Ultrasound Criteria for Carotid Stenosis May Overestimate Severity

SVS: Ultrasound Criteria for Carotid Stenosis May Overestimate Severity

By Neil Osterweil, Senior Associate Editor, MedPage TodayReviewed by Zalman S. Agus, MD; Emeritus Professor at the University of Pennsylvania School of Medicine.
June 07, 2007

Review
BALTIMORE, June 7 -- Patients with internal carotid artery stenosis may be undergoing needless tests and interventions, investigators warned here.

That's because current ultrasound standards for stenoses greater than 50% are based on outmoded information, according to Hisham Bassiouny, M.D., director of the non-invasive vascular lab at the University of Chicago, and colleagues.

Duplex ultrasound velocity thresholds for estimating blood flow through stenotic arteries are too aggressive and tend to overestimate the severity of stenosis, Dr. Bassiouny reported at the Society for Vascular Surgery meeting.

The problem, he said, is that ultrasound standards developed in the 1980s (Strandness criteria) and still in use in the majority of vascular labs in the United States were based on early angiography findings.

"The limitation with angiography is that you had to guess how far the outer wall of the artery was beyond the artery's channel to determine the precise degree of artery blockage," he said. "That was a guess, an estimate. Based upon that subjective estimate, formulas were developed to look at the velocity of blood flow in the artery and determine how much narrowing existed. These formulas became the standard used to this day. However, imaging technology is much better today than when these standards were developed."

The Strandness criteria define greater than 50% stenosis as a peak systolic velocity greater than 125 cm/second, greater spectral broadening throughout systole, and heavy, prominent plaque formation.

The criteria define greater than 80% stenosis as a peak systolic velocity > 125 cm/second, marked spectral broadening and turbulence, severe plaque formation, and end diastolic velocity elevated more than 140cm/second.

To see whether the criteria held up with the use of modern equipment, Dr. Bassiouny and colleagues first compared B-mode ultrasound and computed tomography angiography images performed on 74 patients with internal carotid artery stenosis, in order to validate the accuracy of the ultrasound measurements.

They then evaluated 337 patients with either mild, moderate, or severe internal carotid artery stenoses, looking at the minimal residual lumen and the corresponding outer internal carotid artery or bulb diameter on longitudinal and transverse images. Patients with contralateral occlusion were excluded from the analysis, as were calcified artery segments.

In both the validation sample and the larger study, the highest peak systolic velocity, end diastolic velocity, and ratio of the internal carotid artery to the common carotid artery (ICA/CCA ratio) were recorded.

The investigators determined the optimum threshold for each hemodynamic parameter using receiver operating characteristic curves to predict stenosis of 50% or greater (in 281 patients) and of 80% or greater (in 62 patients) bulb internal carotid artery stenosis.

They found that "there was excellent agreement between B-mode ultrasound and computed tomography angiography (r=0.9, P=0.002)."

But when they looked at the sensitivity, specificity, and positive- and negative-predictive value of B-mode ultrasound, they found that the Strandness criteria would rope in too many patients who did not have serious stenosis.

When both a peak systolic velocity equal to or greater than 155 cm/second and an ICA/CCA ratio of 2 or greater were combined for the detection of at least 50% internal carotid artery stenosis, a positive predictive value of 97% and an accuracy of 82% were obtained. For a stenosis of 80% or greater, and end diastolic velocity of 140 cm/second, a peak systolic velocity 370 cm/second or greater, and an ICA/CCA ratio of 6 or more had acceptable probability values.

"Compared to established velocity thresholds commonly applied in practice (Strandness criteria), a substantially higher peak systolic velocity (155 vs. 125 cm/second) was more accurate for detecting ≥ 50% bulb ICA stenosis," the authors wrote.

"In combination, a PSV of ≥155 cm/second and an ICA/CCA ratio of 2 have excellent predictive value for this stenosis category," they said. "For an ≥ 80% ICA stenosis, an end diastolic velocity of 140 cm/second, a peak systolic velocity of ≥ 370 cm/second, and an ICA/CCA ratio of ≥ 6 are equally reliable."

The authors said that current criteria for duplex ultrasound detection of 50% or greater internal carotid artery stenosis may overestimate carotid bifurcation disease.

"As a result, we've changed the standards in our vascular lab," Dr. Bassiouny said. "We hope these new standards will be adopted everywhere. Such a move would save money and spare at least some patients from unnecessary procedures and tests."

Primary source: Vascular Annual Meeting 2007

Source reference: Shalaan WE et al. "Reappraisal Of Internal Carotid Artery Stenosis Velocity Thresholds Utilizing High-Resolution B-Mode Ultrasound Validated With CTA: Do Current Practice Thresholds Overestimate Carotid