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]