Modern pathology : an official journal of the United States and Canadian Academy of Pathology, Inc
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The clinical and pathologic diagnosis of hypersensitivity pneumonitis has been confounded by conflicting definitions, with two recent guidelines suggesting that hypersensitivity pneumonitis simply be diagnosed as nonfibrotic or fibrotic. Nonfibrotic hypersensitivity pneumonitis is usually characterized by a bronchiolocentric chronic interstitial inflammatory infiltrate, frequently but by no means always with associated granulomas or giant cells. Fibrotic hypersensitivity pneumonitis may take the form of interstitial fibrosis confined to the peribronchiolar regions, or fibrotic nonspecific interstitial pneumonia, or a process similar to and sometimes indistinguishable from usual interstitial pneumonia/idiopathic interstitial fibrosis, but the exact pathologic features that favor a diagnosis of fibrotic hypersensitivity pneumonitis are disputed. ⋯ Clinical and CT features are crucial to the diagnosis of hypersensitivity pneumonitis: sparing of the lung bases, centrilobular nodules, air-trapping, or the triple density sign with fibrosis favor a diagnosis of fibrotic hypersensitivity pneumonitis. At this point there are no molecular tests that reliably separate fibrotic hypersensitivity pneumonitis from other forms of interstitial lung disease. Currently the separation of fibrotic hypersensitivity pneumonitis from usual interstitial pneumonia is crucial to treatment (immunosuppressives for the former, anti-fibrotics for the latter) but this approach is changing and all progressive fibrosing interstitial pneumonias will probably be treated with antifibrotics in the future.