Kidney Stones

A kidney stone is a hard mass developed from crystals that separate from the urine and build up on the inner surfaces of the kidney.

Kidney stones may be as small as a grain of sand or as large as a pearl. Some stones are even as big as golf balls. Stones may be smooth or jagged. They are usually yellow or brown.

Urolithiasis is the medical term used to describe stones occurring in the urinary tract. Gallstones and kidney stones are not related. They form in different areas of the body.

Types

There are four major types of kidney stones.

• Calcium stone. This is the most common type of kidney stones. It contains calcium in combination with either oxalate or phosphate. These chemicals are part of a person's normal diet and make up important parts of the body, such as bones and muscles.
  
• Struvite stone. This type of stone is caused by a bacterial infection in the urinary system. The stone contains ammonia resulting from specific enzymes produced by the bacteria.
  
• Uric acid stone. It may form when there is too much acid in the urine.
  
• Cystine stone. This type of stone is rare. Cystine is one of the building blocks that make up muscles, nerves, and other parts of the body. Cystine can build up in the urine to form a stone. The disease that causes cystine stones runs in families.
  
  

Symptoms

Kidney stones often do not cause any symptoms. Usually, the first symptom of a kidney stone is extreme pain, which occurs when a stone acutely blocks the flow of urine. The pain often begins suddenly when a stone moves in the urinary tract, causing irritation or blockage. Typically, a person feels a sharp, cramping pain in the back and side in the area of the kidney or in the lower abdomen. Sometimes nausea and vomiting occur. Later, pain may spread to the groin.

If the stone is too large to pass easily, pain continues as the muscles in the wall of the tiny ureter try to squeeze the stone along into the bladder. As a stone grows or moves, blood may appear in the urine. As the stone moves down the ureter closer to the bladder, you may feel the need to urinate more often or feel a burning sensation during urination.

If fever and chills accompany any of these symptoms, an infection may be present. In this case, you should contact a doctor immediately.

Diagnosis

Sometimes "silent" stones—those that do not cause symptoms—are found on x rays taken during a general health exam. If they are small, these stones would likely pass out of the body unnoticed.

More often, kidney stones are found on an x ray or sonogram taken on someone who complains of blood in the urine or sudden pain. These diagnostic images give the doctor valuable information about the stone's size and location. Blood and urine tests help detect any abnormal substance that might promote stone formation.

The doctor may decide to scan the urinary system using a special test called a CT (computed tomography) scan or an IVP (intravenous pyelogram). The results of all these tests help determine the proper treatment.

Causes

Normally, urine contains chemicals that prevent or inhibit the crystal formation from the substances that constitute a kidney stone. If the level of inhibiting chemicals is too low, or the concentration of stone constituents (calcium, oxalate, phosphate, ammonia, uric acid, or cystine) is too high, a kidney stone may develop.

The concentrations of chemicals in the urine depend on heredity, drugs, diet, and certain medical conditions. 

Hypercalciuria (elevated calcium level) is inherited. It is the cause of stones in more than half of patients. Calcium is absorbed from food in excess and is lost into the urine. This high level of calcium in the urine causes crystals of calcium oxalate or calcium phosphate to form in the kidneys or urinary tract.

Other causes of kidney stones are hyperuricosuria which is a disorder of uric acid metabolism, gout, excess intake of vitamin D, urinary tract infections, and blockage of the urinary tract. Certain diuretics which are commonly called water pills or calcium-based antacids may increase the risk of forming kidney stones by increasing the amount of calcium in the urine.

Calcium oxalate stones may also form in people who have a chronic inflammation of the bowel or who have had an intestinal bypass operation, or ostomy surgery. As mentioned above, struvite stones can form in people who have had a urinary tract infection. People who take the protease inhibitor indinavir, a drug used to treat HIV infection, are at risk of developing kidney stones. 

Cystinuria and hyperoxaluria are two other rare, inherited metabolic disorders that often cause kidney stones. In cystinuria, too much of the amino acid cystine, which does not dissolve in urine, is voided. This can lead to the formation of stones made of cystine. In patients with hyperoxaluria, the body produces too much of the salt oxalate. When there is more oxalate than can be dissolved in the urine, the crystals settle out and form stones.

In addition, more than 70 percent of people with a rare hereditary disease called renal tubular acidosis develop kidney stones.

Reference:

National Kidney and Urologic Diseases Information Clearinghouse, USA.