Carotid sheath:Internal carotid artery

Internal carotid arises at the bifurcation of the common carotid between C3  and C5 vertebral level. It has no branches outside the skulls and passes straight up in the carotid sheath, beside the pharynx to the carotid canal in the base of the skull.

Relations:

* posterior: sympathetic trunk, superior laryngeal branch of the vagus
* medial: ascending pharyngeal artery
* lateral: internal jugular vein and vagus nerve
* anterior: lower down by lingual and facial veins, the occipital artery and hypoglossal nerve, and higher up by the posterior belly of digastric, stylohyoid ad posterior auricular artery.

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%

Segments

There are seven segments in the Bouthillier classification: 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.

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, whereas the even numbered segments (C2, C4, C6) each have two branches.

* **C1: cervical segment**
	+ Extends from the bifurcation of the common carotid to carotid canal located anterior to the jugular foramen
	+ Superiorly the internal jugular vein, and vagus nerve lie laterally, medially is the pharynx
	+ At the base of the skull the glossopharyngeal, vagus, accessory and hypoglossal nerves lie between the internal carotid artery and the internal jugular vein
	+ Branches: none
* **C2: petrous segment**
	+ From carotid canal to foramen lacerum within the petrous temporal bone
	+ Separate from mdidle ear by a thin plate
	+ Surrounded by extensive sympathetic plexus
	+ Two C2 subsegments joined at genu
		- Short vertical segment [anterior to internal jugular vein (IJV)]
		- "Genu" (where petrous ICA turns anteromedially in front of cochlea)
		- Longer horizontal segment
	+ Branches
		- Vidian artery (artery of pterygoid canal) anastomoses with external carotid artery (ECA)
		- Caroticotympanic artery (supplies middle ear)
* **C3: lacerum segment**
	+ Small segment that extends from petrous apex above foramen lacerum, curving upwards toward cavernous sinus
	+ Turns 90% superiorly following extra dural course. Covered by trigeminal ganglion
	+ No branches
* **C4: Cavernous segment**
	+ Pass from the petrous apex to the dural ring of the anterior clinioid process surrounded by cavernous sinus
	+ Three subsegments joined by two genus (knees)
		- Posterior vertical (ascending) portion to the posterior clinioid
		- Posterior (more medial) genu
		- Horizontal segment to the anterior clinoid process
		- Anterior (more lateral) genu
		- Anterior vertical (subclinoid) segment perforate the dura mater.
	+ Abducens nerve (CN6) is inferolateral
	+ Major branches
		- Meningohypophyseal trunk (arises from posterior genu, supplies pituitary, tentorium and clival dura)
		- Inferolateral trunk arises from horizontal segment, supplies cavernous sinus (CS) dura/cranial nerves;anastomoses with ECA branches through f. rotundum, spinosum, ovale

* **C5: Clinoid segment**
	+ Between proximal, distal dural rings of cavernous sinus
	+ Ends as ICA enters subarachnoid space near anterior clinoid process
	+ No important branches unless OA arises within CS
* **C6: Ophthalmic segment**
	+ Extends from distal dural ring at superior clinoid to just below posterior communicating artery (PCoA) origin
	+ Two important branches
		- OA (originates from anterosuperior ICA, passes through optic canal to orbit; gives off ocular, lacrimal, muscular branches; extensive anastomoses with ECA)
		- Superior hypophyseal artery (courses posteromedially; supplies anterior pituitary, infundibulum, optic nerve/chiasm)
* **C7: Communicating segment**
	+ Extends from below PCoA to terminal ICA bifurcation into anterior cerebral artery (ACA), middle cerebral artery (MCA)
	+ Passes between optic (CN2), oculomotor (CN3) nerves
	+ Major branches
		- Posterior communicating artery
		- Anterior choroidal artery (courses posteromedial, then turns superolateral in suprasellar cistern; enters temporal horn at choroidal fissure; supplies choroid plexus, medial temporal lobe, basal ganglia, posteroinferior internal capsule)
		- Striate arteries

Variant anatomy

* CCA bifurcation location variation
* aberrant ICA: at the petrous segment, can course more poterolaterally and appear as pulsating retrotympanic mass
* congenital absence of the ICA
* retropharyngeal ICA (rare)
* kissing carotids
* persistent carotid-vertebrobasilar anastomoses
	+ persistent hypoglossal artery, arises from ICA at C1-2 level, passes through hypoglossal canal to join BA
	+ proatlantal intersegmental artery: arises from cervical ICA at C2-3 and connects cervical ICA with VA
	+ persistent trigeminal artery: Most common carotid-basilar anastomosis (.02-0.5%) from cavernous segment. Parallels course of CN5, passes posterolaterally around (or through) dorsum sellae.
	+ Persistent otic artery: petrosal to proximal basilar artery via the internal acoustic meatus; very rare; vertebral arteries may be absent or hypoplastic
* lateralised internal carotid artery
* non-bifurcating common carotid (no ICA bulb, and ECA branches arise directly from CCA)
* Persistent stapedial artery (from petrous segment) and terminate as middle meningeal artery. No foramen spinosum in these cases.