Stones During Pregnancy

Incidence of stones during pregnancy

• Overall, no difference in the incidence of symptomatic urinary calculi for pregnant women compared to nonpregnant women of childbearing age
  • Urologic changes in pregnancy
    • Renal blood flow increases, leading to a 30-50% rise in glomerular filtration rate.
      • As a result, the normal ranges of serum creatinine and blood urea nitrogen are ≈25% lower for the pregnant patient.
      • The increased renal blood flow and glomerular filtration rate also increase the filtered loads of sodium, calcium, and uric acid, causing a state of hypercalciuria and hyperuricosuria. However, these potentially lithogenic physiologic changes are offset by an increase in the excretion of urinary inhibitors, such as citrate and magnesium, as well as an increase in urine output
        • Hypercalciuria is further enhanced by placental production of 1,25(OH)2D3, which increases intestinal calcium absorption and secondarily suppresses PTH
      • These metabolic alterations in the urine may contribute to the accelerated encrustation of ureteral stents during pregnancy
        • As a result of these temporary physiologic changes, a metabolic evaluation is not generally undertaken to determine the cause of the stone disease until after the woman has delivered and returned to her baseline state of health.
  • Stone composition
    • One multi-institutional study found that 74% of stones from pregnant women were composed predominantly of calcium phosphate and 26% were predominantly calcium oxalate

Natural History

• Renal colic during pregnancy is a risk for:
  1. Preterm delivery
  2. Premature rupture of membranes

Diagnosis and evaluation

History and physical exam

• Flank pain (most common presenting symptom, usually accompanied by either macroscopic or microscopic hematuria)
• Many of the usual manifesting signs and symptoms may be masked by the patient’s gravid status

Imaging

• Ultrasound
  • Preferred modality in pregnant women
  • If an obstructing calculus cannot be visualized by conventional renal US, transvaginal US can provide imaging of the distal ureter
• MRI
  • Recommended as a second-line imaging test when ultrasonography is nondiagnostic (CW12 p2039)
  • Does not rely on ionizing radiation or contrast medium, making it a potentially attractive tool to evaluate pregnant patients.
  • Because MRI does not visualize calcium, stones are seen as filling defects overlying the high signal intensity of urine.
  • Visualization of smaller stones with this technique is difficult
• Limited Intravenous Pyelography (IVP)
  • Consists of one scout image followed by one plate taken ≈30 minutes after the injection of contrast.
    • Each plain film exposes the fetus to 0.1-0.2 rads, much below the threshold of 1.2 rads, at which the risk begins to increase.
      • Low-dose CT <0.19 rads (1.9 mSV; 1 rad = 10 mSV) (online source)
      • The total pregnancy exposure should not exceed the American College of Obstetrics and Gynecology (ACOG) recommended maximum of 5 rads (2016 AUA Stone Surgery Guidelines)
• Low-dose CT
• Conventional CT
  • Should be avoided during pregnancy due radiation particularly high dose
• Hydronephrosis/hydroureter of Pregnancy
  • Due to (2):
    1. Increased circulating progesterone, which causes relaxation of ureteral smooth muscle, reducing ureteral peristalsis.
    2. Direct compression of the ureters by the gravid uterus
       • Likely the main factor
         • Women with an altered upper urinary tract in whom the ureter does not cross the pelvic brim, such as those with ileal conduit or renal ectopia, do not experience hydronephrosis during pregnancy
  • Right ureter tends to be more dilated than the left
  • Resolves 4-6 weeks post-partum

Management

• First-line: observation in pregnant patients and well controlled symptoms
  • 50-80% of pregnant patients with symptomatic calculi will pass their stones spontaneously
  • A stone event in pregnancy is associated with an increased risk of maternal and fetal morbidity, so patients should be followed closely for recurrent or persistent symptoms.
• Second-line:
  • Pharmacological and surgical intervention should be coordinated with the obstetrician
    • Pharmacological
      • Should MET be considered for the pregnant patient, patient should be counseled that MET has not been investigated in the pregnant population, and the medication is being used for an “off-label” purpose.
      • NSAIDs (e.g., ketorolac) are contraindicated in pregnancy
    • Surgical
      • Ureteral stent or percutaneous nephrostomy tube
        • Disadvantages:
          1. Increased risk for bacteriuria and UTI
          2. Increased risk of stent encrustation/migration
             • Ureteral stents placed in pregnant women should be exchanged every 4 to 6 weeks.
               • For a woman in an early gestational stage, multiple stent changes will be required over the course of the pregnancy.
          3. Stent pain, which can have a negative impact on a patient’s quality of life
          • Many of the same limitations that apply to ureteral stents also apply to nephrostomy tubes in that ≈50% of patients will require exchanges, replacements, or flushing because of dislodgement or obstruction
      • Ureteroscopy
        • Methods to minimize radiation exposure in a pregnant patient (4):
          1. Low dosed and pulsed fluoroscopy§
          2. Collimating to the minimum required visual fluoroscopy field
          3. Using the last image hold feature
             • Has been shown to reduce radiation exposure by reducing the number of repetitive images.
          4. Below-table x-ray source§
             • The uterus is located superior and anterior aspect of the pelvis during the pregnancy
             • X-ray beams that project in a posterior to anterior (PA) direction contribute to less radiation than the beam projected in anterior to posterior (AP) direction because, in PA projection, the X-ray gets attenuated before reaching anteriorly located uterus
               • In the unpregnant- patient, positioning the radiation source of the C-arm under the operating table reduces exposure to the surgeon by reducing scatter radiation but does not change patient exposure.
                 • Scattering of the primary beam from the patient is the primary source of radiation exposure to the operator during endourologic procedures.
                   • Maximizing the distance between the operator and the patient during fluoroscopy is a very effective method of reducing exposure.
          5. X-ray source further away from patient
             • Reduces exposure to fetus but increases scatter to physician
          6. Lead apron placed below the patient's pelvis to shield the fetus§
        • The ACOG guidelines recommend that any nonurgent surgeries, such as URS, should be performed in the second trimester of pregnancy to minimize the risk of preterm contractions and spontaneous abortion

Questions

1. What are potential risks of renal colic during pregnancy?
2. What are the causes of physiologic hydronephrosis in pregnancy?
3. If ureteroscopy is performed during pregnancy, what can be done to reduce radiation exposure to the fetus?

Answers

1. What are potential risks of renal colic during pregnancy?
   1. Premature delivery
   2. Premature rupture of membranes
2. What are the causes of physiologic hydronephrosis in pregnancy?
   1. Compression by gravid ureters
   2. Increased progesterone
3. If ureteroscopy is performed during pregnancy, what can be done to reduce radiation exposure to the fetus?
   1. Place x-ray source below patient
   2. Place lead apron below patient to protect fetus

References

• Wein AJ, Kavoussi LR, Partin AW, Peters CA (eds): CAMPBELL-WALSH UROLOGY, ed 11. Philadelphia, Elsevier, 2015, chap 54