Is gout a form of synovitis?

Gout is a Form of Synovitis Characterized by Monosodium Urate Crystal Deposition

Yes, gout is definitively a form of synovitis caused by monosodium urate (MSU) crystal deposition in the synovial membrane and surrounding tissues. 1, 2

Pathophysiology of Gout as Synovitis

Gout is characterized by:

• Deposition of MSU crystals in the synovial membrane, articular cartilage, and periarticular tissues 3
• Inflammatory response triggered by these crystals in the synovium 4
• Phagocytosis of MSU crystals by synoviocytes, which activates inflammation and increases secretion of pro-inflammatory cytokines (IL-1β, IL-6, IL-8, TNF-α, MCP-1) 4

The natural history of gout progresses through several stages:

1. Asymptomatic hyperuricemia: Elevated serum uric acid without clinical manifestations
2. Acute gouty arthritis: Characterized by sudden-onset synovitis
3. Intercritical gout: Asymptomatic periods between flares, though MSU crystal deposition may continue silently 1, 5
4. Chronic tophaceous gout: Formation of MSU crystal aggregates (tophi) in joints and soft tissues 5

Diagnostic Confirmation of Gouty Synovitis

The gold standard for diagnosing gouty synovitis is:

• Identification of MSU crystals in synovial fluid through joint aspiration and examination under polarized light microscopy 2
• These crystals appear needle-like and show strong negative birefringence 5

The EULAR guidelines strongly recommend:

• A search for MSU crystals in synovial fluid from undiagnosed inflamed joints 1
• Routine examination of all synovial fluid samples for MSU crystals 1

Imaging Evidence of Gouty Synovitis

When synovial fluid analysis is not possible, imaging can help detect both crystal deposition and synovitis:

• Ultrasound: First-line imaging option that can detect both:

  • MSU crystal deposition (double contour sign, tophi)
  • Synovitis in gouty flares 1, 2
  • Sensitivity 63-83% and specificity 76-92% 2

• Dual-Energy CT (DECT):

  • Excellent for detecting MSU crystal deposits
  • Limited evidence for visualizing inflammation/synovitis 1
  • Sensitivity 85-100% and specificity 83-92% 2

Clinical Features of Gouty Synovitis

Acute gouty synovitis typically presents as:

• Sudden monoarthritis with rapid onset and intense pain
• Most commonly affecting the first metatarsophalangeal joint (50% of initial attacks)
• Other common sites: foot, ankle, midtarsal, knee, wrist, finger, and elbow 5
• Rapid development of symptoms reaching maximum within 24 hours 2
• Overlying erythema 2

Important Clinical Considerations

1. Differential diagnosis: Gouty synovitis must be distinguished from other forms of inflammatory arthritis:

   • Septic arthritis (can coexist with gout)
   • Calcium pyrophosphate deposition disease (pseudogout)
   • Rheumatoid arthritis 6

2. Coexisting conditions: Septic arthritis can occur simultaneously with gouty synovitis:

   • Full investigation for sepsis should be undertaken even if MSU crystals are identified 1
   • Synovial fluid Gram stain and culture are essential when infection is suspected 1

3. Diagnostic pitfalls:

   • Serum uric acid levels alone cannot confirm or exclude gout 1, 2
   • Many people with hyperuricemia never develop gouty synovitis 1
   • During acute attacks, serum uric acid levels may be normal 1

Conclusion

Gout is definitively a form of synovitis characterized by MSU crystal deposition in the synovial membrane and surrounding tissues. The inflammatory response to these crystals causes the clinical manifestations of pain, swelling, and erythema. Definitive diagnosis requires identification of MSU crystals in synovial fluid, but imaging modalities like ultrasound and DECT can provide supportive evidence when crystal analysis is not possible.