Kidney stones are small, hard deposits of mineral and acid salts on the inner surfaces of your kidneys.

When a stone moves, blocking the flow of urine from the kidney, the pain can be sudden and excruciating.

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.

Kidney stones can be found unexpectedly if they show up on X-rays when you seek medical care for other problems. Or they may be found during evaluation of recurring urinary infections or bleeding. Until a kidney stone moves into the ureter - the tube connecting the kidney and bladder - you may not know you have it. When a stone moves, blocking the flow of urine from the kidney, the pain can be sudden and excruciating, and these symptoms may occur:

• Pain in the side and back, below the ribs radiating back to the lower abdomen and groin
• Fluctuations in pain intensity, with periods of pain lasting 20 to 60 minutes
• Bloody urine
• Nausea and vomiting
• Persistent urge to urinate

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. 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.

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).

These factors may increase your risk of developing kidney stones:

• Lack of fluids. If you don't drink enough fluids, especially water, your urine is likely to have higher concentrations of substances that can form stones. That's also why you're more likely to form kidney stones if you live in a hot, dry climate or exercise strenuously without replacing lost fluids.
• Family or personal history. If someone in your family has kidney stones, you're more likely to develop stones too. And if you've already had one or more kidney stones, you're at increased risk of developing another.
• Age and sex. Most people who develop kidney stones are between 20 and 70 years of age. Men are more likely to develop kidney stones than are women.
• Diet. A high-protein, high-sodium and low-calcium diet may increase your risk of some types of kidney stones.
• Limited activity. You're more prone to develop kidney stones if you're bedridden or very sedentary for a long period of time. That's partly because limited activity can cause your bones to release more calcium.
• Obesity. High body mass index (BMI), increased waist size and weight gain have been linked to kidney stones in long-term studies of large populations. The relationship is strongest in women.
• High blood pressure. Having high blood pressure doubles your risk of forming kidney stones.
• Gastric bypass surgery, inflammatory bowel disease or chronic diarrhea. Changes in the digestive process affect your absorption of calcium and increase the levels of stone-forming substances in your urine.

DIAGNOSIS

If your doctor suspects you have kidney stones, you're likely to have one or more of the following imaging tests:

• Computerized tomography (CT) scan. This imaging test has become the standard of care for evaluating acute kidney stones. It's rapidly performed, can identify stones regardless of composition and doesn't require the use of contrast dye.
• Abdominal X-ray. An abdominal X-ray can visualize most kidney stones and can help to judge changes in the size of a stone over time.
• Ultrasound. Instead of X-rays, this diagnostic technique combines high-frequency radio waves and computer processing to view your internal organs. It's safe, painless and noninvasive, but it may miss small stones, especially if they're located in a ureter or your bladder.
• Intravenous pyelography (excretory urogram). This study can be useful in determining the location of stones in the urinary system and can define the degree of blockage caused by a stone. A contrast dye is injected into a vein in your arm and a series of X-rays is taken as the dye moves through your kidneys, ureters and bladder.
• In addition, you may need a blood analysis to look for kidney function and excess calcium or uric acid. A 24-hour collection of urine is used to check whether you're excreting too many stone-forming minerals or too few inhibiting substances.

TREATMENT

Treatment for kidney stones varies, depending on the type of stone and the cause. You may be able to move a stone through your urinary tract simply by drinking plenty of water - as much as 2 to 3 quarts (1.9 to 2.8 liters) a day - and by staying physically active. Pain medications, anti nausea drugs and alpha blockers may be used to help you pass a stone.

Stones that can't be treated with more-conservative measures - either because they're too large to pass on their own or because they cause bleeding, kidney damage or ongoing urinary tract infections - may need professional treatment. Procedures include:

• Extracorporeal shock wave lithotripsy (ESWL). This is a commonly used procedure for treating kidney stones. It uses shock waves to break the stones into tiny pieces that are then passed in your urine. ESWL works best for stones in the kidney or upper part of the ureter. This is an outpatient treatment that does require an anesthetic. Your doctor will likely use X-rays or ultrasound to help determine the position of the stone as well as to monitor the status of the stone during treatment.
  
  Following ESWL, you may experience blood in the urine, bruising on the back or abdomen, bleeding around the kidney or other adjacent organs, and discomfort as the stone fragments pass through the urinary tract. In addition, if the stone doesn't shatter completely, you may need a second round of ESWL or ureteroscopic stone removal. After treatment, it may take weeks for all the stone fragments to pass.
• Percutaneous nephrolithotomy. When ESWL isn't effective, or the stone is very large, your surgeon may remove your kidney stone through a small incision in your back using an instrument called a nephroscope.
• Ureteroscopic stone removal. With this procedure, a miniature scope is passed through the urethra (no incisions), into the bladder, and up the ureter to the stone. Lasers or other energy sources can be used to fragment the stone, then the pieces can be removed with grasping tools. These methods work especially well on stones in the lower part of the ureter. Ureteroscopy is an outpatient treatment that requires an anesthetic. Afterward, you may have bloody urine, urinary irritation, and pain in your back or side. If your doctor suspects the stone injured the ureter or swelling may block the kidney, a temporary tube (stent) can be used to assure drainage of urine from the kidney. The tube can be easily removed in the office.

PREVENTION

In many cases, you can prevent kidney stones by making a few lifestyle changes. If these measures aren't effective and blood and urine tests reveal a correctable chemical imbalance or that the stones you have are getting bigger, your doctor may prescribe certain medications.

Lifestyle changes
For people with a history of kidney stones, doctors usually recommend passing at least 2.5 quarts (2.3 liters) of urine a day. To do this, you'll need to drink about 14 cups (3.3 liters) of fluids every day - and even more if you live in a hot, dry climate.

What should you drink? Water is best. Include a glass of lemonade every day, too. Make your own with real lemons, or use a liquid or frozen concentrate, but avoid powdered lemonade mixes. Lemonade increases the levels of citrate in your urine, and citrate helps prevent stone formation.

In addition, if you tend to form calcium oxalate stones, your doctor may recommend restricting foods rich in oxalates. These include rhubarb, star fruit, beets, beet greens, collards, okra, refried beans, spinach, Swiss chard, sweet potatoes, sesame seeds, almonds and soy products. 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.

As a general rule, restricting your intake of calcium doesn't seem to lower your risk. In fact, researchers have found that women with a high calcium intake are less likely to develop kidney stones than are 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.

An exception to this rule occurs when an individual absorbs too much dietary calcium from the intestine. In such a circumstance, restricting calcium intake is useful.

Calcium supplements seem to have the same protective effect as dietary calcium, but only if they're taken with meals.

Medications
Medications can control the level of acidity or alkalinity in your urine and may be helpful in people who form certain kinds of stones. 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) to reduce uric acid levels in your blood and urine and a medicine to keep your urine alkaline. In some cases, allopurinol and an alkalinizing agent may dissolve the uric acid stones.
• Struvite stones. To prevent struvite stones, the first goal is to keep urine free of bacteria that cause infection. Long-term use of antibiotics in small doses may be useful to achieve this goal.
• Cystine stones. Cystine stones are the hardest stones and the most difficult to treat. Your doctor may prescribe certain medications to alkalinize the urine or to bind the cystine in the urine in addition to recommending an extremely high urine output.