Hydronephrosis, a condition that occurs when urine does not drain from a kidney properly, is the primary cause of a dilated renal pelvis. The renal pelvis is the area in the kidney where urine collects before exiting via the ureter, and when the urine gets backed up, it becomes dilated. Infants and adults are the main populations affected by this problem, which is typically the result of a blockage or some other issue that constricts one or both of the ureters. In infants, the cause is usually a congenital defect in the ureters, such as a narrowing of the tubes or a problem with the valves that control urine drainage. For adults, a dilated renal pelvis can result from a number of problems, including pressure from tumors or masses, structural problems within the kidney or ureters, or malfunctions due to disease.
A dilated renal pelvis is fairly common in infants, who may be predisposed to genetic anomalies within their urinary system. They may have one or both ureters that are narrower than normal at the end closest to the kidney, which causes a blockage of the ureteropelvic junction, or UPJ. Another common issue is that the valves that control the flow of urine through the ureters malfunction, causing a reflux of urine back up into the kidneys.
Adults can develop a dilated renal pelvis, though it is less common than in infants, and it can happen for many different reasons. While structural abnormalities can also lead to the condition, it is more apt to occur as a result of other issues in an adult. One common problem is pressure on the ureters, which causes them to narrow and obstruct the UPJ. This may be the result of nearby masses, like tumors or an enlarged prostate, or from swelling in surrounding tissues due to inflammation. Pregnant women frequently develop a dilated renal pelvis, as pressure from the growing fetus can easily put pressure on the nearby ureters and kidneys.
Disease and other medical conditions may also lead to a dilated renal pelvis in adults. The ureters can become blocked by blood clots, kidney stones, or uric acid crystals, obstructing some or all of the urine draining from the kidneys. Diseases that cause inflammation of the urinary tract may be to blame. People who have had surgery on their urinary tract also sometimes have issues afterward with urine drainage from the kidneys.