Subacute Hypersensitivity Pneumonitis

Fig. 21.5 Diffuse alveolar hemorrhage in a 15-year-old boy with IgA nephropathy and pulmonary capillaritis. (a, b) Lung window image of CT (2.5-mm section thickness) scan obtained at level of inferior pulmo-

nary veins shows patchy and extensive parenchymal opacity in both lungs. Please also note small poorly formed centrilobular nodules (arrows) in both lungs

Subacute Hypersensitivity Pneumonitis

Pathology and Pathogenesis

The histopathologic features of subacute HP are a subacute or chronic inßammatory interstitial pneumonia associated with bronchiolitis and small, indistinct, non-necrotizing interstitial granulomas. The histopathology of HP raises a differential diagnosis that includes other cellular interstitial pneumonias, such as NSIP. At low magniÞcation, the surgical lung biopsy shows a moderately dense interstitial inÞltrate, composed of plasma cells and small lymphocytes, causing slight widening of the alveolar walls. A bronchiolocentric distribution may be evident (Fig. 21.3) and thus is called a granulomatous interstitial bronchiolocentric pneumonitis. Typically, the granulomas are small, non-necrotizing, poorly formed, and loosely arranged [23].

Symptoms and Signs

Subacute form of HP is characterized by gradual development of cough and dyspnea over several days and weeks [24]. Some patients present with fever, fatigue, anorexia, and weight loss. Physical examination reveals tachypnea and bibasilar inspiratory crackles.

CT Findings

The most characteristic features of subacute HP on thinsection CT are bilateral patchy areas of GGO, poorly deÞned small centrilobular nodules, and lobular areas of mosaic attenuation on inspiratory images and of air trapping on expiratory CT images [17, 25] (Fig. 21.3). Sometimes, thin-walled cysts can be seen in a small percentage of patients [26].