Current Diagnosis and Management of Hypersensitivity Pneumonitis

Abstract

Hypersensitivity Pneumonitis (HP) one of the most common interstitial lung diseases (ILDs) is characterized by exposure to an inhaled inciting antigen that leads to a host immunologic reaction determining interstitial inflammation and architectural distortion. The underlying pathogenetic mechanisms are unclear. The absence of international shared diagnostic guidelines and the lack of a "gold-standard" test for HP combined with the presence of several clinical and radiologic overlapping features makes it particularly challenging to differentiate HP from other ILDs, also in expert contests. Radiology is playing a more crucial role in this process; recently the headcheese sign was recognized as a more specific for chronic-HP than the extensive mosaic attenuation. Several classification proposals and diagnostic models have been advanced by different groups, with no prospective validation. Therapeutic options for HP have been limited to antigen avoidance and immunosuppressant drugs over the last decades. Several questions about this condition remain unanswered and there is a need for more studies.

Keywords: Antigens; Diagnosis; Fibrosis; High Resolution Computed Tomography; Hypersensitivity Pneumonitis; Lung Diseases, Interstitial; Management; Nintedanib; Pirfenidone; Steroids; Therapy.

Figure 1. Immune response in Hypersensitivity Pneumonitis. Characterized by sensitization and immune memory formation (A), acute/subacute inflammation (B), and chronic inflammation (C).

Figure 2. Radiologic appearance of chronic hypersensitivity pneumonitis at chest high resolution computed tomography. In the inspiratory scans (A–D) architectural distortion, reticular opacities, peribronchovascular interstitial thickening ground-glass opacities and traction bronchiectasis are evident. In the expiratory scans (E–H) the headcheese sign was found.

Figure 3. Histopathologic aspect of chronic Hypersensitivity Pneumonitis (cHP) at hematoxylin and eosin stain: fibrosis with micro-honeycombing, non specific chronic inflammation with peri-bronchial and centrilobular distribution (A, B) and multiple non-caseating giant cell granulomas (C).