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DEFINITION

Hypersensitivity pneumonitis (HP) is an interstitial lung disease believed to be mediated by aberrant cellular and humoral immune responses to various inhaled antigens in susceptible individuals, resulting in inflammatory and/or fibrotic disease of the lung parenchyma and small airways. Its clinical presentation and natural course can be quite variable, and despite the efforts of numerous expert groups and panels, clear consensus on diagnostic criteria and common nomenclature has proven elusive. Previous classification schemes described “acute HP” and “chronic HP” as two forms of the disease, the latter temporally related to the former, despite limited evidence to support this approach. The recognition that prognosis is minimally related to disease duration, and is more dependent upon extent of fibrosis, has led to the recent move to reclassify HP subtypes as “nonfibrotic HP” and “fibrotic HP.” The latter represents a uniquely challenging entity, for multiple reasons discussed below.

EPIDEMIOLOGY AND ETIOLOGIES

The prevalence of HP is quite variable in different populations, presumably because of differing intensity, frequency, and duration of inhalation exposure, probable variations in environmental and occupational patterns, and heterogeneity in host susceptibility (genetic or otherwise). A large number of antigenic exposures are associated with HP, as shown in Table 56-1. At the same time, prevalence data are colored by the variability of diagnostic criteria and the absence of a clear gold standard.1 Once thought to be a relatively rare disease, HP (by any definition) is becoming more frequently recognized, as awareness of the limitations of classic diagnostic criteria has grown. Available studies suggest an overall incidence of HP of 0.3 to 0.9 per 100,000 individuals,2–5 with striking differences in specific populations studied. These data on incidence and prevalence largely pertain to acute (or nonfibrotic) HP, as an entity more easily recognized particularly in association with identifiable antigen exposure. The nuances of this designation, and those of the less recognizable chronic (or fibrotic) entities, will be discussed below.

TABLE 56-1Etiologies of Hypersensitivity Pneumonitis

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