What causes hydronephrosis?

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Hydronephrosis is a condition that puts pressure on the kidneys and can lead to them becoming damaged. Often, something inside or outside of the urinary tract that blocks the urine flow out of the kidneys causes hydronephrosis.

Blockages in the urinary tract that can lead to hydronephrosis include kidney stones or an enlarged prostate. A problem with the muscle where the urethra and bladder connect and that makes urine back up into the kidneys can also cause the condition.

In this article, we look at what hydronephrosis is, along with the main symptoms and causes. We also cover diagnosis, treatment, and complications.


What is hydronephrosis?

Hydronephrosis occurs when urine cannot drain out of the kidneys properly, causing them to swell up or stretch.

Hydronephrosis is a condition affecting one or both of the kidneys. It happens when urine cannot drain out of the kidneys properly, causing them to swell up or stretch.

The bladder, kidneys, and linking tubes are known as the urinary system.

When working correctly, the kidneys filter blood to remove waste products from the body. The kidneys create urine, which carries the waste products down tubes to the bladder. Urine then passes through a tube called the urethra and can be expelled.

Hydronephrosis can develop when there is a problem with the urinary system. It can happen to a person of any age. It usually affects only one kidney but, occasionally, both are involved.


In adults, hydronephrosis may not cause any symptoms, depending on what is the cause. When they do occur, symptoms may include:

  • urinating less often or not as strongly
  • blood in the urine
  • pain in the back, abdomen, or side of the body
  • any symptoms of a urinary tract infection (UTI), such as painful urination, cloudy urine, and a strong urge to urinate
  • fever
  • nausea and vomiting

When hydronephrosis occurs in babies, they mostly have no symptoms. When they do, symptoms may include:

  • multiple UTIs, when the only sign may be an unexplained fever
  • pain in the abdomen or side
  • blood in the urine
  • fever
  • not feeding well
  • lack of energy
  • irritability


What causes hydronephrosis - pregnancy
Hydronephrosis is very common during pregnancy.

One of two main problems causes hydronephrosis.

One of these is called vesicoureteral reflux (VUR). In this condition, the muscular valve where the urethra connects to the bladder does not work correctly. This forces urine to reflux or flow backward into the kidney.

The other problem is an obstruction anywhere in the bladder, kidney, or linking tubes that prevents urine from leaving the kidney. This could be a blockage inside or pressure from something outside the urinary system.

In adults, many things can cause an obstruction, but it is usually due to an underlying medical condition, such as:

  • Pregnancy causing the womb to push against and block the tubes connecting the bladder and kidneys. Hydronephrosis during pregnancy is not unusual.
  • Kidney stones moving out of the kidney where they first formed. If a stone moves into a tube in the urinary system, it can cause a blockage.
  • An enlarged prostate gland wrapping around the urethra between the bladder and penis, which can happen as a person ages. This can compress and obstruct the urethra.
  • Certain cancers affecting the urinary system. These include kidney, prostate, bladder, cervical, or ovarian cancer. If a tumor is pressing against part of the urinary system, it can obstruct the flow of urine from the kidneys.
  • The tubes connecting the bladder and kidneys becoming blocked or narrowed. This may happen because of an injury or infection.
  • Nerves around the bladder being damaged, which can affect how well this organ works.

In babies, the obstruction is usually created when a part of the urinary system develops incorrectly before birth.


Doctors use an ultrasound scan to diagnose hydronephrosis. This type of scan uses sound waves to see the organs inside the body, allowing a doctor to see whether a person’s kidneys are swollen.

Most people will have an ultrasound scan during pregnancy to check the health of the developing fetus. This gives a picture of the fetus and their internal organs. If the kidneys appear swollen, further ultrasound tests will need to be given throughout the pregnancy.

Once the baby is born, ultrasound of the kidneys can be done as in adults.

If the kidneys appear swollen on an ultrasound picture, more tests may be needed. These tests can help to find the underlying cause of hydronephrosis.

Tests can include:

  • urine tests to check for infection or blood
  • blood tests to check for an infection
  • an X-ray of the kidneys to see how urine is moving through the body
  • CT scan to give a 3-D picture of the organs and urinary system

A voiding cystourethrogram is a special X-ray that shows if reflux or obstruction is present. The doctor adds a dye to the urine in the bladder so the path of its flow can be followed on the X-ray. This test is used to diagnose VUR in adults, and babies after birth.



What causes hydronephrosis? - catheter can help
When hydronephrosis is severe, a catheter tube may be used to drain the bladder.

Treatment is based on the underlying cause of hydronephrosis. Doctors will also inform their decisions about treatment on the severity of the condition and symptoms.

When hydronephrosis is severe, the backlog of urine may need to be drained to reduce the pressure and the risk of permanent kidney injury. This is done by inserting a thin tube called a catheter into the bladder or a special tool called a nephrostomy tube into the kidney.

Before treating the underlying cause of hydronephrosis, the doctor may prescribe something to control any pain and antibiotics to treat or prevent infection.

In adults, the underlying problem sometimes fixes itself, and no other treatment is needed.

More often, however, surgery is needed to remove the obstruction or fix the reflux. This can be minimally invasive, using a thin tube with a light and camera and involving only a few small incisions, or it may require larger incisions and instruments.


On some occasions, doctors can treat hydronephrosis with medication.

Hydronephrosis caused by pregnancy usually goes away without treatment once the pregnancy comes to an end.

If hydronephrosis is diagnosed before birth and is not severe, it usually gets better on its own without the need for treatment.

In the first few weeks after birth, a baby may need to have some tests to make sure there are no further problems.

Since babies with hydronephrosis are more likely to develop a UTI, they may be given antibiotics to reduce the risk of developing a urinary infection.

Babies with severe hydronephrosis usually need surgery. In some, the tubes connecting the kidney and bladder may be blocked. Doctors can treat this with a type of surgery known as a pyeloplasty, which is the most common surgical treatment in babies.

A pyeloplasty is done under general anesthetic. The section of the tube that is blocked will be removed, and the two ends joined up.


Untreated hydronephrosis can put a lot of pressure on a person’s kidneys. This can cause permanent kidney damage or even kidney failure. Kidney failure means that the kidneys no longer work.

If both kidneys are affected, a person will need a transplant or dialysis. The latter is a process in which blood is filtered using a machine outside of the body.


If treated early, hydronephrosis will not usually cause any lasting medical problems.

If hydronephrosis in babies is not discovered before birth, it can be harder to diagnose. Being aware of the signs of a UTI in babies can help to spot a possible problem. Early medical advice and treatment should help to ensure a quick recovery.


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