Internal carotid artery

The internal carotid artery (ICA) is a terminal branch of the common carotid artery

It arises most frequently between C3 and C5 vertebral level, where the common carotid bifurcates to form the internal carotid and the external carotid artery (ECA). Just superior to its origin, the ICA has a slight dilatation in is the location of the carotid sinus and body.

Although the majority arise between C3 and C5 vertebral level, a wide variation exists.

  • C1/2: 0.3%
  • C2/3: 3.7%
  • C3/4: 34.2%
  • C4/5: 48.1%
  • C5/6: 13%
  • C6/7: 0.15%

There may be significant asymmetry between left and right ICA origins:

  • level of bifurcation
    • left higher 50%
    • right higher 22%
    • same height 28%
  • orientation of origin
    • dorsolateral or dorsal aspect of CCA: right 82% and left 94%

The internal carotid artery courses posterior to the ECA after its origin and ascends in the neck within the carotid sheath. As it ascends on the pharyngeal wall and the pharyngobasilar fascia, it is consecutively crossed laterally by the pharyngeal branch of the vagus nerve (CN X), glossopharyngeal nerve (CN IX), and the stylopharyngeus and styloglossus muscles.

The internal carotid artery enters the skull base through the carotid canal, where it begins a series of 90° turns which lead it to eventually terminate as the middle and anterior cerebral arteries.

It first turns 90° anteromedially within the carotid canal to run through the petrous temporal bone. It then proceeds to exit the carotid canal and turn 90° superiorly within the cavernous sinus and finally, another 90° turn anteriorly to travel along the roof of the cavernous sinus, where it grooves the body of the sphenoid. Here the abducens nerve is intimately related to the ICA on its lateral side. At the anterior end of the cavernous sinus, the ICA makes another 90° turn superiorly and a final 90° turn posteriorly to pass medial to the anterior clinoid process. At this point, it divides into the middle and anterior cerebral branches and gives off two smaller branches, the anterior choroidal artery and the posterior communicating artery.

There are several classification systems, the most recent of which was described by Bouthillier et al. in 1996 1 (see below). Their classification system is used clinically by neurosurgeons, neuroradiologists and neurologists and relies on the angiographic appearance of the vessel and histological comparison rather than on the embryonic development.

There are seven segments in the Bouthillier classification, which is the recommended classification to be used in the clinical setting:

  1. cervical segment
  2. petrous (horizontal) segment
  3. lacerum segment
  4. cavernous segment
  5. clinoid segment
  6. ophthalmic (supraclinoid) segment
  7. communicating (terminal) segment

Of only historical significance, the Terminologia Anatomica subdivided the ICA into only four parts. This is the classification more commonly found in general anatomy books:

  1. cervical part ("pars cervicalis")
  2. petrous part ("pars petrosa")
  3. cavernous part ("pars cavernosa")
  4. cerebral part ("parts cerebralis") 

There are seven segments in a proposed endovascular/angiography-driven classification from the NYU Langone Medical Center 2. Although not the preferred classification scheme, it is important to become familiar, as the usage of these terms have become common:

  1. cervical segment
  2. petrous segment
  3. cavernous segment
  4. paraophthalmic segment
  5. posterior communicating segment
  6. (anterior) choroidal segment
  7. terminus segment, which is often referred to as "carotid terminus"

Except for the terminal segment (C7), the odd-numbered segments usually have no branches. The even-numbered segments (C2, C4, C6) often have branches, although they are inconstant and usually small, therefore often not visualized even on high-resolution digital subtraction angiography. The exception is the ophthalmic artery, which is seen in nearly all cases 3.

Useful mnemonics to remember the branches of the internal carotid artery are:

  • A VIP'S COMMA
  • Calming voices make intra-operative surgery pleasurable and almost memorable
Anatomy: Head and neck

Anatomy: Head and neck

Anatomy: Brain

Anatomy: Brain

Article information

rID: 4524
Section: Anatomy
Synonyms or Alternate Spellings:
  • Internal carotid artery (ICA)
  • ICA
  • Internal carotid arteries

ADVERTISEMENT: Supporters see fewers/no ads

Cases and figures

  • Figure 1
    Drag here to reorder.
  • Figure 2: ICA segments
    Drag here to reorder.
  • Figure 3: annotated DSA
    Drag here to reorder.
  • Case 1: retropharyngeal ICA (variant)
    Drag here to reorder.
  • Figure 4: normal COW anatomy
    Drag here to reorder.
  • Updating… Please wait.

     Unable to process the form. Check for errors and try again.

     Thank you for updating your details.