Hypersensitivity Pneumonitis: Challenges of a Complex Disease

Abstract

Hypersensitivity pneumonitis (HP) is a complex interstitial lung disease caused by chronic inhalation of a wide variety of antigens in susceptible and sensitized individuals, commonly associated with an occupational exposure. An impressive number of inciting antigens causing hypersensitivity pneumonitis have been found to cover a wide range of occupations. As working practices have changed over time, especially in industrialized countries, new names for occupational HP have emerged. This review emphasizes the main diagnostic issues arising from the high variability of clinical presentation and the broad spectrum of causal antigens. Furthermore, it provides an overview of current methods to unveil possible causes of hypersensitivity pneumonitis, highlights HP's current diagnostic and treatment challenges and the remaining areas of uncertainty, and presents prevention strategies.

Figure 1

Imaging patterns in HP. (a) Axial section of chest high-resolution computer tomography (HRCT) showing ill-defined centrilobular nodules (white asterisk) and areas of air-trapping (arrows) suggesting small airway involvement. (b) Ground-glass opacities and mosaic attenuation (ground glass alternating with air-trapping (arrows)) suggestive of predominant interstitial infiltration. (c) The “three density sign” comprised of lung lobules with normal density (white asterisk), lobules with ground-glass attenuation (arrowheads), and lobules with decreased density due to air-trapping (arrows). (d) Fibrotic HP presented by areas of ground glass accompanied by traction bronchiectasis (black arrow) and honeycombing (white arrow).

Figure 2

An approach to the assessment of clinical, imaging, and pathological features in patients with IPF, fHP, or both as primary diagnostic considerations in the absence of alternative causes (adapted after Marinescu et al. [40]).