Key Points to Remember for Hypersensitivity Pneumonitis Management
The most critical aspect of managing hypersensitivity pneumonitis is identifying and completely eliminating exposure to the inciting antigen, which is essential for preventing disease progression and improving survival outcomes. 1
Disease Understanding
Hypersensitivity pneumonitis (HP) is an immune-mediated lung disease triggered by inhalation of various antigens in susceptible individuals, with both humoral (antigen-specific IgG antibodies) and cellular immune responses (Th1) contributing to the pathology 1
HP can be categorized into two clinical phenotypes: nonfibrotic and fibrotic HP, with different diagnostic approaches and prognoses for each 1
The disease prevalence ranges from 1.67-2.71 per 100,000 in the U.S. population, representing between 2-47% of all interstitial lung disease cases 1
Inciting Agents
Hundreds of potential inciting agents have been identified, generally falling into six categories: bacteria, fungi, mycobacteria, animal/plant proteins, chemicals, and metals 1, 2
Common sources include microorganisms, fungi (especially indoor molds), avian antigens, and low-molecular-weight chemicals like isocyanates 1
Exposure settings can be occupational (e.g., automobile workers exposed to polyurethane), household (e.g., contaminated humidifiers), or recreational (e.g., indoor hot tubs) 1
In approximately 30-60% of cases, the specific inciting agent cannot be identified despite thorough investigation 2
Diagnosis
A thorough exposure history is essential, focusing on establishing the type, extent, and temporal relationship of exposures to symptoms 1
High-resolution CT (HRCT) scan and bronchoalveolar lavage (BAL) fluid lymphocyte cellular analysis are recommended initial diagnostic steps 1
For nonfibrotic HP, the guidelines recommend:
For fibrotic HP, consider BAL for lymphocyte cellular analysis, transbronchial lung cryobiopsy, or surgical lung biopsy when other testing is inconclusive 1
Exposure Assessment and Management
Classify patients based on the likelihood of occupational or environmental antigen exposure to guide management 1
Consider involving occupational medicine specialists and environmental hygienists in the diagnostic workup when the source of exposure is unclear 1
Clinical improvement after antigen avoidance supports the diagnosis of HP, though lack of improvement doesn't rule it out (particularly in fibrotic HP) 1
For occupational exposures, workplace inspection may help identify the antigen source 1
Prognosis and Treatment
Patients with an unidentified inciting antigen have significantly worse survival outcomes (HR 2.08; 95% CI, 1.02-4.24) compared to those with identified antigens 1
Acute HP with early detection and immediate antigen avoidance has an excellent prognosis, while chronic/fibrotic HP may progress despite removal from exposure 2
Fatal HP is uncommon but can occur, particularly in cases with continued exposure or progression to fibrosis despite treatment 3, 4
For severe disease or respiratory failure, prednisone at 1-2 mg/kg/day tapered over 4-8 weeks may be considered 1
Special Considerations
For hot tub-related HP ("hot tub lung"), complete avoidance of indoor hot tub use is recommended 1
For metal workers with HP, avoidance of metalworking fluid exposure is essential 1
Regular maintenance of potential sources (e.g., hot tubs, humidifiers) according to manufacturers' recommendations can help prevent HP 1
Smoking status should be assessed, as there is a higher-than-expected prevalence of emphysema among HP patients, independent of smoking history 1