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Difficulty: MediumOrdering Imaging Studies

A 27-year-old primigravida at 23 weeks of gestation is evaluated in the emergency department for right-sided abdominal pain that started yesterday. The pain was initially dull and located around the umbilicus but has progressively worsened and shifted to the right mid-abdomen over the last twelve hours. She reports severe nausea and has vomited twice. Her temperature is 38.0°C (100.4°F), blood pressure is 112/74 mmHg, and heart rate is 102/min. On physical examination, there is marked tenderness to palpation in the right mid-abdomen, superior to the typical location of McBurney's point. Rebound tenderness is absent. Fetal heart tones are normal. Laboratory analysis shows a leukocyte count of 14,500/mm³ with 82% neutrophils. A graded compression transabdominal ultrasound is performed by an experienced sonographer, but the appendix cannot be visualized due to displacement by the enlarged uterus and acoustic shadowing from overlying bowel gas. Which of the following is the most appropriate next diagnostic step?

  1. Magnetic resonance imaging (MRI) of the abdomen and pelvis without contrastAnswer
  2. B
    Computed tomography (CT) of the abdomen and pelvis with intravenous contrast
  3. C
    Diagnostic laparoscopy
  4. D
    Repeat graded compression transabdominal ultrasonography in 12 hours
  5. E
    Plain radiography of the abdomen

Answer

The most appropriate next diagnostic step is magnetic resonance imaging (MRI) of the abdomen and pelvis without contrast.
In pregnant patients presenting with signs and symptoms of acute appendicitis, the anatomical displacement of the appendix (often shifted superiorly and laterally) makes clinical diagnosis challenging. Graded compression ultrasound is the appropriate first-line imaging modality because it is safe and widely available. However, due to the gravid uterus and overlying bowel gas, the appendix often cannot be adequately visualized. When ultrasound is non-diagnostic, the standard of care is to proceed to magnetic resonance imaging (MRI) of the abdomen and pelvis without gadolinium contrast. MRI has excellent sensitivity and specificity for appendicitis in pregnancy and, importantly, avoids exposing the fetus to ionizing radiation.

Step-by-Step Solution

1
Identify the clinical presentation and context.
The patient is a pregnant woman in her second trimester presenting with classic signs of acute appendicitis, though the pain is displaced superiorly due to the gravid uterus.
Pregnancy alters anatomical landmarks and limits diagnostic testing options due to fetal safety concerns.
2
Evaluate the results of the initial diagnostic workup.
Graded compression ultrasound, the correct initial test, was properly performed but resulted in a non-diagnostic study (appendix not visualized).
Ultrasound is first-line in pregnancy, but it fails to visualize the appendix in a significant percentage of pregnant patients due to uterine size and bowel gas.
3
Determine the optimal secondary imaging modality.
MRI of the abdomen and pelvis without contrast is selected.
MRI provides definitive cross-sectional imaging without exposing the fetus to the ionizing radiation associated with CT scans.

Key Concept

In pregnant patients with suspected acute appendicitis, graded compression ultrasound is the initial imaging test of choice. If the ultrasound is non-diagnostic, magnetic resonance imaging (MRI) without contrast is the preferred next step to establish the diagnosis while avoiding ionizing radiation.
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Ordering Imaging Studies

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