The term hydronephrosis refers to a dilated kidney collecting system. The collecting system of the kidney is where the urine is collected before it drains into the ureter and then the bladder. Hydronephrosis has many causes, some of which block the outflow or urine anywhere along the urinary tract and others which are not associated with blockage.
One of the most common causes of hydronephrosis is when someone is passing a kidney stone. This occurs when a kidney stone passes into the ureter and causes blockage of urine flow. Usually this will be associated with pain and blood in the urine. Often a CT scan will show a dilated collecting system of the kidney on one side which indicates blockage related to the passing stone.
Hydronephrosis can also be seen on both kidneys when someone can’t urinate. This is seen most commonly when there is a large prostate in older men which prevents urination and is called bladder outlet obstruction. Other causes can be from an obstruction of the urethra (where urine passes from the bladder).
Additional common cause of hydronephrosis is when there is a mass or cancer blocking the ureter or bladder. For example, a cancer of the bladder where the ureter enters may cause an obstruction and hydronephrosis of the kidney. A mass of the ovary can press on the ureter and cause obstruction. Any mass that presses on the ureter can cause obstruction and hydronephrosis.
Other causes can include a narrowing or mass where the kidney meets the urethra called a UPJ obstruction. A portion of the kidney collecting system can protrude outside the kidney (extrarenal pelvis) and look like hydronephrosis.
There are mimickers of hydronephrosis as well. For example, a patient can have cysts along the collecting system called parapelvic cysts which can look like hydronephrosis. When the bladder is full, this can cause a backup and appearance of hydronephrosis. Sometimes there is hydronephrosis after a blockage has been relieved which never goes away.
The big concern with a blocked kidney that if untreated is loss of kidney function. Sometimes your doctor will order a nuclear medicine kidney scan to distinguish between a blockage of the kidney and one that simply looks like one. A CT scan will often show a cause of the blockage. Treatment of the blocked kidney depends on the cause. Sometimes a tube or stent will be placed in the ureter so that urine can flow despite a blockage. Often a urology doctor will be involved in the management of hydronephrosis.