Hypersensitivity pneumonitis or extrinsic allergic alveolitis (EAA)

• Inhalation of allergens → hypersensitivity reaction → acute inflammatory cells infiltration into the alveoli
• Causes:
  • Bird-fancier's lung (proteins in bird dropppings)
  • Farmer's lung, mushroom worker's lung (bacteria Micropolyspora faeni and/or fungi Thermoactinomyces vulgaris)
  • Malt worker's lung (fungi Aspergillus clavatus)
  • Sugar can worker's lung (fungi Thermoactinomyces sacchari)

Signs and symptoms

• 4-6 hours post-exposure: fever, dyspnoea, dry cough, fine crackles
• Chronic: finger clubbing, exertional dyspnoea, cor pulmonale, type I respiratory failure

Investigation

• FBC may show neutrophilia
• ABGs, serum antibodies
• CXR: upper zone changes (droppings from the UPPER zone), honeycombing if severe/end-stage
• CT chest: ground-glass opacity, reticular opacity, honeycombing
• Lung function tests: restrictive pattern
• Bronchoalveolar lavage: ↑ lymphocytes and mast cells

Management

• Withdrawal of offending allergens, wearing facemasks
• O₂ as required
• Prednisolone or long-terms steroids

Reference: Oxford Handbook of Clinical Medicine (10th Edition)