Infraoptic anterior cerebral artery: review, report of two cases and an anatomical classification.

Infraoptic course of the pre-communicating anterior cerebral artery (A1) is a rare anomaly. In total, there are 42 examples reported in the literature. We report two further patients. The first had an intradural cerebral aneurysm at the low bifurcation of an internal carotid artery (ICA) with bilateral infraoptic course of A1. The second had right infraoptic course of A1 with associated left parietal cerebral arteriovenous malformation and is the first report of such an association. Overall, 59% of the examples were associated with cerebral aneurysms. Different terminology such as carotid-anterior cerebral artery anastomosis and infraoptic anterior cerebral artery has been used. Having analyzed the reports of infraoptic A1, we found the vascular configurations of the A1 could be better described by classifying them into four types. Such a classification can facilitate analysis of the embryogenesis explanation for this anomaly and the pathogenesis of the associated aneurysms. Besides, such a classification also has some practical implications.