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May–Thurner Syndrome (MTS), also known as Iliac Vein Compression Syndrome, is a vascular compression disorder where the left common iliac vein (LCIV) is compressed between the overlying right common iliac artery (RCIA) and the lumbar vertebra, leading to venous outflow obstruction from the left lower extremity.

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![Iliac vein compression syndrome. Left: the left common iliac vein is compressed by the right common iliac artery; Right: the compressed left CIV is located between the right common iliac artery and the vertebral body.

Liu Y, Zheng H, Wang X, Wang Z, Zhu Q, Wen C and Tong Y (2023) Ultrasound characteristics of abdominal vascular compression syndromes. Front. Cardiovasc. Med. 10:1282597. doi: 10.3389/fcvm.2023.1282597](attachment:87dbb86d-686a-4d46-bd61-ba5097a47960:fcvm-10-1282597-g006.webp)

Iliac vein compression syndrome. Left: the left common iliac vein is compressed by the right common iliac artery; Right: the compressed left CIV is located between the right common iliac artery and the vertebral body.

Pathophysiology

Predominantly affects young to middle-aged women (20–40 years)
True prevalence may be underestimated due to subclinical or nonspecific symptoms
Found incidentally in ~20–25% of autopsies

Pathological triad:

Component	Details
I. .Anatomic compression	RCIA compresses LCIV against L5 vertebral body
II. Endothelial damage	Chronic pulsatile compression causes intimal hyperplasia and fibrosis
III. Venous stasis	Leads to venous hypertension, varicosities, thrombosis

This triad mimics Virchow’s triad: stasis, endothelial injury, and hypercoagulability → risk of DVT.

Clinical features

Presentation	Details
Chronic symptoms	Unilateral left leg swelling, heaviness, varicose veins, venous claudication
Acute presentation	Left-sided iliofemoral DVT (phlegmasia alba dolens or cerulea dolens)
Pain worsening with standing	Postural dependency of venous congestion

![5-year-old female with history of May-Thurner syndrome presenting with acute left lower extremity DVT and skin changes characteristic of phlegmasia cerulea dolens.

Kuo I, Smith J, Abou-Zamzam AM Jr. A multimodal therapeutic approach to phlegmasia cerulea dolens in a pediatric patient. J Vasc Surg. 2011. Jan;53(1):212–5. doi: https://doi.org/10.1016/j.jvs.2010.07.067. Epub 2010 Sep 26. PubMed PMID: 20875715](attachment:9da8e896-24bc-4217-9b10-c53cb6e6f757:42155_2020_150_Fig1_HTML.webp)

5-year-old female with history of May-Thurner syndrome presenting with acute left lower extremity DVT and skin changes characteristic of phlegmasia cerulea dolens.

Complications

Complication	Explanation
Left iliofemoral ‣	Most common complication
‣	Secondary to thrombus propagation
‣	Varicosities, hyperpigmentation, ulceration
‣	Due to reflux into pelvic venous plexus

Radiology

Modality	Imaging features
USD	• Detects deep venous thrombosis (DVT)
• Limited for LCIV visualization due to pelvic location
CTV	• Compression of LCIV: >50% caliber reduction between RCIA and spine
• Post-stenotic dilatation: LCIV proximal to the compressed segment
• Collateral venous channels: Gonadal, lumbar, or pelvic sidewall veins
• Chronic thrombosis: Wall thickening, calcified thrombus, narrowed lumen
MRV	Similar to CT, but avoids radiation
• Extent of thrombus
• Collateral circulation
• Degree of compression
IVUS	• Intra-procedural imaging: Evaluates degree of luminal narrowing
• Identifies: Webs, fibrosis, and stenosis More accurate than venography alone
Catheter-Based Contrast Venography	Traditionally the gold standard, but limited by 2D view and compression underestimation
• Used during endovascular planning