Duplex Diagnosis of Lower Extremity Venous Thrombosis

Pathophysiology of Lower Extremity Venous Thrombosis

The diagnosis of lower extremity deep venous thrombosis (DVT) is a morbid condition and a large function of a noninvasive vascular laboratory. DVT is responsible for as many as 250,000 hospitalizations per year in the United States, and can occur in many different settings, either acquired due to the presence of an external thrombophilic factor or hereditary due to a genetic predisposition. The timely diagnosis of DVT, most of which originate in the deep veins of the leg, and initiation of anticoagulation treatment can prevent pulmonary embolism and recurrent DVT . The timely treatment of DVT can also help prevent or decrease the risk of developing post-thrombotic syndrome (PTS). PTS may develop anytime after DVT but is most common within the first two years, and presents as sudden or insidious onset of lower extremity edema. Duplex examination of those with PTS often reveals no areas of acute thrombosis, but may show recanalized or thickened veins consistent with chronic DVT.

Role of ultrasound in diagnosis of DVT

Duplex/compression ultrasonography has largely supplanted contrast venography as the previous gold standard for the diagnosis of lower extremity DVT. Venography is an invasive procedure requiring both x-ray radiation and nephrotoxic contrast, compared to compression ultrasonography which is noninvasive and comparatively less risky. Compression ultrasound has been shown to have high sensitivity (>95%) and specificity (>95%) for diagnosis of DVT above the knee; however, ultrasound studies may be limited by anatomic areas that are hard to compress (adductor canal) or by conditions that limit viewing or compression (casts or fixation). In addition, viewing and compression of the tibial arteries can be difficult, lowering the predictive value of compression ultrasound below the knee.

Technical Aspects of Lower Extremity Duplex Exam

The primary goals of the ultrasonographer in an ultrasound examination for diagnosis of DVT are to identify the presence or absence of thrombus, characterize any thrombus in the vein lumen, and assess competency of the deep venous valves. Because of the varying depths of the venous system, different probes should be used for assessing the proximal deep veins (midrange frequency transducer) compared to the tibial veins (higher frequency transducer) or the iliac veins (low frequency transducer). The patient should be placed in a dependent position such as to allow the leg veins to dilate. This can either be accomplished with the patient supine in a slight head-up or in a sitting position in a warm room; however, a sitting position may result in difficulty with venous compression testing for the ultrasonographer. The leg is then slightly bent at the knee with external rotation of the hip.

The ultrasonographer should attempt to image around any conditions that limit viewing, such as dressings, casts, or fixation. Some patients with deep venous thrombosis may also have significant pain or tenderness in the leg being examined, rendering it difficult to scan or compress these areas. In these situations the examination may be technically difficult and limited by these external considerations. An incomplete examination of the deep venous system is unable to rule out DVT in the unimaged areas, although indirect inferences about obstruction or occlusion can be made by interpreting the spectral Doppler waveforms of the surrounding areas. Areas able to be imaged but unable to be compressed due to external causes such as pain may be imaged for flow and spectral Doppler analysis; in these cases, patency can be established but a subocclusive acute DVT cannot be ruled out.

Normal Lower Extremity Deep Venous Imaging and Flow

In order to assess the full length of the venous system while performing compression testing, a transverse view is used. The deep venous system is confirmed by locating first the corresponding pulsatile artery, then identifying surrounding vascular structures and correlating them with normal venous anatomy. In a normal vessel, compression of the overlying tissue using the ultrasound probe should result in collapse of the venous bundle while the artery remains circular and pulsatile. The exam begins at the proximal-most visualized portion of the distal external iliac artery at the groin crease and continues distally to the popliteal segment below the knee with compression in the transverse plane every 2-3cm. The exam may then be carried distally to the posterior tibial and peroneal veins depending on lab-specific protocol and accreditation standards, although consensus for medical treatment of calf DVT remains conflicting due to limited sensitivity and specificity as discussed previously. Assessment of flow is then performed in a longitudinal view at each level, examining color flow and spectral Doppler waveforms. Patency at each level is documented using color flow. Spectral Doppler waveforms are analyzed to determine the absence or presence of phasicity in the common femoral vein and proximal femoral vein. Augmentation using a calf squeeze is utilized to elicit a normal acute increase in flow in the popliteal, femoral, and common femoral veins. Absence of compressibility or any of these adjuncts may suggest deep venous thrombosis of that or surrounding deep venous segments.

Iliofemoral

The distal external iliac vein (EIV) becomes the common femoral vein (CFV) below the inguinal ligament and runs concurrently medial to the external iliac artery and common femoral artery. At this point, the confluence of the great saphenous vein with the CFV forms the sapheno-femoral junction – this area is readily recognizable as the saphenous vein and the common femoral artery form small circles anterior and oblique to the larger common femoral vein, resembling the ears and head of a stylized mouse. Thrombosis of the proximal great saphenous vein can be caused by superficial thrombophlebitis or after ablation of the great saphenous for superficial venous reflux, and can result in extension of thrombus into the common femoral vein. The confluence of the profunda vein should also be noted as it joins the common femoral vein, although isolated profunda vein thrombosis is rarely noted and the clinical significance of such a finding is unknown.

Figure 1. Transverse view of the saphenofemoral junction. Common femoral artery: top left; Great saphenous vein: top right

Doppler waveforms in the CFV and the EIV should be phasic following the patient’s respirations due to the transmitted influence of the diaphragm and chest cavity on the relatively low pressure venous system from the vena cava distally through the iliac veins. As in the rest of the deep venous system, absence of compressibility suggests thrombus at the area of compression. However, in the external iliac and common femoral veins, loss of respiratory phasicity may indicate proximal obstruction or occlusion. The proximal iliac veins are difficult to image by ultrasound and are not routinely included in a venous duplex examination; therefore indirect diagnosis of proximal stenosis or occlusion may need to be followed up by further imaging such as a computed tomographic venogram. The clinical presentation of the patient in such a situation is important, as it may identify situations of not only proximal iliac thrombosis but also other diagnoses, such as caval occlusion or correctable anatomic abnormalities such as May-Thurner syndrome (external compression of the left iliac vein by the right iliac artery).

Figure 2. Spectral Doppler flow respiratory variation in the common femoral vein with valsalva maneuver

Femoropopliteal Segment

Distal to the CFV, the femoral vein (formerly known as the superficial femoral vein) parallels the superficial femoral artery, running deep to the sartorius muscle and angling medially to cross through the adductor canal in the distal thigh. Here it enters the popliteal space to become the popliteal vein. In some cases the popliteal vein may be paired; in these situations, patency, compressibility, and augmentation of each paired vein must be examined. Flow through the popliteal vein will not as readily demonstrate the respiratory phasicity of the proximal vessels but will be more dependent on the calf muscle pump. Therefore, in addition to the usual compression test, augmentation to demonstrate patency and flow may be performed by squeezing the calf gently to demonstrate increased flow on spectral or color Doppler, although this is no longer routinely performed in some laboratories.

Figure 3. Increase in femoropopliteal flow with distal augmentation

Calf/Tibial Veins

The anterior tibial vein is not commonly imaged due to a low propensity for deep venous thrombus formation and even lower risk of pulmonary thromboembolism. Anatomically, the posterior tibial and peroneal veins diverge from the tibioperoneal trunk and continue within the gastrocnemius muscle. These deep calf veins are usually paired throughout most of their course and each vein must be examined for compressibility and patency. The posterior tibial veins pass posterior to the medial malleolus, while the peroneal veins run along the fibula. Both sets of veins can be assessed simultaneously in the mid-calf from a medial view. As for the proximal veins, compression and evaluation of flow is the main diagnostic modality for thrombus; however, the specificity of duplex diagnosis of DVT in the calf veins is lower than that for the large proximal veins. Treatment paradigms for calf DVTs remain conflicting, and as such the protocol and utility of assessing for calf-level DVT may differ depending on each lab.

Figure 4. Confluence of the peroneal and posterior tibial veins.

Differentiating Acute from Chronic Thrombosis

Because acute thrombus is more likely to embolize than chronic thrombus, some emphasis has been placed on the ability of ultrasound to distinguish chronicity. Acute thrombus in the deep venous system is characterized on ultrasound as hypoechoic, compared to the brightly hyperechoic chronic thrombus easily visualized on ultrasound. This distinction may aid the practitioner in medical management of deep venous thrombosis. However, in clinical practice, many thrombi demonstrate both acute and chronic characteristics, leading to an age-indeterminate diagnosis.

In addition, many laboratories will note the presence of any hyperechoic “free-floating thrombi”, which are discrete thrombi tethered to the vessel wall by a seemingly tenuous connection. These thrombi can be readily identified as flow will be present on three sides of the thrombus on color Doppler on any view. In some situations, the thrombus itself will be mobile on a stalk, while in other situations the clot may be tethered more tightly to the vessel wall. Although these free-floating thrombi have been implicated as soon-to-be-emboli, the clinical significance remains unclear and the risk for embolization or recurrent embolization is thought to generally be low.

Figure 5. Chronic thrombus in the common femoral vein.

Duplex Diagnosis of Lower Extremity Venous Thrombosis

Final Quiz