Kidney stones are small, solid masses that form when salts or minerals normally found in urine become solid crystals (crystallise) inside the kidney. In most cases, the crystals are too tiny to be noticed, and pass harmlessly out of your body. However, they can build up inside your kidney and form much larger stones.
What causes kidney stones?
In most people the underlying cause isn't known. Some people with kidney stones have lower levels of citrate in their urine, which usually stops the stones forming.
Different kinds of kidney stones form from different salts in the urine.
Calcium stones are the most common type of kidney stone. They are either spiky or large and smooth, and are made up of calcium oxalate or calcium phosphate.
Calcium stones are more common in people who have excess levels of vitamin D or who have an overactive parathyroid gland. People who have medical conditions such as cancer, some kidney diseases, or a disease called sarcoidosis are also more likely to develop calcium stones.
Uric acid stones are smooth, brown and soft. Excess amounts of uric acid can be caused by eating a lot of meat. Conditions such as gout and treatments such as chemotherapy can also increase the risk of getting uric acid stones.
Struvite stones (infection stones) are usually large and have a horn-like shape. They develop when there is too much ammonia in the urine. This can happen if you have a urinary tract infection (UTI), because the bacteria that cause these infections can generate ammonia. This kind of kidney stone is most often found in women.
Cystine stones are yellow and crystalline. They develop if you have high levels of cystine in your urine, which happens if you have a hereditary disorder called cystinuria. Only one in a hundred kidney stones are caused by this condition. Cystine stones tend to develop earlier in life than other kidney stones, usually between the ages of 10 and 30 years.
Who gets kidney stones?
Men are more prone than women, and around half of all people who have previously had a kidney stone will develop another one within five years.
Other risk factors include:
* a family history of kidney stones
* being aged between 20 and 40
* taking certain medicines such as diuretics (water tablets), antacids and thyroid medications
* having only one kidney, or an abnormally shaped kidney
* eating a diet high in protein
* being regularly dehydrated
* having very poor mobility (eg, being confined to bed)
* having a disease of the small intestine or a small intestinal bypass
http://hcd2.bupa.co.uk/fact_sheets/html/Kidney_stones.html
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