Reviewed by Dr. Clement J. Cheng
A kidney stone develops from crystals that separate from urine and build up on the inner surfaces of the kidney. Normally, urine contains chemicals that prevent or inhibit the crystals from forming. These inhibitors do not seem to work for everyone, however, and some people’s bodies form stones. If the crystals remain tiny enough, they will travel through the urinary tract and pass out of the body in the urine without even being noticed.
Kidney stones may contain various combinations of chemicals. The most common type of stone 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.
A less common type of stone is caused by infection in the urinary tract. This type of stone is called a struvite, or infection stone. Much less common are the uric acid stone and the rare cystine stone. Urolithiasis is the medical term used to describe stones occurring in the urinary tract. Other frequently used terms are urinary tract stone disease and nephrolithiasis. Doctors also use terms that describe the location of the stone in the urinary tract. For example, a ureteral stone (or ureterolithiasis) is a kidney stone found in the ureter. To keep things simple, the term "kidney stones" is used throughout this document.
Gallstones and kidney stones are not related. They form in different areas of the body. If a person has a gallstone , he or she is not necessarily more likely to develop kidney stones.
For some unknown reason, the number of persons in the United States with kidney stones has been increasing over the past 20 years. White people are more prone to kidney stones than are black people. Although stones occur more frequently in men, the number of women who get kidney stones has been increasing over the past 10 years, causing the ratio to change. Kidney stones strike most people between the ages of 20 and 40. Once a person gets more than one kidney stone, he or she is more likely to develop others.
Doctors do not always know what causes a stone to form. While certain foods may promote stone formation in people who are susceptible, scientists do not believe that eating any specific food causes stones to form in people who are not susceptible.
A person with a family history of kidney stones may be more likely to develop stones. Urinary tract infections, kidney disorders such as cystic kidney diseases, and metabolic disorders such as hyperparathyroidism are also linked to stone formation. In addition, more than 70 percent of patients with adequate hereditary disease called renal tubular acidosis develop kidney stones. Cystinuria and hyuperoxaluria are two other rare inherited metabolic disorders that often cause kidney stones. In cystinuria, the kidneys produce too much of the amino acid cystine. Cystine does not dissolve in urine and can build up to form stones. 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.
Absorptive hypercalciuria occurs when the body absorbs too much calcium from food and empties the extra calcium 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 (a disorder of uric acid metabolism), gout, excess intake of vitamin D, and blockage of the urinary tact. Certain diuretics (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. Struvite stones can form in people who have had a urinary tract infection.
Kidney stones are more common among young and middle-aged people than elderly people, and are much more prevalent in men than in women. People who live in hot climates are more likely to develop them as well, since dehydration may lead to excess minerals in urine.
