Chronic hypersensitivity pneumonitis (historical)

Chronic hypersensitivity pneumonitis is a legacy term not recommended in the most recent guidelines (c. 2020) ¹². In chronic hypersensitivity pneumonitis, there is radiological evidence of fibrosis and represents the end-stage of repeated or persistent pneumonitis ⁷. Most of the manifestations in this category now fall under fibrotic hypersensitivity pneumonitis.

Terminology

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 ¹².

Pathology

It is considered an immunopathological disorder occurring in susceptible individuals, where both humoral and cellular mechanisms are thought to participate in the development of lung lesions. 

Radiographic features

CT

High-resolution CT of the chest typically reveals indistinct centrilobular peribronchiolar nodular opacities - micronodules) of varying numbers ⁵. Other features include:

• ground-glass opacities
• lobular areas of hyperlucency (mosaic attenuation) caused by bronchiole obstruction (may show a three-density pattern/head cheese sign)
• areas of pulmonary fibrosis and honeycombing

There is often a middle or upper zone predominance of CT findings with sparing of the lung bases, unlike non-specific interstitial pneumonia (NSIP) or usual interstitial pneumonia (UIP), which show a lower zone predominance.

The presence of an extensive reticular pattern, traction bronchiectasis, and honeycombing have been shown to closely correlate with the presence of histologic fibrosis in chronic hypersensitivity pneumonitis ⁶.

Treatment and prognosis

The presence of fibrotic changes confers a poor prognosis ^ref.

Differential diagnosis

In some cases, it may be difficult to differentiate from idiopathic pulmonary fibrosis - UIP cases are also thought to have honeycombing and peripheral or lower lung zone predominance of disease, and less likely to have micronodules ⁴.