What are the key differences between hypersensitivity pneumonitis and asthma?

Distinguishing Hypersensitivity Pneumonitis from Asthma

Hypersensitivity pneumonitis (HP) and asthma are fundamentally different diseases: HP is an immune-mediated interstitial lung disease affecting the alveoli and lung interstitium with restrictive physiology, while asthma is an inflammatory airway disease affecting the bronchi with obstructive physiology and reversible airflow limitation. 1, 2

Core Pathophysiological Differences

Asthma Characteristics

• Airway-centered disease involving bronchial smooth muscle, with inflammation primarily in the conducting airways 1
• Type I allergic reaction mediated by Th2 cells, IgE antibodies, mast cells, and typically eosinophils 1, 3
• Obstructive physiology with variable and reversible airflow limitation, bronchial hyperresponsiveness, and bronchoconstriction 1
• Immediate symptoms (wheezing, breathlessness, chest tightness, cough) that occur during or shortly after exposure 1

Hypersensitivity Pneumonitis Characteristics

• Parenchymal disease affecting the alveoli and lung interstitium, with peribronchiolar inflammation extending into the lung tissue 2, 4
• Type III and IV allergic reactions mediated by Th1 cells, IgG antibodies (not IgE), with predominantly lymphocytic inflammation and granuloma formation 4, 3, 5
• Restrictive physiology with reduced lung volumes and impaired gas exchange 5, 6
• Delayed symptoms typically beginning 4-6 hours after exposure, with fever, chills, dyspnea, and cough 5, 7

Clinical Presentation Distinctions

Timing and Symptom Pattern

• Asthma: Symptoms occur immediately or within minutes of exposure; nocturnal or early morning cough is characteristic; episodes are often reversible spontaneously or with bronchodilators 1
• HP: Symptoms develop 4-6 hours post-exposure with systemic features (fever, chills, malaise); improvement occurs with antigen avoidance over days to weeks 2, 5

Physical Examination

• Asthma: Wheezing on auscultation, prolonged expiratory phase, hyperinflation 1
• HP: Inspiratory crackles (particularly in fibrotic HP), no wheezing unless concurrent airway involvement 8, 5

Diagnostic Testing Differences

Pulmonary Function Tests

• Asthma: Obstructive pattern (FEV1/FVC <0.70), reversibility with bronchodilators (≥12% and 200mL improvement in FEV1), bronchial hyperresponsiveness to methacholine or histamine 1, 9
• HP: Restrictive pattern (reduced FVC and TLC, normal or elevated FEV1/FVC ratio), reduced DLCO; approximately 69% of HP patients show restrictive defects 5, 6

Imaging

• Asthma: Chest X-ray typically normal or shows hyperinflation; HRCT not routinely indicated 1
• HP: HRCT shows characteristic findings including:
  • Nonfibrotic HP: Profuse poorly-defined centrilobular ground-glass nodules, mosaic attenuation with three-density sign, lobular air-trapping on expiration 2
  • Fibrotic HP: Reticular abnormalities with traction bronchiectasis, ground-glass opacities, fibrosis often with mid-to-upper lung predominance 2

Laboratory and Immunologic Testing

• Asthma: Elevated total IgE, positive allergen-specific IgE, peripheral eosinophilia 1
• HP: Precipitating antigen-specific IgG antibodies (not IgE), normal or mildly elevated total IgE 2, 5, 10

Bronchoalveolar Lavage (BAL)

• Asthma: Eosinophilia (though neutrophilic patterns exist in severe asthma) 1, 3
• HP: Lymphocytosis (typically ≥20%, often >30-40%), with reduced CD4/CD8 ratio in many cases 2, 11, 5

Exposure History Distinctions

Asthma Triggers

• Common aeroallergens (dust mites, pollens, animal dander), irritants (smoke, strong odors), exercise, cold air 1
• Symptoms improve rapidly with removal from trigger and bronchodilator use 1

HP Inciting Antigens

• Occupational/environmental organic antigens: thermophilic bacteria, molds (especially Trichosporon asahii in Japan), avian proteins (birds, feathers, down), agricultural exposures 2, 4, 10
• Requires detailed exposure questionnaire focusing on home environment (humidifiers, water damage, birds), occupation, and hobbies 2
• Temporal relationship: symptoms worsen with exposure and improve with prolonged avoidance (days to weeks) 2

Important Overlap and Pitfalls

Coexistence Possibility

• Both conditions can occur simultaneously in the same patient, particularly with certain exposures like metalworking fluids or environmental fungi 12, 8, 13
• HP can involve airways causing bronchiolitis, leading to obstructive features that may mimic asthma 8
• Severe noneosinophilic asthma with neutrophilic inflammation can share features with HP 3

Common Diagnostic Errors

• Misdiagnosing chronic HP as asthma when patients present with chronic cough and dyspnea without recognizing the exposure history 5, 7
• Missing HP in patients with known asthma who develop new restrictive physiology or interstitial changes 8
• Assuming normal chest X-ray excludes HP: HRCT is essential when HP is suspected 2, 5

Algorithmic Approach to Differentiation

When evaluating a patient with respiratory symptoms:

1. Assess timing: Immediate symptoms → consider asthma; delayed symptoms (4-6 hours) with systemic features → consider HP 1, 5

2. Perform spirometry: Obstruction with reversibility → asthma; restriction with reduced DLCO → HP 1, 5, 6

3. Obtain HRCT if: Restrictive physiology, crackles on exam, poor response to asthma therapy, or compelling exposure history 2

4. Detailed exposure assessment: Use structured questionnaire for occupational/environmental antigens; seasonal variation in symptoms (worse in summer for summer-type HP, winter for bird-related HP) 2, 10

5. Consider BAL if diagnosis uncertain: Lymphocytosis ≥20% supports HP; eosinophilia supports asthma 2, 11

6. Antigen-specific testing: IgG antibodies to suspected antigens in HP; IgE testing for asthma allergens 2, 10

Critical caveat: In patients with fibrotic HP, BAL lymphocytosis may be absent, and diagnosis relies more heavily on HRCT pattern, exposure history, and multidisciplinary discussion 2, 11