Dim view of ultrasound for CAS in asymptomatic
Potential harm results from false positives
In light of its excellent safety profile, ultrasound does not usually figure into debates about whether imaging is being overused in this country. However, new recommendations from the U.S. Preventive Services Task Force (USPSTF) suggest there may be at least one indication for which ultrasound is being used inappropriately: screening for carotid artery stenosis (CAS) in asymptomatic patients.1
In recently published clinical guidelines, the USPSTF maintains that while stroke is a leading cause of death and disability in this country, the evidence suggests that the benefits of general screening in asymptomatic patients do not outweigh the potential harms. (See guidelines.) The panel has issued a grade D recommendation, meaning that it advises against routine screening in asymptomatic adults and that it has found at least fair evidence to support this view.
In exploring a large body of evidence regarding CAS and the testing generally used to diagnose the condition, USPSTF concludes that a relatively small proportion of disabling, unheralded strokes is the result of CAS. Further, the panel notes that duplex ultrasonography only has moderate sensitivity and specificity when used to screen for severe CAS, and it produces many false-positive results.
The false-positive results are the chief problem, according to Larry Goldstein, MD, director of Duke University's Center for Cerebrovascular Disease in Durham, NC. "Like any test, it depends what you do with the information," says Goldstein, noting that most of the time the next step is a confirmatory test. Noninvasive tests, such as MR angiography and CT angiography, are also somewhat inaccurate and can lead to false-positive and false-negative results. A higher degree of accuracy is possible, but not without some risk.
"The gold standard test…is cerebral angiography [in which] you put a catheter into the vessel, you inject die, and take a picture of it," Goldstein says. "That alone carries a risk of stroke in general of about 1%, just for doing a diagnostic test."
A second way you could end up harming the patient is by performing an unnecessary operation. "If you have a false-positive test, and you end up doing an operation based on that false-positive test and there is some complication, you have not only put the patient at risk by doing the procedure, but you have also done harm," Goldstein says. "In general, the complication rate for endarterectomy for asymptomatic disease is, on average, 3%...and if you look at national data, the risk of stroke or death after endarterectomy for asymptomatic disease can be as high as 6%."
The USPSTF emphasizes that the recommendation against screening only applies to adults without any neurological signs or symptoms, or any history of strokes or transient ischemic attacks. Less clear, however, is what the clinician should do if he or she hears a bruit upon physical examination of a patient who has no other signs or symptoms. In 1996, the USPSTF reviewed the evidence for ultrasound screening in these patients and found that it had poor reliability and poor sensitivity in these cases.1 However, many experts contend that these types of patients should, in fact, be referred for an ultrasound exam.
There is data to support that you should be screening these patients with bruits, says Robert Mitchell, MD, medical director of the non-invasive vascular lab at Duke University. "I go by that as a clinical indication to do it, and Medicare reimburses for that," Mitchell says. "If you have a clinical indication such as hearing a bruit or pre-syncope, dizziness, lightheadedness — there are several indications where you could get the test done based on that."
Mitchell, who deals exclusively with ultrasound technology in his lab, maintains that ultrasound offers a high degree of accuracy when it is done by experienced sonographers. "There is a propensity for it to over-estimate the level of stenosis based upon velocities, but we have sub-criteria that correct for that," he says. "If [the sonographers] are well-trained and they take the appropriate pictures, they are going to get a very good result, which I think is reproducible."
Mitchell points out that results from the Duke lab are compared to results from MRI and from carotid angiography several times a year, and they match up well. However, he acknowledges that not all labs achieve the same level of accuracy.
Mitchell and Goldstein believe that what the USPSTF may be most concerned about is not physician referrals for ultrasound screening for CAS, but rather the proliferation of community screenings that are being done, often out of vans that travel to different regions, offering a variety of screening tests including ultrasound screening for CAS. Goldstein says, "If you open up your newspaper, you will find vans coming into neighborhoods offering to do screenings for carotid arteries, abdominal aortic aneurisyms, and for peripheral vascular disease. This is money out of our peoples' pockets, and I don't know what the quality of these studies is, but…the data show that there is no utility to doing these general screenings."
Reference
- Screening for Carotid Artery Stenosis: US Preventive Services Task Force Recommendation Statement. Ann Intern Med 2007; 147:854-859.