Материал: Oxford American Handbook of Urology ( PDFDrive )

KIDNEY STONES: EPIDEMIOLOGY 357

Water intake

Low fluid intake (<1200 mL/day) predisposes to stone formation.2 Increasing water hardness (high calcium content) may reduce risk of stone formation, by decreasing urinary oxalate.

Diet

High animal protein intake increases risk of stone disease (high urinary oxalate, low pH, low urinary citrate).3,4 High salt intake causes hypercalciuria. Contrary to conventional teaching, low-calcium diets predispose to calcium stone disease, and high-calcium intake is protective.5

Occupation

Sedentary occupations predispose to stones more than manual work.

2 Borghi L, et al. (1996) Urinary volume, water and recurrences in idiopathic calcium nephrolithiasis: a 5-year randomized prospective study. J Urol 155:839–843.

3 Curhan GC, et al. (1997) Comparison of dietary calcium with supplemental calcium and other nutrients as factors affecting the risk for kidney stones in women. Ann Intern Med 126:497–504.

4 Borghi L, et al. (2002) Comparison of 2 diets for the prevention of recurrent stones in idiopathic hypercalciuria. N Engl J Med 346:77–84.

5 Curhan GC, et al. (1993) A prospective study of dietary calcium and other nutrients and the risk of symptomatic kidney stones. N Engl J Med 328:833–838.

358 CHAPTER 8 Stone disease

Kidney stones: types and predisposing factors

Stones may be classified according to composition (Table 8.1), X-ray appearance, or size and shape.

Other rare stone types (all radiolucent) include indinavir (a protease inhibitor for treatment of HIV), triamterene (a relatively insoluble potas- sium-sparing diuretic, most of which is excreted in urine), and xanthine.

Radiodensity on X-ray

Three broad categories of stones are described, based on their X-ray appearance. This gives some indication of the likely stone composition and helps to some extent to determine treatment options.

However, in only 40% of cases is stone composition correctly identified from visual estimation of radiodensity on plain X-ray.1

Radio-opaque

Opacity implies presence of substantial amounts of calcium within the stone. Calcium phosphate stones are the most radiodense stones, almost as dense as bone. Calcium oxalate stones are slightly less radiodense.

Relatively radiolucent

Cystine stones are relatively radiodense because they contain sulfur (Fig. 8.1). Magnesium ammonium phosphate (struvite) stones are less radiodense than calcium-containing stones.

Completely radiolucent

Uric acid, triamterene, xanthine, indinavir stones are in this category (cannot be seen even on CTU).

Table 8.1 Composition of kidney stones

* The precise distribution of stone types will vary depending on the characteristics of the study population (geographical location, racial distribution, etc.). Hence, the quoted figures do not equate to 100.

†~80% of uric acid stones are pure uric acid; 20% contain some calcium oxalate.

1 Ramakumar S, Patterson DE, LeRoy AJ, et al. (1999) Prediction of stone composition from plain radiographs: a prospective study. J Endosc Urol 13:397–401.

KIDNEY STONES: TYPES AND PREDISPOSING FACTORS 359

Size and shape

Stones can be characterized by their size, in centimeters. Stones that grow to occupy the renal collecting system (the pelvis and one or more renal calyx) are known as staghorn calculi, since they resemble the horns of a stag (Fig. 8.2). They are most commonly composed of struvite—magne- sium ammonium phosphate (being caused by infection and forming under the alkaline conditions induced by urea-splitting bacteria), but may consist of uric acid, cystine, or calcium oxalate monohydrate.

Figure 8.1 A left cystine stone, barely visible just below the midpoint of the 12th rib.

Figure 8.2 A large, right staghorn calculus.