Carotid web

Carotid webs, also known as carotid intimal variant fibromuscular dysplasia, are rare vascular pathologies of the internal carotid artery that are an important cause of cryptogenic and recurrent ischaemic stroke.

Terminology

Carotid webs have had many different names in the literature, including carotid weblike formations or septums, carotid pseudovalvular folds, carotid shelves, carotid diaphragms, and thrombotic carotid megabulbs ¹⁰. "Carotid web" is the most widely used description ¹⁰.

Epidemiology

Carotid webs are thought to be very rare, with two case series finding only 1.0-1.2% of patients who underwent CT angiogram for suspected ischaemic stroke had radiographic evidence of carotid web ^1,2. A retrospective study in 2018 has shown a prevalence of 2.5% in patients with acute ischaemic stroke due to large vessel occlusion ¹¹. It is more commonly found in females ¹¹.

Clinical presentation

Carotid webs clinically manifest as either recurrent ischaemic stroke or recurrent transient ischaemic attack ^1-3, especially in young patients, with one study finding that the recurrence rate of ischaemic stroke was as high as 29% ³. They are otherwise asymptomatic ^1-3.

Pathology

A carotid web is defined as a thin, linear, membrane that extends from the posterior aspect of the internal carotid artery bulb into the lumen, located just beyond the carotid bifurcation ^1-3. Histologically, it is thought to be a rare variant of fibromuscular dysplasia, thus it is also referred to as 'intimal variant' fibromuscular dysplasia ^1-3.

Carotid webs are highly thrombogenic and may be implicated in ischaemic stroke ^1-3. This likely occurs due to stasis of blood and platelet activation just distal to the web, resulting in thrombus formation, which can then embolise intracranially ^1,13. Indeed, one study analysing simulated flow patterns with the use of computational fluid dynamics, showed that this hypothesis was likely, by finding that carotid webs were associated with increased total surface of recirculated blood flow and wall shear stress, both of which could be implicated in thrombogenesis ¹³.

Radiographic features

In one study, approximately one-quarter of carotid webs detected radiographically were thrombosed, and just over half of all patients had bilateral carotid webs ³. Importantly, other radiographic evidence of fibromuscular dysplasia is often not appreciated.

Ultrasound

Carotid webs may be detectable as echogenic lesions on routine carotid Doppler ultrasound ^4,5,10, but this imaging modality is not considered to be as reliable as CT angiography, MR angiography or DSA ^2,5,10. There may or may not be haemodynamically significant stenosis accompanying a carotid web ^4,5,10.

CT

CT angiography is most commonly used to detect and evaluate carotid webs ^1-3,5,6,10. In particular, thin axial and oblique sagittal views are most useful ^1-3,5,6,10. Radiographically, carotid webs are typically described as shelf-like, linear, thin, smooth filling defects, located along the posterior wall of the internal carotid artery bulb, just beyond the carotid bifurcation ^1-3,5,6,10. 

Angiography (DSA)

Digital subtraction angiography is considered the gold-standard imaging modality for detecting carotid webs, whereby contrast is seen to pool in the web, even in venous phases ³. Although this classic finding may not be appreciated on standard posteroanterior or lateral DSA projections, and oblique projections may be required ¹⁰.

However, DSA is less accessible and more invasive than CT or MR angiography and carries inherent risks, thus is not routinely a first-line imaging investigation ³.

MRI

Findings on MR angiography are identical and comparable to those seen in CT angiography ^2,10. MR vessel wall imaging findings are not well described in the literature, but may include peripheral enhancement and central hypoenhancement ¹⁴.

Treatment and prognosis

Given the high risk of recurrent ischaemic stroke, appropriate management of carotid webs is important. Management options include pharmacological therapy, such as the use of antiplatelet agents, or surgical therapy, such as carotid stenting or endarterectomy ². Although evidence is limited to a few case series, one study found that rates of ischaemic stroke were lower in patients given surgical therapy when compared to a group given only pharmacological therapy ^2,7.

A survey performed in 2019 regarding management practices for carotid webs showed that antiplatelet therapy was the most common treatment for patients with either asymptomatic carotid webs or those presenting with a first episode of acute ischaemic stroke ¹². Endarterectomy or stenting was the preferred management for multiple or recurrent ischaemic strokes ¹².

History and etymology

The first description was in 1968 by Rainer et al. ⁸, although the term 'web' was first used in 1973 by Momose and New ⁹.

Differential diagnosis

• spontaneous internal carotid artery dissection flap

  • dissection generally occurs in more distal locations along the internal carotid artery ¹⁰

  • dissection has more irregular borders, and may cause an increase in the arterial diameter from pseudoaneurysm ¹⁰

• atherosclerotic plaque

  • atherosclerotic plaque tends to have more irregular borders, may also be distal to the internal carotid bulb, and may have associated calcifications ¹⁰

See also

• fibromuscular dysplasia