What are the symptoms and treatment of septic arthritis in patients with a history of joint disease or compromised immune systems, including elderly patients and young children?

Symptoms of Septic Arthritis

Septic arthritis presents with acute monoarticular joint pain, swelling, and warmth, though the classic triad of fever, pain, and diminished mobility occurs in only approximately 50% of cases. 1

Clinical Presentation

Cardinal Symptoms

• Acute onset of monoarticular joint pain is the hallmark symptom, typically developing rapidly over hours to days 1, 2
• Joint swelling and erythema accompany the pain, with visible inflammation of the affected joint 3, 2
• Warmth over the joint and tenderness to palpation are characteristic findings 1
• Severely limited range of motion or complete immobility of the joint due to pain and inflammation 1
• Inability to bear weight on the affected joint, particularly in lower extremity involvement 1

Constitutional Symptoms

• Fever >101.3°F (38.5°C) supports the diagnosis but is not always present 1
• Chills and rigors may occur but are poorly sensitive markers for septic arthritis 3
• Constitutional symptoms are notably unreliable - their absence does not exclude septic arthritis 3

Diagnostic Criteria and Laboratory Findings

High-Risk Clinical Features

The American College of Radiology identifies five key diagnostic criteria that, when all are met, approach 100% likelihood of septic arthritis: 1

• Fever >101.3°F
• Erythrocyte sedimentation rate (ESR) ≥40 mm/hour
• White blood cell count ≥12,000 cells/mm³
• Inability to bear weight
• C-reactive protein >2.0 mg/dL

Synovial Fluid Findings

• Synovial fluid white blood cell count ≥50,000 cells/mm³ is highly suggestive of septic arthritis 4, 1
• Purulent joint fluid on aspiration is characteristic, though crystal arthropathies can produce similar findings 1
• Synovial fluid culture is positive in approximately 80% of non-gonococcal septic arthritis cases 4, 1

Age-Specific Presentations

Neonates and Infants

• May present with pseudoparalysis (refusal to move the affected limb) 5
• Often have extension from osteomyelitis into the adjacent joint 5
• Group B streptococcus is the predominant pathogen 1, 5

Children <4 Years

• Kingella kingae is a common causative organism in this age group 4, 1, 5
• More likely to have septic arthritis than osteomyelitis compared to older children 5
• Concurrent osteomyelitis occurs in >50% of pediatric cases, particularly in elbow infections (58%) 1, 5

Elderly Patients

• May have more subtle presentations with less dramatic symptoms 4
• Higher risk of polymicrobial infections 4, 5
• Significantly higher mortality rates (22-69% in patients >79 years) 6

Special Population Considerations

Patients with Prosthetic Joints

• Lower synovial fluid cutoff values apply: as low as 1,100 white blood cells/mm³ with neutrophil differential >64% 3
• Symptoms may be more indolent and less dramatic than native joint infections 4

Immunocompromised Patients

• May lack typical inflammatory signs despite active infection 2
• Risk factors include HIV infection, immunosuppressive medication use, and diabetes mellitus 2, 6

Patients with Sickle Cell Disease

• Salmonella species should be considered as a potential causative organism 4, 5

Critical Pitfalls to Avoid

Coexistent Crystal Arthropathy

• Crystals do not exclude septic arthritis - the two conditions can coexist in 73% of reported cases 1
• Even if monosodium urate (MSU) crystals are identified, septic arthritis must still be ruled out with Gram stain and culture 1

Timing of Evaluation

• Ultrasound performed within 24 hours of symptom onset may yield false-negative results 1
• Plain radiographs are typically normal in early infection (<14 days) and should not be used to exclude septic arthritis 1

Negative Cultures

• Negative joint aspirate culture does not exclude infection - consider percutaneous bone biopsy if clinical suspicion remains high 4, 1
• Peripheral leukopenia may result in lower synovial fluid white blood cell counts despite active infection 3

Differential Diagnosis Considerations

The most common conditions that mimic septic arthritis include: 7

• Crystal arthropathies (gout and pseudogout) - can produce similar inflammatory findings
• Degenerative joint disease/osteoarthritis - particularly when acute flares occur
• Soft tissue rheumatic disorders (rotator cuff tendinitis)
• Reactive arthritis - may present with oligoarthritis and extra-articular features
• Inflammatory arthritis flares in patients with pre-existing rheumatoid arthritis

Inflammatory markers (ESR, CRP) are usually markedly elevated in septic arthritis and help differentiate from non-infectious causes 7