(Stroke. 1995;26:813-815.)
© 1995 American Heart Association, Inc.
Articles |
Asymptomatic Cerebral Embolic Signals in Patients With Carotid Stenosis
Correlation With Appearance of Plaque Ulceration on Angiography
From the Departments of Neurology (L.V., V.L., G.G., A.B.) and Neuroradiology (P.A.), Rangueil University Hospital, Toulouse, France.
Correspondence to Vincent Larrue, MD, Service de Neurologie, Hôpital Rangueil, 31054 Toulouse, France.
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Abstract |
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 Top Abstract IntroductionSubjects and MethodsResultsDiscussionReferences |
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Background and Purpose Asymptomatic cerebral emboli may be detected in patients with carotid stenosis by transcranial Doppler ultrasonography of the middle cerebral artery (MCA). The aim of this study was to determine the angiographic correlates of such embolic signals.
Methods Doppler signals from 48 MCAs in 26 patients with carotid stenosis that was either symptomatic (n=20) or asymptomatic (n=6) were recorded for 40 minutes. The grade of carotid stenosis and the ulcerated or nonulcerated appearance of the plaque were assessed using the criteria of the North American Symptomatic Carotid Endarterectomy Trial.
Results Embolic signals were detected in 8 MCAs from 7 patients; 4 (50%) of these MCAs were clinically symptomatic compared with 16 (40%) without embolic signal. Although there was a trend toward more severe stenosis in the cases with embolic signals, this was not significant (mean±SD, 67±29% versus 55±36%). In contrast, an image of ulceration was found on ipsilateral carotid angiography in 5 cases (63%) with embolic signals and in only 9 cases (23%) without embolic signals (odds ratio, 5.74; 95% confidence interval, 1.15 to 28.79, by multivariate regression analysis).
Conclusions This study demonstrates that the occurrence of embolic signals in patients with carotid stenosis is associated with the appearance of plaque ulceration on angiography.
Key Words: carotid artery diseases • cerebral embolism and thrombosis • ultrasonics
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Introduction |
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TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
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Artery-to-artery embolism is the major mechanism of ischemic stroke in patients with extracranial internal carotid artery disease.1 2 3 Some factors are recognized as increasing the risk of stroke in these patients and are important when deciding on treatment: patients with recent symptoms due to high-grade (>70%) internal carotid artery narrowing benefit from carotid endarterectomy, whereas for most asymptomatic patients and patients with <70% symptomatic stenosis, the benefit is uncertain.4 5 Moreover, a recent report has demonstrated that the presence of an angiographically defined ulceration is associated with increased risk of stroke in medically treated symptomatic patients.6
Transcranial Doppler (TCD) ultrasonography monitoring of the middle cerebral arteries (MCAs) can reveal abnormal high-intensity transient signals (HITS) occurring without any clinical symptoms and indicating cerebral emboli. HITS were first reported in patients with high-grade stenosis by Spencer et al.7 Since then, both animal models8 9 and flow-circuit models10 have allowed definition of the features of HITS caused by solid emboli. Several studies have confirmed that embolic signals can be detected in patients with carotid artery disease,11 12 13 14 15 16 17 and some of these studies have demonstrated an association with the previous occurrence of clinical symptoms and the degree of carotid narrowing.15 17 The aim of our study was to determine whether the ulcerated appearance of the plaque on carotid angiography is a predictor of HITS.
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Subjects and Methods |
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TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
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We reviewed the recordings of TCD monitoring for embolus detection performed between June 1993 and August 1994 in patients who underwent angiography for carotid stenosis. Patients with cardiac sources of emboli and those with thick (>5 mm) or irregular plaques in the aortic arch were excluded by electrocardiography and transesophageal echocardiography. Another patient was excluded because carotid fibromuscular dysplasia was demonstrated on angiography. Twenty-six patients remained (22 men and 4 women). The mean±SD age was 69±10 years (range, 48 to 88 years). Eleven patients presented with either cerebral (10) or retinal (1) transient ischemic attack, 9 had an ischemic stroke, and 6 were asymptomatic. Ischemic symptoms had occurred in the territory of one MCA in 18 patients, in the territory of both MCAs in 1 patient, and in the vertebrobasilar territory in 1 patient. Thus, there were 20 symptomatic and 32 asymptomatic MCAs.
The patients were receiving the following antithrombotic treatments when TCD monitoring was performed: full-dose heparin (5 patients), aspirin (4), ticlopidine (1), either clopidogrel or aspirin (1), low-dose heparin plus aspirin (11), or no antithrombotic treatment (4).
All patients underwent bilateral selective carotid angiography. The angiograms were independently reviewed by two neuroradiologists who were blinded to the clinical data and to the results of TCD monitoring. Carotid stenoses were graded using the method described in the North American Symptomatic Carotid Endarterectomy Trial (NASCET).18 Plaques were further classified according to their ulcerated or nonulcerated appearance, also using NASCET criteria.19
Long-term monitoring of the MCA was performed for 40 minutes on both sides in 22 patients and, due to the lack of a bone window, on only one side in 4 patients. We used a 2-MHz pulsed-wave transducer (Diadop 500, Diatecnic). The Doppler probe was attached with an elastic headband. The depth was chosen from 45 to 50 mm to obtain optimum insonation of the MCA. The audible Doppler shift and the fast-Fourier transformed TCD spectra were continuously observed by an experienced investigator who noted all the events that could be potential sources of artifacts. Video recordings were analyzed off-line by two independent observers (L.V. and V.L.) who were blinded to the clinical data and to the results of carotid angiography. Embolic signals were identified according to their previously defined features12 : short-duration, unidirectional, high-intensity signals visible in the Doppler spectrum, occurring randomly within the cardiac cycle, accompanied by a characteristic "chirping" or "clicking" sound, and without any possible cause of artifact at the same time.
Univariate analysis was performed using the 
2
test for categorical variables and Student's t test for
continuous ones. Significance was set at P<.05. To assess
the effect of selected variables on the probability of HITS, a
multivariate regression analysis using a logistic model was
performed. The explanatory variables included in the initial model were
the previous occurrence of clinical symptoms, carotid stenosis >50%,
and an image of ulceration in the carotid plaque on angiography. A
backward selection procedure allowed only significant variables to be
retained in the model. The cutoff to exclude a variable from the
initial model was defined as a ratio of the absolute value of the
estimate to the standard error smaller than 1. Analysis was conducted
using GLIM software (Royal Statistical Society). Results
are expressed as means±SD.
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Results |
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TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
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HITS were detected in 8 MCAs from 7 patients. The
index for
interobserver agreement was 0.83. The rate of HITS was 2.8±1.9 per
hour. Among symptomatic patients, the time between symptoms and recordings was 31±50 days in patients with HITS and 22±23 days in those without HITS. The difference was not significant. The number of symptomatic patients with HITS was too small to allow assessment of the relationship between the time since the last symptoms and the rate of HITS.
Four (50%) MCAs with HITS were clinically symptomatic compared with 16 (40%) without HITS. The proportion of cases with antithrombotic treatment was exactly the same in cases with HITS (7/8) as in cases without HITS (35/40). The time between angiography and recordings was 4±8 days in cases with HITS and 8±12 days in those without HITS (P=.4).
The degree of carotid narrowing was 67±29% in cases with HITS and
55±36% in those without HITS. This trend was not significant. In
contrast, an image of ulceration was found on ipsilateral carotid
angiograms in 5 cases (63%) with HITS and in only 9 cases (23%)
without HITS (P=.02) (Table 1
). The index
for interobserver agreement in the diagnosis of ulceration was 0.85.
The values of the estimates and standard errors observed in the
regression model are given in Table 2. The backward
procedure selected the ulcerated appearance of the carotid plaque as
the only significant explanatory variable. The corresponding odds ratio
was 5.74, and the 95% confidence interval was 1.15 to 28.79.
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Discussion |
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TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
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The assumption that HITS correspond to solid emboli relies on the following observations: first, in previous studies, these signals occurred distal to emboligenic lesions, whereas they were not detected in control subjects12 16 17 ; second, signals with the same physical features were recorded during the passage of formed-element emboli in both animal models8 9 and flow circuits.10
All the signals detected in our study and in previous ones were asymptomatic.12 14 16 This may be due to the small size of the emboli. Indeed, it has been demonstrated that injections of microspheres of a diameter <105 µm into the common carotid artery of rabbits cause no overt neurological dysfunction.20 Similarly, routine fundoscopic examination may disclose small retinal emboli that have caused no symptoms.21 22
There are strong disparities in the results from different ultrasound laboratories on the incidence of HITS in patients with carotid stenosis. For instance, Grosset et al23 reported HITS in all the patients they monitored. In contrast, Markus et al12 found HITS in only 6 of 25 patients, a proportion close to that observed in our study. These discrepancies may be due to differences in methods, such as TCD and recording equipment, level of background noise, intensity threshold, sample volume, gain, and duration of recording. Moreover, although the features of artifacts from probe motion are well known, the level of false-positives may be higher in unblinded studies.24
Some previous studies also have identified biological variables that are associated with HITS. Siebler et al17 found HITS in 27 of 33 symptomatic patients and in only 9 of 56 patients with asymptomatic stenosis. All patients had stenosis of >70%. It must be noted, however, that symptomatic patients had not been taking antiplatelet drugs for more than 5 days, whereas more than half of the asymptomatic patients were taking antiplatelet treatment at the time of TCD monitoring. This may have decreased the incidence of HITS in asymptomatic patients.25 Ries et al16 reported that they detected HITS in 7 of 36 symptomatic patients and in none of 18 patients with asymptomatic stenosis. Babikian et al15 found HITS in 10 of 37 symptomatic arteries and in only 1 of 34 asymptomatic arteries. In the same sample, they also demonstrated a higher rate of HITS distal to >50% carotid stenosis. However, because they used only univariate analysis, their results do not account for any possibly mutually confounding effects of the two variables.
In our study, neither the previous occurrence of clinical symptoms nor the degree of carotid stenosis were significantly associated with HITS. As the sample was small, these negative results may be due to a lack of power. Moreover, monitoring our symptomatic patients sooner after the occurrence of clinical symptoms might have increased the rate of HITS. Indeed, the findings of Siebler et al14 suggest a decrease with time in the rate of HITS from the last occurrence of ischemic symptoms. However, in our study, multivariate regression analysis, including previous occurrence of clinical symptoms and degree of carotid stenosis as independent variables, revealed that an image of ulceration on carotid angiography was the stronger and only significant predictor of HITS. There is some inaccuracy in the angiographic diagnosis of plaque ulceration: specificity is rather high, but sensitivity is low, ranging from 46% to 56%.19 26 Nevertheless, an image of ulceration on carotid angiography is a powerful predictor of subsequent ipsilateral stroke.6 Assuming that HITS are asymptomatic emboli, our results demonstrate a significant link between the occurrence of asymptomatic cerebral emboli and the presence of an ipsilateral source of emboli. In this regard, HITS detection by TCD monitoring might become a useful and noninvasive tool to assess the thromboembolic risk of carotid plaques. Whether HITS indicate a higher risk for subsequent stroke remains, however, to be determined by the proper longitudinal study.
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Acknowledgments |
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We wish to thank Antoine de Falguerolles (Laboratory of Statistics, Paul Sabatier University) for statistical advice and M. Ségui, C. Antolin, E. Boutonnet, M. Matignon, and J.M. Caumont for their technical assistance.
Received December 19, 1994; revision received February 16, 1995; accepted February 17, 1995.
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