What is the diagnosis and management for a 9-year-old boy with bilateral knee and wrist pain, intermittent fever, and a maculopapular rash, presenting with morning stiffness, limited range of motion (ROM), and a temperature of 38.8 degrees Celsius?

Systemic Juvenile Idiopathic Arthritis (Still's Disease)

This 9-year-old boy most likely has systemic juvenile idiopathic arthritis (Still's disease), and should be started on NSAIDs as first-line therapy with close monitoring for response, escalating to corticosteroids if inadequate improvement within days. 1, 2

Clinical Diagnosis

The presentation is classic for Still's disease based on the following cardinal features:

• High-spiking fever (38.8°C) lasting 2 weeks, which is the hallmark feature present in 95-100% of cases 2
• Salmon-colored maculopapular rash on trunk (back) and proximal limbs (arms), which occurs in 72.7% of Still's disease patients and is characteristically evanescent 3, 1, 2
• Polyarticular arthritis/arthralgia affecting bilateral knees and wrists with morning stiffness and limited ROM, present in 64-100% of cases 3, 2
• Duration of 6 weeks with intermittent fever pattern typical of Still's disease 1

The fever pattern, rash distribution on trunk and proximal limbs, and symmetric polyarthritis involving knees and wrists are pathognomonic for Still's disease rather than other differential diagnoses. 3, 1

Immediate Diagnostic Workup

Essential laboratory investigations to confirm diagnosis and assess severity:

• Complete blood count - expect neutrophilic leukocytosis (often >15,000 cells/L with >80% granulocytes) and thrombocytosis 3, 1
• Inflammatory markers - ESR and CRP will be markedly elevated 1, 2
• Serum ferritin - typically dramatically elevated (often 4,000-30,000 ng/mL, at least 5-fold above normal) with 80% sensitivity 1, 2
• Glycosylated ferritin fraction - <20% has 93% specificity when combined with elevated ferritin 2
• Liver function tests - transaminases commonly elevated 1
• RF and ANA - should be negative to support Still's disease diagnosis 3

Critical Differential Diagnoses to Exclude

Kawasaki disease is unlikely because:

• Age 9 years is atypical (Kawasaki typically affects children <5 years) 3
• Lacks bilateral non-purulent conjunctivitis, oral mucosal changes (cracked lips, strawberry tongue), and extremity changes (erythema/edema of hands/feet) 3
• Rash distribution on trunk/proximal limbs favors Still's disease over Kawasaki's truncal rash with perineal accentuation 3, 1
• Duration of 6 weeks is too prolonged for untreated Kawasaki disease 3

Septic arthritis must be excluded given bilateral joint involvement with fever:

• Bilateral symmetric involvement strongly favors inflammatory arthritis over septic arthritis 3
• Septic arthritis typically presents with single joint involvement, inability to bear weight, and WBC >12,000 with refusal to move the joint 3
• If clinical suspicion exists, arthrocentesis of affected joints is mandatory 3

MIS-C (Multisystem Inflammatory Syndrome in Children) should be considered:

• Obtain SARS-CoV-2 PCR and serology given the overlapping presentation 3
• However, the 6-week duration predating COVID-19 exposure and classic Still's disease features make this less likely 3

Treatment Algorithm

First-line therapy (mild to moderate disease):

• NSAIDs (indomethacin or naproxen preferred over aspirin) as initial monotherapy 3, 2
• Monitor response over 3-7 days 3
• Only 7-15% of patients achieve disease control with NSAIDs alone, so close monitoring is essential 3

Second-line therapy (inadequate NSAID response or moderate-severe disease):

• Oral corticosteroids (prednisolone 0.5-1 mg/kg/day) should be added if fever persists beyond 3-7 days or systemic symptoms worsen 3, 2
• 76-95% of patients respond favorably to corticosteroids 3
• 88-89% of Still's disease patients ultimately require corticosteroids during their disease course 3

Critical Pitfalls and Monitoring

Macrophage Activation Syndrome (MAS) surveillance:

• MAS is a life-threatening complication that can occur at onset, during treatment, or in remission 1, 2
• Monitor for pancytopenia, dramatically elevated ferritin, elevated liver enzymes, coagulopathy, and falling ESR despite worsening clinical status 3
• Requires prompt recognition and aggressive immunosuppressive treatment 3, 2

Do not delay diagnosis waiting for more criteria:

• The Yamaguchi criteria do not require arthritis to be present for diagnosis 1
• Delaying diagnosis while waiting for additional features can lead to unnecessary diagnostic delays and worse outcomes 1

Joint destruction monitoring:

• Wrist involvement is particularly concerning as carpal ankylosis can develop within 1.5-3 years 3
• Patients with polyarthritis and root joint involvement at onset have higher risk of chronic articular disease requiring joint replacement 3