Diagnosis of Iliac Vein Obstruction With Duplex Ultrasound
Criteria used during duplex ultrasound examination to identify iliac vein obstruction.
Venous obstruction most often occurs due to thrombosis, extrinsic compression, or when both conditions are present. Obstruction of the iliac veins has a significant impact because it is responsible for the highest outflow resistance, venous claudication, and higher deep vein thrombosis recurrence. It also causes more prevalent and severe postthrombotic symptoms, impairing the quality of life of the patients, and poses a marked financial burden to society. Therefore, prompt and accurate diagnosis is necessary to facilitate the management of patients.
Obstruction is a dynamic entity associated with increased resistance to outflow due to energy loss when blood travels from one area to another. Most often, symptoms are present when the patient is walking; however, diagnosis of obstruction may be controversial because all imaging tests are performed with the patient in the supine position, which only shows morphologic but not dynamic changes. Venous pressure measurements at rest and during exercise are better indicators of the hemodynamic impact but may underestimate the chronic effect of somewhat compensated obstruction. All tests are patient and operator dependent, but duplex ultrasound may be the most. More importantly, formal training for ultrasound detection of obstruction is lacking, and rigorous training and experience are necessary to perform accurate evaluation. The exam starts at the common femoral vein (CFV) union and finishes with the inferior vena cava (IVC). Diagnosis with duplex ultrasound is based on specific direct and indirect criteria listed in Table 11-9 and detailed in the sections that follow.
Planimetric evaluation and luminal changes are very important because they allow direct imaging and evaluation of the obstruction. Stenosis and occlusion can be differentiated this way. Additionally, the diameter of the veins can be measured, luminal material indicating previous thrombosis with partial or no recanalization can be seen, and extrinsic compression and the type of compression producing the stenosis or occlusion can be assessed. Acute and recurrent thrombosis and extrinsic compression and thrombosis can be directly assessed. When present, the velocity ratio is important because it indicates > 50% diameter stenosis and has been validated by three studies using intravascular ultrasound. However, the absence of the velocity ratio cannot exclude obstruction, as the vein may be occluded, have long stenosis, or be partially recanalized. When velocities or diameters are being measured, it is very important that the pressure applied by the ultrasound transducer does not affect the measurements, as too much pressure may lead to disease overestimation.
When indirect signs are detected, they always indicate some form of obstruction. However, they cannot differentiate between stenosis and occlusion, extrinsic compression, or luminal changes. Therefore, direct imaging of the affected veins is important. The presence of phasic flow and good augmentation cannot exclude obstruction.
Our group and others have identified many patients with obstruction, sometimes with occlusion of iliac vein still having normal CFV phasicity and flow augmentation. Imaging of the inflow and the IVC are important, as this information is needed for proper patient management. The inflow veins, including the femoral, deep femoral, and CFV, are examined for patency because adequate inflow is necessary before stent placement to relieve iliac vein obstruction. Evaluation of the IVC is performed to determine patency. When present, the type of obstruction and anatomic variations such as aplasia, hypoplasia, left-sided cava, and duplication are reported because they are also important for the treatment plan. Understanding the vein anatomy and the surrounding structures is paramount. The areas of vein compression are listed in Table 2.
Many types of compression are observed and do not include other types from tumors, aneurysms, or hematomas. Several duplex ultrasound images are displayed to make an informed diagnosis of iliac vein obstruction. Figure 1 and Figure 2 demonstrate examples of patients with vein compression, and Figure 3 and Figure 4 show patients with different forms of vein occlusion.
Ultrasound is a great method for diagnosing venous obstruction. It is practical, cheap, has no side effects, can be easily repeated, and some dynamic testing is possible. It offers direct imaging as well as indirect criteria, which are very useful for detecting obstruction. However, it is the most operator-dependent imaging method, and there is a lack of formal, rigorous training worldwide. Furthermore, there are no robust diagnostic criteria for defining hemodynamically significant obstruction in a great number of patients. Further, work is needed to establish and also translate such findings with the clinical improvement of the patients.
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Prof. Nicos Labropoulos
Professor of Surgery and Radiology
Stony Brook University Medical Center
Stony Brook, New York