Hydronephrosis is the swelling of a kidney due to a build-up of urine. It happens when urine cannot drain out from the kidney to the bladder from a blockage or obstruction. Hydronephrosis can occur in one or both kidneys.
The main function of the urinary tract is to remove wastes and fluid from the body. The urinary tract has four parts: the kidneys, the ureters, the bladder and urethra. The urine is formed when the kidneys filter blood and remove excess waste materials and fluid. Urine collects into a part of the kidney called the renal pelvis. From the renal pelvis, the urine travels down a narrow tube called the ureter into the bladder. The bladder slowly fills up with urine, which empties from the body through another small tube called the urethra. Hydronephrosis occurs when there is either a blockage of the outflow of urine, or reverse flow of urine already in the bladder (called reflux) that can cause the renal pelvis to become enlarged.
Hydronephrosis may or may not cause symptoms. The main symptom is pain, either in the side and back (known as flank pain), abdomen or groin. Other symptoms can include pain during urination, other problems with urination (increased urge or frequency, incomplete urination, incontinence), nausea and fever. These symptoms depend on the cause and severity of urinary blockage.