Excerpt from Nephrolithiasis

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Background

Nephrolithiasis is a common disease that is estimated to produce medical costs of $2.1 billion per year in the United States. Nephrolithiasis specifically refers to calculi in the kidneys, but this article discusses both renal calculi (Image 1) and ureteral calculi (ureterolithiasis). Ureteral calculi almost always originate in the kidneys, although they may continue to grow once they lodge in the ureter.

Urinary tract stone disease has been a part of the human condition for millennia; in fact, bladder and kidney stones have even been found in Egyptian mummies. Some of the earliest recorded medical texts and figures depict the treatment of urinary tract stone disease.

Pathophysiology

Urinary tract stone disease (Image 3) is likely caused by 2 basic phenomena.

The first phenomenon is supersaturation of the urine by stone-forming constituents, including calcium, oxalate, and uric acid. Crystals or foreign bodies can act as nidi, upon which ions from the supersaturated urine form microscopic crystalline structures. The overwhelming majority of renal calculi contain calcium. Uric acid calculi and crystals of uric acid, with or without other contaminating ions, comprise the bulk of the remaining minority. Other, less frequent stone types include cystine, ammonium acid urate, xanthine, dihydroxyadenine, and various rare stones related to precipitation of medications in the urinary tract.

Other current theories also include renal tubular damage or dysfunction as an important component of the initiation of stone formation. The initial crystal agglomerations likely form in distal collecting tubules that drain into the renal papilla. As these masses grow, they gradually extrude into the collecting system through the papilla and eventually drop off to become free urinary calculi.

Frequency

United States

The lifetime prevalence of urinary tract stone disease in the United States is approximately 10%. The annual incidence of urinary tract stones in the industrialized world is estimated to be 0.2%. The likelihood that a white male will develop stone disease by age 70 years is 1 in 8. Stones of the upper urinary tract are more common in the United States than in the rest of the world. Roughly 2 million patients present on an outpatient basis with stone disease each year in the United States, which is a 40% increase from 1994.

International

The incidence of urinary tract stone disease in developed countries is similar to that in the United States. Stone disease is rare in only a few areas, such as Greenland and the coastal areas of Japan. In developing countries, bladder calculi are more common than upper urinary tract calculi; the opposite is true in developed countries. These differences are believed to be diet-related.

Mortality/Morbidity

• The morbidity of urinary tract calculi is primarily due to obstruction with its associated pain, although nonobstructing calculi can still produce considerable discomfort.
• Conversely, patients with obstructing calculi may be asymptomatic, which is the usual scenario in patients who experience loss of renal function due to chronic untreated obstruction.
• Stone-induced hematuria is frightening to the patient but is rarely dangerous by itself.
• The most morbid and potentially dangerous aspect of stone disease is the combination of obstruction and upper urinary tract infection. Pyelonephritis, pyonephrosis, and urosepsis can ensue.

Race

Urinary tract calculi are far more common Asians and whites than in Native Americans, Africans, African Americans, and some natives of the Mediterranean region. Although some differences may be attributable to geography (stones are more common in hot and dry areas) and diet, heredity also appears to be a factor. This is suggested by the finding that, in regions with both white and nonwhite populations, stone disease is much more frequent in whites.

Sex

• In general, urolithiasis is more common in males (male-to-female ratio, 2-3:1).
• Stones due to discrete metabolic/hormonal defects (eg, cystinuria, hyperparathyroidism) and stone disease in children are equally prevalent between the sexes.
• Stones due to infection (struvite calculi) are more common in women than in men.

Age

• Most urinary calculi develop in patients aged 20-49 years.
• Patients in whom multiple recurrent stones form usually develop their first stones while in their second or third decade of life.
• An initial stone attack after age 50 years is relatively uncommon.

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