Hydronephrosis - Causes 

The two most common reasons that hydronephrosis occurs is either when there is a blockage inside the urinary tract or something disrupts the normal workings of the urinary tract, such as the bladder.

Common causes

In men, the two most common causes of hydronephrosis are:

• non-cancerous swelling of the prostate gland (benign prostatic hyperplasia)
• prostate cancer 

Both conditions can place pressure on the ureters, which can block the flow of urine. The ureters are the tubes that run from your kidneys to your bladder.

In women, common causes of hydronephrosis include:

• pregnancy – during pregnancy the enlarged womb (uterus) can sometimes place pressure on the ureters
• cancers that develop inside the urinary tract, such as bladder cancer or kidney cancer
• cancers that develop inside the reproductive system, such as cervical cancer, ovarian cancer or cancer of the womb

Abnormal tissue growth associated with cancer can place pressure on the ureter or disrupt the workings of the bladder.

Kidney stones are another common cause of hydronephrosis in both men and women. Kidney stones are small stones that are made up of various substances that form in the kidneys. They can sometimes travel out of a kidney and into the ureters where they can block the flow of urine.

Other causes

Other less common causes of hydronephrosis include:

• a blood clot (embolism) – which develops inside the urinary tract
• endometriosis – a condition where tissue that should only grow inside the womb starts to grow outside of the womb; this abnormal growth can sometimes disrupt the urinary tract
• tuberculosis – a bacterial infection that usually develops inside the lungs but in some cases can also spread to the bladder
• damage to the nerves that control the bladder (neurogenic bladder)
• ovarian cysts – fluid-filled sacs that develop inside the ovaries; larger ovaries can sometimes place the bladder or ureter under pressure
• narrowing of the ureter as a result of injury, infection or surgery

Antenatal hydronephrosis

Most cases of antenatal hydronephrosis are thought to be caused by the ureters not developing as quickly as they should. When the ureters first develop they are solid cords of tissue that later transform into tubes with a hollow passageway. A delay in this development can cause temporary hydronephrosis.

Other causes of antenatal hydronephrosis include:

• a blockage – at the point where the ureters join the kidneys; the blockage can be due to a birth defect, although in many cases there is no obvious cause
• primary vesicoureteral reflux – this is a childhood condition where the valve that controls the flow of urine between the bladder and the ureter does not function properly, allowing urine to flow back up to the kidneys; most children will grow out of primary vesicoureteral reflux