E, The left A1 gives rise to a frontopolar branch. The segment of the A1 between the origin of the frontopolar branch and the AComA is hypoplastic. The right A1 is dominant and provides the majority of the flow to both A2s. F, Anterior view. The left A1 is larger than the right. The AComA is short and small. A precallosal artery arises from the left A1-A2 junction near the AComA. The right recurrent artery arises from the frontopolar artery and passes laterally above the carotid bifurcation. The left recurrent artery arises at the level of the AComA. G and H, most common anatomic variant associated with an AComA aneurysm. G, The right A1 is dominant and gives rise to both A2s. The left A1 is hidden behind the optic nerve. The left A2 loops downward between the optic nerves. H, The anterior communicating complex has been elevated to show the hypoplastic left A1. (Images courtesy of AL Rhoton, Jr.)