Arterial pathways of compensation at the cervical level
During stenoses and occlusions of major cervical vessels, blood flow compensation is provided by powerful anastomoses between branches of the external carotid and vertebral arteries. A key vessel in this basin is the occipital artery, which can direct collateral blood flow in any direction.
- In proximal occlusion of the vertebral artery, the deficit is compensated by an inflow from the suboccipital branches of the occipital artery.
- Occlusion of the common carotid artery (CCA) or the proximal internal carotid artery (ICA) is compensated by blood entering the carotid system via the occipital artery from the muscular branches of the vertebral artery.
- A special hemodynamic pattern occurs in proximal occlusion of the CCA (with cessation of antegrade blood flow). In this case, blood from the vertebral artery flows retrogradely into the external carotid, descends to the carotid bifurcation, and then ascends again into the internal carotid artery, maintaining perfusion.
Arterial circle of the brain (circle of Willis)
The circle of Willis is a vascular ring at the base of the brain. Its main function is to maintain continuous perfusion during occlusion or slowly progressive stenosis of a major vessel.
Anatomically, the ring is formed (from anterior to posterior) by the following segments:
- Anterior communicating artery (a. communicans anterior).
- Proximal segment (A1) of the anterior cerebral artery.
- Distal segment of the internal carotid artery.
- Posterior communicating artery (a. communicans posterior).
- Proximal segment (P1) of the posterior cerebral artery.
- Bifurcation of the basilar artery (a. basilaris).
Normally, a decrease in blood flow below the level of the circle is compensated by an increase in collateral flow. However, anatomical variants are common—hypoplasia or absence of segments. If a patient with stenosis has such an anomaly, the formation of collateral blood flow becomes impossible, leading to a hemodynamic infarction.
Callosal and leptomeningeal collaterals
In addition to the circle of Willis, there are additional levels of vascular protection:
- Anastomoses of callosal arteries: connect the carotid and vertebrobasilar systems. For example, in anterior cerebral artery occlusion, the central region can receive blood from the posterior cerebral artery basin.
- Leptomeningeal (meningeal) anastomoses: vascular connections at the level of the pia and arachnoid mater. They unite branches of the anterior, middle, and posterior cerebral arteries, and also connect branches of the three cerebellar arteries.
Venous outflow system
A crucial specific feature of cerebral veins is that they do not run alongside arteries. All venous blood ultimately collects into the dural venous sinuses, running inside its duplicated inner layer.
Superficial outflow proceeds from the cortex through the subarachnoid and subdural spaces via short cortical veins into the sinuses. The most important collateral outflow pathways:
- Superior anastomotic vein of Trolard (Trolard) — connects the superficial middle cerebral vein with the superior sagittal sinus.
- Inferior anastomotic vein of Labbé (Labbé) — located on the lateral surface of the temporal lobe and connects the middle cerebral vein with the transverse sinus.
Deep outflow drains blood from the basal ganglia and thalamus. It is based on a strict hierarchy of confluences:
- The anterior cerebral and deep middle cerebral veins merge into the paired basal vein (of Rosenthal).
- The vein of the septum pellucidum and the thalamostriate vein form the paired internal cerebral vein.
- The basal and internal cerebral veins (two on each side) converge behind the splenium of the corpus callosum and merge into a single short trunk — the great cerebral vein (of Galen).
- From the vein of Galen, blood flows into the straight sinus, and from there into the confluence of sinuses (torcular herophili).
Briefly: answers to topical questions
What anastomoses exist between the external and internal carotid artery basins?
Anastomoses exist between the external and internal carotid artery basins via branches of the facial, superficial temporal, and ophthalmic arteries, as well as through the inferolateral trunk.
Main collateral blood flow pathways:
- Pathway via the facial artery — blood from the external carotid artery enters the facial artery, then the angular artery, and further into the internal carotid artery via an anastomosis with the ophthalmic artery.
- Pathway via the superficial temporal artery — blood from the external carotid artery passes through the superficial temporal and angular arteries into the internal carotid.
- Anastomosis via the inferolateral trunk — connects the external carotid artery with the intracranial portion of the internal carotid artery.
What branches arise from the external carotid artery?
Branches arising from the external carotid artery include the facial, angular, superficial temporal, and occipital arteries.
Main branches:
- Facial artery (a. facialis) — participates in the formation of anastomoses with the internal carotid artery.
- Angular artery (a. angularis) — serves as an intermediate link in collateral blood flow.
- Superficial temporal artery (a. temporalis superficialis) — provides one of the pathways for blood overflow.
- Occipital artery (a. occipitalis) — the most significant branch in the basin of the cervical and occipital muscles, forming collaterals with the vertebral artery.
Which dural venous sinuses drain into the confluence of sinuses (torcular herophili)?
The straight, superior sagittal, and transverse sinuses drain into the confluence of sinuses (torcular herophili).
Sinuses forming the confluence:
- Superior sagittal sinus (sinus sagittalis superior) — collects blood from the convex surface of the brain and merges with the straight sinus.
- Straight sinus (sinus rectus) — drains blood from the deep regions of the brain and proceeds toward the internal occipital protuberance.
- Transverse sinus (sinus transversus) — a paired sinus that also communicates with the confluence of sinuses on both sides.
Formula for deep venous outflow: "2 Internal + 2 Rosenthal = 1 Galen". Four paired veins converge behind the splenium of the corpus callosum and merge into a single great cerebral vein (of Galen).
Class and exam questions
What happens in proximal occlusion of the common carotid artery?
Blood flow in the basin will be maintained retrogradely. Blood from the vertebral artery enters the external carotid, descends to the carotid bifurcation, and then ascends again into the internal carotid artery.
Why are anatomical variants of the circle of Willis dangerous?
When hypoplasia or absence of arterial segments of the circle is combined with stenosis of a major vessel, blood flow compensation is impossible. This directly leads to the development of a hemodynamic brain infarction.
How is the internal cerebral vein formed?
This paired vein is formed by the union of the vein of the septum pellucidum (draining the anterior parts of the lateral ventricles) and the thalamostriate vein.
What else to study on this topic
- Topography of dural venous sinuses (superior sagittal, inferior sagittal, straight, transverse, occipital).
- Anatomical landmarks of the skull base: anterior perforated substance, infundibulum, tuber cinereum, and mammillary bodies.
- Blood supply to the optic chiasm and tuber cinereum via small perforating branches.
- The role of the confluens sinuum in the cerebral venous drainage system.
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