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 ¹¹, including near-total occlusion, subocclusion, incomplete occlusion, functional occlusion, and preocclusive stenosis.

Unfortunately, the term pseudo-occlusion has also been used in this context, mostly before the era of thrombectomy¹. Nowadays, pseudo-occlusion is probably best reserved for 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 ¹¹. Alternatively, the entity has been described by the distal internal carotid artery as small, narrow, or (spuriously/falsely) hypoplastic ¹¹, or with "trickle flow" ⁹.

Epidemiology

Near-occlusions constitute 30% of symptomatic >=50% carotid stenoses (NASCET-grading)¹³. Traditionally, near-occlusion have been thought of as rare, but the diagnosis is easy to overlook, making it appear more rare than it is¹³.  

Radiographic features

Ultrasound

The diagnosis is suggested by a markedly narrow lumen on colour and power Doppler ¹⁰. A slow and dampened (pseudovenous) flow velocity profile suggests full collapse ¹¹. Systolic spikes with absent or reversed diastolic flow can also indicate near-occlusion but is not specific as more distal stenosis/occlusion can have this profile ¹¹. Doppler velocity cannot be relied upon to identify near-occlusion, especially with only partial collapse, where the peak systolic velocity may be misleadingly normal or elevated ^10,11.

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 calibre 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. Traditionally, full collapse appears as a hairline residual lumen, termed the string (or slim) sign. However, new definitions for full collapse have been proposed that better stratify outcomes between those with versus without full collapse ¹². As assessed on CTA, full collapse is met by either of two criteria:

• distal internal carotid artery diameter ≤2.0 mm

• ipsilateral to contralateral distal internal carotid artery diameter ratio ≤0.42.

Angiography

Digital subtraction angiography is the conventional gold standard for evaluating carotid artery stenosis. The angiographic features of near-occlusion are the following ^6,11:

• small extracranial internal carotid artery calibre 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 by traditional criteria appears as a hairline residual lumen, termed the angiographic string (or slim) sign ¹¹.

Radiology report

Near-occlusion should be distinguished from conventional stenoses (non near-occlusions). 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, percent stenosis should not be used - the—the grade of stenosis is "near-occlusion" instead of a percent degree. It is reasonable to highlight that the cause of the use of the term is theindicates a stenosis-caused-related size reduction of the distal ICAinternal carotid artery, specifying whether there is full or partial collapse.

Treatment and prognosis

The risk of stroke with near-occlusion is lower than that seen in severe conventional stenosis ^{1, 14}. The benefit with CEA for symptomatic >50% or >70% stenosis (from the NASCET and ECST trials) are only applicable to conventional stenoses (i.e. after excluding near-occlusions)¹⁴. 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.    

Distinguishing near-occlusion with versus without full collapse likely has prognostic significance in symptomatic patients. Compared to those with conventional ≥50% carotid stenoses, those having near-occlusion with full collapse have a higher risk of recurrent ipsilateral ischaemic stroke or retinal artery occlusion within 28 days, while those having near-occlusion with partial collapse have a lower risk of recurrence ¹².

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 causesNear-occlusion should mainly be distinguished from other grades of steno-occlusive disease:

• conventional severe stenosis: the distal cervical internal carotid artery is not reduced in calibre

• occlusion: the distal cervical internal carotid artery does not fill with contrast on imaging

Causes or mimics of asymmetric small calibre of the extracranial internal carotid artery aside from near-occlusive atherosclerotic disease 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