Internal carotid artery

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

Gross anatomy

Origin

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).

Variations in origin

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%

Asymmetry

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%

Course

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

It first turns 90 degrees anteromedially within the carotid canal to run through the petrous temporal bone. It then proceeds to exit the carotid canal and turn 90 degrees superiorly within the carotid sinus and finally another 90 degree 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 degree turn superiorly and then posteriorly to pass medial to the anterior clinoid process. At this point it divides into the middle and anterior cerebral branches and gives off other smaller branches, such as the anterior choroidal artery and the posterior communicating artery.

Segments

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:

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

Branches

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.

• C1: cervical segment, none
• C2: petrous (horizontal) segment
  • caroticotympanic artery
  • Vidian artery
• C3: lacerum segment, none
• C4: cavernous segment
  • meningohypophyseal trunk
  • inferolateral trunk
  • capsular arteries (of McConnell)
• C5: clinoid segment, none
• C6: ophthalmic (supraclinoid) segment
  • ophthalmic artery
  • superior hypophyseal artery
• C7: communicating segment
  • posterior communicating artery
  • anterior choroidal artery
  • anterior cerebral artery
  • middle cerebral artery

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

Variant anatomy

• aberrant ICA
• congenital absence of the ICA
• retropharyngeal ICA (rare) ⁴
• kissing carotids
• persistent carotid-vertebrobasilar anastomoses
• lateralised internal carotid artery

Related pathology

• ICA dissection
• carotid artery pseudoaneurysm
• carotid stenosis
• carotid web
• free-floating thrombus of the internal carotid artery