KIDNEY STONES

Kidney stones: epidemiology
~10% of Caucasian men will develop a kidney stone by the age of 70. Within 1 year of a calcium oxalate stone, 10% of men will form another calcium oxalate stone, and 50% will have formed another stone within 10 years. The prevalence of renal tract stone disease is determined by factors intrinsic to the individual and by extrinsic (environmental) factors. A combination of factors often contribute to risk of stone formation.

Intrinsic factors
- Age. The peak incidence of stones occurs between the ages of 20 50 years.
- Sex. Males are affected 3 times as frequently as females. Testosterone may cause increased oxalate production in the liver (predisposing to calcium oxalate stones) and women have higher urinary citrate concentrations (citrate inhibits calcium oxalate stone formation).
- Genetic. Kidney stones are relatively uncommon in Native Americans, Black Africans, and US Blacks, and more common in Caucasians and Asians. ~25% of patients with kidney stones report a family history of stone disease (the relative risk of stone formation remaining high after adjusting for dietary calcium intake). Familial renal tubular acidosis (predisposing to calcium phosphate stones) and cystinuria (predisposing to cystine stones) are inherited.

Extrinsic (environmental) factors
- Geographical location, climate, and season. The relationship between these factors and stone risk is complex. While renal stone disease is more common in hot climates, some endogenous populations of hot climates have a low incidence of stones (e.g. Black Africans, Aborigines) and many temperate areas have a high incidence of stones (e.g. Northern Europe and Scandanavia). This may relate to Western lifestyle excess food, inadequate fluid intake, limited exercise combined with a genetic predisposition to stone formation.
- Ureteric stones become more prevalent during the summer, the highest incidence occurring a month or so after peak summertime temperatures, presumably because of higher urinary concentration in the summer (encourages crystallization). Concentrated urine has a lower pH, encouraging cystine and uric acid stone formation. Exposure to sunlight may also increase endogenous vitamin D production, leading to hypercalciuria.
- Water intake. Low fluid intake (<1200ml/day) predisposes to stone formation. 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). High salt intake causes hypercalciuria. Contrary to conventional teaching, low calcium diets predispose to calcium stone disease, and high calcium intake is protective.
Occupation. Sedentary occupations predispose to stones compared with manual workers.

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.

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

Relatively radiolucent
Cystine stones are relatively radiodense because they contain sulphur. Magnesium ammonium phosphate (struvite) stones are less radiodense than calcium containing stones.

Completely radiolucent
Uric acid, triamterene, xanthine, indinavir (cannot be seen even on CTU).

Size and shape
Stones can be characterized by their size, in centimetres. Stones which 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. They are most commonly composed of struvite magnesium ammonium phosphate (being caused by infection and forming under the alkaline conditions induced by urea-splitting bacteria), but may be composed of uric acid, cystine, or calcium oxalate monohydrate.

Kidney stones: mechanisms of formation
Urine is said to be saturated with, for example, calcium and oxalate, when the product of the concentrations of calcium and oxalate exceeds the solubility product (Ksp). Below the solubility product, crystals of calcium and oxalate will not form and the urine is said to be undersaturated. Above the solubility product, crystals of calcium and oxalate should form, but they do not because of the presence of inhibitors of crystal formation. However, above a certain concentration of calcium and oxalate, inhibitors of crystallization become ineffective, and crystals of calcium oxalate start to form. The concentration of calcium and oxalate at which this is reached (i.e. at which crystallization starts) is known as the formation product (Kf) and the urine is said to be supersaturated with the substance or substances in question at concentrations above this level. Urine is described as being metastable for calcium and oxalate at concentrations between the solubility product of calcium and oxalate and the formation product.

The ability of urine to hold more solute in solution than can pure water is due partly to the presence of various inhibitors of crystallization (e.g. citrate forms a soluble complex with calcium, preventing it from combining with oxalate or phosphate to form calcium oxalate or calcium phosphate stones). Other inhibitors of crystallization include magnesium, glycosaminoglycans, and Tamm Horsfall protein.
Periods of intermittent supersaturation of urine with various substances can occur as a consequence of dehydration and following meals.

The earliest phase of crystal formation is known as nucleation. Crystal nuclei usually form on the surfaces of epithelial cells or on other crystals. Crystal nuclei form into clumps a process known as aggregation. Citrate and magnesium not only inhibit crystallization but also inhibit aggregation.