The anterior communicating artery (ACOM) complex is the most common location of intracranial aneurysms. Furthermore, aneurysms in this location account for the most common cause of aneurysmal subarachnoid hemorrhage and have the most complex anatomy of the anterior circulation [1,5]. Indeed, they have a higher risk for rupture than any other aneurysm within the anterior cerebral circulation according to a recent meta-analysis [10].

The formation of intracranial aneurysms is strongly associated with deficiencies in arterial walls at regions of higher wall shear stress. Wall shear stress is believed to account for the weakening of vessel walls and the formation of aneurysms [12]. The high risk of rupture of ACOM aneurysms, regardless of size, suggests the apparent need for intervention. The incidence of ACOM aneurysms has increased in recent years likely due to the improvement of minimally invasive imaging techniques [10].

The ACOM connects the bilateral anterior cerebral arteries between segments A1 and A2 [1]. Due to the proximity of the ACOM to neurological structures, anatomical variation, and flow dynamics, open neurosurgery for treatment poses both technical challenges and risks of neurological deficits [5]. There are also challenges and risks with endovascular approaches, but growing data support this treatment [1,6]. One promising strategy is endoluminal reconstruction with a flow-diverting stent. This approach offers promise for challenging anatomical locations difficult to treat by other strategies. Flow diverters are low-porosity metal stents that decrease the velocity and pressure of intra-aneurysmal blood flow. This results in delayed aneurysm thrombosis with the reconstruction of the parent artery. A flow-redirection endoluminal device (FRED), registered trademark, Microvention Inc, is a flow diverter with a low risk of complications, such as hemorrhage and thromboembolism [9]. FRED consists of an outer expanding meshwork layer and an inner layer that acts as the flow diverter [7,9]. Flow diverters, including FRED, have significantly increased the interventional treatment options for patients with intracranial aneurysms [7]; they have a high success rate for both short and long-term occlusion, up to 95.9% in some studies [7,9].

The fine mesh of FRED reduces the flow within the aneurysm body leading to clot formation. FRED also acts as a scaffold for a new intimal layer enhancing endothelialization which walls off the thrombosed aneurysm. Although the use of detachable coils has significantly increased since their approval in the 1990s, very large or wide-necked aneurysms are significantly challenging in achieving sustained occlusion with coiling alone. Flow diverters reduce the risk of aneurysm rupture by redirecting the blood flow away from the aneurysm sac, decreasing the pressure within the aneurysm [2].

A 68-year-old male with a medical history of diabetes mellitus, atrial fibrillation, hyperlipidemia, and tonsillar cancer status post resection presented with an incidental ACOM aneurysm. The ACOM aneurysm was best visualized on a left internal carotid artery angiogram, although the aneurysm was still visualized on a right internal carotid artery angiogram (see Fig. 1). As seen in Fig. 1, the ACOM aneurysm was boxed-shaped, measuring 3.9 mm wide left-to-right, 4.0 mm wide anterior-to-posterior, and 1.9 mm high neck-to-dome. The ACOM complex appeared to take on an “X” confirmation instead of an “H” shape. Of the two sides, the left anterior cerebral artery was slightly dominant.

ACOM aneurysms are complex and difficult to treat. Due to their high risk of rupture which can lead to neurological deficits, intervention is often recommended. Traditional open neurosurgical interventions, such as clipping, remain an option but pose some challenges. Endovascular embolization offers another approach, and there is a growing toolset available to aid in the treatment of complicated aneurysms. Indeed, the outcomes after endovascular embolization, such as the use of a FRED, have a similar success rate to open surgery [1,5,6,10,12]. Flow diverters have increasingly become a treatment of choice due to high cure rates and low rates of complications [3]. FRED is one option that provides stable positioning and good aneurysm coverage when major arterial branches are located near the aneurysm [8]. Flow diverters cover the intersection of the parent artery and the aneurysm, decreasing the entry of blood into the aneurysm through a low-permeable dual-layer stent [3]. The blood flow will be restricted from entering the aneurysm once the perpendicular force of the FRED is greater than the hemodynamic force. Another mechanism of FRED is the promotion of endothelialization in the parent artery to isolate artery from aneurysm. Flow diverter design plays a critical role in successful endothelialization [2].

This case further highlights the interesting and variable phenomenon of post-flow diversion circulatory remodeling [4,8]. Arterial remodeling represents both physiologic and pathologic changes in the vessel wall structure, usually in response to a change in pressure [11]. Although remodeling is seen in the creation of an aneurysm, FRED facilitates arterial remodeling in the treatment of the aneurysm. Intervention to remodel the arterial wall will result in occlusion of the aneurysm with little to no risk of recurrence if endothelial coverage is complete [8]. Arterial remodeling occurs as a result of said endothelialization, reduced hemodynamic forces within the aneurysm, and flow diversion due to the device. A flow diverter induces a beneficial physiological response to remodel the parent vessel structure. The low flow state within the aneurysm results in coagulation, leading to thrombosis within the aneurysm [11]. Knowing that remodeling is simply a homeostatic mechanism to adapt to changes exerted on the vessel wall, flow diverters can be used to exploit this process.

EuFRED, the largest study to date evaluating FRED, reported a complete occlusion rate in 91.3% of cases after 1 year [3]. Mortality with FRED was reported to be less than 1% and morbidity was reported at 1.5%. EuFRED and other smaller studies found FRED to be safe and effective for major vessels of the circle of Willis [3]. This case also highlights the role flow diverters, and perhaps specifically the dual-layered FRED, have in circulation remodeling. The ACOM was remodeled such that the right A1 supplied the left A2 and the left A1 supplied the right A2. Other data support that “jailing” vessels of substantial size or those which subtend critical tissue remain patent by demand [11].

This case report highlights the successful use of flow diversion for the treatment of a complex aneurysm of the ACOM complex. Flow diverters, such as FRED, are becoming a widely used approach for the treatment of aneurysms which are less suited to conventional open or endovascular techniques. Importantly, this case exemplifies the robust and variable phenomenon of post-flow diversion circulatory remodeling.