The iliac vein compression syndrome is a disorder of venous return to the lower extremities and pelvis caused by compression of the iliac veins and or the presence of abnormal intraluminal adhesion structures. This causes venous reflux disorders in the lower extremities and pelvis, resulting in a series of clinical symptoms and signs. This is why some people call this syndrome Cockett’s syndrome. Iliac vein compression not only causes venous reflux obstruction and lower limb venous hypertension, which is one of the causes of lower limb venous valve insufficiency and superficial varicose veins, but also can cause secondary iliac-femoral vein thrombosis, which is a potential factor for venous thrombosis in the left lower limb. The anatomical relationship between the iliac artery and the iliac vein is the basis of the iliac vein compression syndrome. The bilateral common iliac vein is located on the right side of the 5th lumbar vertebral body in the lower middle plane, and converges into the inferior vena cava and travels up the spine. The right common iliac vein is almost straight and continuous with the inferior vena cava, whereas the left common iliac vein runs transversely from the left side of the pelvis to the right and joins the inferior vena cava at almost right angles before the lumbosacral vertebrae. The abdominal aorta, on the other hand, descends from the left side of the spine and divides into the left and right common iliac arteries in the plane of the inferior border of the 4th lumbar vertebral body, so the right common iliac artery crosses the front of the left common iliac vein and then extends to the lower right side of the pelvis. It has been found that in nearly 3/4 of the population, the right common iliac artery crosses the left common iliac vein at the level of the bilateral common iliac vein confluence; 1/5 of the population is at this point at a mildly superior level, and a minority is below this point. Thus, the left common iliac vein is pushed more or less anteriorly by the physiologic anterior convexity of the lumbosacral spine, while it is pressed posteriorly by the right common iliac artery, which crosses it anteriorly, leaving it in an anatomic position of anterior compression and posterior crowding. When the body is upright and the lumbosacral region is highly anteriorly inclined, the physiological anterior convexity intensifies making the compression more pronounced; when the body is in a sitting position, the compression is relieved or disappears. Occasionally, compression of the left common iliac vein arises from a hypoplastic abdominal aorta, a twisted left common iliac artery, bladder, tumor, and ectopic kidney. 2. abnormal intraventricular luminal structures McMurrich, Erich, and Krumbharr et al. found that the incidence of left common iliac vein compression and intraluminal adhesions was 32, 3%, 23, 8%, and 14%, respectively, after anatomic observation of a large number of cadavers without significant manifestations of left lower extremity venous disease. in 1956, May and Thurner suggested that 22% of autopsies in The presence of crest-like structures within the lumen of the left common iliac vein, which contains fibroblasts, collagen, and a large number of capillaries. Since no such structures were found in the lumen of the fetal common iliac vein, they suggested that this was the result of an acquired response to compression of the left common iliac vein by the right common iliac artery and the 5th lumbar vertebrae.Pinsolle et al. meticulously observed the lumbo-iliac venous junction in 130 cadavers, of which 121 had abnormal structures in the lumen of the left common iliac vein. He classified them into five categories: 1. crests: tiny structures that protrude vertically into the lumen in a sagittal triangular shape at the junction of both common iliac veins. 2, Flap: a bird’s nest-like structure at the lateral border of the common iliac vein. 3.Adhesion: the fusion of the anterior and posterior walls of the veins of a certain length and width. 4.Bridge: long strip-like structure divides the lumen into 2 or 3 parts of different caliber and spatial orientation. 5.Bundle: septum-like structure makes a sieve-like porous change in the lumen. The origin and significance of the abnormal structures within the common iliac vein remain debated. The current preference is to explain it as a result of the close contact between the right common iliac artery, the lumbosacral spine and the left common iliac vein, and the repeated stimulation of the vein wall by the arterial pulsation, which causes chronic injury and tissue reaction of the vein. This view is based mainly on the following: 1, the position of this anatomical structure is fairly constant, always at the level of the point adjacent to the right common iliac artery and the left common iliac vein; 2, the presence of dense fibrous tissue between the arteries and veins; 3, the normal intimal and mesenteric tissue of the lumen is replaced by a neat connective tissue covered with a layer of normal endothelial cells, a structure significantly different from the mechanized thrombus. Another view involves congenital factors and suggests that this luminal abnormal structure is histologically significantly different from similar adhesion structures of neoplastic or inflammatory tissues. Second, embryologically, the right common iliac vein derives exclusively from the right sacral major vein; the left common iliac vein derives from the fusion of the bilateral sacral major veins and often forms 2 or more ducts, and the abnormal intraventricular structure derives from incomplete degeneration of these ducts during development. The presence of this tissue structure has been reported in the literature to have a family history tendency. 3, secondary thrombosis On the basis of the presence of iliac vein compression and luminal abnormal structures, iliac-femoral vein thrombosis can occur secondary to the combination of trauma, surgery, childbirth, malignancy or prolonged bed rest, which slows venous return or increases blood coagulation, etc. Johnson et al. suggest that the pill helps explain the prevalence of iliac vein compression syndrome in young women. Once thrombosis has formed, further inflammation and fibrosis occurs in the compressed and adherent segments of the iliac vein, allowing the iliac vein to progress from partial to complete obstruction. Because of the compression and the presence of abnormal structures in the lumen, it is difficult to recanalize the iliac vein after thrombosis, leaving the left common iliac vein in a long-term occlusion that is difficult to cure.