Deep vein thrombosis (DVT) is a common clinical problem, affecting around 500 000 Americans yearly, according to the CDC. Diagnosing a DVT can be simple, if we master the most important technique of DVT ultrasound–checking for compressibility. In this video, our expert Elizabeth Tenny, from the Stanford Vascular Lab, explains how to diagnose a dangerous clot and what a venous ultrasound beginner can mistake for a DVT.
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The mainstay of deep vein thrombosis or DVT studies is the compression of the deep veins. If the vein is compressible, as seen in this video, there is nothing blocking the lumen. If the vein is partially or non-compressible, there is DVT there and we can further evaluate from the images whether the clot is acute or chronic. Regarding potential to cause of pulmonary embolus, or PE, it is for the first two weeks after DVT formation that the clot is considered acute and at risk of embolizing to the lungs. After two weeks, the clot is considered subacute for a short while and then increasingly chronic with time, during which it either resolves or increasingly adheres to the vein walls, becoming much less of an embolization risk.
Acute DVT, as seen in this transverse image of the femoral vein, appears as a gray or black dilated lumen that is not compressible, as seen in this post-compression image. This technologist pressed so hard that the artery completely compressed but the vein still did not. With acute occlusive DVT when the clot completely blocks the vein, no color will fill the vein, as shown on this longitudinal color view image. In this case, do not perform a distal augment as there may be potential for the clot to dislodge and cause a PE. When the clot becomes more chronic, it appears brighter or whiter on ultrasound and often shrinks. This leads to either scarring of the vein walls or it can result in the vein recanalizing, which means the vein partially reopens and develops increasing compressibility over two to six weeks.
Chronically occluded veins sometimes shrink to very small caliber vessels, but often chronic DVTs result in a partially compressible vein as the clots shrink, as you can see in this video. In long view, a recanalized vein with chronic DVT appears as a bright area that is partially surrounded by color as the shrunken clot allows for partial venous flow. This is an example of a non-obstructive chronic wall scarring from DVT that is no longer significantly obstructing blood flow or at risk of thromboembolization. Superficial veins follow the same guidelines for acute or chronic diagnosis. Notice how this non-compressible great saphenous vein, or GSV, has become quite small due to the presence of a chronic clot.
Compare that to a more recently formed GSV clot, which appears black, relatively dilated and non-compressible. In this superficial vein, you can see a partially occlusive and thus partially compressible chronic clot adhered to the wall. In this video, you can see the bright chronic clot inside the black lumen and the vein is partially compressible due to the shrunken clot. Sometimes due to proximal obstruction, compression, dehydration, or a hypercoagulability disorder, blood cells in the affected leg approach stasis. When this happens, you can sometimes visualize the blood cells swirling around each other on 2D black and white ultrasound, as shown in this video. This is called rouleaux flow, which is a French word that literally translates into roll. As long as the vein is fully compressible, as shown here, there is no blood clot.
However, these low level gray tones within the vein caused by rouleaux flow may initially be concerning for an occlusive or nearly occlusive acute DVT. One source of DVT misdiagnosis is visualization of venous valves, which can look like chronic DVT. Valves are thin, linear and partially mobile structures that do not prevent venous coaptation. This is a video of a proximal femoral vein valve in longitudinal view. This is an image of a proximal femoral vein valve in transverse view next to the superficial femoral artery or SFA. The image on the right shows the femoral vein valve fully compressed so only the artery is clearly visible. In this transverse video, you can see the valves moving within the vein. Valve sites can be dilated, sometimes with mild color defect caused by the valve as seen in this image. Note that there is full color fill in the lumen passing between the valves, indicating there is no DVT present.