Season 4 | Episode 3
Title: Kidney Stones
Merck Manuals medical author: Dr. Glenn M. Preminger, Urologist in Durham, North Carolina, and is affiliated with Duke University Hospital.
Joe McIntyre: Around one in 10 Americans will have a kidney stone at some point in their lives. This prevalence has led to a lot of myths and misconceptions around kidney stones: what causes them? And what prevents them?
Welcome to the Merck Manuals Medical Myths podcast, where we set the record straight on today's most talked about medical topics and questions. I'm your host Joe McIntyre, and on this episode, we're joined by Dr. Glenn Preminger, a professor of urologic surgery and director of the Duke comprehensive Kidney Stone Center at Duke University Hospital. He's with us to debunk common myths surrounding kidney stones and answer questions to help you, or someone you know, lead a healthier life. Dr. Preminger, welcome to the podcast.
Dr. Glenn Preminger: Thank you, Joe. It's a pleasure to be here.
Joe McIntyre: Great, great. Now, I want to start with, you know, first a general question: Can you tell our listeners what exactly a kidney a stone is and how they develop?
Dr. Glenn Preminger: So, kidney stones are actually an accumulation of stone forming salts within the urine, usually made of calcium containing stones like calcium oxalate, or calcium phosphate, and these small crystals can come together in the form of stone that can either sit around in the kidney or pass out by themselves.
Joe McIntyre: Now, how big are kidney stones traditionally? Are some smaller than others, some bigger than others?
Dr. Glenn Preminger: No question that stones can vary in size, and they also vary in composition. What dictates the size of the stone is that most stones, when they start growing, they're attached to the wall of the inside of the kidney. And at some point, that attachment dissolves and the stone, then, is floating free within the kidney and if it stays up there, it can continue to grow.
Joe McIntyre: Now some people who suffer from kidney stones can say it feels like a super painful stomachache, others may feel differently. How do you tell whether the pain you're feeling, or discomfort, is from a kidney stone or something else?
Dr. Glenn Preminger: Most of my patients, especially my female patients, tell me it feels worse than childbirth, which sounds more than a painful stomachache. And really, the discomfort that accompanies a kidney stone is determined by the acuteness or the suddenness of the obstruction that's caused by the stone. So, it's usually a small stone that's been floating around in the kidney that falls from the kidney into the kidney tube, or the ureter, that causes an acute blockage that can cause severe colic, which is accompanied by nausea and vomiting and essentially intractable pain. Whereas, you can have a larger stone that's growing for months and months inside the kidney that's not causing any obstruction, which can grow undetected without any discomfort at all.
Joe McIntyre: Now, you mentioned women relating their pain to childbirth. Do only women suffer from kidney stones? Or is it both men and women?
Dr. Glenn Preminger: So, it is both men and women. One myth, though about the men and women is that, for many years, men had a three times higher incidence of forming kidney stones than women. However, due to changes in diet and lifestyle, in the United States and across the world, we now see that the incidence of kidney stones in women is virtually identical to that of men. So, it's basically equal between the two genders.
Joe McIntyre: Got it. If my parents that, say, suffer from kidney stones, or one of my parents suffer from kidney stones, am I more susceptible to them myself?
Dr. Glenn Preminger: No question, Joe. The patients who have first-line family members, either parents or siblings or aunts and uncles, actually have an increased risk of making stones. But it's not a one-to-one definite prediction that if your parents make a stone, you'll form a stone. It does put you at higher risk, however.
Joe McIntyre: We have heard plenty of rumors, whether true or not, you'll tell us, that soda and tea can cause kidney stones. Is that actually true? What foods do and maybe do not cause kidney stones or are likely to cause kidney stones?
Dr. Glenn Preminger: Well, Joe, it really depends on the patient and what their particular metabolic issues are. Certain foods, specifically calcium and foods that contain a large amount of oxalate like spinach or nuts, can potentially increase the risk of stones. But it really depends on how well the kidney handles those constituents. For example, if some people who eat a lot of dairy as part of their diet with a lot of calcium, that calcium gets taken up into the bone. And therefore, if it's not coming out in the urine, it's not causing a risk for stones. So, it really depends on the individual. Once we have a better idea of what the metabolic risk factors are in a particular patient, we can make specific dietary recommendations.
Joe McIntyre: Yeah, when it comes to those dietary recommendations, are there generally foods to avoid, or reduce your intake of, if you have suffered or are likely to suffer from kidney stones?
Dr. Glenn Preminger: I think that the two major food groups that we discussed are dairy products with calcium. And the bottom line is that for the vast majority of people, we're recommending a normal dairy intake. And what we're talking about here is two to three dairy servings per day, with a dairy serving being a glass of milk, a couple slices of cheese, a small yogurt, or a scoop of ice cream. And, to avoid an excessive amount of foods that have a high amount of oxalate, such as spinach, or rhubarb, or nuts. Now, we're not saying that you have to completely eliminate these foods from your diet. But again, like most other things that are diet related, it's all about moderation. I think another major myth is what you should drink. And truthfully, it's not necessarily what you should drink, it's how much you should drink. We want to keep the kidneys flushed out, and so we therefore encourage our patients to drink anywhere from 80 to 100 ounces of fluid per day. And for the majority of my patients, I'm not as concerned about what they drink as about how much they drink. And there's no doubt that water is the cheapest, it's probably the easiest fluid to drink. But if people don't like water, they could make a homemade mixture of lemonade, or they could put a mix into their water. But juice and soda and other types of fluids, again, in moderation, are very reasonable.
Joe McIntyre: Now, are kidney stones, we've heard this rumor a little bit, are kidney stones more common in summer and in hotter climates, or is that not actually true?
Dr. Glenn Preminger: Well, it is partly true, the fact that we do see a fair amount of stones during the summertime. However, remember that while it makes sense that if you're dehydrated because of the heat or the humidity, that you might make more stones because the urine becomes more concentrated with regards to the stone forming salts, those stones still take months if not years to form. And so, if you pass a stone in July when it's hot, that stone was growing way back in January or even the summer before. So, I don't think that we can attribute heat or humidity to active stone events, but no doubt that they contribute to stone growth.
Joe McIntyre: Now, let's say someone has had kidney stones in the past and fortunately for them had just passed one, is that the end of their kidney stone experience or are they more or less likely to get another kidney stone later in life?
Dr. Glenn Preminger: So, Joe, what we find is that once you form one stone, you are at increased risk for forming additional stones and it's estimated that you have about a 50 to 80% chance of passing another stone within five to 10 years. If someone has formed a stone, at the very least, I would ask them or suggest that they contact either their primary care doctor or their urologist and ask for some general recommendations about what you can do to minimize the risk of having another stone. But other things can increase the risk for stones. As we've discussed before, family history can increase your risk for stones, the type of stone that you might pass can increase the risk for having additional stones, and other medical issues like intestinal problems, inflammatory bowel disease, or perhaps some bone issues can also increase your risk for stones. So, what we normally do is gauge the patient's risk factors for recurrent stone formation, and then we can make specific recommendations about how they can minimize their chance of having another stone.
Joe McIntyre: Dr. Preminger, you mentioned a little while ago that kidney stones form in the kidney, obviously, and they can become detached and float around, I think you said, in the kidney. Is it possible for a kidney stone to form in the kidney and not detach? And essentially, stay there for years or months or whatever it is on end?
Dr. Glenn Preminger: No question, Joe. I see a lot of patients in my practice who have had multiple stones in the past. We perform an evaluation and determine their underlying risk factors for stones, and we get them started on treatment. Many of these patients will have small stones that remain up in the kidney, and in some cases, even big stones. But the stones tend to stay up in the kidneys. And as long as they don't attach, as you mentioned, they should not cause any issues. But the things we're looking for are significant discomfort, infection, or evidence of obstruction of the stone. Those would be reasons that we'd want to remove stones. But just because the stone is sitting up in the kidney, not causing any problems doesn't mean that it needs to be removed.
Joe McIntyre: Is it possible that a kidney stone, once it's causing pain, and I guess ready to want to come out, can be too big to pass naturally through urination? And if that's the case, what happens then?
Dr. Glenn Preminger: So, really, the determinants of whether a stone passes spontaneously enough are really the size and the kind of ‘configuration’ of the stone, if you will. The kidney tube or the ureter will usually accept or pass stones that are about five millimeters in length or less. And once you get to five or six millimeters, the chance of spontaneous stone passage starts to go down dramatically. So, if I see a patient with a one- or two-millimeter stone, they have a 98 to 99% chance of passing that stone, whereas if the stone is seven or eight millimeters, that chance of spontaneous passage is closer to 20%. Now, I've seen patients pass stones that are 10 millimeters in diameter. We usually provide them with some medication that helps to relax the ureter, and with some pain medication to minimize the discomfort. And we can see patients pass stones, because we always find that Mother Nature does a better job than we do in having a stone come out. But no doubt, that the larger the stone, if it gets hung up or if it's causing significant problems for the patient, we have minimally invasive tools, usually telescopes, that we can place into the kidney or into the kidney tube to fragment and remove the stones with minimal discomfort for the patient.
Joe McIntyre: Finally, if our listeners want to go somewhere for more information or are looking for resources about kidney stones, or are even concerned that they may have one, where should they go?
Dr. Glenn Preminger: Sure. So, there's no doubt that resources like the Merck Manual are excellent for more information about kidney stones. I'll also reference the American Urological Association, which is our kind of ‘governing body’ for urologists, who have come out with very specific guidelines on both the medical and the surgical management of kidney stones. They also have information for patients based on kidney stone management, and that would be at the auanet.org, which would be a very valuable resource.
Joe McIntyre: Well, Dr. Preminger, thank you so much for your time and expertise here. It was a really, really enlightening conversation about kidney stones. And for those of us, myself included, who deal with kidney stones regularly, this is even more helpful. For anyone who's listening, for more information on these and hundreds of other medical topics, our listeners can visit merckmanuals.com and Dr. Preminger, I'm going to let you leave our listeners with what we always do with the Merck Manuals.
Dr. Glenn Preminger: Thank you, Joe. It was certainly my pleasure to be here. As you know, medical knowledge is power. Pass it on.