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justawoman · 12-03-2005, 11:56 AM

Causes
Your kidneys are two bean-shaped organs, each about the size of your fist. They're located in back of your abdomen on either side of your spine, and their main function is to remove excess fluid and waste from your blood in the form of urine. The ureters carry urine from your kidneys to your bladder, where it's stored until you eliminate it from your body.

The crystals that lead to kidney stones are likely to form when your urine contains a high concentration of certain substances — especially calcium, oxalate, uric acid and cystine — or low levels of substances that help prevent crystal formation, such as citrate and magnesium. Crystals also may form if your urine becomes too concentrated or is too acidic or too alkaline.

A number of factors can cause changes in your urine, including the effects of heredity, diet, drugs, climate, lifestyle factors and certain medical conditions. Each of the four main types of kidney stones has a different cause:

Calcium stones. Approximately 75 percent to 85 percent of all kidney stones are calcium stones. These stones are usually a combination of calcium and oxalate, a compound that occurs naturally in some fruits and vegetables. A number of factors can cause high concentrations of these substances in urine. Excess calcium, for instance, may result from ingesting large amounts of vitamin D, from treatment with thyroid hormones or certain diuretics, and from some cancers and kidney conditions. You may also have high levels of calcium if your parathyroid glands, which regulate calcium metabolism, are overactive (hyperparathyroidism). On the other hand, certain genetic factors, intestinal bypass surgery and a diet high in oxalic acid may cause excess amounts of oxalate in your body.
Uric acid stones. These stones are formed of uric acid, a byproduct of protein metabolism. You're more likely to develop uric acid stones if you've undergone chemotherapy, you eat a high-protein diet or you have certain genetic factors that predispose you to the condition.
Struvite stones. Found more often in women than in men, struvite stones are almost always the result of chronic urinary tract infections caused by bacteria that produce specific enzymes. These enzymes increase the amount of ammonia in the urine, which is incorporated in the crystals of struvite stones. These stones are often large and have a characteristic stag's horn shape that can seriously damage your kidneys.
Cystine stones. These stones represent only about 1 percent of kidney stones. They form in people with a hereditary disorder that causes the kidneys to excrete excessive amounts of certain amino acids (cystinuria).

Prevention
For people with a history of kidney stones, doctors usually recommend passing at least 2 1/2 quarts of urine a day. To do this, you'll need to drink about 3 1/2 quarts (14 cups) of fluids every day — and even more if you live in a hot, dry climate. Although most liquids count, water is best.

In addition, if you tend to form calcium stones — a combination of calcium and oxalate — your doctor may recommend restricting foods rich in oxalates. These include meats — especially organ meats, such as liver and kidneys — chicken, fish such as herring and anchovies, and asparagus, berries, chocolate, cooked spinach, Swiss chard and rhubarb. What's more, studies show that an overall diet low in salt and very low in animal protein can greatly reduce your chance of developing kidney stones.

Restricting your intake of calcium, on the other hand, doesn't seem to lower your risk. In fact, researchers have found that women with the highest calcium intake are less likely to develop kidney stones than women who consume less calcium. Why? Dietary calcium binds with oxalates in the gastrointestinal tract so that oxalates can't be absorbed from the intestine and excreted by the kidney to form stones.

Calcium supplements seem to have the same protective effect, but only if they're taken with meals. When taken on an empty stomach, the calcium can't bind with the oxalates in food.

Medications
Kidney stone medications control the level of acidity or alkalinity in your urine. The type of medication your doctor prescribes will depend on the kind of kidney stones you have:

Calcium stones. To help prevent calcium stones from forming, your doctor may prescribe a thiazide diuretic or a phosphate-containing preparation. If you have calcium stones because of a condition known as renal tubular acidosis, your doctor may suggest taking sodium bicarbonate or potassium bicarbonate.
Uric acid stones. Your doctor may prescribe allopurinol (Zyloprim, Aloprim) and a medicine to keep your urine alkaline. In some cases, allopurinol may also indirectly dissolve the uric acid stones.
Struvite stones. To prevent struvite stones, the first goal is to keep urine free of bacteria that cause infection. Your doctor may prescribe the drug acetohydroxamic acid (Lithostat) in addition to antibiotics.
Cystine stones. Cystine stones are the hardest stones and the most difficult to treat. Your doctor may prescribe certain medications when all other approaches have failed.