Carotid near-occlusion

Carotid near-occlusion is a special form of severe carotid artery stenosis that results in a partial or complete collapse of the distal internal carotid artery lumen due to underfilling. 

It should not be confused with carotid pseudo-occlusion due to terminal intracranial internal carotid artery occlusion by thromboembolism. 

Terminology

Many synonymous terms have been used ¹¹¹, including near-total occlusion, subocclusion, incomplete occlusion, functional occlusion, and preocclusive stenosis.

Unfortunately, the term pseudo-occlusion has also been used in this context but is probably best reserved from a similar appearance due to terminal internal carotid artery occlusion due to thromboembolism. 

Near-occlusion also overlaps with several terms that describe the degree of proximal narrowing, including critical stenosis, subtotal stenosis, and 99% stenosis ¹¹¹. Alternatively, the entity has been described by the distal internal carotid artery as small, narrow, or (spuriously/falsely) hypoplastic ¹¹¹, or with "trickle flow" ⁹.

Radiographic features

CT

CT angiography is the first-line modality for diagnosing carotid near-occlusion, which is based on the following key features ⁶:

• small extracranial internal carotid artery caliber compared to the contralateral internal carotid artery and to the external carotid artery
• focal severe stenosis with minimal to no luminal contrast opacification

The degree of distal internal carotid artery collapse exists on a spectrum and can be visually subtle when partial. Full collapse appears as a hairline residual lumen, termed the string (or slim) sign.

Angiography

Digital subtraction angiography is the conventional gold standard for evaluating carotid artery stenosis. The angiographic features of near-occlusion are the following ^1,66,11:

• small extracranial internal carotid artery caliber compared to the contralateral internal carotid artery and to the external carotid artery
• delay of contrast filling the distal internal carotid artery
• intracranial collaterals (contrast injection in the contralateral carotid fills intracranial arteries ipsilateral to the near-occlusion)

Full collapse appears as a hairline residual lumen, termed the angiographic string (or slim) sign ¹¹¹.

Radiology report

Near-occlusion should be distinguished from conventional stenoses. The latter are often expressed as percentage luminal narrowing compared to distal unaffected internal carotid artery, based on criteria used in the North American Symptomatic Carotid Endarterectomy Trial (NASCET). When near-occlusion is present, the calculated stenosis would be spuriously low due to distal collapse, potentially leading to inappropriate management.

Treatment and prognosis

The risk of stroke with near-occlusion is lower than that seen in severe stenosis ¹. Existing guidelines recommend treating carotid near-occlusion with best medical therapy ⁸, but recent reviews do not support the superiority of medical therapy alone over carotid artery stenting or endarterectomy ^2,3.

History and etymology

The term near-occlusion in its current meaning was defined in 1997 by the NASCET (North American Symptomatic Carotid Endarterectomy Trial) investigators ⁴. 

Differential diagnosis

Other causes or mimics of asymmetric small caliber of the extracranial internal carotid artery include the following:

• carotid dissection
• cervical carotid pseudo-occlusion due to terminal internal carotid occlusion ^7,9
• post-radiation changes
• circle of Willis variants such as ipsilateral anterior cerebral artery A1 segment hypoplasia/aplasia or contralateral fetal posterior cerebral artery ⁵