Urinary Tract Obstruction

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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