Collateral Cerebral Circulation

Absence of collateral function due to hypoplasia or stenosis of collateral arteries may lead to a higher risk of stroke in patients with severe internal carotid artery occlusive disease

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Essential Neurology


  • Brain constitutes only 2% of the total body weight yet requires 25% of oxygen in every breath.
  • Constant supply of blood is vital to provide oxygen and essential nutrients to brain.
  • Absence of collateral function due to hypoplasia or stenosis of collateral arteries may lead to a higher risk of stroke in patients with severe internal carotid artery occlusive disease.
  • The quality of collateral circulation affects the severity and prognosis of stroke patients.
  • Multiple collateral circulation pathways exist for cerebral arteries.
Circle of Willis is the primary collateral circulation of cerebral arteries that serve as an important intracranial compensatory pathway of collateral circulation.
  • Other pathways of collateral circulation of brain includes:
    • Ophthalmic artery
    • Leptomeningeal arteries
    • Cortical arteries (anastomosis between perforating branches)
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Circle of Willis

  • Circle of Willis named after Thomas Willis who first investigated it in 1962 is the major vascular network for collateral circulation of brain.
  • It is polygonal in shape and is present on base of brain in interpeduncular fossa.
  • It joins the branches of internal carotid arteries and basilar artery to form an anastomosis that ensure the constant supply of blood to brain.
  • Anteriorly, anterior cerebral artery from right and left side gets connected via anterior communicating artery.
  • Posteriorly, basilar artery divides into two posterior cerebral arteries, one on each side. These arteries join the internal carotid arteries of respective side via posterior communicating artery.
  • Anterior cerebral arteries forms the anterolateral boundaries whereas posterior communicating artery forms the lateral boundary
  • Termination of Internal carotid artery forms the lateral corners whereas termination of basilar artery forms the posterior corner of of Circle of Willis
  • Anterior and posterior communicating arteries thus act as a channels that will allow the compensatory blood flow from one side to other in case of internal carotid artery stenosis.

Figure 1: Arterial Circulation of Brain

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Anatomical Variations

  • Almost 21 morphological variations of Circle of Willis have been observed in a study.
  • In one study, 52% of Circle of Willis used in study were found to be anomalous. [S.Iqbal]

Table 1: Common Anatomical Variations of Circle of Willis

Anatomical Configuration*
Normal Circle of Willis
48 %
Attenuated/Hypoplastic Vessels
Accessory Vessels (Duplication/Triplication)
Aberrant Origin of Vessels
Absence of Participating Vessels
*Based upon 50 adult brain samples evaluated
Derived From: Iqbal S. (2013). A comprehensive study of the anatomical variations of the circle of Willis in adult human brains. Journal of clinical and diagnostic research : JCDR7(11), 2423–2427.

Evaluation of Circle of Willis

  • Capacity of Circle of Willis to provide collateral circulation is crucial in cases of compromised circulation of brain
  • Evaluation of patency of collaterals can be done by
    • Common carotid artery compression test (CCA)
    • Transcranial color-coded duplex ultrasonography (TCCD)
    • Digital subtraction angiography (DSA)
    • Doppler ultrasound
    • Magnetic resonance angiography (MRA)
    • Computed tomography angiography (CTA)
    • Conventional Angiography (Gold Standard)
  • Anterior and posterior communicating arteries can be assessed by transcranial ultrasound with a high level of sensitivity and specificity.
  • Adequate working of collateral circulation is primary determinant of cerebral blood flow and perfusion pressure.

Table 2: ASITN/SIR* Collateral Scoring System for Conventional Angiography

Grade 0
No collaterals visible to the ischemic site
Grade 1
Slow collaterals to the periphery of the ischemic site with persistence of some of the defect
Grade 2
Fast collaterals to the periphery of the ischemic site with persistence of some of the defect and to only a portion of the ischemic territory
Grade 3
Collaterals with slow but complete angiographic blood flow of the ischemic bed by the late venous phase
Grade 4
Complete and rapid collateral blood flow to the vascular bed in the entire ischemic territory by retrograde perfusion
*American Society of Interventional and Therapeutic Neuroradiology/Society of Interventional Radiology (ASITN/SIR)

Clinical Importance

  • Circle of Willis is supplied by three major vessels that allows the liberty of damage to anyone without significant compromise in neurovascular circulation.
  • Even though damage to Circle of Willis doesn’t result in significant clinical symptoms, but damage to cerebral arteries results in clinical symptoms as they don’t have frank anatomic reserves.
  • Good collateral circulation is associated with better outcomes of AIS treatment.
  • It also reduces the risk of recurrent stroke in high risk patients with intracranial atherosclerotic stenosis (ICAS).

Further Readings

  • Myron D. Ginsberg, The cerebral collateral circulation: Relevance to pathophysiology and treatment of stroke, Neuropharmacology, Volume 134, Part B, 2018, Pages 280-292, ISSN 0028-3908,


  • Powers, W. J. (1991). Cerebral hemodynamics in ischemic cerebrovascular disease. Annals of Neurology: Official Journal of the American Neurological Association and the Child Neurology Society, 29(3), 231-240.
  • Hedera, P., Bujdakova, J., Traubner, P., & Pancak, J. (1998). Stroke risk factors and development of collateral flow in carotid occlusive disease. Acta neurologica scandinavica, 98(3), 182-186.
  • Hoksbergen, A. W. J., Legemate, D. A., Ubbink, D. T., De Vos, H. J., & Jacobs, N. J. H. M. (1999). Influence of the collateral function of the circle of Willis on hemispherical perfusion during carotid occlusion as assessed by transcranial colour-coded duplex ultrasonography. European journal of vascular and endovascular surgery, 17(6), 486-492.
  • Kardile, P. B., Ughade, J. M., Pandit, S. V., & Ughade, M. N. (2013). Anatomical variations of anterior communicating artery. Journal of clinical and diagnostic research : JCDR, 7(12), 2661–2664.
  • S. A. Gunnal, M. S. Farooqui, R. N. Wabale, "Anatomical Variations of the Circulus Arteriosus in Cadaveric Human Brains", Neurology Research International, vol. 2014, Article ID 687281, 16 pages, 2014.
  • E. Kalsoum, X. Leclerc, A. Drizenko, J.-P. Pruvo, Circle of Willis, Editor(s): Michael J. Aminoff, Robert B. Daroff, Encyclopedia of the Neurological Sciences (Second Edition), Academic Press, 2014, Pages 803-805, ISBN 9780123851581,
  • Zhou, H., Sun, J., Ji, X., Lin, J., Tang, S., Zeng, J., & Fan, Y. H. (2016). Correlation Between the Integrity of the Circle of Willis and the Severity of Initial Noncardiac Cerebral Infarction and Clinical Prognosis. Medicine, 95(10), e2892.
  • Raymond, S. B., & Schaefer, P. W. (2017). Imaging brain collaterals: quantification, scoring, and potential significance. Topics in Magnetic Resonance Imaging, 26(2), 67-75.
  • Liu L, Ding J, Leng X on behalf of the Chinese Society of Cerebral Blood Flow and Metabolism, the Chinese Stroke Association, et alGuidelines for evaluation and management of cerebral collateral circulation in ischaemic stroke 2017Stroke and Vascular Neurology 2018;3:doi: 10.1136/svn-2017-000135
  • Iqbal S. (2013). A comprehensive study of the anatomical variations of the circle of willis in adult human brains. Journal of clinical and diagnostic research : JCDR7(11), 2423–2427.
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Junaid Kalia MD

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Junaid Kalia MD

Founder NeuroCare.AI, Practicing Neurologist, sub-specialized in the field of Neurocritical Care, Stroke & Epilepsy

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