What is the recommended initial imaging test for a pregnant woman presenting with right flank pain?

Imaging for Pregnant Women with Right Flank Pain

Ultrasound should be used as the first-line imaging modality for pregnant women presenting with right flank pain, regardless of trimester. 1, 2

Diagnostic Algorithm for Right Flank Pain in Pregnancy

Initial Imaging

• First step: Renal ultrasound
  • Highest appropriateness rating (8 out of 9) by the American College of Radiology 1
  • No radiation exposure to mother or fetus
  • Can detect hydronephrosis, perinephric fluid, and sometimes stones
  • Visualization rate of urinary stones varies (sensitivity ~80% for clinically significant stones)

Second-line Imaging (if ultrasound is inconclusive)

• MRI without contrast

  • Appropriateness rating of 5 (may be appropriate) 1
  • Excellent for detecting hydronephrosis and perinephric edema
  • Less accurate for directly visualizing stones
  • Can identify alternative diagnoses (in 10-43% of cases) 2
  • High sensitivity (91-100%) and specificity (93-98%) for conditions like appendicitis 2

• Low-dose non-contrast CT

  • Appropriateness rating of 6 (may be appropriate) in second and third trimesters 1
  • Should be considered only when MRI is unavailable and diagnosis remains unclear 2
  • Average fetal radiation exposure ~700 mrads (range 210-1372 mrads) 3
  • Higher sensitivity for stone detection than ultrasound or MRI

Clinical Considerations

Trimester-Specific Approach

• First trimester: Ultrasound → MRI if inconclusive
• Second/third trimester: Ultrasound → MRI if available → Low-dose CT if MRI unavailable and diagnosis critical

Common Pitfalls to Avoid

1. Delaying imaging due to pregnancy concerns

   • Prompt diagnosis is essential to prevent complications like renal damage or sepsis
   • Untreated conditions can pose greater risk to mother and fetus than appropriate imaging

2. Misattributing symptoms to normal pregnancy

   • Right flank pain warrants thorough evaluation 2
   • Physiologic hydronephrosis of pregnancy can complicate diagnosis

3. Overreliance on clinical findings alone

   • Clinical findings like fever and leukocytosis are not reliable enough for diagnosis in pregnancy 2
   • Imaging is essential for accurate diagnosis

4. Using standard-dose CT as initial imaging

   • Unnecessary radiation exposure to fetus
   • Low-dose protocols can reduce exposure while maintaining diagnostic accuracy 3

Differential Diagnosis Considerations

When evaluating right flank pain in pregnancy, consider:

• Urolithiasis (most common cause)
• Pyelonephritis
• Spontaneous rupture of renal pelvis 4
• Appendicitis (if pain radiates to right lower quadrant) 2, 5
• Obstetric causes (placental abruption, round ligament pain)
• Gallbladder disease (if pain is more upper quadrant)

Management Based on Imaging Findings

• For confirmed urolithiasis: Conservative management for stones likely to pass spontaneously
• For obstructive uropathy: Consider ureteral stent placement or percutaneous nephrostomy if complicated by infection
• For non-urologic causes: Management based on specific diagnosis (e.g., appendicitis may require surgical intervention) 2

The 2023 ACR Appropriateness Criteria strongly supports ultrasound as the initial imaging modality for pregnant patients with suspected renal colic, with MRI or low-dose CT reserved for cases where ultrasound is inconclusive 1.