### Date : 2024-11-06 15:53 ### Topic : Hypersensitivity Pneumonitis (HP) #pulmonology ---- **Hypersensitivity Pneumonitis (HP)**, also known as **extrinsic allergic alveolitis**, is an inflammatory lung disease caused by an immune response to inhaled organic antigens, like molds, animal proteins, or plant materials. The inflammation primarily affects the **alveoli** (air sacs in the lungs), resulting in symptoms such as cough, shortness of breath, and fatigue. Repeated exposure to the triggering antigen can lead to chronic lung disease, with progressive scarring and fibrosis. ### Pathophysiology of Hypersensitivity Pneumonitis HP occurs due to an exaggerated immune response involving **both Type III (immune complex-mediated) and Type IV (delayed-type hypersensitivity) hypersensitivity reactions**. 1. **Initial Sensitization**: - Inhaled organic antigens enter the lungs, leading to the production of **IgG antibodies** that bind to the antigens, forming immune complexes. - These immune complexes can trigger inflammation, attracting immune cells, particularly T-cells and macrophages, to the alveoli and small airways. 2. **Acute and Chronic Inflammation**: - Inflammation develops in response to repeated exposure to the antigen, causing alveolitis (inflammation of the alveoli), bronchiolitis, and sometimes granuloma formation. - Over time, continued exposure leads to fibrosis, or scarring of the lung tissue, which impairs lung function. 3. **Progression to Fibrosis**: - In chronic HP, inflammation is replaced by **fibrosis**, leading to a restrictive lung pattern and reduced oxygen exchange. ### Causes and Common Antigens HP is associated with exposure to various antigens, often found in specific environments or occupations. Examples include: - **Farmer’s Lung**: Exposure to moldy hay containing **Thermophilic Actinomycetes** bacteria. - **Bird Fancier’s Lung**: Exposure to bird droppings or feathers from pigeons, parrots, or other birds. - **Humidifier Lung**: Exposure to contaminated humidifiers or air conditioners that aerosolize molds and bacteria. - **Hot Tub Lung**: Exposure to **Mycobacterium avium complex** from poorly maintained hot tubs. These antigens are typically found in occupational or hobby settings where organic material can be aerosolized and inhaled. ### Classification of Hypersensitivity Pneumonitis HP is classified based on the duration and pattern of exposure and symptoms: 1. **Acute HP**: - Symptoms occur **hours** after intense exposure to the antigen. - Common symptoms: Fever, chills, cough, dyspnea, chest tightness, and flu-like symptoms. - Symptoms generally resolve within days after antigen exposure ceases. 2. **Subacute (Subchronic) HP**: - Occurs with intermittent exposure over weeks to months. - Symptoms are more persistent and include chronic cough, shortness of breath, and fatigue. - Chest tightness and mild weight loss may also be seen. 3. **Chronic HP**: - Results from prolonged, low-level exposure to the antigen. - Symptoms are insidious and include progressive dyspnea, chronic cough, and fatigue, often resembling pulmonary fibrosis. - Chronic HP can lead to irreversible lung damage and fibrosis if exposure continues. ### Symptoms of Hypersensitivity Pneumonitis Symptoms vary depending on the type and duration of exposure: - **Acute HP**: Fever, chills, cough, shortness of breath, flu-like symptoms. - **Subacute HP**: Persistent cough, mild weight loss, fatigue, dyspnea. - **Chronic HP**: Severe breathlessness, chronic cough, fatigue, and sometimes clubbing of the fingers (in cases of severe fibrosis). ### Diagnosis of Hypersensitivity Pneumonitis Diagnosis is based on a combination of **clinical history, exposure history, imaging, and lung function tests**: 1. **History and Physical Examination**: - Detailed history of exposure to potential antigens, especially in occupational or hobby settings. - Symptoms aligned with exposure can provide strong diagnostic clues. 2. **Pulmonary Function Tests (PFTs)**: - A restrictive pattern (decreased total lung capacity, TLC) is common, especially in chronic HP. - Reduced DLco (diffusing capacity for carbon monoxide) indicates impaired gas exchange. 3. **Imaging**: - **Chest X-ray**: May show reticular (net-like) or nodular opacities in the upper and middle lung fields. - **High-Resolution CT (HRCT)**: More sensitive and shows ground-glass opacities, mosaic attenuation, centrilobular nodules, and fibrosis in chronic cases. 4. **Bronchoalveolar Lavage (BAL)**: - Bronchoscopy with BAL can show an increase in lymphocytes, supporting an HP diagnosis. High lymphocyte levels in BAL fluid suggest an inflammatory reaction typical of HP. 5. **Lung Biopsy**: - Sometimes required to confirm the diagnosis, especially in chronic or fibrotic HP. - Histological findings include lymphocytic infiltration, non-caseating granulomas, and interstitial fibrosis in chronic cases. 6. **Serum Precipitins**: - Blood tests for antibodies to specific antigens (precipitins) can support the diagnosis, though the presence of antibodies alone does not confirm active disease. ### Treatment of Hypersensitivity Pneumonitis The primary treatment approach is to **identify and avoid the causative antigen**. Additional treatment options include: 1. **Antigen Avoidance**: - This is the cornerstone of HP management. Avoiding exposure can prevent disease progression and improve symptoms. - In occupational settings, proper ventilation, wearing masks, and changing work environments can help reduce exposure. 2. **Corticosteroids**: - Corticosteroids, such as prednisone, can help reduce inflammation, especially in acute or subacute HP. - They are usually given for a short course and tapered off as symptoms improve and antigen exposure is minimized. 3. **Immunosuppressive Agents**: - In chronic or severe cases unresponsive to steroids, immunosuppressive agents such as azathioprine or mycophenolate mofetil may be considered. 4. **Pulmonary Rehabilitation**: - In chronic HP with significant lung damage, pulmonary rehabilitation can improve exercise tolerance and quality of life. 5. **Oxygen Therapy**: - For patients with advanced disease and hypoxemia, supplemental oxygen may be necessary. 6. **Lung Transplant**: - In end-stage fibrotic HP with severe lung dysfunction, lung transplantation may be considered. ### Prognosis The prognosis of HP depends on early diagnosis and effective antigen avoidance: - **Acute HP** generally has a good prognosis with antigen avoidance, and symptoms typically resolve completely. - **Chronic HP** has a poorer prognosis due to irreversible fibrosis and lung scarring. Progressive fibrosis can lead to respiratory failure in severe cases. ### Summary - **Hypersensitivity Pneumonitis (HP)** is an immune-mediated lung disease caused by inhaled organic antigens, leading to alveolar inflammation. - **Types** include acute, subacute, and chronic, depending on the exposure pattern. - **Symptoms**: Cough, shortness of breath, fatigue, and flu-like symptoms in acute cases; chronic cough and breathlessness in chronic cases. - **Diagnosis** is based on history, PFTs, imaging, and sometimes biopsy. - **Treatment** focuses on antigen avoidance, corticosteroids for inflammation, and supportive care for chronic disease. Early diagnosis and elimination of the antigen exposure are critical to preventing progression to chronic, irreversible lung damage in HP. ### Reference: - ### Connected Documents: -