Cryptogenic organising pneumonia (COP) is a rare, idiopathic interstitial lung disease characterised by inflammatory granulation tissue within the distal airways and alveoli, leading to impaired gas exchange.

Aetiology and pathophysiology

• Idiopathic (no identifiable cause)
• Represents an abnormal inflammatory repair response to lung injury
• Formation of granulation tissue plugs (Masson bodies) in:
  • alveoli
  • alveolar ducts
  • distal bronchioles
• Underlying lung architecture is preserved (helps distinguish from fibrotic ILD)

Typical presentation

• Subacute onset (weeks–months)
• Dry cough
• Progressive dyspnoea
• Fever and malaise
• Poor response to antibiotics for suspected pneumonia

Examination

• Often normal
• May have fine inspiratory crackles

Investigations

• Raised inflammatory markers (ESR/CRP ± leukocytosis)
• Chest imaging:
  • Bilateral patchy consolidation ± ground-glass opacities
  • Often peripheral or migratory
• Lung function:
  • Usually restrictive
  • Reduced DLCO

Management