Hydronephrosis

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

See the figure Hydronephrosis: A Distended Kidney.

Normally, urine flows out of the kidneys at extremely low pressure. If the flow of urine is obstructed, urine backs up in 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 kidney function.

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. 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. Causes of this type of obstruction include the following:

Structural abnormalities--for example, when the insertion of the ureter into the renal pelvis is too high
Kinking at this junction resulting from a kidney shifting downward
Stones (calculi) 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 junction of the ureter and renal pelvis or from backflow (reflux) of urine from the bladder. Causes of this type of obstruction include the following:

Stones 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, x-rays, 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 Section 21, Chapter 239)), or rectal impaction with feces
Abnormal contractions of the bladder resulting from a birth defect or an 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. The hydronephrosis 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 produces renal colic--an excruciating, intermittent pain in the flank (the area between the ribs and hip) on the affected side. Partial obstruction may reduce the rate of urine flow. A total stoppage of the flow of urine most often occurs with complete blockage of the ureters from both kidneys or complete blockage of the urethra.

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 shifts downward, causing temporary overfilling of the renal pelvis or temporary blockage of the ureter and producing 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. Urinary tract infections--with pus in the urine, fever, and discomfort in the area of the bladder or kidneys--are fairly common. When the flow of urine is obstructed, stones may form. If both kidneys are obstructed, kidney failure may result.

Diagnosis

Early diagnosis is important, because most cases of obstruction can be corrected and a delay in treatment can lead to irreversible kidney damage. The doctor may suspect hydronephrosis during a physical examination. A distended kidney can sometimes be felt in the flank, particularly if the kidney is greatly enlarged in an infant or a child or a thin adult.

The doctor depends on testing to make the diagnosis. Bladder catheterization (insertion of a hollow, flexible tube through the urethra) is often performed as the first diagnostic test. If the catheter drains a large amount of urine from the bladder, then either the bladder outlet or the urethra is the site of the obstruction.

Ultrasound is a very useful test in most people (particularly children and pregnant women) because it has fewer complications than x-ray studies that use radiocontrast chemicals. Usually it can detect the cause of the obstruction.

Sometimes intravenous urography is used. In this procedure, the kidneys are x-rayed after a radiopaque dye, which can be seen on x-rays, is injected into the bloodstream. X-ray images of the bladder and urethra can be produced after the injected radiopaque dye passes through the kidneys or after this dye is introduced into the urinary tract through the urethra in a procedure called retrograde urography. These tests can provide information about the flow of urine through the kidneys.

An endoscope (a flexible viewing tube) is sometimes used to look at possible sites of obstruction as closely as possible; it can be used to examine the urinary tract.

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.

Treatment and Prognosis

In acute hydronephrosis, urine that has accumulated above the obstruction is drained as soon as possible, usually with a needle inserted through the skin into the kidney. The goals of this urgent drainage are to prevent loss of kidney function or prevent further loss if function is already impaired. The obstruction must also be relieved quickly. The method used depends on the cause, but most obstructions require surgery of some kind. For example, surgery may be needed to remove a stone from the renal pelvis or the ureter.

Complications of acute hydronephrosis, such as urinary tract infections and kidney failure, if present, are treated promptly. The cause of the obstruction that led to acute hydronephrosis is corrected whenever possible.

Urgent treatment of chronic hydronephrosis is usually not required. Chronic hydronephrosis is corrected by draining urine above the obstruction. For example, soft tubes (ureteral stents) may be inserted into the ureter to bypass an obstruction. Complications of ureteral stents can include movement of the tube, infection, irritation, and discomfort.

The cause of the obstruction that led to chronic hydronephrosis is also treated whenever possible. A narrow or abnormal section of a ureter may be surgically removed and the cut ends joined together. Sometimes surgery is needed to remove fibrous tissue from the ureter. If the junction of the ureter and bladder is obstructed, the ureter can be surgically detached, then attached to a different part of the bladder.

If the urethra is obstructed because of an enlarged or cancerous prostate, treatment can include drugs, such as hormone therapy for prostate cancer (see Section 21, Chapter 239), surgery, or enlargement of the urethra with dilators. Other treatments may be needed for stones that block the flow of urine.

Treatment to correct acute hydronephrosis in one or both kidneys is usually successful when the obstruction can be relieved and the kidneys are functioning adequately. The prognosis is less certain for chronic hydronephrosis.