• the collateral cerebral circulation refers to the network of vascular channels that stabilize cerebral blood flow if the main vessel is stenosed or occluded
    • arterial insufficiency may be caused by thromboembolism, hemodynamic compromise, or a combination of these factors
    • reduced anterograde flow changes pressure gradients, and this leads to the opening of the collaterals
  • the quality of the collateral circulation significantly influences the severity and extent of cerebral infarction
    • well-developed and functional collaterals can temporarily replace the function of the occluded artery, thus considerably improving the patient’s prognosis

Classification

Functional

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Anatomic

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Circle of Willis

  • intracranially, the basal arteries are interconnected to form the arterial circle of Willis
  • it is a heptagon consisting of the following:
    • left and right ICA
    • left and right A1 segments connected by an unpaired AComA
    • left and right P1 segments
    • left and right PComA (connecting ICA and P1 segment on each side)
      • PCOM originates at the anterior perforating substance and runs back through the interpeduncular cistern
    • basilar artery tip
  • branches of the circle of Willis also supply the optic chiasm and tracts, infundibulum, hypothalamus, and other structures at the base of the brain
    • medial lenticulostriate arteries (segment A1)
    • perforating branches (from the AComA)
    • thalamoperforating and thalamogeniculate arteries (from the basilar tip, proximal PCA, and PComA)
  • the circle of Willis allows redirection of the blood flow between both sides of the brain and between the vertebrobasilar and the internal carotid artery systems
The circle of Willis
  • a complete circle is present in < 30% of patients; individual anatomical variants are common
    • different diameter of the vessels on the right and left side
    • predominance of carotid or vertebral blood flow
    • significant asymmetry of the whole circuit
    • absence/hypoplasia of one of the arteries
      • AComA is absent in 1% of cases
      • the proximal segment of the ACA is absent in approximately 10%
      • PComA is absent or hypoplastic in 30% of cases
The circle of Willis variants

Leptomeningeal anastomoses (LMAs)

  • LMAs are pial arteries connecting branches of two major cerebral arteries that supply two distinct cortical territories
  • these are most commonly end-to-end anastomoses between the ACA-MCA, ACA-PCA, and MCA-PCA; there are also anastomoses between both ACAs
  • occlusion of a cerebral artery leads to hypoperfusion in the corresponding vascular territory, creating a pressure gradient between the vessels of the unaffected area and the territory of the stenotic/occluded artery, leading to retrograde filling of the affected area
a - PCA, b - LMAs between MCA-ACA, c - LMAs between PCA-MCA, d - tectal plexus between PCA-SCA, e - distal cerbellar artery anastomoses f - ACoA [Liebeskind, 2003]

Factors affecting LMA´s functionality

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Assessment of collateral circulation

CT angiography / DSA

Collateral grading score - left ACA occlusion with complete and fast ACA filling via right ICA and AcoA (grade 4)
mCTA showing poor collaterals in patient with left MCA occlusion

Neurosonology

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STEAL PHENOMENON IN PATIENT WITH RIGHT SUBCLAVIAN ARTERY STENOSIS

Steal syndrome - subclavian artery stenosis on CTA


Stenotic turbulent flow in the subclavian artery


Anterograde flow in the V4 segment of left vertebral artery


Retrograde flow the V4 segment of the right VA


Incomplete steal phenomenon with biphasic flow in basilar artery


Retrograde flow in V2 segment of the right vertebral artery

Retrograde flow in V2 segment of the right vertebral artery

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Collateral cerebral circulation
link: https://www.stroke-manual.com/collateral-cerebral-circulation/