Hypersensitivity Pneumonitis
Introduction
Hypersensitivity Pneumonitis (HP), also known as extrinsic allergic alveolitis, is an inflammatory syndrome affecting the lungs. It is triggered by the inhalation of a variety of organic particles, including bacteria, fungi, and animal proteins, leading to an immune-mediated response. This condition can manifest in acute, subacute, or chronic forms, each presenting with distinct clinical features and requiring different management strategies.
Etiology and Pathogenesis
Hypersensitivity Pneumonitis is primarily caused by repeated exposure to airborne antigens. These antigens can be derived from occupational, environmental, or domestic sources. Common antigens include:
- **Bacteria**: Thermophilic actinomycetes found in moldy hay, compost, and humidifiers.
- **Fungi**: Aspergillus species present in moldy environments.
- **Animal Proteins**: Avian proteins from bird droppings or feathers.
- **Chemicals**: Isocyanates used in the production of polyurethane foams.
Upon inhalation, these antigens reach the alveoli and provoke an immune response characterized by the activation of alveolar macrophages and the release of pro-inflammatory cytokines. This leads to the recruitment of T cells and the formation of granulomas, which are hallmark features of HP.
Clinical Manifestations
The clinical presentation of Hypersensitivity Pneumonitis varies based on the duration and intensity of antigen exposure.
Acute HP
Acute HP typically occurs after a single, heavy exposure to the offending antigen. Symptoms develop within 4-8 hours and include:
- Fever
- Chills
- Malaise
- Cough
- Dyspnea
Physical examination may reveal inspiratory crackles, and chest radiographs often show diffuse, fine nodular opacities.
Subacute HP
Subacute HP results from intermittent or continuous exposure to lower levels of antigen. Symptoms are less severe but more persistent, including:
- Chronic cough
- Progressive dyspnea
- Fatigue
- Weight loss
High-resolution computed tomography (HRCT) scans may show ground-glass opacities and centrilobular nodules.
Chronic HP
Chronic HP develops after prolonged exposure to low levels of antigen. It is characterized by:
- Progressive dyspnea
- Chronic cough
- Fatigue
- Clubbing of fingers
HRCT scans reveal fibrosis, honeycombing, and traction bronchiectasis, indicating irreversible lung damage.
Diagnosis
The diagnosis of Hypersensitivity Pneumonitis involves a combination of clinical, radiological, and immunological findings. Key diagnostic steps include:
- **History and Physical Examination**: Detailed history of exposure to potential antigens and physical examination findings.
- **Radiological Imaging**: Chest radiographs and HRCT scans to identify characteristic lung changes.
- **Pulmonary Function Tests (PFTs)**: Demonstrate restrictive lung disease with reduced diffusing capacity for carbon monoxide (DLCO).
- **Bronchoalveolar Lavage (BAL)**: Increased lymphocytes and presence of specific antigens.
- **Lung Biopsy**: Histopathological examination showing non-caseating granulomas and interstitial inflammation.
Management
The primary approach to managing Hypersensitivity Pneumonitis is the avoidance of the offending antigen. Additional treatment options include:
- **Corticosteroids**: To reduce inflammation and improve symptoms.
- **Immunosuppressive Agents**: Such as azathioprine or mycophenolate mofetil for patients with chronic HP.
- **Supportive Care**: Oxygen therapy and pulmonary rehabilitation for advanced cases.
Prognosis
The prognosis of Hypersensitivity Pneumonitis depends on the stage at which the disease is diagnosed and the effectiveness of antigen avoidance. Acute and subacute forms often resolve with appropriate intervention, while chronic HP can lead to progressive pulmonary fibrosis and respiratory failure.