It is important to recognize that there are 2 types of PCS, primary and secondary as their treatments differ.
Primary PCS is caused by the valves in the ovarian veins not functioning. During pregnancy the ovarian veins increase in size to carry the increased blood flow from the uterus. Afterwards, these veins do not recover and the valves no longer function. This allows blood to leak back into the veins in the pelvis. The internal iliac veins may also be similarly affected.
Secondary PCS is caused by venous compression of other veins that results in blood being diverted into the pelvic veins. Iliac vein compression (also known as May Thurner Synydrome) is the most common secondary cause. Nutcracker syndrome (renal vein compression) is a less common cause. When the iliac or renal veins are compressed, the blood that they normally carry travels through the pelvic veins. The pelvic veins then dilate and enlarge to carry this extra blood.
Secondary PCS due to Iliac Vein Compression: The right common iliac artery is compression the left common iliac vein. There is collateral drainage through the pelvis and also through the ascending lumbar vein.
Secondary PCS due to Left Renal Vein Compression: The left renal vein is compressed between the aorta (not outlined) and the superior mesenteric artery. The left kidney drains mainly through the left ovarian vein which then drains into pelvic varicose veins leading to pelvic venous congestion.
When treating patients with PCS it is critical to understand which veins are leaking and if there is compression of other normal veins. This requires someone who has expertise in transvaginal ultrasound, CT venography, catheter venography and intravascular ultrasound.