Anterior Circulation (ICA, MCA, ACA)

The anterior cerebral circulation supplies the front two-thirds of each cerebral hemisphere and most of the deep gray nuclei. Roughly two-thirds of all ischemic strokes occur in this territory. The patterns of stroke in each artery’s distribution are clinically distinctive, and recognizing them at the bedside often allows the artery to be named before imaging confirms it. This page covers the internal carotid artery and its major branches — the middle cerebral, anterior cerebral, anterior choroidal, and ophthalmic arteries — and the syndromes they produce.

The Internal Carotid Artery

The internal carotid artery (ICA) arises from the common carotid bifurcation in the neck. It ascends through the carotid canal into the cranium, then through the cavernous sinus, and finally enters the subarachnoid space adjacent to the optic chiasm to give off its terminal branches. By convention, the ICA is divided into seven segments (Bouthillier classification):

  • C1 cervical: from bifurcation to entry into the carotid canal.
  • C2 petrous: within the petrous bone.
  • C3 lacerum: brief segment passing through foramen lacerum.
  • C4 cavernous: within the cavernous sinus, with characteristic siphon configuration.
  • C5 clinoid: brief transition.
  • C6 ophthalmic: gives off the ophthalmic artery and superior hypophyseal artery.
  • C7 communicating: gives off posterior communicating and anterior choroidal arteries before bifurcating into ACA and MCA.

Ophthalmic Artery

Arises from the ICA just as it enters the subarachnoid space, passes through the optic canal with the optic nerve, and supplies the retina (central retinal artery), choroid, and orbital structures. Occlusion produces ipsilateral monocular blindness or amaurosis fugax (transient monocular blindness, classically a warning of impending hemispheric stroke).

Posterior Communicating Artery (PComA)

Connects ICA to posterior cerebral artery (PCA), participating in the Circle of Willis. Berry aneurysms at the PComA-ICA junction are common; compression of the adjacent CN III by an enlarging or rupturing PComA aneurysm produces a pupil-involving third nerve palsy — a neurosurgical emergency.

Anterior Choroidal Artery

A small but clinically important branch arising from the ICA distal to PComA. Supplies the posterior limb of the internal capsule, the optic tract, parts of the thalamus, and the choroid plexus. Anterior choroidal infarction produces a characteristic triad of contralateral hemiparesis (internal capsule), contralateral hemisensory loss (thalamus), and contralateral homonymous hemianopia (optic tract).

The Middle Cerebral Artery (MCA)

The largest terminal branch of the ICA. Carries about 80% of carotid flow. Divided into:

  • M1 (horizontal segment): from the carotid bifurcation to the limen insula. Gives off the lenticulostriate arteries — small perforating vessels supplying the basal ganglia and internal capsule.
  • M2 (insular segment): passes over the insula. Typically bifurcates into superior and inferior divisions.
  • M3 (opercular segment): at the Sylvian fissure, distributing over the operculum.
  • M4 (cortical segment): the terminal cortical branches over the lateral hemisphere.

Cortical Territory

The MCA supplies most of the lateral surface of the hemisphere:

  • Lateral frontal lobe (including Broca area, motor cortex face and arm regions).
  • Lateral parietal lobe (including Wernicke area on dominant side).
  • Lateral temporal lobe (most of the superior and middle temporal gyri).
  • Insula.

Deep Territory (Lenticulostriate Arteries)

The lenticulostriate arteries are small perforating vessels from the M1 segment, supplying:

  • Putamen.
  • Lateral globus pallidus.
  • Caudate body and tail.
  • Posterior limb of internal capsule.

These small vessels are vulnerable to hypertensive damage. Lacunar infarcts here produce small, focal deficits (pure motor stroke, sensorimotor stroke, dysarthria-clumsy hand syndrome). Hypertensive intracerebral hemorrhage in the putamen is the most common location for spontaneous intracerebral hemorrhage.

MCA Stroke Syndromes

Proximal M1 Occlusion

Affects the entire MCA territory — cortical plus deep. Contralateral hemiplegia (face, arm, leg) from involvement of both the motor cortex and the posterior limb of internal capsule. Contralateral hemisensory loss. Contralateral homonymous hemianopia (involvement of optic radiations or visual cortex). Dominant side: global aphasia. Non-dominant: hemineglect and anosognosia. Often with conjugate gaze deviation toward the lesion (frontal eye field involvement on intact side).

Superior Division MCA Stroke

Affects frontal MCA territory. Contralateral hemiparesis predominantly affecting face and arm (the leg is supplied by ACA). Broca aphasia if dominant side. Hemineglect if non-dominant.

Inferior Division MCA Stroke

Affects posterior MCA territory (temporal and parietal). Contralateral homonymous hemianopia or superior quadrantanopia. Wernicke aphasia if dominant side. Constructional apraxia and neglect if non-dominant. Less prominent motor weakness.

Deep MCA Stroke (Lenticulostriate Infarct)

Small lacunar infarct producing focal syndromes:

  • Pure motor stroke: contralateral face, arm, leg weakness without other findings. Posterior limb of internal capsule.
  • Sensorimotor stroke: motor and sensory loss together, no cortical features.
  • Dysarthria-clumsy hand: facial weakness, dysarthria, clumsiness of contralateral hand. Posterior limb of internal capsule or basis pontis.

The Anterior Cerebral Artery (ACA)

Smaller than MCA. Divided into:

  • A1: from carotid bifurcation to anterior communicating artery (AComA).
  • A2: from AComA to the genu of the corpus callosum. Gives off the recurrent artery of Heubner (supplying head of caudate, anterior limb of internal capsule).
  • A3 (pericallosal): along the corpus callosum.
  • A4, A5: terminal cortical branches.

Territory

  • Medial frontal lobe.
  • Medial parietal lobe (including paracentral lobule with leg motor and sensory cortex).
  • Anterior corpus callosum.
  • Anterior limb of internal capsule (via recurrent artery of Heubner).
  • Head of caudate (via recurrent artery of Heubner).

ACA Stroke Syndromes

  • Contralateral leg weakness (greater than face or arm) from paracentral lobule involvement.
  • Sensory loss in the leg.
  • Abulia, apathy, akinetic mutism (with bilateral involvement or anterior cingulate damage).
  • Urinary incontinence from medial frontal involvement.
  • Transcortical motor aphasia if dominant side (supplementary motor area).
  • Alien hand syndrome (callosal type) from corpus callosum involvement.

The Circle of Willis

The arterial ring at the base of the brain connecting anterior and posterior circulations. Components:

  • Two anterior cerebral arteries (A1 segments).
  • Anterior communicating artery (AComA) connecting them.
  • Two internal carotid arteries (their terminal segments).
  • Two posterior communicating arteries connecting ICA to PCA.
  • Two posterior cerebral arteries (P1 segments).
  • Basilar artery feeding the PCAs.

The Circle of Willis provides collateral flow that can compensate for occlusion of one feeding artery. Variations are extremely common — a “textbook” complete circle is found in only a minority of brains. Hypoplastic A1, fetal-type PCA (PCA arising mainly from ICA via large PComA), and other variations affect collateral capacity and clinical patterns.

Berry aneurysms preferentially form at branch points of the Circle of Willis. The most common locations: anterior communicating artery (~35%), posterior communicating artery (~30%), middle cerebral bifurcation (~20%).

🔍 Did You Know?

The classical observation that MCA strokes affect face and arm more than leg reflects the somatotopic organization of the motor cortex projected onto the vascular territory. The face and arm representations of motor cortex are on the lateral surface of the hemisphere (MCA territory); the leg representation extends onto the medial surface (ACA territory). A pure cortical MCA stroke therefore produces face and arm weakness with relative leg sparing, while a pure ACA stroke produces leg weakness with relative face and arm sparing. Stroke involving the internal capsule (lenticulostriate territory) affects face, arm, and leg roughly equally because all the corticospinal fibers are tightly packed together at that level. Recognizing these patterns at the bedside often allows the artery to be named within seconds of examining the patient.

Pitfalls and Pearls

  • Cortical MCA stroke spares the leg; ACA stroke spares the face and arm; capsular stroke affects all three equally.
  • Conjugate gaze deviation toward the lesion in acute stroke from frontal eye field involvement (contralateral FEF still working).
  • Painful pupil-involving third nerve palsy is a PComA aneurysm until proven otherwise. Neurosurgical emergency.
  • Anterior choroidal infarction produces a triad of contralateral hemiparesis, hemisensory loss, and homonymous hemianopia.
  • Lenticulostriate infarcts produce lacunar syndromes: pure motor, sensorimotor, dysarthria-clumsy hand.
  • Anterior communicating artery aneurysms are the most common site for aneurysmal SAH.
  • Putaminal hemorrhage is the most common location for hypertensive ICH.
  • Amaurosis fugax is a warning of impending stroke. Always investigate carotid disease.
  • Watershed infarcts are at MCA-ACA or MCA-PCA borders, often from severe hypotension or severe carotid stenosis.

References

  1. Caplan LR. Caplan’s Stroke: A Clinical Approach. 5th ed. Cambridge University Press; 2016.
  2. Mohr JP, Wolf PA, Grotta JC, et al, eds. Stroke: Pathophysiology, Diagnosis, and Management. 6th ed. Elsevier; 2016.
  3. Bouthillier A, van Loveren HR, Keller JT. Segments of the internal carotid artery: a new classification. Neurosurgery. 1996;38(3):425-432.
  4. Brazis PW, Masdeu JC, Biller J. Localization in Clinical Neurology. 7th ed. Wolters Kluwer; 2017.
  5. Standring S, ed. Gray’s Anatomy. 42nd ed. Elsevier; 2021.