Acute monoarthritis and septic arthritis — branching viva

Branching framework

Open with the one-sentence problem representation. This is a febrile child with an acutely hot, held joint, and septic arthritis is the must-not-miss diagnosis until excluded by the synovial fluid. State the urgency aloud — cartilage destruction begins within hours, and the septic hip threatens the femoral head blood supply — before you discuss anything else. The examiner is listening for whether you reach for the aspiration and the theatre before you reach for the antibiotic. [1] [3]

Apply the Kocher criteria to estimate the probability. The four predictors are a fever above 38.5 degrees, an inability to bear weight, an ESR above 40, and a white cell count above 12,000, with the Caird addition of a CRP above 20 milligrams per litre. State the rising probability with each predictor, and be ready for the calibration probe — the Caird prospective study found the original Kocher probabilities may overestimate, so the criteria sharpen suspicion but never rule out sepsis. A child with two or more predictors warrants urgent aspiration. [1] [3]

Branch to the single most important investigation. The joint aspiration, under ultrasound guidance for the hip, sending the synovial fluid for a cell count above 50,000, a Gram stain, and culture, with blood-culture-bottle inoculation for Kingella kingae. State that the aspiration precedes the antibiotic, always, because an antibiotic given first sterilises the specimen. If the examiner probes a culture-negative result in the under-four, name Kingella kingae and the oropharyngeal swab PCR. [1] [7]

Branch to the surgical emergency of the septic hip. The hip is drained by arthrotomy and washout, not managed by aspiration alone, because the intracapsular pus under pressure compresses the retinacular vessels and causes avascular necrosis within hours. Contrast this with the knee, which may be managed by arthroscopic washout or serial aspiration, and cite the Malawian trial showing aspiration was as effective as arthrotomy for the shoulder. The drainage is part of the resuscitation, not a later step. [3]

Branch to the age-stratified empiric antibiotic and the stewardship. Flucloxacillin covers Staphylococcus aureus in every child, a third-generation cephalosporin is added for Kingella kingae in the under-four, gentamicin or cefotaxime is added in the neonate, and vancomycin or clindamycin is added where community-acquired meticillin-resistant S. aureus is prevalent. State the total duration of three to four weeks with an oral step-down once the child is afebrile and the CRP is falling, and the narrowing to a single agent once the organism is known. [9]

Close with the neonate and the stewardship awareness. The neonate presents with sepsis and a pseudoparalysed limb, multiple joints may be involved, and the threshold to image and aspirate is at its lowest. The examiner rewards the candidate who frames the disease around the urgency and the antimicrobial stewardship — cultures before antibiotic, drainage before delay, narrowing and stepping down — because these are the systems-thinking moves the fellowship demands. [3] [9]