SONOGRAPHIC EVALUATION IN CAROTID ARTERY STENOSIS
B. K. Lal
Department of Surgery, New Jersey Medical School, University of Medicine and Dentistry of New Jersey, Newark, New Jersey, USA
Introduction
Over three decades ago, Doppler was introduced as the first ultrasonographic method for evaluation of cerebrovascular disease. Since then, the development of new ultrasound techniques has revolutionized the clinical applications for extracranial carotid disease.
Interpretation of pulse wave Doppler signals invol- ves an analysis of audio signals and the frequency spectrum. It is based on the Doppler frequency shift resulting from moving red blood cells in the carotid artery. In experienced hands, pulse wave Doppler ul- trasound can identify significant luminal narrowing based on increased velocity of blood flow across a stenotic lesion. High-resolution B-mode ultrasound scanning uses linear array transducers (7–12 MHz) to display morphological features of the arterial wall.
Duplex ultrasonography (DUS) combines integrated PW Doppler spectrum analysis and B-mode sonog- raphy. The B-mode image offers information about morphology in addition to serving as a guide for ac- curate PW Doppler velocity measurement. Color Doppler flow imaging based on the direction of flow superimposes color-coded blood flow patterns over the B-mode image. Power Doppler imaging color codes blood flow according to the amplitude of the Doppler signal. Both these modalities afford greater sensitivity to blood flow detection allowing improved detection of near-occlusive stenoses, tor- tuosity, and other morphological abnormalities in the arterial wall. The noninvasive nature of DUS makes this testing modality more attractive than the “gold standard” of cervical angiography. If the diagnosis of significant carotid stenosis can be ac- curately made with DUS, the risk of stroke inherent in angiography (1.2% in the Asymptomatic Carotid Atherosclerosis Study [9]) can be avoided. Cur-
rently, many patients (90% at the authors’ institu- tion) undergo carotid revascularization based solely on a duplex examination.
Indications for testing
Symptomatic patients
Prospective randomized trials have documented the efficacy of carotid endarterectomy (CEA) in reduc- ing stroke in symptomatic carotid stenosis. The North American Symptomatic Carotid Endar- terectomy Trial (NASCET) confirmed that CEA of- fered a 65% relative risk reduction compared to best medical therapy in patients with 70-99% carotid ste- nosis who present with transient ischemic attack (TIA) or non-disabling stroke [23]. Similarly, symp- tomatic patients with a 50–69% carotid stenosis de- rived a 29% relative risk reduction at 5 years [4].
Therefore all patients with hemispheric neurological
symptoms attributable to the carotid circulation
should undergo carotid imaging. This test should be
performed even in the absence of a carotid bruit be-
cause NASCET data revealed that over one third of
patients with high-grade carotid stenosis lacked a ca-
rotid bruit [28]. Testing in asymptomatic patients
with hemispheric infarcts identified on computed to-
mographic (CT) or magnetic resonance (MR) is also
indicated to rule out carotid disease. Patients with
non-lateralizing symptoms such as dizziness or light-
headedness cannot be assumed to be caused by ca-
rotid stenosis. Other symptoms such as cranial nerve
dysfunction (e.g., dysphasia, double vision, or blurred
vision) and drop attacks are more likely to be verte-
bro-basilar in origin.
Asymptomatic patients with a carotid bruit The Asymptomatic Carotid Atherosclerosis Study (ACAS) documented a 53% relative risk reduction in ipsilateral stroke and any perioperative stroke or death at 5 years when comparing CEA with medical therapy for asymptomatic 60% carotid stenosis [9]. Controversy continues regarding who best to screen since widespread imaging of all asymptomatic patients is unlikely to be cost-effective. Most physi- cians have adopted a selective approach; however, it is clear that additional studies are necessary to clar- ify this issue.
The most commonly used selection criteria in- clude the presence of a carotid bruit. Carotid bruits, expected to be found in up to 8.2% of patients 75 years of age [14] have been associated with a three- times increased risk of stroke in a population-based prospective cohort study [33]. One third of patients with an asymptomatic carotid bruit can be expected to harbor a carotid stenosis 50% when studied with DUS [27]. Of note, 21% of patients with a bruit ultimately progressed to 50–79% stenosis; while 27% of patients with 50–79% stenosis progressed to 80% stenosis at 7 years [17]. It is clear that the pres- ence of a carotid bruit should prompt a carotid du- plex ultrasound to screen for carotid stenosis and the patients must be followed regularly for possible dis- ease progression in case their disease is not severe enough to warrant revascularization.
Asymptomatic patients without a bruit
Significant carotid stenosis can exist in the absence of cervical bruit. For this reason, recommendations for carotid duplex scanning have been made based on specific risk factors such as age, hypertension, smoking, coronary artery disease (CAD), and periph- eral arterial occlusive disease (PAD). Significant carotid stenosis was present in 17.5% of patients aged 50 years with CAD, tobacco abuse, and PAD [12]. Claudication and decreased ankle-bra- chial index have both been found to be predictive of carotid stenosis [21]. In a prospective evaluation of 1087 patients undergoing coronary artery bypasses, 17% had carotid stenoses 50% and 5% had steno-
ses 80%. Age 60 years, hypertension, diabetes mellitus, and smoking, increase the risk of signifi- cant carotid stenosis in patients undergoing cardiac surgery [3]. Based on these data, duplex scanning can be recommended selectively based on the pres- ence of several risk factors.
Post-carotid revascularization surveillance
Carotid duplex scanning has been used as a surveil- lance tool for carotid restenosis. Widely varying proto- cols exist for the follow-up of patients after carotid re- vascularization. In addition, despite the absence of conclusive cost-effectiveness data, it is likely that ca- rotid duplex scanning for surveillance in this setting will continue to be a common indication, given the re- stenosis rates associated with CEA [8] and carotid ar- tery stenting [20]. Data supporting surveillance of the contralateral carotid artery after ipsilateral revascula- rization is more forthcoming since a substantial num- ber of contralateral arteries have been observed to prog- ress to high-grade stenoses on follow-up [29]. The optimal frequency of this surveillance has not been de- fined but the author follows patients with 50% ste- noses once a year; and those with 50% disease twice a year.
Quantifying the degree of carotid stenosis
Arteriography is the standard against which all
non-invasive assessments of carotid luminal narrowing
are commonly compared. While several method-
ologies have been proposed for the angiographic
quantification of stenosis, the Committee on Stan-
dards for Noninvasive Vascular Testing of the Joint
Council of the Vascular Societies recommended that
percent diameter reduction should be determined
relative to the distal uninvolved internal carotid ar-
tery (ICA) [30]. Doppler measures that have been
correlated with angiographic stenosis include ICA
peak systolic velocity (PSV), and end-diastolic veloc-
ity (EDV), as well as ratios of ICA PSV and CCA
PSV [1].
with increased carotid volume flow resulting in an overestimation of the severity of disease [31]. Most diagnostic laboratories simply use the well-established criteria of others to diagnose severe carotid stenosis.
However, for the reasons cited above, it is recommend- ed that each laboratory validate its own Doppler cri- teria for clinically relevant stenoses [18]. One such methodology is to subject the vascular laboratory to certification by an independent auditing organization such as the Intersocietal Commission for Accredita- tion of Vascular Laboratories (ICAVL) [6], [16]. Stud- ies comparing the accuracy of duplex ultrasound ex- aminations have noted consistently superior results from accredited versus non-accredited laboratories [6].
A group of experts from different medical spe- cialties met in October 2002 to arrive at a consensus document with regard to the use of DUS in the dia- gnosis of ICA [13]. Table 1 illustrates their recom- mendations when using grayscale imaging and Doppler ultrasound. Technical considerations, diag- nostic stratification, imaging and Doppler param- eters, Doppler diagnostic thresholds, structure and content of the final report, and quality assessment issues were all discussed and suggestions were made.
Recommendations were also made for follow-up of patients at high risk or with asymptomatic carotid stenosis, and research topics were suggested for the future. Ideally, each center should perform its own validation; however, in the absence of such valida- tion, the criteria suggested in Table 1 may serve as a useful guideline.
Ultrasound and carotid artery stenting
Carotid artery stenting (CAS) has emerged as an alter- native to CEA in the management of carotid stenosis under specific high-risk circumstances [32]. The ulti- mate value of CAS when compared with CEA will be based upon ongoing prospective randomized clinical trials [15]. In the interim, however, the number of pa- tients undergoing CAS is increasing rapidly and these patients require intensive follow-up to monitor for in- stent restenosis [20]. US velocity criteria have not been well established for patients undergoing CAS. We have Initially, ICA PSV and the relative distribution
of velocities within the spectral profile based on PW Doppler measurements, were used to define the de- gree of narrowing of the ICA [5]. The University of Washington criteria were subsequently refined with the addition of EDV [28]. Accordingly, stenoses were classified as mild (1–15%), moderate (16–49%), severe (50–99%) and occluded. These criteria re- mained useful until the publication of the results of the North American Symptomatic Carotid Endarter- ectomy Trial (NASCET) [23], [4], and the Asymp- tomatic Carotid Atherosclerosis Study (ACAS) [9].
The NASCET demonstrated an early and signifi- cant benefit associated with carotid endarterectomy (CEA) in symptomatic patients with an ipsilateral ICA stenosis 70%, and a significant benefit over a longer follow-up in symptomatic patients with ICA stenosis of 50–69%. Additionally, the ACAS conclu- ded that asymptomatic patients with an ICA stenosis
60% could also benefit from CEA. These new cri- teria did not correspond to the categories previously defined by researchers at the University of Washing- ton. Using receiver operator characteristic (ROC) curves to compare sensitivity, specificity, positive pre- dictive value (PPV) and negative predictive value (NPV) for new criteria to define degrees of stenosis relevant to clinical management, Faught et al. [10]
concluded that the combination of a PSV 130 cm/s and an EDV 100 cm/s defined a stenosis of 70–99%. Using a similar approach, Moneta et al.
[22] concluded that an ICA PSV/common carotid artery PSV ratio 4.0 provided optimal accuracy for the diagnosis of a stenosis of 70–99%. Yet a third set of criteria for the same degree of stenosis were pro- posed by Carpenter et al. [7] such that a combination of PSV 210 cm/s, EDV 70 cm/s, ICA PSV/CC PSV 3.0, and ICA EDV/CCA EDV 3.3 was most accurate. It is clear that DUS results are more accurate when a single cutoff point for stenosis is used (e.g., 50% or 60%), or when a broad cate- gory is used rather than a small range of stenosis (e.g., 50–99% vs. 60–69%).
Recent studies demonstrate that Doppler criteria are influenced by equipment used [11], laboratories [2] and the technologist performing the test [24].
Additionally, contralateral disease has been associated
reported that the introduction of a stent into the ICA alters arterial biomechanical properties such that the resultant stent-arterial complex has decreased compli- ance [19]. Additional studies have demonstrated that when velocity criteria for non-stented ICAs were used in stented ICAs, several normal diameters were diag- nosed as having in-stent stenosis [25], [26]. We have also reported that normal luminal diameters in recently stented ICAs were most accurately defined by revised velocity criteria: PSV 150 cm/sec with ICA PSV/CCA PSV ratio 2.16 [19]. Correlation studies with evolving in-stent restenosis (ISR) and angio- graphic diameter reduction will help define velocity criteria for increasing grades of stenosis in the stented ICA. Meanwhile, the author’s laboratory routinely reg- isters baseline velocities immediately post-CAS, and utilizes a combination of planimetric measurements on B-mode imaging and rising intra or peri-stent veloc- ities as indicators of evolving ISR during follow-up.
Testing procedure
The DUS examination must include a blood pres- sure measurement in both arms. A blood pressure differential of 20 mmHg is indicative of possible brachiocephalic, subclavian or axillary artery steno- sis. Testing proceeds with the patient in the supine position with the head turned away from the side being examined. The carotid sheath travels along the anterior border of the sternocleidomastoid muscle and this is where imaging can commence. The com- mon carotid artery is identified at its origin in the
base of the neck just above the clavicle and followed to the carotid bifurcation usually located below the mandible. Abnormal locations of the bifurcation must be reported since they impact on the extent of surgical exposure (mandibular subluxation during CEA for high bifurcations) or decision regarding the type of revascularization to be offered (CAS for extremely high bifurcations).
The transducer may be placed anterior, medial, or posterior to the sternocleidomastoid muscle to opti- mize flow visualization. Any luminal or wall abnormal- ities should be reported. Velocity measurements are recorded with an angle of insonation of 60°. Com- monly, velocity measurements are obtained at the prox- imal and distal common carotid artery (CCA), exter- nal carotid artery (ECA); ICA (proximal, middle, and distal); and vertebral. Every effort should be made to follow the ICA as far cephalad as possible. The loca- tion of the distal extent of the plaque should be report- ed since this again impacts on the revascularization procedure. If the plaque continues cephalad into the distal ICA it entails extensive surgical dissection in the distal carotid triangle.
The vertebral arteries can be found posterior to the CCA between the vertebral bodies. While the tortuous course of these arteries makes diagnosis of a stenosis difficult; every effort must be made to re- port antegrade versus retrograde (i.e., reverse) flow.
Validation
The standard technique of validating DUS is through comparison to angiography. The goal for a definitive
Table 1. Representative Doppler ultrasound velocity criteria for the diagnosis of internal carotid artery stenosis
Primary parameters Additional parameters
Degree of stenosis (%) ICA PSV (cm/sec) ICA/CCA PSV ratio ICA EDV (cm/sec)
Normal 125 2.0 40
50 125 2.0 40
50–69 125–230 2.0–4.0 40–100
70–99 230 4.0 100
Near occlusion High/low/undetectable Variable Variable
Occlusion Undetectable Not applicable Undetectable
Adapted from: Grant EG et al.: Carotid artery stenosis: Gray-scale and Doppler ultrasound diagnosis. Society of Radiologists in Ultrasound con- sensus conference. Radiology 229: 340–346 (2003).
much less because the highest velocities are seen at the exit of the stenosis. Planimetry, however, comple- ments velocity measurements and these methods should be used together. It is an especially important adjunct in the follow-up of stented carotid arteries.
Conclusions
Carotid DUS is the established method for diagnosing a cervical carotid artery stenosis. In medical centers with a vascular laboratory that has established an excellent PPV, angiography may be avoided before embarking on carotid revascularization. Other laboratories may em- phasize the screening aspect of their testing and will have a higher NPV. Patients diagnosed in this setting must undergo a confirmatory study prior to undergoing revascularization. The criteria used for diagnosing spe- cific categories of stenoses vary between laboratories.
The hallmark of a good laboratory is one that estab- lishes its own criteria and continually validates them in an ongoing quality assurance program.
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