Internal carotid artery (ICA)

C1 - Cervical segment

Extends from the bifurcation of the common carotid artery at C3-C5 level to the carotid canal at the base of the temporal bone.


C2 - Petrous segment

Courses within the carotid canal of the temporal bone. Turns anteromedially forming a genu in front of the cochlea. Exits the carotid canal at the petrous apex.


Branches:

  1. Caroticotympanic artery
    • Anastomoses with branches of ECA:
      • Inferior tympanic artery (from the ascending pharyngeal artery).
      • Stylomastoid artery (from the posterior auricular or occipital artery).
      • Deep auricular or anterior tympanic arteries (from the maxillary artery).
    • Supplies:
      • Mucosa lining of anterior wall of tympanic cavity.
      • Contributes to the tympanic plexus supplying the middle ear.
      • Eustachian tube
      • Bony walls of the carotid canal and tympanic cavidy.

Anatomic variants:

  • Aberrant ICA courses more posterolaterally and appears as a mass in hypotympanum. May be mistaken for glomus tympanicum tumor.
  • Persistent stapedial artery. Arises from the petrous portion of the internal carotid artery and crosses through the middle ear.

C3 - Lacerum segment

Extends from petrous apex to cavernous sinus. Runs above the foramen lacerum.


Branches:
  1. Vidian artery (artery of the pterygoid canal)
    • Enters the vidian canal and exits at the pterygopalatine fossa.
    • Anastomoses with maxillary artery.
    • May sometimes arise from C2 segment of ICA.
    • Supplies:
      • Pterygoid canal structures including the vidian nerve.
      • Adjacent skull base and sphenoid region.
      • Contributes to sphenoid sinus, palatine bone and sometimes nasopharynx.
      • May contribute to upper pharynx, pharyngeal muscles and Eustachian tube.

C4 - Cavernous segment

Lies within the cavernous sinus. Divided into posterior, horizontal and anterior segments. The trigeminal ganglion lies posteriorly to C4 segment. Abducens nerve lies inferolaterally to it.


Branches:


  1. Meningohypophyseal trunk (MHT)
    • Arises from posterior genu of C4.
    • May be a single or several trunks.
    • Branches:
      1. Inferior hypophyseal artery - supplies posterior pituitary.
      2. Lateral clival arcade - anastomoses with jugular division branches of ascending pharyngeal artery.
      3. Medial clival arcade - anastomoses with hypoglossal division branches of ascending pharyngeal artery.
      4. Marginal tentorial arcade - supplies free edge of the tentorium cerebelli.
      5. Lateral tentorial arcade - around the superior petrosal sinus along the petrous ridge.
      6. Deep recurrent meningeal arcade - anastomoses between MHT and ILT in the margin between anterior and posterior clinoids.
      7. Anastomoses with the trigeminal artery.

  1. Inferolateral trunk (ILT)
    • Arises laterally from the horizontal part of C4 as a single or multiple trunks.
    • Supplies cavernous sinus dura and cranial nerves III, IV, V1, V2, VI, branches pass through foramen ovale and rotundum.
    • Forms multiple anastomoses with branches of maxillary artery, middle and accessory middle meningeal artery, MHT, ophthalmic artery and jugular and hypoglossal divisions of ascending pharyngeal artery.

Anatomic variants:

  • Persistent trigeminal artery: persistent fetal carotid-vertebrobasilar anastomosis. There are three variants according to the Saltzman classification:
    • Type I: the trigeminal artery supplies the distal vertebrobasilar arteries and the ipsilateral PCom is hypoplastic or absent.
    • Type II: the trigeminal artery supplies both superior cerebellar arteries and the PCAs are supplied by the PCom on both sides.
    • Type III: one PCA is supplied by the ipsilateral PCom, the other PCA and both SCAs are supplied by the trigeminal artery.

C5 - Clinoid segment

Segment of ICA that exits cavernous sinus and runs to the subarachnoid space near the anterior clinoid process.


The ICA passes through two dural rings as it transitions from the cavernous sinus to the subarachnoid space:

  1. Proximal dural ring: at the roof of the cavernous sinus.
  2. Distal dural ring: forms the roof of the clinoid segment and is the true dural boundary between the extradural clinoid ICA and the intradural ophthalmic ICA.

The clinoid ICA does not give off any major branches. Small meningeal branches may arise as anatomic variation supplying anterior clinoid process and adjacent dura, or pituitary capsule.


C6 - Ophthalmic segment

Extends from superior to the anterior clinoid process to proximal to the posterior communicating artery (PCom).


Branches:
  1. Ophthalmic artery
    • Originates anterosuperiorly from ICA. Runs through the optic canal.
    • Branches:
      1. Central retinal artery - terminal branch without any collaterals. Enters the optic nerve sheath approximately 10–15 mm posterior to the globe and runs centrally in the terminal portion of the optic nerve. Enters the globe at the optic disk where it branches radially to supply the inner retinal layers.
      2. Short posterior cilliary arteries - pierce the sclera near the optic nerve and supply the choroid, optic nerve head, and posterior sclera.
      3. Long posterior cilliary arteries - pierce the sclera near the optic nerve and supply the anterior choroid, ciliary body, and iris.
      4. Anterior cilliary arteries - branch off from muscular branches of the ophthalmic artery and course alongside the rectus muscles, piercing the sclera near the limbus. Form the major arterial circle of the iris, along with the long posterior ciliary arteries. Supply the conjunctiva, sclera, ciliary body and iris and contribute significantly to the limbal vascular arcade.
      5. Lacrimal artery - collateralize with maxillary artery anterior deep temporal branches through transosseous and muscular routes.
      6. Anterior ethmoidal artery - runs through the anterior ethmoid foramen. Supplies:
        • Nasal cavity (upper nasal septum and lateral nasal wall)
        • Dorsum of the external nose
        • Medial orbital wall
        • Anterior ethmoid
        • Dura of the anterior cranial fossa
        • Anastomoses with posterior ethmoidal artery, sphenopalatine artery, greater palatine artery, facial artery, and dorsal nasal artery.
        • Contributes to Kiesselbach’s plexus on the anterior nasal septum (a common site of epistaxis).
      7. Posterior ethmoidal artery - runs through the posterior ethmoid foramen. Supplies:
        • Nasal cavity (superior portion of the nasal septum and superior nasal concha).
        • Posterior ethmoid
        • Superior part of the sphenoid sinus
        • Dura of the posterior part of the anterior cranial fossa, near the cribriform plate.
        • Anastomoses with anterior ethmoidal artery, sphenopalatine artery, and occasionally with branches from the posterior lateral nasal arteries.
      8. Recurrent tentorial branch - collateral network with anteromedial branch of ILT, connecting orbit to middle cranial fossa.
      9. Recurrent meningeal branch - branch from the lateral (lacrimal) division which can collateralize with ophthalmic branch of the middle meningeal artery.
      10. Anterior frontal meningeal branch - supplies the dura of the frontal convexity.
      11. Inferior branches - supply orbital soft tissues, can collateralize with branches from maxillary artery and angular artery (terminal branch of facial artery).

  1. Superior hypophyseal artery. Arises medially or superomedially just distal to the distal dural ring, within the subarachnoid space and there are often multiple small branches rather than a single artery. Runs posteromedially.
    • Supplies:
      • Anterior pituitary
      • Infundibulum
      • Sellar diaphragm and adjacent dura over the pituitary
      • Cisternal portion of the optic nerve
      • Superior aspect of optic chiasm
      • Median eminence of the hypothalamus

C7 - Communicating segment

Terminal segment extending from the origin of the posterior communicationg artery. Passes between optic nerve (inferolaterally to it) and oculomotor nerve (inferomedially to it).


Branches:
  1. Posterior communicating artery (PCom)
    • Part of the circle of Willis, it forms an anastomosis between ICA and posterior cerebral artery
    • Passes inferior to the optic tract and optic chiasm, often crosses over or near the oculomotor nerve
    • Perforating branches from PCom supply:
      • Optic chiasm and tract
      • Mammillary bodies and posterior hypothalamus
      • Part of thalamus
      • Subthalamus
      • Part of the cerebral peduncle
    • Common anatomic variant: fetal PCom. The PCom is larger than the P1 segment of the posterior cerebral artery. About 25% prevalence.

  1. Anterior choroidal artery
    • Originates from the posterior wall of the ICA between the PCom and the ICA termination. Courses laterally and inferior to the optic tract, runs posterolaterally in the perimesencephalic cistern above the uncus and enters the choroidal fissure.
    • Supplies:
      • Optic tract
      • Posterior limb of internal capsule
      • Tail of caudate nucleus
      • Posteromedial part of the putamen and lateral part of globus pallidus
      • Lateral geniculate nucleus and lateral thalamus
      • Amygdala and part of anterior hippocampus
      • Lateral aspect of cerebral peduncle
      • Choroid plexus of the temporal horn

  1. Anterior cerebral artery

  1. Middle cerebral artery