Typically, urine passes from the kidney through a tube called a ureter that drains into the bladder, and then out of the body. But, sometimes urine backs up or remains inside the kidney or in the ureter. That's when hydronephrosis can develop.

Some common causes of hydronephrosis include:

• Partial blockage in the urinary tract. Urinary tract blockages often form where the kidney meets the ureter. Less commonly, blockages may occur where the ureter meets the bladder.
• Vesicoureteral reflux. Vesicoureteral reflux happens when urine flows backward through the ureter from the bladder up into the kidney. Typically, urine flows only one way in the ureter. Urine flowing the wrong way makes it difficult for the kidney to empty properly and causes the kidney to swell.

Less-common causes of hydronephrosis include kidney stones, a tumor in the abdomen or pelvis, and problems with nerves that lead to the bladder.

Your health care provider may refer you to a doctor who specializes in conditions affecting the urinary system (urologist) for your diagnosis.

Tests for diagnosing hydronephrosis may include:

• A blood test to evaluate kidney function
• A urine test to check for signs of infection or urinary stones that could cause a blockage
• An ultrasound imaging exam, during which your doctor can view the kidneys, bladder and other urinary structures to identify potential problems
• A specialized X-ray of the urinary tract that uses a special dye to outline the kidneys, ureters, bladder and urethra, capturing images before and during urination

If necessary, your doctor may recommend additional imaging exams, such as a CT scan or MRI. Another possibility is a test called a MAG3 scan that evaluates function and drainage in the kidney.

Treatment for hydronephrosis depends on the underlying cause. Although surgery is sometimes needed, hydronephrosis often resolves on its own.

• Mild to moderate hydronephrosis. Your doctor may opt for a wait-and-see approach to see if you get better on your own. Even so, your doctor may recommend preventive antibiotic therapy to lower the risk of urinary tract infections.
• Severe hydronephrosis. When hydronephrosis makes it hard for the kidney to function — as can happen in more-severe hydronephrosis or in hydronephrosis that involves reflux — surgery may be recommended to fix a blockage or correct reflux.

Left untreated, severe hydronephrosis can lead to permanent kidney damage. Rarely, it can cause kidney failure. But hydronephrosis typically affects only one kidney and the other kidney can do the work for both.