Introduction/purpose The intracranial arterial vasculature is known to have an abundance of variability, especially in and around the Circle of Willis. A functional absence of the anterior communicating artery (ACom), an artery that connects the paired A1 segment anterior cerebral arteries (ACAs), is a common variation. This usually manifests with normal, bilaterally symmetric ACAs with paired A1 segments, A2 segments, and so on. In rare cases, the A1 segments can join to form a singular A2 segment, resulting in what is termed an “azygous” ACA. This variation has been reported in 0.4–1% of the population, and is similar to the standard arterial anatomy in lower primates. In humans, azygous configuration of the ACA has been associated with other midline abnormalities such as dysgenesis of the corpus collosum, lobar holoprosencephaly, and septooptic dysplasia, as well as arteriovenous malformations and (anecdotally, at least) aneurysms.

Materials and methods In our practice, we see the azygous ACA configuration extremely uncommonly. We performed a retrospective review of all neurointerventional angiographic procedures performed in our department during a 10 year period, comprising over 20,000 angiograms and embolizations. Patients were identified as having an azygous configuration ACA through a search of the medical records, which were reviewed with the angiogram report (and images, where available).

Results Among this set of patients, we found a total of 45 patients (0.225%) with azygous ACA configuration. Of these 45 patients, 14 (31.1%) had an associated aneurysm, either at the A1 confluence, or the origin of the pericallosal artery. Four of these aneurysms (28.5%) presented with rupture, with the remainder discovered on workup of other aneurysms, or incidentally during other imaging procedures. Half of the ruptured aneurysms were treated with endovascular coiling and half were treated with surgical clipping, all with technically excellent results. An additional 4 unruptured azygous ACA-associated aneurysms received treatment during this time (2 via coiling and 2 via surgical clipping), also with excellent results. Two of our patients with an azygous ACA were also found to have brain AVMs, one of whom also had an azygous ACA-associated aneurysm. The remaining 6 patients with azygous ACA-associated aneurysms were untreated and demonstrated no aneurysm growth or other high-risk features and have remained clinically stable.

Conclusion Azygous configuration of the A2 segment anterior cerebral artery is an uncommonly seen anatomic variant, and our experience is similar to the published occurrence rates (0.2% at our institution). This finding has been associated with several conditions and, in our experience, is very commonly associated with aneurysms of the ACA (31%), with 28.5% of these aneurysms presenting with rupture. When an azygous configuration of the ACA is identified it is important to take care in studying the ACA thoroughly to look for associated aneurysms.

Disclosures A. Nicholson: None. D. Cooke: None. M. Amans: None. F. Settecase: None. S. Hetts: None. C. Dowd: None. R. Higashida: None. V. Halbach: None.