The nutcracker phenomenon (NCP), also known as left renal vein entrapment refers to a situation of impeded outflow from the left renal vein (LRV) into the inferior vena cava (IVC) as a result of compression and is often accompanied by demonstrable lateral (hilar) dilatation and mesoaortic narrowing. It can be a common incidental finding on routine CT imaging and many patients can be asymptomatic. 

If associated with relevant symptoms, it is then known as the nutcracker syndrome.