The iliac veins are large blood vessels deep within the pelvis that collect deoxygenated blood from the legs and lower abdomen, returning it toward the heart. Iliac vein compression (IVC) occurs when these soft-walled veins are squeezed or narrowed by an adjacent, firmer structure, typically an artery or bone. This obstruction hinders the normal flow of blood from the lower extremities, increasing pressure below the blockage. While some narrowing is common, IVC only becomes a medical concern when the compression is severe enough to impede blood return and cause symptoms.
May-Thurner Syndrome
The most frequent cause of iliac vein narrowing is May-Thurner Syndrome (MTS), also known as Cockett’s Syndrome. This anatomical condition involves a specific crossover point where the right common iliac artery passes directly over the left common iliac vein. The vein is compressed, or “sandwiched,” between the artery in front and the hard surface of the fifth lumbar vertebra or pelvic bone behind it.
The artery carries blood under high pressure and exerts a chronic, repetitive compressive force against the softer vein wall with every heartbeat. This continuous pulsatile pressure causes irritation and shear stress to the inner lining of the vein (endothelium). In response to this chronic injury, the vein attempts to heal itself by depositing excess collagen, leading to a buildup of scar tissue.
This internal scarring manifests as fibrous bands or spurs (intimal hyperplasia) that project into the vein’s lumen, further narrowing the vessel from the inside. The combination of the external arterial squeeze and the internal fibrous spurs significantly reduces the cross-sectional area of the left iliac vein. This chronic anatomical restriction is the main driver of symptomatic iliac vein compression.
Although this anatomical arrangement is common, occurring in over 20% of the population, only a small fraction of individuals develop symptomatic disease. The left iliac vein is overwhelmingly affected due to its unique path under the right artery. The condition often remains undiagnosed until it progresses to a complication like deep vein thrombosis.
Other Structural Obstructions
While May-Thurner Syndrome is the most common anatomical cause, other physical structures within the abdomen and pelvis can also compress the iliac veins, leading to impaired blood flow. These other causes are generally acquired or secondary to another disease process. A significant category of obstruction involves masses that exert external pressure on the vein, such as tumors or malignancies.
These masses may originate in the retroperitoneal space or the pelvis and physically squeeze the iliac vein against the bony spine or other firm structures. Similarly, enlarged lymph nodes (lymphadenopathy) can press on the vein, often in response to infection or cancer. Retroperitoneal fibrosis, the buildup of dense, non-cancerous scar tissue behind the abdominal lining, is another cause.
This fibrous tissue can encase and constrict the iliac vein, leading to narrowing independent of the iliac artery’s position. Previous pelvic trauma, such as a severe fracture, or complications from prior abdominal or pelvic surgery can also cause external compression or scarring that deforms the vein.
Factors That Increase Risk
Several factors can increase a person’s risk of developing symptomatic iliac vein compression or suffering its most severe complication, deep vein thrombosis (DVT). The condition is diagnosed more frequently in women, particularly those between the ages of 20 and 40. Pregnancy is a risk factor because the growing uterus places substantial pressure on the pelvic veins, exacerbating any pre-existing anatomical compression.
Medical conditions that promote blood clotting (hypercoagulable states) also increase the likelihood of symptomatic disease once compression exists. Prolonged periods of immobilization, such as extended travel or bed rest, can slow blood flow and increase the risk of a clot forming at the site of the narrowing. Patients with a high degree of iliac vein compression, specifically greater than 70% narrowing, have an elevated risk of developing DVT.
How Vein Compression Causes Symptoms
The narrowing of the iliac vein creates a bottleneck for blood returning from the leg. This obstruction results in blood stasis, which is the slowing or pooling of blood in the veins below the point of compression. According to Virchow’s triad, this sluggish blood flow is a condition that predisposes the patient to the formation of blood clots.
The most concerning consequence is the development of Deep Vein Thrombosis (DVT), a blood clot that forms in the deep veins of the leg and can extend into the iliac vein itself. DVT formation can suddenly and severely block the vein, leading to acute symptoms like painful leg swelling and warmth. Iliac vein compression is considered an underdiagnosed anatomical risk factor for iliofemoral DVT.
If the compression is chronic or a DVT has partially resolved, the resulting persistent high pressure in the leg veins leads to Chronic Venous Insufficiency (CVI). This venous hypertension causes chronic symptoms that worsen over time, including persistent leg swelling, a heavy or aching sensation, and venous claudication (pain when walking). Over years, the sustained high pressure can cause skin changes such as discoloration (hyperpigmentation), prominent varicose veins, and the formation of venous ulcers.