This case represents pulmonary and mediastinal manifestations of sarcoidosis, which is a non-caseating granulomatous multisystemic disease.
Relevant aspects of this condition:
- although any age group may be involved, it is usually seen in patients between 20 and 40 years of age
- approximately 50% of patients are asymptomatic
- histopathology study shows multiple granulomas in a characteristic lymphatic and perilymphatic distribution, typically non-caseating and coalescent. Sarcoid granulomata are distributed mainly along the lymphatics in the bronchovascular sheath and, to a lesser extent, in the interlobular septa and subpleural lung regions
- sarcoidosis is often termed the "great mimicker" as there is a vast range of HRCT presentations
- lymphadenopathy is present in 80-90% of patients: generally tends to be symmetrical with hilar and middle mediastinal involvement
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, it is important to realise that the terms acute, subacute and chronic lie on a continuum.
Symptoms 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.
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
- heterogeneous or small nodular opacities: with a predominance for the mid to lower lung zones
- patchy air space opacification
- 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