Description
The circle of Willis (coW) connects the cerebral blood flow (CBF) from the internal carotid arteries (ICAs) (80% of CBF) with the vertebrobasilar system (VBS) (20% of CBF) in a ring of vessels beneath the hypothalamus.
The circular arrangement of the cerebral arteries creates a collateral system of blood flow that protects the brain from ischemia when one part of the circle becomes blocked or narrowed.
- The coW consists of the following arteries (see Figure 1):
- The basilar artery along with the two vertebral arteries are referred to as the vertebrobasilar system. It supplies blood to the coW.
- The cerebral circulation can be divided into anterior and posterior vessels:
- The anterior circulation is supplied by the ICAs (arising from the common carotid arteries). The first branch is the ophthalmic artery, followed by the PCOM, and then the anterior choroidal artery. The ICAs terminate into the anterior and middle cerebral arteries.
- The posterior circulation is supplied by the VBS. The vertebral arteries (arising from the subclavian arteries) supply circulation to the spinal cord and give off branches to the cerebrum, midbrain, pons, medulla, and cerebellum. Branches of the vertebral arteries include the anterior spinal artery, paired posterior spinal arteries, and the posterior inferior cerebellar artery. The vertebral arteries then unite to form the basilar artery. The basilar artery terminates by dividing into paired PCA that supply the medial temporal lobe, occipital lobe, and parts of the thalamus.
- The anatomy of the coW is highly variable in adults. These variations are largely the result of fetal development in utero. As early as seven weeks postconception, a preliminary form of the coW is present with approximately equal diameters of all vessels. As the fetus grows so do the components of the coW, eventually assuming the adult morphology (2). The main differences between the fetal and adult coW anatomy are the diameters of the PCOM and the circular part of the PCA. The coW is complete in only 42.1–47% of individuals (3).
- It is possible that in some individuals a fetal-type PCA (one supplied primarily by the ICA rather than the vertebral artery) will persist. ICA volume flow is significantly increased and basilar artery flow significantly decreased. In those with absent A1 segments of the anterior cerebral artery, flow between the ICAs is largely asymmetrical (6).
Physiology/Pathophysiology
- Approximately 95% of all aneurysms can be found near the coW.
- Cerebral aneurysms are classified by size, shape, and location. Size is considered to be one of the most important determinants of future rupture. The ISUIA trial (a large international prospective study) evaluated 5-year cumulative cerebral aneurysm rupture risk based on location and size (4).
- Size (anterior/posterior%):
- <7 mm (0/2.5%)
- 7–12 mm (2.6/14.5%)
- 13–24 mm (14.5/18.4%)
- Shape:
- Saccular (most common): The aneurysm has an outpouching.
- Berry: A saccular aneurysm with a stem or neck.
- Fusiform: Without an outpouching or stem.
- Size (anterior/posterior%):
- Location:
- 85% are located in the anterior circulation
- Anterior communicating artery (most common site of rupture in anterior circulation)
- Anterior cerebral arteries
- Bifurcation of the ICAs
- Middle cerebral arteries
- PCOM
- 15% are located in the posterior circulation
- PCA
- Basilar artery bifurcation
- 85% are located in the anterior circulation
- Ischemia, thrombosis, or hemorrhage of a cerebral artery is known as a cerebrovascular accident (CVA). It is defined as a new focal or global neurological (motor or sensory) deficit lasting >48 hours.
- A transient ischemic attack (TIA) is characterized by a sudden vascular-related focal neurologic deficit that resolves within 24 hours.
- Occlusion of a specific major intracranial artery will result in predictable clinical neurologic deficits.
- Isolated infarction of the anterior cerebral artery is uncommon.
- Carotid stenosis can lead to TIAs and CVAs if collateral CBF is not adequate.
- Subclavian steal syndrome is a phenomenon of transient cerebral ischemic attacks following ipsilateral arm exercise in the presence of severe stenosis (>80%) or occlusion of the subclavian artery proximal to the origin of the vertebral artery. It is a flow-related phenomenon.
- At rest, the arm becomes dependent on the collateral vessels from the head, shoulder, and neck.
- During exercise, blood flows retrograde from the ipsilateral vertebral artery to the distal subclavian artery to supply the arm. In doing so, blood is diverted from the circle of Willis and can result in transient cerebral ischemia.
- Arteriovenous malformation (AVM) is an abnormal connection between arteries and veins; whereby high-pressure arteries connect directly to low-pressure veins without passing through a capillary network. They are most often congenital lesions that can be found anywhere in the body. They are usually asymptomatic. In rare circumstances, cerebral lesions may bleed or expand, leading to CNS symptoms. The most common symptoms of cerebral AVMs are seizures, headache, and pain.
- Moyamoya syndrome is a disease in which the lumen of certain cerebral arteries (mainly the ICAs up to the middle and anterior cerebral arteries) overgrow inwardly to the point of constriction or blockage. This leads to angiogenesis of collateral blood vessels to compensate for lack of blood flow in the region. The collateral network of small blood vessels has the appearance of a "puff of smoke" or "moyamoya," in Japanese. Problems may arise when these small fragile networks hemorrhage, aneurysm, or clot off.
- Cerebral vasculature imaging (i.e., angiography, MagneticResonance Angiogram (MRA) and Computed Tomography Angiography (CTA)) may be utilized preoperatively or intraoperatively to demonstrate vascular occlusions or emboli at vessel bifurcations after an acute CVA, as well as to diagnose and treat aneurysms.
- It may be vitally important to evaluate the cerebral circulation prior to certain surgeries via angiography. for example, in endovascular stent-graft repair for thoracic aortic disease, it is commonly necessary to extend the stent-graft beyond the left subclavian artery (LSA). If the contralateral vertebral artery is hypoplastic and the coW is incomplete or when the left vertebral artery significantly participates in the supply of the superior portion of the anterior spinal artery, then coverage of the LSA may disrupt collateral pathways to the posterior cerebral circulation and spinal cord; this results in serious neurological complications. In order to avoid such debilitating complications, careful imaging of the right vertebral artery up to the basilar artery may be mandatory preoperatively (7).
- In certain situations, neurologic complications can be minimized when a prophylactic carotid subclavian bypass is performed, as well as avoidance of intraoperative hypotension (8).
- During carotid endarterectomy (CEA) surgery, an incomplete coW formation predisposes approximately one-sixth of individuals to cerebral ischemia during carotid clamping or transient closure of the carotid artery. In those with coexisting contralateral ICA occlusion, the risk of cerebral ischemia rises more than threefold (9).
- Middle cerebral artery blood flow velocities can be measured in real-time by transcranial Doppler (TCD) to detect and quantify embolic signals intraoperatively or at the bedside. It can be beneficial in situations such as patients status post-CEA and/or patients undergoing thrombolytic therapy.
- TCD predictors of stroke after CEA include emboli during wound closure, >90% decrease in middle cerebral artery peak systolic velocity at cross-clamping, and >100% increase of the pulsatility index of the Doppler signal at clamp release (10).