OVERVIEW OF CEREBRAL CIRCULATION
- Brain receives blood predominantly via two major systems.
- Carotid circulation constitutes anterior supply.
- Vertebro-basilar circulation constitutes posterior supply.
- Anastomosis occurs at Circle of Willis.
- Secondary anastomoses occur through smaller leptomeningeal vessels.
- Diencephalon supplied by end-arteries lacking adequate anastomoses; occlusions yield devastating effects.
- Watershed zones lie between major arterial territories; vulnerable to decreased cerebral perfusion pressure rather than focal occlusion.
ANTERIOR CIRCULATION
- Composed of Internal Carotid Artery (ICA) and major branches.
- Supplies ipsilateral brain hemispheres.
Internal Carotid Artery (ICA)
- Perfuses optic nerve and retina via ophthalmic artery branch.
- Bifurcates into Middle Cerebral Artery and Anterior Cerebral Artery.
Middle Cerebral Artery (MCA)
- Proximal segment (M1) gives rise to lenticulostriate arteries.
- M1 segment bifurcates in sylvian fissure into superior and inferior divisions (M2 branches).
- Cortical branches supply lateral hemispheric surface, sparing frontal pole, superomedial border, lower temporal/occipital poles.
MCA Vascular Territories
| Artery / Segment | Structures Supplied |
|---|
| Lenticulostriate Arteries | Putamen, outer globus pallidus, posterior limb internal capsule, adjacent corona radiata, caudate nucleus (majority). |
| Superior Division (M2) | Frontal cortex, superior parietal cortex. |
| Inferior Division (M2) | Inferior parietal cortex, temporal cortex. |
Anterior Cerebral Artery (ACA)
- Divided into precommunal (A1) and postcommunal (A2) segments.
- A1 connects ICA to anterior communicating artery.
- A2 located distal to anterior communicating artery.
ACA Vascular Territories
| Artery / Segment | Structures Supplied |
|---|
| Precommunal (A1) Deep Branches | Anterior limb internal capsule, anterior perforate substance, amygdala, anterior hypothalamus, inferior head of caudate nucleus. |
| Postcommunal (A2) Cortical Branches | Frontal pole, superomedial border of frontal and parietal lobes. |
Anterior Choroidal Artery
- Arises directly from ICA.
- Supplies posterior limb of internal capsule.
- Supplies white matter posterolateral to internal capsule (geniculocalcarine fibers).
POSTERIOR CIRCULATION
- Composed of paired vertebral arteries, basilar artery, paired Posterior Cerebral Arteries (PCA).
- Vertebral arteries join at pontomedullary junction forming basilar artery.
Vertebral Arteries
- Consist of four segments (V1-V4).
- V1: Origin to C6 transverse foramen.
- V2: Traverses foramina C6 to C2.
- V3: Traverses C1, pierces dura at foramen magnum.
- V4: Ascends to join contralateral vertebral artery.
- V4 gives rise to Posterior Inferior Cerebellar Artery (PICA).
- PICA supplies lateral medulla, inferior cerebellar surface.
Basilar Artery
- Formed by joined V4 segments.
- Supplies base of pons, superior cerebellum.
- Divides into distinct branch groups.
Basilar Artery Branches
| Branch Type | Quantity | Structures Supplied |
|---|
| Paramedian Branches | 7-10 | Wedge of pons on either side of midline. |
| Short Circumferential Branches | 5-7 | Lateral two-thirds of pons, middle and superior cerebellar peduncles. |
| Bilateral Long Circumferential | Paired | Cerebellar hemispheres (via Superior Cerebellar Artery, Anterior Inferior Cerebellar Artery). |
Posterior Cerebral Artery (PCA)
- Originates from basilar artery bifurcation (75% cases).
- Originates from ipsilateral ICA via posterior communicating artery (20% cases).
- Originates bilaterally from ICA (5% cases - termed fetal PCA).
- Segmented into P1 (precommunal) and P2 (distal to posterior communicating artery).
PCA Vascular Territories
| Artery / Segment | Structures Supplied |
|---|
| P1 Penetrating Branches | Midbrain, subthalamus, thalamus (via thalamogeniculate, artery of Percheron, posterior choroidal arteries). |
| P2 Cortical Branches | Medial temporal lobes, occipital lobes, calcarine cortex, visual association areas. |
SMALL-VESSEL / PENETRATING MICROCIRCULATION
- Originate from MCA stem, Circle of Willis (A1, anterior/posterior communicating, P1 segments), basilar, vertebral arteries.
- Vessel size ranges 30-300 micrometers.
- Penetrate deep gray and white matter of cerebrum and brainstem.
- Highly susceptible to atherothrombotic occlusion or lipohyalinotic thickening causing lacunar infarcts.
REGIONAL BRAINSTEM BLOOD SUPPLY
- Extensive overlapping arterial supply dictates distinct clinical stroke syndromes based on specific occlusions.
Medullary Supply
- Medial Medulla: Branches of vertebral or lower basilar artery.
- Lateral Medulla: Vertebral artery, Posterior Inferior Cerebellar Artery (PICA), superior/middle/inferior lateral medullary arteries.
Pontine Supply
- Inferior Pons (Medial): Paramedian branch of basilar artery.
- Inferior Pons (Lateral): Anterior Inferior Cerebellar Artery (AICA).
- Mid Pons (Medial): Paramedian branch of midbasilar artery.
- Mid Pons (Lateral): Short circumferential artery.
- Superior Pons (Medial): Paramedian branches of upper basilar artery.
- Superior Pons (Lateral): Superior Cerebellar Artery.
Midbrain Supply
- Medial Midbrain: Paramedian branches of upper basilar and proximal PCA.
- Lateral Midbrain: Small penetrating arteries arising from PCA.
