Subacute hypersensitivity pneumonitis develops when hypersensitivity pneumonitis continues beyond the acute phase (i.e. continues for weeks to months). While some publications suggest the disease to needs to prevail for between 1-4 months to fall into this category 6, it is important to realise that the terms acute, subacute and chronic lie on a continuum.
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 characterised by the presence of cellular bronchiolitis, non-caseating granulomas, and bronchiolocentric interstitial pneumonitis with a predominance of lymphocytes.
As in the acute phase, chest x-ray can be frequently normal.
There can be considerable overlap in HRCT chest findings of acute and subacute hypersensitivity pneumonitis. Typically described features in the subacute phase include:
ground-glass and nodular opacities in a centrilobular distribution
- in transition from acute to subacute disease, poorly defined air-space opacities may be replaced by well-defined reticular or nodular opacities 1
- these nodular densities may be very well defined 1
- heterogeneous or small nodular opacities: with a predominance for the mid to lower lung zones
- patchy air space opacification 8
- irregular linear opacities are less common on CT
- air trapping on expiratory imaging is a non specific but helpful ancillary finding - this reflects associated bronchiolitis, this can in turn lead to a mosaic attenuation pattern
- thin-walled lung cysts can be seen in a small percentage of patients 2
Diffuse homogeneous opacities are usually not seen during the subacute phase of the disease.
Treatment and prognosis
Steroids are often given for acute exacerbations and for prophylaxis against recurrence. Early diagnosis and removal of the offending antigen it still considered crucial in prevention of recurrent disease and progression to fibrosis.
- progression to chronic hypersensitivity pneumonitis
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- 2. Franquet T, Hansell DM, Senbanjo T et-al. Lung cysts in subacute hypersensitivity pneumonitis. J Comput Assist Tomogr. 2003;27 (4): 475-8. Pubmed citation
- 3. Remy-Jardin M, Remy J, Wallaert B et-al. Subacute and chronic bird breeder hypersensitivity pneumonitis: sequential evaluation with CT and correlation with lung function tests and bronchoalveolar lavage. Radiology. 1993;189 (1): 111-8. Radiology (abstract) - Pubmed citation
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- 7. Lima MS, Coletta EN, Ferreira RG et-al. Subacute and chronic hypersensitivity pneumonitis: histopathological patterns and survival. Respir Med. 2009;103 (4): 508-15. doi:10.1016/j.rmed.2008.12.016 - Pubmed citation
- 8. Silver SF, Müller NL, Miller RR et-al. Hypersensitivity pneumonitis: evaluation with CT. Radiology. 1989;173 (2): 441-5. Radiology (abstract) - Pubmed citation