Anatomical triangles defining routes to anterior communicating artery aneurysms: the junctional and precommunicating triangles and the role of dome projection

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Anterior communicating artery (ACoA) aneurysms are common intracranial aneurysms. Despite advances in endovascular therapy, microsurgical clipping remains an important treatment for aneurysms with broad necks, large size, intraluminal thrombus, complex branches, or previous coiling. Anatomical triangles identify safe corridors for aneurysm access. The authors introduce the A1-A2 junctional triangle and the A1-A1 precommunicating triangle and examine relationships between dome projection, triangular corridors of access, and surgical outcomes.


Preoperative catheter and CT angiograms were evaluated to characterize aneurysm dome projection. Aneurysm projection was categorized into quadrants and octants. Preoperative, intraoperative, and postoperative factors were correlated to aneurysm dome projection and patient outcomes using univariate and multivariate analyses.


A total of 513 patients with microsurgically treated ACoA aneurysms were identified over a 13-year period, and 400 had adequate imaging and follow-up data for inclusion. Surgical clipping was performed on 271 ruptured and 129 unruptured aneurysms. Good outcomes were observed in 91% of patients with unruptured aneurysms and 86% of those with ruptured aneurysms, with a mortality rate < 1% among patients with unruptured aneurysms. Increasing age (p < 0.01), larger aneurysm size (p = 0.03), and worse preoperative modified Rankin Scale score (p < 0.01) affected outcomes adversely. Aneurysms projecting superiorly and posteriorly required dissection in the junctional triangle, and multivariate analysis demonstrated worse clinical outcomes in these patients (p < 0.01).


Anteriorly and inferiorly projecting aneurysms involve only the precommunicating triangle, are simpler to treat microsurgically, and have more favorable outcomes. Superior and posterior dome projections make ACoA aneurysms more difficult to visualize and require opening the junctional triangle. Added visualization through the junctional triangle is recommended for these aneurysms in order to facilitate dissection of efferent branch arteries, careful clip application, and perforator preservation. Dome projection can be determined preoperatively from images and can help anticipate dissection routes through the junctional triangle.

ABBREVIATIONS ACA = anterior cerebral artery; ACoA = anterior communicating artery; FpA = frontopolar artery; ICG = indocyanine green; mRS = modified Rankin Scale; OfA = orbitofrontal artery; RAH = recurrent artery of Heubner; SAH = subarachnoid hemorrhage.

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  • Tables S1–S3 (PDF 460 KB)

Article Information

Correspondence Michael T. Lawton: Barrow Neurological Institute, Phoenix, AZ.

INCLUDE WHEN CITING Published online April 5, 2019; DOI: 10.3171/2018.12.JNS183264.

Disclosures The authors report no conflict of interest concerning the materials or methods used in this study or the findings specified in this paper.

© AANS, except where prohibited by US copyright law.



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    ACoA aneurysm dome projections. Axial (A) and sagittal (B) views of the ACoA dome projection. The vertical angle in the sagittal view was used to categorize ACoA aneurysm projection into 4 quadrants: anterior (Ant.), superior (Sup.), posterior (Pos.), and inferior (Inf.). By shifting the vertical angle by 45° and adding a horizontal angle determined by rightward or leftward projection, we further categorized ACoA aneurysm projection into 8 3D octants referenced to the side of craniotomy approach (C and D). L-ICA = left internal carotid artery (ICA); L-MCA = left middle cerebral artery (MCA); R-ICA = right ICA; R-MCA = right MCA. Copyright Kenneth Probst. Published with permission.

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    ACA triangles. The precommunicating triangle is formed by the right and left terminal A1 segments with the ACoA acting as the apex and the optic chiasm as the base. The junctional triangle is formed by the intersection of the distal A1 ACA segment and the proximal A2 ACA segment. The ACoA originates from the apex of the triangle in the midline, and the gyrus rectus lies against this A1-A2 intersection. The precommunicating segment of the ACA courses horizontally across the optic nerves to the midline, whereas the postcommunicating segment courses vertically along the rostrum of the corpus callosum. CN = cranial nerve. Copyright Kenneth Probst. Published with permission.

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    Distribution of aneurysm size by dome projection. Aneurysm dome projection, quadrant and size correlation to ruptured presentation, surgical outcome, percent gyrus rectus resection, craniotomy approach, intraoperative rupture, use of temporary (Temp) clipping, need for clip adjustment after ICG injection, and average temporal clip time. Graph marks actual dome size and projection, with X noting unfavorable outcome and the circles noting favorable outcome. Green text represents favorable characteristics of the quadrant, and red text represents challenging characteristics of the quadrant. Color does not correlate to statistical significance. p values can be found in Table S2. Copyright Kenneth Probst. Published with permission.

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    Representative ACoA aneurysms. Inferiorly projecting aneurysms (A) are found in the A1-A1 triangle and can adhere to the chiasm or dive into the interoptic triangle to block access to the contralateral A2. This can increase the rate of intraoperative rupture and hinder dissection of the distal aneurysm neck at the contralateral A1 segment and limit proximal control. In superiorly projecting ACoA aneurysms (B), the dome conceals the contralateral A2 segment. To search for the contralateral A2 ACA we first search in the superior corner of the triangle, above the aneurysm dome, which can require aggressive widening of the triangle, gyrus rectus resection, and the use of an orbito-pterional craniotomy. Posteriorly projecting aneurysms (C) push the ACoA perforators inferiorly into the inferior corner of the triangle putting them in jeopardy during permanent clipping. Extensive dissection and inspecting with ICG intraoperatively for complete visualization is imperative, and clip changes can be needed to ensure preservation of blood flood through the perforators. Copyright Kenneth Probst. Published with permission.




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