116 Urinary Tract Obstruction

A patent urinary tract is necessary for optimal kidney function. Under normal circumstances, urine passes unimpeded from the renal pelvises to the tip of the urethra. Obstruction can occur anywhere along this pathway and may lead to both acute and progressive kidney parenchymal damage.

Several definitions may be encountered when considering urinary tract obstruction:

• Obstructive uropathy refers to disorders that interfere with drainage of the urine. Obstructive uropathy can result from pathology within the urinary tract itself (intrinsic obstruction) or from pathology originating outside the urinary tract that causes external compression of the system (extrinsic obstruction). It may be acute or chronic and either partial or complete; the resulting symptom complex typically depends on both the acuity and severity.

• Obstructive nephropathy refers to cases in which obstructive uropathy causes a decline in renal function.

• Hydronephrosis refers to dilatation of the urinary collecting system, with renal parenchymal changes. Typically, however, the term is used to describe any dilatation of the urinary tract, regardless of renal parenchymal involvement. Hydronephrosis is usually, but not exclusively, seen in obstructive disorders. Nonobstructive pathogenesis of hydronephrosis includes vesicoureteral reflux or excessive flow through the collection system, such as with habitual water drinking or diabetes insipidus.

Epidemiology

Urinary tract obstruction is a common disorder. On autopsy, 3.1% of adults have hydronephrosis.¹ Data from the Healthcare Cost and Utilization Project’s National Inpatient Sample (based on ICD-9 codes) indicate that 1.75% of all hospital discharges are complicated by either hydronephrosis or obstruction.² When hydronephrosis is excluded, urinary tract obstruction occurs in approximately 1% of hospital discharges.² Urinary tract obstruction accounts for approximately 10% of community-acquired acute kidney failure³^–⁵ and is a factor in 2.6% of acute kidney failure cases in the intensive care setting.⁶

Etiology

Many disorders may lead to urinary tract obstruction. A useful classification is to first divide causes by the level of obstruction: upper (from the renal pelvis to the ureterovesicular junction) or lower (from the bladder to the urethra) urinary tract. This approach may then be refined into intrinsic versus extrinsic causes.

Congential Causes

Congenital anomalies may result in obstruction at various levels of the urinary tract. This discussion will be limited to congenital ureteropelvic junction obstruction (UPJO), as this is the congenital disorder most likely to present in adulthood.

Congenital UPJO is usually due to disease intrinsic to the urinary tract. Often an adynamic segment of ureter results in failure of peristalsis at the ureteropelvic junction (UPJ).⁷ Ureteral kinks or valves are another intrinsic cause of UPJO. Potential extrinsic causes of UPJO include abnormal rotation of the kidney during development, leading to ureteral compression and entrapment of the ureter by blood vessels, although significant controversy exists regarding the latter.^7,⁸

Patients with UPJO often present with intermittent abdominal or flank pain, sometimes accompanied by nausea and vomiting. Alternatively, patients may present with hematuria or azotemia, or the condition may be uncovered after an imaging study done for an unrelated problem.

The widespread use of maternal prenatal ultrasound has lead to more antenatal diagnosis of UPJO. The diagnosis may be made by ultrasound, intravenous urography, or in equivocal cases, isotope renography (see later imaging section).

Acquired Causes

There are many causes of acquired obstructive uropathy which may affect the urinary tract at any location from the renal tubules to the tip of the urethra. This discussion will consider upper and lower urinary tract obstruction separately, further dividing the causes into intrinsic versus extrinsic.

Obstruction at the level of the renal tubules may be due to crystal-induced disease, uric acid nephropathy (as in the tumor lysis syndrome), or cast nephropathy due to multiple myeloma. Crystal-induced nephropathy has been classically described with sulfadiazine, acyclovir, indinavir, triamterene, and methotrexate.⁹ Newer literature also implicates orlistat¹⁰ and ciprofloxacin.¹¹

Nephrolithiasis is a common cause of upper urinary tract obstruction at the level of the ureter, with the size of the stone determining the likelihood of obstruction. Stones ≤2 mm, 3 mm, 4 to 6 mm and larger than 6 mm will pass spontaneously 97%, 86%, 50%, and 1% of the time, respectively.¹² Typically the obstruction occurs at one of the three narrowest portions of the ureter: the UPJ, the ureterovesicular junction (UVJ), or at the point where the ureter crosses over the pelvic brim. The obstruction is usually, but not always, acute and symptomatic. Neoplasms, blood clots, and sloughed renal papillae are rarer causes of intrinsic obstruction at the level of the ureter.

The causes of intraluminal obstruction at the level of the bladder are similar to those affecting the ureter, with urolithiasis, blood clots, and neoplasms being most common. Worldwide, infection with Schistosoma hematobium with resulting fibrosis is a common cause of bladder obstruction.¹³ Although rare in industrialized nations, it should be suspected in patients from endemic areas such as Africa and the Middle East.

Intramural Causes

Obstruction due to intramural causes is most often seen in the lower urinary tract. Disorders affecting the neuromuscular control of bladder emptying, such as cerebrovascular accidents,¹⁴ spinal cord injury,¹⁵ multiple sclerosis,¹⁶ and diabetic neuropathy¹⁷ may lead to bladder outlet obstruction. Multiple medications, including anticholinergics, opioid analgesics, nonsteroidal antiinflammatory agents, α-adrenoreceptor antagonists, benzodiazepines, and calcium channel blockers have also been associated with urinary retention.¹⁸ Stricture of the urethra may also lead to obstruction.

One potential intramural cause affecting the upper tract is ureteral stricture due to genitourinary tuberculosis.

Extrinsic Compression

Pregnancy is typically associated with right-sided dilation of the renal pelvis, calyx, and ureter. Hormonal mechanisms and mechanical compression from an enlarging uterus and an enlarging ovarian vein plexus have been implicated in these changes.¹⁹ Clinically meaningful obstruction from the gravid uterus is extremely rare.

Malignancies may cause obstruction by several different mechanisms. Local ureteric compression may be seen in metastatic cancers of the cervix, bladder and prostate, as well as with expanding retroperitoneal soft-tissue masses. Alternatively, the ureters may be compressed or encased by metastatic retroperitoneal lymphadenopathy from a distant primary.²⁰

Retroperitoneal fibrosis may lead to obstruction of one or both ureters via inflammation. It is an uncommon disorder, with a reported incidence rate of 1.3 case per million population and a male/female ratio of 3.3 : 1.²¹ Although the majority of these cases are idiopathic (>75%),²² numerous conditions are suspected to cause retroperitoneal fibrosis, including malignancies, medications, infection, trauma, or radiation.²³ Treatment of idiopathic retroperitoneal fibrosis is initially with steroids, but recurrences are common. Case reports describe the use of cyclophosphamide, azathioprine, colchicine, mycophenolate, or tamoxifen for treatment relapses or steroid-resistant disease, although conclusive data are absent.²² Abdominal aortic aneurysms (AAA) may also cause obstruction due to compression of the ureter or via inflammation. A recent series evaluated 999 cases of inflammatory AAA and found preoperative hydronephrosis in 7.4%.²⁴

Extrinsic compression of the lower urinary tract is more common in males. The etiology is usually either benign prostatic hypertrophy or prostate cancer.

The clinician must always bear in mind that hydroureter and/or hydronephrosis may be absent in obstruction due to retroperitoneal processes. Thus, one must maintain a high degree of suspicion and use alternative imaging modalities when considering these disorders.

The etiology of urinary tract obstruction is summarized in Box 116-1.

Box 116-1

Causes of Urinary Tract Obstruction

Extrinsic Causes

Upper urinary tract:

Pregnancy

Malignancy

Retroperitoneal fibrosis

Abdominal aortic aneurysms

Lower urinary tract:

Benign prostatic hypertrophy

Prostate cancer

Clinical Presentation

The clinical presentation of urinary tract obstruction depends on the location, duration, and severity of obstruction and may therefore be quite variable.

Pain

Acute ureteral obstruction often presents with severe flank pain, otherwise known as renal colic. This is usually due to urolithiasis but may be due to other causes of ureteral obstruction (see earlier). Obstruction causes increased intraluminal pressure and spasm of the ureteral muscles, which are responsible for the colicky pain.²⁵ Partial ureteral obstruction may present with a chronic dull pain. Bladder outlet obstruction may lead to distention and subsequent abdominal discomfort.

Changes in Urine Output

One pitfall in the diagnosis of obstruction is the expectation that patients will be anuric. While true of patients with obstruction of all functioning renal mass—complete bilateral ureteral obstruction, complete obstruction of a solitary functioning kidney, or complete obstruction distal to the bladder neck—this is not the case in patients with less severe disease. The degree of urine output does not reliably predict the presence or absence of obstruction; patients may present with normal urine output or even polyuria due to the effects of obstruction on renal salt and water handling (reviewed later).

Lower Urinary Tract Symptoms

Obstruction of the lower urinary tract often presents with some or all of a predictable constellation of symptoms known collectively as lower urinary tract symptoms, or LUTS. LUTS include voiding symptoms (difficulty urinating, incomplete emptying), postmicturition symptoms (post-void dribbling), and storage symptoms (urgency, frequency, hesitancy, incontinence).²⁶ Alternatively, patients with lower urinary tract obstruction may be asymptomatic.

Kidney Dysfunction

If asymptomatic, the initial clue to underlying obstruction may be an elevated creatinine level on a blood sample drawn for an unrelated reason. The fact that urinary tract obstruction may be asymptomatic mandates its inclusion in the differential diagnosis of unexplained kidney failure. If blood work is not obtained during the course of the obstruction, the kidney function may deteriorate such that the first presentation is with uremic symptoms and need for dialysis.

Infection

The urinary retention associated with lower urinary tract obstruction provides an excellent culture medium for bacteria. Patients may present with cystitis, pyelonephritis, or sepsis. An obstructing renal stone may also be a nidus for infection. Recurrent infection should raise suspicion for possible anatomic abnormalities, especially in men. In one study, 25 out of 83 men (30%) with a febrile urinary tract infection (UTI) had anatomic lesions in the lower urinary tract, supporting imaging of the lower tract in men with this presentation.²⁷ More recent data refute this finding in men younger than 45 years old.²⁸

Laboratory Values

There are no laboratory values specific to obstruction. Blood tests may show no abnormalities or may show values consistent with kidney failure, such as elevated blood urea nitrogen, creatinine, potassium, and phosphorus levels and decreased calcium, bicarbonate, and hemoglobin values. The blood tests may also be indicative of a renal tubular acidosis (see later). The urinalysis may be bland or may include red blood cells (in the setting of a stone or malignancy) or white blood cells (in the setting of infection). An experienced observer may also be able to discern crystals in a freshly voided urine. The fractional excretion of sodium (FE_Na) may be less than 1% in acute obstruction, but it is generally greater than 1% when the obstruction is chronic, owing to renal tubular dysfunction.

Imaging in Urinary Tract Obstruction

Various imaging modalities may be used to diagnose obstruction: plain abdominal radiography, ultrasound, CT, intravenous urography, retrograde pyelography, and nuclear scanning. It is important to understand the indications and limitations of each modality.

Plain Abdominal Radiography

Abdominal radiography (kidney, ureter, and bladder [KUB]) is often the first imaging modality preformed in patients with acute flank pain. Although most stones are composed of calcium and should in theory be visible, only 59% of stones are detected on plain film.²⁹ Compared to CT scanning, the sensitivity and specificity of abdominal films were 45% to 59% and 77%, respectively.²⁹ Further, plain films may not always be able to differentiate phleboliths from calculi. This limits the utility of plain abdominal films to the diagnosis of recurrent disease in those with known radioopaque stones.

Ultrasound

Ultrasound (US) is inexpensive, does not expose the patient to radiation, and is typically readily available. Its accuracy in detecting hydronephrosis makes US a good screening tool for obstruction in the patient with unexplained kidney failure, or the patient with suspected lower urinary tract obstruction (Figure 116-1). US has been largely superseded by noncontrast CT in the detection of nephrolithiasis and stone-related obstruction. When CT is used as a reference, US has a sensitivity of 24% and a specificity of 90% for the detection of kidney stones and is likely to miss those less than 3 mm.³⁰ Another disadvantage of US compared to CT is that bowel gas may obscure visualization of the ureters.³¹ Thus despite its ability to detect hydronephrosis, US may be limited in its ability to demonstrate the cause or site of an obstruction. Other conditions such as peripelvic cysts and renal artery aneurysms may mimic hydronephrosis on US.³¹ These conditions are easily distinguished via CT scanning.