1. Introduction
Traditional risk factors for stroke include advanced age, hypertension, diabetes mellitus, hyperlipidemia, cardiac or coronary artery disease, obesity, and tobacco smoking [
1]. In 2010, the American Heart Association introduced a framework for ideal cardiovascular health called Life’s Simple 7, which are seven predictors (body weight, physical activity, nonsmoking, diet, total cholesterol level, blood pressure, and fasting blood glucose level) of heart health [
2]. Adherence to Life’s Simple 7 may effectively reduce primary and secondary cardiovascular risks in the general population [
3].
The aforementioned traditional risk factors also contribute to the development of carotid atherosclerosis, which may serve as a marker of the general atherosclerotic process and is associated with cardiovascular and cerebrovascular events [
4,
5,
6]. The prevalence of carotid plaque formation has been increasing worldwide [
7]. Carotid atherosclerosis screening may help identify high-risk patients who would benefit the most from intensive medical therapy. Spence et al. reported that 63% of all asymptomatic individuals had carotid plaque progression and that these patients had twice the risk of those with stable plaque [
8,
9]. Carotid sonography is the most convenient, noninvasive, and reproducible tool for the evaluation of carotid atherosclerosis, and its use is recommended for all patients who have experienced an ischemic stroke or a transient ischemic attack (TIA). However, the procedure is not frequently performed on patients with the aforementioned traditional risk factors in whom no cerebrovascular event has occurred. As the prevalence of asymptomatic carotid artery stenosis is low in the general population and because this disease is associated with a relatively small proportion of all stroke cases, the US Preventive Service Task Force (USPSTF) recommends a moderate level of caution against screening asymptomatic individuals (i.e., adults without a history of stroke or a neurologic sign or symptom of a TIA) for carotid artery stenosis [
10]. The European Society of Cardiology does not recommend the systematic screening of carotid artery stenosis because it does not improve disease outcomes in patients with diabetes mellitus without a history of cerebrovascular disease [
11]. However, the prevalence of carotid artery stenosis is considerably higher in patients with multiple risk factors than in the general population [
7]. The European Society of Cardiology and the European Atherosclerosis Society still regard carotid plaque as class IIa evidence in the risk stratification of cardiovascular disease [
12]. We recently demonstrated that asymptomatic individuals with carotid risk scores of ≥7 had a 50% risk of moderate-grade or high-grade carotid atherosclerosis (MHCA), highlighting the need for comprehensive carotid sonography [
13].
Point-of-care ultrasound (POCUS), the concept of an ultrasound stethoscope, is defined as ultrasonography brought to the patient and performed by the provider in real time [
14]. It is rapid, cost effective, and portable; therefore, it has diverse uses in medical settings, particularly in the emergency department, with applications ranging from screening and diagnosis to procedural guidance and monitoring, and it has become crucial for shared clinical decision making [
15]. However, its diagnostic accuracy varies depending on the skill of the operator, quality of the ultrasound device, location of the target organ, and time required for the procedure. Rapid carotid POCUS screening may not be necessary for symptomatic patients who require comprehensive carotid sonography. It is more valuable for use on asymptomatic individuals or individuals with vascular risk factors. Therefore, in this study, we evaluated the validity and reliability of carotid POCUS for detection of carotid atherosclerosis in asymptomatic individuals with multiple risk factors in an outpatient setting.
4. Discussion
We demonstrated the validity of outpatient POCUS for the rapid (≤3 min) screening of carotid atherosclerosis in high-risk patients. In our cohort, 50% of the patients with carotid risk scores of ≥7 were diagnosed as having MHCA (laboratory hCPS > 5). sCPSs were more likely to be overestimated (underestimated) in patients with low (high) laboratory sCPSs. The participants’ outpatient sCPSs exhibited satisfactory agreement, strong correlations, and excellent reliability with their laboratory sCPSs. Higher carotid risk scores indicate higher sCPSs and hCPSs.
Plaque scores, rather than plaque morphology, are widely used to predict plaque burden and vascular risk. Various scoring systems with satisfactory predictive ability have been developed for the quantitative assessment of carotid plaques [
21,
26,
27,
28,
29,
30]. We previously evaluated the hCPSs of the respondents of our community survey because of the convenience of calculation; this score can be calculated by simply summing the maximal plaque thicknesses measured on the near and far walls at each of the four areas on both sides of the carotid arteries without consideration of plaque morphology [
13]. In the present study, we selected sCPS for outpatient POCUS because it is a convenient, time-saving, and semiquantitative approach, with no requirement for measuring plaque thickness. The average time required for the calculation of sCPS through outpatient carotid POCUS may be shortened to <3 min if the assessment is performed with the patient in a suitable sitting position; in this position, the patient does not need to change posture except to turn the body to an appropriate angle to facilitate the detection of the bilateral carotid arteries. The wireless ultrasound device used enables physicians to move the scanner freely without any limitation related to distance from the tablet. Thus, physicians can perform rapid bedside carotid POCUS screening as part of a physical examination, similar to the stethoscope-based examination of the patient’s heart and breathing sounds.
The overestimation of outpatient sCPSs was more likely in patients with lower laboratory sCPSs. This might be because the apparent (visual) local thickening (<50%) of the intima–media layer may be regarded as a carotid plaque alongside a relatively smooth and normal carotid wall. The underestimation of outpatient sCPSs was more likely in patients with higher laboratory-derived sCPSs. Segmental plaques across the CCA bifurcation and ICA were categorized into one area on rapid POCUS but two areas on laboratory comprehensive sonography, in which the landmark of the vessel walls could be clearly visualized without any time limit, with a score of 1 and 2, respectively. However, this discrepancy in scores did not affect the consistency of the evaluation results.
We demonstrated satisfactory agreement and a strongly positive linear correlation (r = 0.956) between outpatient and laboratory sCPSs. An ICC of 0.954 with 95% confidence interval of 0.925–0.972 between the outpatient and laboratory sCPS measurements suggests considerably high similarity between the measurement results, which indicated that the measurements had excellent reliability (ICC > 0.90) [
24]. We further found significantly positive linear correlations between outpatient and laboratory-derived sCPSs and laboratory-derived hCPSs. The calculation of hCPS is more time consuming than that of sCPS; nevertheless, hCPS offers a higher level of detail and a wider range of plaque scores to better reflect atherosclerosis compared with sCPS. These findings suggest that sCPS is a suitable plaque score for the POCUS-based screening of carotid atherosclerosis in the OPD.
Certain hypoechoic plaques may be overlooked when real-time B-mode carotid ultrasound is performed without color Doppler flow imaging, but the detection rate can be increased with improved pixel resolution of real-time B-mode imaging. Jang et al. reported that 39 of 801 (5%) asymptomatic individuals had hypoechoic plaque [
23]. Furthermore, Polak et al. observed that 856 of 4886 (17.5%) asymptomatic individuals aged ≥ 65 years had hypoechoic plaques without any visible echogenic cap in the ICA [
31]. Among them, 30 individuals (0.6%) had a stroke within 3.3 years. Only a small portion of ICA hypoechoic plaques without any visible echogenic cap escape detection; nonetheless, this is problematic because hypoechoic plaques are associated with vascular events and thus regarded as high-risk plaques. Currently, most newly developed pocket-size POCUS devices facilitate high-resolution real-time B-mode imaging with color Doppler flow imaging. Another unusual scenario is distal ICA stenosis or occlusion without any apparent plaque within the examined ICA range; this condition can only be diagnosed on the basis of increased resistance and reduced blood flow volume noted during Doppler flow imaging. However, Yang et al. [
32] reported that the incidence rate of the aforementioned condition was considerably low, accounting for only 0.5% of all laboratory carotid sonography assessments.
Although no special medications are prescribed for mild- or moderate-grade carotid atherosclerosis, patients’ awareness of their subclinical carotid atherosclerosis may help reduce their cardiovascular risk, possibly by enhancing their compliance with medications and encouraging them to implement healthy lifestyle changes [
33]. Asymptomatic individuals with MHCA require intensive therapy. Spence et al. formulated a new paradigm of “treating arteries instead of treating risk factors”. They observed that more intensive treatment of plaque progression resulted in a successful reversed portion of plaque progression versus regression in asymptomatic patients with carotid stenosis, considerably decreasing the incidence of cardiovascular events [
34]. They emphasized that treating arteries without examining plaques is comparable to treating hypertension without measuring blood pressure.
The USPSTF reported that the harms of screening asymptomatic individuals for carotid artery stenosis outweigh the benefits [
10]. Extensive laboratory-based comprehensive carotid sonography for asymptomatic individuals with one or more risk factors is not recommended because of the low incidence of carotid atherosclerosis and low cost-effectiveness of this procedure. Nevertheless, the noninvasive screening of subclinical atherosclerosis should be implemented to promote early diagnosis in high-risk patients [
10]. In a study including 5808 adults from the United States, Baber et al. observed that the risks of major adverse cardiac events increased with carotid plaque burden and coronary artery calcification [
5]. They recommended further cost-effective analyses to define the optimal roles of these complementary noninvasive techniques. We previously indicated the requirement for comprehensive carotid sonography in patients with carotid risk scores of ≥7, which indicates a >50% risk of MHCA. In the present study, we further recommend outpatient POCUS-based sCPS calculation for the screening of patients with multiple risk factors, which might be extended to patients with a lower carotid risk score, depending on clinical judgment. Outpatient POCUS can facilitate the rapid and cost-effective screening of carotid burden in patients with one or more risk factors. This tool may help avoid unnecessary comprehensive carotid ultrasonography in patients with carotid risk scores of ≥7 and considerably low sCPSs and also help identify patients with carotid risk scores of <7 but and high sCPSs. At present, POCUS devices are not widely used. The development of wireless, high-resolution, and cost-effective pocket-size ultrasound devices opens up new avenues for future prospective studies on the advantages and novel applications of outpatient carotid POCUS for patients with various risk factors.
This study has some limitations. First, a skilled and experience clinician is required to perform outpatient carotid POCUS with the patient in a sitting position, particularly if the location of CCA bifurcation is relatively high. Second, we did not perform intrarater and interrater reliability tests. Ultrasound is a highly operator-dependent procedure; therefore, sonographers must receive rigorous training. Richter et al. reported excellent interrater agreement (experienced and certified vascular neurologists) and satisfactory inter method agreement (compared with angiography) for the sonographic grading of extracranial ICA stenosis [
35]. Andersen et al. focused on outpatient cardiac auscultation performed for the detection of cardiac murmur by using a stethoscope; they reported that both intrarater agreement and agreement with the reference considerably varied among general practitioners, cardiologists, and medical students [
36]. They found that >5 years of experience in clinical practice and cardiology specialty was strongly associated with agreement between the obtained and reference results of cardiac murmur. In the study of Suttie et al., a trained medical student, an emergency department resident, and emergency department physicians used POCUS to detect carotid artery stenosis in patients with acute stroke or TIA presenting to the emergency department [
37]. The authors reported that the results of carotid POCUS had a low to moderate association with those of computed tomography angiography for detecting ≥ 50% stenosis. Similarly, Saxhaug et al. conducted a carotid POCUS study in the ward, wherein experienced cardiologists evaluated patients hospitalized because of acute stroke or TIA for carotid artery stenosis [
38]. Their study indicated satisfactory agreement between the handheld ultrasound device and conventional duplex ultrasound or computed tomography angiography for the classification of carotid stenosis. In the present study, carotid POCUS was performed by a stroke neurologist specializing in neurosonology and with >30 years of clinical experience. Laboratory carotid sonography was performed by experienced sonographers with ≥10 years of relevant experience, and the results were interpreted by another stroke neurologist with ≥5 years of relevant experience. Sufficient training is crucial for quality to be maintained. Nevertheless, studies could further examine the interrater agreement studies of carotid POCUS to help clarify the benefits of its clinical application. Third, as previously mentioned, hypoechoic plaque or distal ICA stenosis or occlusion is likely to be overlooked during real-time B-mode imaging. To save time, we performed only the visual assessment of plaques for calculating the participants’ outpatient sCPSs even though color Doppler flow imaging and Doppler flow detection are built-in functions of the POCUS device used in this study. Laboratory hCPSs can be accurately determined through the measurement of plaque thickness, but this process is time consuming. Nevertheless, the correlation we observed between sCPS and hCPS was strong.