Renal calculi can form anywhere in the urinary tract, but they most commonly develop in the renal pelvis or calyces. Calculi form when substances that normally are dissolved in the urine (such as calcium oxalate and calcium phosphate) precipitate. Renal calculi vary in size and may be solitary or multiple.
About 1 in 1,000 Americans require hospitalization for renal calculi. They're more common in men than women and are rare in blacks and children.
The development of the stones is related to decreased urine volume or increased excretion of stone-forming components such as calcium, oxalate, urate, cystine, xanthine, and phosphate. The stones form in the urine collecting area (the pelvis) of the kidney and may range in size from tiny to staghorn stones the size of the renal pelvis itself.
The cystine stones (below) compared in size to a quarter (a U.S. $0.25 coin) were obtained from the kidney of a young woman by percutaneous nephrolithotripsy (PNL), a procedure for crushing and removing the dense stubborn stones characteristic of cystinuria.
Signs and Symptoms
Diagnosis is based on clinical features and the results of various tests. For example, kidney-ureter-bladder (KUB) radiography reveals most renal calculi, and excretory urography helps confirm the diagnosis and determine the size and location of calculi.
Kidney ultrasonography is easily performed, noninvasive, and nontoxic. It's used to detect obstructive changes, such as unilateral or bilateral hydronephrosis and radiolucent calculi not seen on the KUB radiography.
Urine culture of a midstream specimen may indicate pyuria, a sign of urinary tract infection. A 24hour urine collection is evaluated for calcium oxalate, phosphorus, and uric acid excretion levels; three separate collections, along with blood samples, are needed for accurate testing.
Calculus analysis shows mineral content.
Other diagnostic test results may suggest the cause of calculus formation:
Appendicitis, cholecystitis, peptic ulcer, and pancreatitis must be ruled out as sources of pain before the diagnosis can be confirmed.
Because 90% of renal calculi are smaller than 5 mm in diameter, treatment usually involves encouraging their natural passage through vigorous hydration (more than 3 L [3.2 qt]/24 hours). Other treatment measures include administration of antimicrobial agents for infection (varying with the cultured organism); analgesics, such as meperidine or morphine, for pain; and diuretics to prevent urinary stasis and further calculus formation (thiazides decrease calcium excretion into the urine). Methenamine mandelate is given to suppress calculus formation when infection is present.
Measures to prevent recurrence include a diet of adequate calcium intake, often combined with oxalate-binding cholestyramine, for absorptive hypercalciuria; parathyroidectomy for hyperparathyroidism; administration of allopurinol for uric acid calculi; and daily oral doses of ascorbic acid to acidify the urine. High-risk groups should be identified and monitored closely. Other preventive measures include adequate hydration (2.5 to 3 L [2.6 to 3.2 qt]/24 hours), early mobilization of patients, repositioning, and exercise for immobilized or patients with inadequate mobility.
Calculi too large for natural passage may require removal. A calculus lodged in the ureter may be removed by inserting a cystoscope through the urethra and then manipulating the calculus with catheters or retrieval instruments. Extraction of calculi from other areas, such as the kidney calyx or renal pelvis, may necessitate a flank or lower abdominal approach. Two other methods, percutaneous ultrasonic lithotripsy and extracorporeal shock wave lithotripsy, shatter the calculus into fragments for removal by suction or natural passage.
Cystine calculi are difficult to treat without surgical intervention or an invasive procedure. If electrohydraulic ultrasound isn't effective, the calculi are surgically removed.
If there is a history of stones, fluids should be encouraged to produce adequate amounts of dilute urine (usually 6 to 8 glasses of water per day). Depending on the type of stone, medications or other measures may be recommended to prevent recurrence.
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