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Cerebrovascular Anatomy

What You'll Learn

Anterior vs. posterior circulation anatomy and key branches
ICA segments (Bouthillier C1–C7) and their clinical significance
Circle of Willis components and clinically significant normal variants
Vascular territory stroke syndromes for each major artery
Localization: cortical vs. subcortical, anterior vs. posterior, lateral vs. medial
Blood supply of key structures (internal capsule, thalamus, brainstem)
Watershed (border zone) infarct patterns and mechanisms
Intracranial aneurysm sites and their classic presentations

HighYield Pearls

ICA Bouthillier segments (C1–C7): cervical → petrous → lacerum → cavernous → clinoid → ophthalmic → communicating; cavernous sinus contains CN III, IV, V1, V2 in lateral wall and CN VI medially next to ICA
Recurrent artery of Heubner (branch of A1/A2 junction) → head of caudate + anterior limb of internal capsule → contralateral face/arm weakness + abulia
Lenticulostriate perforators (from M1) → basal ganglia + posterior limb internal capsule; classic pure motor lacunar stroke; eyes deviate toward the lesion in large MCA strokes
Artery of Percheron — variant single thalamoperforator from one P1 supplying both medial thalami ± midbrain → bilateral paramedian thalamic infarct + coma + vertical gaze palsy
Top-of-basilar syndrome → bilateral PCA + thalamic + midbrain infarcts → cortical blindness, amnesia, vertical gaze palsy, decreased consciousness
Anterior choroidal artery triad (from supraclinoid ICA): contralateral hemiparesis + hemisensory loss + homonymous hemianopia (posterior limb IC + optic tract + LGN)
AICA → lateral pons + internal auditory artery → ipsilateral sensorineural hearing loss + vertigo + facial weakness + Horner; PICA → lateral medullary (Wallenberg)
Artery of Adamkiewicz — dominant anterior radiculomedullary feeder, usually T9–T12 (left-sided ~75%); injured in aortic surgery/dissection → thoracolumbar cord infarct
Anterior spinal artery syndrome → paraplegia + bilateral pain/temp loss + sphincter dysfunction with preserved dorsal columns (vibration/proprioception)
Watershed (border zone) — ACA/MCA cortical strip → “man-in-a-barrel” proximal arm weakness; internal watershed (corona radiata) → chronic carotid stenosis, hemodynamic mechanism
Cavernous sinus thrombosis → orbital pain + chemosis + proptosis + multiple CN palsies (III/IV/V1/V2/VI) + fever; emergency — IV antibiotics ± anticoagulation
Empty delta sign on post-contrast CT → superior sagittal sinus thrombosis (most common CVT site)
Moyamoya → bilateral progressive supraclinoid ICA stenosis + basal collateral “puff of smoke”; ischemic in children, hemorrhagic in adults
Symptomatic carotid stenosis 70–99% → CEA within 2 weeks (NASCET); CAS reserved for high surgical risk

🔍 Quick ReferenceArterial / circle of Willis · Perforators · Venous / spinal

Arterial / Circle of Willis

Bouthillier C1–C7 → ICA segments (cervical → petrous → lacerum → cavernous → clinoid → ophthalmic → communicating)
Fetal PCA (10–20%) → P1 hypoplastic, PCom supplies PCA → carotid stroke can cause occipital infarct
Persistent trigeminal artery → most common carotid–basilar anastomosis (embryologic remnant)
Hypoplastic A1 → bilateral ACA territory infarct from single ACom aneurysm clip
Top of the basilar → bilateral PCA + thalamic + midbrain infarct + vertical gaze palsy
AICA territory → lateral pons + labyrinth (sudden deafness + vertigo)
SCA territory → ipsilateral cerebellar ataxia, dysarthria, nausea/vomiting, contralateral pain/temp loss (CN IV palsy can occur near SCA/PCA region but is not a classic hallmark)
Macular sparing → PCA occlusion with MCA collateral to occipital pole

Perforators / variants

Recurrent artery of Heubner → head of caudate + anterior limb IC (A1/A2 junction)
Lenticulostriates (M1) → basal ganglia + posterior limb IC → pure motor lacunar stroke
Artery of Percheron → bilateral paramedian thalami ± midbrain → coma + vertical gaze palsy
Anterior choroidal artery → hemiparesis + hemisensory + hemianopia triad (posterior limb IC + optic tract + LGN)
Thalamoperforators (P1) → medial thalamus; thalamogeniculate (P2) → lateral thalamus (Déjerine-Roussy)
Moyamoya → “puff of smoke” basal collaterals with bilateral supraclinoid ICA stenosis
Man-in-a-barrel → bilateral ACA/MCA cortical watershed (proximal arm weakness, hands spared)

Venous / spinal

Vein of Trolard → superior anastomotic vein (superficial cortical → superior sagittal sinus)
Vein of Labbé → inferior anastomotic vein (temporal cortex → transverse sinus)
Vein of Galen → deep venous drainage (internal cerebral & basal veins → straight sinus)
Empty delta sign → superior sagittal sinus thrombosis on post-contrast CT
Cavernous sinus → CN III/IV/V1/V2 in lateral wall, CN VI medial next to ICA
Cavernous sinus thrombosis → chemosis + proptosis + ophthalmoplegia + V1/V2 sensory loss
Artery of Adamkiewicz → dominant thoracolumbar cord feeder (T9–T12); aortic surgery risk
Anterior spinal artery syndrome → paraplegia + dissociated sensory loss with preserved dorsal columns
Posterior spinal artery syndrome → bilateral dorsal column loss (vibration/proprioception)
T4–T8 thoracic cord → watershed zone for hypoperfusion infarct

Anterior Circulation

Common Carotid Artery (CCA)

Anatomy

Right CCA: from brachiocephalic (innominate) artery; Left CCA: directly from aortic arch
Bifurcates at ~C4 level (angle of jaw) into ECA and ICA
Carotid body (at bifurcation): chemoreceptor (O₂, CO₂, pH); innervated by CN IX
Carotid sinus (proximal ICA): baroreceptor; innervated by CN IX (Hering nerve)

Clinical Significance

Carotid bifurcation = most common site of cerebral atherosclerosis (62%), followed by vertebral artery origin (15%), MCA origin (10%)
Carotid sinus hypersensitivity → syncope with head turning or tight collars
Carotid body tumor (paraganglioma) → painless pulsatile neck mass at angle of jaw

External Carotid Artery (ECA)

Key Branches

Internal maxillary → middle meningeal artery: enters via foramen spinosum; rupture → epidural hematoma (lens-shaped, temporal)
Superficial temporal artery: STA-MCA bypass in Moyamoya; biopsied in giant cell arteritis
Ascending pharyngeal: Supplies CN IX, X, XI at jugular foramen; can be source of embolism during embolization procedures
Facial artery: ECA-ICA collateral pathway (via angular artery ↔ ophthalmic artery)
Occipital artery: ECA-vertebral collateral pathway

Clinical Significance

ECA branches form collateral pathways to ICA and vertebral territories — can rescue flow in chronic ICA occlusion
ECA feeders are commonly recruited by dural arteriovenous fistulas and meningiomas

Internal Carotid Artery (ICA)

Segments (Bouthillier C1–C7)

Segment	Name	Key Branches	Clinical Significance
C1	Cervical	None	Most common site for atherosclerosis and dissection; no branches = distinguishes from ECA on angiography
C2	Petrous	Caroticotympanic, vidian	Through carotid canal in temporal bone; vulnerable in skull base fractures
C3	Lacerum	None (small periosteal)	Short segment over foramen lacerum; transition zone
C4	Cavernous	Meningohypophyseal trunk, inferolateral trunk	Within cavernous sinus; adjacent to CN III, IV, V1, V2, VI; aneurysm → CCF with proptosis, chemosis, CN VI palsy
C5	Clinoid	None	Short transition; enters subarachnoid space through dural ring
C6	Ophthalmic	Ophthalmic artery, superior hypophyseal	Ophthalmic a. = first major intradural branch → retina + optic nerve; superior hypophyseal arteries supply optic chiasm/optic nerve region, infundibulum, and pituitary region. Sheehan syndrome is postpartum anterior pituitary ischemic necrosis from shock/hypovolemia, not a focal superior hypophyseal artery stroke syndrome.
C7	Communicating	PCom, AChA	Terminates as ACA + MCA at the “carotid T”; PCom aneurysm → CN III palsy

Syndromes

“Carotid T” occlusion: Terminal ICA at ACA/MCA bifurcation → devastating combined ACA + MCA syndrome
Ipsilateral monocular blindness + contralateral hemiparesis = ICA localization
“Man-in-a-barrel”: Bilateral arm weakness with spared legs — bilateral ACA/MCA watershed from bilateral ICA disease or cardiac arrest

🧪 ICA Dissection

Important cause of young stroke; trauma, neck manipulation, or spontaneous (connective tissue disorders: Ehlers-Danlos type IV, Marfan, fibromuscular dysplasia)
Classic triad: Ipsilateral headache/neck pain + partial Horner’s (miosis + ptosis WITHOUT anhidrosis — sympathetics on ICA wall, sudomotor fibers follow ECA) + delayed ischemic symptoms
Imaging: CTA/MRA “flame-shaped” tapering; fat-sat MRI shows crescent sign (intramural hematoma)
Can also cause ipsilateral CN XII palsy (ICA in close proximity to hypoglossal nerve in upper cervical space)

Ophthalmic Artery

Anatomy

First major intradural branch of ICA (C6); enters orbit via optic canal alongside CN II
Key branches: Central retinal artery (end artery → retina), posterior ciliary arteries (optic nerve head, choroid), lacrimal artery

Syndromes

Amaurosis fugax: Transient monocular vision loss (“curtain coming down”) — hallmark of ICA disease; Hollenhorst plaques (cholesterol emboli) on fundoscopy
CRAO: Sudden painless monocular blindness; pale retina with “cherry red spot” (fovea perfused by choroidal circulation); in elderly consider GCA

Anterior Ischemic Optic Neuropathy (AION)

Feature	Arteritic (A-AION / GCA)	Non-Arteritic (NA-AION)
Age	Typically >70	Typically 50–70
Mechanism	Giant cell arteritis → posterior ciliary artery inflammation	Hypoperfusion of posterior ciliary arteries; nocturnal hypotension
Disc appearance	Pallid (chalky white) disc edema	Hyperemic disc edema; “disc at risk” (small crowded disc, no cup)
Visual field	Severe vision loss; altitudinal defect	Altitudinal defect (usually inferior); less severe
Systemic clues	Jaw claudication, scalp tenderness, elevated ESR/CRP, PMR	Vascular risk factors (HTN, DM, OSA)
Key action	Emergent steroids before biopsy to prevent fellow eye involvement	No proven treatment; control risk factors
Fellow eye risk	Very high without treatment (days–weeks)	~15% over 5 years

Anterior Choroidal Artery (AChA)

Anatomy

From supraclinoid ICA (C7) just distal to PCom — long, narrow, highly vulnerable end artery
Territory: Posterior 2/3 of posterior limb of IC, internal globus pallidus, optic tract, lateral geniculate body, medial temporal lobe (hippocampus), amygdala

Syndromes

Classic AChA triad: Contralateral hemiparesis + hemisensory loss + homonymous hemianopia WITHOUT cortical signs
Absence of aphasia/neglect distinguishes AChA from MCA stroke — key board differentiator
Variable presentation due to rich anastomotic network; can present as an isolated lacunar-like syndrome

Middle Cerebral Artery (MCA)

Segments

M1 (horizontal/sphenoidal): Gives off lenticulostriate arteries (6–12 deep penetrators → basal ganglia, posterior limb IC, corona radiata)
M2 (insular): Bifurcates (or trifurcates) into superior and inferior divisions in Sylvian fissure
M3 (opercular): Over the opercula
M4 (cortical): Terminal cortical branches

Territory

Lateral frontal (motor cortex for face/arm, frontal eye fields, Broca’s area)
Parietal (sensory cortex, angular and supramarginal gyri)
Superior temporal (Wernicke’s area) and insula
Deep: putamen, outer globus pallidus, posterior limb IC, corona radiata

Variants

Early bifurcation: M1 bifurcates near its origin — can mimic M2 occlusion on angiography
Accessory MCA: Arises from A1 or ACA — provides additional supply to MCA territory

Syndromes

Complete MCA (M1): Face/arm > leg weakness, hemisensory loss, homonymous hemianopia, aphasia (dominant) or neglect (nondominant), gaze deviation toward lesion
Superior division: Face/arm weakness + Broca’s aphasia (dominant); contralateral lower facial droop
Inferior division: Wernicke’s aphasia or hemispatial neglect, homonymous hemianopia (or superior quadrantanopia), minimal motor deficit
Gerstmann syndrome (dominant angular gyrus — inferior division): Finger agnosia + acalculia + right-left disorientation + agraphia

Lenticulostriate Arteries

Arise from M1 — “arteries of stroke” (Chârcot)
End arteries (no collaterals) → occlusion = lacunar infarcts; rupture = putaminal hemorrhage (most common hypertensive ICH)
Lateral lenticulostriates (from M1) vs. medial lenticulostriates (from A1/ACA — Heubner’s territory)

💎 Board Pearl

MCA is the most commonly affected vessel in ischemic stroke. M1 occlusion with good leptomeningeal collaterals may present with cortical signs (aphasia/neglect) without dense hemiplegia — collaterals rescue cortex but lenticulostriates have no collaterals. Gerstmann syndrome = dominant angular gyrus (inferior MCA division).

Anterior Cerebral Artery (ACA)

Segments

A1: ICA → ACom (gives off medial lenticulostriates)
A2: ACom → pericallosal/callosomarginal bifurcation; gives off recurrent artery of Heubner
A3–A5: Distal cortical branches (pericallosal, callosomarginal)

Territory

Anterior 3/4 of medial hemisphere (leg/foot motor and sensory cortex)
Medial-orbital frontal lobe, anterior cingulate gyrus
Anterior 4/5 of corpus callosum
Heubner’s artery: Head of caudate, anterior limb IC, anterior putamen

Variants

Azygos ACA: Single midline A2 supplies both hemispheres — occlusion → bilateral ACA syndrome
Bihemispheric ACA: One A2 supplies both medial hemispheres (if contralateral A1 hypoplastic)

Syndromes

A1 occlusion: Usually well tolerated (ACom cross-flow). If both ACAs from single A1 or azygos ACA → bilateral ACA infarcts with paraplegia, abulia, akinetic mutism
A2 occlusion: Contralateral leg >> arm/face weakness and sensory loss, urinary incontinence, abulia, alien limb phenomenon, grasp reflex, transcortical motor aphasia (dominant)
Heubner’s occlusion: Contralateral face/arm weakness (caudate head → abulia + behavioral changes)

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