The internal carotid artery (ICA) is one of the two terminal branches of the common carotid artery (CCA) which supplies the intracranial structures. The other terminal branch is the external carotid artery (ECA), which is somewhat larger in caliber than the ICA, and gives off several branches to supply structures within the neck, extracranial head and face.

In most cases, the carotid bifurcation occurs between the levels of the C3 and C5 vertebrae, or between the levels of the hyoid bone and upper border of the thyroid cartilage ⁹. However, there is wide variation: the bifurcation can be higher than the hyoid or C2-3 level and as low as the cricoid cartilage or C6-7 level ⁹. Moreover, the level of the bifurcation may be asymmetric.

Segments

There are several classification systems, the most common of which was described by Bouthillier et al. in 1996 ¹and is integrated into the Terminologia Neuroanatomica ¹⁰. There are seven segments in the Bouthillier classification:

C1: cervical segment
C2: petrous (horizontal) segment
C3: lacerum segment
C4: cavernous segment
C5: clinoid segment
C6: ophthalmic (supraclinoid) segment
C7: communicating (terminal) segment

The labels C1-C7 are not universally recognized and the descriptive terms are preferred for reporting. In addition, sometimes the term "paraclinoid" is used, encompassing both the clinoid and ophthalmic segments ⁸.

An older but simpler classification by Gibo used the following labels ²:

C1: extracranial segment
C2: petrous segment
C3: cavernous segment (terminates at distal dural ring)
C4: supraclinoid segment (entire intradural portion, with 3 subsegments)
- ophthalmic
- communicating
- choroidal

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

cervical part ("pars cervicalis")
petrous part ("pars petrosa")
cavernous part ("pars cavernosa")
cerebral part ("pars 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:

cervical segment
petrous segment
cavernous segment
paraophthalmic segment
posterior communicating segment
(anterior) choroidal segment
terminus segment, which is often referred to as "carotid terminus"

The term "carotid siphon" is sometimes used to refer to the cavernous plus clinoid or paraclinoid internal carotid artery segments ¹⁰.

Course

The cervical segment of the ICA courses posterior, lateral, or posterolateral to the ECA after its origin ⁹and ascends in the neck within the carotid sheath. As it ascends on the pharyngeal wall and the buccopharyngeal 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 (C1 segment) 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 as the C2 segment to run through the petrous temporal bone. As it exits the carotid canal it lies superior to the foramen lacerum (C3 segment) and then turns 90° superiorly and then immediately another 90^o turn anteriorly to groove the body of the sphenoid and enter the medial aspect of the cavernous sinus, at the petrolingual ligament (C4 segment). Within the cavernous sinus the abducens nerve is intimately related to the artery on its lateral side. At the anterior end of the cavernous sinus, the ICA makes another 90° turn superiorly through the proximal dural ring (C5 segment) and a final 90° turn posteriorly to pass medial to the anterior clinoid process, continuing through the distal dural ring (C6 segment). The terminal ICA (C7 segment) begins just proximal to the origin of the posterior communicating artery. The ICA gives off this artery posteriorly as well as the anterior choroidal artery before abruptly dividing into the middle and anterior cerebral branches.¹

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.