Nutcracker syndrome is the left renal vein compression syndrome, also known as nutcracker phenomenon, refers to the left renal vein between the aorta and superior mesenteric artery by mechanical compression, the left renal vein blood flow return obstruction caused by the phenomenon of left renal venous hypertension and genital vein syndrome, manifested as hematuria and/or proteinuria, abdominal pain and other clinical symptoms. 1972 Schepper reported the left renal hemorrhage caused by this syndrome. Since then, there have been several reports in China and abroad, and it has received increasing attention. Pathogenesis】We know that the blood vessels in the human body, like the four-way roads, have a certain direction. The left renal vein runs between the abdominal aorta and the superior mesenteric artery, and these two arteries form an angle of 40-60 degrees, and the left renal vein just passes through this angle. Anatomically, the right renal vein injects directly into the inferior vena cava with a short and straight stroke. In contrast, the left renal vein needs to cross the angle between the ventral aorta and the mesentery and cross the front of the abdominal aorta before it can be injected into the inferior vena cava, so the left renal vein is much longer than the right renal vein. In normal time, the angle between superior mesenteric artery and abdominal aorta is filled by mesentery, fat, lymph nodes and peritoneum, so that the left renal vein is not squeezed. When puberty is faster, rapid growth in height, hyperextension of the spine, rapid change in body shape or renal prolapse, the left renal vein has a bad time in this angle and will be squeezed, causing changes in blood flow and corresponding clinical symptoms. 【Clinical manifestations】 The disease occurs in men from adolescence to about 40 years old, the onset of children is distributed in 4 to 7 years old, the most frequent age is seen in 13 to 16 years old, both male and female can be seen. The main symptoms of Nutcracker phenomenon are hematuria and proteinuria, of which asymptomatic hematuria is more easily detected by the naked eye, and submicroscopic hematuria is found in urinalysis during routine physical examination, which can be aggravated during strenuous exercise or upright position, sometimes accompanied by left abdominal pain or lumbago. Some patients also have hematuria with proteinuria, which can range from trace to 2+ and may show intermittent or postural exacerbation. In addition, varicocele can occur in men and irregular menstrual bleeding can occur in women. Cystoscopy, arterial DSA and arteriovenous angiography are invasive and not easily accepted by children, and CT/MRI is expensive, so abdominal ultrasound should be preferred. 1.Urine red blood cell morphology examination: including phase contrast microscopy and RBC distribution volume. Urinary RBC is normal morphology, i.e. non-renal hematuria. 2.Color multispectral ultrasonography: the distal segment of the left renal vein stenosis is more than twice as wide as the internal diameter of the stenosis in two-dimensional supine position, and the internal diameter of the dilated part is more than 4 times wider than the internal diameter of the stenosis after 15-20 minutes in posterior spinal extension position, the angle between the superior mesenteric artery and abdominal aorta is within 9°, the blood flow velocity of the dilated segment of the left renal vein is less than 0.09m/s, and the blood flow velocity of the stenosis is 5 more than 5 times of the stenosis. 3.Renal venography: It is used to directly observe the compression and dilatation of left renal vein, and also can directly measure the internal pressure of inferior vena cava and left renal vein, when both of them are greater than 0.4kpa, left renal vein hypertension can be considered. However, renal venography can cause false negatives due to the influence of hemodynamic and some technical factors, and it is not easy to observe the collateral circulation. 4.Arterial DSA: It can clearly show the left renal vein compression and dilated left renal vein, and at the same time, it can show the situation of collateral circulation, which is more accurate and comprehensive for the diagnosis of nutcracker phenomenon, and other vascular malformations, tumors and other lesions can be excluded in the performance of left renal arteriogram. 5.CT and MRI: Enhanced scan in the corresponding plane can show the anatomical relationship between the abdominal aorta and superior mesenteric artery and the compressed left renal vein, and sometimes can show the varicose veins of the ovary and spermatic cord. 6. Cystoscopy: blood spray or hematuria at the left ureteral orifice is seen. Treatment and prognosis】 No special treatment, regular follow-up is needed. Generally, with the increase of fat and connective tissue at the angle between superior mesenteric artery and abdominal aorta with age, or the establishment of collateral circulation, the stasis can be improved and the symptoms are relieved. Generally, with normal renal function and normal growth and development, they can participate in general school activities, and those with more pronounced hematuria are exempt from sports training. Some authors consider surgery as an indication when there is massive hematuria and persistent pain. The long-term prognosis of this disease has not been reported. An abdominal ultrasound examination and renal function tests once every 1 to 2 years are necessary. To avoid missing the diagnosis of true glomerular disease, such as IgA nephropathy with predominant hematuria, which is the most common in pediatric patients.