An azygos anterior cerebral artery is an uncommon-to-rare variant of the circle of Willis where the two A1 segments of the anterior cerebral artery (ACA) join to form a single trunk. As a result, there is no anterior communicating artery. 

The clinical importance of an azygos artery:

• it may be associated with a number of brain anomalies such as dysgenesis of corpus callosum and holoprosencephaly, and a predisposition to aneurysm formation
• occlusion of an azygos artery may result in infarcts of the bilateral medial frontal lobes and corpus callosum