MTS develops through three stages, starting with (1) asymptomatic left CIV compression, leading to (2) the formation of a venous spur, and finally resulting in (3) left lower extremity DVT.[9] Most patients live with MTS, without ever having a DVT. They may develop left lower extremity venous hypertension without ever noticing it. Rarely, they may develop phlegmasia cerulea dolens.[10] Subtle signs and symptoms like left lower extremity tightness, which resolves after sleeping overnight, mild swelling, hyperpigmentation, telangiectasias, or venous ulceration, can be secondary to MTS. However, none of these signs are specific to MTS. 

MTS is known to occur more commonly in the second and third decades of life. Young women are more commonly affected compared to men. Due to the proximity with the lower lumbar vertebrae, MTS should be suspected in patients with left lower extremity DVT.[1][7] Although MTS is an anatomical variant that is present since birth, the presence of transient risk factors like pregnancy or postpartum, prolonged immobilization, post-surgery, or secondary to oral contraceptive pills is needed to precipitate a DVT.[1] A high index of suspicion is needed, especially when a young woman presents with the left lower extremity DVT in the setting of the aforementioned transient risk factors, and the hypercoagulability workup is negative.[7]  

MTS is best diagnosed with the use of imaging. Various modalities for imaging are discussed here[11]:

1. Ultrasound (US) Doppler: This is the most common technique used in the emergency department to diagnose a DVT. However, technical difficulties in assessing the inferior vena cava (IVC) and iliac vein may limit their utility. In addition to this, it is very challenging to diagnose iliac vein compression on a US Doppler. The high velocity of blood in the common iliac vein may be an indicator of iliac vein compression; however, this exam is dependent on technical expertise.[12]

2. CT venography: It has a higher sensitivity and specificity to detect iliac vein compression nearing 95%. It is also useful in ruling out other causes of iliac vein compression, like lymphadenopathy, hematoma, and cellulitis.[13] However, a common pitfall with CT venography is that it cannot account for the volume status of the patient and hence can overestimate the degree of compression in a dehydrated patient.[9]

3. Magnetic Resonance venography (MRV): MRI/MRV has been proposed as an alternative to diagnose MTS. However, a single MRV may not be sufficient to diagnose MTS due to variability of LCI compression over time, and may also be limited by cost.[14]

4. Venography with intravascular US (IVUS): This is the gold standard to diagnose MTS. IVUS provides a real-time evaluation of the vessel lumen, the accurate size of the luminal diameter, and provides information regarding the structural changes in the vessel wall.[15] In addition to this, IVUS can also provide information regarding the chronicity of the thrombus, which could help in deciding management (for example: to perform thrombolysis of acute clot burden or not). IVUS can also localize guidewires during challenging recanalizations in a patient with multiple venous collaterals and assist in accurate placement of stents.[11] The biggest advantage of IVUS is that in venous studies, contrast is not needed, which reduces the chances of contrast related nephropathy and allergies. 

All patients with acute thrombosis undergo catheter-directed thrombolysis, after which the endovascular stent is placed. Berger et al. were the first to report the safety and efficacy of vascular stenting in MTS.[16] Multiple studies have shown that vascular angioplasty is not good enough for MTS.[17][18] The guidelines by the Society of Interventional Radiology and the Society of Vascular Surgery recommend iliac venous stenting in the setting of external iliac vein compression.[19][20] Catheter-directed thrombolysis increases the risk of bleeding. 

Absolute contraindications to catheter-directed pharmacologic thrombolysis include

• Active internal bleeding
• Cerebral infarction
• Neurological and eye procedures 
• Head trauma within three months
• Presence of an intracranial tumor, aneurysm, or vascular malformation

Relative contraindications include

• Major trauma, surgery or obstetrical delivery within 10 days
• Uncontrolled hypertension (systolic greater than 180 mmHg or diastolic greater than 110 mmHg)
• Gastrointestinal bleeding within 3 months
• Pregnancy
• An infected venous thrombus
• Severe renal or liver disease
• Hemorrhagic diabetic retinopathy
• Bleeding diathesis 

Open surgical thrombectomy, especially in the setting of MTS, is losing favor. This is shown by the fact that surgical thrombectomy was done for only 4.1% of all patients with MTS related DVT compared to 75% of all patients before 2000.[8] The operative dissection, morbidity from surgery and anesthesia, and poor surgical outcomes go against surgical dissection.[21] Surgical resection is mostly reserved for patients who fail endovascular procedures. In the rare patients with an occluded left iliac vein who fails an endovascular procedure, a saphenofemoral crossover bypass, cross pelvic venous bypass (the Palmae Dale procedure), femorofemoral prosthetic bypass, femorocaval bypass, ilioilial prosthetic bypass, and aortic elevation can be pursued.[7]

Anticoagulation forms an important part of the management of patients with MTS who present with iliofemoral DVT. A delay in starting anticoagulation is associated with an increased risk of life-threatening pulmonary embolism (PE). Low molecular weight heparin or fondaparinux is preferred over unfractionated heparin to reduce the risk of bleeding and heparin-induced thrombocytopenia. Major societies still recommend vitamin-K antagonist (VKA), warfarin for anticoagulation in patients with iliofemoral thrombosis.[22] Factor Xa inhibitors have also been approved for the management of DVT; however, lack of good prospective data prevented their use in patients with iliofemoral thrombosis.[23][24][25] Recently, a multicenter randomized trial demonstrated the safety of rivaroxaban in patients with iliofemoral vein thrombosis.[25] In this study, approximately 50% of patients were diagnosed with MTS. Although the difference in major bleeding events could not reach statistical significance, the risk of bleeding was quite low in the rivaroxaban group compared to the warfarin group (2.9% versus 9.4%, HR, 0.31; 95% CI, 0.03–2.96; p = 0.31).[25] Another prospective registry-based study led by Sebastian et al. also reported similar efficacy for patients who received catheter-directed thrombolysis and stent placement followed by anticoagulation with rivaroxaban.[26] Anticoagulation alone is not enough in patients with thrombosis of iliofemoral vein secondary to MTS. The results from the CaVenT trial and subgroup analysis of the ATTRACT trial showed conclusively that catheter-directed thrombolysis with anticoagulation is superior to anticoagulation alone.[27][28] 

The top three main causes of iliac vein compression other than MTS are the following.

• Malignancy or lymphadenopathy
• Hematoma
• Cellulitis

Other than these any other anatomical condition compressing the iliofemoral vein must be considered in the differential

• Uterine enlargement from fibroids, cancer, or pregnancy, and also pelvic masses
• Aortoiliac aneurysm
• Retroperitoneal fibrosis
• Osteophyte

Along with these, alternate causes of thrombosis must be considered in any young patient presenting with a blood clot. All patients who present with thrombosis must undergo age-appropriate cancer screening. 

MTS remains undetectable or clinically silent in the majority of patients. Multiple cadaveric studies on unselected subjects have shown a much higher prevalence compared to the actual thrombosis events reported in the alive patients. However, in patients with left-sided iliofemoral thrombosis, the prevalence of MTS has been consistently reported to be quite high. Post-thrombotic syndrome (PTS) is a common complication of MTS.

Post-thrombotic syndrome is a common complication of PTS, which adds to the morbidity of the patients. Comerota et al. demonstrated that residual thrombosis after catheter-based thrombolysis is positively associated with the development of PTS.[29] A residual thrombus and recurrent DVT are the strongest predictors for PTS in a patient with any kind of DVT.[30] The same principles also apply for iliofemoral DVT. Thrombolysis (mechanical or pharmacological) may reduce the risk of PTS.[29][31] Patients with PTS can benefit from knee or thigh-high compression stocking, applying a pressure of 30 to 40 mmHg.[7] A delay in anticoagulation for the DVT increases the risk of life-threatening pulmonary embolism. Anticoagulation is associated with bleeding risks. 

MTS is more prevalent than thought; however, most patients do not exhibit the signs or symptoms of MTS. The most common presentation is iliofemoral DVT, which should always raise the suspicion for MTS or iliofemoral compression syndrome. Healthcare teams must maintain a high index of suspicion for MTS in patients presenting with left-sided iliofemoral vein thrombosis. Patients must be made aware of the signs and symptoms of DVT and PE so that they can return to seek care at the onset of symptoms. They must also be taught about the post-thrombotic syndrome and given adequate counseling on how to wear the compression stockings. All patients should also be educated regarding the risk of bleeding with DOAC's, warfarin, and parenteral agents and how to seek care in case of uncontrolled or extensive bleeding. 

A few pearls regarding MTS

• Although MTS accounts for only 2% to 5% of all patients presenting with DVT, multiple autopsy studies have shown that the actual prevalence is as high as 14% to 32% in the general population.
• Despite the high prevalence, MTS remains clinically silent in most patients.
• Iliofemoral DVT is the most common presentation of MTS.
• Young women are at a higher risk of developing DVT compared to men. 
• A transient risk factor is usually present in patients with MTS, which precipitates the thrombotic event. 
• MTS has been associated with patent foramen ovale and cryptogenic stroke. 
• Prompt anticoagulation with low-molecular-weight heparin or fondaparinux is needed to prevent PE.
• Although most societies recommend warfarin for long term anticoagulation, recent studies show that rivaroxaban is equally effective and has a lower risk of major bleeding (although this has not reached statistical significance in clinical studies).
• Venogram with IVUS is the gold standard in diagnosing MTS. In addition to diagnosis, it also helps in the treatment of DVT secondary to MTS.
• Catheter-directed thrombolysis, followed by stent placement, is the treatment of choice. 
• Surgical resection of thrombus is falling out of favor. It is reserved for patients in whom endovascular treatment fails
• Anticoagulation alone is not sufficient to treat patients with DVT secondary to MTS. It should be combined with catheter-directed thrombolysis. 
• Post-thrombotic syndrome is the most common adverse event after developing iliofemoral DVT. Residual thrombus after thrombolysis and stent placement is positively associated with the risk of developing PTS.

The diagnosis of MTS or iliofemoral compression syndrome requires a high index of suspicion. All physicians, starting from the team in the emergency department (where most patients with DVT usually present) or those in the primary care office, must be aware of this condition. A dedicated team of radiologists and vascular surgeons, along with support staff (nurses, anesthesiologists, etc.) are needed to provide acute care to such patients. Also, post thrombolysis and stent placement, a dedicated team is required to follow up with patients to help in rehabilitation and early detection of post-thrombotic syndrome. The risk of anticoagulation must be discussed with the patient by a vascular surgeon or a hematologist. Pharmacists review prescriptions, check for interactions, and counsel patients about their use and side effects.

The treatment modalities discussed in this article have undergone phase II and phase III trials with level I evidence as described above.