Internal carotid artery

The internal carotid artery 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%

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, whereas the even numbered segments (C2, C4, C6) each have two branches.

• C1: cervical segment, none
• C2: petrous (horizontal) segment
  • caroticotympanic artery
  • vidian artery
• C3: lacerum segment, none
• C4: cavernous segment
  • meningohypophyseal trunk
  • inferolateral trunk
• 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

A useful mnemonic to remember the branches of the internal carotid artery is:

• A VIP'S COMMA

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