What is the management of hypersensitivity pneumonitis?

Management of Hypersensitivity Pneumonitis

The cornerstone of hypersensitivity pneumonitis management is complete and permanent antigen avoidance, which must be prioritized before any pharmacologic intervention, as this represents the only truly effective treatment and carries the best medical prognosis. 1, 2, 3

Initial Classification and Risk Stratification

Immediately classify the patient based on the presence or absence of fibrosis, as this fundamentally determines treatment approach and prognosis 1, 2:

• Non-fibrotic HP: Better prognosis with potential for complete recovery; responds well to antigen avoidance alone 2, 4
• Fibrotic HP: Significantly worse prognosis with limited response to immunosuppression; may require antifibrotic therapy 1, 5, 6

Antigen Identification and Avoidance Strategy

Exposure Assessment

Obtain a detailed environmental and occupational exposure history focusing on the type, extent, and temporal relationship between exposures and symptoms 1, 7. Common sources include 7:

• Avian antigens (birds, feather bedding)
• Indoor molds and contaminated humidifiers
• Hot tubs (mycobacterial exposure)
• Occupational exposures (metalworking fluids, isocyanates)

For occupational exposures, involve an occupational medicine specialist and environmental hygienist during workup, especially when the source is unclear 1, 7.

Antigen Remediation

Complete and definitive antigen avoidance is mandatory 1, 2:

• For hot tub-related HP: Remove indoor hot tubs completely or move outdoors 2, 7
• For occupational exposures: Complete workplace avoidance may be necessary 7
• For avian exposure: Remove all birds and feather-containing materials from the home 7

Critical pitfall: Partial avoidance is insufficient—continued low-level exposure can perpetuate disease progression despite treatment 1, 2.

Pharmacologic Management

Non-Fibrotic HP

For severe disease or respiratory failure, initiate prednisone 1-2 mg/kg/day tapered over 4-8 weeks 2, 7. Corticosteroids may hasten recovery and improve gas exchange but must be combined with antigen avoidance 2.

For mycobacterial HP (hot tub lung), consider antimycobacterial therapy for 3-6 months if symptoms persist after antigen avoidance 2.

Fibrotic HP

The treatment algorithm differs substantially 1, 5:

1. First-line: Antigen avoidance remains paramount 1, 2
2. Second-line: Immunosuppressive therapy when complete antigen avoidance cannot be achieved or disease progresses 2, 6
3. Third-line: Antifibrotic agents for progressive disease despite immunosuppression 1, 5, 6

Mycophenolate mofetil or azathioprine have demonstrated benefit in altering FVC decline slope and improving DLCO in fibrotic HP 7.

Key distinction from IPF: In IPF, antifibrotic therapy is first-line, whereas in fibrotic HP, antigen remediation and immunosuppression precede antifibrotic consideration 1, 5.

Monitoring and Response Assessment

Monitor lung function (FVC%, DLCO%) regularly to assess treatment response 1, 2:

• In non-fibrotic HP, improvement typically occurs within 3-4 months after antigen avoidance 1, 2
• Clinical improvement after antigen avoidance in fibrotic HP is associated with decreased mortality (HR 0.18; 95% CI 0.04-0.77) 2

Important caveat: Lack of clinical improvement with antigen avoidance does not rule out HP, particularly in fibrotic disease where irreversible damage may have occurred 1, 2.

Prognostic Factors

Patients with unidentified inciting antigens have significantly worse survival (HR 2.08; 95% CI 1.02-4.24) compared to those with identified antigens 7. This underscores the critical importance of thorough exposure assessment 1.

Management of Treatment Failure

If disease worsens during corticosteroid taper 7:

1. Re-evaluate thoroughly for ongoing or unidentified antigen exposure
2. Return to the last effective corticosteroid dose
3. For fibrotic HP, consider steroid-sparing agents (mycophenolate mofetil or azathioprine)
4. Recognize that fibrotic HP often has limited response to immunosuppression alone

Critical error to avoid: Escalating immunosuppression without ensuring complete antigen avoidance will result in treatment failure 2, 7.