Corpus: Internal carotid artery

The internal carotid artery, or ICA for short, is a branch of the common carotid artery. After passing through the carotid canal, it supplies the anterior part of the brain, as well as the eye and its appendages. It also sends smaller branches to the forehead and nose.

2.1. ...according to anatomical course

Based on its course and the surrounding anatomical structures, the internal carotid artery can be divided into four sections from caudal to cranial:

• Cervical part (Pars cervicalis): Extends from the carotid bifurcation to the entry into the base of the skull.
• Petrous part (Pars petrosa): Begins at the entry into the carotid canal, courses through the petrous part of the temporal bone, initially vertically and then horizontally across the foramen lacerum to the tip of the petrous pyramid.
• Cavernous part (Pars cavernosa): Within the cavernous sinus, the ICA makes an S-shaped curve known as the carotid siphon and lies close to the optic chiasm.
• Cerebral part (Pars cerebralis): This intradural section begins after the ICA pierces the dura mater at the level of the anterior clinoid process or at the exit of the ophthalmic artery. It courses below the optic nerve, posteriorly and superiorly, up to the "carotid T," where the posterior communicating artery and anterior choroidal artery branch off.

2.2. ...according to Bouthillier

In addition to the anatomical classification, the Bouthillier classification is used in neurosurgery and divides the internal carotid artery into seven segments:

• C1: Cervical segment: Begins at the carotid bifurcation and ends at the entry into the carotid canal.
• C2: Petrous or horizontal segment: Extends within the carotid canal and ends at the posterior edge of the foramen lacerum.
• C3: Lacerum segment: Begins at the exit from the carotid canal and extends to the petrolingual ligament (a periosteal extension of the carotid canal).
• C4: Cavernous segment: Courses within the cavernous sinus, featuring a vertical part, a posterior convex bend, a horizontal part, and an anterior convex bend.
• C5: Clinoidal segment: Formed by the incomplete proximal and the complete distal dural ring.
• C6: Ophthalmic segment: Extends between the distal dural ring (intradural) and the proximal exit of the posterior communicating artery.
• C7: Terminal or communicating segment: Begins before the exit of the posterior communicating artery and ends at the carotid T.

2.3. ...according to NYU classification

The NYU classification (2014) is particularly used in neuroradiology for the angiographic assessment of aneurysms. It also divides the ICA into seven segments:

• Cervical segment: Analogous to C1.
• Petrous segment: Includes C2 and C3. The petrolingual ligament is not visible angiographically, so the anterior part of the petrous edge is used as a landmark.
• Cavernous segment: Extends from the presumed location of the petrolingual ligament to the proximal dural ring.
• Paraophthalmic segment: Extends from the proximal dural ring to just before the exit of the posterior communicating artery.
• Posterior communicating segment: Extends from just before the exit of the posterior communicating artery to the exit of the anterior choroidal artery.
• Anterior choroidal segment: Begins at the departure of the anterior choroidal artery.
• Terminal segment: Extends distal to the exit of the anterior choroidal artery to the carotid T.

In addition to these classifications of the ICA, there are other older classifications, such as the Fischer classification.

3.1. Cervical Part

The internal carotid artery arises from the common carotid artery at approximately the level of the fourth cervical vertebra (C4). The bifurcation point is also known as the carotid bifurcation, where the carotid body is located, which contains chemoreceptors. The origin of the ICA is slightly widened to form the carotid sinus, which contains baroreceptors in its wall. The mostly branchless cervical section ascends almost straight to the base of the skull in the lateral pharyngeal space, along with the internal jugular vein and the vagus nerve. The ICA is separated from the external carotid artery by the styloglossus and stylopharyngeus muscles. Shortly before entering the petrous bone, the artery forms a slight curve.

3.2. Petrous Part

This approximately 3 cm long section ascends within the carotid canal of the petrous bone, with the canal forming the anterior wall of the tympanic cavity. The artery then bends medially (carotid bend). Within the canal, the ICA is surrounded by the internal carotid venous plexus. Small caroticotympanic branches run through bony tubules into the tympanic cavity. A thin bony lamella and the dura separate the ICA from the trigeminal ganglion at the apex of the petrous pyramid. Above the fibrocartilaginous basilar membrane, the ICA enters the cavernous sinus.

3.3. Cavernous Part

Within the cavernous sinus, the ICA lies against the lateral surface of the sphenoid body and follows an S-shaped curve (carotid siphon) from posteroinferior to anterosuperior. The siphon knee marks the transition to the cerebral part.

3.4. Cerebral Part

The ICA pierces the dura mater between the optic chiasm and the anterior clinoid process, entering the carotid cistern of the subarachnoid space. It runs from anteroinferior to posterosuperior and finally divides into its four cerebral branches.

The cervical section of the ICA typically has no branches. Rarely, it may give off the inferior thyroid artery and other head arteries. The following branches arise in the other sections:

Petrous Part

• Caroticotympanic branch: To the tympanic cavity
• Occasionally, the artery of the pterygoid canal originates from this section

Cavernous Part

• Basal and marginal tentorial branches: To the tentorium cerebelli
• Cavernous sinus branch
• Inferior hypophyseal artery: To the pituitary gland
• Trigeminal ganglion branch: To the trigeminal ganglion

Cerebral Part

• Ophthalmic artery: Enters the optic canal, usually medial and inferior to the optic nerve
• Superior hypophyseal artery: Enters the pituitary gland from above via the chiasmatic cistern near the pituitary stalk
• Clival branches, meningeal branch, uncal artery: Irregular branches supplying the dura in the area of the cavernous sinus and surrounding regions, as well as parts of the uncus parahippocampalis
• Cerebral arteries:
  • Posterior communicating artery
  • Anterior choroidal artery
  • Anterior cerebral artery (terminal branch)
  • Middle cerebral artery (terminal branch)

There are several arterio-arterial anastomoses, particularly with the external carotid artery. During the gradual stenosis of the ICA, these collateral circuits can expand, potentially even reversing the flow in arteries close to the brain. If these collaterals are sufficient, the stenosis may not cause clinical symptoms.

Examples of such collaterals include:

• External carotid artery → Facial artery → Angular artery → Dorsal nasal artery → Ophthalmic artery → Internal carotid artery (ophthalmic collaterals)
• External carotid artery → Occipital artery → Meningeal arteries → Vertebral artery (occipital anastomosis)
• External carotid artery → Superficial temporal artery → Frontal branch → Lateral palpebral artery / Supraorbital artery / Supratrochlear artery

In the event of hemorrhage, the following anastomoses should also be considered:

• External carotid artery → Sphenopalatine artery → Locus Kiesselbachi ← Anterior ethmoidal artery ← Ophthalmic artery ← Internal carotid artery
• External carotid artery → Blood supply to the temporal bone (stylomastoid artery / inferior tympanic artery / deep auricular artery / posterior tympanic artery / superior tympanic artery / anterior tympanic artery) ← Carotid artery ← Internal carotid artery

There are also anastomoses between the three major cerebral arteries (anterior, middle, and posterior cerebral arteries) via the anterior communicating artery and the posterior communicating arteries. The connection between the supply areas of the right and left ICA and the supply area of the vertebral arteries is known as the cerebral arterial circle (Circle of Willis).

The most significant pathology of the ICA is its narrowing (stenosis) due to systemic arteriosclerosis. A complication of ICA stenosis is stroke. Temporary reductions in blood flow can lead to impaired consciousness, visual disturbances, and dizziness.

• Shapiro et al. Toward an Endovascular Internal Carotid Artery Classification System, American Journal of Neuroradiology, 2014
• Shield. Angiography. Georg-Thieme-Verlag, 1994
• Schünke, Schulte, Schumacher: Prometheus. Learning atlas of anatomy - head, neck and neuroanatomy, Thieme Verlag, Stuttgart 2018
• Pschyrembel - Arteria carotis interna, retrieved on 23/01/2023