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Hypersensitivity Pneumonitis

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Overview

Formerly called extrinsic allergic alveolitis, hypersensitivity pneumonitis (HP) is characterised by non-IgE-mediated immune inflammation leading to inflammation of the lung tissue in people who have been sensitised by repeated inhalation of triggering antigens (e.g. dust, animal dander etc.). Chronic HP can lead to lung fibrosis.

Risk Factors and Associations

  • Farmer’s lung – due to exposure to mouldy hay and Saccharopolyspora rectivirgula
  • Bird fancier’s lung – due to exposure to avian proteins e.g. pigeons and parrots
  • Cheese-worker’s lung – due to exposure to cheese mould
  • Chemical worker’s lung – due to exposure to compounds used in the manufacture of plastics and rubber
  • Some drugs – nitrofurantoin and methotrexate

Epidemiology

  • Usually seen in adults, but bird fancier’s lung can occur in children
  • Mean age of presentation is in the 60s

Presentation

Acute hypersensitivity pneumonitis

In acute HP, symptoms usually start 4-8 hours after exposure and resolve quickly, usually within days. Feature are:

Chronic hypersensitivity pneumonitis

In chronic HP, symptoms usually start weeks to months after exposure. Features are:

  • Shortness of breath
  • Productive cough
  • Weight loss
  • Clubbing

Investigations

All patients

  • Chest x-ray:
    • May show infiltrates or fibrosis
  • CT chest:
    • May show ground-glass shadowing
  • Spirometry and lung function testing:
    • Usually shows a restrictive pattern
    • Transfer factor (TLCO): reduced
  • Blood serology testing to causative antigen:
    • Usually shows the presence of IgG to specific antigen
  • Full blood count (FBC) and differential:
    • Normal, and as this is a non-IgE-mediated process, there is no eosinophilia

Management

All patients

  • 1st-line: avoid precipitating factor
  • Consider using corticosteroids

Complications

  • Progressive decline in lung function and fibrosis
  • Hypoxia
  • Death

Prognosis

  • In acute HP, most patients completely recover when exposure to the precipitating factor stops
  • Chronic HP can lead to progressive and irreversible lung fibrosis and can carry a high mortality rate

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