Anatomical variants of the anterior segment of the circle of Willis: association with cerebral aneurysms

Abstract

The circle of Willis is a vascular network that supplies blood to the brain and surrounding structures. One of its main functions is to maintain the irrigation through the anastomoses when a certain condition compromises the flow of any of its branches. It is composed of the anterior cerebral, anterior communicating, internal carotid, posterior cerebral, and posterior communicating arteries. Its conformation varies in the population. More than 30-60% of people have a different composition than the usual, with hypoplasia of one or more vessels being the most common anatomical variant. This is defined as the existence of less than 1 mm of external caliber in the anterior communicating and posterior cerebral arteries and the arterial segments A1 of the anterior cerebral and P1 of the posterior cerebral arteries. The posterior cerebral artery is the most frequently hypoplastic, followed by the P1 segment of the posterior cerebral artery, the A1 segment of the anterior cerebral artery, and the anterior communicating artery.

The formation of intracranial aneurysms is more common in the anterior communicating artery; and the unilateral predominance of the ACA trunk, aplasia/hypoplasia of the A2 segment, and hypoplasia of the A1 segment have been determined as risk factors for its appearance. This last variant constitutes the main risk factor for rupture of intracranial aneurysms of the anterior communicating artery, since it increases blood flow on the opposite side causing an increase in intramural pressure that favors the development of the aneurysm.