Screening for asymptomatic carotid artery stenosis

INTRODUCTION — Progression of atheromatous plaque at the cervical carotid artery bifurcation results in luminal narrowing, often accompanied by ulceration. This process can be asymptomatic but may lead to ischemic stroke or transient ischemic attack from embolization, thrombosis, or hemodynamic compromise.

This topic will review the role of screening for asymptomatic carotid atherosclerotic disease. The management of asymptomatic carotid disease is discussed separately. (See "Management of asymptomatic extracranial carotid atherosclerotic disease".)

Other aspects of carotid occlusive disease are reviewed elsewhere. (See "Evaluation of carotid artery stenosis" and "Management of symptomatic carotid atherosclerotic disease".)

ASYMPTOMATIC CAROTID DISEASE

Definitions — Asymptomatic cervical carotid atherosclerotic disease refers to the presence of atherosclerotic narrowing of one or both of the extracranial internal carotid arteries in individuals without a history of carotid territory ischemic stroke or transient ischemic attack (TIA) [1-3]. Carotid stenosis is also often considered asymptomatic if the patient has not had an ipsilateral carotid territory ischemic stroke or TIA within the prior six months [4].

Symptomatic carotid disease is defined as focal neurologic symptoms (eg, amaurosis fugax, contralateral weakness or numbness of an extremity or the face, dysarthria or aphasia, spatial neglect, homonymous visual loss) in the distribution of a carotid artery with a significant stenosis.

Importantly, nonspecific neurologic symptoms (eg, dizziness, lightheadedness) are not indicative of carotid stenosis. Therefore, patients with these symptoms in isolation should be considered as asymptomatic with regard to carotid disease even if they are found to have carotid artery stenosis. (See "Management of symptomatic carotid atherosclerotic disease", section on 'Definition of symptomatic disease'.)

Prevalence — The prevalence of asymptomatic cervical carotid stenosis is low in the general population, but increases with age, which is the most important risk factor.

●In a 2010 meta-analysis of four population-based studies (including the CHS) with individual data from over 23,000 participants, the prevalence estimates of asymptomatic cervical carotid stenosis varied according to age [5]. The prevalence of severe stenosis (defined as ≥70 percent of the lumen diameter) was quite low, ranging from <1 percent for men and women age <70 year up to approximately 3 percent for men and 1 percent for women age ≥80 years (figure 1). The prevalence of at least moderate stenosis (defined as ≥50 percent of the lumen diameter) was somewhat higher.

●A 2020 meta-analysis of worldwide population-based studies identified 59 eligible studies and found that the global prevalence of ≥50 percent stenosis of the extracranial carotid artery was 1.5 percent (95% CI 1.1-2.1), increased with age from 30 to 79 years, and was higher in men than women [6].

●A Chinese national cross sectional study not included in the 2020 meta-analysis reported that among people ≥40 years of age, the prevalence of a >50 percent carotid artery stenosis was 0.4 percent (95% CI 0.3-0.4%) [7].

Stroke risk — The most feared consequence of carotid atherosclerosis is ischemic stroke. The estimated risk of ipsilateral stroke in prospective studies that followed patients with asymptomatic cervical internal carotid artery atherosclerosis (stenosis ≥50 percent) receiving optimal medical management is ≤1 percent annually, as discussed separately (see "Management of asymptomatic extracranial carotid atherosclerotic disease", section on 'Risk of stroke and cardiovascular events'). The risk of stroke may be higher in individuals with severe carotid artery stenosis (70 to 99 percent) compared with individuals with moderate (50 to 69 percent) carotid artery stenosis [8]. However, in a retrospective study of a community-based cohort of patients with asymptomatic severe (70 to 99 percent) carotid stenosis diagnosed between 2008 and 2012 who did not undergo carotid revascularization, the estimated annual rate of ipsilateral carotid-related acute ischemic stroke was 0.9 percent [9].

Asymptomatic carotid atherosclerosis is also a marker of increased risk for myocardial infarction and vascular death. Thus, asymptomatic carotid atherosclerosis is considered a risk equivalent for coronary heart disease. (See "Overview of established risk factors for cardiovascular disease", section on 'Noncoronary atherosclerotic disease'.)

Therapeutic options — All patients with carotid atherosclerotic disease should undergo intensive medical therapy, which includes several strategies to reduce their cardiovascular risk. Periodic clinical follow-up to evaluate compliance with medical therapies and to evaluate for symptoms and signs of TIA or stroke is also important. Medical management and the role of carotid revascularization with endarterectomy or stenting is reviewed in detail separately. (See "Management of asymptomatic extracranial carotid atherosclerotic disease".)

SCREENING

Our approach — Consistent with national guidelines (see 'Recommendations of others' below), we suggest not screening asymptomatic individuals for carotid artery stenosis with vascular imaging tests. Although there have been no randomized controlled trials evaluating the utility of screening for carotid artery stenosis, general screening for carotid stenosis in asymptomatic individuals does not appear to be warranted based upon the following observations [10-12]:

●The prevalence of asymptomatic carotid stenosis in the population is low; for severe (≥70 percent) carotid stenosis, the prevalence increases with age from approximately 0 to 3 percent. (See 'Prevalence' above.)

●The annual risk of ipsilateral stroke in patients with asymptomatic carotid artery stenosis is relatively low (≤1 percent annually). (See 'Stroke risk' above.)

●There are no validated, reliable clinical characteristics or markers that identify a subset of asymptomatic individuals likely to have carotid stenosis who also would benefit from carotid revascularization.

●The routine use of duplex ultrasonography to detect asymptomatic carotid stenosis, a low-prevalence condition found in approximately 1 percent of the general population, would result in many more false-positive results than true-positive results, which in turn could lead to many unnecessary interventions, including additional testing and carotid revascularization, with its attendant morbidity and mortality [13,14]. As shown in data from large administrative databases and surgical registries, both carotid endarterectomy and carotid stenting for asymptomatic carotid stenosis are associated with an increased 30-day risk of stroke and death. These risks are variable between surgeons and centers, but are in the range of 1.5 to 3.5 percent for carotid endarterectomy and 2.6 to 5.1 percent for carotid artery stenting [15]. Complication rates may be higher in low-volume settings.

●For patients with asymptomatic carotid stenosis, the absolute reduction in stroke risk with carotid endarterectomy was small when compared with the now-outdated medical treatment employed in the randomized trials of the time (2004 and earlier). This small benefit of revascularization may have been reduced or eliminated with subsequent advances in medical therapy. (See "Management of asymptomatic extracranial carotid atherosclerotic disease", section on 'Intensive medical therapy and follow-up'.)

Recommendations of others — As already discussed (see 'Our approach' above), we suggest not screening asymptomatic individuals for carotid artery stenosis with vascular imaging tests. Our recommendation is in general agreement with most national and society guidelines, though there are some differences among them:

●The 2021 United States Preventive Services Task Force (USPSTF) statement, reaffirming its 2014 position [16], recommends against screening for asymptomatic carotid artery stenosis in the general population [17,18].

●The 2014 guidelines for the primary prevention of stroke from the American Heart Association/American Stroke Association indicate that screening low-risk populations for asymptomatic carotid artery stenosis is not recommended [19].

●Joint 2011 guidelines from multiple US societies (including the American College of Cardiology, American Heart Association, American Stroke Association, American College of Radiology, and the Society for Vascular Surgery) advise that carotid duplex ultrasonography "is not recommended for routine screening of asymptomatic patients who have no clinical manifestations of or risk factors for atherosclerosis" [20]. However, they note that it is reasonable to screen (with duplex ultrasonography) asymptomatic individuals who have a carotid bruit, and that duplex ultrasonography screening of the carotid arteries "may be considered" for patients who have symptomatic atherosclerotic disease in another vascular bed (ie, peripheral arterial disease, coronary disease, or aortic aneurysm), or have two or more risk factors for atherosclerotic disease.

Benefits and harms of screening — The potential benefit of carotid screening depends on identifying people with asymptomatic carotid stenosis whose stroke risk would be reduced with carotid revascularization or more intensive medical treatment [10]. The potential harms associated with screening include risks associated with the screening procedure itself and risks associated with carotid revascularization or other interventions [21]. The risks of screening studies also include false positive findings and the need for confirmatory testing. The reliability of carotid duplex screening is variable among laboratories and is operator-dependent [22]. False-positive tests cause patient anxiety and the potential for additional testing and unnecessary surgical procedures.

Carotid tests — In the absence of symptoms, screening patients for carotid artery stenosis has not been shown to be effective at a population level. As noted above, we suggest not screening asymptomatic individuals for carotid artery stenosis with vascular imaging tests (see 'Our approach' above). Screening studies for carotid artery stenosis have used two approaches: noninvasive imaging of the carotid artery (eg, carotid duplex ultrasonography [DUS], magnetic resonance angiography [MRA], or computed tomography angiography [CTA]) and auscultation for carotid bruits during the physical examination. Cerebral angiography is the gold standard for imaging the carotid arteries, but it is not appropriate for screening because of its invasive nature, attendant risk of complications, high cost, and risk of morbidity. (See "Evaluation of carotid artery stenosis", section on 'Catheter cerebral angiography'.)

Noninvasive imaging — There are many noninvasive techniques available to identify and quantify carotid stenosis, including carotid DUS, MRA, contrast-enhanced MRA (CEMRA), and CTA. (See "Evaluation of carotid artery stenosis".)

The accuracy of these noninvasive imaging modalities is not well-studied in subjects with asymptomatic carotid stenosis, but all have high sensitivities and specificities for diagnosing 70 to 99 percent internal carotid artery stenosis in patients with ipsilateral carotid territory ischemic symptoms. (See "Evaluation of carotid artery stenosis", section on 'Choice of imaging test'.)

Carotid auscultation — A carotid bruit is an audible sound arising from turbulent blood flow. Interobserver agreement in detecting a carotid bruit by physicians is relatively high (kappa = 0.67) [23]. A meta-analysis suggested that the presence of a carotid bruit increases the risk of cerebrovascular disease [24]. However, a bruit alone is a poor predictor of either underlying carotid stenosis or stroke risk in asymptomatic patients, as reflected by the following:

●In a meta-analysis of symptomatic and asymptomatic patients, the sensitivity and specificity of a carotid bruit for a 70 to 99 percent stenosis carotid lesion were 53 percent and 83 percent, respectively [25].

●The Framingham Heart Study found that an asymptomatic carotid bruit was associated with an approximate doubling of the expected stroke risk [26]. However, the majority of strokes occurred in a vascular territory different from the carotid bruit, suggesting that a carotid bruit is a marker of generalized atherosclerosis.

●The Systolic Hypertension in the Elderly Program (SHEP) found that the presence of a carotid bruit was associated with a nonsignificant overall relative risk of 1.29 for stroke over a mean follow-up of 4.2 years [27].

●A study of 241 older nursing home residents (mean age 86 years) found that 12 percent had asymptomatic carotid bruits [28]. The three-year cumulative incidence of cerebrovascular events was similar for patients with and without bruits (10 and 9 percent, respectively). Furthermore, baseline carotid bruits disappeared in 60 percent of surviving residents; loss of the bruit was not due to a cerebrovascular event.

The poor predictive value of carotid bruits in asymptomatic individuals is in part related to the low prevalence of significant carotid stenosis in this population.

Bruits are a better indicator of general atherosclerotic disease than of stroke risk [26,29-31]. The rate of myocardial infarction and cardiovascular death in patients with carotid bruits is twice that of patients without carotid bruits [31]. Patients with carotid artery disease are more likely to die from cardiovascular than cerebrovascular disease [32,33]. Nonetheless, risk stratification to identify patients at high risk for cardiovascular disease is most commonly based upon the Framingham risk score, the pooled cardiovascular risk calculator (calculator 1) [34,35], or similar risk assessment tools and not the presence or absence of carotid bruits. (See "Atherosclerotic cardiovascular disease risk assessment for primary prevention in adults: Our approach" and "Screening for coronary heart disease".)

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Stroke in adults" and "Society guideline links: Occlusive carotid, aortic, renal, mesenteric, and peripheral atherosclerotic disease".)

SUMMARY AND RECOMMENDATIONS

●Definition of asymptomatic carotid stenosis – Asymptomatic cervical carotid atherosclerotic disease refers to the presence of atherosclerotic narrowing of the extracranial internal carotid artery in individuals without a history of recent ipsilateral carotid territory ischemic stroke or transient ischemic attack (TIA). The cutoff used to define clinically significant carotid artery stenosis varies among studies, ranging from ≥50 to ≥70 percent stenosis. (See 'Definitions' above.)

●Prevalence – The prevalence of asymptomatic carotid stenosis is low in the general population, but increases with age, which is the most important risk factor, and is higher in men than in women. (See 'Prevalence' above.)

●Risk of stroke with asymptomatic disease – The most important consequence of carotid atherosclerosis is ischemic stroke; however, the estimated risk of stroke directly attributable to an identified stenosis in patients with asymptomatic carotid atherosclerosis (stenosis ≥50 percent) is ≤1 percent annually. (See 'Stroke risk' above.)

●Screening not recommended – In the absence of a practical strategy for targeting screening to individuals with high risk, we suggest not screening asymptomatic individuals for carotid artery stenosis (Grade 2B). This recommendation is based on the low prevalence of asymptomatic carotid stenosis, the low annual risk for stroke in patients with asymptomatic carotid stenosis, and the high estimated rate of false-positive results with noninvasive screening that could lead to unnecessary additional testing or interventions. (See 'Our approach' above and 'Benefits and harms of screening' above.)