Hypersensitivity pneumonitis (HP) is an inflammation of the alveoli and distal bronchioles caused by an immune response to inhaled allergens.
Occupational exposure to organic dust is the key epidemiological factor - most commonly including Actinomycetes bacteria, animal proteins, or reactive chemicals.
Diagnosis requires a high index of suspicion when evaluating individuals with interstitial lung disease or recurrent flu-like illness.
Immunologic reaction to inhaled antigen is corroborative.
No pathognomonic tests.
Treatment involves avoidance of causative agent and use of corticosteroids.
Hypersensitivity pneumonitis (HP), also known as extrinsic allergic alveolitis, is the result of non-IgE mediated immunologic inflammation. HP is caused by repeated inhalation of nonhuman protein, which can be of natural plant or animal origin or can be the result of a chemical conjugated to a human airway protein, such as albumin. The inflammation of HP manifests itself in the alveoli and distal bronchioles. Historically, HP has been categorized as acute, subacute, or chronic, depending on the frequency, duration, and intensity of exposure and the duration of illness. However, these categories are not easily separated. The American Thoracic Society, Japanese Respiratory Society, and Latin American Thoracic Association diagnostic guidelines published in 2020 classify patients as having fibrotic or nonfibrotic HP. Classification is determined by the predominant presence or absence of fibrosis on radiologic and/or histopathologic exam. Many cases of HP are caused by occupational exposure.
History and exam
Key diagnostic factors
- exposure to avian protein antigen, especially excrement
- mold in work environment
- occupational exposure to chemicals
Other diagnostic factors
- weight loss/anorexia
- bibasilar rales
- diffuse rales
- viral infection
- exposure to avian protein antigen
- exposure to mold antigen
- exposure to bacterial antigen
- exposure to diisocyanate (e.g., epoxy resin)
- exposure to acid anhydride antigen (e.g., paint refinisher)
- exposure to metalworking fluid
- nitrofurantoin, methotrexate, roxithromycin, and rituximab
- herbal supplements with ayurvedic medicine
1st investigations to order
- Chest x-ray
- CT chest
- serum IgG
- pulmonary function test
- diffusing lung capacity of carbon monoxide
Investigations to consider
- bronchoalveolar lavage
- lung biopsy
Leslie Grammer, MD
Northwestern University Feinberg School of Medicine
Ernest S. Bazley Asthma and Allergy Center
Clinic Practice Director
Northwestern Medical Faculty Foundation
LG declares that she has no competing interests.
Jordan Fink, MD
Professor of Pediatrics
Medical College of Wisconsin
JF is an author of a number of references cited in this topic.
Alec Chessman, MD
Professor of Family Medicine
Medical University of South Carolina
AC declares that he has no competing interests.
Toby M. Maher, MB, MSc, PhD, MRCP
Consultant Respiratory Physician
Interstitial Lung Disease Unit
Royal Brompton Hospital
TMM has received consultancy fees from GSK, Actelion, and Philips Respironics. He has received lecture fees from Astra Zeneca and holds an unrestricted industry-academic grant from GSK.
- Viral pneumonia
- Bronchiolitis obliterans
- Diagnosis of hypersensitivity pneumonitis in adults. An official ATS/JRS/ALAT clinical practice guideline
- Japanese guidelines for occupational allergic diseases 2020
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