Page 78 - Libro 2

58 PART 2 — CEREBROVASCULAR
In the major North American CEA, trials the sever- ity of internal carotid stenosis was calculated from arteriograms by comparing the diameter of the mini- mal residual lumen at the stenotic site to the diameter of the normal distal cervical internal carotid.14 This approach to measuring the stenosis is now often re- ferred to as the NASCET method. The categories of carotid stenosis in the University of Washington crite- ria were developed long before the CEA trials by com- paring the diameter at the stenotic site to an estimate of the diameter of the normal carotid bulb. However, because the bulb often has a larger diameter than the distal internal carotid, the two methods of measur- ing stenosis do not give the same percentage of an- giographic stenosis for the same lesion. Calculations of angiographic stenosis using the distal internal ca- rotid as the reference vessel result in lower stenosis percentages than calculations using the bulb as the reference site. This effect is particularly striking for lesions in the middle of the stenosis range, and the differences decrease with increasing stenosis severity.
Recognizing the wide variability in the perfor- mance and interpretation of carotid duplex scans, a panel of authorities from a variety of medical spe- cialties met in 2002 to develop a consensus regarding the key components of the carotid ultrasound exami- nation and the most appropriate criteria for clas- sification of disease.18 The panelists recommended the consistent use of relatively broad categories to classify the degree of internal carotid stenosis. The panel also concluded that Doppler parameters are relatively inaccurate for subcategorizing stenoses
TABLE 4-3
of less than 50% diameter reduction and recom- mended that these lesions be reported under a single stenosis category. They noted that although PSV is a primary parameter for interpretation, its measure- ment is subject to significant variability. In order to minimize this variability, it was recommended that Doppler waveforms be obtained with an insonation angle as close to 60° as possible, but not exceeding 60°, and the sample volume should be placed within the area of maximal stenosis. Additional parameters such as the ICA/CCA ratio and EDV were regarded as secondary parameters. A summary of the consensus panel criteria is given in Table 4-3. It is important to emphasize that these criteria have not been sub- jected to retrospective or prospective evaluation and do not represent the results of any one laboratory or study. However, they can serve as a reference for those laboratories that have not been able to inter- nally validate their own criteria.
As previously stated, criteria used for classifying ICA disease cannot be applied for lesions in the CCA or ECA. However, sites of significant stenosis in these vessels can still be identified by the presence of plaque on B-mode imaging and associated focal increases in velocity in Doppler waveforms. A stenosis of more than 50% can be inferred by the presence of a focally increased PSV followed by poststenotic turbulence.
COLOR AND POWER DOPPLER FINDINGS
The ability to display flow information using color Doppler and power Doppler has improved remarkably
Consensus Panel Recommendations for Classification of Internal Carotid Artery Stenosis18
Normal: The ICA PSV is less than 125 cm/s and there is no visible plaque or intimal thickening. Normal arteries should also have an ICA/CCA ratio of less than 2.0 and ICA EDV of less than 40 cm/s.
ICA stenosis 􏰁50% is present when the ICA PSV is less than 125 cm/s and there is visible plaque or intimal thickening. Such arteries should also have an ICA/CCA ratio of less than 2.0 and an ICA EDV of less than 40 cm/s.
ICA stenosis of 50% to 69% is present when the ICA PSV is 125 to 230 cm/s and there is visible plaque. Such arteries should also have an ICA/CCA ratio of 2.0 to 4.0 and an ICA EDV of 40 to 100 cm/s.
ICA stenosis 􏰀70% to 99% but less than near occlusion is present when the ICA PSV is more than 230 cm/s and there is visible plaque with lumen narrowing on grayscale and color Doppler imaging. The higher the PSV, the more likely (higher positive predictive value) that there is severe disease. Such stenoses should also have an ICA/CCA ratio of more than 4.0 and an ICA EDV of more than 100 cm/s.
Near occlusion of the ICA: The velocity parameters may not apply. “Preocclusive” lesions may be associated with
high, low, or undetectable velocity measurements. The diagnosis of near occlusion is therefore established primarily by demonstration of a markedly narrowed lumen with color Doppler. In some near occlusive lesions, color or power Doppler can distinguish between near occlusion and occlusion by demonstrating a thin wisp of flow traversing the lesion.
Occlusion: There is no detectable patent lumen on grayscale imaging and no flow with pulsed Doppler, color Doppler, or power Doppler. Near occlusive lesions may be misdiagnosed as occlusions when only grayscale ultrasound and pulsed Doppler spectral waveforms are used.
ICA, internal carotid artery; PSV, peak systolic velocity; CCA, common carotid artery; EDV, end diastolic velocity; ICA/CCA ratio, maximal ICA PSV divided by the maximal CCA PSV.