Are Ground-Glass Opacities on Chest High-Resolution Computed Tomography a Manifestation of Airway Disease?

To the Editor:

We commend the American Thoracic Society, Japanese Respiratory Society, and Asociación Latinoamericana del Tórax for the publication of the first hypersensitivity pneumonitis (HP) clinical practice guidelines (1). The guidelines propose two distinctive subtypes of HP: nonfibrotic and fibrotic, as determined by the presence of radiological and/or histopathological fibrosis.

In their Table 5, the authors propose that a “nonfibrotic typical HP pattern” on chest computed tomography (CT) requires the identification of at least one feature of lung infiltration—mosaic attenuation or ground-glass opacities (GGOs)—plus at least one abnormality indicative of small airway disease—ill-defined centrilobular nodules or air trapping.

However, as the authors pointed out, mosaic attenuation per se is not specific for lung infiltration, because in diseases that affect both the lung parenchyma and the small airways (e.g., HP), mosaic attenuation can be due to either GGOs or air trapping and the difference is established by expiratory CT images (1).

Therefore, to be more specific and to avoid confusion, would it not be better to use either GGOs or mosaic attenuation due to GGOs as the sole criterion for lung infiltration on Table 5, because they represent the same phenomenon?

On the other hand, in their Table 6, the authors propose that a “fibrotic typical HP” pattern on CT requires the identification of at least one feature of lung fibrosis in a specific distribution, and at least one abnormality indicative of small airway disease: ill-defined centrilobular nodules and/or GGOs, or mosaic attenuation.

However, mosaic attenuation is not specific for air trapping, because it can be due to GGOs, particularly in the setting of HP; therefore, to avoid confusion, would it not be better to use either air trapping or mosaic attenuation due to air trapping on expiratory CT images as a criterion for small airway disease?

Finally, in their Table 6 the authors included GGOs as an abnormality indicative of small airway disease in a “fibrotic typical HP” pattern on CT. However, they stated in their Table 4 that GGOs reflect an infiltrative lung disease, which is in agreement with the Fleischner Society, which considers GGOs to be caused by partial filling of airspaces, interstitial thickening (due to fluid, cells, and/or fibrosis), partial collapse of alveoli, increased capillary blood volume, or a combination of these (2). We would appreciate clarification of the seemingly contrasting definition of GGOs.