Hydronephrosis is distention (dilation) of the kidney with urine, caused by backward pressure on the kidney when the flow of urine is obstructed.

• Kidney stones (see Stones in the Urinary Tract) are common causes of urinary tract obstruction.
• When hydronephrosis occurs quickly, people may have excruciating pain, most often in the flank (the area between the ribs and the hips).
• When hydronephrosis occurs more gradually, people may have no symptoms or experience attacks of dull, aching discomfort in the flank.
• Doctors initially use bladder catheterization (or ultrasonography) to detect hydronephrosis, and they may use ultrasonography or another imaging test to determine the site of the blockage.
• Treatment depends on the cause of the obstruction.

Hydronephrosis: A Distended Kidney In hydronephrosis, the kidney is distended because the flow of urine is obstructed and urine backs up in the kidney's small tubes and central collecting area (renal pelvis).

Normally, urine flows out of the kidneys at extremely low pressure. If the flow of urine is obstructed, urine backs up behind the point of blockage, eventually reaching the small tubes of the kidney and its collecting area (renal pelvis), distending the kidney and increasing the pressure on its internal structures. The elevated pressure from obstruction may ultimately damage the kidney and can result in loss of its function. When the flow of urine is obstructed, urinary tract infections are fairly common and stones are more likely to form. If both kidneys are obstructed, kidney failure may result.

Long-standing distention of the renal pelvis and ureter can also inhibit the rhythmic muscular contractions that normally move urine down the ureter from the kidney to the bladder (peristalsis). Scar tissue may then replace the normal muscular tissue in the walls of the ureter, resulting in permanent damage.

Causes

Hydronephrosis commonly results from an obstruction located at the junction of the ureter and renal pelvis (ureteropelvic junction). Causes of this type of obstruction include the following:

• Structural abnormalities—for example, a birth defect in which the insertion of the ureter into the renal pelvis is too high or there is inadequate development of the ureteral muscles (congenital ureteropelvic junction obstruction)
• Kinking at the ureteropelvic junction resulting from a kidney shifting downward (ptosis of the kidney)
• Stones (calculi) or a blood clot in the renal pelvis
• Compression of the ureter by bands of fibrous tissue, an abnormally located artery or vein, or a tumor

Hydronephrosis can also result from an obstruction below the ureteropelvic junction or from backflow (reflux) of urine from the bladder. Causes of this type of obstruction include the following:

• Stones in the ureter
• Blood clot in the ureter
• Tumors in or near the ureter
• Narrowing of the ureter resulting from a birth defect, an injury, an infection, radiation therapy, or surgery
• Disorders of the muscles or nerves in the ureter or bladder
• Formation of fibrous tissue in or around the ureter resulting from surgery, radiation therapy, or drugs (especially methysergide)
• Bulging of the lower end of the ureter into the bladder (ureterocele)
• Cancers of the bladder, cervix, uterus, prostate, or other pelvic organs
• Obstruction that prevents urine flow from the bladder to the urethra, resulting from prostate enlargement (most often caused by a condition called benign prostatic hyperplasia—see Prostate Disorders: Benign Prostatic Hyperplasia (BPH)) or rectal impaction with feces
• Abnormal contractions of the bladder resulting from a birth defect or a spinal cord or nerve injury

Hydronephrosis of both kidneys can occur during pregnancy as the enlarging uterus compresses the ureters. Hormonal changes during pregnancy may aggravate the problem by reducing the muscular contractions that normally move urine down the ureters. This condition, commonly called hydronephrosis of pregnancy, usually ends when the pregnancy ends, although the renal pelvis and ureters may remain somewhat distended afterward.

Symptoms

Symptoms depend on the cause, location, and duration of the obstruction. When the obstruction begins quickly (acute hydronephrosis), it usually causes renal colic—an excruciating, intermittent pain in the flank (the area between the ribs and hip) on the affected side. Obstruction on one side does not reduce urine flow. Obstruction can stop or reduce urine flow if blockage affects the ureters from both kidneys or if it affects the urethra. Obstruction of the urethra or bladder outlet may cause pain, pressure, and distention of the bladder.

People who have slowly progressive (chronic) hydronephrosis may have no symptoms, or they may have attacks of dull, aching discomfort in the flank on the affected side. Sometimes a kidney stone temporarily blocks the ureter and causes painful hydronephrosis that occurs intermittently.

Hydronephrosis may cause vague intestinal symptoms, such as nausea, vomiting, and abdominal pain. These symptoms sometimes occur in children when hydronephrosis results from a birth defect in which the junction of the ureter and renal pelvis is too narrow (ureteropelvic junction obstruction).

People who have urinary tract infections may have pus in the urine, fever, and discomfort in the area of the bladder or kidneys.

Diagnosis

Early diagnosis is important, because most cases of obstruction can be corrected and because a delay in treatment can lead to irreversible kidney damage. Doctors may suspect hydronephrosis because of a person's symptoms and sometimes because of findings discovered during a physical examination. A distended kidney can occasionally be felt in the flank, particularly if the kidney is greatly enlarged in an infant or a child or a thin adult. A distended bladder can sometimes be felt in the lower part of the abdomen just above the pubic bone.

Doctors depend on testing to make the diagnosis. Bladder catheterization (insertion of a hollow, flexible tube through the urethra) is often the first diagnostic test done in people with renal colic, pelvic pressure, or distention. If the catheter drains a large amount of urine from the bladder, then either the bladder outlet or the urethra is obstructed. Many doctors do ultrasonography to determine whether the bladder is filled with a large amount of urine before doing bladder catheterization.

If the presence or site of obstruction is in doubt, various imaging tests can be done to identify evidence of obstruction such as hydronephrosis or a site of blockage. For example, ultrasonography is a very useful test in most people (particularly children and pregnant women) because it is fairly accurate and does not expose the person to any radiation. Computed tomography (CT) scanning is an alternative. It is rapid and highly accurate, particularly at identifying stones. Other imaging tests, such as intravenous urography, may be performed to identify the site of obstruction, if it is not visible with ultrasonography or CT.

An endoscope (a rigid or flexible telescope) is sometimes used to look at possible sites of obstruction as closely as possible. An endoscope can be used to examine the urinary tract.

Blood and urine tests are done. Blood test results are usually normal, but tests may reveal high levels of urea nitrogen (sometimes called BUN), creatinine, or both if obstruction affects both kidneys. Results from an analysis of urine (urinalysis) are usually normal, but white blood cells and red blood cells may be present when a stone or a cancer is the cause of obstruction or when the obstruction is complicated by an infection.

Prognosis

Permanent kidney damage is unlikely to result unless both kidneys are obstructed for at least a few weeks. The prognosis is less certain for chronic hydronephrosis.

Treatment

Treatment usually aims to relieve the cause of obstruction. For example, if the urethra is obstructed because of an enlarged or cancerous prostate, treatment can include drugs, such as hormone therapy for prostate cancer (see Prostate Disorders: Prostate Cancer), surgery, or enlargement of the urethra with dilators. Other treatments, such as lithotripsy or endoscopic surgery, may be needed for stones that block the flow of urine. If the cause of obstruction cannot be rapidly corrected, particularly if there is infection, kidney failure, or severe pain, the urinary tract is drained. In acute hydronephrosis, urine that has accumulated above the obstruction can be drained with a soft tube inserted through the skin into the kidney (nephrostomy tube) or by insertion of a soft plastic tube that connects the bladder with the kidney (ureteral stent). Complications of nephrostomy tubes or ureteral stents can include displacement of the tube, infection, and discomfort.

Urgent relief of chronic hydronephrosis is usually not required. Complications of hydronephrosis, such as urinary tract infections and kidney failure, if present, are treated promptly.

Last full review/revision August 2007 by Glenn M. Preminger, MD