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.
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.
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
Systemic upset - fever, malaise
Can be severe respiratory failure in extreme cases
Usually within 4-8 hours of exposure and resolving in a few days of removal of trigger
Progressive exertional breathlessness
Sometimes systemic upset (e.g. weight loss)
Usually progressive over several months but may have shown episodes of acute flare ups. Can progress to features of chronic respiratory disease, such as cor pulmonale Signs
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
Non specific rise in inflammatory markers
Neutrophilia rather than eosinophilia
Serum antibody (IgG) precipitin - usually not very specific
Diffuse small nodules or infiltrates
Ground glass appearance
Chronic may show more upper and mid zone reticulation
High resolution CT
Patchy ground glass changes
More invasive investigations may have a role in some cases (under specialist guidance).
Bronchial alveolar lavage
Pulmonary function tests
The key principles are:
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.