Kidney stones have been described as more painful than childbirth. The severe pain that occurs when these stones abruptly move into the tight passageway between the kidney and the bladder frequently drives otherwise stoic men and women directly to the emergency room.
Approximately 10 percent of the US population has had a kidney stone, and the incidence of stones appears to be increasing. This increase may be due to the large amount of protein in the American meat-based diet, particularly among men 35 to 45, which is the age group most affected by kidney stones. Below, Dr. Stephen Leslie, an assistant clinical professor in the department of urology at the Medical College of Ohio, and the co-author of The Kidney Stone Handbook, discusses why kidney stones form and how they can be prevented and treated.
What is a kidney stone?
A kidney stone is caused by crystals that form in the kidney and in the urine. These crystals tend to stick together and eventually literally form a stone, something as hard as any rock you might find out in your driveway.
A kidney stone attack is marked by sudden, very severe pain, usually in the side radiating towards the groin. It occurs when a stone, which may have been there for weeks or months, gets stuck in the passage between the kidney and bladder and causes an obstruction.
Why do kidney stones form?
The simple answer is that they form when there are more minerals and chemicals in the urine than the urine can reasonably dissolve. The most common contributing factor is not having enough fluid in your system. Another common cause is having high levels of several different chemicals that seem to promote stones. These include calcium, uric acid and oxalate.
Is the tendency to develop kidney stones inherited?
There is a strong tendency for related family members to get stones. For example, if one brother had a stone, there's a 50-50 chance the other one will. If one parent had a stone, there is roughly a 25 percent chance that a male offspring will, compared to about 12 percent chance that someone in the general population will form a stone.
Why is the condition more common in men than women?
We're not absolutely sure. The most likely answer has to do with what we eat and average general size. Since the typical man is substantially larger than the typical woman, the amount of waste product that men excrete is also somewhat higher. Meanwhile, the urinary system in men and women is roughly the same size.
Another explanation has to do with diet. Men tend to be more meat-and-potatoes, while women tend to eat more vegetables. Meat protein may increase risk of stone formation.
There is also thought to be some protection from the female hormone, estrogen. Estrogen seems to naturally increase the citrate level, which inhibits stone formation, and provides some other protection.
What medical conditions are associated with kidney formation?
Hypercalciuria is a condition in which someone absorbs a much higher percentage of calcium than usual from the diet or excretes too much calcium into the urine. This can significantly increase the risk of kidney stones.
Another risk factor is hyperparathyroidism, which is a glandular problem. A hormone from the parathyroid gland in the neck helps regulate calcium in the blood. In someone with hyperparathyroidism, this gland goes haywire and produces a very high level of hormones. One of the results is too much calcium in the blood and eventually this ends up in the urine. Fortunately, this condition is relatively rare.
Can dietary calcium affect stone formation?
About 70 percent of stones are calcium-based. We used to recommend that everyone who had calcium-based stones decrease their calcium intake. We no longer recommend lowering the calcium intake, however, because calcium is important for the operation of muscles and for bone strength, and because some calcium in the diet is even good for preventing kidney stone formation. Calcium and a chemical called oxalate, which is found in fruits and vegetables, bind very tightly together, and form some of the hardest kidney stones. And oxalate is a much stronger promoter of stone growth and formation than calcium. If you don't eat enough calcium, the oxalate in your intestinal tract will not be bound up by calcium, so more of this oxalate will be available for absorption. As a result, if you decrease your calcium intake, you end up raising your risk of stones by increasing the amount of oxalate that is absorbed.
Are some calcium supplements better for people with a history of kidney stones than others?
There is some evidence that calcium citrate may be a slightly better calcium supplement for calcium stone-formers. The theory is that the citrate portion of that particular supplement—being a stone inhibitor—tends to negate any stone-promoting effect from the calcium. I'm not sure how significant this is, but it seems to make sense, so usually I will recommend calcium citrate to my kidney stone patients who need a calcium supplement.
If you've had kidney stones, it may be better to take the calcium supplement without vitamin D, which is usually recommended to improve calcium absorption. It is better to allow the calcium to stay in the intestinal tract a little longer, where it can help bind oxalate, which is a more significant promoter of stones. If you have the vitamin D, the calcium will tend to be absorbed further up in the intestinal tract, and you won't see that added benefit.
Why are kidney stones so painful?
The degree of the pain has nothing to do with the size of the stone; it has to do with the degree of the blockage. Although the level of pain can vary, kidney stones can cause the most severe pain we know of. We think the actual cause of the pain is a combination of stretching of the ureter—the hollow tube between the kidney and the bladder—and associated muscle spasms.
The good news is that the pain of a severe attack usually disappears within 24 hours, even if the stone hasn't moved. But there are medications that are quite effective at controlling the pain.
How often will the stones pass by themselves?
More than 70 percent of kidney stones will pass by themselves. The size of the stone is a good predictor of how likely it is the stone will pass on its own: Usually stones that are 4 mm (about 3/16 of an inch) and smaller will pass by themselves.
For larger stones that are not likely to pass by themselves, or for stones that are continuing to cause problems after 24 hours, some kind of surgical procedure is usually recommended. The typical patient will get a stent. During the procedure, a small tube is threaded up the ureter into the kidney. The other end goes through the ureter and into the bladder. By diverting the urine around any blockage, the stent gets rid of the severe pain from the stones. But the stent itself can be somewhat uncomfortable, and it's usually there only temporarily.
Definitive treatment usually involves one of two or three modalities. The most commonly used one is something called ESWL, or extracorporeal shockwave lithotripsy. With this treatment, a machine outside the body focuses energy on the stone and breaks it up into little pieces, which can then be passed painlessly. In about an hour, most stones are broken up into very small fragments.
The nice thing is that the energy passes through the soft tissue of the body without any doing harm. With older machines, the shockwaves were delivered while patient lay in a water bath or tub, but with modern machines patients don't have to get wet. Various methods of creating the shockwaves exist, including electromechanical, electrohydraulic and piezoelectric energy. But the basic concept is a pressure wave focused right on the stone.
The other method for dealing with stones is ureteroscopy. Ureteroscopy involves the use of a very long, very thin telescope that, under anesthetic, can be placed through the bladder and actually go into the ureter so that the physician can see the stone. At that point, we can use a variety of implements to either grab it and pull it out or break it into little pieces, if it's too big.
How does one determine what is causing the patient's stones?
One way is to do a 24-hour urine collection and measure the chemistry of the urine. That way you can tell whether the oxalate and uric acid levels are normal, if citrate level is adequate, etc. It's best done when the patient is on their usual diet, because that's what they were doing when they made the stone in the first place. If they are eating something that is high in one of these minerals, that's the time to identify it.
Do you recommend saving the stone for testing?
We always try to collect the stone or stone fragments for testing so we know what the stone's made of. Testing the stone can be easier than talking patients into doing a 24-hour urine collection because that is difficult and inconvenient. If you miss one sample, the whole 24-hour sample is not valid.
In order to collect their stones and stone fragments, we recommend that patients urinate through some type of strainer. The one I tend to recommend is a very fine-meshed brine shrimp net from an aquarium supply store.
If your first stone is made of a certain mineral, is it likely that another one will have that same composition?
It's likely, but not necessarily the case. You may make significant modifications in your diet in response to having the first stone, and that will result in a chemistry change. Or using a medication or a dietary supplement can change the chemistry. We may find that we've fixed one problem, but created a new one. If you limit one particular beverage or food, any substitute food item may have new chemical properties and risk factors. For example, if a patient reduces their meat intake but eats more vegetables, their uric acid level from the meat may decrease to normal, but their oxalate intake is likely to rise, which could increase their overall risk of stones. We usually recommend a periodic retest to make sure that the treatment still effective, and that a new problem hasn't developed.
How likely is someone to have a recurrence of kidney stones?
There's a 70 percent to 80 percent likelihood of a recurrence at some point, and a roughly 50 percent likelihood of a recurrence within five years. Obviously, it varies according to sex, age and the underlying problem.
But with an optimal prevention plan based on a 24-hour urine analysis, the recurrence rate can be reduced by up to 99 percent.
How can people prevent recurrence?
The first thing to do is to increase fluid intake. People who have had kidney stones should do whatever it takes to get roughly 50 percent above the usual average urinary output, which is around 1300 cc's (44 ounces). A 50 percent increase brings fluid intake up to about a half a gallon, and that's what we shoot for in most stone-formers. For people who absolutely can't drink any more water, the next best thing would be lemonade made from real lemon juice. Lemon juice is very rich in citrate, so you get that added benefit.
People should also reduce salt intake. Salt causes fluid retention. We don't want the fluid to stay in the body; we want the fluid to be excreted in the urine. The second problem is that it changes some of the body's chemistry, especially with regard to calcium. So you are going to end up excreting more calcium if your salt intake is high.
When we have more specific information about what the chemistry of the stone and, in particular, what the chemistry of the patient's urine is really like, then we can be a lot more specific in our recommendations. That can only be determined by a 24-hour urine that is done to look at these types of chemistries.
For people who have low citrate, which inhibits stone formation, there is a specific potassium citrate supplement and that can be given as needed in order to get the level up.
For people with stones containing uric acid, which is a waste product associated with protein found in meat, we try to reduce the protein intake. This allows more uric acid in the urine to dissolve. There's also a disease called gout, which causes a problem in the way the liver handles uric acid, so in these people, the body makes much more uric acid that it would normally. Specific medication is required in those patients.
In people with oxalate-containing stones, we try to limit the amount of the very high-oxalate foods that the patient will take in. The most common are chocolate, green leafy vegetables, rhubarb, nuts, strong tea and coffee. There are many other fruits and vegetables that tend to have relatively high oxalate levels, but those are the worst offenders.
We don't want to impose overly tight dietary restrictions, just encourage reasonable moderation in both the higher-oxalate foods and meat protein.