May-Thurner Syndrome
S. Elissa Altin, MD
Gabriella Wilson, MD
I. Introduction
A. History May-Thurner syndrome (MTS) is a syndrome of venous outflow obstruction due to extrinsic compression by the arterial system against bony structures. In 1908, McMurrich first described isolated left lower extremity swelling due to left iliac vein compression. In this cadaver study, he observed that adhesions of the iliac veins were present in about one-third of the cadavers studied. Of these, the vast majority of adhesions attached to the iliofemoral vessels were present on the left side. This study thus proposed that adhesions could be a possible explanation for the left-sided predilection for iliofemoral thrombosis formation that Virchow first noted in 1851. Almost 40 years after McMurrich, Ehrich and Krumbhaar performed an autopsy study that revealed increased collagen and elastin deposition of the left common iliac vein, causing obstruction and possibly contributing to the increased prevalence of left-sided iliofemoral thrombosis.1
B. Studies
1. A hallmark study was conducted by May and Thurner in 1957 that proposed an anatomic variant as the underlying cause for increased prevalence of left-sided iliofemoral venous thrombosis. They evaluated 430 cadavers and noted that 22% of the cadavers exhibited compression of the left common iliac vein between the right common iliac artery and the fifth lumbar vertebra. They postulated that this anatomic abnormality could be an explanation for this side preference of iliofemoral deep vein thrombosis (DVT) first described by Virchow. They hypothesized that both pulsation and mechanical compression of the overlying right common iliac artery in these variants leads to focal stenosis of the wall of the underlying left common iliac vein. They referred to this area of stenosis as a “venous spur” and also proposed that this “spur” might play an instigative role in lower extremity venous outflow obstruction.2
2. The association between iliac vein compression and postthrombotic syndrome (PTS) was shown by Cockett et al in 1967.3 They found that this anatomic variant may serve as both the main initiating factor in iliofemoral venous thrombus formation and the limiting factor in vessel recanalization after thrombosis. This study investigated 48 cases of postthrombotic iliac venous obstruction without inferior vena cava involvement and found that of these 48 cases, 39 were confined to the left leg. Of these, 33 showed the level of obstruction occurring at the junction where the right common iliac artery crosses the left common iliac vein.
C. Setting of Iliac Vein Thrombosis In the setting of iliac vein thrombosis, the vessel can either completely or incompletely recanalize. The subsequent thrombophlebitis triggers an acute inflammatory response within the vessel, which leads to scarring of the vein. This process drives recanalization and, in the majority of cases, results in incomplete recanalization. In this setting, adequacy of venous outflow depends on whether or not the body is able to develop sufficient collateral circulation. In the event of incomplete recanalization and inadequate collateral formation, the result is iliac vein obstruction manifesting clinically as PTS.
II. Epidemiology
A. May-Thurner Syndrome, Illiac Vein Compression Syndrome, or Cockett Syndrome
1. The presence of a compressed left common iliac vein by the overlying right common iliac artery is widely referred to as May-Thurner syndrome (MTS), although some refer to this anatomic variant as iliac vein compression syndrome or Cockett syndrome. MTS is most commonly seen in patients aged 20-40 years, disproportionately affecting females more than males. The autopsy studies of May and Thurner in the 20th century demonstrated left common iliac vein compression in 22% of cadavers, although the actual prevalence of this anatomic variant in the general population remains unknown.4
2. A study performed by Kibbe et al attempted to better define the prevalence of left common iliac vein compression in the asymptomatic population by performing a retrospective review of CT scans to determine the presence or absence of left iliac vein compression in patients without underlying evidence of or risk for iliac vein compression or DVT. Of the 50 CT scans reviewed in this group, mean left iliac vein compression was 35.5% ± 2.4%. Additionally, 24% of patient CT scans reviewed demonstrated at least 50% compression of the left iliac vein.5
B. May-Thurner Syndrome and Deep Vein Thrombosis MTS-associated DVT accounts for approximately 2%-3% of overall cases of lower extremity DVT, and MTS has only been diagnosed in 2%-5% of patients with venous disease of the lower extremity.4,6 However, there are other studies that suggest a higher prevalence than this. In an MRI study of 24 patients with unilateral left lower extremity edema, 37% of these patients had evidence of MTS on magnetic resonance venography (MRV).7 Another study on venous registry data found that in about 62% of patients with acute iliofemoral DVT, “spurlike lesions” were found.8 This finding may suggest that the association of left-sided iliofemoral DVT with left common iliac vein compression is more common than previously thought.9 The true association between MTS and lower extremity venous disease is not well defined and is likely underestimated.
III. Pathophysiology
MTS refers to extrinsic compression of the iliac venous system by an overlying iliac arterial vessel against the lumbar vertebrae. In the vast majority of cases, this involves compression of the left common iliac vein between the fifth lumbar vertebra and the overlying right common iliac artery.4 However, other variants have been identified and include the right iliac artery compressing the inferior vena cava, right internal iliac artery compressing the right iliac vein, and left internal iliac artery compressing the left iliac vein. The compression and pulsation of the overlying artery causes increased deposition of elastin and collagen in the iliac vein wall and intimal proliferation, which leads to the formation of a “spur” that can cause complete or partial occlusion of the iliac vein. Over time, venous thrombosis may occur and without proper recanalization and development of collateral circulation, venous outflow obstruction and venous hypertension result.3
IV. Clinical Presentation
A. History
1. Patients with the anatomic variation underlying MTS are usually asymptomatic until thrombosis and venous outflow obstruction occur, usually in the second to fourth decades of life. The location of the symptoms is important to determine, as this suggests which vessels may be involved. If pain is the primary symptom, the patient should be prompted to localize the pain to the calf, thigh, groin, and/or pelvis. Additionally, when MTS and iliofemoral DVT are considered on the differential diagnosis, a full history should be taken to include the presence of any prior DVTs including location, age of occurrence, provoking factors, treatment course, and complications.9 Finally, the history should also investigate any current risk factors for DVT, including recent surgery, prolonged immobilization, extended travel with prolonged motionlessness, hormone replacement therapy/oral contraceptive pill use, pregnancy, clotting disorder, malignancy, smoking, and obesity, as these risk factors have been implicated in MTS syndrome.
2. The clinical presentation is characterized by either acute thrombosis with unilateral edema or more chronic progressive left lower extremity edema and pain with development of chronic venous insufficiency. Lower extremity skin changes due to venous stasis may develop, such as hyperpigmentation, skin induration, and subcutaneous fat inflammation, consistent with lipodermatosclerosis.4 As the venous outflow obstruction persists, patients may demonstrate signs of chronic venous insufficiency, including venous ulceration, varicose veins, and superficial venous thrombophlebitis. Additionally, those with longstanding venous outflow obstruction can also develop venous claudication. Finally, though rare, it is important to recognize that MTS can also present with complications, including pulmonary embolus and left common iliac vein rupture.6 Suspicion for MTS should increase when a patient presents with a history concerning for DVT of the entire limb, primary complaints of venous claudication, and recurrent DVT in the same location.9
B. Physical Examination On physical examination, both lower extremities should be examined specifically for assessment of edema, erythema, tenderness, skin changes, varicose veins, and ulcers. Additionally, all lower extremity peripheral pulses should be palpated, and the pelvis and lower abdomen should be examined for the presence of varicose veins or any other changes consistent with chronic venous stasis. Following physical examination, duplex ultrasound can help determine compressibility of the common femoral vein with Doppler waveform phasicity to compare affected leg with unaffected.9
V. Diagnosis
A. Differential Diagnosis Clinical suspicion may lead a clinician to consider MTS on the differential diagnosis for unilateral lower extremity edema, but there are many conditions that may present similarly to MTS. Therefore, various imaging techniques have proven
useful in both evaluating for the presence of iliac vein compression and also excluding other possible conditions that may mimic MTS. Currently color Doppler ultrasound (CDUS), conventional venography, computed tomography imaging (CT), magnetic resonance venography (MRV), and intravascular ultrasound (IVUS) are the most commonly used imaging techniques for evaluation of chronic venous insufficiency and possible MTS. These techniques differ in cost, invasiveness, and their ability to visualize the iliofemoral vessels, identify compression of the iliac vein, and rule out other causes of unilateral lower extremity edema. Although conventional venography remains the gold standard for diagnosis, these other modalities are noninvasive and clinically useful.
B. Imaging
1. Color Doppler Ultrasound
Although many imaging modalities are currently used in the workup of MTS, color doppler ultrasound (CDUS) is the most cost-effective and noninvasive imaging modality for evaluation of lower extremity superficial and deep veins and is therefore commonly used as a starting point for diagnostic imaging, with parameters described by Labropoulos et al. They investigated 37 patients with lower extremity swelling with or without pain using duplex ultrasound, venography, and IVUS to localize and diagnose venous stenosis and define criteria for diagnosis. Using a 2-4 MHz transducer to evaluate the iliac veins and inferior vena cava, an angle of insonation of <60° was applied in B-mode setting for comparison of vein diameter reduction. They found that a peak vein velocity ratio of >2.5 across the stenosis could most reliably predict the presence of a pressure gradient of 3 mm Hg, which represented a >50% reduction in luminal diameter. They identified the following parameters for diagnosis of central venous stenosis: mosaic color indicating poststenotic dilatation, abnormal Doppler signal at the site of stenosis, asymmetry of Doppler waveform of the affected extremity compared with unaffected extremity, poor flow augmentation, and low amplitude signals.10
Duplex ultrasound has been a mainstay in diagnostic workup for DVT and chronic venous insufficiency and is particularly useful for evaluation of the calf, popliteal, and femoral veins. In comparison, pelvic veins are more difficult to visualize with duplex due to bowel gas, body habitus, or location of the veins with respect to the bladder or adipose tissue. Additionally, CDUS can identify most cases of iliofemoral DVT by assessing for vessel patency but lacks the sensitivity to detect nonocclusive thrombosis of the common iliac vein and as a result may be insufficient to diagnose MTS.11 In a prospective study quantifying the accuracy of duplex imaging in patients suspected of having DVT, duplex imaging could adequately assess the common and external iliac vein in only 47% of cases. For the external iliac vein alone, though, 79% of cases could be adequately assessed, underscoring the difficulty of visualizing the common iliac vein with CDUS within the pelvis.12
2. Venous Phase CT and Magnetic Resonance Venography
CT imaging in the venous phase provides better visualization of the pelvic vessels in comparison with CDUS and is considered a sensitive and specific test for diagnosing iliofemoral thrombosis. CT is able to assess for vessel obstruction due to extrinsic compression within the pelvis not only from MTS, but also from malignancy, hematoma, or fibrosis as well. Limitations include visualization distortion in cases of chronic DVT where fibrosis within the vein may alter the vessel structure.11 Additionally, CT imaging is costly, requires the use of IV contrast, is relatively contraindicated during pregnancy, and is technically difficult to time in the venous phase because of variations of cardiac output and the degree of stenosis.9 Finally, degree of luminal compression may be affected by the patient’s volume status, with dehydration overestimating degree of compression.
MRV is another imaging technique that, similarly to CT imaging, provides improved visualization of the pelvic vessels. MRV is also comparable to CT scan in its ability to rule out extrinsic compression of the iliac veins and is considered superior at assessing the anatomy of the iliofemoral venous system. A primary advantage of MRV over CT venography is use of contrast agents that remain within the vascular system longer, eliminating the contrast timing difficulty encountered with venous phase CT imaging.9 Additionally, enhancements including spin-echo imaging can evaluate inflammatory changes in the vessel wall that may help differentiate between acute and chronic thrombus.13 MRV does share some limitations with CT scanning, including cost, contrast use, and availability. Also, vessels that demonstrate turbulent flow, such as areas above bifurcations, may be indistinguishable from filling defects on MRV. Finally, despite their high sensitivity and specificity, both MRV and CT imaging are limited by level of resolution, which in some instances may not be sufficient to pick up small spurs or subtle webs in the common iliac vein. Given these limitations, MRV seems to be more useful in patients with a low pretest probability of MTS who have mild disease.
3. Contrast Venography Invasive contrast venography allows direct visualization in the evaluation of iliac vein obstruction in suspected MTS and is the gold standard for diagnosis. This involves injection of dye via venous access (common femoral or popliteal) through a catheter with fluoroscopic visualization.13 It can help identify the location and extent of occlusion or thrombosis, assess for any concomitant malformations, and assess the chronicity of the occlusion. Additionally, it allows for pressure measurement across a suspected stenosis via pullback gradient. Although there is no formal guideline in the literature on what degree of pressure gradient reflects a hemodynamically significant stenosis, most studies suggest that 2-3 mm Hg gradient is sufficient to cause symptoms.9 Intervention is possible at the time of venography, including thrombolysis, balloon angioplasty, and stenting.11
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