Internal carotid artery

Dr Jeremy Jones et al.

The internal carotid artery is a terminal branch of the common carotid artery

Gross anatomy


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%

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%

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

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.

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

Variant anatomy

Related pathology

Related articles

Head and neck anatomy

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