In this chapter, we will be covering the vascular anatomy of the brain, brainstem, and spinal cord. Additionally, we will be covering typical stroke presentations. After reviewing this chapter, one will (1) better understand the anterior and posterior cerebral vascular systems, (2) be able to identify stroke syndromes based on the neurological deficit, and (3) be able to identify vascular territories on imaging.

If you want to learn more about the pathophysiology and management of stroke check out our Vascular Neurology chapter.

Author: Brian Hanrahan MD

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Figure 1: The Cerebral Vascular System
Diagram of the Cerebral Vascular System with the circle of willis
The Cerebral Vascular System

Introduction

  • The cerebral vascular system can be subdivided into anterior and posterior circulatory systems.
    • Two internal carotid arteries (ICAs) provide the anterior circulation.
    • The vertebral arteries/basilar artery provides the posterior circulation.
  • Cerebral angiogram (DSA) provides the best imaging quality to identify any vascular pathology.
    • CT angiograms and MRA imaging can also be used to evaluate vascular anatomy but is less sensitive than a cerebral angiogram.

Anterior Circulation

  • The left common carotid artery branches off the aortic arch while the right common carotid artery branches off the brachiocephalic artery.
    • The common carotid artery divides into external and internal carotid arteries at the level of the fourth cervical vertebra.
      • The external carotid supplies structures in the neck, face, and meninges.
      • The internal carotid supplies the anterior portion of the brain, eye, optic nerve, anterior pituitary, and meninges.
  • The internal carotid artery has seven segments (according to the Bouthillier Classification):
Add terminal branches
    • Cervical (C1)
    • Petrous (C2)
    • Lacerum (C3)
    • Cavernous (C4)
    • Clinoidal (C5)
    • Ophthalmic (C6)
    • Communicating (C7)
    • Cervical (C1)
    • Petrous (C2)
      • Caroticotympanic artery
      • Vidian artery
    • Lacerum (C3)
    • Cavernous (C4)
      • Meningohypophyseal trunk
      • Inferolateral trunk
      • Capsular arteries of McConnell (variable)
    • Clinoidal (C5)
    • Ophthalmic (C6)
      • Ophthalmic artery
      • Superior hypophyseal artery
    • Communicating (C7)
      • Posterior communicating artery (PCOM)
      • Anterior choroidal artery
      • Anterior cerebral artery (ACA)
      • Middle cerebral artery (MCA)

(Note: Gibo and Fisher Classifications number these slightly different)

Diagram of the anatomy of the Internal Carotid Artery Segments on MRA imaging
Internal Carotid Artery (ICA) Segments on MRA imaging

  • The first branch of the internal carotid artery after it enters the skull is the ophthalmic artery.
      • The ophthalmic artery supplies the eyeball, ocular muscles, and adjacent structures.
      • If an embolus obstructs the ophthalmic artery it may cause irreversible blindness. If vision symptoms are transient it is called amaurosis fugax.
  • The terminal branches of the communicating segment of the ICA are the anterior cerebral artery (ACA), middle cerebral artery (MCA), posterior communicating artery (PCOM), and anterior choroidal artery.
    • The PCOM connects anterior and posterior circulatory systems.
    • The anterior choroidal artery supplies numerous deep cortical structures including the posterior limb of the internal capsule and optic tract.
      • A lesion to the anterior choroidal artery can lead to contralateral homonymous hemianopia, contralateral hemiparesis, and hemisensory loss.

Anterior Cerebral Artery (ACA)

  • The ACA supplies the mesial frontal, medial parietal lobes. There are numerous small perforating branches, including the recurrent artery of Heubner.
    • The recurrent artery of Heubner supplies the caudate nucleus and anterior limb of the internal capsule.
      • Remember, the posterior limb is supplied by the anterior choroidal artery, a branch directly off of the ICA.
      • Both the anterior and posterior limbs also get additional blood supply from the lateral lenticulostriate branches of the MCA.
  • The most anterior portion of the ACA is called A1. The A2 segment begins after the anterior communicating artery branches off.
  • The anterior communicating artery connects the two anterior cerebral arteries.
  • A unilateral ACA occlusion can lead to contralateral weakness and numbness of the lower extremity.
    • Other possible symptoms include amotivation, apathy, akinetic mutism, transcortical motor aphasia, alien hand syndrome, and incontinence.

Middle Cerebral Artery (MCA)

  • The MCA supplies the temporal lobe, lateral frontal lobe, and most of the parietal lobe.
    • Lenticulostriate perforators supply the posterior and anterior limb of the internal capsule, globus pallidus, and corona radiata.
      • A posterior limb of the internal capsule stroke can cause contralateral weakness of the face, arm, and leg.
  • The first segment of the MCA is called M1.
    • A proximal M1 lesion can lead to contralateral face/arm weakness/numbness, global aphasia (dominant hemisphere), homonymous hemianopia, and ipsilateral gaze preference.

  • An M1 occlusion can sometimes be seen on a CT scan. It is called a “hyperdense MCA sign”.
Left Hyperdense MCA Sign
CT scan, axial image showing a left hyperdense MCA sign
  • The middle cerebral artery (MCA) branches into two major M2 branches, the superior and inferior divisions, in the Sylvian fissure.
    • The superior division supplies the lateral inferior frontal lobe, which includes Broca’s area, and the superior parietal lobe.
      • Dominant superior M2 infarcts lead to expressive aphasia. Non-dominant superior M2 infarcts can lead to a neglect syndrome such as anosognosia or hemineglect. 
      • The inferior division supplies the superior temporal gyrus, which includes Wernicke’s area, and the inferior parietal lobe.

Posterior Circulation

  • The vertebral arteries arise from the subclavian arteries. The vertebral arteries travel through the transverse foramen of cervical vertebrae from C6 to C1. After exiting the transverse foramen of the C1 cervical vertebrae the two vertebral arteries converge with one another to become the basilar artery.
  • Prior to becoming the basilar artery, each vertebral artery gives off two main branches; the posterior inferior cerebellar artery (PICA) and the anterior spinal artery (ASA).
    • The PICA supplies the inferior cerebellum and lateral medulla.
      • A lesion to the PICA leads to a lateral medullary (Wallenberg) syndrome.
    • The ASA supplies the anterior two-thirds of the spinal cord.
      • An ASA lesion will lead to quadriparesis and loss of pain and temperature distal to the lesion. The posterior aspect of the spinal cord that supplies the dorsal column-medial lemniscus pathway will remain intact. Because of this, sensation to light touch and proprioception are not affected by ASA lesions.

Basilar Artery

  • The basilar artery enters the skull along the brainstem and terminates as two posterior cerebral arteries.
  • The anterior inferior cerebellar artery (AICA) and superior cerebellar artery (SCA) are key branches of the basilar artery that supply important cerebellar and brainstem regions.
    • The AICA supplies the anterior cerebellum, inferior lateral pons, and the middle cerebellar peduncle.
    • The SCA supplies the superior cerebellar peduncle, superior portion of the cerebellum, and superior lateral pons
    • There are also small pontine perforators off the basilar that supply the medial pons. If injured, these perforators can cause small ischemic strokes that have profound neurological deficits.
  • A vessel occlusion at the top of the basilar will lead to coma due to damage to the brainstem as well as possible bilateral medial thalamus damage from blockage of the thalamic-subthalamic arteries (the first branch of the PCA).
Basilar Occlusion
This is a CT angiogram, axial image. Note the two internal carotid arteries anteriorly. Posteriorly, there is no contrast where the basilar artery should be. This is consistent with a basilar artery occlusion.

Posterior Cerebral Artery (PCA)

  • The PCA supplies the occipital lobe, posterior medial temporal lobes, inferior parietal lobe, splenium of the corpus callosum, and most of the thalamus.
    • Three out of the four blood vessels that supply the thalamus branch off the PCA.
  • Unilateral PCA territory infarcts can present with contralateral homonymous hemianopia, visual agnosia, and/or prosopagnosia.
  • Bilateral PCA territory infarcts can present with cortical blindness or Anton syndrome.
Acute Right PCA Stroke
Acute Right PCA Stroke

Circle of Willis

  • The circle of Willis represents the arrangement of blood vessels within the brain that allows for collateral circulation. This collateral circulation helps maintain adequate blood flow to brain parenchyma whenever one of its contributing arteries is deficient.
    • The ACA, ICA, PCA, anterior communicating, and posterior communicating arteries are considered to be parts of the circle of Willis.
  • Branch points between these regions are generally susceptible to the development of saccular aneurysms, especially the anterior and posterior communicating arteries.
    • Ruptured aneurysms will cause subarachnoid hemorrhage.

High yield cerebral blood vessels and their associated brainstem or cortical stroke symptoms.

Watershed Infarcts

  • Cerebral watershed infarcts, also known as border zone infarcts, involve the regions between two vascular territories.
  • They occur secondary to cerebral hypoperfusion or systemic hypotension.
  • On imaging, watershed infarcts will present with a “string of pearls” or wedge-shaped regions of ischemia located at the juncture between two vascular territories.
  • The two most important ones to know are the ACA-MCA and MCA-PCA territories.
    • ACA-MCA watershed infarcts present with ischemia in the frontal lobes
    • MCA-PCA watershed infarcts are seen in the parieto-occipital regions.
      • Possible clinical presentations for MCA-PCA watershed infarcts include prosopagnosia and Balint syndrome (the triad of simultanagnosia, oculomotor apraxia, and optic ataxia).

References

  1. Balami, J S., Chen, R.L., Buchan, A.M., Stroke syndromes and clinical management, QJM: An International Journal of Medicine, Volume 106, Issue 7, July 2013, Pages 607–615.
  2. Ioannides K, Tadi P, Naqvi IA. Cerebellar Infarct. [Updated 2019 Mar 24]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2019 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK470416/.
    Javed K, M Das J. Neuroanatomy, Posterior Cerebral Arteries. [Updated 2019 Feb 11]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2019 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK538474/.
  3. Juergenson I, Mazzucco S, Tinazzi M. A typical example of cerebral watershed infarct. Clin Pract. 2011;1(4):e114. Published 2011 Nov 18. doi:10.4081/cp.2011.e114.
  4. Khanni JL, Casale JA, Koek AY, Espinosa Del Pozo PH, Espinosa PS. Artery of Percheron Infarct: An Acute Diagnostic Challenge with a Spectrum of Clinical Presentations. Cureus. 2018;10(9):e3276. Published 2018 Sep 10. doi:10.7759/cureus.3276
  5. Kobayashi, Shunsuke, et al. “Clinical Symptoms of Bilateral Anterior Cerebral Artery Territory Infarction.” Journal of Clinical Neuroscience, vol. 18, no. 2, 2011, pp. 218–222., doi:10.1016/j.jocn.2010.05.013.
  6. Mangla, R., et al. (2011). “Border zone infarcts: pathophysiologic and imaging characteristics.” Radiographics 31(5): 1201-1214.
  7. Navarro-Orozco D, Sánchez-Manso JC. Neuroanatomy, Middle Cerebral Artery. [Updated 2019 Jan 16]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2019 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK526002/.
  8. Schmahmann JD. Vascular syndromes of the thalamus. Stroke. 2003;34(9):2264-78.
  9. Waxman SG. eds. Clinical Neuroanatomy, 28e New York, NY: McGraw-Hill; http://accessmedicine.mhmedical.com/content.aspx?bookid=1969&sectionid=147036871.

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