Sonographic Hallmarks of Nutcracker Syndrome

Nutcracker syndrome (NCS) occurs when the left renal vein (LRV) is compressed between the aorta and superior mesenteric artery (SMA), which can manifest as hematuria, proteinuria, flank pain, and pelvic venous congestion as a sequela of renovenous hypertension. In the absence of symptoms, LRV compression is referred to as nutcracker anatomy or the nutcracker phenomenon. Though there are no currently formally accepted diagnostic pathways, US, CT, and MRI are frequently used to assess patients with NCS. The US is often the first imaging modality to be used in the workup of NCS. Doppler US is a way to measure blood flow velocity to suggest this diagnosis noninvasively. Two main US findings have been described in patients with NCS: a caliber reduction in the LRV >50% where it crosses the abdominal aorta and elevated peak systolic velocity at the level of obstruction. The anteroposterior diameter of the LRV hilar portion to its aortomesenteric portion of the LRV is compared with a ratio >4 described with NCS. In addition, the peak flow velocity of the LRV at the site of narrowing is compared to the flow velocity proximal to the narrowing, with a ratio greater than 5 described in patients with NCS.