Prevalence of femoral vein duplication: systematic review and metaanalysis

Femoral vein is the continuation of the popliteal vein beyond the adductor hiatus into the thigh. It ascends together with the femoral artery within the adductor canal and into the femoral triangle, where it is joined by the profunda femoris vein to form the common femoral vein. This terminates as the external iliac vein behind the inguinal ligament⁽¹⁾. There are several variations in the anatomy of the deep venous system of the lower limb, one of such variations being duplication of the femoral veins.

The presence of duplicated femoral vein (DFV) is an important anatomical variation which has been shown to result in false negative compression ultrasound results during evaluation for deep venous thrombosis^(2–4). The ultrasound probe may be focused on one of the branches that does not have a thrombus, whereas the thrombus may be lodged in the other branches. This may result in misdiagnosis with serious consequences such as pulmonary embolism, post-thrombotic syndrome and death. It is therefore important to be wary of this variation in anatomy during ultrasonography.

Lower extremity deep venous thrombosis (DVT) is a common cause of morbidity and mortality, with a prevalence of 1 in 1,000 people and accounting for up to 100,000 deaths annually⁽⁵⁾. The most feared complication is pulmonary thromboembolism, when a thrombus becomes dislodged and travels to the pulmonary arteries. This may lead to sudden death if not diagnosed early⁽⁶⁾. Early diagnosis and intervention are, therefore, vital to prevent mortality. Lower extremity DVT is classified into proximal and distal DVT. Proximal DVT occurs in the deep veins above the knee (external iliac, femoral and popliteal), whereas distal DVT occurs in the deep veins below the knee without any involvement of the proximal veins^(7,8). Proximal extension of distal DVTs can be found in up to 15% of cases⁽⁹⁾. The proximal DVTs are more likely to dislodge and cause clinically significant morbidity and mortality⁽⁷⁾. Current guidelines from The Society of Radiologists in Ultrasound recommend a comprehensive duplex ultrasound scan of all the lower limb veins from the thigh to the ankle when evaluating for DVT⁽¹⁰⁾.

Contrast venography has historically been considered the gold standard for the diagnosis of DVT, but it is too invasive and hence less frequently used. As such, Doppler ultrasound is currently used as the standard first-line imaging modality for diagnosing DVT owing to its safety, reliability, accuracy, ease of access, and non-invasiveness^(5,10,11). Proximal lower extremity thrombi are commonly diagnosed by performing an ultrasound of the femoral-saphenous veins with compression. The diagnostic criteria for DVT by ultrasonography include incompressibility of a venous segment, lack of flow, visualization of thrombus, and abnormal spectral pattern^(5,12). Adequate knowledge of the normal and variant anatomy of the femoral vein is important in order to correctly identify the vein and make proper diagnosis. The presence of duplicated femoral veins has been reported before but the findings have been varied, with a prevalence ranging from 5–46%^(4,13,14). This metaanalysis, therefore, sets out to determine the pooled prevalence of duplicated femoral veins.

This metaanalysis demonstrated that the pooled prevalence of DFV was high at 19.7%, with significant interstudy heterogeneity present. The heterogeneity could partially be explained by the difference in the modalities used in the evaluation of double femoral vein. For instance, ultrasonography is highly operator-dependent, and accurate interpretation relies on well-trained operators and techniques used⁽²⁶⁾. This could potentially account for the high prevalence observed in the ultrasonography group. The significantly lower prevalence in the MDCT group (6%) can be explained by the patient characteristics in two of the three studies. The study subjects in the two studies by Bastarrika et al., 2007⁽¹⁷⁾ and Redondo et al., 2009⁽²³⁾ had Klippel–Trénaunay syndrome, which is a congenital vascular anomaly associated with aplasia or hypoplasia of the deep venous systems. For instance, 50% of the patients included in the study by Bastarrika et al., 2007⁽¹⁷⁾ either had aplasia or hypoplasia involving the femoral vein, hence the low prevalence of duplication. The cadaveric studies may be a reflection of the true prevalence of duplicated femoral veins since they are based on thorough explorative evaluation. However, only two cadaveric studies were included in the study and the sample size was limited in one of them.

Clinicians need to be wary of the presence of DFV, as it could potentially impact on the diagnostic accuracy of compression ultrasonography when evaluating for DVT in the lower limbs. There is a possibility of failure to identify the clot in one arm of the double vein if the arm being evaluated has no clots, resulting in false-negative results^(3,4). Using venography, Streaton et al., 1998⁽²⁾, showed that it was more likely to get a false negative femoropopliteal thrombosis on ultrasonography in cases of DFV compared to single veins. This observation was, however, not statistically significant. There is also a possibility of clinically silent DVTs without complete occlusion since there are collateral vessels⁽¹³⁾. A good proportion of these silent DVTs remain undiagnosed and may result in pulmonary embolism and death⁽²⁷⁾.

It is also postulated that duplication of the vein potentially results in an overall reduction in the velocity of blood flow along the duplicated veins, although this hypothesis has not been proven. Dona et al., 2010⁽³⁾ established that femoral vein duplication resulted in an overall increase in venous cross-sectional area by about 42% compared to non-duplicated veins. They calculated a mean decrease in venous flow velocity by about 36%, with the assumption that venous flow rate must remain constant. This reduction in velocity could contribute to relative stasis, which could potentially predispose to DVT formation compared to non-duplicated veins.

The superficial femoral vein is also widely used as an arterial or venous vascular graft. Its applications include the repair of conventional infected grafts of mycotic aneurysms, aortoiliac reconstruction in occlusive arterial disease, and as a peripheral bypass material^(28,29). Segments of about 5–9 mm in diameter can be used as internal iliac grafts^(18,30). In this systematic review, 4 studies reported the mean diameters of DFV. Dona et al., 2000 (mean diameter of 7.2 mm) Fereira, 2015 (20 mm), Casella (25.4% of duplicates had diameters greater than 6 mm) and Gordon (4.9 mm)^(3,18,19,25). Except for the findings by Gordon et al.⁽¹⁹⁾, duplicated femoral veins can be used successfully as vascular grafts given the adequacy of their diameters, conferring an advantage in these cases.

From the studies analyzed, only 5 reported on the bilaterality of occurrence of duplicated femoral vein, with a pooled prevalence of 15.9%. This could be a pointer to the occurrence of duplications in the contralateral limb in cases where clinical imaging detects duplication in one limb.

This study was limited by the small number of available studies on the prevalence of double femoral vein, and by the persistently high interstudy heterogeneity despite subgroup analysis. However, it still provides a comprehensive metaanalysis and systematic review on the currently available data on the prevalence of duplicated femoral vein.

Duplication of the femoral vein is a common variation in the lower limbs. Routine watch-out should be practiced especially when performing lower limb Doppler ultrasound studies in cases of DVT in order to avoid misdiagnosis and improve diagnostic accuracy.