Kidney stones are stones formed from chemicals in urine that crystallize and stick together. Some cause problems, and others we may never know they're there.

If you've ever passed a kidney stone, you're not likely to forget the experience - it can be excruciatingly painful. What's more, kidney stones (renal lithiasis) are increasingly common.

Not all kidney stones cause symptoms. They're often discovered when you have X-rays for an unrelated condition or when you seek medical care for problems such as blood in your urine or recurring urinary tract infections. The pain becomes agonizing only when a stone breaks loose and begins to work its way down from your kidneys to your bladder.

• Urinary tract infection - (UTI)

Kidney stones usually form when your urine becomes too concentrated. This causes minerals and other substances in urine to form crystals on the inner surfaces of your kidneys. Over time, these crystals may combine to form a small, hard mass, or stone.

Small stones can cause some discomfort as they pass out of the body. Regardless of size, stones may pass out of the kidney, become lodged in the tube that carries urine from the kidney to the bladder (ureter), and cause severe pain that begins in the lower back and radiates to the side or groin. A lodged stone can block the flow of urine, causing pressure to build in the affected ureter and kidney. Increased pressure results in stretching and spasm, which cause severe pain.

Most small kidney stones pass into your bladder without causing any permanent damage. Still, it's important to determine the underlying cause so that you don't form more stones in the future. In many cases, you can prevent kidney stones simply by drinking more water and making a few dietary changes.

Signs and symptoms

You're not likely to have signs and symptoms unless a kidney stone is large, causes a blockage, is associated with an infection or is being passed. Then the most common symptom is an intense, colicky pain that may fluctuate in intensity over periods of 5 to 15 minutes. The pain usually starts in your back or your side just below the edge of your ribs. As the stone moves down the ureter toward your bladder, the pain may radiate to your groin. If the stone stops moving, the pain may stop too. Other signs and symptoms may include:

• Bloody, cloudy or foul-smelling urine

• Nausea and vomiting

• Persistent urge to urinate

• Fever and chills if an infection is present

• vomiting

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).

Risk factors

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, work in a hot environment such as a commercial kitchen 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, sex and race. Most people who develop kidney stones are between 20 and 40 years of age. Men are more likely to develop kidney stones than women are, although for unknown reasons the number of women with kidney stones is increasing. In addition, white Americans are at higher risk of kidney stones than are black Americans.

• Certain diseases. Rare, inherited diseases such as renal tubular acidosis and cystinuria can increase your risk of kidney stones. So can more common disorders such as gout, chronic urinary tract infections, cystic kidney disease and hyperparathyroidism.

• Certain medications. Medications can have variable effects on stone formation. For example, diuretics may increase your risk of developing kidney stones in some situations and decrease it in others. If you're at risk, check with your doctor or pharmacist about any medications you take.

• Diet. A diet that's high in protein (meat, chicken and fish) and low in fiber (fruits, vegetables and whole grains) 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 period of time. That's because limited activity can cause your bones to release more calcium.