Kidney stones are a topic near and dear to my heart as I've had the distinct pleasure of passing four of them. Stones are also known as calculi, from the Latin for pebble. They can form and stay in the kidneys (renal calculi or nephrolithiasis) or move down the ureters, the tubes connecting the kidneys to the bladder (ureteral calculi or urolithiasis). Stones may also be found in the bladder.

The ureters are very small tubes that contain smooth muscle cells. These cells contract to help move the urine from the kidneys to the bladder. When a stone is too large to pass down the ureter, it can partially or completely block the flow of urine, causing pressure to build up. This pressure, along with contractions of the muscles in the ureter, results in severe pain called ureteral colic.

The peak onset of kidney stones is in the third and fourth decades. It is rare after age 60. Men have about a 12 percent lifetime chance of developing a kidney stone while women have a seven percent chance. Interestingly, stones are more common in the Southeast United States. The chance of developing a recurrence of stones is 14 percent at one year, 35 percent at five years, and 52 percent at ten years.

Stones form when the urine becomes supersaturated. This means that compounds in the urine become so concentrated that they start to form crystals. These eventually grow to form stones. Maintaining adequate fluid intake to keep the urine diluted is therefore very important in reducing the risk of stone formation. Other types of stones may be formed by infection in the kidneys.

There are four basic chemical types of stones that are associated with over 20 different conditions, too numerous to discuss here. Most stones contain calcium. Chemical evaluation of stones and urine identifies the cause in 95 percent of cases. This allows for specific therapy in most patients that can reduce recurrence rates by up to 90 percent. Even though most stones contain calcium, dietary restriction of calcium usually is not required.

Symptoms of kidney stones vary based on the size of the stone and its location. Renal calculi often have no symptoms. Ureteral calculi, on the other hand, can be extremely painful. Women who have given birth and also had stones often say the stones are more painful. Renal colic accounts for 2 million physician visits in the U.S. each year. The pain is often accompanied by nausea and vomiting. It's also common for patients to be very restless and they often seek relief by pacing back and forth.

The pain can be located anywhere from the back to the groin, testicle or vulva. The progression of the pain often follows the anatomy of the urinary tract - it may start in the back or abdomen and move downward as the stone moves down the ureter. The pain may not move if the stone becomes stuck in the ureter. The most common places for stones to get lodged are where the ureter crosses over the pelvic bone and where it enters the bladder.

Normal X-rays may reveal some stones, depending on their composition. Most people get a special type of X-ray called a helical CT (CAT) scan that is very accurate in determining the location and size of the stone as well as giving much more information about the health of the ureter, kidney and other abdominal organs.

Small stones (less than 4 millimeters) usually pass spontaneously 80 to 85 percent of the time. Medications such as tamsulosin (Flomax) may be given to help speed the process. Larger or irregularly shaped stones may become stuck and need to be removed, especially if they are over 8 millimeters in diameter. This can be done by inserting a lighted tube called a ureteroscope through the urethra, into the bladder and up the ureter. An instrument can then be used to grasp the stone and pull it out. Some stones need to be broken up into smaller pieces to allow them to pass. This can be done by using external sound waves (lithotripsy), or internally using an instrument inserted through the ureteroscope.

Dr. John Roberts is a local physician. His column appears weekly in the Times, and he has a daily health tip on the front page. Dr. Roberts is one of the owners of Sagamore News Media, parent company of The Times.