The Nutcracker Syndrome is a commonly-used term for a condition better described as aorto-mesenteric compression of the left renal (kidney) vein (LRV) causing pain in the left flank and microscopic amounts of blood in the urine. The volume of blood flow through the kidney is high in order for the kidney to filter the blood. This high volume of blood flow must return to the heart, and anything which partially blocks return of venous blood to the heart will cause the blood to build up high pressures within the veins at the kidney. This higher venous pressure may cause the left flank pain and leakage of blood cells into the urine. These symptoms occur only in a small number of cases of severe compression of the LRV though mild to moderate degrees of aorto-mesenteric compression of the LRV with no symptoms are common.
The renal vein which drains the blood from the left kidney passes several cm from the kidney toward the right side of the upper abdomen to drain blood from the kidney into a much larger vein, the inferior vena cava, as the blood flows back toward the heart. The LRV usually passes through a narrow angle between two arteries, the abdominal aorta and the superior mesenteric artery. Since the arteries are filled with blood under much higher pressure than the pressure in the renal vein, the LRV may be compressed by the arteries. Thus, the renal vein is the “nut” compressed in the angle between two much stronger objects like a simple nutcracker.
Other veins near the kidney such as the adrenal veins or the gonadal (ovarian or testicular) veins serve as alternate pathways for the renal blood flow to pass back to the heart. Sometimes, reversal of blood flow in the gonadal vein will allow blood to pass into the pelvis causing pelvic pain and tenderness or pain with intercourse (dyspareunia) in the female and varicose veins of the testicular vein (varicoceles) near the left testicle. Each of these conditions may also be due to failure of one-way valves in the ovarian or testicular vein unrelated to LRV compression.
Treatment of LRV compression is not recommended except for cases which produce significant symptoms. The traditional treatment is to move (transpose) the LRV to drain into another vein or to place a bypass graft to carry the LRV blood flow to another vein. Placement of a stent into the LRV seems attractive as a minimally-invasive procedure, but the long-term results are not yet known since stenting of the LRV has not been performed in a significant number of patients until recently.