Collateral vessels may modify the effects of cerebral ischemia
There is usually not enough redundancy in the blood vessels of the brain to support function if one vessel is suddenly occluded. If there were, ischemic strokes would be far less frequent.

Many smaller penetrating brain vessels such as the lenticulostriate branches of MCA that supply the basal ganglia and internal capsule, as well as the penetrating branches from vessels on the brain surface that supply deep white matter, are terminal arteries. This means that they form few if any connections (anastamoses) with other arteries.  When they are occluded, the brain regions they supply will therefore become ischemic.

Other cerebral vessels, however, can form anastomoses that potentially could protect the brain from infarction, or limit the amount of damage, by providing alternative routes for blood to reach brain regions threatened with ischemia.
Common and important anastomoses can occur between:
  • External carotid and internal carotid via branches of the ophthalamic artery
  • The major intracranial vessels via the circle of Willis (for example one carotid might supply parts of the contralateral hemisphere by connections through the anterior cerebral and anterior communicating vessels)
  • Muscular branches of cervical arteries and the extracranial vertebral or external carotid arteries
  • Small cortical branches of ACA, MCA, and PCA, or branches of the major cerebellar arteries

Diagrams

How effective the collateral circulation can be in supporting blood flow and hence neurologic function depends on the size of the vessels. The smaller the diameter of the collaterals, the less likely it is that they will be able to carry enough blood to prevent infarction. The speed at which the occlusion of a vessel occurs therefore can play a role in determining whether collateral circulation can save the region it supplies from infarction. If a major vessel is slowly being occluded by atherosclerosis, there may be time for preexisting collateral channels to enlarge to the point where they can support major blood flow. This may happen, for instance, when there is gradual occlusion of the internal carotid artery in the neck by the build up of atherosclerotic plaque.  In this situation retrograde flow may develop from the external carotid through the ophthalmic artery into the intracranial internal carotid, bypassing the blockage. Some people with one or both internal carotid arteries largely or entirely occluded show no neurologic deficits thanks to collateral circulation around the orbit.

The circle of Willis is highly over-rated as a source of effective collateral circulation
At first glance, the anatomy of the circle of Willis suggests that blood can be easily shunted from one side of the brain to the other, or from the carotid to the vertebral-basilar system or vice versa. In a majority of cases, however, one or more of the vessels that forms the circle is narrow. Functionally, this means that the circular connections could not carry enough blood to compensate for most abrupt arterial blockages. For example, they would provide little 'protection' to brain tissue if a cardiac embolus were to suddenly block a major vessel. By contrast, the circle may be able to compensate for slowly developing arterial occlusions produced by build-up of atherosclerotic plaque, since even tiny connecting arteries can enlarge over time in response to increasing hemodynamic demands.

Before we leave the circle of Willis, we should point out that places where large vessels branch or bifurcate are favorite locations for the formation of saccular ('berry') aneurysms. In these locations the muscular part of the arterial wall is relatively weak, and balloon-like swellings that are prone to rupture may occur. The greatest number of aneurysms involve the anterior portion of the circle of Willis: the junction of the anterior communicating artery with ACA, the junction of the internal carotid and posterior communicating artery, the division of MCA into superior and inferior branches within the Sylvian fissure. If an aneurysm ruptures, blood under high pressure is forced into the subarachnoid space which contains the circle of Willis, and possibly into the interior of neighboring brain substance as well.