Chronic Hypersensitivity Pneumonitis: Important Considerations
Chronic Hypersensitivity Pneumonitis: Important Considerations
The classic pathology of hypersensitivity pneumonitis includes the triad of cellular bronchiolitis, a lymphoplasmacytic interstitial infiltrate and poorly formed non-necrotizing granulomas. Small patches of organizing pneumonia are also commonly seen. However, it is becoming increasingly clear that hypersensitivity pneumonitis manifests with other pathologic patterns. First, any of the above components may occur in isolation and the full triad is present in less than half the cases. When the bronchiolitis occurs in isolation, cases mimic obliterative bronchiolitis. Pathologists interpret NSIP if only the lymphoplasmacytic interstitial infiltrate is present. Other studies show that fibrotic NSIP, organizing pneumonia, usual interstitial pneumonia (UIP) and airway centric fibrosis are all potential pathologic patterns (see below).
The pathologic patterns associated with hypersensitivity pneumonitis:
This morphologic diversity is reflected in the imaging as well and likely explains the ability of hypersensitivity pneumonitis to mimic other ILDs. In addition, the granulomatous inflammation that is the hallmark of typical hypersensitivity pneumonitis becomes less prominent, the more fibrotic the disease. Differentiating IIP, especially IPF, from chronic hypersensitivity pneumonitis on pathology is thus challenging. However, distinguishing features have been reported (Table 1) and an experienced pulmonary pathologist can differentiate the two conditions the vast majority of the time. HRCT clues that suggest hypersensitivity pneumonitis is more likely than IPF include the absence of lower lobe predominance, the presence of lobular areas of air-trapping and the presence of centrilobular nodules (Fig. 1).
(Enlarge Image)
Figure 1.
(a) Coronal image from an HRCT of a patient with chronic hypersensitivity pneumonitis demonstrating the absence of lower lobe predominance and some scattered centrilobular nodules. (b) HRCT cross-sectional image from the same patient demonstrating lobular areas of air-trapping.
The Expanded Pathologic Spectrum of Hypersensitivity Pneumonitis
The classic pathology of hypersensitivity pneumonitis includes the triad of cellular bronchiolitis, a lymphoplasmacytic interstitial infiltrate and poorly formed non-necrotizing granulomas. Small patches of organizing pneumonia are also commonly seen. However, it is becoming increasingly clear that hypersensitivity pneumonitis manifests with other pathologic patterns. First, any of the above components may occur in isolation and the full triad is present in less than half the cases. When the bronchiolitis occurs in isolation, cases mimic obliterative bronchiolitis. Pathologists interpret NSIP if only the lymphoplasmacytic interstitial infiltrate is present. Other studies show that fibrotic NSIP, organizing pneumonia, usual interstitial pneumonia (UIP) and airway centric fibrosis are all potential pathologic patterns (see below).
The pathologic patterns associated with hypersensitivity pneumonitis:
Typical
The triad of lymphoplasmacytic interstitial infiltrates, cellular bronchiolitis, poorly formed granulomas
Other potential pathologic patterns associated with hypersensitivity pneumonitis
Usual interstitial pneumonia
Cellular nonspecific interstitial pneumonia
Fibrotic nonspecific interstitial pneumonia
Bronchiolitis
Airway centric fibrosis
Organizing pneumonia
This morphologic diversity is reflected in the imaging as well and likely explains the ability of hypersensitivity pneumonitis to mimic other ILDs. In addition, the granulomatous inflammation that is the hallmark of typical hypersensitivity pneumonitis becomes less prominent, the more fibrotic the disease. Differentiating IIP, especially IPF, from chronic hypersensitivity pneumonitis on pathology is thus challenging. However, distinguishing features have been reported (Table 1) and an experienced pulmonary pathologist can differentiate the two conditions the vast majority of the time. HRCT clues that suggest hypersensitivity pneumonitis is more likely than IPF include the absence of lower lobe predominance, the presence of lobular areas of air-trapping and the presence of centrilobular nodules (Fig. 1).
(Enlarge Image)
Figure 1.
(a) Coronal image from an HRCT of a patient with chronic hypersensitivity pneumonitis demonstrating the absence of lower lobe predominance and some scattered centrilobular nodules. (b) HRCT cross-sectional image from the same patient demonstrating lobular areas of air-trapping.