Microsurgically critical anomaly of the anterior communicating artery complex during the pterional approach to a ruptured aneurysm: double fenestration of the proximal A2 segments.

A 50-year-old man presented with a ruptured aneurysm of the anterior communicating artery (ACoA) complex anomaly with the neck of the aneurysm located in the upper ACoA. The right pterional approach was performed 2 days after admission. Retraction of the frontal lobe exposed the lower half of the ACoA complex consisting of the origins of the bilateral A2 segments, and the apparent neck of the aneurysm was clipped. However, further exposure of the bilateral A2 segments revealed double fenestration of the proximal A2 segments, a subtype of duplication of the ACoA accompanied by a bridging artery between the upper and lower ACoA. The limited visualization of this ACoA complex anomaly with an aneurysm had led to the proximal origin of the bridging artery being mistaken for the neck of the aneurysm. Such double fenestration of the proximal A2 segments is a microsurgically critical anomaly of the ACoA complex, because the bridging artery mimics the neck of an aneurysm when visualized by the pterional approach.