What is hydronephrosis?

Hydronephrosis is a condition, affecting about 1 in 100 babies, where urine overfills or backs up into the kidney, causing the kidney to swell. Infants with hydronephrosis may be diagnosed before (prenatal) or after (postnatal) birth.

In many of the children who are diagnosed prenatally, the condition disappears spontaneously by the time of birth or soon after. In children who have mild or sometimes moderate hydronephrosis, kidney function is commonly unaffected and the condition may resolve over a period of time after delivery.

Hydronephrosis affects the drainage of urine from the urinary system — the kidneys, ureters, bladder, and urethra. When the urinary system is impaired, this can cause the urine to back up and the kidney to swell. Typically, hydronephrosis is caused by either something blocking urine flow or by urine leaking backward through the urinary system (reflux).

Your doctor will describe your child’s hydronephrosis as mild, moderate, or severe — based on how much the kidney is stretched and how much the urinary flow is impaired — and will tell you whether your child’s hydronephrosis affects one kidney (unilateral) or both kidneys (bilateral). More than half of the cases resolve by the time the baby is born or soon after.

What are the symptoms of hydronephrosis?

Most babies with hydronephrosis have no symptoms. Older children may also have no symptoms if they have mild or moderate hydronephrosis, and the condition may disappear on its own.

If your child has moderate to severe hydronephrosis, some symptoms may include:

  • pain in the abdomen
  • pain in the side (flank pain)
  • blood in the urine (hematuria)

A child with hydronephrosis may develop a UTI. Symptoms of a urinary tract infection can include the following:

  • strong urge to use the bathroom
  • painful urination
  • cloudy urine
  • back pain
  • fever
  • vomiting

If your infant has had multiple urinary tract infections (UTIs) with (or without) a fever, it could indicate some kind of obstruction or reflux in the urinary system. However, UTIs can be difficult to spot in infants: In many cases, multiple, unexplained fevers are the only sign.

Older children may have more recognizable symptoms of UTIs, including a strong urge to urinate, painful urination, or cloudy urine. If your child tends to get repeat UTIs, you may want to have him evaluated for possible urinary tract obstruction.

What are the causes of hydronephrosis?

Two types of problems cause hydronephrosis. One is obstruction, where urine is physically prevented from draining out of the kidney. The obstruction, or blockage, can occur at any point in the urinary system from the kidney down to the urethra. The second is reflux, in which urine flows back up into the kidney.

Blockage (obstruction)

  • Ureteropelvic junction (UPJ) obstruction: A blockage at the point where the kidney joins the ureter (the thin tube that carries urine to the bladder). A narrowing at the top of the ureter is usually the cause.
  • Ureterovesical junction (UVJ) obstruction: A blockage at the point where the ureter joins the bladder.
  • Posterior urethral valves (PUV): A congenital condition, found only in boys, in which there are abnormal flaps of tissue in the urethra, causing bladder obstruction. This type of obstruction is also associated with vesicoureteral reflux. See below.
  • Ureterocele: A bulge in the ureter that can obstruct part of the kidney and sometimes the bladder.
  • Vesicoureteral reflux (VUR): A backwash of urine that happens when the muscles at the junction of the ureter and bladder aren’t working properly and allow urine to flow back up toward the kidney with bladder filling or emptying.

Other causes of hydronephrosis

  • Ectopic ureter: A rare condition where a ureter doesn’t connect to the bladder in the normal location.
  • Unknown: In more than half of the children who are prenatally diagnosed with hydronephrosis, the condition resolves itself and the cause is never known.

How we care for hydronephrosis

At Boston Children’s Hospital, we are here to help. Our physicians and nurses are trained in pediatric urology and have extensive experience with hydronephrosis. We are honored to be recognized by U.S. News & World Report as #1 in the nation, and have the largest pediatric urology service in the world.