Treatment of Hypersensitivity Pneumonitis
The cornerstone of HP treatment is complete and permanent removal from the inciting antigen exposure, which is the only intervention proven to improve survival; corticosteroids are reserved for severe non-fibrotic disease or as adjunctive therapy in fibrotic disease when antigen avoidance alone is insufficient. 1
Initial Classification and Risk Stratification
Before initiating treatment, classify HP based on the presence or absence of fibrosis, as this fundamentally determines therapeutic approach and prognosis 2, 1:
- Non-fibrotic HP: Purely inflammatory disease with potential for complete recovery
- Fibrotic HP: Mixed inflammatory-fibrotic or purely fibrotic disease with significantly worse prognosis and limited reversibility 2
Determine whether an inciting antigen (IA) has been identified, as this critically impacts both management strategy and survival outcomes 2, 1.
Antigen Identification and Avoidance Strategy
Environmental Assessment
Conduct a comprehensive exposure history covering three domains 2:
- Occupational exposures: Request material safety data sheets; consider occupational medicine consultation for workplace-related cases 2
- Home environment: Assess for water damage, mold, HVAC contamination, humidifiers, evaporative coolers, and pet birds 2
- Recreational exposures: Hot tubs, swimming pools, hobbies involving organic dusts 2
For suspected occupational HP, involve an occupational medicine specialist and certified environmental hygienist during diagnostic workup to determine exposure likelihood, assist with worker removal from exposure, and recommend workplace modifications 2.
Complete Antigen Removal
For mycobacterial-related HP (hot-tub lung), indoor hot tubs must be moved outdoors or completely removed—partial measures are insufficient 1. Clinical improvement after antigen avoidance in fibrotic HP is associated with dramatically decreased mortality (HR 0.18; 95% CI 0.04-0.77) 1.
Pharmacological Management
Non-Fibrotic HP
For severe disease or respiratory failure: Prednisone 1-2 mg/kg/day tapered over 4-8 weeks 1, 3. Corticosteroids may hasten recovery and improve gas exchange but must be combined with antigen avoidance 1.
For mycobacterial HP: If symptoms persist after antigen removal, consider antimycobacterial therapy for 3-6 months 1.
For mild-moderate disease: Antigen avoidance alone is often sufficient; monitor closely for improvement within 3-4 months 1, 3.
Fibrotic HP
The treatment paradigm differs fundamentally from non-fibrotic disease 2, 4:
- Primary intervention: Antigen remediation remains critical even in established fibrosis 1, 4
- Immunosuppressive therapy: Consider when complete antigen avoidance cannot be achieved or disease progresses despite avoidance 1. However, recent evidence suggests fibrotic HP patients demonstrate decline in FVC% and DLCO% over one year regardless of immunosuppression use 5
- Antifibrotic agents: Should be considered secondarily in progressive fibrotic disease, particularly with UIP-like patterns 2, 4, 5
- Steroid-sparing agents: Mycophenolate mofetil or azathioprine for patients requiring prolonged immunosuppression 3
Critical caveat: Immunosuppression that is beneficial in fibrotic HP is harmful and should be avoided in idiopathic pulmonary fibrosis, making accurate distinction between these entities essential 2, 4.
Monitoring and Follow-Up
Assess response to treatment through serial pulmonary function testing 1, 3:
- FVC% predicted and DLCO% predicted at baseline, 3-4 months, and regularly thereafter 1, 3
- In non-fibrotic HP, improvement typically occurs within 3-4 months after antigen avoidance 1, 3
- In fibrotic HP, stabilization rather than improvement may be the realistic goal 5
Perform continued surveillance for exposure and reinforce antigen avoidance education at every visit 1.
Critical Pitfalls to Avoid
Do not rely on clinical improvement with medical therapy alone to confirm or exclude HP diagnosis—this is unreliable and may lead to misdiagnosis 1, 3.
Do not assume failure to improve with antigen avoidance rules out HP, especially in fibrotic disease where irreversible damage may have occurred 1.
Do not escalate immunosuppression before ensuring complete cessation of antigen exposure—this is the most common and consequential error in HP management 1, 3.
Do not overlook the need for steroid-sparing agents in patients requiring prolonged immunosuppression to minimize corticosteroid toxicity 3.
For unidentified antigen cases (occurring in 30-50% of referral center patients), more aggressive immunosuppression may be necessary, though prognosis remains guarded 2, 1.