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.
Hit: 819 times
 Print Health Information
Health Home
| |
Bookmark Health Information
Make Health Information Homepage
Advertise with Draligus.org