Deep venous thrombosis (DVT) tops the differential diagnosis list for unilateral lower extremity edema, but another entity could imitate or even cause a DVT. May-Thurner syndrome is caused by compression of the left common iliac vein by the overlying right iliac artery, resulting in impeded venous blood flow from the left lower extremity. The left leg becomes edematous, causing discomfort and concern. Early recognition of May-Thurner could prevent a DVT and provide symptomatic relief.
Introduction
The aorta travels left of the inferior vena cava as it descends through the abdomen, such that when it bifurcates, the right common iliac artery must cross over the left common iliac vein, near the bifurcation of each central vessel (Fig. 1).1 Because an artery has a much thicker wall and is under much greater pressure than a vein, when pushed together the vein becomes the compressed vessel. If the vein is sufficiently compressed against the pelvic rim, the increased pressure can cause edema in the left lower extremity.2 Virchow’s Triad describes an increased risk of venous thrombosis with stasis, hypercoagulability, and vessel intimai injury, the first of which is present with any venous obstruction.3 There is evidence that intimai injury may also take place in the form of a spur when the compression occurs over time. Despite many collateral vessels throughout the pelvis, significant compression can produce symptomatic edema.
Because the etiology of May-Thurner is left common iliac vein compression by the overlying right common iliac artery, the diagnosis should only be entertained when the edematous leg is on the left. There is a female predominance, and in 1965, Cockett and Thomas4 generally found it in young women between the second and fourth decades. Surgery, pregnancy, and prolonged bed rest have all been mentioned as possible acute precipitating factors. Based on autopsies, May and Thurner found a 22% prevalence of a spur, the name they gave to the intravenous endothelial changes seen at the point of compression near the mouth of the left common iliac vein.5 May-Thurner patients may present only after a deep venous thrombosis (DVT) has formed, well past the primary prevention stage. Because iliac vein DVT are found more than twice as often on the left than the right, May-Thurner compression could be responsible for many of these cases.6
In one case, a 27-year-old active duty Hispanic woman presented with 4 months of left lower extremity swelling and ankle discomfort when running. The patient had no other symptoms, including chest pain, respiratory difficulty, or calf tenderness. The patient had a negative medical history, including surgery, trauma, pregnancy, oral contraception, current medication use, smoking, allergies, abnormal bleeding, or family history of coagulopathy. The patient was not obese, weighing 113 pounds and a height of 61 inches. Physical examination revealed 2+ pitting edema to the left knee, but a negative Roman’s sign. There was no inguinal lymphadenopathy or other abnormality on physical examination. The differences in circumference of the ankle and calf from the left to the right were 2 and 1.5 cm, respectively, with only trace difference at the knee (Fig. 2). An ultrasound was done that showed no evidence of DVT. Ultrasonography demonstrated a patent deep venous system from the popliteal to the left common iliac vein. However, during pelvic venogram via micropuncture catheter access of the left femoral vein, significant extrinsic compression of the left common iliac vein was noted under the crossing right common iliac artery (Figs. 3 and 4).
Diagnosis
The patient with symptomatic chronic left common iliac compression will typically present with symptoms, many of which are similar to those found in patients with a DVT. Swelling, pain in the distal lower extremity, engorged varicose veins, and even venous stasis skin changes can all be seen. An inability to wear a tight fitting shoe may be the initial complaint. Difficulty in performing physical exercise such as running can be seen in athletes or active duty military members secondary to the discomfort of having one leg swollen compared with the unaffected right side. Unlike a DVT, whose onset is generally acute, MayThurner would be expected to present in a more chronic fashion. However, a patient may fail to notice significant swelling until an event such as the wearing of a new shoe forces recognition of the difference. A precipitating event, such as the rapid anatomical change that accompanies the weight gain of pregnancy, is present in some cases.
When a patient presents with unilateral lower extremity edema, the clinician should attempt to rule in or out DVT. Most commonly, a compression/Doppler ultrasound is used because of its relative simplicity and noninvasiveness. Impedence plethysmoography is not used as often as ultrasound, but appears to have most of the same benefits and limitations as ultrasound in finding a DVT. Magnetic resonance imaging can also show thrombosis in the pelvis, but has not been shown to be superior to other, less expensive, choices. Venography is the gold standard for ruling in or out DVT and May- Thurner syndrome, but is invasive and painful.7 Intravascular ultrasonography is a relatively new test performed by placing a transducer in a large vessel using the Seldinger technique at an entry location in the groin.8 It can be used to detect a DVT or measure cross-sectional areas of vessels. In some studies, it has shown promising iliac vein results, but may be less widely available than more conventional tests.9
Fig. 1. Diagram showing right common iliac artery crossing over left common iliac vein.
Fig. 2. Case patient with left lower extremity edema.
When a patient presents with unilateral lower extremity edema, the clinician must consider the possibility of a DVT because of its potential for pulmonary embolism (PE). If the ultrasound shows a DVT, although prevention of PE is the top priority, May-Thurner might still be considered as a possible etiology of the DVT. If no DVT is seen on Doppler/compression ultrasound, given its 97% sensitivity, the clinician must run the differential diagnosis list while acknowledging the inherent 3% false-negative rate.10 The history and physical should include questions about personal and family bleeding disorders, respiratory symptoms, trauma, infection, and travel. The physical examination should document size difference between the extremities and the presence or absence of Homan’s sign, in which ankle dorsiflexion provokes calf pain.11 If the person has spent time in the East Indies or Africa, one could consider the possibility of lymphedema secondary to filariasis. Cancer metastatic to the lymph system can also cause secondary lymphedema. A tumor, such as a thigh lipoma, or a psoas or gluteal abscess could cause swelling in the proximal part of the lower extremity. Warmth, fever, erythema, and localized pain could indicate infection. Acute arthritis, a popliteal artery aneurysm, or ruptured Baker’s cyst could all cause unilateral leg swelling in the absence of trauma.12 Gout is a common cause of swelling and pain of the distal lower extremity, but does not normally cause measurable differences in calf diameter. If the history, physical, and ultrasound provide no evidence of DVT in a young woman with left lower extremity edema, May-Thurner syndrome should be considered.
Treatment
A literature search using OVID/MEDLINE and MDConsult revealed no widely accepted recommendations for patients suffering from May- Thurner syndrome who have not yet experienced a DVT. However, because stasis is one of the three pillars of Virchow’s Triad, such patients could be at increased risk. There are several options in DVT prevention. Decompression of the left common iliac vein via vascular surgery or interventional radiology can diminish the stasis. If an invasive procedure is not chosen, anticoagulation may be appropriate. If a patient has suffered a first DVT, anticoagulant treatment is indicated for a minimum of 3 to 6 months in patients in whom it is not contraindicated. With a second DVT and no treatable cause, anticoagulation is generally used indefinitely.13 In the case above, aspirin was chosen for DVT prophylaxis, at least until a more permanent treatment is undertaken. Aspirin is generally contraindicated for those with a history of one recent or multiple DVT because other anticoagulants are more effective, but no evidence is readily available for DVT prophylaxis in those with May-Thurner syndrome and no history of DVT.14 Aspirin was shown in the PE Prevention trial to prevent DVT compared with placebo in hip fracture patients.15 Although study has shown aspirin to be safe during the second and third trimester of pregnancy, the risks of taking aspirin during the first trimester are not known and aspirin is listed as a Class D medication in pregnancy, and counseling in such cases is appropriate if the patient is contemplating pregnancy.16
If DVT risk is significant, such as in the case of a patient with a history of DVT, the use of oral warfarin (Coumadin), a vitamin K antagonist, or an injectable low molecular weight heparin, an antithrombin III protagonist such as enoxaparin (Lovenox), is indicated. For those who are athletes, travelers to remote places where blood transfusion might be risky, or military members who \must pass stringent physical fitness testing to be ready for deployment to remote places, warfarin may not be an acceptable choice because of the frequent laboratory monitoring required. Enoxaparin requires daily or twice daily injections that have been shown to reduce DVT risk to a level equal to or better than warfarin in many postsurgical situations, but may have a slightly increased risk of major bleeding compared with warfarin.17 None of these situations can be directly compared with the iliac vein compression found in May-Thurner syndrome. In terms of cost, warfarin and low molecular weight heparin have been shown to be similar when the laboratory costs associated with warfarin are included, although both are much more expensive than aspirin. Several authors have suggested a possible link between air travel with limited limb movement and DVT, and many experts recommend some light exercise during air travel. However, no studies have definitively shown a reduction in DVT through regular cardiovascular exercise involving the legs, such as running. In fact, running may actually be more difficult when one extremity is edematous.
Fig. 3. Case patient’s venogram showing left common iliac vein compression. Note catheter just medial to left femoral head, dark left common iliac vein, and lighter area between vein and inferior vena cava at site of compression by right common iliac artery.
Fig. 4. Case patient’s venogram (oblique) showing left common iliac vein compression. Note light groove between dark left common iliac vein and inferior vena cava.
There is no definitive treatment for May-Thurner syndrome, although there are several promising treatments. Multiple journal articles report small numbers of successful venous stent placement, usually involving a Wallenstent. Immediate and short-term results are quite good, with 71% primary patency at 2 years.18 However, there are no long-term patency rates. A vascular surgical option can also be undertaken. Transection, movement, and reanastomosis of the right common iliac artery have been successful in a few cases.19 Another procedure involves bypassing the vein, which has also shown promising results in a few cases.20 Because of the relative rarity of this diagnosis, there are no long-term head-to-head studies comparing vascular surgery versus stent placement, therefore, no specific algorithm recommendation can be made after a review of the literature. In the case above, the consulting surgeon recommended no treatment, although referral for stent placement was offered.
Conservative management simply involves pain control with or without DVT prophylaxis. Because there is no known spontaneous resolution of May-Thurner, this could be a difficult decision for a competitive athlete or active duty military member who experiences discomfort during strenuous exercise.
Conclusion
The top of the differential diagnosis list for unilateral left lower extremity edema should be a DVT, but with or without sonographic evidence of DVT, another possibility is that stasis from May-Thurner could be the etiology of the symptoms. As treatment options improve, these patients may soon have more definitive choices, impacting the decision to continue a successful military career.
References
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Guarantor: LCDR David A. Boyd, MC USN
Contributor: LCDR David A. Boyd, MC USN
Department of Family Practice, Naval Hospital Jacksonville NAS, Jacksonville, FL 32214.
This manuscript was received for review in May 2003. The revised manuscript was accepted for publication in January 2004.
Reprint & Copyright by Association of Military Surgeons of U.S., 2004.
Copyright Association of Military Surgeons of the United States Dec 2004
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