Stones (calculi) are hard masses that form anywhere in the urinary tract and may cause pain, bleeding, or an infection or block of the flow of urine.
Depending on where a stone forms, it may be called a kidney stone, ureteral stone, or bladder stone. The process of stone formation is called urolithiasis, renal lithiasis, or nephrolithiasis.
Every year, about 1 of 1,000 adults in the United States is hospitalized because of stones in the urinary tract. Stones are more common among middle-aged and older adults and men. Stones vary in size from too small to be seen with the naked eye to 1 inch (2.5 centimeters) or more in diameter. A large, so-called staghorn (because of its many projections that resemble a deer's antlers), stone may fill almost the entire renal pelvis (the central collecting chamber of the kidney) and the tubes that drain into it (calices).
A urinary tract infection may result when bacteria become trapped in urine that pools above a blockage. When stones block the urinary tract for a long time, urine backs up in the tubes inside the kidney, causing excessive pressure that can distend the kidney (hydronephrosis—see Obstruction of the Urinary Tract: Hydronephrosis: A Distended Kidney) and eventually damage it.
Types of stones:
Stones are made of minerals in the urine that form crystals. Sometimes the crystals grow into stones. About 85% of the stones are composed of calcium, and the remainder are composed of various substances, including uric acid, cystine, or struvite. Struvite stones—a mixture of magnesium, ammonium, and phosphate—are also called infection stones, because they form only in infected urine.
Stones may form because the urine becomes too saturated with salts that can form stones or because the urine lacks the normal inhibitors of stone formation. Citrate is such an inhibitor because it normally binds with the calcium that is often involved in forming stones.
Stones are more common among people with certain disorders (for example, hyperparathyroidism, dehydration, and renal tubular acidosis) and among people whose diet is very high in animal-source protein or vitamin C or who do not consume enough water or calcium. People who have a family history of stone formation are more likely to have calcium stones and to have them more often. People who have undergone surgery for weight loss (bariatric surgery) may also be at increased risk of stone formation.
Rarely, drugs (including indinavir) and substances in the diet (such as melamine) cause stones.
Stones, especially tiny ones, may not cause any symptoms. Stones in the bladder may cause pain in the lower abdomen. Stones that obstruct the ureter or renal pelvis or any of the kidney's drainage tubes may cause back pain or renal colic. Renal colic is characterized by an excruciating intermittent pain, usually in the area between the ribs and hip, that spreads across the abdomen and often extends to the genital area and inner thigh. The pain tends to come in waves, gradually increasing to a peak intensity, then fading, over about 20 to 60 minutes. The pain may radiate down the abdomen toward the groin or testis or vulva.
Other symptoms include nausea and vomiting, restlessness, sweating, and blood in the urine. A person may have an urge to urinate frequently, particularly as a stone passes down the ureter. Chills, fever, and abdominal distention sometimes occur.
Doctors usually suspect stones in people with renal colic. Sometimes doctors suspect stones in people with tenderness over the back and groin or pain in the genital area without an obvious cause. Finding blood in the urine supports the diagnosis. Occasionally, the symptoms and physical examination findings are so distinctive that no additional tests are needed, particularly in people who have had urinary tract stones before. However, most people are in so much pain and have symptoms and findings that make other causes for the pain seem likely enough that testing is necessary to exclude these other causes.
Helical (also called spiral) computed tomography (CT) that is done without the use of radiopaque contrast material is usually the best diagnostic procedure. CT can locate a stone and also indicate the degree to which the stone is blocking the urinary tract. CT can also detect many other disorders that can cause pain similar to the pain caused by stones. The main disadvantage of CT is that it exposes people to radiation. Still, this risk seems prudent when possible causes include another serious disorder that would be diagnosed by CT, such as an aortic aneurysm or appendicitis. Newer CT devices that limit exposure to radiation are sometimes used.
Ultrasonography is an alternative to CT and does not expose people to radiation. However, ultrasonography, compared with CT, more often misses small stones (especially when located in the ureter), the location of urinary tract blockage, and other, serious disorders that could be causing the symptoms.
X-rays of the abdomen expose people to much less radiation than does CT, but x-rays are much less accurate in diagnosing stones and can only show calcium stones. When doctors suspect the person has a calcium stone, x-rays are an alternative to confirm the presence of a stone or to see how far a stone has travelled.
Excretory urography (previously called intravenous urography or intravenous pyelography) is a series of x-rays taken after intravenous injection of a radiopaque dye. This test can detect stones and accurately determine the degree to which they are blocking the urinary tract, but it is time-consuming and involves the risks of exposure to the dye (for example, an allergic reaction or worsening of kidney failure). Excretory urography is rarely used if CT or ultrasonography are available.
Urinalysis is usually done. It may show blood or pus in the urine whether or not symptoms are present.
For people with diagnosed stones, doctors often do tests to determine the type of stones. People should attempt to retrieve stones that are passed. They can retrieve stones by straining all urine through a paper or mesh filter. Stones found can be analyzed. Depending on the type of stone, urine tests and tests to measure levels of calcium, hormones, and other substances in the blood may be necessary.
In a person who has passed a calcium stone for the first time, the likelihood of forming another stone is about 15% within 1 year, 40% within 5 years, and 80% within 10 years. Measures to prevent the formation of new stones vary, depending on the composition of the existing stones.
Drinking large amounts of fluids—8 to 10 ten-ounce (300-milliliter) glasses a day—is recommended for prevention of all stones. Other preventive measures depend somewhat on the type of stone.
People with calcium stones may have a condition called hypercalciuria, in which excess calcium is excreted in the urine. For these people, measures that decrease the amount of calcium in the urine can help prevent formation of new stones. One such measure is a diet that is low in sodium and high in potassium. Calcium intake should be about normal–—1,000 to 1,500 milligrams daily (see Electrolyte Balance: Calcium). The risk of a new stone forming is actually higher if the diet contains too little calcium, so people should not try to eliminate calcium from their diet. However, people may need to avoid sources of excess calcium such as antacids that contain calcium. Thiazide diuretics, such as chlorthalidone or indapamide, also reduce the concentration of calcium in the urine in such people. Potassium citrate may be given to increase a low urine level of citrate, a substance that inhibits calcium stone formation. Restricting dietary animal protein may help reduce urinary calcium and the risk of stone formation in many people with calcium stones.
A high level of oxalate in the urine, which contributes to calcium stone formation, may result from excess consumption of foods high in oxalate, such as rhubarb, spinach, cocoa, nuts, pepper, and tea, or from certain intestinal disorders (including some kinds of weight loss surgery). Calcium citrate, cholestyramine, and a diet that is low in fat and in oxalate-containing food may help to reduce urinary oxalate levels in some people. Sometimes pyridoxine decreases the amount of oxalate the body makes.
In rare cases, when calcium stones result from hyperparathyroidism, sarcoidosis, vitamin D toxicity, renal tubular acidosis, or cancer, the underlying disorder must be treated.
For stones that contain uric acid, a diet low in animal sources of protein (such as meat, poultry, and fish) is recommended, because animal protein increases the level of uric acid in the urine. If a change in diet is not effective, allopurinol may be given to reduce the production of uric acid. Potassium citrate should be given to all people who have uric acid stones to make the urine alkaline, because uric acid stones mainly form when urine acidity increases. Maintaining a large fluid intake is also very important.
For stones made of cystine, urinary cystine levels must be kept low by maintaining a large fluid intake and sometimes taking alpha-mercaptopropionylglycine (tiopronin) or penicillamine.
People with recurrent struvite stones may need to take antibiotics continually to prevent urinary tract infections. Acetohydroxamic acid may also be helpful in people with struvite stones.
Small stones that are not causing symptoms, blockage of the urinary tract, or an infection usually do not need to be treated and often pass on their own. Larger stones (over three sixteenths of an inch [5 mm]) and those that are closer to the kidney are less likely to pass on their own.
The pain of renal colic may be relieved with nonsteroidal anti-inflammatory drugs (NSAIDs). If the pain is severe, opioids are sometimes needed.
Drinking plenty of fluids or receiving large amounts of fluids intravenously has been recommended to help stones pass, but it is not clear that this approach is helpful. Alpha-adrenergic blockers (such as tamsulosin) may help the stone pass. Once a stone has passed, no other immediate treatment is needed.
Sometimes when a blockage is severe, doctors insert a temporary tube (stent) in the ureter to bypass the obstructing stone. Doctors insert a hollow viewing instrument (cystoscope) into the bladder and pass the stent through the cystoscope and into the opening of the ureter. The stent is pushed up past the obstructing stone. The stent is left in place until the stone can be removed (for example, by surgery). Alternatively, doctors may drain the blockage by inserting a drainage tube through the skin into the collecting chamber of the kidney (nephrostomy tube).
Often, a stone in the renal pelvis or uppermost part of the ureter that is ½ inch (1 centimeter) or less in diameter can be broken up by shock waves directed at the body by a sound wave generator (a procedure called shock wave lithotripsy). The pieces of stone are then passed in the urine. Sometimes, a stone is removed with grasping forceps using an endoscope (viewing tube) through a small incision in the skin, or the stone can be shattered into fragments using a probe from a lithotripsy machine and then the pieces are passed in the urine. Often, a laser is used to break up the stone. When a laser is used, the procedure is called holmium laser lithotripsy.
Small stones in the lower part of the ureter that require removal may be removed with a small tube (called a ureteroscope, a kind of endoscope) that is inserted into the urethra and through the bladder. In some instances, the ureteroscope can also be used with a device to break up stones into smaller pieces that can be removed with the ureteroscope or passed in the urine (a procedure called intracorporeal lithotripsy). Most commonly, holmium laser lithotripsy is used.
Uric acid stones may sometimes be dissolved gradually by making the urine more alkaline (for example, with potassium citrate taken for 4 to 6 months by mouth), but other types of stones cannot be dissolved this way. Sometimes, larger stones that are causing an obstruction may need to be removed surgically.
Struvite stones usually need to be removed by endoscopic surgery. Antibiotics are not helpful for urinary tract infections until the stones are completely removed.
Last full review/revision February 2013 by Glenn M. Preminger, MD