Hypersensitivity Pneumonitis

Definition:  A class of lung disease based on a lung reaction to antigen or possibly antigen-antibody complexes.  The reaction may be acute, subacute or chronic.  The antigenic stimuli may be fungi, spores (eg. farmer’s lung, mushroom worker's lung), or a host of antigenic stimuli often related to occupational or avocational activities.

Pathophysiology:  Most often the antigen is inhaled and the reaction is maximal in the most ventilated parts of the lungs that being the lung bases.  More subacute and chronic changes lead to inflammatory reactions and fibrosis in the mid and ultimately the upper lungs which may show chronic end-stage changes.

CXR Findings:  The most characteristic changes are due to acute inhalation with air space changes in the lower lungs.  Pleural effusions and adenopathy are not typically seen.  More chronic or repeated exposures lead to airspace changes, small airways changes, fibrosis; rarely, may lead to severe lung distortion, fibrosis and honeycombing (end stage lung changes) usually in the upper lungs.

Clues: The disease is usually not suspected by the radiologist without a clinical history of exposure.  The changes are mixed and phasic and any individual presentation is therefore non-specific.

"Aunt Sophies":

Acute phase: pneumonia, pulmonary edema, lung hemorrhage, or aspiration pneumonia

Subacute and chronic changes: atypical edema, infections especially granulomatous infections(TB) and post-inflammatory scarring