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Obstructive uropathy is structural or functional hindrance of normal urine flow, sometimes leading to renal dysfunction (obstructive nephropathy). Symptoms, less likely in chronic obstruction, are pain radiating to the T11 to T12 dermatomes, anuria, nocturia, or polyuria. Diagnosis is based on results of bladder catheterization, ultrasonography, CT, cystourethroscopy, cystourethrography, or pyelography, depending on the level of obstruction. Treatment, depending on cause, may require prompt drainage, instrumentation, surgery (eg, endoscopy, lithotripsy), hormonal therapy, or a combination of these modalities.
Each year about 2/1000 people in the US are hospitalized for obstructive uropathy. The condition has a bimodal distribution. In childhood, it is due mainly to congenital anomalies of the urinary tract. Incidence then declines until after age 60, when incidence rises, particularly in men because of the increased incidence of benign prostatic hyperplasia (BPH) and prostate cancer. Overall, obstructive uropathy is responsible for about 4% of end-stage renal disease. Hydronephrosis is found at postmortem examination in 2 to 4% of patients.
Etiology
Many conditions can cause obstructive uropathy, which may be acute or chronic, partial or complete, and unilateral or bilateral (see Table 1: Obstructive Uropathy: Causes of Obstructive Uropathy ). In children, the most common causes are anatomic abnormalities (including urethral valves or stricture and stenosis at the ureterovesical or ureteropelvic junction). In young adults, the most common cause is a calculus. In older adults, the most common causes are BPH or prostate cancer, retroperitoneal or pelvic tumors, and calculi. Obstruction may occur at any level, from the renal tubules (casts, crystals) to the external urethral meatus. Proximal to the obstruction, effects may include increased intraluminal pressure, urinary stasis, UTI, or calculus formation (which may also cause obstruction). Obstruction is much more common in males, but acquired and congenital urethral strictures and meatal stenosis occur in both males and females. In females, urethral obstruction may occur secondary to a tumor or as a result of stricture formation after radiation therapy, surgery, or urologic instrumentation (usually repeated dilation).
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Table 1
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| Causes of Obstructive Uropathy |
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Category
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Examples
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Anatomic abnormalities
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Abnormal anterior or posterior valve (urethra)
Contracture of the vesical neck (bladder)
Diverticulum (urethra)
Injury: Pelvic fracture, straddle injury (urethra)
Meatal stenosis (urethra)
Paraphimosis (urethra)
Phimosis (urethra)
Polyp (ureter)
Stricture (urethra)
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Compression due to extrinsic masses or processes
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Female reproductive system: Pregnancy, uterine prolapse, tumor, abscess, Gartner's duct cyst, tubo-ovarian abscess
GI tract: Crohn's disease (via inflammation or abscess), diverticular abscess, appendiceal abscess, tumor (including pancreatic), abscess, cyst
GU tract: Periurethral abscess, benign prostatic hyperplasia, fibrosed chronic prostatitis, prostate cancer
Blood vessels: Aneurysm, aberrant vessel, retrocaval ureter, puerperal ovarian vein thrombophlebitis
Retroperitoneum: Fibrosis (idiopathic, surgical, drug-induced), TB, sarcoidosis, lymphoma, metastatic tumor, lymphocele, hematoma, pelvic lipomatosis
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Functional abnormalities
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Neurogenic disorders or drugs (bladder)
Ureteropelvic or ureterovesical junction dysfunction, bladder neck dysfunction
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Mechanical obstruction of the lumen of the urinary tract
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Blood clot (renal pelvis or ureter)
Fungus ball (renal pelvis or ureter)
Renal papillae (renal pelvis or ureter)
Uric acid crystals (renal tubule)
Urolithiasis (renal pelvis or ureter)
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Pathophysiology
Pathologic findings consist of dilation of the collecting ducts and distal tubules and chronic tubular atrophy with little glomerular damage. Dilation takes 3 days from the onset of obstructive uropathy to develop; before then, the collecting system is relatively noncompliant and less likely to dilate. Obstructive uropathy without dilation can also occur when fibrosis or a retroperitoneal tumor encases the collecting systems, when obstructive uropathy is mild and renal function is not impaired, and in the presence of an intrarenal pelvis.
Obstructive nephropathy:
Obstructive nephropathy is renal dysfunction (renal insufficiency, renal failure, or tubulointerstitial damage) resulting from urinary tract obstruction. The mechanism involves, among many factors, increased intratubular pressure, local ischemia, and, often, UTI. Obstruction may result in type 1 renal tubular acidosis due to reduced distal hydrogen secretion probably because of a defect in the hydrogen ion transporter. In this case, Na wasting can occur and predispose to ECF volume depletion. If obstruction is bilateral, nephropathy may result in renal insufficiency. Renal insufficiency may rarely occur when obstruction is unilateral because autonomic-mediated vascular or ureteral spasm may affect the functioning kidney.
Symptoms and Signs
Symptoms and signs vary with the site, degree, and rapidity of onset of obstructive uropathy.
Pain is common when obstruction acutely distends the bladder, collecting system (ie, the ureter plus the renal calyces), or renal capsule. Upper ureteral or renal pelvic lesions cause flank pain or tenderness, whereas lower ureteral obstruction causes pain that may radiate to the ipsilateral testis or labia. The distribution of kidney and ureteral pain is usually along T11 to T12. Acute complete ureteral obstruction (eg, an obstructing ureteral calculus) may cause severe pain accompanied by nausea and vomiting. A large fluid load (eg, from beer drinking or osmotic diuresis due to an IV contrast agent) causes dilation and pain if urine production increases to a level greater than the flow rate through the area of obstruction. Pain is typically minimal or absent with partial or slowly developing obstructive uropathy (eg, congenital ureteropelvic junction obstruction, pelvic tumor). Hydronephrosis may occasionally produce a palpable flank mass, particularly in massive hydronephrosis of infancy and childhood.
Urine volume does not diminish in unilateral obstruction unless it occurs in the only functioning kidney. Absolute anuria occurs with complete obstruction at the level of the bladder or urethra. Partial obstruction at that level may cause difficulty voiding or abnormalities of the urine stream. In partial obstruction, urine output is often normal and is rarely increased. Increased urine output with polyuria and nocturia occur if the ensuing nephropathy causes impaired renal concentrating capacity and Na reabsorption. Long-standing nephropathy may also result in hypertension.
Infection complicating obstruction may cause dysuria, pyuria, urinary urgency and frequency, referred kidney and ureteral pain, costovertebral angle tenderness, fever, and, occasionally, septicemia.
Diagnosis
Obstructive uropathy should be considered in patients with any of the following:
The history may suggest symptoms of BPH, prior cancer, or urolithiasis. Because early relief of obstruction usually achieves the best outcome, diagnosis should be as rapid as possible.
Urinalysis and serum chemistries (serum electrolytes, BUN, creatinine) should be obtained. Other tests are done depending on symptoms and suspected level of obstruction. Infection with urinary obstruction requires immediate evaluation and treatment.
In an asymptomatic patient with long-standing obstructive uropathy, urinalysis may be normal or reveal only a few casts, WBCs, or RBCs. In a patient with acute renal failure who has a normal urinalysis, bilateral obstructive nephropathy should be considered.
If serum chemistries indicate renal insufficiency, obstruction is probably bilateral and severe or complete. Other findings in bilateral obstruction with nephropathy may include hyperkalemia. Hyperkalemia may result from type 1 renal tubular acidosis due to decreased hydrogen ion and K secretion by distal segments of the nephron.
Evaluation of suspected urethral obstruction:
If urine output is diminished or if there is a distended bladder or suprapubic pain, bladder catheterization should be done. If catheterization results in a normal flow of urine or if the catheter is difficult to pass, a urethral obstruction (eg, prostatic enlargement, stricture, or valve) is suspected. Patients with such findings should have cystourethroscopy along with voiding cystourethrography.
Voiding cystourethrography shows nearly all bladder neck and urethral obstructions as well as vesicoureteral reflux, adequately displaying the anatomy and the volume of urine left in the bladder after voiding (postvoiding residual volume).
If symptoms of urethral obstruction are absent or if cystourethroscopy and voiding cystourethrography show no obstruction, the site of obstruction is presumed to be at the ureters or proximal to them.
Evaluation of ureteral or more proximal obstruction:
Patients undergo imaging tests to detect the presence and site of obstruction. The choice and sequencing of tests depend on the clinical scenario.
Abdominal ultrasonography is the initial imaging test of choice in most patients without urethral abnormalities because it avoids potential allergic and toxic complications of contrast agents and allows assessment of associated renal parenchymal atrophy. Ultrasonography is aimed at detection of hydronephrosis. However, the false-positive rate is 25% if only minimal criteria (visualization of the collecting systems) are considered in the diagnosis. Also, absence of hydronephrosis (and false-negative results) can occur if obstruction is early (in the first few days) or mild or if retroperitoneal fibrosis or tumor encases the collecting system, preventing dilation of the ureter.
CT is sensitive for diagnosing obstructive nephropathy and is used when obstruction cannot be shown by ultrasonography or by intravenous urography. Unenhanced helical CT is the modality of choice. It is particularly accurate for obstruction due to ureteral calculi. The combination of ultrasonography, plain abdominal x-ray, and, if necessary, CT reveals obstructive uropathy in > 90% of patients, but ultrasonography and CT may not be able to differentiate hydronephrosis from multiple renal or parapelvic cysts.
Duplex Doppler ultrasonography can usually show unilateral obstructive uropathy in the first few days of acute obstruction before the collecting system dilates by detecting an increased resistive index (a reflection of increased renal vascular resistance) in the affected kidney. This modality is less useful in obesity and in bilateral obstruction, which cannot be distinguished from intrinsic renal disease.
IVU (contrast urography, intravenous pyelography [IVP], excretory urography) has been largely superseded by CT and MRI (with or without contrast). However, when CT cannot identify the level of obstructive uropathy and when acute obstructive uropathy is thought to be caused by calculi, sloughed papilla, or a blood clot, IVU or retrograde pyelography may be indicated.
Antegrade or retrograde pyelography is preferred to studies that involve vascular administration of contrast agents in the azotemic patient. Retrograde studies are done through a cystoscope, whereas antegrade studies require placement of a catheter percutaneously into the renal pelvis. Patients with intermittent obstruction should be studied when they are having symptoms; otherwise, the obstruction may be missed.
Radionuclide scans also require some renal function but can detect obstruction without the use of contrast agents. When a kidney is assessed as nonfunctioning, a radionuclide scan can determine perfusion and identify functional renal parenchyma. Because this test cannot detect specific areas of obstruction, it is mainly used in conjunction with diuresis renography to evaluate hydronephrosis without apparent obstruction.
MRI can be used when avoiding ionizing radiation is important (eg, in young children or pregnant women). However, it is not superior in accuracy to ultrasonography or CT.
Evaluation of hydronephrosis without apparent obstruction:
Testing may be necessary to determine whether back or flank pain is caused by obstruction in patients who have hydronephrosis but no obvious obstruction revealed by other imaging tests. Testing may also be done to detect otherwise unrecognized obstruction in patients with incidentally recognized hydronephrosis.
In diuresis renography, a loop diuretic (eg, furosemide 0.5 mg/kg IV) is given before a radionuclide renal scan (or an IVU). The patient must have sufficient renal function to respond to the diuretic. If obstruction is present, the rate of washout of the radionuclide (or contrast agent) from the time the tracer appears in the renal pelvis is reduced to a half-life of > 20 min (normal is < 15 min). If the renogram is negative or equivocal but the patient is symptomatic, a perfusion pressure flow study is done via percutaneous insertion of a catheter into the dilated renal pelvis, followed by fluid perfusion into the pelvis at 10 mL/min. The patient is in a prone position. If obstructive uropathy is present, in spite of the marked increase in urine flow, the rate of washout of the radionuclide during renal scanning is delayed, and there will be further dilation of the collecting system on IVU and elevation of the renal pelvic pressure to > 22 mm Hg during perfusion. A renogram or perfusion study that causes pain similar to the patient's initial complaint is interpreted as positive. If the perfusion study is negative, the pain probably has a nonrenal cause. False-positive and false-negative results are common for both tests.
Prognosis
Most obstruction can be corrected, but a delay in therapy can lead to irreversible renal damage. How long it takes for nephropathy to develop and how reversible nephropathy is vary depending on the underlying pathology, the presence or absence of UTI, and the degree and duration of the obstruction. In general, acute renal failure due to a ureteral calculus is reversible, with adequate return of renal function. With chronic progressive obstructive uropathy, renal dysfunction may be partially or completely irreversible. Prognosis is worse if UTI remains untreated.
Treatment
Treatment consists of eliminating the obstruction by surgery, instrumentation (eg, endoscopy, lithotripsy), or drug therapy (eg, hormonal therapy for prostate cancer). Prompt drainage in hydronephrosis is indicated if renal function is compromised, UTI persists, or pain is uncontrollable or persistent. Lower obstructive uropathy may require catheter or more proximal drainage. Indwelling pigtail ureteral catheters can be placed for acute or long-term drainage in selected patients. Temporary drainage using a percutaneous technique may be needed in severe obstructive uropathy, UTI, or calculi. Intensive treatment for UTI and renal failure is imperative.
In the case of hydronephrosis without evident obstruction, surgery should be considered if the patient has pain and a positive diuretic renogram. However, no therapy is necessary in an asymptomatic patient with a negative diuretic renogram or with a positive diuretic renogram but normal renal function.
Last full review/revision April 2007 by Ralph E. Cutler, MD
Content last modified April 2007
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