Understanding Hydronephrosis: 5 Essential Basics

A quick guide to understanding hydronephrosis

MilitaryHealth, National Museum of Health and Medicine. Flickr.com; Creative Commons 2.0 License


As the name suggests, "hydro" meaning water, and "nephro" meaning kidney, implies a condition where the kidneys or the urinary tract become swollen and enlarged due to too much urine buildup. The condition can affect either one or both kidneys. In the affected patient, it can be seen at any stage of life including in unborn babies in the uterus (This is called antinatal hydronephrosis and is discussed elsewhere).

For most adults though, it is a diagnosis they might receive after an ultrasound or CT scan of the kidneys picks up the finding (unless they are symptomatic, of course). 


To understand hydronephrosis, it is essential to have a bit of understanding about the urinary tract's anatomy (click here for picture).   The term includes the path urine takes from where it is made to its ultimate expulsion from the body.  Therefore, we start with the kidneys where urine production occurs, from where it goes into the ureters (tubes that connect the kidney to the urinary bladder), and this urine is then collected in the urinary bladder.  Once the bladder contracts, urine enters another tube called the urethra which leads urine to the outside.


Since the condition involves too much back up of urine in the kidney, it is understandable that any downstream blockage in the urinary tract can cause it.

 Essentially, what we see is an increase in the volume of the urine drainage system.

However, you don't always need a physical structure obstructing the normal flow of urine.  You could sometimes have a functional obstruction in which case something alters the the normal contraction and relaxation pattern of the bladder.

 This could cause urine to flow back from the bladder towards the kidney rather than to the outside.


Now that we understand what hydronephrosis is and the basic mechanism why it happens, the following brief list of things that could do it will make a little more sense:

  • Kidney stones, when they obstruct the flow of urine (remember you could have nonobstructive stones that will not necessarily do this)
  • Strictures, or narrowing of the urinary tract anywhere from the kidney down to the urethra
  • Any abnormal masses along the urinary tract.  These could include cancers of the kidney, ureter, or the bladder.
  • In men, both prostate cancer and noncancerous enlargement of the prostate, called benign prostatic hyperplasia can do this
  • You don't necessarily need cancers/tumors along the urinary tract.  Cancers outside but in close proximity can also impinge on the urinary tract and restrict the outflow of urine.  An example would include ovarian cancer or cancer of the uterus in women
  • Hydronephrosis is a common finding during pregnancy due to the enlarged uterus putting pressure on the ureters
  • Besides structural obstruction, functional obstruction can occur without a physical mass being present along the urinary tract.  The classic cause is when bladder contraction is disrupted because of damage to the nerves that control it.  This is often seen in people with uncontrolled diabetes


The answer to this question depends on how quickly one develops hydronephrosis.  In situations where hydronephrosis happens very slowly, like from a slow growing cancer, the affected patient may not even have any symptoms at all.  However in situations where a sudden blockage occurs, let's say from an obstructing kidney stone, the symptoms could include severe flank pain, abdominal pain, decrease in urine output, vomiting, blood in the urine, etc.

Fever from a superimposed urinary tract infection is also possible. 


Treatment depends on the cause and the severity of the hydronephrosis.  The details are covered here.

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