Hydronephrosis means “water in the kidney”, and is meant to describe the condition where the drainage system of the kidney is dilated. The outer portion of the kidney is the “meat “ of the kidney , where blood is continuously filtered to remove excess salts, water, and metabolic by-products. The Urine is then collected in the main funnel called the renal pelvis, and drains down the ureter to the bladder.

Hydronephrosis can occur from a number of reasons, including obstruction, ureteral reflux, or from a non-obstructive, non-refluxing physiologic dilation. Congenital obstruction is not usually a complete blockage, but rather, a narrowing of the ureter, most commonly where the kidney joins the ureter (ureteropelvic junction- UPJ) or where the distal ureter joins the bladder (ureterovesical junction – UVJ). If there were a complete blockage of the ureter, the kidney would stop functioning. As a result of the narrowing, though, the renal collecting system develops back pressure, and it stretches out, causing the appearance of hydronephrosis on imaging studies. If there is a bladder neck obstruction from a urethral valve in a boy, or urethral atresia in a girl, then one would expect to see both ureters and kidneys dilated (bilateral hydroureteronephrosis).

The most common causes of hydronephrosis are varying congenital obstructions, but development of obstruction can occur, by the dropping of a kidney stone, or from a sphincter muscle obstruction due to malfunctioning nerves. Scarring from infection or injury can also cause obstruction. Cancers are extremely rare in children.

Because congenital obstruction is not complete, it is important to determine whether the apparent narrowing would be suspected of causing renal injury; that is, whether it is significant or not. Oftentimes, the dilated appearance remains stable or improves without intervention. The exact cause of a congenital UPJ obstruction is often unknown.