With the wide application and improvement of our prenatal ultrasound diagnosis technology, most children with duplicated kidney malformation are diagnosed prenatally, and many parents become anxious about whether to keep or abort their children with duplicated kidney, and whether they must be treated after birth. And how to treat it? When facing the disease, parents seem to be helpless and confused, so I would like to give a brief introduction of Duplex kidney in children so that we can have a more comprehensive understanding of it. What is Duplex Kidney? Duplex kidney malformation is a common congenital malformation of the urinary tract, often accompanied by duplicate ureteral malformation. In duplicated kidney malformation, the upper half of the kidney and the lower half of the kidney are mostly fused together, and each of the upper and lower halves has a separate renal pelvis, ureter and blood vessels. The incidence is about 1/125, mostly in children, and the ratio of male to female is about 1:2. The incidence of left side is slightly more than the right side, and the bilateral incidence accounts for about 20% of all duplicated renal malformations. Repetitive renal malformation can be divided into complete repetitive renal malformation and incomplete repetitive renal malformation (i.e. “Y” type repetitive kidney), and the incidence of the latter is three times that of the former. A complete duplicated kidney malformation is one in which the normal ureter and the abnormal ureter open separately in the bladder or elsewhere, and the ureteral opening in the upper half of the kidney is generally lower than that in the lower half (Weigert-Meyer’s law). Incomplete duplicated kidney malformations are those in which the normal ureter and the abnormal ureter converge and open together in the bladder (“Y” shape). Pathogenesis of duplicated kidney In the fourth week of embryonic life, a ureteral bud emanates from the lower end of the mesonephric duct, which is the primitive tissue that forms the ureter, renal pelvis and calyces. If two ureteral buds or one ureteral bud branches prematurely from the middle renal duct, complete and incomplete duplicated renal malformations are formed, respectively. The two ureters of a duplicated renal malformation may each open at a normal or abnormal site, or they may converge at different sites to form a “Y” shaped ureter. The ectopic opening of the ureter is usually found in the upper half of the kidney in duplicated renal malformation, and the ectopic opening of the ureter is usually located below the normal opening, mostly in the posterior urethra, seminal vesicles and ejaculatory ducts in male children, and in the urethra, vagina or vestibule in female children. Therefore, the common pathological changes of duplicated kidney are ectopic openings or cysts in the upper ureter, lower ureteral reflux, or stenosis of the pyeloureteral junction (UPJO) in the lower kidney, while stenosis of the pyeloureteral junction (UPJO) in the upper kidney is very rare. Clinical symptoms of duplicated kidney: Most children with duplicated kidney malformation have no clinical symptoms. Common clinical symptoms include urinary tract infection, lumbar pain, hydronephrosis and intermittent dribbling of urine (urinary incontinence). In males, the ectopic openings of the ureter are usually located in the posterior urethra, seminal vesicles and ejaculatory ducts, so there are usually no symptoms of urinary incontinence, and they are often seen for urinary tract infections. In children with combined ureteral cysts, the opening of the ureteral cyst may be located in the bladder or urethra, which may cause urinary obstruction and difficulty in urination, resulting in hydronephrosis and impaired function of the upper half of the kidney. Common imaging tests for the diagnosis of duplicated kidney: ①Ultrasound: it is the most common test, which can initially reflect the size, shape and presence of hydronephrosis and ureteral dilatation of duplicated kidney malformation, and is the preferred method of diagnosis. ②Isotope: It is the most sensitive technical gold standard for evaluating kidney function, which can better reflect the function of bilateral kidneys as well as upper and lower half kidneys, and provide strong evidence for clinical treatment whether to preserve or cut the kidney. (iii) Magnetic resonance urogram (MRU): Its multidimensional scan and reconstruction can visually reflect hydronephrosis and the degree of ureteral dilatation, especially in the diagnosis of children with duplicated renal malformations with ectopic ureteral openings and ureteral cysts, due to other imaging examinations. ④Intravenous urography (IVU): it can reflect renal function and partially show both renal pelvises and ureters. It is generally suitable for older children, and for small infants IVU is poorly visualized due to low GFR. ⑤ CT examination: better sensitivity than ultrasound and IVU imaging, can clearly show both kidneys and ureters in duplicated renal malformation, but we do not use it as a routine examination because of the effect of CT radiation. (6) Excretory cystourethrography (VCUG): In children with duplicated renal malformation, VCUG is needed to understand the presence or absence of ureteral reflux and cyst size, and to provide advice on the choice of treatment plan. Treatment principles of duplicated kidney: If there are no clinical symptoms and both kidneys are functioning well, no treatment is needed for duplicated kidney malformation. If a child with duplicated kidney malformation has clinical symptoms, progressive increase in hydronephrosis or significantly impaired kidney function, surgery is required. Treatment methods for duplicated kidney: ① Duplicated hemianephrectomy and ureterectomy: Applicable to those with non-functioning duplicated hemianephrosis or those with functional hemianephrosis but with severely dilated renal pelvis and ureter. (ii) Repeat upper and lower ureteral end-lateral anastomosis: for older children, isotope shows that the kidney is functional, it is worth preserving the hemianephrosis tissue, and MRU shows that the ureter is not severely dilated and distorted. Intraoperatively, the diseased ureter is sutured to the normal ureter so that it forms an artificially incomplete duplicate kidney (Y-shaped). (iii) Single duplicate ureteral bladder reimplantation: for children with isotopes showing that the kidney is functional and it is worthwhile to preserve half of the kidney tissue, and it is impossible to perform a duplicate ureteral end-lateral anastomosis. ④ Transcystoscopic ureteral cystotomy: for ectopic ureteral cysts of the duplicated kidney leading to bladder outlet obstruction, difficulty in urination and severe infection and sepsis in the duplicated upper kidney. ⑤ Repeat ureteral end-lateral anastomosis + lower ureteral bladder reimplantation: for children with lesions in both the repeat upper and lower ureter, i.e., ectopic opening or cyst in the upper ureter and reflux in the lower ureter.