The urinary tract consists of the kidneys, ureters, bladder, and urethra. Urinary
calculi may form anywhere along the urinary tract. The main problem occurs when the
stones block the drainage of the kidney resulting in urinary obstruction. It has been
estimated that approximately 3% of the adult population in the United States will develop
urinary calculi at least once in their lifetime. Urinary calculi are responsible for 10%
of urological hospital admissions per year, accounting for approximately 400,000 plus
patient hospitalizations. It also accounts for a significant number of visits to the
hospital emergency departments and absenteeism from work each year. Upper urinary tract
calculi are more common in adults than children and are seen more often in men than in
women. Bladder stones are rare in women and children, but are common in men due to the
greater incidence of lower urinary tract obstruction, e.g., prostate enlargement. Ten
percent of males will experience kidney stones during their lifet
ime, which is almost three times the incidence in females. Caucasians are affected more
commonly than other racial groups.
Kidney stones, one of the most painful of the urologic disorders, are not a product of
modern life. Scientists have found evidence of kidney stones in a 7,000-year-old Egyptian
mummy. Unfortunately, kidney stones are one of the most common disorders of the urinary
tract; more than 1 million cases were diagnosed in 1996. An estimated 10 percent of people
in the United States will have a kidney stone at some point in their lives. Men tend to be
affected more frequently than women.
Most kidney stones pass out of the body without any intervention by a physician. Stones
that cause lasting symptoms or other complications may be treated by various techniques,
most of which do not involve major surgery. Also, research advances have led to a better
understanding of the many factors that promote stone formation.
A kidney stone is a hard mass developed from crystals that separate from the urine and
build up on the inner surfaces of the kidney. Normally, urine contains chemicals that
prevent the crystals from forming. These inhibitors do not seem to work for everyone,
however, so some people form stones. If the crystals remain tiny enough, they will travel
through the urinary tract and pass out of the body in the urine without being noticed.
Kidney stones may contain various combinations of chemicals. The most common type of
stone contains calcium in combination with either oxalate or phosphate. These chemicals
are part of a person's normal diet and make up important parts of the body, such as bones
and muscles.
A less common type of stone is caused by infection in the urinary tract. This type of
stone is called a struvite or infection stone. Much less common are the uric acid stone
and the rare cystine stone.
Urolithiasis is the medical term used to describe stones occurring in the urinary
tract. Other frequently used terms are urinary tract stone disease and nephrolithiasis.
Doctors also use terms that describe the location of the stone in the urinary tract. For
example, a ureteral stone (or ureterolithiasis) is a kidney stone found in the ureter. To
keep things simple, however, the term "kidney stones" is used throughout this fact
sheet.
Gallstones and kidney stones are not related. They form in different areas of the body.
If you have a gallstone, you are not necessarily more likely to develop kidney stones.
For unknown reasons, the number of people in the United States with kidney stones has
been increasing over the past 20 years. White Americans are more prone to develop kidney
stones than African Americans. Although stones occur more frequently in men, the number of
women who get them has been increasing over the past 10 years, causing the ratio to
change. Kidney stones strike most typically between the ages of 20 and 40. Once a person
gets more than one stone, others are likely to develop.
Doctors do not always know what causes a stone to form. While certain foods may promote
stone formation in people who are susceptible, scientists do not believe that eating any
specific food causes stones to form in people who are not susceptible.
A person with a family history of kidney stones may be more likely to develop stones.
Urinary tract infections, kidney disorders such as cystic kidney diseases, and metabolic
disorders such as hyperparathyroidism are also linked to stone formation.
In addition, more than 70 percent of people with a rare hereditary disease called renal
tubular acidosis develop kidney stones.
Absorptive hypercalciuria occurs when the body absorbs too much calcium from food and
empties the extra calcium into the urine. This high level of calcium in the urine causes
crystals of calcium oxalate or calcium phosphate to form in the kidneys or urinary
tract.
Other causes of kidney stones are hyperuricosuria (a disorder of uric acid metabolism),
gout, excess intake of vitamin D, and blockage of the urinary tract. Certain diuretics
(water pills) or calcium-based antacids may increase the risk of forming kidney stones by
increasing the amount of calcium in the urine.
Usually, the first symptom of a kidney stone is extreme pain. The pain often begins
suddenly when a stone moves in the urinary tract, causing irritation or blockage.
Typically, a person feels a sharp, cramping pain in the back and side in the area of the
kidney or in the lower abdomen. Sometimes nausea and vomiting occur. Later, pain may
spread to the groin.
If the stone is too large to pass easily, pain continues as the muscles in the wall of
the tiny ureter try to squeeze the stone along into the bladder. As a stone grows or moves,
blood may appear in the urine. As the stone moves down the ureter closer to the bladder,
you may feel the need to urinate more often or feel a burning sensation during
urination.
If fever and chills accompany any of these symptoms, an infection may be present. In
this case, you should contact a doctor immediately.
Sometimes "silent" stones--those that do not cause symptoms--are found on x-rays taken
during a general health exam. These stones would likely pass unnoticed.
Fortunately, surgery is not usually necessary. Most kidney stones can pass through the
urinary system with plenty of water (2 to 3 quarts a day) to help move the stone along.
Often, you can stay home during this process, drinking fluids and taking pain medication
as needed. The doctor usually asks you to save the passed stone(s) for testing. (You can
catch it in a cup or tea strainer used only for this purpose.)
If you've had more than one kidney stone, you are likely to form another; so prevention
is very important. To prevent stones from forming, your doctor must determine their cause.
He or she will order laboratory tests, including urine and blood tests. Your doctor will
also ask about your medical history, occupation, and eating habits. If a stone has been
removed, or if you've passed a stone and saved it, the laboratory can analyze it to
determine its composition.
A simple and most important lifestyle change to prevent stones is to drink more
liquids--water is best. If you tend to form stones, you should try to drink enough liquids
throughout the day to produce at least 2 quarts of urine in every 24-hour period.
People who form calcium stones used to be told to avoid dairy products and other foods
with high calcium content. But recent studies have shown that foods high in calcium,
including dairy foods, help prevent calcium stones. Taking calcium in pill form, however,
may increase the risk of developing stones.
You may be told to avoid food with added vitamin D and certain types of antacids that
have a calcium base. If you have very acidic urine, you may need to eat less meat, fish,
and poultry. These foods increase the amount of acid in the urine.
The doctor may prescribe certain medications to prevent calcium and uric acid stones.
These drugs control the amount of acid or alkali in the urine, key factors in crystal
formation. The drug allopurinol may also be useful in some cases of hypercalciuria and
hyperuricosuria.
Another way a doctor may try to control hypercalciuria, and thus prevent calcium stones,
is by prescribing certain diuretics, such as hydrochlorothiazide. These drugs decrease
the amount of calcium released by the kidneys into the urine.
Some patients with absorptive hypercalciuria may be given the drug sodium cellulose
phosphate, which binds calcium in the intestines and prevents it from leaking into the
urine.
Surgery should be reserved as an option for cases where other approaches have failed or
shouldn't be tried. Surgery may be needed to remove a kidney stone if it
- Does not pass after a reasonable period of time and causes constant pain
- Is too large to pass on its own or is caught in a difficult place
- Blocks the flow of urine
- Causes ongoing urinary tract infection
- Damages kidney tissue or causes constant bleeding
- Has grown larger (as seen on followup x-ray studies)
