Calculus, renal

Stone formation may be due to metabolic abnormality, structural disorders or recurrent urinary tract infection. Metabolic causes include hypercalciuria, hyperplasia, hyperuricaemia and cystinuria renal. Hypercalciuria may be resorptive (due to bone lysis, as in primary hyperparathyroidism, malignancy, Cushings syndrome, immobilization and Pagets disease absorptive (due to excessive uptake from the gastrointestinal tract, which may be idiopathic or secondary to sarcoidosis, hypervitaminosis D or milk alkali syndrome); or renal (due to excess excretion of calcium from the kidney, which may be idiopathic or secondary to type I renal tubular acidosis). Structural abnormalities which predispose to stone formation include pyelocalyceal diverticulum, ureteropelvic junction obstruction, horseshoe kidney, renal ectopia and polycystic kidney disease.

Urinary tract calculi are composed of mucoprotein matrix and a crystalline aggregate of varying chemical composition. Few stones occur in pure form: most are crystal mixtures. Most calculi are composed of calcium oxalate, calcium phosphate and magnesium ammonium phosphate. Uric acid and urate stones comprise approximately 10% of stones in the United States. Less frequent are calculi of cystine, xanthine, matrix and fibrin. Although each of these constituents has a characteristic radiographic appearance when pure, crystalline admixtures produce varied images on radiographs. Nonetheless, analysis of the size, shape and appearance of a calculus as it appears on radiographs often provides useful clues to its pathogenesis.

Calcium oxalate calculi are usually small (1.5 cm or less in diameter), smooth or mamillated, round or oval, and densely opaque.

Calcium phosphate (apatite) rarely forms a pure calculus but commonly combines with either calcium oxalate or magnesium ammonium phosphate, imparting a concentrically laminated appearance to such mixed stones. Apatite is an important constituent of calculi present in infected, alkaline urine. A pure apatite calculus is radiopaque and may be difficult to distinguish radiographically from a pure calcium oxalate calculus.

Magnesium ammonium phosphate hexahydrate (struvite) calculi are associated with an alkaline urine and are almost always accompanied by, and are the result of, a urea-splitting bacillary urinary tract infection, usually with Proteus mirabilis. Pure stuviite or infection stones are rare and of relatively low radiopacity but are often laminated with more dense calcium salts, usually calcium phosphate, resulting in a magnesium-ammonium-calcium phosphate or triple-phosphate stone.

Uric acid calculi, when pure, are radiolucent. However, over half of such calculi are adventitiously covered with calcium oxalate or, less commonly, calcium phosphate, imparting varying degrees of radiopacity. Factors favouring formation of uric acid calculi are hyperuricosuria and an acid, concentrated urine. When pure, uric acid stones tend to be relatively small and smooth and are often disc-shaped. Large uric acid calculi can attain a staghorn configuration.

Cystine stones account for approximately 2% of urinary calculi. They are rarely pure in composition, and as a result may be single, multiple or of the staghorn variety.

Imaging

Because approximately 90% of all urinary trac ureterectasis is usually also present if ureteric obstruction has been present for more than a few hours. The ureteric lumen just distal to a stone often appears narrowed due to inflammation and oedema of the ureteric mucosa. This narrowing may give the false impression of a ureteric stricture that has prevented further distal migration of the stone. Lesser degrees of ureteric obstruction may be apparent only on an erect radiograph, which shows delayed or diminished drainage of opacified urine from the affected ureter above the site of obstruction. Rarely a relatively small calculus may reside in the ureter for a variable period of time without producing any urographic abnormality. Patients harbouring such nonobstructive calculi are usually asymptomatic.

Ultrasonography can differentiate renal calculi from other causes of pyelocalyceal filling defects such as tumours or blood clots. The sonographic diagnosis of a calculus is based on the demonstration of a highly echogenic focus that produces an acoustic shadow. Stones as small as 0.5 cm can be reliably detected in this manner. This technique assumes greatest importance when faced with a nonopaque filling defect on urography. Tumours and clots lack a distal acoustic shadow.

Recently, it has become apparent that noncontrast helical CT (HCT) has major advantages over intravenous urography in the evaluation of patients with urolithiasis. Noncontrast HCT studies can be completed much more rapidly than intravenous urography, as there is no need for oral or intravenous contrast administration or other patient preparation. Virtually all types of urinary tract calculi contain enough calcium to be visibly hyperdense on noncontrast HCT. It has been demonstrated that stones are more accurately detected with noncontrast HCT than standard radiography, nephrotomography, intravenous urography, or sonography. Noncontrast HCT provides no direct functional information. The primary finding indicative of a ureteric stone is visualization of the stone within the ureter. For accurate detection of a ureteric stone, the ureter should be followed caudally on sequential HCT images. In addition to primary visualization of a ureteric stone, secondary signs are important for diagnosis. These signs include unilateral ureteric dilatation, unilateral perinephric soft-tissue changes, symmetric intrarenal collecting system dilatation, and unilateral renal enlargement. The major difficulty in interpreting noncontrast HCT images of patients with acute flank pain is in distinguishing between phleboliths and ureteric stones, particularly in the pelvis. The soft-tissue "rim" sign describes a circumferential soft-tissue ring around a calcification. This sign is thought to be indicative of ureteric wall oedema at the site of an impacted stone. The presence of this sign strongly indicates that the calcification represents a ureteric stone rather than a phlebolith. The absence of a rim sign is less useful. Another secondary sign that is useful for differentiating stones from phleboliths is the "comet-tail" sign. This term describes a curvilinear soft-tissue band extending from the suspect calcification on serial images. This soft-tissue band is thought to represent a vein in which the phlebolith has developed, therefore indicating that the associated calcification is a phlebolith rather than a ureteric stone. Ureteric obstruction if so severe, or of such long standing, that urographic opacification of the ureter is insufficient for diagnosis and is not forthcoming even with delayed filming, retrograde ureterography is usually preferred to determine the cause for the radiological findings.

MR imaging has not yet had a major impact in this area because stones lack mobile protons and generate no signal with standard MR imaging techniques.

Complications

The acutely obstructing upper urinary tract calculus rarely gives rise to significant complications, because its dramatic clinical presentation usually leads to prompt medical attention. On the other hand, the chronically stone-bearing kidney passed or removed calculi is an essential component of this therapy. Uric acid stones, often of considerable size, can be treated with good results orally with urinary alkalinizing agents and increased fluid volumes. Some cystine stones gradually dissolve with oral medication, and prevention of recurrent stones in this condition is moderately successful. Recurrent urinary tract infections related to struvite stones may be controlled with antibiotics in those patients who are suboptimal surgical candidates.

Spontaneous passage of asymptomatic stones down the ureter and into the bladder is nearly always the preferred outcome in patients with nephrolithiiasis. Calculi that are less than 4 mm in diameter will pass without intervention in more than 80% of cases, with the success rate falling to 20% in stones over 8 mm in diameter.

The surgical therapy of symptomatic calculi changed dramatically in the 1980s.The successive clinical introductions of percutaneous stone removal techniques, transurethral ureteroscopy, and extracorporeal shock wave lithotripsy resulted in greatly reduced patient morbidity and hospital stays. The classic indication for surgical intervention (pain, obstruction or infection) are unchanged. The role of classic, open-flank surgical procedures for stone disease has diminished greatly and now accounts for only 1-2% of all stone interventions.

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