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Internal Carotid Arteries

Anatomy || Importance in Disease || Overview || Related Articles ||
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Anatomy





The two internal carotid arteries (ICA) provide almost all of the blood flow to the frontal, parietal, and temporal lobes, as well as many of the deep subcortical strucutures of the brain.

The internal carotid arteries are classically broken into seven segments. These segments are based on angiographic and anatomical location; the segments also provide a framework for thinking about the diseases and conditions that affect the internal carotids at different points along their course. The segments are as follows:

    Extradural segments (ie: outside the lining of the brain)

  1. Cervical
  2. Petrous
  3. Lacerum
  4. Cavernous

    Intradural segments (ie: inside the lining of the brain)

  5. Clinoid
  6. Ophthalmic (aka: supraclinoid)
  7. Communicating (aka: terminal)

Let’s discuss the relevant anatomy of each segment below....

The cervical portion of the ICA is in the neck (ie: near the “cervical” spine). It usually branches off of the common carotid artery between the third and fifth cervical vertebrae. Normally it has no branches, but occasionally vessels that are supposed to regress/disappear in-utero may be present. These vessels include the proatlantal intersegmental artery (an embryologic connection between the cervical ICA and the vertebral artery) and the hypoglossal artery (an embryologic connection between the cervical ICA and the basilar artery).

The cervical portion ascends towards the head where it enters the petrous portion of the temporal bone. At this point it becomes known as the “petrous” ICA. The petrous segment has two characteristic features on angiography: a vertical and a horizontal section. The vertical section lies behind the eustachian tube of the ear. The vessel then takes an abrupt turn forward and towards the midline of the head (ie: anteromedially) becoming the “horizontal” petrous ICA.

The petrous ICA infrequently has branches that can be seen on angiograms. The most commonly visualized vessel, if present, is the vidian artery (aka: artery of the pterygoid canal). This artery is usually a branch of the external carotid artery (ECA), but in some people it is a branch of the petrous ICA.

The “genu” of the petrous ICA is the point where the horizontal and vertical segments meet. A small vessel known as the caroticotympanic artery branches off the genu and supplies the tympanic cavity. It is not normally seen on formal angiograms. Finally, small unnamed perforating blood vessels supply the temporal bone; they are branches off the bottom (inferior) portion of the horizontal segment.

The lacerum portion is a very short segment that begins once the ICA exits the carotid canal of the temporal bone; the lacerum segment ends as soon as the vessel pierces the cavernous sinus. It has no branches.

The cavernous segment of the ICA forms an S-shaped curve known as the “carotid siphon”. The cavernous ICA has several important branches. The first branch is the meningohypophyseal trunk. This vascular trunk gives rise to three additional vessels that may or may not be visualized on angiograms. The first branch is the tentorial artery (aka: the artery of Bernasconi and Casinari), the second is the dorsal meningeal artery, and the third is the inferior hypophyseal artery.

The second inconstant artery that branches off the cavernous ICA is the artery of the inferior cavernous sinus, which is also known as the “inferolateral trunk”. This trunk and its branches supply the dura of the cavernous sinus, as well as the cranial nerves that course through it (ie: the third, fourth, fifth, and sixth nerves).

The least constant branches of the cavernous ICA are McConnell’s capsular arteries. They include the inferior and anterior capsular arteries that send blood to the sella turcica, which is the bony cave that houses the pituitary gland.

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Overview

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References and Resources

(1) Bouthillier A, van Loveren HR, Keller J. Segments of the Internal Carotid Artery: A New Classification. Neurosurgery. 38(3):425-433, March 1996.

(2) Osborn AG. Introduction to Cerebral Angiography. Harper and Row: Philadelphia, 1980.

(3) Harris FS, Rhoton AL. Anatomy of the cavernous sinus. A microsurgical study. J Neurosurg. 1976 Aug;45(2):169-80.

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