Subacute hypersensitivity pneumonitis (historical)

Last revised by Joshua Yap on 30 Apr 2023

Subacute hypersensitivity pneumonitis (also known as subacute extrinsic allergic alveolitis) is a legacy term not recommended in the most recent guidelines (c. 2020) 10. It develops when hypersensitivity pneumonitis continues beyond the acute phase (i.e. continues for weeks to months) and still has the potential to resolve with treatment. While some publications suggest the disease needs to prevail for between 1-4 months to fall into this category 6, it is important to realize that the terms acute, subacute, and chronic lie on a continuum.

Although the symptomatic disease has been classically divided into acute, subacute, and chronic types, given contradictory definitions on what exactly constitutes each phase, the condition is subtyped into non-fibrotic hypersensitivity pneumonitis and fibrotic hypersensitivity pneumonitis 10.

Symptoms in the subacute phase of hypersensitivity pneumonitis are similar to, but less severe than, those in the acute phase. Symptoms are often prolonged over weeks to months. Patients may experience recurrent episodes of acute symptoms superimposed on a background of deteriorating respiratory function.

Subacute hypersensitivity pneumonitis usually results from intermittent or continuous exposure to low doses of antigen and is histologically characterized by the presence of cellular bronchiolitis, non-caseating granulomas, and bronchiolocentric interstitial pneumonitis with a predominance of lymphocytes.

On imaging, the features are mostly those of an inflammatory process (alveolitis) and, therefore, indistinguishable from the acute phase. Remember that the condition lies on a continuum and, depending on the time definition used to call it subacute, early fibrotic changes may be also described. 

Steroids are often given for acute exacerbations and for prophylaxis against recurrence. Early diagnosis and removal of the offending antigen are still considered crucial in the prevention of recurrent disease and progression to fibrosis.

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