What is monoarthritis?

Monoarthritis: Definition, Diagnosis, and Management

Monoarthritis is defined as inflammation of a single joint, characterized by pain, swelling, and limited range of motion, which can be acute or persistent (lasting longer than 3 months). 1

Clinical Presentation and Characteristics

• Definition: Inflammation limited to a single joint, though in practice it may sometimes involve 2-3 joints (technically oligoarthritis) 2
• Duration: May be acute or persistent (>3 months) 3
• Common sites: Large joints (knee, ankle, wrist, elbow, hip) and small joints (MTP joints, PIP joints) 1
• Clinical features:
  • Joint pain, swelling, and tenderness
  • Reduced range of motion
  • Local warmth and erythema (especially in infectious causes)
  • Morning stiffness (duration varies by etiology)

Differential Diagnosis

Monoarthritis has a broad differential diagnosis that requires systematic evaluation:

1. Infectious causes (highest priority due to destructive potential):

   • Bacterial septic arthritis
   • Mycobacterial infection
   • Fungal arthritis
   • Viral arthritis
   • Lyme disease

2. Crystal-induced arthropathies:

   • Gout
   • Calcium pyrophosphate deposition disease (pseudogout)

3. Traumatic causes:

   • Joint injury
   • Internal derangement

4. Inflammatory/Autoimmune:

   • Early presentation of rheumatoid arthritis
   • Spondyloarthropathies
   • Reactive arthritis
   • HCV-related arthritis 1

5. Other causes:

   • Tumor (pigmented villonodular synovitis)
   • Osteonecrosis
   • Foreign body synovitis

Diagnostic Approach

History and Physical Examination

• Duration of symptoms
• Pattern of joint involvement
• Morning stiffness duration (>30 min suggests inflammatory cause)
• Recent infections, trauma, or tick exposure
• Family history of rheumatic diseases
• Complete joint examination to confirm monoarticular involvement

Laboratory Investigations

• Blood tests:

  • Complete blood count
  • Erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) 1
  • Rheumatoid factor (RF) and anti-citrullinated protein antibodies (ACPA) 1, 4
  • Uric acid levels (for suspected gout)
  • HLA-B27 (if spondyloarthropathy suspected) 1

• Joint aspiration (arthrocentesis):

  • Essential for diagnosis, especially to rule out infection 5
  • Synovial fluid analysis:
    • Cell count and differential
    • Gram stain and culture
    • Crystal examination
    • Glucose and protein levels

Imaging

• Plain radiographs:

  • First-line imaging for all cases of monoarthritis 1
  • May show joint effusion, erosions, osteophytes, chondrocalcinosis

• Advanced imaging (when indicated):

  • Ultrasound: Detects effusions, synovitis, erosions
  • MRI: Evaluates soft tissues, bone marrow edema, early erosions
  • Nuclear medicine studies: For suspected infection or inflammatory process

Special Considerations

• Synovial biopsy: Recommended in persistent monoarthritis when diagnosis remains unclear after initial evaluation 1
• In aseptic monoarthritis: Consider rheumatic fever in high-risk populations 1

Management Approach

Immediate Management

1. Rule out septic arthritis first - this is a medical emergency

   • Joint aspiration and empiric antibiotics if infection suspected
   • Hospitalization for suspected septic arthritis

2. Symptomatic relief:

   • Rest and immobilization of affected joint
   • NSAIDs for pain and inflammation
   • Local ice application

Specific Management Based on Etiology

1. Infectious monoarthritis:

   • Appropriate antibiotics based on culture results
   • Joint drainage (repeated aspirations or surgical drainage)

2. Crystal-induced arthritis:

   • Acute gout: NSAIDs, colchicine, or corticosteroids
   • Chronic management: Urate-lowering therapy for gout

3. Persistent inflammatory monoarthritis:

   • Intra-articular corticosteroid injections
   • Disease-modifying antirheumatic drugs (DMARDs) may be considered in persistent cases 6, 3
   • Biological therapy has shown promising results in refractory cases 6

4. HCV-related monoarthritis:

   • Typically non-erosive and seronegative
   • May respond to conventional DMARDs 1

5. Surgical options:

   • Synovectomy for persistent cases unresponsive to medical therapy 3
   • Joint replacement for advanced joint destruction

Prognosis

• Approximately 50% of persistent inflammatory monoarthritis cases are self-limiting 3
• Others may evolve into oligo- or polyarticular disease
• Prognosis is generally better than polyarthritis 3
• Early diagnosis and treatment of infectious causes is crucial to prevent joint destruction and systemic complications

Pitfalls and Caveats

1. Delayed diagnosis of septic arthritis can lead to rapid joint destruction and significant mortality - always consider this diagnosis first 5

2. Monoarthritis may be the initial presentation of what will later develop into polyarticular disease (e.g., rheumatoid arthritis) 2

3. Persistent monoarthritis is often resistant to standard therapies used for polyarticular disease and requires specialized management 3

4. Synovial fluid analysis is essential and should not be delayed by imaging studies when infection is suspected 5

5. Repeated evaluations may be necessary as the clinical picture evolves over time