Nearly 12 percent of men and 5 percent of women in the United States will feel a kidney stone during their life—and it's not pleasant. Although they're often described as "the worst pain I've ever experienced," kidney stones rarely cause long-term damage. Kidney stones can occur at any age, but men are more likely to develop stones between the ages of 40 and 70, while women are more likely to develop a kidney stone in their 50s. Often kidney stones are a recurring problem; having had one kidney stone puts you at increased risk for a second.
Medically known as urolithiasis or nephrolithiasis, kidney stones can form anywhere along the urinary tract, including the kidneys, bladder, ureter, or urethra. A kidney stone starts as a microscopic crystal. Over time, the crystal enlarges or multiple crystals can aggregate to form a single large kidney stone. Pain develops when the stone breaks away and becomes lodged in the urinary tract, either partially or totally blocking the passage of urine. Other common names for a kidney stone include renal calculus, renal stone, ureteral calculus or stone, bladder calculus, or urethral stone. Kidney stones are totally unrelated to gallstones, which form in the gallbladder.
When a stone is in the kidneys, many people are totally unaware of its presence. These stones are often incidentally discovered on an X-ray or other imaging technique performed for an unrelated medical issue. If shape and size permit, kidney stones sometimes pass without significant discomfort after they break loose. However, even a small stone can wreak havoc as it moves down the urinary tract.
Each year nearly 3.3 million Americans seek out medical care for kidney stone removal and pain relief at a cost of about $5.3 billion per year. Treatment strategies include waiting for the stone to pass, physically breaking the stone into fragments, or surgically removing it; often some combination of the three approaches is required. Since people who have had one kidney stone are likely to develop more, kidney stone treatment involves not only addressing the immediate problem but also learning how to prevent formation of additional kidney stones.
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The urinary tract includes the kidneys, ureter, bladder, and the urethra. Urine is produced in the kidneys, two bean-shaped organs the size of your fist located in the lower back, flanking the spine. The kidneys release urine into the ureter, a small muscular tube that can dilate up to 7 millimeters in diameter (about 1/4 of an inch) and connects the kidney to the bladder where urine is stored. When the bladder is full, we release the urine into a narrow tube, the urethra, which channels the urine out of the body.
Each day, nearly one quarter of the blood that's pumped by the heart flows through the kidneys into a network of tiny blood vessels that filter the blood. Together the kidneys contain 1 million tiny structures called nephrons that filter this blood, directing the waste products into tubules. In the tubules the wastes are further processed and partially reabsorbed to generate roughly 1 to 3 quarts of urine that the body needs to eliminate each day. The waste products in the blood are derived from the foods we eat and the normal breakdown of tissue in the body. Not only do the kidneys keep the blood chemically balanced by converting waste products into urine and discharging excess water, but they also produce hormones that are needed to stimulate bone marrow to make red blood cells (erythropoietin), regulate blood pressure (renin), and help maintain healthy calcium levels in the bones and body (calcitriol).
In general, kidney stones form when specific compounds in the urine become overly concentrated, causing minerals to crystallize. This usually happens near the end of tubules, before fluid leaves the kidney. Initially, the crystals are the size of tiny grains of sand, but over time they can aggregate to form a pebble about a half inch in diameter up to the size of a golf ball. Stones can be relatively smooth and round or irregularly shaped, like an antler. There are four main types of kidney stones, each of which is associated with different chemical conditions in the urinary tract.
- Calcium oxalate stones make up nearly80 percent of the kidney stones in the United States. These stones, composed of calcium oxalate or calcium oxalate mixed with calcium phosphate, are hard, like a piece of gravel. Calcium oxalate stone formation is driven by the relative concentrations of certain chemicals in the urine; too much calcium, too much oxalate (a chemical compound that occurs naturally in many plants we eat), too little citrate (a chemical that inhibits minerals from crystallizing in the urine), or highly concentrated urine from decreased fluid intake can lead to the formation of calcium oxalate stones. In many families, there appears to be a genetic predisposition for developing the conditions that favor calcium stone formation, but the genetics are not well understood. Other more rare causes of calcium stone formation include ingesting excessive amounts of vitamin D or calcium, hyperactive parathyroid glands, sarcoidosis (a condition causing small nodule lesions to form in the body), intestinal bypass surgery, or chronic inflammation of the bowel.
- Uric acid stones tend to form in concentrated, acidic urine. About 5 to 10 percent of stones consist of uric acid, a byproduct of protein metabolism. Uric acid stones are associated with early stages of diabetes, a high-protein diet, chronic diarrhea, gout, and genetic factors—such as certain Middle Eastern ethnicities—that can predispose one to developing uric acid stones.
- Pure calcium phosphate stones tend to form in urine that is too alkaline, a condition often caused by renal tubular acidosis. Renal tubular acidosis can be an inherited genetic disorder or arise later in life in association with other disease processes. Overly alkaline urine can also result from chronic ingestion of high quantities of antacids.
- Struvite stones are relatively soft and almost always the result of chronic urinary tract infections caused by specific strains of bacteria. These bacteria produce enzymes that change the composition of urine by increasing the concentration of ammonia and the pH. The resulting ammonia-rich and alkaline environment favors the formation of magnesium ammonium phosphate crystals, the minerals composing struvite stones. Struvite stones tend to grow quickly, incorporating the bacteria in the stone itself. Struvite stones must be removed surgically to eliminate the bacteria causing the urinary tract infection.
- Cystine stones are rare and are associated with cystinuria, an inherited metabolic disorder. In affected individuals, the kidneys are not able to reabsorb cystine, leading to high levels of citrate in the urine, where it can crystallize to form cystine kidney stones. With this rare disease, cystine stones can start forming in young infants and continue throughout their life. Cystine stones are one of the physically hardest types of kidney stone.
The risk factors for kidney stones fall into two categories: those you can control through lifestyle choices and those traits you are born with. Having a risk factor doesn't mean that you'll develop a disease or condition. Medical research continues to reveal how risk factors interact to influence a person's health and life span. However, understanding your risk allows you to balance the value you place on your health with the risk that may compromise your health in the future. In the United States, the incidence of kidney stones is on the rise, especially in young children. Many experts attribute this increase to changes in lifestyle such as poor diet and a decrease in water consumption.
Risk factors for forming a kidney stone include:
- Familial or personal history of kidney stones. If a first-degree relative (mother, father, or sibling) has had a kidney stone, your risk of developing a kidney stone increases dramatically. If you have had a kidney stone, your risk of developing a subsequent one increases dramatically. The risk of developing subsequent kidney stones increases with each episode.
- Lack of fluids. You're more likely to form stones if you live in a hot dry climate, work in a hot environment like a commercial kitchen, exercise strenuously without replacing fluids, or habitually drink relatively low amounts of fluid.
- Elevated calcium levels in the urine. Elevated calcium levels in the urine increase the likelihood of developing a kidney stone. This is often the result of a genetic condition called idiopathic hypercalciuria.
- Renal tubular acidosis, cystinuria, gout, chronic urinary tract infections, and hyperparathyroidism are the more common diseases that are associated with kidney stones.
- High-protein and low-fiber diet increases the risk of forming a kidney stone.
- Obesity and sudden weight gain increase the risk of developing a kidney stone. Gastric bypass surgery, inflammatory bowel disease or chronic diarrhea can cause a change in the chemistry of your urine that increases the likelihood of developing kidney stones.
Last reviewed on 10/13/09
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