Organizing Pneumonia (OP)

• What is it?
  • OP is a form of chronic inflammatory lung disease that involves the respiratory bronchioles, alveolar ducts, and alveoli.
• Classified as:
  • A type of interstitial lung disease (ILD).
• Caused by:
  • Secondary OP: Associated with a known cause such as
    • infection,
    • drugs,
    • radiation, or
    • systemic diseases.
  • Cryptogenic OP (COP): Idiopathic with no identifiable cause.
• Resulting in:
  • Impaired gas exchange and restrictive lung disease.
• Characterized by:
  • Patchy alveolar and bronchiolar inflammation.
  • Formation of fibroblastic plugs (Masson bodies).
  • Potential progression to fibrosis.
• Anatomically affecting:
  • Respiratory bronchioles, alveolar ducts, and alveoli.
• Pathophysiology:
  • Initial injury occurs at the level of the respiratory bronchioles and alveolar ducts.
  • Inflammatory response leads to granulation tissue formation within these structures and extends into alveoli.
  • Failure of resolution results in organizing fibrosis.
• Diagnosis:
  • Clinical:
    • Symptoms include dyspnea, cough, and sometimes fever or weight loss.
    • Lack of response to antibiotics for suspected pneumonia.
  • Imaging:
    • Radiologic Principles:
    • Applied Anatomy:
    • Diverse Findings
      • Parts: Respiratory bronchioles, alveolar ducts, alveoli.
      • Size: Lesions often span multiple secondary pulmonary lobules.
      • Shape: Rounded, nodular, linear, or perilobular.
      • Position: Can involve any lung region without lobar preference.
      • Character:
        • Ground-glass opacities
        • Reticulations
        • Consolidation,
        • Atoll sign (reverse halo sign):
          • Central ground-glass opacity surrounded by a
          • ring of consolidation,
          • historically considered characteristic but not pathognomonic.
        • Nodules
        • Masses
        • Despite involvement of bronchioles mosaic attenuation and air trapping is not a common feature
      • Time: Subacute onset over weeks to months.
    • CXR:
      • Patchy areas of consolidation.
      • Peripheral and lower lobe predominance.
    • CT:
      • Key Findings:
        • Consolidation with ground-glass opacity.
        • Perilobular pattern (arcade-like sign).
        • Atoll sign (reverse halo sign).
        • Small nodules or masses.
        • Absence of honeycombing.
    • PET-CT:
      • Hypermetabolic activity in inflamed or fibrotic regions.
      • Useful to differentiate OP from neoplastic lesions.
    • Other relevant modalities:
      • MRI has limited utility but may show high signal intensity on T2-weighted images.
  • Laboratory findings:
    • PFTs: Restrictive pattern with reduced diffusion capacity (DLCO).
    • Bloods: Elevated inflammatory markers (e.g., ESR, CRP).
    • Pathology: Histology reveals intra-alveolar granulation tissue and organizing fibrosis.
• Treatment:
  • First-line: Corticosteroids (e.g., prednisone).
  • Address underlying cause if secondary OP.
  • Antibiotics for concurrent infection.
• Prognosis:
  • Generally favorable with appropriate treatment.
  • Relapses can occur, requiring long-term monitoring.
• Radiological Implications:
  • Follow-up imaging to monitor treatment response.
  • Recognition of patterns (e.g., Atoll sign) is crucial for differentiation from other diseases.
• Key Points and Pearls:
  • OP can mimic infectious pneumonia but does not improve with antibiotics.
  • Imaging Has Diverse findings:
    • • OP can appear as
        • bronchial wall thickening
          ground-glass opacities
        • patchy consolidations,
        • nodules,
        • masses , or combinations thereof.
          • Atoll sign (reverse halo sign): Once thought to be pathognomonic, it is now known to occur in other conditions (e.g., invasive fungal infections, vasculitis).
          • Arcade like Sign
            • Perilobular pattern
        • Can present as focal masses, raising suspicion for malignancy.
  • Multidisciplinary approach often required for diagnosis and management.