Hydronephrosis means “water in the kidney”, and is meant to describe the condition where the drainage system of the kidney is dilated.
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.
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).