The Willis circle is an anatomical formation of blood vessels, localized on the basis of the brain, which provides redistribution of blood between different vascular pools in the presence of insufficient blood supply to the brain. It is a closed polygon, which is a vascular anastomosis.
The Willis circle is formed from the anterior and posterior connecting arteries (PSA and ZCA), the initial segments of the anterior and posterior cerebral artery (PMA and ZMA), the supraclavicular segment of the internal carotid artery (ICA) and the distal section of the main artery (OA).
Collateral circulation between the systems of the carotid arteries (for example, ICA and PMA of the right side — PSA — ICA and PMA of the left side) is performed in its anterior section — between the carotid artery system and vertebrobasilar system (ICA — ZCA — ZMA — OA). )
It is worth noting that a normally developed and completely closed Willis circle is found in only half of the world's population. Widespread are hypoplasia or aplasia of ZMA and PMA, as well as hypoplasia of PSA and ZSA. In the anterior section, abnormalities are detected slightly less frequently than abnormalities in the structure of the posterior section.
PSA is very variable in its structure, size and location: options are possible from complete absence to the formation of a wide fistula at the site of contact of the PMA, it is also possible to double or triple the PSA. “Anterior trifurcation” of the anterior cerebral artery is very rare: both PMAs start from the same ICA. In this case, the Willis circle is open in the anterior region. A rather rare anomaly is the presence of an additional PMA called the Wilder artery.
The posterior section normally occurs only in about half of the world's population. Anomalies of the posterior region can be very diverse: a large scatter of sizes, asymmetry in the branching of arterial branches, the absence of segments or even arteries. The most common anomaly is a difference in the diameters of the vertebral arteries: hypoplasia of one with simultaneous hypertrophy of the other. At the present stage, it is believed that one vertebral artery dominates, and the second is recessive in terms of their total contribution to the blood flow of the vertebrobasilar basin. Permissible asymmetry of 50%. ZCA in diameter is usually slightly smaller than PSA. Hypoplasia of CSA is also quite common.
A variant of the development of the Willis circle is a pronounced anomaly in the structure of the intracranial vertebrobasilar system: the vertebral arteries of both sides are not completely combined into the main artery, anastomoses of irregular structure are possible or the arteries go in isolation. In the case of aplasia of the main artery or ZSA, the openness of the posterior part of the arterial circle of the brain is formed. It is from the back that there are perforating branches heading to the midbrain and diencephalon. It turns out that an open Willis circle can be in the front segment and in the rear. Moreover, it can be fully opened (connecting arteries are absent) and incompletely (with hypoplasia or narrowing).
Changes in the normal structure of the arterial circle of the brain can be either congenital or acquired - the result of the adaptation of certain parts to the changed hemodynamic conditions in various pathologies of the main vessels of the head, which often develop in these cases. Assessment of individual characteristics in the structure of the vascular bed is carried out according to angiography and is very important in neurological practice. It is necessary to distinguish when making a diagnosis of changes in the arteries: which are the cause and which develop as a result of pathological changes.