Porth's Essentials of Pathophysiology, 4e

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Kidney and Urinary Tract Function

U N I T 7

Mechanisms of Renal Damage The destructive effects of urinary obstruction on kidney structures are determined by the degree (i.e., partial ver- sus complete, unilateral versus bilateral) and the dura- tion of the obstruction. The two most damaging effects of urinary obstruction are stasis of urine, which predis- poses to infection and stone formation, and progressive dilation of the renal collecting ducts and renal tubular structures, which causes destruction and atrophy of renal tissue. A common complication of urinary tract obstruction is infection. Stagnation of urine predisposes to infection, which may spread throughout the urinary tract. When present, urinary calculi serve as foreign bodies and con- tribute to the infection. Once established, the infection is difficult to treat. It often is caused by urea-splitting organisms (e.g., Proteus, staphylococci) that increase ammonia production and cause the urine to become alkaline. 27 Calcium salts precipitate more readily in stag- nant alkaline urine; thus, urinary tract obstructions also predispose to stone formation. In situations of severe partial or complete obstruc- tion, the impediment to the outflow of urine causes dilation of the renal pelvis and calyces associated with progressive atrophy of the kidney. 4,5 Even with com- plete obstruction, glomerular filtration continues for some time. Because of the continued filtration, the caly- ces and pelvis of the affected kidney become dilated, often markedly so. The high pressure in the renal pel- vis is transmitted back through the collecting ducts of the kidney, compressing renal vasculature and causing renal atrophy. Initially, the functional alterations are largely tubular, manifested primarily by impaired urine- concentrating ability. Only later does the GFR begin to diminish. Hydronephrosis Hydronephrosis refers to urine-filled dilation of the renal pelvis and calyces associated with progressive atrophy of the kidney due to obstruction of urine outflow. 4,5,27 The degree of hydronephrosis depends on the duration, degree, and level of obstruction. In far-advanced cases, the kidney may be transformed into a thin-walled cystic structure with parenchymal atrophy, total obliteration of the pyramids, and thinning of the cortex (Fig. 25-14). The condition is usually unilateral; bilateral hydrone- phrosis occurs only when the obstruction is below the level of the ureterovesical junction. When the obstruc- tion affects the outflow of urine from the distal ureter, the increased pressure dilates the ureter, a condition called hydroureter . Bilateral hydroureter may develop as a complication of bladder outflow obstruction due to prostatic hyperplasia (see Chapter 39). Clinical Features The manifestations of urinary obstruction depend on the site of obstruction, the cause, and the rapidity with which the condition developed. Most of the early symp- toms are produced by the underlying pathologic pro- cess. Urinary tract obstruction encourages the growth

of microorganisms and should be suspected in persons with recurrent urinary tract infections. Complete or partial unilateral hydronephrosis may remain silent for long periods because the unaffected kidney can maintain adequate function. Obstruction may provoke pain due to distention of the collecting system and renal capsule. Whereas, acute supravesical obstruction, such as that due to a kidney stone lodged in the ureter, is associated with excruciating pain. More insidious causes of obstruction, such as narrowing of the ureteropelvic junction, may produce little pain but cause total destruction of the kidney. Complete bilateral obstruction results in oliguria and anuria and renal failure. Acute bilateral obstruction may mimic prerenal failure. With partial bilateral obstruction, the earliest manifestation is an inability to concentrate urine, reflected by polyuria and nocturia. Hypertension is an occasional complication of urinary tract obstruc- tion. It is more common in cases of unilateral obstruction in which renin secretion is enhanced, probably second- ary to impaired renal blood flow. In these circumstances, removal of the obstruction often leads to a reduction in blood pressure. When hypertension accompanies bilat- eral obstruction, it is volume related. The relief of bilat- eral obstruction leads to a loss of volume and a decrease in blood pressure. In some cases, relieving the obstruc- tion does not correct the hypertension. Early diagnosis of urinary tract obstruction is impor- tant because the condition usually is treatable and a delay in therapy may result in permanent damage to the kidneys. Diagnostic methods vary with the symptoms. Ultrasonography has proved to be the single most use- ful noninvasive diagnostic modality for urinary obstruc- tion. Radiologic methods, CT scans, and intravenous urography may also be used. Other diagnostic methods, such as urinalysis, are used to determine the extent of renal involvement and the presence of infection. Treatment of urinary tract obstruction depends on the cause. Urinary stone removal may be necessary, or FIGURE 25-14. Hydronephrosis. Bilateral urinary tract obstruction has led to conspicuous dilation of the ureters, pelves, and calyces. The kidney on the right shows severe cortical atrophy. (From Jennette JC. The kidney. In: Rubin R, Strayer DS, eds. Rubin’s Pathology: Clinicopathologic Foundations of Medicine. 6th ed. Philadelphia, PA: Wolters Kluwer Health | Lippincott Williams &Wilkins; 2012:801.)