ILD / DPLD

Hypersensitivity Pneumonitis (HP)

Added by Dr. Andrew · Last updated 2024-01-15

Overview

Hypersensitivity pneumonitis (extrinsic allergic alveolitis) is an immune-mediated ILD caused by inhalation of organic antigens in susceptible individuals. It can present as acute, subacute, or chronic (fibrotic) forms. Chronic fibrotic HP has a prognosis similar to IPF.

Key Points

1Common antigens: bird proteins (bird fancier's lung), thermophilic actinomycetes (farmer's lung), Trichosporon (summer-type HP in Japan)
2Acute HP: fever, chills, dyspnea 4–8 hours after antigen exposure; resolves with avoidance
3Chronic HP: progressive fibrosis, may develop UIP pattern — can be indistinguishable from IPF
4HRCT: upper/mid-lobe predominant GGO + centrilobular nodules + mosaic attenuation (air trapping)
5BAL: lymphocytosis >20% (often >40%) — key differentiator from IPF
6Treatment: antigen avoidance (most important) + corticosteroids for acute/subacute

Clinical Pearls

Fibrotic HP with UIP pattern: may need antifibrotics (nintedanib shown effective in INBUILD trial)
Serum precipitins: positive in 50–90% of bird fancier's lung but not diagnostic alone
Inhalation challenge: gold standard but rarely performed clinically
Hot tub lung: Mycobacterium avium complex in hot tub water — a form of HP

Board High-Yield

Exam Focus

BAL lymphocytes >20% argues strongly against IPF, favors HP
Antigen avoidance is the most important treatment — must identify and remove exposure
INBUILD trial: nintedanib slows FVC decline in progressive fibrosing ILDs including HP
Chronic fibrotic HP: upper lobe fibrosis + lower lobe GGO + air trapping on expiratory CT
Farmer's lung: thermophilic actinomycetes in moldy hay — most common form of HP

ABIM Exam Weight

10%

Diffuse Parenchymal Lung Disease represents approximately 10% of the PCCM certification exam.