This is probably due to a ureteropelvic junction obstruction which is a partial blockage of the connection of
a renal pelvis with the ureter, the ureteropelvic junction (UPJ). This impedes drainage and causes urine to
back up in the kidney leading to increased pressure, flank pain and, in some cases, kidney damage. Most often,
this blockage is the result of an abnormality of kidney development occurring prior to birth (congenital). The
muscle of the obstructed ureteropelvic junction is poorly developed and, sometimes, replaced with scar tissue.
Both kidneys are involved in 10% of cases. In some, the obstruction may have occurred after an injury or
inflammation related to a kidney stone. Occasionally, a blood vessel crossing over the UPJ can contribute to
blockage. UPJ obstruction may be diagnosed at any age. Routine ultrasound during pregnancy can detect fullness
of the fetal renal pelvis (hydronephrosis). Some infants are found to have UPJ obstruction when abnormal
swelling in the abdomen or flank is discovered or urinary infection develops. In adults, it may be identified
during the investigation of intermittent flank or abdominal pain often made worse by drinking large volumes of
fluid. Some patients develop bloody urine or stones in the affected kidney.