ADD-ONS / ANATOMY

Created 10/11/2022, last revision 12/01/2023

Extracranial cerebral arteries

• ICA usually courses posterior, lateral, or posterolateral to the ECA
• the ICA enters the skull via the carotid canal (C1, extracranial segment)
• it turns anteromedially as the C2 (petrous) segment  to run through the petrous temporal bone
  • in the petrous part of the temporal bone, the artery runs medially
  • it gives off small branches to the middle ear cavity (caroticotympanic arteries)
  • caroticotympanic arteries anastomose with the anterior tympanic branch of the internal maxillary and with the posterior tympanic branch of the stylomastoid artery
• ICA exits the carotid canal superiorly (C3, lacerum segment)
• later it turns anteriorly and enters the medial part of the cavernous sinus (C4, cavernous segment)
• then ICA turns superiorly (C5, clinoid segment)
• finally, it turns posteriorly along the anterior clinoid process and continues as a C6 segment (ophthalmic)
  • ophthalmic artery arises in this segment and enters the orbit together with the optic nerve (via the optic canal)
• the terminal ICA (C7 segment) begins proximal to the origin of the PComA
  • distal to the ophthalmic artery, the posterior communicating artery (PComA) and anterior choroidal artery arise  (AChA may originate from MCA)
  • the terminal segment divides into two arteries – anterior cerebral anterior (ACA) and middle cerebral artery (MCA)
• there are several classifications of ICA segments (see tab) – preferably use descriptive terms instead of simple labels (C1-7)

→ see complete anatomy at KENHUB

• middle cerebral artery (MCA) is (together with ACA) a terminal branch of the internal carotid artery
• the MCA supplies many deep brain structures, the majority of the lateral surface of the cerebral hemispheres, and the temporal pole of the brain
• MCA travels from the base of the brain through the lateral sulcus (of Sylvius) and terminates on the lateral surface of the brain; the surgical classification divides the middle cerebral artery into four segments (M1-M4)
  • M1  (sphenoidal/horizontal) segment – originates at the terminal bifurcation, travels along the posterior margin of the lesser wing of sphenoid bone (ala minor ossis sphenoidal), and usually has 2 terminal branches
    • MCA gives off small arteries penetrating the anterior perforated substance (substantia perforata anterior) – central branches/lateral lenticulostriate arteries, which supply the anterior part of the thalamus,  internal capsule, and the basal ganglia
    • MCA communicates with the posterior cerebral artery (PCA) via the posterior communicating artery (PComA)
    • originally the MCA genu was determined as the end of the M1 segment (bifurcation usually starts here but can be placed more distally) = it is more practical to use bifurcation as a distal border of the M1 segment
  • M2 (insular) segment begins at the point of MCA bifurcation (or genu) and comprises a superior and inferior trunk. It terminates at the level of the circular sulcus of the insula
  • M3 (opercular) segment – ascends toward the surface of the brain. It courses over the inner surfaces of the parietal and temporal portions of the insular operculum and reaches the surface of the Sylvian fissure
  • M4 (terminal, cortical) segment – starts at the external surface of the Sylvian fissure and travels over the surface of the cerebral hemisphere

• the anterior cerebral artery (ACA) is a terminal branch of the ICA
• ACA supplies the medial aspect of the cerebral hemispheres back to the parietal lobe
• the left and right anterior cerebral arteries are connected by the anterior communicating artery (AComA), forming the anterior portion of the circle of Willis
• ACA is divided into 5 segments:
  • A1 segment (horizontal/pre-communicating) originates from the internal carotid artery and extends to the AComA
    • medial lenticulostriate arteries – arise from the A1 segment and supply the globus pallidus and medial portion of the putamen
      • shorter, thinner, and fewer in number than the lateral lenticulostriate arteries, which arise from the M1 segment of MCA
        
      • should not be confused with the perforating branches from the A1/A2 intersection (including the recurrent artery of Heubner)
    • the recurrent artery of Heubner (RAH; AKA the medial striate artery) is the largest perforating branch routinely seen on angiography
      • it may arise distal to the AComA, at the level of the AComA, or less frequently proximal to the AComA
      • 0.8 mm in diameter
      • it supplies the head of the caudate nucleus, the medial portion of globus pallidus, the anterior crus of the internal capsule, the anterior hypothalamus, the basal nucleus of Meynert
      • its occlusion can occur during clipping of the AComA aneurysm
        
      • variants: absent or duplicated/triplicated/quadruplicated
  • A2 (infracallosal) segment – extends from the AComA along the rostrum of the corpus callosum to the genu
    • orbitofrontal artery  arises near AComA
    • frontopolar artery
      
  • A3  (precallosal) segment – extends around the genu of the corpus callosum and bifurcates into callosomarginal and pericallosal arteries (continuation of ACA)
    • callosomarginal artery can arise anywhere along the A1-A4 segments; it further branches into the medial frontal arteries (anterior, intermediate, posterior), and the paracentral artery  
      
    • sometimes A2 segment is very short and  the pericallosal and callosomarginal trunks arise just distal to the AComA;  it may look like an ACA “quadrifurcation”
  • 4/5 segment (pericallosal artery) – continues above the body of the corpus callosum

• unpaired artery, formed by the confluence of the vertebral arteries (V4 segments)
• localized in the pontine basilar sulcus
• branches:
  • anterior inferior cerebellar artery (AICA)
  • labyrinthine artery (only in 15%) – accompanies n. VIII. through the internal acoustic meatus to the inner ear; most commonly (in 85% of cases), it originates from AICA
  • superior cerebellar artery (SCA) – arises from the rostral portion of the basilar artery, just below the PCA
    
  • perforating arteries supplying the brainstem
• BA terminates by branching into posterior cerebral arteries (PCAs)

• posterior cerebral artery (PCA) is a terminal branch of the basilar artery
• PCA crosses the oculomotor nerve, curves around the cerebral peduncle, and goes above the tentorium to supply the inferomedial temporal lobe and the occipital lobe (including the visual cortex)
  • in the initial segments, it gives off branches to the middle part of the mesencephalon, a substantial part of the thalamus, and the lateral geniculate nucleus  
  • the macular part of the visual cortex is usually supplied from both PCA and MCA
• P1 segment (pre-communicating)
  • originates at the termination of the basilar artery and ends at the origin of the PComA
  • occlusion of the P1 segment (proximal to PComA) leads to the infarction of the cerebral peduncle, thalamus, medial temporal, and occipital lobes
• P2 segment (post-communicating)
  • terminates as it enters the quadrigeminal cistern
  • peduncles and thalamus are spared in occlusions localized in the distal P2 segment  
    
• P3 segment (quadrigeminal)​ and P4 segment (cortical)
  

1 – thalamo-subthalamic arteries (paramedian branches), 2 – posteromedial choroidal arteries, 3 – thalamogeniculate artery, 4 – posterolateral choroidal arteries, 5 – polar (thalamotuberal) artery, 6 – temporal branches, 7 – occipitotemporal artery , 8 – parieto-occipital artery and calcarine artery

• hypoplasia (usually on the right side) is a common variant of the norm 
• significant branches:
  • PICA (can originate from BA as well)
  • anterior and posterior spinal arteries
• atherosclerosis typically involves the V0 and V4 segments
• dissection commonly affects the V3 segment