The left renal vein entrapment syndrome, commonly known as the “nutcracker syndrome” is a rare and probably underdiagnosed condition. The term refers to the compression of the left renal vein between the aorta and the superior mesenteric artery. This results in increased left renal vein pressure and peak velocity, formation of collateral veins and their subsequent rupture into the calyceal fornices and development of hematuria. Patients may also suffer from albuminuria, lumbar/left-flank pain, and men may show varicocele. Women may also experience pelvic varices, chronic pelvic pain, dyspareunia, dysuria or dysmenorrhea. The nutcracker syndrome refers to symptomatic disease, while nutcracker phenomenon simply refers to its radiographic findings in absence of symptoms.
Although other modalities, such as venography and CT angiography are used to diagnose this condition, researchers in the last decade have developed useful criteria for diagnosing nutcracker syndrome with ultrasound. While increased LRV diameter alone is not specific for nutcracker syndrome diagnosis, spectral and doppler ultrasound can provide extra information about flow and velocity. The most useful ultrasound measure of stenosis of the LRV is the ratio of peak systolic flow velocity in the proximal vs hilar portions of the vein. The cutoff for diagnosis is set by various studies at > 3.7-5.0 in children and > 5.0 in adults, allowing near 100% sensitivity and 90% specificity. Venography remains the gold standard for diagnosis of this condition, but CT or MR angiography are the most common follow-up studies after ultrasound diagnosis.
Treatment for LRV entrapment remains controversial. In children with LRV entrapment, one possible recommendation for treatment is the allowance of a gradual increase in BMI that comes with normal development. This theoretically provides increased soft tissue volume to pad the LRV from compression by the SMA. In adults, options include stenting. renocaval reimplantation, SMA transposition, and saphenous vein or synthetic bypass grafting.