What is an internal carotid aneurysm?

Internal Carotid Aneurysms: Definition and Classification

An internal carotid aneurysm is defined as an abnormal focal dilation of the internal carotid artery with attenuation of the vessel wall, which can occur anywhere along its course from the cervical segment through the intracranial portions. 1

Anatomical Classification

Internal carotid aneurysms are classified based on their location along the course of the internal carotid artery:

1. Extracranial Internal Carotid Aneurysms

   • Located in the cervical portion of the internal carotid artery
   • Typically begin approximately 3-3.5 cm from the common carotid artery bifurcation 2
   • May present as a neck mass or cause local compression symptoms

2. Intracranial Internal Carotid Aneurysms

   • Further subdivided by specific segments:
     • Cavernous segment aneurysms - located within the cavernous sinus
     • Paraclinoid aneurysms - near the ophthalmic artery origin
     • Communicating segment aneurysms - including posterior communicating artery aneurysms
     • Bifurcation aneurysms - at the terminal bifurcation of the internal carotid artery 3

Morphological Classification

Internal carotid aneurysms are also classified by their morphology:

• Saccular aneurysms - most common type, appearing as a pouch-like outpouching
• Fusiform aneurysms - dilations involving the entire circumference of the vessel
• Giant aneurysms - those measuring ≥25 mm in diameter 1
• Dissecting aneurysms - resulting from a tear in the arterial wall

Size Classification

According to standard guidelines, aneurysms are categorized by size 1:

• Small aneurysms: ≤5 mm
• Medium aneurysms: >5 mm and <15 mm
• Large aneurysms: ≥15 mm and <25 mm
• Giant aneurysms: ≥25 mm

Clinical Significance by Location

The location of internal carotid aneurysms has important clinical implications:

• Cavernous segment aneurysms have extremely low rupture rates (approximately 0% annually) and are often managed conservatively 4
• Communicating segment and bifurcation aneurysms carry higher rupture risks and more frequently require intervention 1
• Posterior communicating artery aneurysms represent about 30% of all internal carotid aneurysms 5

Diagnostic Considerations

When evaluating internal carotid aneurysms:

• CTA has high sensitivity (>90%) for aneurysm detection and is recommended as the initial screening test 4
• MRA is preferred for follow-up of untreated or coiled aneurysms and for screening high-risk populations 4
• Digital subtraction angiography remains the reference standard with highest spatial resolution 4
• Dome-to-neck ratio and neck-to-artery ratio are critical measurements that influence treatment decisions 1

Treatment Implications

The anatomical location significantly impacts treatment approach:

• Middle cerebral artery aneurysms are often more amenable to surgical clipping 1
• Posterior circulation aneurysms typically have better outcomes with endovascular coiling 1
• Cavernous segment aneurysms of the internal carotid artery are difficult to treat surgically but may be relatively easily treated with coil embolization 1

Understanding the precise anatomical classification of internal carotid aneurysms is essential for appropriate risk stratification and treatment planning.