What are kidney stones?

If you've ever passed a kidney stone, you're not likely to forget the experience — it can be excruciatingly painful. What's more, kidney stones (renal lithiasis) are a fairly common disorder and there's evidence these stones have plagued humans for millennia. Scientists have found traces of kidney stones in mummies more than 7,000 years old. Today, more than 1 million cases are reported each year in the United States, and that number is steadily increasing. Your chance of developing at least one kidney stone in your lifetime is 10 percent.

Kidney stones often occur when urine becomes too concentrated. This causes minerals and other substances in your urine to form crystals on the inner surfaces of your kidneys. Over time these crystals may combine to form a small, hard mass. Sometimes this mass (stone) breaks off and passes into the ureter, one of the two thin tubes that carry urine from your kidneys to your bladder.


About 80 percent of stones are a combination of calcium and oxalate (oxalic acid), a substance found in many fruits, vegetables and grains. Most other stones are composed of uric acid, which is a byproduct of protein metabolism. A few are made of ammonia crystals (struvite) and result from urinary tract infections. About 1 percent of stones are composed of the amino acid cystine and occur in people who have an inherited disorder.

Not all kidney stones cause symptoms. In fact, it's not unusual for stones to be discovered in the kidneys during X-rays for an unrelated problem. They may also be discovered when you seek medical care for blood in your urine, recurring urinary tract infections, or a vague pain or ache in your side — all common symptoms of kidney stones. It's only when a stone breaks loose and begins to work its way down the ureter that the pain becomes agonizing.

Most small kidney stones pass into your bladder without causing any permanent damage. Still, it's important to determine and treat the underlying cause so that you don't form more stones in the future. Fortunately, you may be able to prevent kidney stones simply by increasing the amount of water you drink and making a few dietary changes.

Signs and symptoms

You may not have symptoms unless a kidney stone is large or causes a blockage or an infection. Then the most common symptom is an intense, colicky pain that may fluctuate in intensity over a period of 5 to 15 minutes. The pain usually starts in your back or your side just below the edge of your ribs. As the stone moves down the ureter toward your bladder, the pain may radiate to your groin. If the stone stops moving, the pain may stop too. Other signs and symptoms may include:

• Bloody, cloudy or foul-smelling urine
  
  
• Nausea and vomiting
  
  
• Persistent urge to urinate
  
  
• Fever and chills if an infection is present

• Calcium stones. Approximately 75 percent to 85 percent of all kidney stones are calcium stones. These stones are usually a combination of calcium and oxalate and may occur if you have too much of either of these substances in your urine. A number of factors can cause high calcium concentrations in urine, including large amounts of vitamin D — which may cause your body to absorb too much calcium — drugs such as thyroid hormones and some diuretics, certain cancers, and some kidney conditions. You may also have high levels of calcium if your parathyroid glands, which regulate calcium, are overactive (hyperparathyroidism). You may have increased levels of oxalate if you eat a lot of foods high in this substance or as a result of certain genetic factors. People who have had intestinal bypass operations also are at increased risk of oxalate stones.
  
  
• Uric acid stones. As the name suggests, these stones are formed of uric acid, a byproduct of protein metabolism. A diet high in meat may cause excess amounts of uric acid in your urine. You may also have high levels of uric acid in your blood and urine if you have received or are receiving chemotherapy.
  
  
• Struvite stones. Found mainly in women, struvite stones are almost always the result of chronic urinary tract infections caused by bacteria that secrete specific enzymes. These enzymes increase the amount of ammonia in the urine, which makes up the crystals in struvite stones. These stones are often large and have a characteristic stag's horn shape, which can cause serious damage to your kidneys.
  
  
• Cystine stones. These stones represent only about 1 percent of kidney stones. They form in people with a hereditary disorder that causes the kidneys to excrete excessive amounts of certain amino acids (cystinuria).

• Family or personal history. If someone in your family has kidney stones, you're more likely to develop these stones too. And if you've already had one or more kidney stones, you're at increased risk of developing another. The recurrence rate is about 3 percent after the first stone and 6 percent after the second.
  
  
• Age, sex and race. Most people who develop kidney stones are between 20 and 40 years of age. Men are more likely to develop kidney stones than women are, although for unknown reasons the number of women with kidney stones is increasing. In addition, whites are at higher risk of kidney stones than are blacks in America.
  
  
• Certain diseases. Rare, inherited diseases such as renal tubular acidosis and cystinuria can increase your risk of kidney stones. So can more common disorders such as gout, chronic urinary tract infections, cystic kidney disease and hyperparathyroidism.
  
  
• Certain medications. Medications can have variable effects on stone formation. For example, diuretics may increase your risk of developing kidney stones in some situations and decrease it in others. If you're at risk, check with your doctor or pharmacist about any medications you take.
  
  
• Missing kidney. Although most people have two kidneys, approximately 1 in every 1,500 babies are born with only one kidney. Many people live full, healthy lives with a single kidney, but they do have an increased risk of kidney stones.
  
  
• Diet. A diet that's high in protein (meat, chicken and fish) and low in fiber (fruits, vegetables and whole grains) may increase your risk of some types of kidney stones.
  
  
• Lack of fluids. If you don't drink enough fluids, especially water, your urine is likely to have higher concentrations of substances that can form stones. That's also why you're more likely to develop kidney stones if you live in a hot, dry climate, work in a hot environment such as a commercial kitchen or exercise strenuously, and don't replenish lost fluids.
  
  
• Limited activity. You're more prone to develop kidney stones if you're bedridden or very sedentary for a period of time. That's because limited activity can cause your bones to release more calcium.

• Extracorporeal shock wave lithotripsy (ESWL). This is the most commonly used surgical procedure for kidney stones. It uses shock waves to break the stones into small crystals that are then passed in your urine. In some cases you may be partially submerged in a tub of water during the procedure. In others, you may lie on a soft cushion. You won't be hurt by the shock waves, and you won't feel them. But a loud noise is produced each time a shock wave is generated, and you'll wear earphones to protect your hearing.
  
  Your doctor will likely use X-rays or ultrasound to help determine the position of your stone as well as to monitor the status of the stone during treatment. In many cases the stone will begin to crumble after 200 to 400 shock waves.
  
  Complications that may occur with ESWL include blood in the urine, bruising on the back or abdomen, and discomfort as the stone fragments pass through the urinary tract. In addition, if the stone doesn't shatter completely, you may need another treatment such as a second ESWL or ureteroscopic stone removal. After treatment it may take months for all the stone fragments to pass.
  
  
• Percutaneous nephrolithotomy. When ESWL isn't effective, or the stone is very large, your surgeon may remove your kidney stone through a small incision in your back using an instrument known as a nephroscope.
  
  
• Ureteroscopic stone removal. Your surgeon may use this procedure to remove a stone lodged in one of the thin tubes leading from your kidneys to your bladder. He or she does this by passing a small instrument known as a ureteroscope through your bladder into the ureter. The stone is then snared and removed. It may also be shattered using ultrasound, a laser or a technique known as electrohydraulic lithotripsy. These work especially well on stones in the ureter that can't be treated using other methods.
  
  
• Parathyroid surgery. Some calcium stones are caused by overactive parathyroid glands, which are located on the four corners of your thyroid gland, just below your Adam's apple. When these glands produce too much parathyroid hormone, your body's level of calcium can become too high, resulting in excessive excretion of calcium in your urine. Most often, this is the result of a small benign tumor in one of your four parathyroid glands. A doctor can surgically remove the tumor.

• Calcium stones. To help prevent calcium stones from forming, your doctor may prescribe a thiazide diuretic or a phosphate-containing preparation. If you have calcium stones because of a condition known as renal tubular acidosis, your doctor may suggest taking sodium bicarbonate or potassium bicarbonate.
  
  
• Uric acid stones. Your doctor may prescribe allopurinol (Zyloprim, Aloprim) and a medicine to keep your urine alkaline. In some cases allopurinol may also indirectly dissolve the uric acid stones.
  
  
• Struvite stones. To prevent struvite stones, the first goal is to keep urine free of bacteria that cause infection. Your doctor may prescribe the drug acetohydroxamic acid (Lithostat) in addition to antibiotics.
  
  
• Cystine stones. Cystine stones are the hardest stones and the most difficult to treat. Your doctor may prescribe certain medications when all other approaches have failed.

— April 17, 2002 —