The renal pelvis is the structure that drains urine from the kidneys. The tube that carries the urine from the renal pelvis into the bladder is called the ureter. The narrow area where these two structures meet is called the ureteropelvic junction (UPJ). A blockage in the UPJ can inhibit the flow of urine.
Blockage of the UPJ can be present from birth. It is a common urinary tract abnormality. Most of the time the problem appears in childhood. In some people, however, symptoms may not show up until after puberty. Obstruction can also result from a kidney stone lodged in the area or from injury.
In between 10 and 40 percent of cases, UPJ obstruction occurs on both sides of the body. Twenty to forty percent of children with this condition have a backflow of urine into the kidney.
UPJ obstruction can be caused by the following:
There is some suspicion that the problem can be inherited. However, this has not been proven conclusively.