Hypersensitivity pneumonitis

Hypersensitivity pneumonitis (HP) is an inflammation of the alveoli and distal bronchioles caused by an immune response to inhaled allergens.

Exposure to an antigen is the key epidemiologic 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.

Various tests may be used to corroborate a presumptive diagnosis, but no test is pathognomonic.

Treatment involves avoidance of the causative agent and use of corticosteroids.

HP, also known as extrinsic allergic alveolitis, is the result of non-IgE mediated immunologic inflammation. HP is caused by repeated exposure to an antigen, usually through inhalation. Potential antigens include proteins of natural plant or animal origin, a chemical conjugated to a human airway protein (such as albumin), and some drugs (via ingestion).[1]Hamblin M, Prosch H, Vašáková M. Diagnosis, course and management of hypersensitivity pneumonitis. Eur Respir Rev. 2022 Feb 9;31(163):210169. https://publications.ersnet.org/content/errev/31/163/210169 http://www.ncbi.nlm.nih.gov/pubmed/35140104?tool=bestpractice.com [2]Barnes H, Corte TJ, Keir G, et al. Diagnosis and management of hypersensitivity pneumonitis in adults: a position statement from the Thoracic Society of Australia and New Zealand. Respirology. 2024 Dec;29(12):1023-46. https://onlinelibrary.wiley.com/doi/10.1111/resp.14847 http://www.ncbi.nlm.nih.gov/pubmed/39467777?tool=bestpractice.com ​​​ 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.[3]Churg A. Hypersensitivity pneumonitis: new concepts and classifications. Mod Pathol. 2022 Jan;35(suppl 1):15-27. https://www.modernpathology.org/article/S0893-3952(22)00068-0/fulltext http://www.ncbi.nlm.nih.gov/pubmed/34531525?tool=bestpractice.com [4]Calaras D, David A, Vasarmidi E, et al. Hypersensitivity Pneumonitis: Challenges of a Complex Disease. Can Respir J. 2024 Jan 18;2024:4919951. https://onlinelibrary.wiley.com/doi/10.1155/2024/4919951 http://www.ncbi.nlm.nih.gov/pubmed/38283656?tool=bestpractice.com ​​​​​​ 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.[5]Raghu G, Remy-Jardin M, Ryerson CJ, et al. Diagnosis of hypersensitivity pneumonitis in adults. An Official ATS/JRS/ALAT clinical practice guideline. Am J Respir Crit Care Med. 2020 Aug 1;202(3):e36-69. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7397797 http://www.ncbi.nlm.nih.gov/pubmed/32706311?tool=bestpractice.com Many cases of HP are caused by occupational exposure.[6]Quirce S, Vandenplas O, Campo P, et al. Occupational hypersensitivity pneumonitis: an EAACI position paper. Allergy. 2016 Jun;71(6):765-79. http://onlinelibrary.wiley.com/doi/10.1111/all.12866/full http://www.ncbi.nlm.nih.gov/pubmed/26913451?tool=bestpractice.com [7]Kongsupon N, Walters GI, Sadhra SS. Occupational causes of hypersensitivity pneumonitis: a systematic review and compendium. Occup Med (Lond). 2021 Oct 1;71(6-7):255-9. https://academic.oup.com/occmed/article/71/6-7/255/6346534 http://www.ncbi.nlm.nih.gov/pubmed/34370035?tool=bestpractice.com