Internal carotid artery anatomy

ICA Segments - The Scenic Route

• C1: Cervical
  • No major branches.
• C2: Petrous
  • Caroticotympanic artery.
• C3: Lacerum
  • Traverses foramen lacerum; often considered part of the petrous segment.
• C4: Cavernous
  • Meningohypophyseal trunk.
• C5: Clinoid
  • Transitional segment.
• C6: Ophthalmic
  • Ophthalmic artery.
• C7: Communicating
  • Posterior communicating artery (PCom).

⭐ The cavernous segment (C4) is encased by the cavernous sinus, alongside CN III, IV, V1, V2, and VI. Aneurysms here can cause multiple cranial nerve palsies.

Cavernous Segment - Neurological Crossroad

• Traverses the cavernous sinus, a dural venous sinus beside the sella turcica.
• Forms an S-shaped bend known as the carotid siphon.
• Structures passing through the sinus:
  • Internal Carotid Artery (ICA)
  • Abducens Nerve (CN VI), positioned most medially, adjacent to the ICA.
• Nerves in the lateral wall (superior to inferior):
  • Oculomotor Nerve (CN III)
  • Trochlear Nerve (CN IV)
  • Ophthalmic Nerve (CN V1)
  • Maxillary Nerve (CN V2)
• 📌 Mnemonic (Lateral Wall): Oh To Obtain More (Oculomotor, Trochlear, Ophthalmic, Maxillary).

⭐ Aneurysm or thrombosis of the cavernous ICA most commonly affects the Abducens nerve (CN VI) first, due to its intimate medial position, causing lateral rectus palsy.

Supraclinoid Branches - Brain's Main Feeders

• Ophthalmic Artery (OphA): First branch after cavernous sinus. Enters orbit via optic canal with CN II. Supplies the eye and orbital contents.
• Posterior Communicating Artery (PCoA): Connects ICA to the Posterior Cerebral Artery (PCA), linking anterior and posterior circulations.
• Anterior Choroidal Artery (AChA): Supplies deep structures often missed.
  • Optic tract, posterior limb of internal capsule, globus pallidus, lateral geniculate body.
  • Occlusion leads to contralateral hemiparesis, hemianesthesia, and homonymous hemianopsia.
• Terminal Bifurcation: ICA divides into:
  • Anterior Cerebral Artery (ACA)
  • Middle Cerebral Artery (MCA)

📌 Mnemonic: "Only Pimps And Criminals Make Allowances" (Ophthalmic, PCoA, Ant. Choroidal, MCA, ACA)

⭐ Exam Favorite: Aneurysms of the PCoA are a classic cause of isolated, painful CN III (Oculomotor) palsy. This presents with a "down and out" eye position, ptosis, and a fixed, dilated pupil ("blown pupil") due to compression of parasympathetic fibers running on the nerve's surface.

Clinical Correlations - Arterial Trouble Spots

• Saccular (Berry) Aneurysms:

  • Occur at arterial branch points in the Circle of Willis.
  • Most common cause of non-traumatic subarachnoid hemorrhage (SAH).
  • Presents as "worst headache of my life."
  • Common sites: Anterior Communicating A. (ACOM), Posterior Communicating A. (PCOM), MCA bifurcation.

• Carotid Artery Stenosis:

  • Atherosclerotic narrowing, often at the carotid bifurcation.
  • Can lead to transient ischemic attacks (TIAs) like amaurosis fugax or watershed strokes.

⭐ An aneurysm of the Posterior Communicating Artery (PCOM) can compress the oculomotor nerve (CN III), causing a "down and out" eye with mydriasis.

High‑Yield Points - ⚡ Biggest Takeaways

• The Internal Carotid Artery (ICA) arises from the common carotid and is a key component of the Circle of Willis.
• It has four main segments: cervical, petrous, cavernous, and cerebral.
• The ophthalmic artery is the first major intracranial branch.
• The ICA terminates into the Anterior (ACA) and Middle Cerebral Arteries (MCA).
• The carotid siphon, a bend within the cavernous sinus, is a frequent location for aneurysms.
• It links the anterior and posterior circulations through the Posterior Communicating Artery.