\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e9 (20.5, 44)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eInfarction\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e3 (6.8, 44)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eBrainstem compressive symptoms\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e15 (34.1, 44)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eHydrocephalus\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e6 (13.6, 44)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003ePrevious interventions\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e25 (52.1, 48)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eMaximum diameter, median (IQR, total number)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e25.5 (20-34.5, 42)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eFlow alteration\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eFlow preservation with aneurysm obliteration\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e7 (12.3, 57)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eMild flow reduction\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e44 (77.2, 57)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eModerate flow reduction\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e4 (7.0, 57)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eMaximum flow reduction\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e2 (3.5, 57)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eBypass\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eYes\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e34 (59.6, 57)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eChanged aneurysm type\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eObliteration with branch\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e7 (26.9, 26)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eNo change but flow reduction\u003csup\u003eb\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e1 (3.8, 26)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSide wall type\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e9 (34.6, 26)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eStagnant junction type\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e7 (26.9, 26)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eBlind alley type\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e2 (7.7, 26)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003ePostoperative complications\u003csup\u003ec\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e27 (52.9, 51)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eInfarction\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e18 (35.3, 51)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eCranial nerve palsy\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e4 (7.8, 51)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTransient neurologic deterioration\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e8 (15.7, 51)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eBrainstem compression\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e4 (7.8, 51)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eDelayed rupture\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e2 (3.9, 51)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eFollow-up image\u003csup\u003ed\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e33 (57.9, 57)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eEnlargement\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e4 (12.1, 33)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eIncomplete obliteration\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e4 (12.1, 33)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eVirtually complete obliteration\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e10 (30.3, 33)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eComplete obliteration\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e15 (45.5, 33)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eFollow-up period (months), median (IQR, total number)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e12 (6-42.25, 48)\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colspan=\"2\" nameend=\"c2\" namest=\"c1\"\u003e\u003cp\u003ea: Data are reported as No. (%, total number) unless otherwise indicated. Total number is the sum of cases with available data.\u003c/p\u003e\u003cp\u003eb: To reduce the hemodynamic burden in a patient with bilateral internal carotid artery occlusion, superficial temporal artery-middle cerebral artery bypass surgery was performed bilaterally.\u003c/p\u003e\u003cp\u003ec: Each complication was counted separately.\u003c/p\u003e\u003cp\u003ed: Among the 24 cases without follow-up images, 14 were not described in the original article, and 10 were due to mortality, with 8 of those attributed to complications.\u003c/p\u003e\u003cp\u003eNo., number; IQR, interquartile range; SAH, subarachnoid hemorrhage\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003e\u003cb\u003eClinical and radiological outcomes\u003c/b\u003e\u003c/p\u003e\u003cp\u003eFigure\u0026nbsp;3 shows changes in preoperative and last follow-up mRS scores. Preoperative mRS was undocumented in five cases; of these, two had a last mRS of 3 and three had an mRS of 6. Among the 52 patients with available preoperative scores, 42.3% (22) improved, 26.9% (14) were unchanged, and 30.8% (16) worsened. A good outcome (mRS 0\u0026ndash;2) was seen in 49.1% (28 of 57), and a poor outcome (mRS 4\u0026ndash;6) in 31.6% (18 of 57). Infarction was the most frequent complication, occurring in 35.3% (18 of 51) (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). At final imaging follow-up, 75.8% (25 of 33) showed favorable obliteration. Median follow-up was 12 months (IQR, 6\u0026ndash;42.25).\u003c/p\u003e\u003cp\u003e\u003cb\u003eCFD experiments for various flow alteration situations\u003c/b\u003e\u003c/p\u003e\u003cp\u003eBased on the blood flow velocities and time-averaged wall shear stress (TAWSS) results for the 11 experimental configurations, each strategy had distinct effects on aneurysm hemodynamics (Online Resource 2). The maximum and mean TAWSS values in the aneurysm are summarized in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. In the baseline state, very high TAWSS values were observed in the stenosis region just proximal to the aneurysm, reaching 530 dynes/cm\u0026sup2;. The flow from the BA created an impingement jet at the apex of the aneurysm, producing a TAWSS of 290 dynes/cm\u0026sup2;.\u003c/p\u003e\u003cp\u003eThe \u0026ldquo;stagnant junction type with bifurcation\u0026rdquo;, which can be achieved by performing only proximal occlusion, lowered the maximum TAWSS to 64 dynes/cm\u0026sup2; from 290 dynes/cm\u0026sup2; in the original state, with the mean value reduced to just 10% of its original. However, it increased TAWSS in the PCoAs to 760 dynes/cm\u0026sup2;. The \u0026ldquo;stagnant junction type with a branch\u0026rdquo; achieved significantly lower maximum and mean TAWSS values in the aneurysm than the \u0026ldquo;stagnant junction type with bifurcation,\u0026rdquo; at 15 and 0.05 dynes/cm\u0026sup2;, respectively.\u003c/p\u003e\u003cp\u003e Among the side wall types, the \u0026ldquo;side wall type (acute angled)\u0026rdquo; recorded the lowest maximum and mean TAWSS values, marginally lower than those of the \u0026ldquo;side wall type (obtuse angled).\u0026rdquo; TAWSS analysis revealed that the \u0026ldquo;side wall types with a branch or a bifurcation\u0026rdquo; and the \u0026ldquo;side wall type (straight)\u0026rdquo; exhibited relatively higher TAWSS values in the aneurysm region compared to the \u0026ldquo;side wall types (acute or obtuse angled).\u0026rdquo; In the \u0026ldquo;stagnant junction type\u0026rdquo; and \u0026ldquo;blind alley type,\u0026rdquo; CFD simulations showed extremely slow and stagnant blood flow in the aneurysm.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eSummary of maximum and mean time-averaged wall shear stress values at the aneurysm region\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"3\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eExperiments\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eMax TAWSS at the aneurysm (dynes/cm\u003csup\u003e2\u003c/sup\u003e)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eMean TAWSS at the aneurysm (dynes/cm\u003csup\u003e2\u003c/sup\u003e)\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eOriginal state\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e290.29\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e2.68\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eStagnant junction type with a bifurcation\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e64.66\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0.21\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eStagnant junction type with a branch\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e15\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0.05\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSide wall type with a bifurcation\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e67.83\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0.3\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSide wall type with a branch (acute)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e46.44\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0.52\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSide wall type with a branch (obtuse)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e24.26\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0.21\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSide wall type (straight)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e18.69\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0.08\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSide wall type (obtuse angled)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e9.19\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0.06\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSide wall type (acute angled)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e8.1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0.02\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eStagnant junction type\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e1.01\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e1.3 \u0026times; 10⁻\u0026sup3;\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eBlind alley type\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e5.5 \u0026times; 10⁻\u0026sup3;\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e4.3 \u0026times; 10⁻⁵\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colspan=\"3\" nameend=\"c3\" namest=\"c1\"\u003e\u003cp\u003eTAWSS, time-averaged wall shear stress.\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eIn this systematic review of flow alteration for the treatment of complex basilar apex aneurysms (CBAAs), a good outcome was observed in 49.1% of patients, while 69.2% experienced either improvement or no change in their mRS scores. Among those with available follow-up imaging, 75.8% achieved favorable obliteration. Postoperative complications occurred in over half of the cases, with infarction being the most common. Eight cases resulted in death due to complications. However, given that more than 80% of patients with untreated giant BAAs die and the remainder are severely disabled [39], flow alteration may be an alternative option when conventional treatments are deemed difficult or infeasible. Moreover, computational fluid dynamics (CFD) simulations suggest that, beyond simply reducing aneurysmal inflow, optimizing the balance and direction of inflow may mitigate hemodynamic shear stress and enhance aneurysm control. These insights might guide the selection of flow-alteration strategies; however, careful consideration of the requisite surgical techniques and associated risks remains crucial.\u003c/p\u003e\u003cp\u003e\u003cb\u003eReview of flow alteration methods for CBAAs\u003c/b\u003e\u003c/p\u003e\u003cp\u003eIn this study, we included all treatment approaches that modify the existing flow architecture, ranging from flow preservation to flow redirection and reduction. Even in cases of CBAAs, \u0026ldquo;flow preservation with aneurysm obliteration\u0026rdquo; may be achieved. Although the number of such cases was limited, our systematic review suggested a relatively lower complication rate (28.6%, 2 of 7) with this approach compared to other methods. Flow redirection is considered when the aneurysm cannot be completely excluded from the circulation, aiming to alter the lesion\u0026rsquo;s hemodynamics and impact its progression [13, 32].\u003c/p\u003e\u003cp\u003eProximal occlusion, which has been employed for a long time [7], can be achieved surgically or endovascularly. Endovascular occlusion has targeted both bilateral vertebral arteries (VAs) and the BA, though outcomes are variable and may risk compromising brainstem perforators [12, 44]. Surgical occlusion allows direct visualization, potentially reducing this risk. Kellner et al. reported nine BAA cases treated with surgical BA occlusion after successful balloon test occlusion; all but one had favorable long-term outcomes [19]. When collateral flow is inadequate, bypass may be used to augment circulation [14, 40].\u003c/p\u003e\u003cp\u003eIn unclippable vertebrobasilar system aneurysms, the outcomes of deliberate occlusion of the BA or VA have been linked to the size of the PCoAs [39]. This correlation likely reflects not only the adequacy of collateral flow but also the degree of flow passing through the aneurysm orifice. When both PCoAs have a diameter ratio of \u0026lt;\u0026thinsp;0.45, BAAs show no change or occlusion following proximal occlusion; completely thrombosed aneurysms and partially thrombosed aneurysms had a ratio above 0.6 and a ratio between 0.46 and 1, respectively [6]. These findings suggest that the greater the flow passing through the aneurysm orifice after proximal occlusion, the lower the likelihood of subsequent aneurysm occlusion. Similarly, Nagasawa et al. performed simulations in a BAA model and found that a high PCoA diameter ratio was associated with markedly increased intra-aneurysmal stagnation [29]. Other studies have reported that persistent blood flow within the aneurysm can result in a lack of treatment response or even rupture [41, 46].\u003c/p\u003e\u003cp\u003eMiyamoto et al. introduced moderate and maximum flow reduction techniques, hypothesizing that reducing and stagnating blood flow at the aneurysm orifice aids in aneurysm obliteration [28]. By diminishing both inflow and outflow, they reported a high rate of aneurysm obliteration and favorable clinical outcomes. Although the number of cases was small, the theoretical rationale appears sound, but these methods carry considerable risk due to the complexity of staged surgical procedures.\u003c/p\u003e\u003cp\u003eComplication risk is an important consideration in flow alteration. In this study, 52.9% (27 of 51) experienced complications; however, 69.2% showed improved or stable mRS scores, suggesting not all complications had lasting effects. Early thromboembolic brainstem ischemia and aneurysm thrombosis with mass effect may occur [39]. One case developed a giant PCA aneurysm from long-term hemodynamic changes [24], highlighting the need for continued follow-up.\u003c/p\u003e\u003cp\u003e\u003cb\u003eHemodynamic considerations in various flow alteration methods\u003c/b\u003e\u003c/p\u003e\u003cp\u003ePrevious studies have demonstrated that both high and low WSS can influence aneurysm formation, progression, and rupture at different stages [26, 27, 37, 45]. However, persistently high WSS can lead to wall degradation, delamination, and imbalances in collagen remodeling, increasing the risk of rupture [5, 17, 26, 45]. In large or giant aneurysms, the complex intra-aneurysmal flow can create localized regions of high WSS that exacerbate wall injury [17].\u003c/p\u003e\u003cp\u003eOur CFD simulations demonstrated that proximal occlusion effectively reduces both maximal and mean TAWSS within the aneurysm and mitigates impingement jets (resulting in \u0026ldquo;stagnant junction type with a bifurcation\u0026rdquo; or \u0026ldquo;side wall type with a bifurcation\u0026rdquo; configurations). Nevertheless, compared with other flow alteration techniques, proximal occlusion continued to exhibit relatively high hemodynamic stress. This mechanism might explain previous reports of poor prognosis following proximal occlusion when persistent blood flow remains within the aneurysm [41, 46]. In particular, when a PCoA diameter ratio is nearly 1, the \u0026ldquo;stagnant junction type with a bifurcation\u0026rdquo; is theoretically associated with a favorable outcome [6, 29, 39]. However, based on our simulation findings, additional flow reduction might be even more beneficial.\u003c/p\u003e\u003cp\u003eBy contrast, for the \u0026ldquo;side wall type with a bifurcation,\u0026rdquo; the inflow direction and hemodynamic burden may be unfavorable for aneurysm control. In one report of a large BAA treated with bilateral VA occlusion, jet flow from the P1 segment continued to enter the aneurysm, as it washed out the earlier thrombosed region [1]. Shojima et al. also described a CBAA that became a \u0026ldquo;side wall type with a bifurcation\u0026rdquo; after BA occlusion and ruptured 6 months later; based on CFD analysis, the authors concluded that flow dynamics could worsen in certain cases [38]. Moreover, our simulation revealed highly elevated TAWSS within the remaining PCoA, which could increase the risk of de novo aneurysm formation in the long term [24]. Thus, in side wall configurations, the angle between the inlet and the aneurysmal sac\u0026mdash;as well as the associated flow burden\u0026mdash;may play a pivotal role in treatment outcomes. Indeed, for \u0026ldquo;side wall type without a branch,\u0026rdquo; the angled configuration demonstrated more favorable results than the straight configuration. A likely explanation is that the outlet in the angled configuration, supplying the SCA, requires less flow than the straight configuration, which supplies the PCA.\u003c/p\u003e\u003cp\u003eFinally, compared with the \u0026ldquo;stagnant junction type with a bifurcation,\u0026rdquo; the \u0026ldquo;stagnant junction type with a branch\u0026rdquo; configuration demonstrated approximately one-fourth the maximal and mean TAWSS. The impact of reducing the flow burden for a single SCA in the \u0026ldquo;stagnant junction type with a bifurcation\u0026rdquo; is greater than in the \u0026ldquo;side wall type with a bifurcation.\u0026rdquo; In addition, both the \u0026ldquo;blind alley type\u0026rdquo; and \u0026ldquo;stagnant junction type\u0026rdquo; nearly eliminate flow through the aneurysm. Consequently, not only reducing inflow but also balancing and redirecting flow may yield theoretically better outcomes. Yet achieving these configurations often entails complex surgical techniques and concomitant risks that must be carefully weighed.\u003c/p\u003e\u003cp\u003e\u003cb\u003eLimitations\u003c/b\u003e\u003c/p\u003e\u003cp\u003eAlthough we aggregated prior data, CBAAs remain rare, limiting case numbers. Treatment methods varied, constraining generalizability. Due to time and resource constraints, only one case was used for CFD simulations, reducing representativeness. Boundary conditions were extrapolated from the literature (not directly measured) [4, 6, 38, 43], and rigid-wall assumptions omitted real viscoelastic effects. However, to our knowledge, this is the first systematic review of this condition. Our comprehensive simulations of various flow‑alteration strategies offer potentially valuable insights. Larger cohorts and more advanced CFD models are needed to validate and expand these findings.\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eGiven the severe natural course of CBAAs, flow alteration strategies may serve as viable alternatives when conventional treatment options are not feasible. CFD simulations may assist in identifying optimal strategies that maximize aneurysm control while minimizing procedural risk.\u003c/p\u003e"},{"header":"Abbreviations","content":"\u003cp\u003eBAAs, basilar apex aneurysms\u003c/p\u003e\n\u003cp\u003eCBAAs, complex basilar apex aneurysms\u003c/p\u003e\n\u003cp\u003eCFD, computational fluid dynamics\u003c/p\u003e\n\u003cp\u003emRS, modified Rankin Scale\u003c/p\u003e\n\u003cp\u003ePRISMA, Preferred Reporting Items for Systematic Reviews and Meta-Analyses\u003c/p\u003e\n\u003cp\u003eTAWSS, time-averaged wall shear stress\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eCompeting Interests\u003c/h2\u003e\u003cp\u003eThe authors have no relevant financial or non-financial interests to disclose.\u003c/p\u003e\u003c/p\u003e\u003cp\u003e\u003ch2\u003eAuthour Contributions\u003c/h2\u003e\u003cp\u003e Conceptualization: June Ho Choi, Amirhossein Arzani, Jae Sung Ahn; Data curation: June Ho Choi, Mahmoud Elhadidy, Minwoo Kim, Sung-Pil Joo, Sang Hyo Lee; Formal analysis and investigation: June Ho Choi, Mahmoud Elhadidy; Methodology: June Ho Choi, Mahmoud Elhadidy, Amirhossein Arzani; Project administration: Michael T. Lawton, Amirhossein Arzani, Jae Sung Ahn; Resources: June Ho Choi, Mahmoud Elhadidy, Sung-Pil Joo, Sang Hyo Lee, See Un Lee, Jae Seung Bang, Amirhossein Arzani; Supervision: Michael T. Lawton, Amirhossein Arzani, Jae Sung Ahn; Writing - original draft preparation: June Ho Choi, Mahmoud Elhadidy; Writing - review and editing: June Ho Choi, Mahmoud Elhadidy, Wonhyoung Park, Jung Cheol Park, Byung Duk Kwun, Jae Seung Bang, Michael T. Lawton, Amirhossein Arzani, Jae Sung Ahn.\u003c/p\u003e\u003c/p\u003e\u003cp\u003e\u003ch2\u003eEthics Approval\u003c/h2\u003e\u003cp\u003e All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards. This is a retrospective review study. Collection of retrospective data was approved by Institutional Review Board of each center.\u003c/p\u003e\u003c/p\u003e\u003cp\u003e\u003cstrong\u003eInformed consent\u003c/strong\u003e\u003cp\u003e\u003cb\u003eT\u003c/b\u003ehe need for patient consent was waived because of the retrospective nature of the study.\u003c/p\u003e\u003ch2\u003eClinical trial number\u003c/h2\u003e\u003cp\u003e Not applicable.\u003c/p\u003e\u003c/p\u003e\u003ch2\u003eFunding\u003c/h2\u003e\u003cp\u003eNo funding was received for this research.\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eConceptualization: June Ho Choi, Amirhossein Arzani, Jae Sung Ahn; Data curation: June Ho Choi, Mahmoud Elhadidy, Minwoo Kim, Sung-Pil Joo, Sang Hyo Lee; Formal analysis and investigation: June Ho Choi, Mahmoud Elhadidy; Methodology: June Ho Choi, Mahmoud Elhadidy, Amirhossein Arzani; Project administration: Michael T. Lawton, Amirhossein Arzani, Jae Sung Ahn; Resources: June Ho Choi, Mahmoud Elhadidy, Sung-Pil Joo, Sang Hyo Lee, See Un Lee, Jae Seung Bang, Amirhossein Arzani; Supervision: Michael T. Lawton, Amirhossein Arzani, Jae Sung Ahn; Writing - original draft preparation: June Ho Choi, Mahmoud Elhadidy; Writing - review and editing: June Ho Choi, Mahmoud Elhadidy, Wonhyoung Park, Jung Cheol Park, Byung Duk Kwun, Jae Seung Bang, Michael T. Lawton, Amirhossein Arzani, Jae Sung Ahn.\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e\u003cp\u003eWe would like to thank Editage (www.editage.co.kr) for English language editing.\u003c/p\u003e\u003ch2\u003eData availability\u003c/h2\u003e\u003cp\u003eAll data related to this article are available from the corresponding author upon reasonable request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eAln\u0026aelig;s MS, Mardal K-A, Bakke S, Sorteberg A (2015) Computational fluid dynamics evaluation of flow reversal treatment of giant basilar tip aneurysm. Interv Neuroradiol J Peritherapeutic Neuroradiol Surg Proced Relat Neurosci 21:586\u0026ndash;591. doi: 10.1177/1591019915597415\u003c/li\u003e\n\u003cli\u003eBarrow DL, Samson DS, D\u0026rsquo;Ambrosio A, Solomon RA, Lawton MT (2005) Surgical clipping of complex basilar apex aneurysms: A strategy for successful outcome using the pretemporal transzygomatic transcavernous approach. Comments. 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AJNR Am J Neuroradiol 24:1226\u003c/li\u003e\n\u003cli\u003eXiang J, Natarajan SK, Tremmel M, Ma D, Mocco J, Hopkins LN, Siddiqui AH, Levy EI, Meng H (2011) Hemodynamic-morphologic discriminants for intracranial aneurysm rupture. Stroke 42:144\u0026ndash;152. doi: 10.1161/STROKEAHA.110.592923\u003c/li\u003e\n\u003cli\u003eYasui T, Komiyama M, Iwai Y, Yamanaka K, Matsusaka Y, Morikawa T, Ishiguro T (2004) Regrowth and fatal rerupture despite proximal occlusion after coil embolization of a ruptured large basilar bifurcation aneurysm--case report. Neurol Med Chir (Tokyo) 44:587\u0026ndash;590. doi: 10.2176/nmc.44.587\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"aneurysm, basilar apex aneurysm, complex intracranial aneurysm, computational fluid dynamics, flow alteration, flow reduction","lastPublishedDoi":"10.21203/rs.3.rs-7248899/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7248899/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eComplex basilar apex aneurysms (CBAAs) present a significant challenge due to their unfavorable natural history and difficulty with conventional treatments. This study aimed to provide insights into flow alteration strategies by combining a systematic review using PRISMA methodology with a multicenter experience from South Korea. We analyzed 57 cases, finding that flow preservation with aneurysm obliteration was performed in 12.7%, while mild, moderate, and maximum flow reduction were applied in 77.2%, 7.0%, and 3.5% respectively. Outcomes showed that 75.8% of patients with available imaging achieved satisfactory aneurysm obliteration. A good clinical outcome (mRS 0\u0026ndash;2) was observed in 49.1% of cases. However, poor outcomes (mRS 4\u0026ndash;6) were reported in 31.6%, with a mortality rate of 17.5%. Beyond simply reducing intra-aneurysmal flow, computational fluid dynamics (CFD) simulations revealed that alterations in flow balance and direction significantly influenced hemodynamic stress. Given the severe prognosis of CBAAs, flow alteration strategies can serve as viable alternatives when conventional treatments are not feasible. Furthermore, CFD simulations might hold promise in identifying optimal strategies that can maximize aneurysm control while minimizing procedural risks.\u003c/p\u003e","manuscriptTitle":"Flow alteration strategies for complex basilar apex aneurysms: multicenter experience, systematic review, and insights from computational fluid dynamics","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-10-10 15:12:07","doi":"10.21203/rs.3.rs-7248899/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"291a4ef2-78b0-4639-afc2-96b36d8e8059","owner":[],"postedDate":"October 10th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2025-11-09T02:23:08+00:00","versionOfRecord":[],"versionCreatedAt":"2025-10-10 15:12:07","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7248899","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7248899","identity":"rs-7248899","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}