Young, pregnant patients presenting with loin or back pain and associated hydronephrosis on imaging create a diagnostic dilemma. The challenge is establishing the causal link between the pain and imaging while limiting or minimizing radiation exposure. Although back pain in pregnancy is common, it can be secondary to physiological changes in pregnancy or pathological causes, which may be obstetric or non-obstetric in nature. Kidney stones are the most common cause of pathological pain (1).
The incidence of kidney stones is increasing and affects 10% of people at some point in their lives. A stone event occurs in 1 out of every 200–1500 pregnancies, with 80%–90% presenting in the second and third trimesters (2). Although the incidence of stones is no higher in pregnant women than in the non-pregnant population, a stone event during pregnancy represents a unique clinical situation that poses risk to both the mother and the fetus.
Pregnancy is a complex state that alters the risk factors for stone formation. Increased stone promoters are offset by an increase in stone inhibitors. Urinary stasis promotes stone formation, and hydronephrosis during pregnancy occurs in 90% of pregnant women, often physiological from compression at the pelvic brim by the growing uterus and smooth muscle relaxation induced by elevated progesterone levels (2). These normal anatomical changes during pregnancy can make it challenging to determine the diagnosis.
On the basis of symptoms alone, a preliminary diagnosis can be made but can be incorrect in at least 30% of patients (2). Ultrasound remains the first-line imaging modality given its lack of radiation exposure. Although the specificity is as high as 94%, it has a low sensitivity of up to 45%, therefore failing to rule out stones. In one study examining kidney stones in pregnancy, only 60% of stones were identified using ultrasound (1–3).
This leads to a further diagnostic dilemma: what to do next. The best second-line imaging is controversial, with magnetic resonance imaging (MRI) previously being the recommended choice for its lack of radiation. However, it is often limited by availability and relies on secondary signs of a stone such as dilation (2).
Low-dose computed tomography of the kidney, ureter, and bladder (CT KUB) is increasingly being used because it has a higher positive predictive rate (>95%) than an ultrasound or MRI (2) but raises the question of the effect of radiation on the fetus. The good news is that fetal risk of anomalies, growth restriction, and abortion have not been reported with radiation exposure <50 mGy, which is endorsed by The American College of Obstetricians and Gynecologists (4, 5). The risk of carcinogenesis as a result of in utero exposure to ionizing radiation is unclear but is probably very small. A CT intravenous pyelogram has 10–15 mSv used for investigation of hematuria; however, a low-dose CT KUB has an average dose of 1–3 mGy, well below the concerning levels mentioned above (2).
Pregnancy is a complex state that alters the risk factors for stone formation.
The American Urological Association (5) and the European Association of Urology (6) advise that CT KUB should not be withheld if clearly indicated because the material benefit for early and accurate diagnosis may outweigh the theoretical harm to the fetus. Locally, we are endorsing the use of low-dose CT KUB in pregnancy as a second-line imaging modality following discussion in a multidisciplinary setting, including radiology, urology, and obstetrics specialties.
Managing proven kidney stones in pregnancy is a complex situation. Intrarenal stones will be managed conservatively along with uncomplicated ureteric stones. However, in the case of infection, uncontrolled pain, or worsening kidney function, an urgent de-obstructive procedure must be performed, either a stent or nephrostomy tube. Definitive stone treatment will be deferred until the woman is postpartum, or if an intervention is required, ureteroscopy or laser lithotripsy is safest in the second trimester.
References
- 1.↑
Rashid P. The role of imaging in pregnancy associated loin pain. BJUI Knowledge, 2015. https://doi.org/10.18591/BJUIK.0401
- 2.↑
Thakur APS, et al. Management of ureteric stone in pregnancy: A review. Afr J Urol 2020; 26:60. https://afju.springeropen.com/articles/10.1186/s12301-020-00070-5
- 3.↑
Semins M, Matlaga B. Kidney stones during pregnancy. Nat Rev Urol 2014; 11:163–168. doi: 10.1038/nrurol.2014.17
- 4.↑
The American College of Obstetricians and Gynecologists. Committee opinion No. 723: Guidelines for diagnostic imaging during pregnancy and lactation. Obstet Gsynecol 2017; 130:e210–e216. https://journals.lww.com/greenjournal/pages/results.aspx?txtKeywords=Guidelines+for+diagnostic+imaging+during+pregnancy+and+lactation
- 5.↑
Fulgham P, et al. Clinical effectiveness protocols for imaging in the management of ureteral calculous disease: AUA technology assessment. J Urol 2013; 189:1203–1213. doi: 10.1016/j.juro.2012.10.031
- 6.↑
European Association of Urology. Guidelines. Urolithiasis. https://uroweb.org/guidelines/urolithiasis/chapter/introduction