Extrinsic Allergic alveolitis (Hypersensitivity Pneumonitis)

EAA not typically characterized by wheeze and bronchospasm. It is caused by a Type IV and III immune reaction.

ℹ️ About

• Extrinsic Allergic Alveolitis (EAA) is an allergic response to inhaled antigens, often of animal or fungal origin.
• This leads to a lymphocytic inflammatory response with potential for fibrosis in the lungs.
• It typically presents as a restrictive lung defect with interstitial shadowing on imaging.
• Antigen avoidance is the primary treatment strategy.
• EAA accounts for 2% of occupational lung diseases, mostly affecting agricultural workers.
• Over 300 antigens have been identified as triggers.

Main Issues

• Extrinsic: Caused by exposure to substances outside the body.
• Allergic: Involves an abnormally heightened immune response to common substances.
• Alveolitis: Refers to inflammation within the small air sacs of the lungs (alveoli).

🧬 Aetiology

• Antigen exposure triggers an acute inflammatory reaction in the bronchioles and alveoli.
• It is not an allergic reaction as seen with asthma but involves a Type IV hypersensitivity reaction with granuloma formation.
• Type III reactions involve antigen-specific antibodies and complement activation, contributing to immune complex deposition.
• Levels of interleukins 1 and 8 and tumour necrosis factor-alpha are increased during inflammation.

Common Exposures and Associated Diseases

🩺 Clinical Features

• Acute Form:
  • Onset of symptoms like fever, chills, chest tightness, breathlessness, and malaise.
  • Typically begins 4-8 hours after antigen exposure, often worsening after the first day back at work.
  • Weekend or work absence may provide symptom relief, giving a diagnostic clue.
• Chronic Form:
  • Gradually progressive dyspnea, weight loss, and reduced exercise tolerance.
  • Can develop over months or years with persistent antigen exposure.
  • Inspiratory crackles, especially in the upper lobes, tachypnea, and cyanosis if severe.
• Historical Clues: Recent job change, moving to a new home, using a new hot tub, or having pets (especially birds) can be relevant.

Differential Diagnosis

• Asthma: Features such as wheeze and bronchospasm are more prominent in asthma, whereas EAA presents with a delayed hypersensitivity pattern and systemic features.

🔎 Investigations

• Blood Tests: Elevated neutrophils and raised ESR.
• CXR: Mid-zonal opacities, bilateral reticulonodular shadowing, and progression to fibrosis.
• Respiratory Function Tests: Restrictive lung defect with reduced transfer factor (diffusion capacity).
• Blood Tests for Precipitins: Detect antibodies against the offending antigen in serum.
• HRCT Chest: Ground-glass opacities and reticulation.
• Bronchoalveolar Lavage: Increased CD8+ T cells.
• Lung Biopsy: Lymphocytic infiltration, granuloma formation, and interstitial fibrosis.

⚠️ Complications

• Progression to chronic restrictive lung disease if exposure continues.
• Development of respiratory failure, cor pulmonale, and pulmonary hypertension.

Factors Supporting Diagnosis

• Known exposure to an offending antigen.
• Presence of precipitating antibodies to the antigen.
• Recurrent episodes of symptoms, particularly after exposure.
• Inspiratory crackles on examination.
• Symptoms starting 4-8 hours after exposure.
• Unexplained weight loss over time.

💊 Management

• Antigen Avoidance: The primary treatment is avoiding the causative antigen. In cases where complete avoidance is impossible, positive pressure masks may be worn.
• Steroids: Oral corticosteroids may be used to reduce inflammation, but are not a replacement for antigen avoidance. A reducing dose may be trialed in chronic cases.
• Compensation: Individuals with significant lung disease due to occupational exposure may be eligible for compensation, particularly in severe or chronic cases.