Giant partially thrombosed intracranial aneurysms present clinical challenges marked by difficulty in management, attributed to their unfavorable natural history and the absence of well-established treatment approaches [8]. Currently, no consensus exists on the optimal management of these aneurysms despite advancements in intracranial aneurysm treatment. Although associated with relatively high mortality and morbidity, microsurgical clipping offers the advantage of immediate complete occlusion with remarkable durability, resulting in significantly lower recurrence and retreatment rates [11]. Parent vessel occlusion, featuring a low complication rate and positive outcomes often requires sophisticated bypass surgery, not widely available due to the technical skill required [8]. In contrast, endovascular treatments, despite the numerous device developments, are known for high recurrence rates [7]. Flow diverters have emerged as a promising approach for endovascular treatment of intracranial aneurysms, particularly effective for large and giant aneurysms. However, their use carries the risk of delayed intraparenchymal hemorrhage which can be fatal [3].

The Woven EndoBridge (WEB) device (MicroVention, Tustin, CA, USA) is a promising option for wide-necked bifurcation/terminal aneurysms, traditionally challenging with conventional coiling techniques [2]. However, the use of the WEB device in giant, partially thrombosed intracranial aneurysms is rare, and evidence of its efficacy in such cases is limited [1].

In this technical note, we present successful treatment using a combination of stent-assisted WEB embolization and flow reversal by bilateral vertebral artery trapping for a symptomatic giant partially thrombosed basilar artery (BA) aneurysm.

A 62-year-old male presented to our hospital with a 30-minute episode of left-sided hemiparesis and dysarthria. Brain computed tomography (CT) revealed a rounded, high-attenuation lesion in the interpeduncular cistern (Fig. 1A), and magnetic resonance imaging (MRI) showed a partially thrombosed aneurysm at the tip of the BA (Fig. 1B-D). Diagnostic cerebral angiography indicated canalized portion sized 10.01×10.96 mm with a 7.42 mm wide-neck (Fig. 2).

A two-stage endovascular treatment was planned for the symptomatic wide-neck giant thrombosed aneurysm at the basilar tip. In the first stage, we performed a stent-assisted WEB embolization of the aneurysm with left vertebral artery occlusion. Initially, we placed one microcatheter in the left P1 and another (Via-33; MicroVention) in the aneurysmal sac. Then, we deployed a WEB-SL (WEB-single layer, 10×5 mm) inside the sac and a Enterprise stent (4×30 mm; Codman Neuro, Raynham, MA, USA) was implanted from the left P1 to the BA (Fig. 3A). Subsequently, the dominant left vertebral artery was internally trapped (Fig. 3B). On the follow-up angiogram, the flow of the left P1 and left posterior inferior cerebellar artery (PICA) was intact, with flow stasis observed on the aneurysm sac (Fig. 3C and D).

Two weeks after the first surgery, we performed a second surgery to attempt occlusion of the right vertebral artery. Before inducing general anesthesia, we performed a balloon test occlusion (Fig. 4A). Following the insertion and inflation of a Scepter XC balloon catheter into the right vertebral artery, we confirmed intact retrograde flow in the posterior circulation via the posterior communicating artery (Fig. 4B and C). Approximately 15 minutes later, we conducted brainstem and cerebellar function tests, detecting no signs of neurological deficits. Then, internal trapping of the right vertebral artery under general anesthesia and intraoperative monitoring was performed (Fig. 4D). On the final angiogram, we observed occlusion of the BA aneurysm sac and intact flow from the anterior circulation to the posterior circulation (Fig. 4E and F).

The patient remained stable and was discharged 1 week after the second-stage procedure. Follow-up transfemoral cerebral angiography was performed 7 months later and revealed no signs of aneurysm sac recanalization, along with the intact flow of the upper BA via the posterior communicating artery (Fig. 5). Furthermore, assessments of brainstem and cerebellar function revealed no abnormalities.

Giant thrombosed aneurysms exhibit a distinctive pathology that sets them apart from typical aneurysms. The pathogenesis involves recurrent bleeding from the vasa vasorum, leading to an increase in aneurysm size and proliferation of neomembranes and new vessels [10]. Similarly, the pathogenic mechanism of partially thrombosed aneurysms likely involves intramural neovascularization within the aneurysm wall. This process includes recurrent endothelial damage due to turbulent blood flow, followed by subsequent healing of the aneurysm wall [5]. Failure to achieve complete aneurysmal isolation through clipping or to facilitate intraluminal thrombosis and endothelization via flow diversion poses a risk of recurrence [12]. In a study involving patients with large or giant aneurysms, recurrence rates were 44% (72/164) and 32.5% (26/80) for simple and stent-assisted coiling, respectively. Similarly, retreatment rates were 37.2% (61/164) and 26% (21/80) for simple and stent-assisted coiling respectively [4]. Another study on patients with very large or giant aneurysms found that additional treatment was required in 10% (14/140) of cases in the anterior circulation, compared to a higher 23% (10/44) in the posterior circulation. Notably, basilar tip aneurysms required further treatment in 21% (3/14) of the cases [6]. Therefore, considering the complex pathogenesis of giant partially thrombosed aneurysms and the high recurrence rates associated with conventional treatments, we chose to employ the WEB device for its advantages in promoting flow diversion-based endothelialization, diverging from typical stent-assisted coiling. To further enhance the flow diversion effects, we additionally integrated stent assistance with the WEB device.

The WEB device, designed for intrasaccular flow diversion in wide-necked bifurcation or terminal intracranial aneurysms, disrupts intra-aneurysmal flow and induces thrombosis within the aneurysm sac [1,2]. Although its global use increased post-Food and Drug Administration (FDA) approval, research on its effectiveness in partially thrombosed aneurysms is limited. Kabbasch et al. [9] found it technically successful and safe for large, complex aneurysms but not effective against recurrence in partially thrombosed aneurysms, with a 100% recurrence rate (6/6), compared to no recurrences in non-thrombosed aneurysms (0/2). This suggests that the WEB device alone may not suffice for giant partially thrombosed aneurysms without additional flow reversal techniques like parent artery occlusion.

Flow diversion or reversal plays a crucial role in managing giant bifurcation aneurysms. In flow diversion, blood flow within the aneurysm is altered, leading to intrasaccular thrombus formation and, ultimately, isolation of the aneurysm sac from circulation via a scaffold-like structure. Conversely, flow reversal blocks direct blood flow through the parent vessel feeding the aneurysm, effectively isolating the aneurysm sac from circulation and inducing intrasaccular clot formation.

Giant thrombosed aneurysms in the posterior circulation can cause various cranial nerve dysfunctions and symptoms of brainstem compression, the manifestations of which include ocular motor nerve dysfunction leading to double vision, facial sensory loss, lower cranial nerve dysfunction, bulbar palsies, paresis, and, in some cases, obstructive hydrocephalus [5]. A study reported a 100% mortality rate in patients with a significant mass effect, emphasizing the critical nature of a worsening mass effect on the brain stem and their potential for an extremely unfavorable prognosis [13]. Therefore, the treatment goal extends beyond preventing rupture to eliminate associated mass effects and prevent additional symptoms. Although bilateral vertebral artery trapping may appear aggressive, in cases of giant BA aneurysms with significant mass effects, the potential for recurrence to severely impair the patient’s quality of life warrants a decisive initial treatment strategy.

In our case, to prevent complications, including possible infarction, we performed a balloon occlusion test to confirm symptoms while awake before the surgery and evoked potential monitoring was performed after initiating general anesthesia to maximize safety.

We effectively treated a complex giant partially thrombosed aneurysm located near the brainstem utilizing a dual approach, combining intrasaccular flow diversion with a WEB device and flow reversal achieved through bilateral vertebral artery trapping.