A kidney stone is a hard, crystalline mineral material formed within the kidney or urinary tract. Kidney stones are a common cause of blood in the urine (hematuria) and often severe pain in the abdomen, flank, or groin. Kidney stones are sometimes called renal calculi.
The condition of having kidney stones is termed nephrolithiasis. Having stones at any location in the urinary tract is referred to asurolithiasis, and the termureterolithiasis is used to refer to stones located in the ureters.
Who is at risk for kidney stones?
Anyone may develop a kidney stone, but people with certain diseases and conditions (see below) or those who are taking certain medications are more susceptible to their development. It is estimated that one out of every 10 people in the U.S. will develop stones in the urinary tract at some point in their lives. Most urinary stones develop in people 20-49 years of age, and those who are prone to multiple attacks of kidney stones usually develop their first stones during the second or third decade of life.
In residents of industrialized countries, kidney stones are more common than stones in the bladder. The opposite is true for residents of developing areas of the world, where bladder stones are the most common. This difference is believed to be related to dietary factors. Urinary tract stones are about three times more common in males than in females. The prevalence of kidney stones begins to rise when men reach their 40s, and it continues to climb into their 70s. A Caucasian male has a one in eight chance of developing a kidney stone by age 70. People who have already had more than one kidney stone are prone to developing further stones.
A family history of kidney stones is also a risk factor for developing kidney stones. Kidney stones are more common in Asians and Caucasians than in Native Americans, Africans, or African Americans.
Uric acid kidney stones are more common in people with chronically elevated uric acid levels in their blood.
A small number of pregnant women (about one out of every 1,500-3,000 pregnancies) develop kidney stones, and there is some evidence that pregnancy-related changes may increase the risk of stone formation. Factors that may contribute to stone formation during pregnancy include a slowing of the passage of urine due to increased progesterone levels and diminished fluid intake due to a decreasing bladder capacity from the enlarging uterus. Healthy pregnant women also have a mild increase in their urinary calcium excretion. However, it remains unclear whether the changes of pregnancy are directly responsible for kidney stone formation or if these women have another underlying factor that predisposes them to kidney stone formation.
What causes kidney stones?
Kidney stones form when there is a decrease in urine volume and/or an excess of stone-forming substances in the urine. The most common type of kidney stone contains calcium in combination with either oxalate or phosphate. Other chemical compounds that can form stones in the urinary tract include uric acid and the amino acid cystine.
Dehydration from reduced fluid intake or strenuous exercise without adequate fluid replacement increases the risk of kidney stones. Obstruction to the flow of urine can also lead to stone formation. In this regard, climate may be a risk factor for kidney stone development, since residents of hot and dry areas are more likely to become dehydrated and susceptible to stone formation.
Kidney stones can also result from infection in the urinary tract; these are known as struvite or infection stones.
A number of different medical conditions can lead to an increased risk for developing kidney stones:
- Gout results in chronically increased amount of uric acid in the blood and urine and can lead to the formation of uric acid stones.
- Hypercalciuria (high calcium in the urine), another inherited condition, causes stones in more than half of cases. In this condition, too much calcium is absorbed from food and excreted into the urine, where it may form calcium phosphate or calcium oxalate stones.
- Other conditions associated with an increased risk of kidney stones include hyperparathyroidism, kidney diseases such as renal tubular acidosis, and some inherited metabolic conditions, including cystinuriaand hyperoxaluria. Chronic diseases such as diabetes and high blood pressure (hypertension) are also associated with an increased risk of developing kidney stones.
- People with inflammatory bowel disease or who have had an intestinal bypass or ostomy surgery are also more likely to develop kidney stones.
- Some medications also raise the risk of kidney stones. These medications include some diuretics, calcium-containing antacids, and the protease inhibitor indinavir (Crixivan), a drug used to treat HIVinfection.
- Dietary factors and practices may increase the risk of stone formation in susceptible individuals. In particular, inadequate fluid intake predisposes to dehydration, which is a major risk factor for stone formation. Other dietary practices that may increase an individual's risk of forming kidney stones include a high intake of animal protein, a high-salt diet, excessive sugar consumption, excessive vitamin D supplementation, and possible excessive intake of oxalate-containing foods such as spinach. Interestingly, low levels of dietary calcium intake may alter the calcium-oxalate balance and result in the increased excretion of oxalate and a propensity to form oxalate stones.
