Hypersensitivity pneumonitis

Last reviewed: 6 Jun 2022

Last updated: 25 Jun 2021

Hypersensitivity pneumonitis 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.

Immunological reaction to inhaled antigen is corroborative.

No pathognomonic tests.

Treatment involves avoidance of causative agent and use of corticosteroids.

Definition

Hypersensitivity pneumonitis (HP), also known as extrinsic allergic alveolitis, is the result of non-IgE mediated immunological inflammation. HP is caused by repeated inhalation of non-human 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. The clinical manifestations of HP depend on the concentration and frequency of exposure. The clinical syndromes - acute, sub-acute, and chronic HP - present differently.[1][2] Some authors are now reporting that there are only two clinical syndromes; however, they vary as to whether they group sub-acute with the acute or the chronic form.[3][4] Many cases of HP are caused by occupational exposure.[5]

History and exam

Key diagnostic factors

exposure to avian protein antigen, especially excrement
mould in work environment
occupational exposure to chemicals
dyspnoea
non-productive cough
productive cough

More key diagnostic factors

Other diagnostic factors

fever/chills
malaise
weight loss/anorexia
bibasilar rales
diffuse rales
clubbing

Other diagnostic factors

Risk factors

smoking
viral infection
exposure to avian protein antigen
exposure to mould 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

More risk factors

Diagnostic investigations

1st investigations to order

immunological response to causative antigen
FBC
erythrocyte sedimentation rate
albumin
Chest x-ray (CXR)
CT chest
pulmonary function test
diffusing lung capacity of carbon monoxide

More 1st investigations to order

Investigations to consider

bronchoalveolar lavage
lung biopsy

More investigations to consider

Treatment algorithm

ACUTE

all patients

Contributors

Authors

Leslie Grammer, MD

Professor

Northwestern University Feinberg School of Medicine

Director

Ernest S. Bazley Asthma and Allergy Center

Clinic Practice Director

Allergy-Immunology

Northwestern Medical Faculty Foundation

Chicago

Disclosures

LG declares that she has no competing interests.

Peer reviewers

Jordan Fink, MD

Professor of Pediatrics

Allergy-Immunology Division

Medical College of Wisconsin

Milwaukee

Disclosures

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

Charleston

Disclosures

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

London

Disclosures

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.

Differentials
- Viral pneumonia
- Sarcoidosis
- Bronchiolitis obliterans
More Differentials
Guidelines
- Japanese guidelines for occupational allergic diseases 2020
- Occupational hypersensitivity pneumonitis: an EAACI position paper
More Guidelines
Log in or subscribe to access all of BMJ Best Practice

Use of this content is subject to our disclaimer

Hypersensitivity pneumonitis

Summary

Definition

History and exam

Key diagnostic factors

Other diagnostic factors

Risk factors

Diagnostic investigations

1st investigations to order

Investigations to consider

Treatment algorithm

all patients

Contributors

Authors

Leslie Grammer, MD

Disclosures

Peer reviewers

Jordan Fink, MD

Disclosures

Alec Chessman, MD

Disclosures

Toby M. Maher, MB, MSc, PhD, MRCP

Disclosures

Differentials

Guidelines

Log in or subscribe to access all of BMJ Best Practice

Log in or subscribe to access all of BMJ Best Practice

Log in to access all of BMJ Best Practice