Transient synovitis and the irritable hip — formative SAQs

SAQ 1 (10 marks) — The five-year-old with a post-viral limp

Stem: A five-year-old boy is brought to the emergency department with a one-day history of a limp and a sore right hip, two days after a viral upper respiratory infection. He is afebrile, alert, and well-looking. He walks with an antalgic limp but he bears weight. His white cell count is 9,200 per microlitre, his CRP is 8 milligrams per litre, and his ESR is 18 millimetres per hour. An ultrasound shows a small right hip effusion. Outline your assessment, your diagnostic reasoning, and your management with the safety-net. [1] [6]

Model answer

Assessment and diagnostic reasoning (4 marks). This child fits the classic picture of transient synovitis — a well-looking boy in the peak age group of three to eight years, an acute limp following a viral illness, an afebrile and well appearance, and a preserved ability to bear weight cautiously. The critical step is to apply the Kocher criteria to exclude septic arthritis. The four original 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. This child meets none of the four — he is afebrile, he bears weight, his ESR is 18, and his white cell count is 9,200 — and his CRP of 8 is well below the Caird cut-off of 20 milligrams per litre. With zero predictors the probability of septic arthritis is below one per cent, and the small ultrasound effusion in a well-looking child supports transient synovitis. [1] [3]

Management (4 marks). Once sepsis has been excluded by the score and the markers, the management is conservative. The child is advised to rest and to avoid weight bearing until the pain subsides, with a return to normal activity as the symptoms allow. A non-steroidal anti-inflammatory drug such as ibuprofen at five milligrams per kilogram three times a day, to a maximum of thirty milligrams per kilogram per day, reduces the inflammation and the pain, and simple analgesia such as paracetamol at fifteen milligrams per kilogram every four to six hours is added as needed. No antibiotics are given, because the process is non-infectious and sterile. [6]

Safety-net (2 marks). The family is told that transient synovitis resolves within one to two weeks, and that a return of fever, a refusal to bear weight, or a worsening of the pain demands urgent review, because these are the features of a declaring septic arthritis or a missed mimic. A routine review at one to two weeks is arranged. A limp that persists beyond two weeks is not a slow transient synovitis but a different diagnosis such as early Perthes disease, and it prompts repeat radiographs and specialist referral. [9]

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

Stem: An eighteen-month-old girl presents with a two-day limp and a reluctance to walk. She has a low-grade fever of 37.9 degrees Celsius and a mildly irritable but non-toxic appearance. Her white cell count is 11,500 per microlitre, her CRP is 32 milligrams per litre, and her ESR is 28 millimetres per hour. The ultrasound shows a right hip effusion, and the aspirated synovial fluid has a cell count of 18,000 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. [6] [9]

Model answer

Diagnostic reasoning (5 marks). This child sits in the under-four age group where Kingella kingae is the commonest organism and the one most easily missed. Her low-grade fever and her modestly raised inflammatory markers — a CRP of 32 above the Caird cut-off of 20, but an ESR below 40 and a white cell count below 12,000 — give her one Kocher predictor (the raised CRP is the Caird addition, and her single original predictor is arguably none, since fever is 37.9 below 38.5). The synovial cell count of 18,000 with a neutrophil predominance is higher than expected for classic transient synovitis, which typically has a count below 15,000, and the culture-negative result at 48 hours in this age group raises Kingella kingae, which grows poorly on routine solid media. Yagupsky showed that the young child with a culture-negative hip effusion is not safely labelled transient synovitis. The diagnosis is pursued by inoculating the synovial fluid into blood-culture bottles and by a polymerase chain reaction assay, with an oropharyngeal swab considered. [6] [9]

Management (5 marks). Because sepsis is not excluded by a negative early culture in this age group, the child is managed with a high index of suspicion. She 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 polymerase chain reaction. She is monitored clinically and biochemically, and the CRP is repeated at 48 to 72 hours to confirm the trend. The blood-culture-bottle cultures and the assay are followed to confirm the organism and to guide the targeted therapy and the duration. The hip is reviewed with the orthopaedic team, and the drainage decision — serial aspiration, arthroscopy, or observation — is guided by the clinical course and the confirmed organism. The conservative rest-and-review pathway of classic transient synovitis is reserved for the child in whom sepsis is excluded with confidence, which is not this child until the extended cultures and the assay return. [9]