Epidemiology

Etiology

• Combined type III hypersensitivity reaction and type IV hypersensitivity reaction with genetic predisposition
• Inhalation of organic particles (< 5 microns), primarily through occupational exposure
• Farmers are frequently affected

Pathophysiology

Clinical features

• Acute
  • Flu-like symptoms: fever, chills, malaise, cough, headache
  • Dyspnea without wheezing
  • Chest tightness
  • Diffuse fine crackles and a late inspiratory squawk upon auscultation
  • Symptoms subside within hours to days after removal of the inciting antigen.
• Chronic
  • Insidious onset of fatigue, productive cough, progressive dyspnea, cyanosis
  • Bilateral rales
  • Weight loss
    • Due to increased respiratory activity

Diagnostics

High-resolution CT

• Honeycombing (irreversible fibrotic changes) with or without emphysema
• Thickening of alveolar septa
• Traction bronchiectasis or bronchiolectasis
• Ground-glass opacities with inspiratory mosaic attenuation

Additional studies

• Bronchoalveolar lavage (BAL): Lymphocytosis (e.g., ≥ 20–30%) is highly sensitive for HP.
• Pulmonary function testing: restrictive pattern
• Lung biopsy
  • Supportive findings: noncaseating granulomas with lymphocytes and polynuclear giant cells

Differential diagnostics

• Noncaseating granuloma: vs sarcoidosis
• Clinical features and HRCT: vs idiopathic pulmonary fibrosis
  • IPF: Affected patients are usually older (eg, age >50) and granulomas are not expected on lung biopsy.

Treatment