Atlas of pathophysiology, 2 Edition

Part II - Disorders

Renal Calculi

Renal calculi, or stones (nephrolithiasis), can form anywhere in the urinary tract, although they most commonly develop on the renal pelves or calyces. They may vary in size and may be single or multiple.

Age Alert

Renal calculi are more common in men than women and rarely occur in children. Calcium stones generally occur in middle-aged men with a familial history of stone formation.

Causes

Exact cause unknown

Predisposing factors

·   Dehydration

·   Infection

·   Changes in urine pH (calcium carbonate stones, high pH; uric acid stones, lower pH)

·   Obstruction to urine flow leading to stasis in the urinary tract

·   Immobilization causing bone reabsorption

·   Metabolic factors, such as hyperparathyroidism, renal tubular acidosis, elevated uric acid, defective oxalate metabolism

·   Dietary factors such as increased intake of calcium or of oxalate-rich foods

·   Renal disease

Pathophysiology

Calculi form when substances that are normally dissolved in the urine, such as calcium oxalate and calcium phosphate, precipitate. Dehydration may lead to renal calculi as calculus-forming substances concentrate in urine.

Stones form around a nucleus or nidus in the appropriate environment. A stone-forming substance (calcium oxalate, calcium carbonate, magnesium, ammonium, phosphate, uric acid, or cystine) forms a crystal that becomes trapped in the urinary tract, where it attracts other crystals to form a stone. A high urine saturation of these substances encourages crystal formation and results in stone growth. The pH of the urine affects the solubility of many stone-forming substances. Formation of calcium oxalate and cystine stones is independent of urine pH. Most stones are calcium oxalate or a combination of oxalate and phosphate.

Stones may form in the papillae, renal tubules, calyces, renal pelves, ureter, or bladder. Most are less than 5 mm in diameter and are usually passed in the urine. Staghorn calculi (casts of the calyceal and pelvic collecting system) can continue to grow in the pelvis, extending to the calyces, forming a branching stone, and ultimately resulting in renal failure if not surgically removed.

Calcium stones are the smallest. Although 80% are idiopathic, they frequently occur in patients with hyperuricosuria. Prolonged immobilization can lead to bone demineralization, hypercalciuria, and stone formation. In addition, hyperparathyroidism, renal tubular acidosis, and excessive intake of vitamin D or dietary calcium may predispose a person to renal calculi.

Struvite (magnesium, ammonium, and phosphate) stones are often precipitated by an infection, particularly with Pseudomonas or Proteus species. These urea-splitting organisms are more common in women. Struvite calculi can destroy renal parenchyma.

Signs and symptoms

·   Severe pain from obstruction

·   Nausea and vomiting

·   Fever and chills from infection

·   Hematuria when calculi abrade a ureter

·   Abdominal distention

·   Anuria from bilateral obstruction or obstruction of only kidney

Diagnostic test results

·   Kidney-ureter-bladder (KUB) radiography shows most renal calculi.

·   Excretory urography confirms the diagnosis and determines the size and location of calculi.

·   Kidney ultrasonography detects such obstructive changes as unilateral or bilateral hydronephrosis and radiolucent calculi not seen on KUB radiography.

·   Urine culture shows pyuria.

·   A 24-hour urine collection determines levels of calcium oxalate, phosphorus, and uric acid excretion.

·   Calculus analysis determines mineral content.

·   Serial blood calcium and phosphorus levels diagnose hyperparathyroidism.

·   Blood protein levels determine the level of free calcium unbound to protein.

Treatment

·   Fluid intake greater than 3 L/day to promote hydration

·   Antimicrobial agents; vary with the cultured organism

·   Analgesics, such as meperidine or morphine

·   Diuretics to prevent urinary stasis and further calculus formation; thiazides to decrease calcium excretion

·   Acetohydroxamic acid to suppress calculus formation when infection is present

·   Cystoscopy and manipulation of calculus to remove stones too large for natural passage

·   Percutaneous ultrasonic lithotripsy and extracorporeal shock wave lithotripsy or laser therapy to shatter the calculus into fragments for removal by suction or natural passage

·   Surgical removal of cystine calculi or large stones

·   Placement of urinary diversion around the stone

Type-specific treatment

·   Low-calcium diet

·   Oxalate-binding cholestyramine

·   Parathyroidectomy

·   Allopurinol for uric acid calculi

·   Daily small doses of ascorbic acid to acidify urine

P.367

TYPES OF RENAL CALCULI

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