Osteomyelitis and discitis — branching viva

Candidate brief

You are the general paediatric registrar in the emergency department. You have eight minutes to answer the examiner's questions on the child below. The viva branches through diagnosis, investigation, management, and then to a second scenario. Be prepared to justify each answer with the underlying pathophysiology and the relevant evidence. [1] [4]

Opening scenario

A three-year-old boy presents with three days of fever, increasing left leg pain, and refusal to walk. He holds the left leg still and cries when you palpate the distal tibial metaphysis. His C-reactive protein is 95 milligrams per litre. His plain radiograph is normal. [1] [4]

Branch 1 — Diagnosis and pathophysiology. What is your working diagnosis? Explain the anatomical basis for the metaphyseal site of infection, and explain why the normal radiograph does not exclude your diagnosis. [1] [4]

Branch 2 — Investigation. What is the gold-standard imaging investigation, and what does it reveal? Why is the C-reactive protein the most useful marker for monitoring the response? What is the role of blood cultures, and what organism-specific consideration applies in this age group? [1] [6]

Branch 3 — Empirical antibiotics. Outline your empirical antibiotic regimen. Which organisms must you cover? How does Kingella kingae modify the regimen? When do you adjust for local methicillin-resistant Staphylococcus aureus rates? [1] [6]

Branch 4 — The intravenous-to-oral switch. State the criteria for the switch. What is the total duration of therapy for uncomplicated disease? What evidence supports this strategy? What are the risks of an overlong intravenous course? [1] [12]

Branch 5 — Failure to improve. The child remains febrile at 48 hours and the C-reactive protein has risen. What are the three possibilities, and what is your response? What does a subperiosteal abscess on magnetic resonance imaging demand? [1] [4]

Pivot scenario — The toddler who refuses to sit

The examiner now introduces a two-year-old girl who refuses to walk, sit, or crawl for five days, holds her spine rigid, and cries when her lumbar spine is palpated. She is afebrile with a C-reactive protein of 22. [9]

Branch 6 — Discitis. What is the diagnosis? What imaging confirms it, and what is the critical complication that must be excluded? How does the management differ from long-bone osteomyelitis? [9]

Branch 7 — Kingella kingae. Why is Kingella kingae an important consideration in this child, and how is the diagnostic yield optimised? [6]

Examiner's marking key

Strong answers will:

• Name acute haematogenous osteomyelitis and explain the metaphyseal capillary hairpin loop as the seedbed [4]
• State that magnetic resonance imaging is the gold standard and that a normal early radiograph does not exclude the diagnosis [1]
• Name Staphylococcus aureus and Kingella kingae as the age-stratified organisms and describe the empirical regimen [1] [6]
• State the three switch criteria (afebrile, improving, C-reactive protein falling by at least 50 per cent) and the three-to-four-week total course [1] [12]
• Identify the subperiosteal abscess as a surgical indication in the child who fails to improve [4]
• Name discitis, recognise the epidural abscess as the critical complication, and describe Kingella kingae as a milder but easily missed cause [9] [6]

Weak answers will:

• Accept the normal radiograph as excluding osteomyelitis [1] [4]
• Fail to include Kingella kingae in the microbiological differential of a child under four years [6]
• State a fixed four-to-six-week intravenous course rather than the response-guided oral-switch strategy [1] [12]
• Miss the subperiosteal abscess as the reason for failure to improve and the indication for surgery [4]