Acute monoarthritis and septic arthritis — formative SAQs

SAQ 1 (10 marks) — The three-year-old with a febrile limp and a held hip

Stem: A three-year-old boy is brought to the emergency department refusing to walk, holding his right hip flexed, abducted, and externally rotated. He has a fever of 39.2 degrees Celsius. He had a viral upper respiratory infection a week ago. His white cell count is 15,400 per microlitre, his CRP is 78 milligrams per litre, and his ESR is 55 millimetres per hour. An ultrasound shows a right hip effusion. Outline your assessment, investigations, and immediate management. ^{[1] [3]}

Model answer

Assessment and probability of sepsis (2 marks). This child meets all four Kocher criteria — a fever above 38.5 degrees, an inability to bear weight, an ESR above 40, and a white cell count above 12,000 — together with a CRP above 20 milligrams per litre, the strongest single predictor from the Caird prospective study. The probability of septic arthritis is very high, and the child is treated as having a septic hip until proven otherwise. The preceding viral illness does not make this transient synovitis; the inflammatory markers and the refusal to bear weight override the history. ^{[1] [3]}

Investigations (3 marks). The single most important investigation is the urgent joint aspiration of the hip, under ultrasound guidance, sending the synovial fluid for a cell count and differential, a Gram stain, and culture, with direct inoculation into blood-culture bottles to improve the yield of fastidious organisms such as Kingella kingae. Blood cultures and a full infective screen are drawn before any antibiotic is given. The blood tests already confirm the raised inflammatory markers and provide the baseline for tracking the response. Plain radiographs exclude the mimics, and magnetic resonance imaging is considered if adjacent osteomyelitis is suspected. ^{[1] [7]}

Immediate management (5 marks). The cultures and the aspiration are obtained first, before any antibiotic. The septic hip is then taken to theatre for arthrotomy and washout as a surgical emergency, because the intracapsular pus under pressure threatens the femoral head blood supply from the medial circumflex femoral artery and avascular necrosis follows within hours to days. Analgesia and intravenous fluids are given as resuscitation. Empiric intravenous antibiotics are started moment cultures are drawn: flucloxacillin to cover Staphylococcus aureus, plus a third-generation cephalosporin such as cefotaxime or ceftriaxone to cover Kingella kingae in this under-four child. The total duration is three to four weeks, stepping down to oral once the child is afebrile, clinically improved, and the CRP is falling. The CRP is repeated to confirm the response, and the child is followed up to exclude avascular necrosis. ^{[3] [9]}