Osteomyelitis and discitis — formative SAQs

SAQ 1 — The febrile child with a painful limb (10 marks)

Stem: A three-year-old boy presents to the emergency department with three days of fever, increasing pain in the right leg, and a refusal to walk. On examination he is febrile at 39 degrees, holds the right leg still, and has exquisite tenderness over the distal tibial metaphysis. His C-reactive protein is 95 milligrams per litre, his erythrocyte sedimentation rate is 55 millimetres per hour, and his white cell count is 16,000. Plain radiographs are normal. [1] [4]

a) State the most likely diagnosis and explain why the normal radiograph does not exclude it. (2 marks) [1] [4]

b) Outline the pathophysiology of this condition, explaining the anatomical basis for the metaphyseal site of infection. (3 marks) [4]

c) Describe the appropriate empirical antibiotic regimen for this child, including the organisms that must be covered and the rationale for the age-stratified microbiology. (3 marks) [1] [6]

d) State the criteria for switching from intravenous to oral antibiotic therapy and the recommended total duration of treatment for uncomplicated disease. (2 marks)

SAQ 2 — The toddler who refuses to walk or sit (10 marks)

Stem: A two-year-old girl is brought by her parents because she has refused to walk, sit, or crawl for the past five days. She holds her back rigid and cries when you palpate her lumbar spine. She is afebrile and her C-reactive protein is 22 milligrams per litre. She recently had an upper respiratory infection. [9] [6]

a) What is the most likely diagnosis, and what is the gold-standard imaging investigation? (2 marks) [9]

b) Describe the typical epidemiology, clinical features, and microbiology of this condition in children. (4 marks) [9] [10]

c) Outline the management, including the empirical antibiotic strategy and the key complication that must be excluded on imaging. (2 marks) [9] [1]

d) Explain why Kingella kingae is an important consideration in this age group and how the diagnosis is optimised. (2 marks)

Model answer — SAQ 1

a) The most likely diagnosis is acute haematogenous osteomyelitis of the distal tibia. Plain radiographs are normal in the first seven to fourteen days because the cortical and periosteal changes take time to develop. The radiograph's role is to exclude fracture, tumour, and mimics, not to confirm osteomyelitis. A child with clinical suspicion requires magnetic resonance imaging regardless of the plain film. [1] [4]

b) In acute haematogenous osteomyelitis, bloodborne bacteria lodge in the terminal capillary loops of the metaphyseal circulation, which arise from the nutrient artery and turn sharply at the growth plate to drain into the slow-flowing venous sinusoids. This hairpin architecture creates a zone of slow, turbulent blood flow and low oxygen tension in which bacteria can adhere and multiply. Once established, the infection raises intraosseous pressure, compresses the blood supply, and tracks through the cortical Haversian canals to the subperiosteal space. [4]

c) Empirical therapy must cover Staphylococcus aureus (the leading organism at all ages) and Kingella kingae (the leading cause under four years). An anti-staphylococcal beta-lactam such as intravenous cefazolin or flucloxacillin is the backbone, with a third-generation cephalosporin such as ceftriaxone added to cover Kingella kingae. Where local methicillin-resistant Staphylococcus aureus rates are high, clindamycin or vancomycin is substituted. Blood cultures are obtained before antibiotics to guide definitive therapy. [1] [6]

d) The oral switch is made when the child is afebrile, clinically improving (less pain, less swelling, willing to bear weight), and the C-reactive protein is falling by at least 50 per cent from peak. The total course is three to four weeks for uncomplicated disease. The de Graaf feasibility study and the 2021 PIDS/IDSA guideline support this shorter, response-guided strategy. [1] [12]

Model answer — SAQ 2

a) The most likely diagnosis is discitis (spondylodiscitis). Magnetic resonance imaging of the spine is the gold-standard investigation. It reveals high signal in two adjacent vertebral bodies and the intervening disc and excludes the critical complication of epidural abscess with cord compression. [9]

b) Discitis classically affects children aged one to five years and presents with refusal to walk, sit, or crawl, a rigid spine, and crying when the spine is palpated. Fever is often low-grade or absent, and inflammatory markers may be only mildly elevated. The lumbar spine is most often affected. The microbiology is dominated by Staphylococcus aureus and Kingella kingae, and the recent upper respiratory infection supports Kingella kingae as the likely organism. [9]

c) Management comprises empirical antibiotics covering Staphylococcus aureus and Kingella kingae, immobilisation, and analgesia, typically for two to four weeks. The critical complication to exclude on magnetic resonance imaging is epidural abscess with spinal cord compression, which presents with neurological signs and demands urgent surgical decompression. [9] [1]

d) Kingella kingae is the leading cause of osteoarticular infection in children under four years. It colonises the pharynx and causes transient bacteraemia following viral upper respiratory infections. Its presentation is milder than staphylococcal infection, with lower fevers and lower inflammatory markers, which can delay diagnosis. The organism is fastidious on routine culture, and the yield is improved with enriched media or nucleic-acid amplification assays. [6]