From the Guidelines
Hypersensitivity pneumonitis treatment should prioritize identifying and avoiding the offending antigen, and for acute episodes, corticosteroids are the mainstay of therapy, typically starting with prednisone 0.5-1 mg/kg/day (usually 40-60 mg daily) for 1-2 weeks, followed by a gradual taper over 2-6 weeks depending on response, as recommended by the most recent guidelines 1. The disease is an inflammatory lung condition caused by repeated inhalation of environmental antigens that trigger an immune response in susceptible individuals.
- Key aspects of management include:
- Identifying and avoiding the offending antigen, which may include molds, bird proteins, or chemical compounds in the home or workplace
- Classifying patients based on the likelihood of an occupational or environmental inciting antigen exposure, and considering the inclusion of an occupational medicine specialist and an environmental hygienist during the multidisciplinary diagnostic workup 1
- Classifying the disease as fibrotic or nonfibrotic based on the presence or absence of fibrosis on high-resolution computed tomography (HRCT) of the chest, as this provides important prognostic information 1
- Treatment options:
- Corticosteroids for acute episodes, with a typical starting dose of prednisone 0.5-1 mg/kg/day (usually 40-60 mg daily) for 1-2 weeks, followed by a gradual taper over 2-6 weeks depending on response
- Long-term immunosuppression may be necessary in chronic or progressive cases, often with prednisone maintenance (5-10 mg daily) or steroid-sparing agents like mycophenolate mofetil (1-1.5 g twice daily) or azathioprine (2 mg/kg/day) 1
- Additional considerations:
- Pulmonary rehabilitation is beneficial for patients with significant impairment
- Regular follow-up with pulmonary function tests every 3-6 months is essential to monitor disease progression
- The disease results from a complex type III and type IV hypersensitivity reaction, where repeated antigen exposure leads to immune complex formation and T-cell mediated inflammation, causing alveolar damage and potentially irreversible fibrosis if not properly managed 1
From the Research
Definition and Classification of Hypersensitivity Pneumonitis
- Hypersensitivity pneumonitis (HP) is a granulomatous, inflammatory disease of the lungs caused by the inhalation of antigenic organic particles or fumes 2.
- HP can be classified into acute, subacute, and chronic forms, with acute HP resulting from intermittent, high-level exposure to the inducing antigen, and chronic HP mostly originating from long-term, low-level exposure 3.
- The disease may present as an acute, subacute, or chronic illness, with episodes of acute and subacute HP usually resolving following cessation of antigen exposure 2.
Pathogenesis and Immunologic Response
- The pathogenesis of HP involves both type III and type IV hypersensitivity reactions that are mediated by immune complexes and Th1 T cells, respectively 2.
- Proinflammatory cytokines and chemokines activate alveolar macrophages, cause an influx of CD8+ lymphocytes into the lungs, facilitate granuloma formation, and promote the development of pulmonary fibrosis 2.
- The immunologic response includes activated macrophages and CD8+ cytotoxic lymphocytes, and bronchoalveolar lavage fluid reveals marked lymphocytosis with a ratio of CD4+ to CD8+ cells of <1 4, 5.
Diagnosis and Predictors
- A careful environmental and occupational history and establishment of exposure to a known inciting antigen are key factors in making the diagnosis of HP 2.
- Six significant predictors have been identified that provide approximately 95% diagnostic accuracy, including exposure to a known offending allergen, positive precipitating antibodies to the offending antigen, recurrent episodes of symptoms, inspiratory crackles on lung auscultation, symptoms occurring 4-8 hours after exposure, and weight loss 4, 5.
- Serum precipitating antibodies, bronchoalveolar lavage, and lung biopsy may be helpful in making the diagnosis 2.
Treatment and Management
- Avoidance of organic antigen exposure is the most important factor in the management of HP 2, 3.
- Corticosteroids are indicated for the treatment of severe acute and subacute HP and for chronic HP that is severe or progressive 2.
- Immunosuppressive therapy, such as azathioprine or mycophenolate mofetil, may be used in chronic HP, with a decreased incidence of treatment-emergent adverse events, but no difference in lung function decline or survival compared to prednisone alone 6.