A Prospective Study of the Diagnostic Accuracy of Noninvasive Imaging of Carotid Stenosis
Abstract & Commentary
Source: Nederkoorn PJ, et al. Preoperative diagnosis of carotid artery stenosis: Accuracy of noninvasive testing. Stroke. 2002;33:2003-2008.
Nederkoorn and colleagues prospectively studied the diagnostic accuracy of duplex ultrasound (DUS), magnetic resonance angiography (MRA), and a combination of these tests with digital subtraction catheter arteriography (DSA). The study population comprised 350 consecutive symptomatic (amaurosis fugax, TIA, or stroke) patients admitted to 3 hospitals in The Netherlands. Observers who measured stenosis were blinded for clinical information and other test results. Results of DUS and MRA tests were compared with DSA as the reference standard. The measured stenoses were divided into categories (0-29%, 30-49%, 50-69%, 70-99%, and 100%). Only measurements of internal carotid arteries (ICA) on the symptomatic side were analyzed.
MRA (sensitivity 92%, specificity 76%) showed a slightly better accuracy than DUS (sensitivity 88%, specificity 76%) in the diagnosis of ICA stenosis. MRA and DUS were in agreement in diagnosing 70-99% stenosis in 229 (84%) patients. In 16% of patients, the diagnosis of severe stenosis was unclear because the MRA and DUS results disagreed as to the degree of ICA stenosis present. Both MRA and DUS had a tendency to overestimate the degree of stenosis compared with DSA. In all 24 cases in which MRA and DUS overestimated the degree of stenosis as 70-99%, the degree of stenosis by DSA was only one category lower (50-69%). The diagnostic test results of the single noninvasive tests and the combination are listed in the table (see Table). Nederkoorn et al found that although MRA was slightly better than DUS in the diagnosis of ICA stenosis, the best accuracy was achieved when both tests were in agreement. Therefore, they recommend that both DUS and MRA be performed.
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Commentary
Ever since carotid endarterectomy was shown to be beneficial in both symptomatic and asymptomatic patients with severe ICA stenosis, the diagnosis of a 70-99% stenosis, as distinct from 100% occlusion and a less than 70% stenosis, has become crucial. Although previous studies compared noninvasive techniques with conventional catheter arteriography, improvements in MRA technology by the introduction of contrast enhancement made an up-to-date prospective study necessary. Nederkoorn et al found that using the combination strategy of DUS and MRA yielded the highest accuracy, an approach already adopted by most clinicians. In this study, both DUS and MRA tended to overestimate the degree of stenosis, for example; in 2 and 3 instances, DUS and MRA, respectively, misdiagnosed total occlusion when only severe stenosis existed. This low error rate of about 1% is reassuring and indicates that in most cases, DSA is not required to rule out the presence of trickle flow when both DUS and MRA diagnose total ICA occlusion.
In an additional 24 patients, both DUS and MRA agreed that endarterectomy for severe ICA stenosis was indicated when DSA classified the degree of stenosis as only moderate (50-69%). Based on current information from NASCET (Barnett HJ, et al. N Engl J Med. 1998; 339:1415-1425) and The European Carotid Surgery Trial (European Carotid Surgery Trialists’ Collaborative Group. Lancet. 1998;351:1379-1387), these patients with moderate ICA stenosis will have only limited benefit from carotid endarterectomy. Faced with such a high rate of error for distinguishing moderate from severe ICA stenosis, most clinicians will opt for a confirmatory study such as DSA or, more likely, "just will wait a period of time and then repeat noninvasive studies." The emerging role of CT-angiography in such instances also needs to be established.
The information provided by this study is important and useful to clinicians who must make therapeutic decisions for individual patients based on the precise estimation of the degree of ICA stenosis. At present, as always, clinical judgment is required. —John J. Caronna
Dr. Caronna, Vice-Chairman, Department of Neurology, Cornell University Medical Center, Professor of Clinical Neurology, New York Hospital, is Associate Editor of Neurology Alert.
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