Skip to content

BASICS

Description

  • HP is a diffuse inflammatory disease of lung parenchyma caused by immunologic reaction to inhaled aerosolized antigens.
  • Also known as extrinsic allergic alveolitis (EAA).
  • Classified by time course:
  • Acute: symptoms within 4–12 hours post heavy exposure, resolving within days to weeks.
  • Subacute: insidious symptoms over days to weeks with continual low-level exposure.
  • Chronic: prolonged, progressive disease leading to fibrosis and respiratory failure.

Epidemiology

  • Incidence: ~0.9 per 100,000
  • Prevalence varies by exposure:
  • Farmers: 1-19%
  • Bird fanciers: 6-20%
  • Others: 1-8%
  • Increasingly recognized cause of fibrotic interstitial lung disease.

ETIOLOGY AND PATHOPHYSIOLOGY

Immunologic Mechanisms

  • Type III hypersensitivity: immune complex deposition (IgG-mediated).
  • Type IV hypersensitivity: T-cell mediated delayed response.
  • Inhaled antigens form immune complexes triggering complement activation.
  • Cellular inflammation involves CD8+ T cells predominance.

Common Antigens by Exposure

  • Farming: thermophilic actinomycetes (e.g., Faenia rectivirgula), molds.
  • Bird exposure: droppings, feathers, serum proteins.
  • Wood, grain, textiles, chemicals, plastics, humidifiers, and other occupational/environmental sources.
  • Hot tub lung: Mycobacterium avium complex.
  • Numerous specific antigenic sources detailed in occupational categories.

Genetics

  • No definitive susceptibility genes.
  • Possible involvement of TNF-Ξ± and MHC class II genes.

RISK FACTORS

  • Exposure to organic antigens.
  • Viral infections during exposure.
  • Nonsmokers have higher incidence than smokers; smoking may modify immune response.
  • Smokers tend toward chronic disease with worse outcomes.

DIAGNOSIS

Clinical Criteria (not validated but widely used)

  1. Exposure history to known antigens.
  2. Clinical symptoms consistent with HP.
  3. Radiologic imaging showing interstitial lung disease.
  4. Immunologic evidence of sensitization (serum precipitins, BAL lymphocytosis).

Clinical Presentation

  • Acute: fever, chills, myalgia, cough, dyspnea, malaise within hours of exposure.
  • Chronic: progressive cough, dyspnea, fatigue, weight loss over months to years.

Physical Exam

  • Acute: fever, tachypnea, diffuse fine crackles.
  • Chronic: inspiratory crackles, hypoxia, clubbing, weight loss.

Differential Diagnosis

  • Acute infectious pneumonia.
  • Asthma, aspiration.
  • Chronic: sarcoidosis, COPD, IPF, tuberculosis, lymphoma, collagen vascular diseases.

Diagnostic Tests

Laboratory and Immunologic

  • Serum precipitins (antigen-specific antibodies): supportive but not diagnostic (up to 40% positive in asymptomatic exposed).
  • Bronchoalveolar lavage (BAL): lymphocytosis >50%, CD8+ predominance, low CD4/CD8 ratio.

Pulmonary Function Tests (PFTs)

  • Restrictive pattern with decreased diffusion capacity (DLCO).
  • Obstructive patterns possible.

Imaging

  • Chest X-ray: nonspecific; ground-glass, nodules, interstitial changes.
  • High-resolution CT (HRCT):
  • Acute: ground-glass opacities, poorly defined centrilobular nodules, air trapping.
  • Chronic: fibrosis, honeycombing, bronchiectasis, volume loss.

Lung Biopsy (if needed)

  • Transbronchial: noncaseating granulomas, peribronchiolar inflammation.
  • Surgical: organizing pneumonia, fibrosis, multinucleated giant cells.

TREATMENT

General Measures

  • Strict avoidance of offending antigen is primary treatment.
  • Outpatient management except in severe acute pneumonitis.

Medications

First Line

  • Corticosteroids (e.g., prednisone 20–50 mg daily for 1–2 weeks then taper) for symptomatic relief during exacerbations.
  • Do not improve long-term prognosis.

Second Line

  • Bronchodilators/inhaled corticosteroids for wheeze and chest tightness.
  • Oxygen therapy as needed.
  • Lung transplantation for end-stage disease.

ISSUES FOR REFERRAL

  • Pulmonologist and immunologist consultation recommended.

ADMISSION AND INPATIENT CARE

  • For severe respiratory compromise, hypoxia, or need for invasive diagnostics.

ONGOING CARE

Follow-Up

  • Weekly to monthly based on severity.
  • Serial chest imaging, PFTs, antibody titers for monitoring.

Diet

  • No specific restrictions.

Patient Education

  • Loss of symptom awareness with chronic exposure is possible.
  • Smoking does not protect once disease is established; may worsen prognosis.
  • Use personal protective equipment in high-risk occupations.

PROGNOSIS

  • Acute HP: good prognosis if antigen removal is early.
  • Chronic HP: fibrosis predicts poorer outcomes.
  • Corticosteroids improve symptoms acutely but do not alter long-term disease course.

COMPLICATIONS

  • Progressive pulmonary fibrosis.
  • Respiratory failure.
  • Cor pulmonale and right heart failure.

REFERENCES

  1. Spagnolo P, Rossi G, Cavazza A, et al. Hypersensitivity pneumonitis: a comprehensive review. J Investig Allergol Clin Immunol. 2015;25(4):237-250.
  2. D'souza RS, Donato A. Hypersensitivity pneumonitis: an overlooked cause of cough and dyspnea. J Community Hosp Intern Med Perspect. 2017;7(2):95-99.

Clinical Pearls

  • Skin testing is not useful in diagnosis.
  • Suspect HP in patients with unexplained cough, dyspnea, restrictive/diffusion defects, and history of antigen exposure.
  • HP can mimic viral respiratory illness or asthma exacerbation; misdiagnosis delays therapy.
  • Smoking protects against developing HP but worsens prognosis once established.
  • Protective equipment in occupational settings is essential to prevent HP.
  • Chronic HP is a recognized mimic of other fibrotic lung diseases.