Nutcracker phenomenon, also known as left renal vein compression syndrome, was first reported by Schepper in 1972 and gradually attracted attention; according to statistics, nutcracker phenomenon is mostly seen in children and adolescents, the age of onset is 4-20 years old, it is more common in males, the male to female ratio is about 24:5, the most frequent age is seen in 13-16 years old, it is one of the common causes of non-renal hematuria in children, the left renal vein The left renal vein travels at the angle between the abdominal aorta and the superior mesenteric artery, and the left renal vein just passes through this angle. The normal anatomical relationship of the left renal vein The inferior vena cava is located to the right of the abdominal aorta, and the right renal vein injects directly into the inferior vena cava with a short, straight stroke. The left renal vein, on the other hand, passes through the angle between the aorta and the superior mesenteric artery, crosses the anterior aspect of the abdominal aorta, and then injects into the inferior vena cava, making the left renal vein much longer than the right renal vein. In normal conditions: The angle between the superior mesenteric artery and the abdominal aorta is filled with mesentery, fat, lymph nodes and peritoneum so that the left renal vein is not compressed. Under normal conditions, the superior mesenteric artery begins at the abdominal aorta and forms an angle of 25° to 60° with it. If factors that narrow this angle exist or occur, the left renal vein and duodenum can be compressed and lead to left renal vein compression syndrome. Clinical manifestations: When there is rapid pubertal growth, rapid height growth, hyperextension of the spine, drastic changes in body shape or renal prolapse, the left renal vein will have a bad time in this angle and will be compressed, causing changes in blood flow and corresponding clinical symptoms. The hematuria or proteinuria produced is usually upright, that is, hematuria or proteinuria appears when the body is upright and disappears in the horizontal position, mostly seen with the more lean and tall adolescents, and rare in those over 30 years old. Hematuria or proteinuria tends to appear after strenuous exercise and in the evening, and can be recurrent. It has the characteristics of non-glomerular-derived hematuria, but a few patients can show glomerular-derived hematuria. The hematuria or proteinuria is usually mild, but in rare cases, carnal hematuria is present, and hematuria and proteinuria do not usually occur together. Clinical diagnosis: The diagnosis can be made by the following two tests: 1. 90% or more of urine red blood cell morphology is orthomorphic; that is, non-renal hematuria; 2. Ultrasound examination shows compression of the left renal vein. The diagnostic criteria are: the proximal internal diameter of the dilated site before the left renal vein stenosis in the supine position is more than 3 times wider than the internal diameter of the stenosis; after 15-20 minutes in the posterior spinal extension position, the internal diameter of the dilated site is more than 4 times wider than the internal diameter of the stenosis, and the diagnosis can be made by taking two positions. These two tests are simple, easy to perform and effective. Prognosis: Some domestic scholars believe that the nutcracker sign in children is a temporary phenomenon in adolescence and has a good prognosis. As children grow older, effective collateral circulation is established, bruising is improved, and adipose tissue around the beginning of the superior mesenteric artery is increased, all of which relieves the local compression of the renal veins, and hematuria disappears without special treatment. However, since some triggers such as strenuous exercise and cold can trigger hematuria or cause recurrent episodes of hematuria, these triggers should be avoided so that the child can pass through puberty smoothly.