Internal carotid artery (Anterior circulation)

General

Bouthlier classification of ICA

View of the ICA after sylvian fissure split following pterional craniotomy showing major arterial vessels at base of brain

Cervical C1

No branches

Separation: External orifice of carotid canal

Petrous C2

Caroticotympanic artery

Embryonic hyoid artery remnant
Aberrant course can cause a retrotympanic pulsating mass
Anastomosis with inferior tympanic artery (branch of ascending pharyngeal artery)

Vidian artery

Artery of the pterygoid canal
Travel through vidian canal and foramen laceraum
Anastomosis with ECA

Separation: Foramen lacerum

Lacerum C3

Intercavernous C4

ICA enters the posterior part of the cavernous sinus, where it runs forwards.

ICA is surrounded by sinus endothelium and supported by trabeculae

Carotid siphon: U-shaped section of the ICA, formed from mostly the cavernous portion

Segments

Ascending Portion

Posterior Genu

Horizontal Portion

Anterior Genu

3 branches

Meningohypophyseal trunk (largest branch) (A)

From ascending portion

Branches

Dorsal meningeal artery (clival branch): CN6 + part of clivus

Main branches

Lateral Tentorial Artery (B)

Along the lateral edge of the tentorium,
In hemodynamic balance with

Petrosquamosal branches (L) of the Middle meningeal Artery AND
Occipital Artery (O)

Marginal Tentorial Artery (C) a.k.a. Tentorial artery of Bernasconi and Cassinari along the free edge;
Inferior Hypophyseal Artery (D), branches into:

Hypophyseal branches (E)

Supplying the posterior and parts of anterior pituitary
Anastomosing with each other

Inferior Clival Branch (F)

Descending along the dorsum sella
It is in hemodynamic balance with the ascending clival braches (K) of the Ascending Pharyngeal

Lateral Clival artery (G), branching into:

Lateral Branch of the Lateral Clival artery (H)

Runs alongside the Superior Petrosal Sinus

Medial Branch of the Lateral Clival Artery (I)

Runs alongside the Inferior Petrosal Sinus
Is in haemodynamic balance with Jugular branches (J) of the Ascending Pharyngeal

Dorsal meningeal artery (clival branch): CN6 + part of clivus

Only half the tent is drawn here. The checkered vessel connecting MHT with the Basilar Artery (P) is the Trigeminal Artery, which may come off the MHT or ILT.

Inferior view of the pituitary after the bony sella has been removed

Inferolateral trunk (Aka: artery of the inferior cavernous sinus)

From anterior genu
Anastomosis with

Maxillary artery ( through artery of foramen rotundum
MMA + Ophthalmic artery

Travels/supply to: floor of the middle cranial fossa /cranial nerves of Cavernous sinus

inferior lateral cavernous sinus wall
Tentorium (via tentorial branch)
CN III,IV, VI and Gasserian (trigeminal) ganglion (via Foramen Ovale/spinosum to supply)

It is in balance with middle and accessory meningeal arteries

I, McConnell's capsular arteries

Medial trunk
McConnell's capsular arteries

Supplies

Sellar region, in particular, to the sella dura mater and the anterior pituitary capsule

Branches into

Anterior capsular artery
Inferior capsular artery

Courses inferior

Least constant of the three major branches (30% present) of the intercavernous ICA

Endoscopic anterior view of the sellar region and left internal carotid artery and cavernous sinus

Separation: Proximal dural ring

Clinoidal segment

The clinoidal carotid segment is a transitional segment between the cavernous sinus before the ICA exits through the distal ring and enters the subarachnoid space

Two rings are fused posteriorly, they are variably separated anteriorly, giving rise to two regions medially and laterally.

The lateral region may be considered to be extradural and extracavernous.
On the medial side, the space between a redundant portion of distal dural ring and ICA is termed the carotid cave.

Although usually extradural, rupture of carotid cave aneurysm extending superiorly out of the cave may result in subarachnoid haemorrhage

Distal dural ring sits inclined posterior-medially and is continuous medially with the falciform ligament and with the dura overlying the superior-medial aspect of the anterior clinoid process

Surgically understanding of this region is important as access to proximal aneurysms at or near the origin of the ophthalmic artery will require an anterior clinoidectomy

Distal ring is not visible radiologically and its location can only be estimated.

Clinically this region is important as the distal dural ring differentiates aneurysms that cause subarachnoid haemorrhage versus those that cause carotico-cavernous fistulae.

Proximal dural ring and distal dural ring arises above and below the anterior clinoid process

Right anterior clinoid is being cut off.

Anatomical Exposure and Regions

The optic strut is exposed in the anterior part of the clinoidal triangle.
The clinoid segment is exposed in the midportion.
The roof of the cavernous sinus is exposed in the posterior part.
The posterior bend of the internal carotid artery (ICA) and the origin of the meningohypophyseal trunk are exposed in the infratrochlear triangle.

Dural Rings and Membranes

The upper margin of the clinoid segment is surrounded by the upper dural ring.

The upper dural ring is formed by the dura extending medially from the upper surface of the anterior clinoid.

The lower dural ring defines the lower margin of the clinoid segment.
The dura on the lower margin of the anterior clinoid is referred to as the carotidoculomotor membrane.

The carotidoculomotor membrane separates the lower surface of the anterior clinoid from the upper surface of the oculomotor nerve.

This membrane extends medially to form the lower dural ring.

Internal Carotid Artery (ICA) and Branches

The meningohypophyseal trunk arises near the posterior bend of the ICA.
The meningohypophyseal trunk gives rise to the tentorial and dorsal meningeal arteries.
The inferolateral trunk arises from the horizontal segment of the intracavernous carotid.

Cranial Nerve Pathways

The abducens nerve passes through Dorello’s canal.
The abducens nerve passes between the lateral surface of the intracavernous carotid and the medial side of the ophthalmic nerve.
The inferolateral trunk passes above the abducens nerve.

0m. ange—. CN Ill > • Me Iin. Tria ptic S Supratroch. Trian rian mromed. Triang. — Lateral view of cavernous sinus.

CN Ill Supratroch. Triang. Men. A. . Hyp. Tr. Ihr h. Triang. Upper Ri CN Il Lower Rin e — Enlarged view of the clinoidal, oculomotor, supratrochlear, and infratrochlear cavernous sinus triangles.

Separation: Distal dural ring

Ophthalmic segment C5

Ophthalmic artery (Has a bayonet shape- can be used to identify on DSA)

Origin

just beyond the distal dural ring
inferior to the optic nerve and anterior clinoid process

enters the orbit through the optic foramen.

1st intracranial ICA branch

0.5% arises from MMA (Meningo-opthalmic artery)

arising from the middle meningeal artery and entering the orbit through the superior orbital fissure may replace the ophthalmic artery
Can lead to blindness if MMA is embolized

Branches

Ocular branches: Supplies retina, choroid of eye, optic nerve

Central Retinal artery
Ciliary artery

Orbital branches

Lacrimal artery: Lacrimal gland, conjunctiva

Recurrent meningeal artery turns backwards through superior orbital fissure to anastomose with MMA

Muscular artery: extraocular muscle, orbital periosteum

Extraorbital branches: Extensive anastomosis with ECA for collateral intracranial flow during proximal ICA occlusion

Supraorbital
Anterior and posterior Ethmoidal

Anterior ethmoidal is a landmark for the frontal sinus

Dorsal Nasal
Medial frontal
Palpebral
Superior trochlear

Aneurysm arising from the ICA distal to the ophthalmic artery: points upwards towards the optic nerve

Superior hypophyseal artery

Several small arteries arsing from posterior medial portion of intradural ICA

Courses under optic nerve

Supplies

Pituitary

Parts

Ant Pituitary
Pituitary stalk
Tuber cinereum

Anastomosis (forms hypophyseal portal system)

With contralateral side
Inferior hypophyseal artery

Optic nerve and chiasm (inferior surface)

Aneurysm

Mimics a pituitary adenoma
arising here points inferiorly and medially
Of note, aneurysms within the clinoid, ophthalmic and hypophyseal region are frequently termed ‘paraopthalmic’ aneurysms, reflecting the difficulty in their angiographic localization.

Communicating C6

Posterior communicating artery

Branches

7 perforator evenly distributed along PComA courses superior medially to supply

Post hypothalamus
Ant Thalamus
Sub thalamic
Post limb of internal capsule

Course

arises from the dorsolateral aspect of the ICA, and connects after approximately 1 cm, to the posterior cerebral artery,
has an entirely cisternal trajectory; however, it is encased in a sleeve of arachnoid membrane
runs posteromedially below the tuber cinereum above the Sella turcica and oculomotor nerve, close to the dura overlying the posterior clinoid process, before piercing the Lilliquist membrane of the interpeduncular cistern to join the posterior cerebral artery.
close proximity to the oculomotor nerve- surgical (involving the pupil) third nerve palsy.

Aneurysm

Arise from post wall of ICA just distal to PComA origin , and point posteriorly towards CN III

Variant: Present in 50% cases

Absent (hypoplastic PComA): 30%

Foetal variant of PCA: 20%

P1 has a smaller diameter than the PComA and the PCA arises predominantly from the ICA
Foetal PCA fail to regress
Occurs 1/3 of hemispheres 25% of people
danger that if anterior circulation is occluded the occipital lobe will be ischaemic

Infundibular: 10%

Looks like an aneurysm but is not
Funnel shaped dilatation observed at junction between PComA and ICA
Infundibular has a vessel at the apex of the infundibular "aneurysm"

Normal

Aneurysmal

Infundibular

Images:

1, corpus callosum
2, fornix column and anterior commissure
3, foramen of munro
4, thalamus
5, aqueduct
6, ACA
7, MCA
8, Anterior choroidal artery (cisternal segment)
9, distal portion of the cisternal segment of the anterior choroidal artery (on the upper portion of the posteromedial surface of the uncus) entering the temporal horn
10, ICA
11, PCOM
12, P2A
13, P1
14, Tegmentum/pons
15, 4th ventricle
16, Rhinal sulcus

1, Acom
2, MCA
3, semilunar gyrus
4, Cisternal segment of anterior choroidal artery
5, inferior choroidal point
6, Posterior medial surface of uncus
7, P2P
8, ambient gyrus
9, P2A
10, ICA
11, PCOM
12, parahippocampal gyrus
13, P1
14, Fusiform gyrus (Collateral gyrus)
15, pons

1, Isthmus of cingulate gyrus
2, P3
3, P3
4, Inferior temporal artery
5, Parahippocampal gyrus
6, collateral gyrus (fusiform gyrus)
7, Rhinal sulcus
*, P2P

Choroidal C7

Anterior choroidal artery (ACh)

Origin

(1–3 mm distant to the PComA origin) usually arises from the posterior aspect of the ICA

Course:

Lateral to optic tract, curves medially to be inferomedial to optic tract --> curve laterally to run along the lateral aspect of the optic tract --> circumvents the cerebral peduncles to reach the lateral geniculate body --> traverses in the posterolateral direction above the uncus to enter the choroidal fissure, at the plexal point.

Plexal point is always constant

Two segments

Cisternal segment:

Extends from its origin until the choroidal fissure; measures ~2.5 cm

Plexal/Intraventricular segment (3-10 perforators)

Plexal point: where anterior choroidal artery enter temporal horn

Supplies

Visual system: Inferior optic chiasm, Posterior portion of optic tract, Optic radiation, Lateral geniculate body
Temporal lobe: Uncus, Parahippocampal gyrus, Amygdala, Choroid plexus, Temporal horn, Atrium
Basal ganglia: Globus pallidus medial, Tail of caudate, Internal capsule (genu)
Diencephalon: Subthalamus, Thalamus (Lateral ventrolateral nucleus, Lateral ventroanterior nucleus)
Midbrain: Middle 1/3 of cerebral peduncle, Upper red nucleus, Substantia nigra

Very rare to have perforating branches outside of origin of anterior choroidal artery

Anastomoses with lateral posterior choroidal artery

Anterior choroidal artery-Clinical

Use to be ligated to tx Parkinson's in the past
Aneurysm: located superior/superiorlaterally to origin of anterior choroidal artery
Stroke → Anterior choroidal artery syndrome: (3H)

Hemisensory loss
Hemiplegia
Homonymous hemianopia

Difference between PCOM vs Anterior choroidal artery

Features	Anterior choroidal	PComA
Origin	More distal	More proximal
Size	Smaller	Larger
Direction of travel	Has a superior hump (plexal point) where it passes through the inferior choroidal point to enter the temporal horn.	Goes up and down the straight back up and usually bifurcates
Relation between each	More lateral	More medial

Terminating

Bifurcates in the area below the anterior perforated substance at the medial end of the sylvian fissure

Into

ACA (Anterior Cerebral Artery)

MCA (Middle cerebral artery)