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). Just superior to its origin, the ICA has a slight dilatation in is the location of the carotid sinus and body.

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 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 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 thena final 90 degree 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 2  smaller branches, 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) (variable)
• 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

Usefulmnemonics 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