Kidney stones are abnormal, hard, chemical deposits that form inside the kidneys. This condition also is called nephrolithiasis or urolithiasis.
Kidney stones are often as small as grains of sand. They pass out of the body in urine without causing discomfort.
However, the deposits can be much larger-the size of a pea, a marble or even larger. Some of these larger stones are too big to be flushed from the kidney.
Some kidney stones manage to travel into the ureter. This is the narrow tube between the kidney and bladder. The stones may become trapped in the ureter. Trapped kidney stones can cause many different symptoms. These include:
Blocked urine flow
Bleeding from the walls of the urinary tract
There are several different types of stones. They form for a variety of reasons. Kidney stones are grouped into four different families, based on their chemical composition:
Calcium oxalate stones - These stones account for most kidney stones. Several factors increase the risk of calcium oxalate stone formation in the kidney:
Low urinary volume
High concentrations of calcium in the urine
High concentrations of oxalate in the urine
Low amounts of citrate in the urine (citrate acts to inhibit stone formation)
Medical conditions that increase the risk of calcium oxalate kidney stones include:
Extra parathyroid hormone (hyperparathyroidism)
High uric acid levels in the blood (as in people who have gout, )
Surgery for obesity
Struvite stones - These stones are made of magnesium and ammonia (a waste product). They are related to urinary tract infections caused by certain bacteria. Struvite stones are less common now that urinary tract infections are better recognized and treated. Struvite stones are more common in women than in men. They develop frequently in people who have long-term bladder catheters.
Uric acid stones - Uric acid stones form because of an abnormally high concentration of uric acid in the urine. They are more likely to occur in people who have gout because of an over production of uric acid. Gout is a disorder in which uric acid builds up in the blood and gets deposited in joints.
Cystine stones - These rare stones are the least common type of kidney stones. They are composed of the amino acid cystine. Cystine is a building block of proteins. Cystine stones are caused by an inherited defect.
Very small kidney stones may pass out of the body in the urine without causing symptoms.
Larger stones may become trapped in the narrow ureter. This can cause:
Severe pain in the back or side
Nausea and vomiting
Blood in the urine (urine may look pink, red, or brown)
The location of pain may shift downward, closer to the groin. This usually indicates that the stone has traveled downward in the ureter and is now closer to the bladder. As the stone approaches the bladder, you may feel:
A stronger urge to urinate
A burning sensation when urinating
When stones pass out of your body in your urine, you may see the stones exit.
Your doctor will ask you about your symptoms. He or she will ask about any change in the color of your urine. Your doctor will want to know about your family history of kidney stones, and whether you have had gout.
Your doctor will check your urine for red blood cells. He or she may order a computed tomography (CT) scan or ultrasound. The CT scan can show an actual stone. The ultrasound usually cannot detect the actual stone. But an ultrasound can show swelling of the kidney and/or ureter which indicates that the stone is obstructing urine flow.
If you are able to collect a stone that you passed from your urine, your doctor will send the stone to a laboratory for chemical analysis. Blood and urine tests may be done to identify a treatable cause of the stone.
If you do not have any symptoms and you find a small kidney stone in your urine, strain out the stone and save it for your doctor. Your doctor can send the stone to a medical laboratory for chemical analysis.
When a kidney stone becomes trapped in the ureter, it may remain there until your doctor removes it. Or, it eventually may move downward and pass on its own. It can take hours, days or weeks for a stone to pass.
As a rule, the smaller the stone, the more likely it is to pass on its own. The larger the stone, the greater the risk that it will remain trapped in the ureter. A trapped stone can significantly obstruct the flow of urine.
In general, you can help to prevent kidney stones by drinking plenty of fluids and avoiding dehydration. This dilutes your urine and decreases the chance that chemicals will combine to form stones.
You can prevent calcium oxalate stones by eating low-fat dairy products and other calcium-rich foods. Taking calcium supplements, however, can increase the risk of stone formation.
People who excrete too much oxalate into their urine should avoid eating foods high in oxalate. These foods include beets, spinach, chard and rhubarb. Tea, coffee, cola, chocolate and nuts also contain oxalate, but these can be used in moderation. Eating too much salt and meat can cause more kidney stones to form.
After your doctor receives an analysis of the chemical composition of your kidney stones, he or she can suggest medications or changes in your diet that will help to prevent stones from forming in the future.
Certain medications may increase the risk of stones. So your doctor may want to adjust your regular medications if you have had kidney stones.
In many cases, a trapped kidney stone eventually flushes out of the urinary tract on its own, especially if you drink plenty of fluids. With a doctor's supervision, it is likely that you can remain at home. You can take pain medicine as needed until the stone dislodges and flushes away.
In some cases, it is necessary to have your stone removed or broken into fragments that can pass more easily. This may be the case if:
The stone is too large to pass on its own
Your pain is severe
You have infection
You have significant bleeding
Doctors have several options for destroying stones lodged in the urinary tract:
Extracorporeal lithotripsy - Shock waves applied externally break kidney stones into smaller fragments. The fragments are then swept away in the urine stream.
Percutaneous ultrasonic lithotripsy - A narrow, tube-like instrument is passed through a small incision in the back to the kidney. There, ultrasound breaks up the kidney stones. The stone fragments are then removed.
Laser lithotripsy - A laser breaks up stones in the ureter. The stones then pass on their own.
Ureteroscopy - A very small telescope is inserted into the urethra as it makes its way to the bladder. The doctor finds the opening of the affected and guides the scope up the ureter until it reaches the stone. The stone is then either fragmented or removed.
It is rare that surgery is required to remove a kidney stone.
Once a kidney stone has been removed, you can sometimes prevent new stones from forming with medications or changes in diet.
Call your doctor whenever you have:
Severe pain in your back or side, with or without nausea and vomiting
Unusually frequent urination or a persistent urge to urinate
A burning and discomfort when urinating
Urine that is colored pink or tinged with blood
Trapped kidney stones can lead to a urinary tract infection. Call your doctor if you have fever and chills, or if your urine becomes cloudy or foul smelling.
The prognosis varies from person to person. Up to half of people who pass a kidney stone will never pass a second. For people with a recurrent kidney stone, the prognosis depends on the cause of the kidney stones and the response to preventive therapies.
National Institute of Diabetes and Digestive and Kidney Disorders
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Urology Care Foundation
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National Kidney Foundation
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New York, NY 10016