Kidney stones are one of the most common
disorders of the urinary tract. Kidney stones or renal stones are
gravel-like collections of chemicals that may appear in any area of the
urinary system, from the kidney to the bladder. They may be small or
large, single or multiple. One in every 20 people develops a kidney
stone at some point in their life.
The condition of having kidney stones is termed nephrolithiasis. Having stones at any location in the urinary tract is referred to as urolithiasis.
Types of stones-
Calcium oxalate stones
The most common type of kidney stone is composed of calcium oxalate crystals, occurring in about 80% of cases, and the factors that promote the precipitation of crystals in the urine are associated with the development of these stones.
Common cause is increased consumption of calcium in diet. However, current evidence suggests that the consumption of low-calcium diets is actually associated with a higher overall risk for the development of kidney stones. This is perhaps related to the role of calcium in binding ingested oxalate in the gastrointestinal tract. As the amount of calcium intake decreases, the amount of oxalate available for absorption into the bloodstream increases; this oxalate is then excreted in greater amounts into the urine by the kidneys. In the urine, oxalate is a very strong promoter of calcium oxalate precipitation, about 15 times stronger than calcium.
Uric acid (urate)
Multiple kidney stones composed of uric acid and a small amount of calcium oxalate.
About 5-10% of all stones are formed from uric acid. This might be occurring when fluid intake is less. They may occur when uric acid excretion is increased in urine. This is often found with gout and chemotherapy.
Other types of kidney stones are composed of struvite (magnesium, ammonium and phosphate); calcium phosphate; and cystine.
The formation of struvite stones is associated with the presence of urea-splitting bacteria, most commonly Proteus mirabilis (but also Klebsiella, Serratia, Providencia species). These organisms are capable of splitting urea into ammonia, decreasing the acidity of the urine and resulting in favorable conditions for the formation of struvite stones. Struvite stones are always associated with urinary tract infections.
Kidney stones form when there is a decrease in urine volume and/or an excess of stone-forming substances in the urine.
Dehydration from reduced fluid intake or strenuous exercise without adequate fluid replacement increases the risk of kidney stones. Obstruction to the flow of urine can also lead to stone formation. Kidney stones can also result from infection in the urinary tract.
A number of different medical conditions can lead to an increased risk for developing kidney stones:
1. Gout results in an increased amount of uric acid in the urine and can lead to the formation of uric acid stones.
2. Hypercalciuria (high calcium in the urine), another inherited condition, causes stones in more than half of cases. In this condition, too much calcium is absorbed from food and excreted into the urine, where it may form calcium phosphate or calcium oxalate stones.
3. Other conditions associated with an increased risk of kidney stones include hyperparathyroidism, kidney diseases such as renal tubular acidosis, and some inherited metabolic conditions including cystinuria and hyperoxaluria. Chronic diseases such as diabetes and high blood pressure (hypertension) are also associated with an increased risk of developing kidney stones.
4. People with inflammatory bowel disease or who have had an intestinal bypass or ostomy surgery are also more likely to develop kidney stones.
5. Some medications also raise the risk of kidney stones. These medications include some diuretics, calcium-containing antacids, and the protease inhibitor indinavir (Crixivan), a drug used to treat HIV infection.
Environmental and dietary factors-
Low urine volumes.
High ambient temperatures.