What is the appropriate workup for flank pain in a pregnant woman at 32 weeks gestation?

Workup for Flank Pain at 32 Weeks Pregnancy

Ultrasonography of the kidneys and bladder should be the first-line imaging study for evaluating flank pain in a pregnant woman at 32 weeks gestation, as it provides reasonable sensitivity for detecting urinary stones without exposing the fetus to radiation. 1, 2

Initial Assessment

Primary Differential Diagnosis

• Urolithiasis (kidney stones)
• Physiologic hydronephrosis of pregnancy
• Pyelonephritis
• Spontaneous adrenal hemorrhage (rare but serious) 3

Immediate Evaluation

• Vital signs with attention to fever and blood pressure
• Complete urinalysis and urine culture
• Blood tests: creatinine, uric acid, ionized calcium, sodium, potassium, complete blood count, and C-reactive protein 1

Imaging Algorithm

1. First-line: Renal ultrasonography with color Doppler

   • Evaluates for hydronephrosis, stones, and ureteral obstruction
   • Safe for mother and fetus with no radiation exposure
   • Sensitivity ~45% with specificity of ~94% for ureteral stones 1
   • Look for indicators requiring intervention:
     • Renal pelvis anteroposterior diameter >16.5 mm
     • Resistive index (RI) of kidney >0.70
     • Difference in RI between affected and normal kidneys >0.04 2

2. Second-line (if ultrasound inconclusive):

   • Magnetic resonance urography (MRU) without contrast
   • Provides better visualization of collecting system and potential obstruction
   • No radiation exposure to fetus 1

3. Last-resort (if diagnosis remains unclear and treatment decisions depend on accurate diagnosis):

   • Low-dose CT without contrast
   • Higher sensitivity (93.1%) and specificity (96.6%) than ultrasound 1
   • Used more commonly in second and third trimesters when benefits outweigh risks 1, 4
   • Average fetal radiation exposure with low-dose protocol: ~700 mrads 4

Important Clinical Considerations

Physiologic vs. Pathologic Hydronephrosis

• Physiologic hydronephrosis occurs in >80% of pregnant women, more commonly on the right side 1
• Poor correlation between hydronephrosis and flank pain intensity 5, 6
• Presence of hydronephrosis alone should not be considered etiologic for flank pain without other clinical evidence 5

Risk Assessment for Urolithiasis

• Clinical predictors that increase likelihood of urolithiasis:
  • Left-sided symptoms (adjusted OR 44.3)
  • Difference in calf circumference ≥2 cm (adjusted OR 26.9)
  • First-trimester presentation (adjusted OR 63.4) 1

Management Considerations

• If symptoms are well-controlled, observation is first-line therapy
• For intractable pain or evidence of obstruction, ureteroscopy is preferred over prolonged stenting 2, 7
• Ureteral stenting may be necessary for temporary decompression but requires frequent exchanges due to rapid encrustation during pregnancy 2

Monitoring and Follow-up

• For conservative management: follow with ultrasound every 2-4 weeks to monitor hydronephrosis progression 2
• Monitor for signs of preterm labor, as stone events in pregnancy carry increased risk of maternal and fetal morbidity 2
• Coordinate care with obstetrics for any interventional management 2

Pitfalls to Avoid

• Do not assume hydronephrosis is the cause of flank pain without other supporting evidence 5
• Avoid NSAIDs for pain management as they are contraindicated in pregnancy 2
• Do not delay imaging if there is fever, as this may indicate pyelonephritis requiring urgent treatment
• Consider rare but serious causes like spontaneous adrenal hemorrhage in cases of severe, acute-onset flank pain 3