Stones in the Urinary Tract

Stones (calculi) are hard masses that form anywhere in the urinary tract and may cause pain, bleeding, obstruction of the flow of urine, or an infection.

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 in older adults and men. 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 calcium that is often involved in forming stones. About 80% of the stones are composed of calcium; the remainder are composed of various substances, including uric acid, cystine, and struvite. Stones are more common in people with certain diseases (for example, hypertension and short bowel syndrome) and in people whose diet is very high in protein or who do not consume enough water. Struvite stones--a mixture of magnesium, ammonium, and phosphate--are also called infection stones, because they form only in infected urine.

Stones vary in size from too small to be seen with the naked eye to 1 inch or more in diameter. A large so-called staghorn stone may fill almost the entire renal pelvis 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, producing excessive pressure that can distend the kidney (hydronephrosis) and eventually damage it.

Symptoms

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 its drainage tubes may cause back pain or renal colic. Renal colic is characterized by an excruciating intermittent pain, usually in the flank (the area between the ribs and hip), that spreads across the abdomen, often to the genital area and inner thigh.

Other symptoms include nausea and vomiting, abdominal distention, chills, fever, and blood in the urine. A person may have an urge to urinate frequently, particularly as a stone passes down the ureter.

Diagnosis

Stones that cause pain are generally diagnosed on the basis of the symptoms of renal colic, together with tenderness over the back and groin or pain in the genital area without an obvious cause. Usually, no additional tests are needed, unless the pain persists for more than a few hours or the diagnosis is uncertain. Urinalysis may show blood or pus in the urine whether or not symptoms are present.

X-rays or an ultrasound of the abdomen can show stones made of calcium, cystine, and struvite, but usually not of uric acid. If needed, other diagnostic procedures can be performed. In intravenous urography, a radiopaque dye, which is visible on x-rays, is injected into a vein and travels to the kidneys where it outlines uric acid stones so they can be seen on x-rays. In retrograde urography, the radiopaque dye is introduced into the urinary tract through the urethra. Computed tomography (CT) is usually reserved for situations in which the diagnosis has not been made with other procedures.

Treatment

Small stones that are not causing symptoms, obstruction, or an infection usually do not need to be treated. Drinking plenty of fluids or receiving large amounts of fluids intravenously if oral intake is not feasible increases urine production and helps wash out some stones; once a stone is passed, no other immediate treatment is needed. The pain of renal colic may be relieved with nonsteroidal anti-inflammatory drugs (NSAIDs) or opioids.

Often, a stone in the renal pelvis or uppermost part of the ureter that is ½ inch or less in diameter can be broken up by ultrasound waves directed at the body by a sound wave generator (a procedure called extracorporeal shock wave lithotripsy). The pieces of stone are then passed in the urine. Sometimes, a stone is removed with grasping forceps through a small incision in the skin (percutaneous nephrostolithotomy), or the stone can be fragmented using a lithotripsy probe.

See the figure Removing a Stone With Sound Waves.

Small stones in the lower part of the ureter may be removed with a small, flexible scope (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 stones up into smaller pieces that can be removed with the ureteroscope or passed in the urine (a procedure called intracorporeal lithotripsy).

Uric acid stones are sometimes 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.

Prevention

Measures to prevent the formation of new stones vary, depending on the composition of the existing stones. These stones are analyzed, and levels in the urine of substances that can form stones are measured.

Drinking large amounts of fluids--8 to 10 ten-ounce glasses a day--is recommended. Following a diet moderately low in calcium and taking sodium cellulose phosphate, a resin, may help, but such measures can cause levels of calcium to become too low. Many people with calcium stones have a condition called hypercalciuria, in which excess calcium is excreted in the urine. Thiazide diuretics, such as hydrochlorothiazide, reduce the concentration of calcium in the urine and help prevent the formation of new stones in such people. Potassium citrate may be given to increase a low urine level of citrate, a substance that inhibits calcium stone formation.

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. A change in diet may help, and the underlying disorder can be treated.

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 meat, fish, and poultry is recommended, because these foods increase the level of uric acid in the urine. 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 form when urine acidity increases.

Struvite stones usually need to be removed by extracorporeal shock wave lithotripsy or surgery. Antibiotics are not helpful for urinary tract infections until the stones are removed.