Nutcracker syndrome is the compression of left renal (kidney) vein, usually between abdominal aorta and superior mesenteric artery. The abdominal aorta and superior mesenteric artery look like the two levers of a nutcracker, squeezing renal vein. Nutcracker syndrome is also called left renal vein entrapment syndrome. Blood in urine and flank pain on left side are symptoms of this syndrome.