Willis circle and variations of its structure

The Willis circle is an anatomical formation of vessels localized on the basis of the brain, which ensures the redistribution of blood between the various 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 connective arteries (PSA and ЗСА), the initial segments of the anterior and posterior cerebral arteries (ПМА and ЗМА), the supraglinic segment of the internal carotid artery (BCA) and the distal part of the main artery (OA).

In its anterior section, there is collateral circulation between the carotid artery systems (for example, ICA and PMA on the right side - PSA - BCA and PMA of the left side), and in the posterior compartment - between the carotid artery system and the vertebrobasilar system (BCA - ЗСА - ЗМА - ОА ).

It is worth noting that the normally developed and completely closed Willis circle is found only in half the world's population. Widespread are hypoplasia or aplasia of ZMA and PMA, as well as hypoplasia of PSA and PSA. In the anterior part, abnormalities are detected slightly less frequently than anomalies in the structure of the posterior part.

PSA is very variable in its structure, size and location: options are possible from complete absence to the formation of a broad fistula at the point of contact of the PMA, possibly also doubling or tripling PSA. Very rarely there is an "terior trifurcation "of the anterior cerebral artery: both PMAs start from one ICA. In this case, the circle of Willis becomes open in the front section. Quite a rare anomaly is the presence of additional PMA, which is called the Wilder artery.

The rear department is normally found only in about half of the world's population. Anomalies of the posterior part can be very diverse: a large scatter of dimensions, asymmetric divergence of branches of the arteries, the absence of segments or even arteries. The most common anomaly is the 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 bloodstream of the vertebrobasilar basin. Admissible asymmetry is 50%. DSA by diameter is usually slightly less than PSA. Also quite common is HSA hypoplasia.

The variant of the Willis circle development is a pronounced anomaly of the structure of the intracranial vertebrobasilar system: there is no complete union of the vertebral arteries of both sides in the main artery, anastomoses of the wrong structure or arteries are possible in isolation. In the case of aplasia of the main artery or ACA, an opening is formed in the posterior part of the arterial circle of the brain. It is from the back part that there are perforating branches going to the middle and intermediate brains. It turns out that an open circle of Willis can be in the front segment, and in the back. Moreover, it can be completely opened (connective arteries absent) and incompletely (with hypoplasia or constriction).

Changes in the normal structure of the arterial circle of the brain can be both congenital and acquired - the result of adaptation of certain parts of it to the changed conditions of hemodynamics with various pathologies of the main vessels of the head, which often develop in these cases. Evaluation of individual features in the structure of the vascular bed is based on angiography and is very important in neurological practice. It is necessary to distinguish when making a diagnosis changes in the arteries: what are the causes, and which develop as a result of pathological changes.