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May-Thurner syndrome — also called iliocaval compression syndrome, Cockett syndrome or iliac vein compression syndrome — occurs secondary to compression of the left iliac vein by the overriding right iliac artery. Virchow was the first author to be credited with describing iliac vein compression. It was not until 1957 that May and Thurner brought much attention to the anatomic variant thought responsible for Virchow’s observation. They found that the right iliac artery compressed the left iliac vein against the fifth lumbar vertebra in 22–32% of 430 cadavers.1 These terms may be used interchangeably, but they all describe the phenomenon of left-sided vein compression by the right iliac artery causing left iliofemoral deep venous thrombosis (DVT).
Etiology and Incidence
The incidence of May-Thurner syndrome is unknown and ranges from 18–49% among patients with left-sided lower extremity DVT.2 Close to 600,000 hospitalizations occur in the United States each year due to DVT. DVT is more common in the left lower extremity than the right, and May-Thurner syndrome is considered to be a risk factor for patients with left-sided iliofemoral DVT. May and Thurner postulated that the chronic pulsations of the overriding right iliac artery led to the development of a “spur” in the vein wall, and that this spur would result in partial venous obstruction (Figure 1). Chronic trauma to the inner side of the vein wall due to adjacent arterial pulsations leads to the accumulation of elastin and collagen, contributing to spur formation.3 In addition to the chronic arterial pulsations, mechanic compression of the iliac vein by the thick-walled overriding iliac artery leads to extensive local intimal proliferation, impaired venous return and venous thrombosis.4 In addition to the mechanical alterations to the vessel wall, hypercoaguable states, when tested, are found in the majority of patients. Kolbel et al5 found underlying hypercoaguable disorders in 67% of patients screened prior to treatment of chronic iliac vein occlusion. Left iliac vein compression is the most common variant seen in May-Thurner syndrome; however, several other variants have been described in the literature. Compression of the left common iliac vein by the left internal iliac artery,6 compression of the right common iliac vein by the right internal iliac artery,7 compression of the inferior vena cava by the right common iliac artery8 and right-sided May-Thurner syndrome in a patient with a left-sided inferior vena cava9 have all been described.
Patients with May-Thurner syndrome typically present with unilateral (left) lower extremity edema and pain. A propensity for this disorder is seen in young women in their second to fourth decade of life, after prolonged immobilization or pregnancy. Because of the chronic nature of the disease process, patients may also present with stigmata associated with post-thrombotic syndrome such as pigmentation changes, varicose veins, chronic leg pain, phlebitis and recurrent skin ulcers.4 The clinical stages of iliac vein compression were described by Kim et al,10 and include: Stage I, asymptomatic iliac vein compression; Stage II, development of a venous spur; Stage III, development of left iliac vein DVT.
The diagnosis of May-Thurner syndrome is based on the clinical presentation of left lower extremity swelling and pain in association with radiologic evidence of compression. This being said, diagnosis of May-Thurner syndrome may not always be straightforward. Doppler ultrasound will detect if a DVT is present in the iliac vessels, but is unable to visualize iliac vein compression and spurs. Other diagnostic modalities include helical abdominal computed tomography (CT), CT venography, magnetic resonance venography (MRV), intravenous ultrasound (IVUS) and conventional venography. Kibbe et al11 used abdominal helical CT scanning to determine the incidence of left common iliac vein compression in an asymptomatic population (Figure 2). They found that two-thirds of all patients studied had at least 25% compression of the left iliac vein. The authors concluded that compression of the left iliac vein may be a normal anatomic finding, and that abdominal CT scanning is accurate in determining if left iliac vein compression is present. There are, however, limitations to the use of abdominal CT scanning in determining if iliac vein compression is present. The CT scans were obtained during the arterial phase of the intravenous bolus, which limits the type of vessel reconstruction and analysis that can be performed. CT venography may be used as an effective adjunctive modality when there is a known DVT. Chung et al12 found that CT venography was just as specific and highly sensitive in the diagnosis of DVT compared with ultrasound and accurately delineated venous anatomy and the extent of thrombus present. A limitation of CT venography involves the inability to control for the volume status of the patient, which could lead to overemphasis of the degree of compression of the left iliac vein in a dehydrated patient. The traditional “gold standard” for diagnosis of May-Thurner syndrome is conventional venography, which can be diagnostic and therapeutic when endovascular therapy is used (Figure 3.). Non-invasive imaging methods are being used increasingly to diagnose DVT and iliac compression. The aforementioned imaging modalities may help in planning catheter-directed thrombosis without the initial need for conventional venography. These non-invasive imaging modalities are simple, efficient and cost-effective in diagnosing DVT associated with iliac compression.3
May-Thurner syndrome patients are commonly asymptomatic, and it is therefore unrecognized until symptoms develop. Treatment of symptomatic May-Thurner syndrome has evolved over the years from traditional open repair to less invasive endovascular repair. Treatment is aimed at clearing the thrombus present to prevent post-thrombotic syndrome and to correct the underlying compression of the left iliac vein. Untreated, a significant majority of adults with May-Thurner syndrome and thrombosis develop debilitating post-thrombotic syndrome.13 Historically, several surgical procedures have been used to ameliorate symptoms and correct the underlying compression such as venovenous bypass with autologous vein, creation of a tissue sling to elevate the overriding right iliac artery, retropositioning of the iliac artery and excision of the intraluminal spur with patch venoplasty. Traditional open repair has yielded variable results, and with the advent of endovascular technology and technique, mainstay therapy now includes a combination of surgical and endovascular approaches.
The first known report of treatment of May-Thurner syndrome solely by endovascular means was by Berger14 et al in 1995, who successfully placed a venous stent to relieve iliac compression. Several subsequent studies have demonstrated efficacy in the treatment of iliac vein compression with thrombectomy and endovascular stenting.15,16 The initial treatment of patients with documented iliofemoral thrombosis in the setting of iliac vein compression is to decrease the clot burden by use of thrombectomy. Traditionally, surgical thrombectomy alone was used. However, if a stent was not placed subsequently, reocclusion occurred in approximately 70% of patients treated.17 Catheter-directed thrombolysis with urokinase or t-PA is very effective in reducing clot burden by dissolving the thrombus present (Figure 4). Localized thrombolytic therapy is commonly performed to decrease the risk of major bleeding that can be seen with the use of systemic thrombolytic therapy. Alternatively, mechanical thrombolysis has been used to reduce lytic infusion time and complications. Following thrombectomy, angioplasty with stent placement is used to correct the venous obstruction. Stent implantation versus angioplasty alone is more effective in relieving venous obstruction.18 Recanalization of the involved segment involves passing a wire through the occlusion and in general predilating the vessel. Self-expandable stents are used in the venous system as they can cover long distances, are easy to re-sheath and have adequate durability. Balloon-expandable stents may be used if needed (insufficient response to predilatation/self-expandable stent). Extending the stent into the inferior vena cava (IVC) may be done without increasing the risk of contralateral iliac vein occlusion.19 However, some studies have found that extending the iliac stent below the inguinal ligament in the common femoral vein to ensure venous inflow could increase the risk of in-stent restenosis.20,21 These results, though, have been controversial and there is to date no consensus on the risk of venous stenting below the inguinal ligament. Long-term 5-year patency rates for patients undergoing stent implantation range from 74–80%.5,21 Patency can be affected by the amount of occlusion in the iliac vein. Patients who have chronic complete occlusion (as in patients with May-Thurner) have lower patency rates when compared to those with just stenotic obstructions.20 This has led to an increase in combined surgical and endovascular approaches for patients with complete venous occlusions in the late 1990s and early 2000. Surgical intervention typically involved common femoral venectomy, complete thrombus removal with stent placement. The results shown by Kolbel et al5 and Wahlgren et al22 have shown that now similar results can be accomplished with endovascular stenting alone. Several complications occur with endovascular stenting including stent migration, stent fracture, retroperitoneal hemorrhage and early in-stent thrombosis. Patients are routinely anticoagulated following thrombectomy and venous stent placement for up to 6 months to minimize in-stent restenosis. There are several factors associated with in-stent restenosis, despite the use of anticoagulation, which include recent trauma, thrombotic disease, thrombophilia and stenting below the inguinal ligament.23
May-Thurner syndrome continues to challenge medical practitioners to this day. It can have debilitating sequelae resulting from post-thrombotic syndrome. More and more, helical CT scanning is becoming the standard for diagnosis of iliofemoral vein thrombosis. Management of May-Thurner syndrome has evolved over the past few decades favoring endovascular management as the primary treatment. With early recognition and aggressive management, May-Thurner syndrome can be a well-managed disease.
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Omar Al-Nouri, DO, MS* and Ross Milner, MD§
From the *Department of Surgery and the §Department of Vascular and Endovascular Surgery, Loyola University Medical Center, Maywood, Illinois.
The authors report no conflicts of interest regarding the content herein.
Address for correspondence: Al-Nouri, DO, MS, Loyola University Medical Center, 2160 S. First Avenue, Maywood, IL 60153.