Gary Curhan, MD, SCD
Kidney stones are actual stones that form in the kidney. They may eventually pass and come out in the urine. But they do form in the kidney. There's all different types of kidney stones. There are calcium oxalate stones, uric acid stones, cystine stones. And the type of stone depends on a lot of different factors.
Twenty percent of men and 10% of women will form at least one stone during their lifetime. And at any one time in the United States, over 19 million individuals have had a history of kidney stones. So this is extremely common.
Many people actually have what we call asymptomatic or painless kidney stones. So the stone has actually already formed when it's still in the kidney and hasn't passed yet. So usually stones that are just sitting in the kidney do not cause any symptoms at all. But once the stone breaks off and then drops into the ureter, that's the tube that connects the kidney to the bladder, then people usually experience severe pain.
So kidney stones form when there's too much or too little of some things in the urine. So the way I look at urine is that it's just water with a bunch of stuff dissolved in it. And as long as the material stays dissolved, then when you urinate it just comes out as the liquid. But sometimes there's precipitation, which means there's too much of something in this water. And no longer can it stay dissolved, so crystals form.
There are a number of different groups that are more or less likely to form kidney stones. So the group that's most likely to form stones are white males, usually between the ages of about 40 and 60. But younger males and older males can form stones as well. And even a man as old as 75 or 80 could form his first stone.
The likelihood of stone formation is slightly less in women compared to men and also depends on age. So a younger woman has a slightly higher risk than an older woman. But at any age, women are less likely to form stones than men.
There are number of risk factors. So the risk factors for calcium oxalate stones are a low calcium diet. And it may seem a little counterintuitive that eating less calcium would be a risk factor for calcium oxalate stones. But, in fact, a study that we published for the first time more than 20 years ago, and was based at Brigham and Women's Hospital, found that individuals with a higher dietary calcium intake actually had a lower risk of kidney stones.
Other factors that are associated with a higher risk of kidney stones include lower intake of potassium-rich foods, so lower intake of fruits and vegetables, and also if people drink less. So as I mentioned before, urine is really just water with other things dissolved in it. The main determinant of your urine volume is how much you drink. And for people that drink less, then the urine gets more concentrated. And that makes it more likely that crystallisation will occur.
Patients are identified as kidney stones in two ways. If they develop severe -- what we call renal colic, or that's the pain of passing a kidney stone, most the time it's so severe that they'll end up in an emergency room or at least call their doctor. And the way the definitive diagnosis is made is by X-ray. Most people have a CAT scan or a computed tomography scan. And there, we can actually see the kidney stone. And then other individuals who aren't having any symptoms, if they have a radiology study for another reason, then a asymptomatic stone may be identified.
So if an individual has an asymptomatic stone, how you manage it really depends on the size and the location. For small stones, usually we don't do anything. If the stones are larger, then it would probably be helpful for them to have a conversation with the urologist to decide about whether preemptive treatment of the existing stone would be helpful. The separate question is whether those people still should be evaluated to figure out how to prevent them from getting new stones? And I recommend that they still go ahead and do that, even though they haven't yet passed the stone.
Urologists have different ways of removing or breaking up that stone. So there are noninvasive approaches. One is called ESWL, or extracorporeal shock wave lithotripsy, which is offered at Brigham and Women's Hospital. And that's a way to break up a existing kidney stone without having to do something invasive
And then there's other approaches that are a little bit more invasive. Urologists now have these endoscopes that they can put into the body and go up and actually break up the existing stones or remove them. With anesthesia, it's considered day surgery and actually can be quite effective.
I really want to emphasize is that kidney stones really can be prevented. And that's what usually nephrologists do, medical kidney specialists such as myself. And we evaluate the patient a number of different ways. One is by taking a careful history to find out about how many stones they've had. Another is about their diet, family history, other lifestyle conditions that may relate to stone formation. And then we also order blood tests to find out if there's a systemic cause to why they may be forming kidney stones.
And based on that, we can make a number of recommendations. Some are based on dietary modifications and then the other is with some that are very effective in reducing new stone formation.
Now, kidney stones are also potentially related to other long-term adverse outcomes. And we recently published a study, again based on the cohorts from Brigham and Women's Hospital, that individuals with a history of kidney stones are more likely to subsequently develop cardiovascular disease, even after adjusting for all these other risk factors. The exact connection still isn't clear. But it is another reason why stone prevention may be important.
We've been very fortunate here to have the largest and the longest going studies of how to prevent kidney stones. These involve the very large cohorts based at the Channing Division of Network Medicine at Brigham and Women's Hospital. These include the Nurses' Health Study I, Nurses' Health Study II, and Health Professionals Follow-up Study. These involve several hundred thousand people from across the United States that we've been following since the mid-1980s.
And we have updated information about diet and a number of other lifestyle factors, that allow us to keep figuring out and identifying additional risk factors for stone formation. This work has actually changed the guidelines, including the recent American Urological Association guidelines on prevention of kidney stones.
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