Chest ImagingHypersensitivity pneumonitis
(also called extrinsic allergic alveolitis), an allergic lung disease caused by the inhalation of antigens contained in a variety of organic dusts. The
radiographic and
pathological abnormalities seen in patients with hypersensitivity pneumonitis are quite similar, regardless of the organic antigen responsible; these abnormalities can be classified into acute,
subacute and chronic stages. In the acute stage, heavy exposure to the inciting antigen can cause
diffuse ill-defined air-space
consolidation visible on radiographs; this reflects alveolar filling by neutrophils, eosinophils, lymphocytes and large mononuclear cells, or obstructive pneumonitis. After
resolution of the acute abnormalities, or between episodes of acute exposure, a fine
nodular pattern is often visible on radiographs. This pattern is characteristic of the
subacute stage of hypersensitivity pneumonitis. The
nodular appearance correlates with the presence of alveolitis,
interstitial infiltrates, small granulomas and cellular bronchiolitis; histological abnormalities are usually most severe in a peribronchiolar distribution. The chronic stage of hypersensitivity pneumonitis is characterized by the presence of
fibrosis, which may develop months or years after the initial exposure.
The HRCT findings of hypersensitivity pneumonitis depend on the stage of disease. In the subacute stage, weeks to months following first exposure to the antigen, typical findings include patchy ground glass opacity and small ill-defined nodules, usually centrilobular in distribution (Fig.1). Chronic hypersensitivity pneumonitis is characterized by the presence of fibrosis, although findings of active disease are often superimposed. Findings of fibrosis in patients with chronic disease most often show a midlung zone predominance or are evenly distributed throughout the upper, mid and lower lung zones.
RW