The anatomical path of the internal carotid artery.
The contents of the intracranial cavity are largely supplied by the circle of Willis anastomosis. The internal carotid arteries and vertebral arteries supply the anterior and posterior circulatory components of the anastomosis respectively. This article will focus on the internal carotid artery (ICA) and it’s complex journey from the extra-cranial region to the intracranial space.
The ICA arises from the bifurcation of the common carotid artery, usually at the upper border of the thyroid cartilage. Bifurcation may, less frequently occur above or below this level.
Unlike the external carotid, which leaves the carotid sheath and proceeds to give a multitude of branches to structures in the neck and head. The ICA ascends laterally to the pharyngeal wall, medial to the digastric muscle, anterior to the cervical transverse processes, giving off no branches (typically) and remains in the carotid sheath, finally entering the carotid canal on the underside of the petrous part of the temporal bone, medial to the styloid process and anterior to the jugular foramen.
Once in the canal the ICA ascends vertically approximately 1cm and then takes an almost 90 degree bend and heads anteromedially, passing in front of the anterior wall of the middle ear. As it passes, caroticotympanic arterial branches of the ICA pass through the anterior wall of the middle ear and form an anastomosis with the anterior tympanic artery from the maxillary artery and the stapedial artery. These arterial branches are accompanied by caroticotympanic nerve branches from the sympathetic plexus around the ICA heading towards the promontory. Meanwhile, the ICA continues in its canal until it arrives above the foramen lacerum, at the apex of the petrous part of the temporal bone. The bottom of the foramen lacerum is plugged with connective tissue. The ICA enters above the foramen and takes a 90 degree turn up to pierce through the floor of the cavernous sinus.
In the cavernous sinus it bends 90 degrees forwards and heads again in an anteromedial direction, heading towards the most anterior, medial and superior part of the roof of the cavernous sinus. While it travels through the cavernous sinus, it is accompanied by the abducens nerve laterally. At the end of its journey through the cavernous sinus, it bends 90 degrees again, heading towards the ventral (inferior) surface of the brain by travelling through the roof of the cavernous sinus. It passes through two rings of dural tissue and is then in the subarachnoid space, just medial to the anterior clinoid processes. After passing the second dural ring, the ICA gives off the ophthalmic artery, and then proceeds to bend 90 degrees heading caudally. As it bends, it gives off the posterior communicating artery and it’s terminal branches, the anterior cerebral and middle cerebral arteries.
Aneurysms of the ICA
The ICA, becomes intracranial after it enters the carotid canal. During its path towards the brain, it also goes from being extradural, to travelling through a dural sinus and then eventually sitting in the subarachnoid space. Therefore, aneurysms at different points of the ICA can present in significantly different ways. An aneurysm in the carotid canal may cause erosion in to the middle ear and present with hearing loss, pulsatile tinnitus and headaches. Whilst an aneurysm that ruptures inside the cavernous sinus will create an arteriovenous fistula and no doubt pain and diplopia, due to compression of the abducens nerve, followed by other cranial nerves in the lateral wall of the cavernous sinus (oculomotor, trochlear and the ophthalmic and maxillary divisions of the trigeminal nerve) . An aneurysm of the ICA, after it leaves the dural roof of the cavernous sinus, past the distal dural ring and arachnoid layer will cause a devastating subarachnoid haemorrhage, often resulting in the patient presenting with a thunderclap headache, meningeal irritation and a rapid deterioration in conscious level.
The anatomical pathway of the ICA is particularly relevant during angiography and therefore it is useful to have a method to classify and identify different segments of the ICA. This helps to provide a common framework for communication between clinicians. There are several classifications, though by far the most commonly used is the Bouthillier classification. This divides the ICA into 7 anatomical regional segments, and numbers of the segments ascend in the same direction as the flow of blood. On its tortuous path through the intracranial cavity, 2 branches tend to come off each of the even numbered segments, the exception is the last segment (C7), which has 4 branches. The following table describes the classification in more detail:
|From the common carotid bifurcation to the carotid canal|
|C2 Petrous||The carotid canal to the foramen lacerum|
|C3 Lacerum||As the ICA traverses the foramen lacerum, up to the petroclinoid ligament.|
|C4 Cavernous||From the petroclinoid ligament to the proximal dural ring. The ICA is now within the cavernous sinus, with the abducens nerve lateral to it.|
|C5 Clinoid||Between the proximal and distal dural ring (roof of the cavernous sinus) Transition zone from dural to subarachnoid space.|
|C6 Ophthalmic||From the distal dural ring to the origin of the posterior communicating artery. The ICA is now in the subarachnoid space.|
From the posterior communicating to the ICA bifurcation into the MCA and ACA
Branches of the ICA
There are usually ten branches that come off the ICA, this includes those arising during its pathway through the cranium and the terminal branches. As mentioned earlier, two branches arise off each even numbered Bouthillier segment, the exception being the final segment, which is an odd numbered segment and has 4 branches. The following mnemonic can be used to recall all the branches: Can’t Vesalius Imitate My Stylish Original PAAMphlet.
|C2 Petrous||Caroticotympanic, Vidan|
|C4 Cavernous||Inferohypophyseal, Meningohypophyseal|
|C6 Ophthalmic||Superior hypophyseal, Ophthalmic|
|C7 Communicating||Pcom, Anterior choroidal, ACA and MCA|
ICA Internal carotid artery, MCA Middle cerebral artery, ACA Anterior cerebral artery and Pcom Posterior communicating artery.
Netter’s Atlas of Human Anatomy, Frank Netter