Kidney stones

Kidney stones (renal lithiasis) are small, hard deposits of mineral and acid salts on the inner surfaces of your kidneys. Normally, the substances that make up kidney stones are diluted in the urine. When urine is concentrated, though, minerals may crystallize, stick together and solidify. The result is a kidney stone. Most kidney stones contain calcium.

Passing kidney stones can be excruciating. The pain they cause typically starts in your side or back, just below your ribs, and radiates to your lower abdomen and groin.

Painful as they are, kidney stones usually cause no permanent damage. Medical intervention — apart from pain medication — is often unnecessary.

Still, it's important to find out what type of kidney stone you have and why it developed. Some of the underlying causes of kidney stones can be treated to prevent new stones from forming. If no specific treatment exists, you may be able to stave off additional kidney stones simply by drinking more water and making a few dietary changes.

Symptoms

Until a kidney stone moves into the ureter — the tube connecting the kidney and bladder you may not know you have it. At that point, these signs and symptoms may occur:

Pain in the side and back, below the ribs

Fluctuations in pain intensity, with periods of pain lasting 20 to 60 minutes

Pain waves radiating from the side and back to the lower abdomen and groin

Bloody, cloudy or foul-smelling urine

Pain on urination

Nausea and vomiting

Persistent urge to urinate

Fever and chills if an infection is present

Kidney stones that don't cause these symptoms may show up on X-rays when you seek medical care for other problems, such as blood in your urine or recurring urinary tract infections.

Type of stones

Most kidney stones contain crystals of more than one type. Determining the type that makes up the bulk of the stone  usually a combination of calcium compounds — helps identify the underlying cause. The best preventive approach after your first kidney stone also depends partly on the stone's composition.

Calcium stones.
Roughly four out of five kidney stones are calcium stones, usually in the form of calcium oxalate. Oxalate is found in some fruits and vegetables, but the liver produces most of the body's oxalate supply. Dietary factors, high doses of vitamin D, intestinal bypass surgery and several different metabolic disorders can increase the concentration of calcium or oxalate in urine.

Struvite stones.
Found more often in women, struvite stones are almost always the result of urinary tract infections. Struvite stones may be large enough to fill most of a kidney's urine-collecting space, forming a characteristic stag's-horn shape.

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 eat a high-protein diet. Gout also leads to uric acid stones. Certain genetic factors and disorders of the blood-producing tissues also may predispose you to the condition.

Cystine stones.
 These stones represent only a small percentage of kidney stones. They form in people with a hereditary disorder that causes the kidneys to excrete excessive amounts of certain amino acids (cystinuria).

Causes

Kidney stones form when the components of urine — fluid and various minerals and acids — are out of balance. When this happens, your urine contains more crystal-forming substances, such as calcium and uric acid, than the available fluid can dilute. At the same time, your urine may be short of substances that keep crystals from sticking together and becoming stones. Kidney stones are also prone to develop in highly acidic or highly alkaline urine.

Problems in the way your system absorbs and eliminates calcium and other substances create the conditions for kidney stones to form. Sometimes, the underlying cause is an inherited metabolic disorder or kidney disease. Gout promotes specific types of kidney stones, as does inflammatory bowel disease. So do some drugs, including furosemide (Lasix), used in treating heart failure and high blood pressure; topiramate (Topamax), an anti-seizure drug; and indinavir (Crixivan), which is used to treat human immunodeficiency virus, the cause of AIDS.

It's common, however, for kidney stones to have no definite, single cause. A number of factors, often in combination, create the conditions in which susceptible people develop kidney stones.

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