A child with a limp and a swollen knee
A 7-year-old girl is brought in with a 3-week history of limping and a swollen right knee. She is otherwise well, afebrile, and has no rash. Her mother reports she has been less keen to walk and sometimes cries at night. Examination: warm swollen right knee with a moderate effusion, restricted extension, and mild tenderness over the distal femur. No other joints are swollen. She has no recent travel, no tick bite, and is fully vaccinated. Laboratory tests: ESR 42 mm/h, CRP 25 mg/L, white cell count 9.8 × 10⁹/L, platelets 420 × 10⁹/L, haemoglobin 118 g/L. Blood cultures are pending. Radiograph of the knee shows soft tissue swelling and a possible small effusion, with no bony destruction.
1. What is your differential diagnosis and what is the most urgent condition to exclude?
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Most urgent to exclude: septic arthritis and osteomyelitis. A hot swollen joint with restricted movement, night pain and raised inflammatory markers in a child must be treated as septic until proven otherwise. Although she is afebrile and blood cultures are pending, the Kocher criteria (non-weight-bearing, fever >38.5°C, ESR >40, WCC >12) are partly met (she is limping and ESR 42), so the pre-test probability is not negligible. Joint aspiration for Gram stain and culture is the definitive step if there is any doubt; do not delay if the joint is hot and the child is unwell.
Other differentials:
- Juvenile idiopathic arthritis (oligoarticular JIA) - the most likely non-infectious cause: age under 16, arthritis in one joint for more than 6 weeks (approaching), no systemic features. Chronic uveitis is a risk and requires slit-lamp screening.
- Lyme arthritis - if exposure history were present; serology would be indicated.
- Reactive arthritis - usually follows a GI or GU infection and is oligoarticular.
- Osteomyelitis - distal femoral tenderness suggests this; MRI is the imaging of choice.
- Malignancy (leukaemia, osteosarcoma, Ewing sarcoma) - night pain and a single bone lesion warrant imaging; check blood film.
- Trauma or toddler's fracture - less likely with this inflammatory picture.
2. If septic arthritis and malignancy are excluded and oligoarticular JIA is diagnosed, outline the initial management and the screening this child requires.
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Initial management:
- Intra-articular corticosteroid injection (e.g. triamcinolone hexacetonide) into the affected knee is first-line for oligoarticular JIA and often induces prolonged remission; it is done under sedation or general anaesthetic in children.
- NSAIDs (e.g. naproxen 10-15 mg/kg/day in two doses) for symptom control while awaiting response.
- Physiotherapy to maintain range of motion and prevent contracture; address the flexion deficit.
- Escalation if inadequate response: methotrexate (10-15 mg/m² weekly) is the standard second-line DMARD; biologics (e.g. etanercept) if refractory. Systemic steroids are generally avoided in oligoarticular disease.
Screening and monitoring:
- Slit-lamp examination for chronic anterior uveitis at diagnosis and then every 3-4 months for at least the first years - uveitis is asymptomatic and can cause blindness; risk is highest in young girls with oligoarticular, ANA-positive disease.
- ANA testing - ANA positivity identifies the uveitis-risk subgroup.
- Baseline blood count, liver and renal function before methotrexate; ophthalmic surveillance continues throughout.
- Growth monitoring, as chronic inflammation and steroids affect growth.
- Multidisciplinary follow-up with paediatric rheumatology, ophthalmology and physiotherapy; involve the family in education about the chronic relapsing course.