Hypersensitivity Pneumonitis

- Hypersensitivity pneumonitis, also known as extrinsic allergic alveolitis, is a loosely defined term describing diffuse granulomatous interstitial lung disease caused by the inhalation of various agents, including dust, fungus, molds, or chemicals.
- Bird fancier's lung is the most common type and is caused by exposure to high levels of proteins found in feathers or droppings of various species of birds.
- Farmer's lung is caused by exposure to thermophilic actinomyces in hay, straw, or grain.
- Hypersensitivity pneumonitis can be acute, subacute, or chronic. 

- Nonspecific: chills, cough, fever, malaise, dyspnea; if chronic, weight loss

- Rales on lung exam, changes on CXR, definitive diagnosis by HRCT
- DDx includes NSIP, respiratory bronchiolitis, Langerhans granulomatosis, and sarcoid

- Small particles deposit in small airways, causing allergic granulomatous reaction.
- Granulomas are not histologically diagnostic, are small, may be difficult to find, not tightly organized as in sarcoid.

- Acute syndrome is reversible; chronic form may lead to pulmonary fibrosis

Imaging Findings
- Chiefly characterized by ground-glass opacities, centrilobular nodules and mosaic perfusion
- Lung cysts may be seen; emphysematous changes more common than fibrosis
- Fibrotic changes can be seen in chronic HP
- Changes most prominent in midlung, spares costophrenic angles
- Mediastinal adenopathy 

Case #1. Patchy mosaic attenuation and patchy ground glass opacities characteristic of HP.