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 the moment the 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]

SAQ 2 (10 marks) — The eighteen-month-old with a culture-negative swollen knee

Stem: An eighteen-month-old girl presents with a two-day history of a swollen, warm right knee and a reluctance to walk. She is afebrile, mildly irritable, with a CRP of 28 milligrams per litre and a white cell count of 10,800 per microlitre. The synovial fluid aspirate shows a cell count of 32,000 per microlitre with a neutrophil predominance, but the Gram stain and the standard culture are negative at 48 hours. Discuss your diagnostic reasoning and management. [7] [3]

Model answer

Diagnostic reasoning (4 marks). This is a young child in the peak age group for Kingella kingae, which presents with a deceptively mild, low-fever or afebrile picture and modestly raised inflammatory markers. Kingella kingae grows poorly on routine solid media, so standard cultures are often negative, and the organism is best detected by inoculating the synovial fluid into blood-culture bottles or by a polymerase chain reaction assay. Ceroni showed that an oropharyngeal swab PCR detects the organism in the carrier child with an osteoarticular infection, and this is requested. The differential includes transient synovitis, but the synovial cell count of 32,000 with a neutrophil predominance is higher than expected for transient synovitis and supports an inflammatory or septic process rather than a simple irritable knee. [7] [3]

Management (6 marks). Because sepsis is not excluded by a negative early culture in this age group, the child is treated empirically with intravenous antibiotics covering Kingella kingae — a third-generation cephalosporin such as cefotaxime or ceftriaxone, which also covers Staphylococcus aureus and Streptococcus pyogenes — while awaiting the extended cultures and the PCR. The knee, unlike the hip, may be managed by serial needle aspiration or arthroscopic washout rather than open arthrotomy, and the drainage decision is made with the orthopaedic team. The oropharyngeal swab PCR is sent to confirm Kingella kingae. The child is monitored clinically and biochemically, and the CRP is repeated at 48 to 72 hours to confirm the fall. Once the organism is confirmed and the child is improving, the antibiotic is streamlined and the course is completed orally, typically over a total of two to three weeks for confirmed Kingella kingae infection, which usually has an excellent response to beta-lactams. If the cultures and the PCR remain negative and the child improves on empiric therapy, the course is completed and the child is followed to exclude a persistent or atypical process. [7] [9]