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.
Immunological 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 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. Historically, HP has been categorised 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 non-fibrotic HP. Classification is determined by the predominant presence or absence of fibrosis on radiological and/or histopathological examination. Many cases of HP are caused by occupational exposure.
History and exam
Key diagnostic factors
- exposure to avian protein antigen, especially excrement
- mould 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 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
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
- 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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