Acute abdominal imaging decision — OSCE

Candidate brief

You are the paediatric registrar in the emergency department. An 8-year-old boy presents with an 18-hour history of central abdominal pain migrating to the right iliac fossa, anorexia, and a low-grade fever. He is alert and perfusing well, with focal tenderness and voluntary guarding in the right lower quadrant. The registrar asks you to outline the imaging decision. [3]

Task

1. Define the clinical question that the imaging must answer. (1 minute)
2. Choose the first-line imaging modality and explain the reasoning. (2 minutes)
3. Apply the pARC risk calculator to stratify the probability of appendicitis. (2 minutes)
4. State the role of the abdominal radiograph in this presentation and explain why it is not the first test. (2 minutes)
5. State the ALARA principle and summarise the Pearce 2012 radiation-risk evidence. (2 minutes)
6. Outline the next steps for the equivocal ultrasound and the computed tomography decision. (1 minute) [3]

Examiner guidance — the expected answer

The clinical question is whether this is acute appendicitis. The first-line imaging modality is ultrasound, because it uses no ionising radiation and it directly visualises the appendix. The abdominal radiograph is not the first test because it has a very low yield in suspected appendicitis, it cannot see the appendix in the well child, it rarely changes the management, and it delivers a small radiation dose without benefit. [3]

The pARC calculator stratifies the probability from the age, the sex, the pain duration and migration, the right-lower-quadrant tenderness, the guarding, the rebound, the anorexia, and the nausea. The high-probability child proceeds to surgical review and the intermediate-probability child has the ultrasound as the deciding test. [4]

ALARA means the dose is As Low As Reasonably Achievable, because the young child is more radiation-sensitive. The Pearce 2012 study of nearly 180,000 children with CT scans estimated a small but significant increase in leukaemia and brain tumour risk with the cumulative dose, which underpins the radiation-aware practice and the Image Gently campaign. [1][2]

The computed tomography is the appropriate next step when the ultrasound is equivocal and the clinical concern persists, and the scan is dose-modified to the size of the child, limited to the region of the question, and the intravenous contrast is used when the soft-tissue detail is needed. [3]

Extension questions

• How would the imaging decision differ if this were an 8-month-old infant with episodic colicky pain and a current-jelly stool? (Expected: suspected intussusception, ultrasound first, enema reduction. [5])
• How would the imaging decision differ if this were a 5-week-old infant with bilious vomiting? (Expected: suspected malrotation with midgut volvulus, upper gastrointestinal contrast study, surgical emergency. [6])
• How would you counsel the family about the radiation dose of the computed tomography scan if it becomes necessary? (Expected: the dose is kept as low as possible, the study is justified by the clinical question, and the benefit outweighs the small risk. [1][2])