Key Facts

Organising pneumonia (OP) is a clinicopathological entity characterised by intraluminal plugs of granulation tissue (Masson bodies) in distal bronchioles, alveolar ducts, and alveoli. When no cause is identified, it is termed cryptogenic organising pneumonia (COP).

Epidemiology

COP typically presents in the 5th-6th decade. There is no clear sex predominance. Incidence is approximately 1-2 per 100,000 per year. It accounts for 2-4% of ILD diagnoses in specialist centres. Smoking is not a major risk factor (unlike IPF).

Aetiology

• Cryptogenic (COP): no identifiable cause (~60% of cases)
• Secondary OP: infection (bacterial, viral including COVID-19), drug-induced (amiodarone, bleomycin, statins, nitrofurantoin), CTD (RA, dermatomyositis, polymyositis, SLE), post-radiation, post-transplant, haematological malignancy

Pathophysiology

Alveolar epithelial injury triggers an inflammatory response with intra-alveolar fibrin exudation. Fibroblasts and myofibroblasts proliferate within the airspaces forming organised granulation tissue plugs (Masson bodies). Crucially, unlike UIP/IPF, the underlying lung architecture is preserved — this explains the excellent steroid responsiveness and reversibility. The process involves excessive TGF-β and PDGF signalling but without the irreversible honeycombing seen in UIP.

Typical Presentation

• Subacute illness (weeks to months) mimicking community-acquired pneumonia that fails to respond to antibiotics
• Non-productive cough, malaise, low-grade fever, dyspnoea
• Weight loss in ~50% of patients
• Inspiratory crackles on auscultation (less prominent than IPF)
• Clubbing is rare (unlike IPF)

Atypical Presentations

• Solitary nodule/mass mimicking lung cancer
• Rapidly progressive fulminant OP (rare, may require ICU)
• Incidental radiographic finding

Red Flags

• Failure to respond to antibiotics for "pneumonia" (should prompt consideration of OP)
• Rapidly progressive dyspnoea (fulminant OP — consider AIP in differential)
• Haemoptysis (consider malignancy, vasculitis)
• Systemic features suggesting CTD (joint pain, rash, Raynaud's)

Bedside

• SpO₂: may be low on exertion
• Detailed history: drug exposure, CTD symptoms, infection history, radiation exposure

Bloods

• FBC: mild leucocytosis, ESR/CRP raised in >80%
• Autoimmune screen: ANA, ENA, RF, anti-CCP, myositis panel (to exclude CTD-OP)
• LFTs: may be mildly deranged
• Blood cultures: if infective cause suspected

Imaging

• CXR: bilateral patchy consolidation, often peripheral
• HRCT: bilateral peripheral/peribronchial consolidation, ground-glass opacity; reverse halo (atoll) sign in ~20%; migratory pattern on serial imaging; lower lobe predominance

Special Tests

• Pulmonary function tests: restrictive pattern (↓FVC, ↓TLC), ↓DLCO
• BAL: mixed cellularity — lymphocytes 20-40%, neutrophils and eosinophils mildly raised; CD4/CD8 ratio usually low
• Transbronchial biopsy: may show Masson bodies but sampling error common
• Surgical lung biopsy (VATS): gold standard for definitive diagnosis if transbronchial biopsy non-diagnostic

Non-pharmacological

• MDT discussion: essential for diagnosis confirmation
• Remove causative agent: stop offending drug, treat underlying infection
• Pulmonary rehabilitation: for persistent functional limitation

Pharmacological

• First-line: prednisolone 0.75-1 mg/kg/day (max 60 mg) for 4-8 weeks, then taper over 6-12 months (typical total duration 12 months)
• Relapse on taper: restart at previous effective dose; consider steroid-sparing agents
• Steroid-sparing agents (for relapsing/steroid-dependent disease): azathioprine 2-3 mg/kg/day, mycophenolate 1-1.5 g BD, or cyclophosphamide
• Macrolide antibiotics: clarithromycin 250-500 mg BD or azithromycin — some evidence for mild disease or steroid-sparing effect

Surgical/Interventional

• Lung biopsy (VATS): for diagnostic confirmation if non-invasive workup inconclusive

Referral Criteria

• All cases to ILD MDT for diagnostic confirmation
• Respiratory specialist follow-up for steroid taper monitoring
• Consider transplant referral for rare refractory/fulminant cases

COP has an excellent prognosis with steroids — >80% show complete or near-complete radiographic and functional recovery. Relapse rate is 30-50% during steroid taper, but relapses typically respond to re-escalation. Only 5-10% have progressive disease refractory to treatment. Fulminant OP (rare) carries high mortality (~50%) and may require ICU support. Secondary OP prognosis depends on the underlying cause.

Steroid Tapering Schedule for COP

Key Differentiating Features from IPF