Understanding the Circle of Willis: Anatomy and Variations with Clinical Implications

The human brain is the main controlling organ of the body. So, the normal and varied anatomy of the brain, including its vascular supply, is very important for neurosurgeons for the management of neurovascular diseases of the brain. Now, the brain is supplied by mainly irrigated by the internal carotid artery and basilar-vertebral arteries and branches of these arteries form an arterial polygon known as the circle of Willis at the base of the brain. Standard configuration of the circles of Willis is present in only one-third of the population only emphasising that quite a large population suffers from variation in the formation of the circle of Willis. The main variations of the circle of Willis include asymmetry, fenestration, duplication, aplasia, agenesis, curved, kinked and tortuous course of the arteries forming the circle of Willis. These variations may lead to aneurysm, stroke and may cause ischemia of parts of the brain supplied by arteries forming the circle of Willis. These clinical conditions may be fatal. So, to manage these clinical conditions of the brain, thorough knowledge of the normal and varied anatomy of the circle of Willis is very important for neurosurgeons. The aim of this chapter is to expound the normal anatomy along with the variations associated with the arteries forming the circle of Willis so that vascular diseases of the brain can be managed successfully with minimum complications.