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Kidney Stones During Pregnancy: Imaging & Diagnosis

Author Melissa Malena covers Kidney Stones During Pregnancy: Imaging & Diagnosis  on BackTable OBGYN

Melissa Malena • May 29, 2024 • 39 hits

Kidney stones can be difficult to treat, especially in pregnant patients where stone visualization, diagnosis, and treatment require unique considerations. Pregnancy especially affects kidney stone imaging modalities as radiation exposure endangers the fetus. Expert urogynecologists Dr. Suzette Sutherland and Dr. Alana Desai of the University of Washington discuss their strategies for stone imaging in the pregnant patient, and share their recommendations on differentiating between pregnancy-related hydronephrosis and true blockages.

This article features excerpts from the BackTable OBGYN Podcast. We’ve provided the highlight reel in this article, but you can listen to the full podcast below.

The BackTable OBGYN Brief

• Contrary to previous beliefs, pregnancy does not increase the rate of kidney stone occurrence, although it might increase the risk of stone-related complications.

• The preferred modality for kidney stone imaging in pregnant patients is ultrasound, followed by T2 weighted MRI without contrast.

• CT scans can be used as a third line imaging tool in pregnant patients, as long as gestational age supersedes fifteen weeks.

• Hydronephrosis, caused by increased progesterone levels, can occur as early as six weeks and continue throughout the pregnancy as the uterus enlarges.

• Obstructing stones and pregnancy related hydronephrosis both commonly present as flank pain, requiring practitioners to differentiate between the two through a detailed physical examination and patient history.

Kidney Stones During Pregnancy: Imaging & Diagnosis

Table of Contents

(1) Kidney Stones in Pregnancy: Is the Incidence Really Higher?

(2) Kidney Stone Imaging During Pregnancy: Ultrasound, MRI & CT

(3) Differentiating Between Pregnancy Related Hydronephrosis & Kidney Stones

Kidney Stones in Pregnancy: Is the Incidence Really Higher?

Despite previous beliefs that pregnant patients are at a higher risk for developing kidney stones, current research shows that the incidence in pregnancy is consistent with that of the general population. However, a 2021 Mayo Clinic study suggests that pregnancy increases the risk of a first-time symptomatic stone, with risk peaking around the time of delivery. During pregnancy, hydronephrosis due to increased progesterone levels often induces widespread ureteral dilation which increases the risk of preexisting stone migration and obstruction. According to Dr. Desai and Dr. Sutherland, pregnancy does not increase the likelihood of stone formation but can increase the risk of stone-related complications.

[Dr. Suzette Sutherland]
There's just so many worrisome details to consider, not only the urological details that we consider in the non-pregnant patient, but now we're worried about not only mom, but the fetus. Let's just start to dive into some basics about kidney stones and pregnancy. What's the real incidence of kidney stones during pregnancy? Is the incidence really higher? If so, what are the factors that increase that incidence?

[Dr. Alana Desai]
Sure. Previously thought to not be a higher risk in pregnant patients than the general population, because of the difficulties associated with managing them. The incidence may seem higher, but typically it's not higher than the general population. In a 2020 claims-based analysis of 1.4 million pregnancies, stone disease was found to be diagnosed in only 1% of all pregnancies.

There is some newer evidence from an observational study at Mayo Clinic in 2021, however, that pregnancy does increase the risk of a first-time symptomatic stone, with that risk peaking close to delivery. It does improve about a year after delivery, but there's still a bit higher risk compared to age-match controls going forward. It is also the most common non-obstructive because for hospital admissions, complicating 1 in 200 to 1 in 2,000 pregnancies.

[Dr. Suzette Sutherland]
That's a lot lower than I think most people think about. Certainly you don't see it all the time, but I guess maybe it just makes such an impression on us as urologists when we do see it in the pregnant patient that we think of it. Historically, people have thought that the incidence, maybe, or your chance of having a kidney stone as a woman is higher when you're pregnant because of maybe some hormonal changes or dilation of the ureters. Is any of that then true?

[Dr. Alana Desai]
There are both anatomical and physiological changes that occur during pregnancy due to the enlarging uterus. You can get just hydronephrosis, which typically occurs in about 90% of people by the third trimester, but it can also occur as early as a sixth week. That's just due to increased hormone production, namely progesterone. Because of this dilation, there's also an increased risk of stone migration and obstruction. Stones are twice as likely to be located in the ureter when diagnosed during pregnancy, just because of that migration.

Even when not caused by stones, hydronephrosis in pregnancy can also lead to symptoms. The dilation from the stasis of the urine along with the elevated progesterone causing further ureter dilation and reducing ureteral peristalsis, both of these can promote the formation of stone crystals or urinary crystals. This is further accelerated by the increased GFR, plasma, and the plasma flow rate. These do lead to an increased lithogenic factors, including urinary calcium oxalate and uric acid. The hypercalceria or higher calcium in the urine is also exacerbated by placental production of vitamin D to meet the demands of the fetus.

However, even though these lithogenic factors are increased by this increased flow, there is also an increased excretion of stone inhibitors such as glycoprotein, nephrocalcine, and tamhorsol that should compensate for the lithogenic factors. Again, a stone formation is not necessarily increased in pregnant patients because it is balanced by the stone inhibitors. Again, like you said, it potentially just feels like it's an increased risk in pregnant patients.

[Dr. Suzette Sutherland]
Is it maybe fair to say the incidence of developing a stone isn't necessarily higher in the non-pregnant female, but you described some other factors that might make them move during pregnancy are more likely that they try to start to pass. They may become more problematic during the pregnancy, whereas they were just nice and quiet up in the kidney previously. Is that fair?

[Dr. Alana Desai]
I think that is a potential. Again, with these findings from the Mayo Clinic, it's tough to know whether stone formation itself is increased or just a symptomatic event. Again, with that stone passage, like you said.

Listen to the Full Podcast

Urolithiasis in Pregnancy: Balancing Risks & Management with Dr. Alana Desai on the BackTable OBGYN Podcast)
Ep 43 Urolithiasis in Pregnancy: Balancing Risks & Management with Dr. Alana Desai
00:00 / 01:04

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Kidney Stone Imaging During Pregnancy: Ultrasound, MRI & CT

Determining the ideal imaging modality to view kidney stones in pregnant patients poses a unique challenge. While ultrasound is the first line imaging technique for stone visualization, many stones will not be visible. Implementing resistive indices in Doppler ultrasounds can help to distinguish between true obstruction and physiologic hydronephrosis. In around 15 percent of pregnant patients, the ureteral jets will not be visible so diagnosis requires a combination of imaging, history, and laboratory findings.

If concrete visualization of the stone is required, AUA and EAU guidelines suggest utilizing MRI without contrast as a second line imaging modality. T2 weighted scans are the most amenable to differentiating between hydronephrosis and stone blockage. Although generally avoided, CT scans can be used as a third line imaging modality, as long as the gestational age of the fetus surpasses 15 weeks to avoid dangerous radiation doses during critical phases of development.

[Dr. Suzette Sutherland]
Then we know that sometimes stones don't show up well on ultrasound, but of course, the thinking is if they're so small they don't show up, they're not going to be problematic. What size do you think makes that differentiating factor about whether it's really going to be a problematic stone or not, if you can see it on ultrasound?

[Dr. Alana Desai]
Sure. I think this varies by patient, body habitus, and stone size. As you said, a lot of stones don't show up on ultrasound. There are additional findings in the absence of actually visualizing a stone with an ultrasound. There are other features of ultrasound that we paid attention to in this diagnosis. One, hydronecrosis, of course, but you can evaluate ureteral presence, absence, delay. The other thing that people look at with pregnant stones is the resistive indices. It's a calculation using peak systolic velocity and diastolic velocity, using Doppler ultrasound. This can help distinguish true obstruction from physiologic hydronephrosis of pregnancy.

One of the other things you can use is the difference between kidneys. If there's a difference between more than 0.06 between resistive indices between kidneys, that's a very high sensitivity and specificity for obstruction of the ipsilateral kidney. In addition, like I said, to hydronephrosis and presence of stone, you can look at these other features of ultrasound and help improve the diagnostic accuracy of the ultrasound.

[Dr. Suzette Sutherland]
How easy actually in a pregnant patient who's farther along in her pregnancy is it to see ureteral jets, down low?

[Dr. Alana Desai]
They can just be missing in general and up to 15% of patients. It's not the only factor. When you're presented with a pregnant stone case, there's so many factors that you really try to hone in on to determine whether it is actually a stone or not. These are just the, sort of you take the constellation of historical factors, laboratory findings, imaging, and try to piece it together while reducing your risk to the mom and fetus or minimizing your risk.

[Dr. Suzette Sutherland]
If you're not finding anything definitively on with those modalities, but your suspicion is still pretty high, you have a pretty pained patient in front of you and your suspicion is high. When do you move on to CAT scan, radiation? What are the true radiation risks to the fetus with these modalities?

[Dr. Alana Desai]
Sure. Both AUA and EAU guidelines suggest MRI as a second line imaging modality without contrast. MRI using T2-weighted images can help to differentiate between physiologic and pathologic hydronephrosis during pregnancy. There are theoretical risks associated with this, such as the thermal effect of radiofrequency pulses, but the American College of Radiology have determined the dose to be safe in all pregnant patients. This is also recommended by the ACOG as well, saying that the teratogenic effects are minimal.

[Dr. Suzette Sutherland]
Do you find that today that is very widely used among general practitioners, MRI for stones?

[Dr. Alana Desai]
Yes, that is a good question. There are disadvantages owing to limited availability, especially after hours and prolonged acquisition time. There are certain protocols, like HAITH protocol, in which the test can be completed in less than 15 minutes. It is probably not widely used, but it is second line imaging. Now, low dose CT can be used as a second or third line option, but you do want to wait until at least the second trimester if you do need that. In the short term, the concern for radiation exposure are the potential teratogenic effects on the developing fetus.

Of course, this varies based on the radiation dose, the gestational age of the fetus. If one has to perform a CT scan, the best time to avoid it is between the second and 15th week of gestation when the radiation effects of the fetus are highest. Of course, appropriate patient counseling and involving the patient in this decision making process should occur. Longer term, there is some evidence that intrauterine radiation exposure can increase later risk of childhood cancers.

Differentiating Between Pregnancy Related Hydronephrosis & Kidney Stones

Hydronephrosis can be caused by an obstructing stone or as a complication of pregnancy. It can develop as early as the sixth week of pregnancy due to increased progesterone levels. As pregnancy progresses, hydronephrosis may result from mechanical obstruction caused by the gravid uterus and the growing fetus. Early pregnancy hydronephrosis can complicate the diagnostic process for kidney stones since both conditions often present with intense flank pain.

To differentiate between pregnancy-related hydronephrosis and an obstructing kidney stone, Dr. Desai recommends starting with a thorough physical examination and patient history. Kidney stone disease is characterized by a combination of flank pain, nausea, vomiting, microhematuria, gross hematuria, fever, and chills. When dealing with a pregnant patient, Dr. Desai advises evaluating for kidney stones independently of the pregnancy and following the diagnostic protocol as if the patient were not pregnant.

[Dr. Suzette Sutherland]
You used a term, I'm trying to think back to the term that you used, but hydronephrosis, I think of pregnancy is a term that we talk about. When you use that term, of course we could have hydronephrosis if we have an obstructing stone, but during pregnancy, are you just referring to the fact that the baby also is laying on the ureter during those later trimesters? Is there more to it in general, even during early pregnancy that it's common to see a slight hydronephrosis on one side or the other?

[Dr. Alana Desai]
Right. Both early in pregnancy, like I said, can occur up to as early as the sixth week of pregnancy. That's due to the increased progesterone that can be caused because of some of the ureteral dilation in itself. Then later in pregnancy is when hydronephrosis can be seen because of the gravid uterus itself. A mechanical obstruction, whereas in earlier pregnancy, it's this physiologic sort of response to the progesterone.

[Dr. Suzette Sutherland]
That's interesting because we often as urologists get referred these patients early on in their pregnancy. We know the fetus isn't that big yet. Pushing on the ureter causing the hydro, but we're being asked to evaluate them for a stent because they have some hydro. This is very important information to know that this is pretty common within six weeks already of pregnancy just because of hormonal changes.

[Dr. Alana Desai]
Exactly. Most people probably know this, but it is more common on the right side because of the production of the sigmoid colon on the left side. Up to 90% on the right side, it can occur in 67% on the left.

[Dr. Suzette Sutherland]
Then if we are suspecting that a pregnant woman, she's having some pain on one side or the other, how do we best differentiate between maybe they're just a little bit of hydronephrosis from the pregnancy versus a truly alarming, maybe obstructing stone? What's the best method of evaluation to start with?

[Dr. Alana Desai]
You have to start with your very basic history and physical examination. Again, as you said, you can't have leg pain just from pregnancy itself or back pain commonly. The most common presenting symptom of kidney stone disease is flank pain, about 80% to 100% present like this, followed by nausea and vomiting. Again, typical symptoms of pregnancy. You can also see other signs of stone disease such as microhematuria and gross hematuria, and then less commonly, fevers and chills.

Typically, again, you start with your basic history. Sometimes they'll tell you, I had an acute, sudden episode of pain in my back, and then it wrapped around to my abdomen, went to my groin. A little while later, I felt like I had to go to the bathroom. It just sounds like a classic stone history. That is very important. I think it's important to evaluate pregnant patients as you would a non-pregnant patient, taking those key events that happened during stone passage, and then, of course, considering the fact that there is a fetus involved. If there is fetal distress, then you have to take that into consideration. Initially, when you evaluate, it is much like a non-pregnant patient in taking that good history in laboratory evaluation as we normally would.

Podcast Contributors

Dr. Alana Desai discusses Urolithiasis in Pregnancy: Balancing Risks & Management on the BackTable 43 Podcast

Dr. Alana Desai

Dr. Alana Desai is an associate professor with UW Medicine in St. Louis, Missouri.

Dr. Suzette Sutherland discusses Urolithiasis in Pregnancy: Balancing Risks & Management on the BackTable 43 Podcast

Dr. Suzette Sutherland

Dr. Suzette Sutherland is the director of female urology with UW Medicine in Seattle, Washington.

Cite This Podcast

BackTable, LLC (Producer). (2024, January 10). Ep. 43 – Urolithiasis in Pregnancy: Balancing Risks & Management [Audio podcast]. Retrieved from https://www.backtable.com

Disclaimer: The Materials available on BackTable.com are for informational and educational purposes only and are not a substitute for the professional judgment of a healthcare professional in diagnosing and treating patients. The opinions expressed by participants of the BackTable Podcast belong solely to the participants, and do not necessarily reflect the views of BackTable.

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