Chronic Hypersensitivity Pneumonitis: Important Considerations in the Work-up of This Fibrotic Lung Disease

Craig S. Glazer


Curr Opin Pulm Med. 2015;21(2):171-177. 

In This Article

Abstract and Introduction


Purpose of review Chronic hypersensitivity pneumonitis is increasingly recognized as an important mimic of other fibrotic lung diseases. This review will summarize recent data regarding the importance and difficulty of determining causative exposures both for accurate diagnosis and prognosis, and describe the expanded pathologic spectrum of the disease, the effects of fibrosis on prognosis and challenges in the diagnostic evaluation.

Recent findings Several recent publications show the potential pathologic patterns induced by chronic hypersensitivity pneumonitis are broader than the classic triad of bronchiolitis, interstitial infiltrates and granulomas. Other pathologic patterns include nonspecific interstitial pneumonia, usual interstitial pneumonia, organizing pneumonia, bronchiolitis and airway centric fibrosis. Detecting a causative antigen in fibrotic hypersensitivity pneumonitis is challenging but critically important both for accurate diagnosis and improved prognosis. The prognosis in hypersensitivity pneumonitis worsens in the presence of fibrosis, but it remains significantly better than idiopathic pulmonary fibrosis.

Summary Hypersensitivity pneumonitis is increasingly recognized as an important cause of fibrotic interstitial lung disease. Hypersensitivity pneumonitis demonstrates a remarkable tendency to mimic other idiopathic interstitial pneumonias. A detailed exposure history remains a cornerstone of diagnosis and management.


Hypersensitivity pneumonitis is a diffused parenchymal lung disease caused by repeated exposure and sensitization to a variety of organic and chemical antigens. Despite the efforts of two recent collaborations of international experts, including an National Heart Lung and Blood Institute (NHLBI)/Organization for Rare Diseases (ORD) workshop, a unifying definition or set of diagnostic criteria remains elusive. The NHLBI/ORD workshop stated: 'hypersensitivity pneumonitis, also known as extrinsic allergic alveolitis, is a complex health syndrome of varying intensity, clinical presentation and natural history. Hypersensitivity pneumonitis is the result of an immunologically induced inflammation of the lung parenchyma in response to inhalation exposure to a large variety of antigens'.[1] Establishing a diagnosis is challenging as no gold standard exists. Diagnosis relies on integration of a variety of factors including history, laboratory, radiologic and pathologic abnormalities.

Although the clinical phenotype of classic hypersensitivity pneumonitis is well described,[2] the importance of hypersensitivity pneumonitis as a mimic of other diffused parenchymal lung diseases, especially fibrotic interstitial lung diseases (ILDs), is becoming increasingly apparent. Hypersensitivity pneumonitis is now recognized as one of the most common ILDs along with nonspecific interstitial pneumonia (NSIP), autoimmune disease-related ILD and idiopathic pulmonary fibrosis (IPF).[3,4] A recent cohort study from Denmark found hypersensitivity pneumonitis was the third most common ILD.[5] In a recent American Thoracic Society (ATS) project on NSIP, the diagnosis was changed from idiopathic NSIP to hypersensitivity pneumonitis by multidisciplinary review in 59 of the 193 cases reviewed.[6] Another study on the value of multidisciplinary review in the diagnoses of ILD showed hypersensitivity pneumonitis was the third most common disease after IPF and NSIP.[7] Sarcoid patients were not included in that trial. Others report multidisciplinary review changes the initial pathologic diagnosis between 20 and 40% of the time and the most common change is from an idiopathic interstitial pneumonia (IIP) to hypersensitivity pneumonitis.[8] The latest IPF guidelines state that 'it is of particular importance to evaluate patients thoroughly for possible chronic hypersensitivity pneumonitis, since such patients may mimic IPF'.[9] Finally, the latest international consensus classification of IIPs states that hypersensitivity pneumonitis and IIP are frequently confused.[10] This review will thus focus on chronic fibrotic hypersensitivity pneumonitis with particular attention to new data regarding exposure, the importance of fibrosis to prognosis, the expanded pathologic spectrum of the disease and the limitations of some common diagnostic tests.