Many parents are confused when the doctors tell them that their child's kidney is "hydronephrotic". This is such a big medical word and doctors will often use other words such as "enlarged" or "dilated" or even "obstructed" to mean the same thing. While these words are less medical, families still don't understand the implications. Like many things in medicine the word "hydronephrosis" can mean many things all dependant on whether the kidney is mildly, moderately, or severely affected. Even when severe, hydronephrosis isn't always a bad thing. Hydronephrosis comes from a combination of Greek words that mean "water in the kidney"; of course the word doesn't refer to water but rather urine.

The kidney filters blood, and through that filtration process produces urine. Once produced, the urine flows out of the kidney into the ureters, which carry the urine to the bladder. Urine is then stored in the bladder for a while before being expelled from the body through the urethra during urination.

Hydronephrosis can be explained by comparing it to a dam in a creek. Imagine a narrow spot in the ureter draining the kidney like a dam in a creek. The dam holds back water in the creek until the water level gets so high that it flows over the dam. Hydronephrosis refers to the dilation of the drainage system (kidneys) above a narrow spot. This "obstruction" is only partial, and urine will continue to flow, even in very severe cases of hydronephrosis.

Occasionally, Pediatric Urologists need to perform surgery to decrease hydronephrosis by cutting out the narrow portion and reconstruct the system to allow better flow. Though many kidneys are hydronephrotic, few need surgical treatment. Because some hydronephrotic kidneys end up with operations, doctors become concerned when the x-ray report suggests that a kidney has hydronephrosis. But parents should remember that most dilations are mild and likely of no long-term consequence. If the tube draining the kidneys (ureters) becomes a bit bigger in diameter, that is like lowering the dam. The pond above the dam becomes smaller (the hydronephrosis improves) and nothing bad happens to the kidney.

Steinhardt, George F., "Ureteropelvic Junction Obstruction." Pediatric Urology Practice. Eds. Gonzales, Edmond T. and Stuart B. Bauer. New York: Lippincott Williams & Wilkins, 1999 pp 181-204.