Fibrotic hypersensitivity pneumonitis (fHP) is a chronic, often progressive fibrosing form of hypersensitivity pneumonitis and is also often categorised as a form of interstitial lung disease. Manifestations previously categorised as chronic hypersensitivity pneumonitis now fall under this category. As with other forms of hypersensitivity pneumonitis it is caused by inhaled antigenic exposures. 

Radiographic features

CT 

The associated lung fibrosis can frequently demonstrate bronchocentricity ⁵

The three-density pattern, previously referred to as the "head cheese sign," is characterised by distinct areas of lobular air-trapping, ground-glass opacity, and normal lung tissue. This finding was initially deemed highly specific for fibrotic hypersensitivity pneumonitis. However, other conditions or disease combinations causing small airway obstruction and ground-glass opacities may present similarly.

The collaborative American Thoracic Society (ATS), Japanese Respiratory Society (JRS), and Latin American Thoracic Association (ALAT) guidelines suggest using the three descriptors "typical", "compatible with", or "indeterminate for" fibrotic hypersensitivity pneumonitis ³:

• typical hypersensitivity pneumonitis

  • coarse reticulation and minimal honeycombing in a random axial distribution with no zonal predominance

  • evidence of small airway disease

• compatible with hypersensitivity pneumonitis

  • variable pattern and/or distribution of lung fibrosis:

    • basal and subpleural predominance

    • upper zone predominance

    • central or peribronchovascular predominance

  • fibrotic ground-glass attenuation seen alone or in association with small airway disease

• indeterminate for hypersensitivity pneumonitis

  • usual interstitial pneumonia (UIP) pattern

  • nonspecific interstitial pneumonia (NSIP) pattern

  • organising pneumonia-like pattern

  • truly indeterminate findings