29 - Obstructive Uropathy

Authors: Macfarlane, Michael T.

Title: Urology, 4th Edition

Copyright 2006 Lippincott Williams & Wilkins

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Chapter 29

Obstructive Uropathy

Obstruction to urine flow can occur anywhere in the urinary tract. Proximal to the obstruction, pressures within the collecting system and renal tubules will increase. Ultimately renal injury will result because of cellular atrophy and necrosis if the obstruction to urine flow is not relieved. Acute obstruction will produce distention of the bladder, ureter, or renal pelvis that is generally associated with pain. However, a slowly progressing obstruction can result in massive dilatation of the collecting system with no clinical symptoms. Recovery of some renal function can generally be expected in cases of complete unilateral ureteral obstruction if flow is restored within 6 weeks.

Causes of Obstruction

The most frequent causes of obstruction in adult men are stones, benign prostatic hyperplasia, prostate cancer, ureteropelvic junction (UPJ) obstruction, and ureteral strictures, whereas in women, the most frequent causes are pregnancy, stones, UPJ obstruction, pelvic malignancies, and surgical trauma to the ureters. In children, UPJ obstruction, ureterovesical junction obstruction (e.g., ectopic ureters), ureteroceles, urethral valves, and stones are most frequently responsible for urinary obstruction.

Causes of obstruction can be more systematically evaluated by location.

Bladder Outlet Obstruction

Mechanical	Functional
Benign prostatic hyperplasia	Neurogenic (detrusor-sphincter dyssynergia)
Urethral stricture	Pharmacologic
Bladder neck contracture
Posterior urethral valves
Ureteroceles
Urethral stone
Foreign body

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Ureteral Obstruction

Intrinsic Obstruction	Extrinsic Obstruction
Stones	Pregnancy
Tumors	Tumor
Strictures	Retroperitoneal fibrosis
(tuberculosis, radiation, previous instrumentation)	Crossing blood vessel to lower pole of kidney (UPJ)
Adynamic segment at UPJ	Retrocaval ureter
	Pelvic lipomatosis
	Iatrogenic ligature
UPJ, ureteropelvic junction.

Diagnosis

The diagnosis of lower urinary tract obstruction must always be the first consideration in the patient who presents with oligoanuria. The simple passage of a urethral catheter or percutaneous suprapubic catheter is both diagnostic and therapeutic. Upper urinary tract obstruction will require more specialized diagnostic techniques.

Renal Ultrasound

Renal ultrasound is a quick, noninvasive method of identifying upper-tract obstruction and can be used in the setting of an elevated creatinine level. Occasionally upper-tract obstruction may not cause significant hydronephrosis and may result in a false negative renal ultrasound.

Excretory Urography

An intravenous urogram is the method of choice for demonstrating upper-tract obstruction in the patient with a normal serum creatinine level. It gives anatomic insight into the location and etiology of the obstruction. Obstruction is evident by a prolonged nephrogram, delayed calyceal filling, and dilatation of the collecting system proximal to the site of obstruction. With severe obstruction, no function may be seen; however, it is important to obtain delayed films for at least several hours to look for delayed excretion.

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Retrograde and Anterograde Pyelography

The retrograde injection of contrast up the ureter during cystoscopy is the gold standard for ruling out ureteral obstruction. It can define the lowermost extent of any ureteral obstruction and can be used in the setting of renal failure or contrast hypersensitivity. Anterograde injection of contrast via a percutaneous nephrostomy also is an option under these circumstances.

Nuclear Renograms

Mercaptoacetylglycine (MAG3) or diethylenetriaminepentaacetate (DTPA) renal scans are useful in the evaluation of the patient with suspected obstruction. They can be used even in the setting of contrast hypersensitivity or mildly impaired renal function. MAG3 or hippuran scans are more useful in the setting of moderate renal failure. With the addition of a diuretic such as furosemide (Lasix), renal scans can usually differentiate partial obstructions and dilated, but unobstructed, systems (e.g., megacalyces or megaureter).

Pressure/Flow Perfusion: The Whitaker Test

Normal pressure differential between the renal pelvis and bladder during a constant-flow infusion of the renal pelvis at 10 mL/min should be less than15 cm H₂O. Pressures of more than 22 cm H₂O suggest obstruction.

Management

Relief of urinary tract obstruction should be obtained expeditiously. Prospects for recovery of function are directly related to degree and duration of the obstruction. Temporary catheter drainage of the upper or lower urinary tract should be provided until definitive repair can be performed. The kidney's potential for recovery after long-standing obstruction also can be determined before undertaking a major surgical procedure by providing temporary catheter drainage and monitoring creatinine clearance.

Postobstructive Diuresis

After the relief of long-standing urinary tract obstruction with elevated blood urea nitrogen (BUN) and creatinine, a physiologic

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diuresis can be expected. This is usually a self-limiting nephropathy, which can be managed with proper fluid and electrolyte replacement. Patients with very high BUN and creatinine levels or urine output greater than 200 mL/hour for 2 consecutive hours are at increased risk of a pathologic diuresis, which will require monitoring and management. Occasionally, it can lead to severe hypovolemia and even shock. Three different physiologic types of diuresis are described.

Types of Diuresis

Urea Diuresis

Urea diuresis is most common and occurs secondary to the osmotic load of the urea retained during obstruction. The excretion of excess urea and water after relief of obstruction is self-limiting, lasting only 24 to 48 hours, and requires little attention.

Salt Diuresis

Salt diuresis is the second most common variety. The excess total body salt and water that is retained during obstruction is generally excreted in a self-limiting fashion until a normovolemic state is reached. However, a salt-and-water diuresis may occasionally continue in a pathologic manner well after normal fluid balance has been established. It is therefore important to monitor and replace fluid and electrolytes carefully during a salt diuresis to avoid severe dehydration and salt depletion.

Water Diuresis

Water diuresis occurs as a self-limited nephrogenic diabetes insipidus in the absence of an expanded total body water. This is believed to occur rarely.

Diagnosis

Measure urine sodium, potassium, and osmolarity. The osmolarity should approximate twice the urine sodium and urine potassium (in mEq/L) plus the urea.

Urea Diuresis 2 (U_Na + U_K) = <(1 ) Osmolarity

Salt Diuresis 2 (U_Na + U_K) = >(1 ) Osmolarity

Treatment

A urea diuresis is generally self-limiting and will cease when the BUN returns to normal. Monitoring output and electrolytes is usually sufficient. A salt diuresis, conversely, will require more aggressive management because of the potential for prolonged

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duration of more than 72 hours. Careful monitoring of input and output, serum and urine electrolytes, and central venous pressures is appropriate. If the diuresis persists after the patient's edema fluid has been excreted, then replacement therapy should be started. Intravenous replacement should approximate the electrolyte concentration of the urine, giving 0.5 mL for each milliliter of urine on an hourly basis. Overzealous fluid replacement can result in perpetuation of a self-limiting salt diuresis.

Idiopathic Causes of Obstruction

Retroperitoneal Fibrosis

Retroperitoneal fibrosis is a chronic inflammatory process of unknown etiology in most cases. It produces a fibrotic retroperitoneal mass that is typically situated in the region between the renal pedicles and the sacral promontory. As the process progresses, it easily compresses the ureters as it drags them medially, causing obstructive uropathy and hydroureteronephrosis above the level of the sacral promontory. It affects both male and female patients in a 2:1 ratio, usually in the fifth and sixth decades. Its etiology is unknown in the vast majority of cases; however, some conditions associated with its occurrence include the use of certain drugs (methysergide, methyldopa, hydralazine, -blockers, etc.), chemicals (Avitene, talcum powder), inflammatory processes [perianeurysmal inflammation, chronic inflammatory bowel disease, gonococcal infection (GC), tuberculosis, syphilis], radiation injury, and malignancies.

Diagnosis

Pain is the most frequent presenting complaint; it is typically a dull ache in the lower back, flank, or abdomen that radiates to both lower quadrants, the umbilicus, or testicles. The excretory intravenous urogram will demonstrate medial deviation of the middle third of the ureter with proximal hydroureteronephrosis secondary to extrinsic ureteral compression. Computed tomography scan may show a dense retroperitoneal mass anterior to the great vessels. Histologic diagnosis by biopsy must be performed to rule out malignancy.

Treatment

Initial management entails decompression of the upper urinary tract with indwelling ureteral stents or percutaneous nephrostomy

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tubes. Definitive management will require abdominal exploration for a diagnostic biopsy and ureteral lysis and lateralization or intraperitonealization of the ureters. A course of corticosteroid therapy (prednisone) has been advocated by some to retard progression of the disease and to prevent its recurrence.

Pelvic Lipomatosis

Pelvic lipomatosis is a rare proliferative process of unknown etiology that involves the mature fat of the pelvic retroperitoneum. It occurs primarily in overweight men (predominantly black) in the third to sixth decades and is usually discovered incidentally, with no presenting symptoms. The diagnosis is made by the typical radiolucency of lipomatous tissue surrounding the bladder on kidney, ureter, and bladder (KUB) films. The intravenous urogram usually demonstrates normal upper tract and vertical elongation of the bladder with a pear or teardrop shape. On occasion, ureteral obstruction with hydroureteronephrosis has been noted. A computed tomography scan is ideal for the diagnosis. Treatment is generally conservative and consists of diet control and massive weight reduction.