The circle of Willis is formed by an arterial polygon as the internal carotid and vertebral systems anastomose around the optic chiasm and infundibulum of the pituitary stalk. This communicating pathway allows equalization of blood-flow between the two sides of the brain, and permits anastomotic circulation, should a part of the circulation be occluded.
Vessels comprising the circle of Willis
- left and right internal carotid arteries (ICA)
- horizontal (A1) segments of the left and right anterior cerebral arteries (ACA)
- anterior communicating artery (ACOM)
- left and right posterior communicating arteries (PCOM)
- horizontal (P1) segments of left and right posterior cerebral arteries (PCA)
- basilar artery (tip)
The anterior circulation is comprised of vessels 1-3 and their branches; the posterior circulation arises from vessels 4-6 and their branches.
The basilar artery divides at the upper border of the pons to form the left and right PCAs. From each ICA, a PCOM arises at the anterior perforated substance and runs back through the interpeduncular cistern to join the ipsilateral PCA. Each ICA also gives off an ACA. The ACAs are united by the ACOM, a small vessel that runs in the chiasmatic cistern (below the rostrum of the corpus callosum), to complete the circle.
Branches of the circle of Willis (supply optic chiasm and tracts, infundibulum, hypothalamus and other structures at base of brain):
- medial lenticulostriate arteries (from A1 segment of ACA)
- thalamoperforating and thalamogeniculate arteries (from basilar tip, proximal PCAs and PCOMs)
- perforating branches (from the ACOM)
A complete circle of Willis (in which no component is absent or hypoplastic) is only seen in 20 - 25% of individuals. Posterior circle anomalies are seen in nearly 50% of anatomical specimens.
- hypoplasia of one or both PCOMs : 34%
- hypoplastic / absent A1 segment of ACA
- origin of PCA from the ICA, with absent / hypoplastic P1 segment : 17% (foetal PCOM)
- infundibular dilatations of the PCOM origin : 10%
Congenital absence of one or both ICAs may occur but is rare. If one ICA is absent, intrasellar intercarotid communicating arteries are common and there is a high incidence of associated aneurysms.
- persistent primitive trigeminal artery (PPTA), seen in 0.1 - 0.6% of cerebral angiograms; bilateral PTAs extremely rare. PTA arises where the ICA exits the carotid canal to enter the cavernous sinus; it then runs posterolaterally along the trigeminal nerve (41%), or crosses over or through the dorsum sellae (59%) before joining the basilar artery. Usually associated with a small PCOM and vertebrals, as well as a hypoplastic basilar caudal to the anastomosis of PTA. (increased incidence of AVMs and aneurysms)
- persistent primitive hypoglossal artery (PPHA), seen in 0.027 - 0.26% of cerebral angiograms. It courses through the hypoglossal canal, parallel to the nerve, connecting the cervical ICA with the basilar artery. When present, it is functionally a single artery that supplies the brainstem and cerebellum (often associated with aneurysms).