Porth's Essentials of Pathophysiology, 4e

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Disorders of Renal Function

C h a p t e r 2 5

to treat urinary tract infections. Most stones that are less than 5 mm in diameter pass spontaneously. All urine should be strained during an attack in the hope of retrieving the stone for chemical analysis and determi- nation of type. This information, along with a careful history and laboratory tests, provide the basis for long- term preventive measures. A major goal of treatment in persons who have passed kidney stones or have had them removed is to prevent their recurrence. Prevention requires investigation into the cause of stone formation using urine tests, blood chemistries, and stone analysis. Underlying disease conditions, such as hyperparathyroidism, are treated. Adequate fluid intake reduces the concentra- tion of stone-forming crystals in the urine and needs to be encouraged. Calcium supplementation with calcium salts such as calcium carbonate and calcium phosphate also may be used to bind oxalate in the intestine and decrease its absorption. Thiazide diuret- ics lower urinary calcium by increasing tubular reab- sorption so that less remains in the urine. Drugs that bind calcium in the gut (e.g., cellulose phosphate) may be used to inhibit calcium absorption and uri- nary excretion. Depending on the type of stone that is formed, dietary changes, medications, or both may be used to alter the concentration of stone-forming elements in the urine (Table 25-2). For example, persons who form cal- cium oxalate stones may need to decrease their intake of foods that are high in oxalate (e.g., spinach, Swiss chard, cocoa, chocolate, pecans, peanuts). Because of associated electrolyte disturbances and altered urine chemistry, obese persons are predisposed to hyperuri- cemia, gout, hypercitraturia, and uric acid stones. 31 Weight loss may improve or undermine management of these stones; however, it can be detrimental if associ- ated with a high animal protein diet, laxative abuse, rapid loss of lean tissue, or poor hydration. High acid diets, such as the Atkins diet, increase the risk of uric acid stones. Measures to change the pH of the urine also can influence kidney stone formation. In persons who lose the ability to acidify or lower the pH of their urine, there is an increase in the divalent and trivalent forms of urine phosphate that combine with calcium to form calcium phosphate stones. The formation of uric acid stones is increased in acid urine; stone formation can be reduced by raising the pH of urine to 6.0 to 6.5 with potassium alkali (e.g., potassium citrate) salts. In some cases, stone removal may be necessary. Several methods are available for removing kidney stones: ureteroscopic removal, percutaneous removal, and extracorporeal lithotripsy. 28 All of these proce- dures eliminate the need for an open surgical procedure, which is another form of treatment. Open stone surgery may be required to remove large calculi or those that are resistant to other forms of removal. A nonsurgical treat- ment, called extracorporeal shock-wave lithotripsy, uses acoustic shock waves to fragment calculi into sandlike particles that are passed in the urine over the next few

days. Because of the large amount of stone particles that are generated during the procedure, a ureteral stent (i.e., a tubelike device used to hold the ureter open) may be inserted to ensure adequate urine drainage.

SUMMARY CONCEPTS

■■ Obstruction of urine flow can occur at any level of the urinary tract. Among the causes of urinary tract obstruction are developmental defects, pregnancy, infection and inflammation, kidney stones, neurologic defects, and prostatic hypertrophy. ■■ Obstructive disorders produce stasis of urine, increase the risk for infection and calculi formation, and produce progressive dilation of the renal collecting ducts and renal tubular structures, which cause renal atrophy. ■■ 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. Unilateral hydronephrosis may remain silent for long periods because the unaffected kidney can maintain adequate function. With partial bilateral obstruction, the earliest manifestation is an inability to concentrate urine, reflected by polyuria and nocturia. Complete bilateral obstruction results in oliguria, anuria, and renal failure. ■■ Kidney stones are a major cause of upper urinary tract obstruction.The development of kidney stones is influenced by the concentration of stone components in the urine, the ability of the stone components to complex and form stones, and the presence of substances that inhibit stone formation. ■■ There are four types of kidney stones: calcium (i.e., oxalate and phosphate) stones, which are associated with increased serum calcium levels; magnesium ammonium phosphate (i.e., struvite) stones, which are associated with urinary tract infections; uric acid stones, which are related to elevated uric acid levels; and cystine stones, which are seen in cystinuria.Treatment measures depend on stone type and include adequate fluid intake to prevent urine saturation, dietary modification to decrease intake of stone-forming constituents, treatment of urinary tract infections, measures to change urine pH, and the use of diuretics that decrease the calcium concentration of urine.