Hypersensitivity Pneumonitis
Last updated 21st September 2022. Tom Heaton
A.K.A. Extrinsic allergic pneumonitis
This is a group of conditions of inflammation of the lungs due to inhaled antigen.
This is diffuse and granulomatous in nature and occurring in those with previous sensitisation.
The condition can range from acute to subacute to chronic.
This is a group of conditions of inflammation of the lungs due to inhaled antigen.
This is diffuse and granulomatous in nature and occurring in those with previous sensitisation.
The condition can range from acute to subacute to chronic.
Pathophysiology
It is not fully understood but seems to involve both type 3 and type 4 hypersensitivity processes (immune complex and cell mediated).
The small size of the precipitating antigens often means that it is the distal airways that are affected i.e. alveoli and distal bronchioles.
It is not atopic in nature and not associated with an elevation in IgE or eosinophils.
On a histological level there is often a combination of interstitial inflammatory infiltrate, bronchiolitis, organising pneumonia with non caseating granulomas.
In chronic disease there may be fibrosis.
Causes
There are a wide number of recognised triggers, of varying rarity.
One method for categorisation can be based on the nature of this antigen.
Animal protein
The small size of the precipitating antigens often means that it is the distal airways that are affected i.e. alveoli and distal bronchioles.
It is not atopic in nature and not associated with an elevation in IgE or eosinophils.
On a histological level there is often a combination of interstitial inflammatory infiltrate, bronchiolitis, organising pneumonia with non caseating granulomas.
In chronic disease there may be fibrosis.
Causes
There are a wide number of recognised triggers, of varying rarity.
One method for categorisation can be based on the nature of this antigen.
Animal protein
- Bird fancier’s lung - from bird feathers and droppings
- Rat lung - protein in rat droppings
- Farmer’s lung - from Saccharopolyspora rectivirgula in mouldy hay
- Malt worker’s lung - from Aspergillus clavatus in mouldy malt
- Cheese worker’s lung - from exposure to Penicillium casei in cheese mould
- Mushroom worker’s lung - from exposure to thermophilic actinomycetes
- Hot tub lung
- Various chemicals found in paints and manufacturing processes
Presentation
Acute
Chronic
Can progress to features of chronic respiratory disease, such as cor pulmonale
Signs
The history will likely yield a culprit antigen.
Some occupations are therefore clearly more at risk e.g. malt worker
- Breathlessness
- Dry cough
- Chest tightness
- Systemic upset - fever, malaise
- Can be severe respiratory failure in extreme cases
Chronic
- Progressive exertional breathlessness
- Dry cough
- Sometimes systemic upset (e.g. weight loss)
Can progress to features of chronic respiratory disease, such as cor pulmonale
Signs
- Diffuse crackles
- Fever
- Rarely clubbing in chronic disease
The history will likely yield a culprit antigen.
Some occupations are therefore clearly more at risk e.g. malt worker
Investigations
Bloods
Imaging
More invasive investigations may have a role in some cases (under specialist guidance).
- Non specific rise in inflammatory markers
- Neutrophilia rather than eosinophilia
- Serum antibody (IgG) precipitin - usually not very specific
Imaging
- CXR
- Diffuse small nodules or infiltrates
- Ground glass appearance
- Apical sparing
- Chronic may show more upper and mid zone reticulation
- Diffuse small nodules or infiltrates
- High resolution CT
- Patchy ground glass changes
- Micronodules
- Patchy ground glass changes
More invasive investigations may have a role in some cases (under specialist guidance).
- Bronchial alveolar lavage
- Pulmonary function tests
- Lung biopsy
Management
The key principles are:
Supportive care may be needed in some severe cases e.g. hypoxia.
In many other cases, simple removal from the precipitant will result in improvement in acute cases.
This is less clear in chronic cases where there may be a degree of irreversible fibrosis.
Corticosteroids are the mainstay of specific therapy, although there is a lack of strong evidence for this.
- Supportive care
- Removal of precipitant
- Consideration of immune modulation
Supportive care may be needed in some severe cases e.g. hypoxia.
In many other cases, simple removal from the precipitant will result in improvement in acute cases.
This is less clear in chronic cases where there may be a degree of irreversible fibrosis.
Corticosteroids are the mainstay of specific therapy, although there is a lack of strong evidence for this.
Links & References
- Knott, L. Hypersensitivity pneumonitis. Patient UK. 2020. https://patient.info/doctor/extrinsic-allergic-alveolitis-pro
- Hypersensitivity pneumonitis, in Oxford handbook of respiratory medicine. Oxford University Press. 2005
- Hypersensitivity pneumonitis. BMJ best practice.
- Occupational hypersensitivity pneumonitis: an EAACI position paper. 2016. https://onlinelibrary.wiley.com/toc/13989995/2016/71/6
