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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 any disorder that interferes with drainage of the urine. 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 kidney function.

  • Hydronephrosis refers to dilatation of the urinary collecting system and 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. Common anatomic variants, including the presence of an extrarenal pelvis and duplicated collecting systems, are also associated with hydronephrosis on imaging.

Epidemiology

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

Etiology

Many disorders may lead to urinary tract obstruction. A useful classification is to first separate pathology arising from within the urinary tract itself (intrinsic obstruction) from those diseases that arise outside the urinary tract causing external compression of the system (extrinsic obstruction). This discussion will also consider upper (from the renal pelvis to the ureterovesicular junction [UVJ]) and lower (from the bladder to the urethra) urinary tract obstruction separately.

Intrinsic obstruction

Intrinsic urinary tract obstruction may be the result of pathology within the lumen (intraluminal) or within the walls of the collecting system (intramural).

Intraluminal causes

Obstruction at the level of the renal tubules may be the result of crystal-induced disease, uric acid nephropathy (as in tumor lysis syndrome), or cast nephropathy caused by multiple myeloma. Crystal-induced nephropathy has been classically described with sulfadiazine, acyclovir, indinavir, triamterene, and methotrexate. Published case reports also implicate orlistat and ciprofloxacin.

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, 4–6 mm, and >6 mm will pass spontaneously 97%, 86%, 50%, and 1% of the time, respectively. Typically, the obstruction occurs at one of the three narrowest portions of the ureter: the ureteropelvic junction (UPJ), the UVJ, or the point where the ureter crosses over the pelvic brim. Neoplasms, blood clots, and sloughed renal papillae are rarer causes of intraluminal 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 the most common. Worldwide, infection with Schistosoma haematobium with resulting fibrosis is a common cause of bladder obstruction. Although schistosomiasis is rare in industrialized nations, it should be suspected in patients from endemic areas such as Africa and the Middle East presenting with urinary tract obstruction.

Intramural causes

Congenital malformations of the genitourinary tract can cause intramural obstruction of the upper urinary tract. UPJ obstruction (UPJO) warrants specific mention, as it is the most common congenital genitourinary disorder likely to present in adulthood. Kinks, valves, or an adynamic segment of ureter results in failure of peristalsis at the UPJ. The widespread use of maternal prenatal ultrasound (US) has led to increased antenatal diagnosis of UPJO. The diagnosis can be made by US, intravenous urography (IVU), or in equivocal cases, isotope renography (see later imaging section).

Another intramural cause of upper urinary tract obstruction is ureteral stricture caused by genitourinary tuberculosis (GU TB). Although rare in the developed world, GU TB complicates up to 40% of patients with extrapulmonary tuberculosis. Hematogenous spread of mycobacteria can seed the renal cortex and ureters, causing inflammation with resultant fibrosis, obstruction, and secondary infections. Seeding of the retroperitoneum and bladder can also lead to complications in patients with GU TB.

More common are disorders affecting the neuromuscular control of the lower urinary tract such as cerebrovascular accidents, spinal cord injury, multiple sclerosis, and diabetic neuropathy, which may lead to bladder outlet obstruction. Multiple medications, including anticholinergics, opioid analgesics, nonsteroidal antiinflammatory agents, alpha-adrenoreceptor agonists, benzodiazepines, and calcium channel blockers, have also been associated with urinary retention. Stricture of the urethra may also lead to obstruction.

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 ovarian vein plexus have been implicated in these changes. Clinically meaningful obstruction from the gravid uterus is extremely rare.

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

Retroperitoneal fibrosis may lead to the obstruction of one or both ureters via inflammation. It is an uncommon disorder, with a reported incidence rate of 1.3 cases per million population and a male:female ratio of 3.3:1. Although the majority of these cases are idiopathic (>75%), numerous conditions are suspected to cause retroperitoneal fibrosis, including malignancies, medications, infection, trauma, radiation, and IgG4-related systemic disease. , Treatment of idiopathic retroperitoneal fibrosis is initially with steroids, but recurrences are common. Case reports have described the use of cyclophosphamide, azathioprine, colchicine, mycophenolate, or tamoxifen for treatment relapses or steroid-resistant disease, although conclusive data are absent.

Abdominal aortic aneurysms (AAAs) may also cause obstruction caused by compression of the ureter or via inflammation. A published series of 999 cases of inflammatory AAA found preoperative hydronephrosis in 7.4% of these cases. The clinician must always bear in mind that hydroureter and/or hydronephrosis may be absent in obstruction caused by retroperitoneal processes. Thus one must maintain a high degree of suspicion and use alternative imaging modalities when considering these disorders.

Extrinsic compression of the lower urinary tract is more common in males, and the cause is usually either benign prostatic hypertrophy (BPH) or prostate cancer. The etiology of urinary tract obstruction is summarized in Box 101.1 .

BOX 101.1
Causes of Urinary Tract Obstruction

Intrinsic causes

Intraluminal

Renal tubules

  • Crystal-induced disease

  • Uric acid nephropathy

  • Cast nephropathy (in multiple myeloma)

Upper urinary tract

  • Nephrolithiasis

  • Neoplasms

  • Blood clots

  • Sloughed renal papillae

Lower urinary tract

  • Urolithiasis

  • Blood clots

  • Neoplasms

  • Schistosomiasis

Intramural

Upper urinary tract

  • Congenital ureteropelvic junction obstruction

  • Genitourinary tuberculosis

Lower urinary tract

  • Disorders affecting neuromuscular control

    • Cerebrovascular accident

    • Spinal cord injury

    • Multiple sclerosis

    • Diabetic neuropathy

  • Medications

    • Anticholinergic agents

    • Opiates

    • Nonsteroidal antiinflammatory agents

    • Alpha-adrenoreceptor antagonists

    • Benzodiazepines

    • Calcium channel blockers

  • Urethral structure

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