Internal carotid artery

Dr Francis Deng ◉ and Dr Jeremy Jones ◉ et al.

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

There are several classification systems, the most recent of which was described by Bouthillier et al. in 1996 ¹. 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:

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 only 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 ("parts 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"

Course

The cervical segment of the ICA 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 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 (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 (C5 segment) and a final 90° turn posteriorly to pass medial to the anterior clinoid process (C6 segment). The terminal ICA (C7 segment) abruptly divides into the middle and anterior cerebral branches and gives off two smaller posterior branches, the anterior choroidal artery and the posterior communicating artery.

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.