Anomalous formation of the circulus arteriosus and its clinico-anatomical significance.

Circulus arteriosus (Circle of Willis) is an anastomotic ring which unites the internal carotid and vertebrobasilar systems. It slows down the blood before it reaches the brain and helps in collateral circulation. Anatomic study reveals the variation in the pattern and calibre of vessels which make up the circulus arteriosus. The greatest variation in calibre occurs in the posterior communicating artery. The posterior cerebral artery is divided into three portions by the posterior communicating artery; these are- P1, P2, and P3. Three basic configurations of the posterior communicating artery has been described; foetal, transitional and adult. In foetal configuration: the diameter of the P1 segment of posterior cerebral artery is less than the diameter of posterior communicating artery so that the blood supply to the occipital lobes is mainly via the internal carotid arteries. Thirty five circulus arteriosus of the formalin embalmed brains were studied during the period of three years. Posterior cerebral and posterior communicating arteries were traced from their origins to terminations. It is revealed that in 8.6% of cases the formation of the circulus arteriosus was anomalous. The calibre of the pre communicating (P1) segment of posterior cerebral artery was smaller than the posterior communicating artery. It appeared that the posterior cerebral artery is the continuation of posterior communicating artery, thus the branch of the internal carotid artery. Anomalies of the formation of circulus arteriosus leads to stroke. The neurosurgical importance of this study lies during the exposure of the region for different purposes. Knowledge of the vascular variations will increase the success of the procedure.