Cryptogenic organising pneumonia typically presents in older non-smokers with central ground-glass opacities with peripheral consolidation in a subpleural and lower lobe distribution, demonstrating Masson bodies within small airways, alveolar ducts, and alveoli.
Description
Cryptogenic organising pneumonia (COP), previously known as bronchiolitis obliterans organising pneumonia (BOOP), is a distinct entity among the idiopathic interstitial pneumonias. COP is characterised by the presence of granulation tissue within the lumens of small airways, alveolar ducts, and some alveoli, often associated with a variable degree of interstitial and airspace infiltration by mononuclear cells.
Pathogenesis
The pathogenesis of COP is not fully understood but is thought to involve damage to the lung’s small airways and alveoli, leading to an inflammatory response and resulting in the formation of the granulation tissue that characterises the disease. This process is largely limited to the lungs’ distal bronchioles, alveolar ducts, and alveoli.
Epidemiology, Risk Factors & Associations
- No clear risk factors have been identified.
- COP can occur at any age but is most common in individuals aged 50-60 years.
- There is no gender predilection.
- Associated with rheumatoid arthritis-associated thoracic disease.
- No association with smoking.
Clinical Features
- Symptoms are often subacute, developing over weeks to months, and include cough, fever, malaise, and shortness of breath.
- Physical examination may reveal crackles but is often unremarkable.
Complications
- COP is generally not associated with an increased risk of malignant transformation.
- If untreated, it can progress to severe respiratory distress and failure.
Pathological Features
Histopathology
- Macroscopic: Lung architecture may appear normal or show areas of consolidation. No interstitial fibrosis or honeycoming.
- Microscopic: Presence of polypoid plugs of loose organising connective tissue within bronchioles, alveolar ducts, and alveoli (Masson bodies).
Radiological Features
General Features
- Unilateral or bilateral patchy areas of consolidation, often migratory with a peripheral, subpleural or peribronchovascular distribution
- Reverse halo sign/Atoll sign – characteristically demonstrates central ground-glass opacities with peripheral consolidation, often with a subpleural distribution (seen only in 20%)
- No significant lymphadenopathy or pleural effusion.
- CT tends to demonstrate more extensive findings than plain chest radiography.
XR
- May demonstrate diffuse or patchy areas of consolidation.
CT
- Non-contrast:
- Patchy peripheral or peribronchial consolidation, ground-glass opacities, nodules
- Lower lobes are frequently involved
- Some cases demonstrate subpleural sparing
- Air bronchograms with mild cylindrical bronchial dilatation
- Opacities migrate, changing location and size
- Reverse halo sign
- Lung volumes are mostly preserved.
- Arcade-like sign – curved or arched bands of consolidation distributed around the structures surrounding the secondary pulmonary lobules representing perilobular fibrosis
Grading and Staging
There is no recognised grading or staging system for COP.
Diagnosis
Diagnosis is made based on the combination of clinical, radiological, and histopathological findings. It often requires lung biopsy to confirm.
Differential Diagnosis
- Chronic eosinophilic pneumonia: Typically presents in a middle-aged woman with a history of asthma. It also manifests as peripheral consolidation, but the distribution is often more upper lobe predominant.
- Pneumonia (bacterial, viral, or fungal): Typically acute in onset with symptoms of fever, productive cough, and leukocytosis. Microbiology tests can help differentiate.
- Lung cancer: Most often seen in older individuals with a smoking history. Usually presents as a solitary mass with or without hilar/mediastinal lymphadenopathy.
Management
Management typically involves corticosteroid therapy, which leads to improvement in the majority of cases. In refractory cases, other immunosuppressive drugs may be used.