What is a kidney stone?
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 as urolithiasis, and the term ureterolithiasis 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. Urinary tract stones are more common in men than in women. It is estimated that about 12% of men and 7% of women in the U.S. will develop stones in the urinary tract at some point in their lives. About 20 million people seek medical care each year because of kidney stones. 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. People who have already had more than one kidney stone are prone to developing further stones.
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. People who live in the southern or southwestern regions of the U.S. have a higher rate of kidney stone formation than those living in other areas. Over the last few decades, the percentage of people with kidney stones in the U.S. has been increasing; the reason for this is not well understood.
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 (hyperuricemia).
A small number of pregnant women (about one out of every 1,500-3,000 pregnancies) develop kidney stones, and there is some evidence thatpregnancy-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. About 75% of kidney stones are calcium stones. Other chemical compounds that can form stones in the urinary tract include uric acid, magnesium ammonium phosphate (which forms struvite stones; see below), 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. Metabolic abnormalities, including inherited disorders of metabolism, can alter the composition of the urine and increase an individual’s risk of stone formation.
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 other 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 are also more likely to develop kidney stones.
- Those who have undergone intestinal bypass or ostomy surgeryare also at increased risk for 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 HIV infection.
- 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.
What are kidney stones symptoms and signs?
While some kidney stones may not produce symptoms (known as “silent” stones), people who have kidney stones often report the sudden onset of excruciating, cramping pain in their low back and/or side, groin, or abdomen. Changes in body position do not relieve this pain. The abdominal, groin, and/or back pain typically waxes and wanes in severity, characteristic of colicky pain (the pain is sometimes referred to as renal colic). It may be so severe that it is often accompanied by nausea and vomiting. The pain has been described by many as the worst pain of their lives, even worse than the pain of childbirth or broken bones. Kidney stones also characteristically cause blood in the urine. If infection is present in the urinary tract along with the stones, there may be fever and chills. Sometimes, symptoms such as difficulty urinating, urinary urgency, penile pain, or testicular pain may occur due to kidney stones.
How are kidney stones diagnosed?
The diagnosis of kidney stones is suspected when the typical pattern of symptoms is noted and when other possible causes of the abdominal or flank pain are excluded. Imaging tests are usually done to confirm the diagnosis. A helical CT scan without contrast material is the most common test to detect stones or obstruction within the urinary tract. Formerly, an intravenous pyelogram (IVP; an X-ray of the abdomen along with the administration of contrast dye into the bloodstream) was the test most commonly used to detect urinary tract stones, but this test has a greater risk of complications, takes longer, and involves higher radiation exposure than the non-contrasted helical CT scan. Helical CT scans have been shown to be a significantly more effective diagnostic tool than the IVP in the diagnosis of kidney or urinary tract stones.
In pregnant women or those who should avoid radiation exposure, anultrasound examination may be done to help establish the diagnosis.
How can kidney stones be prevented?
Rather than having to undergo treatment, it is best to avoid kidney stones in the first place when possible. It can be especially helpful to drink more water, since low fluid intake and dehydration are major risk factors for kidney stone formation.
Depending on the cause of the kidney stones and an individual’s medical history, dietary changes or medications are sometimes recommended to decrease the likelihood of developing further kidney stones. If one has passed a stone, it can be particularly helpful to have it analyzed in a laboratory to determine the precise type of stone so specific prevention measures can be considered.
People who have a tendency to form calcium oxalate kidney stones may be advised to limit their consumption of foods high in oxalate, such as spinach, rhubarb, Swiss chard, beets, wheat germ, and peanuts.
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