Harvey's Garden

Asthma

5 min read

Etiology

  • Risk factors for asthma include:
    • Family history of asthma
    • Past history of allergies
    • Atopic dermatitis
    • Low socioeconomic status
  • Allergic asthma (extrinsic asthma) vs Nonallergic asthma (intrinsic asthma)
    • Allergic asthma (extrinsic asthma)
      • Cardinal risk factor: atopy
      • Environmental allergens: pollen (seasonal), dust mites, domestic animals, mold spores
      • Allergic occupational asthma from exposure to allergens in the workplace (e.g., flour dust)
    • Nonallergic asthma (intrinsic asthma)
      • Viral respiratory tract infections (one of the most common stimuli, especially in children)
      • Cold air
      • Physical exertion (laughter, exercise-induced asthma)
      • Gastroesophageal reflux disease (GERD): often exists concurrently with asthma
      • Chronic sinusitis or rhinitis
      • Medication: aspirin/NSAIDS (aspirin-induced asthma), beta blockers
      • Stress
      • Irritant-induced occupational asthma (e.g., from exposure to solvents, ozone, tobacco or wood smoke, cleaning agents)

Tip

  • Allergic asthma (extrinsic asthma): A type of asthma triggered by allergens (e.g., pollen, dust mites, mold spores, pet allergens). Typically onset in childhood.
  • Nonallergic asthma (intrinsic asthma): A type of asthma that typically develops in patients > 40 years of age.

Pathophysiology

Common underlying pathophysiology

Asthma is an inflammatory disease driven by T-helper type 2 cells (Th2-cell) that manifests in individuals with a genetic predisposition. It consists of the following three pathophysiologic processes:

  1. Bronchial hyperresponsiveness
  2. Bronchial inflammation
    • Symptoms are primarily caused by inflammation of the terminal bronchioles, which are lined with smooth muscle but lack the cartilage found in larger airways.
  3. Endobronchial obstruction caused by:
    • Increased parasympathetic tone
      • Reversible bronchospasm
      • Increased mucus production
      • Mucosal edema and leukocyte infiltration into the mucosa with hyperplasia of goblet cells
    • Hypertrophy of smooth muscle cells

Type-specific pathophysiology

  • Allergic asthma
    • IgE-mediated type 1 hypersensitivity to a specific allergen
    • Characterized by mast cell degranulation and release of histamine after a prior phase of sensitization
  • Nonallergic asthma
    • Irritant asthma: irritant enters lung → ↑ release of neutrophils → submucosal edema → airway obstruction
    • Aspirin-induced asthma (NSAID-exacerbated respiratory disease) is characterized by the Samter triad:
      • Inhibition of COX-1 → ↓ PGE2 → ↑ leukotrienes and inflammation → submucosal edema → airway obstruction
      • Chronic rhinosinusitis with nasal polyposis
      • Asthma symptoms

Clinical features

Subtypes and variants

  • Allergic asthma
    • Most common phenotype
    • Begins with intermittent symptoms in childhood
    • Triggered by allergens
    • Usually associated with atopy (e.g., eczema, rhinitis)
    • Responds well to ICS-containing treatment
  • Nonallergic asthma
    • Less common than allergic asthma
    • Triggered by, e.g., viral upper respiratory tract infections, cold air, GERD
    • Not associated with atopy
    • Responds poorly to ICS-containing treatment
  • Cough variant asthma: a type of asthma characterized by chronic dry cough without other typical symptoms of asthma
    • Cough often worsens at night.
  • Aspirin-exacerbated respiratory disease
  • Occupational asthma

Differential diagnostics

FeatureCardiogenic “Asthma” (Heart Failure)Bronchial Asthma
PathophysiologyLV failure → pulmonary edema → airway narrowing.Airway inflammation (IgE/eosinophilic) → bronchoconstriction.
Patient ProfileOlder patient, history of CHF, HTN, MI.Younger patient, history of atopy (eczema, allergies).
Key SymptomsOrthopnea, PND, pink frothy sputum.Nocturnal cough, triggered by allergens/exercise.
Physical ExamJVD, S3 gallop, bibasilar crackles/rales.Diffuse expiratory wheezing, often normal between exacerbations.
Key Diagnostics↑ BNP, Kerley B lines & cardiomegaly on CXR.PFTs show reversible obstruction post-bronchodilator.
ManagementDiuretics (e.g., Furosemide), Nitrates, O2, BiPAP.Bronchodilators (e.g., Albuterol), Corticosteroids.

Diagnostics

Spirometry

  • Supportive findings: Expiratory airway limitation: i.e., ↓ FEV1 and ↓ FEV1/FVC ratio
  • Bronchodilator Responsiveness Testing:
    • Used when the patient has abnormal baseline spirometry showing obstruction (FEV1/FVC ≤70%)
    • Tests if the obstruction is reversible (suggesting asthma) or fixed (suggesting conditions like COPD)
    • Not useful if current spirometry is normal, even if the patient has asthma symptoms
    • Quick test: perform spirometry → give bronchodilator → repeat spirometry
  • Bronchial Challenge Testing:
    • Identify airway hyperresponsiveness and bronchoconstriction in response to direct, nonallergic stimuli (e.g., methacholine, histamine) or indirect stimuli (e.g., exercise, hyperventilation).
    • Used when patient has symptoms suggestive of asthma but normal baseline spirometry
    • Tests for airway hyperresponsiveness by attempting to provoke bronchospasm
    • Particularly useful for patients with intermittent symptoms who are asymptomatic during office visits
    • More time-intensive: involves giving increasing doses of the provocative agent and measuring response

Treatment

Approach

1. Chronic Asthma Management (Stepwise)

  • Foundation: Control inflammation with an Inhaled Corticosteroid (ICS). Therapy is stepped up or down based on symptom control.
  • Preferred Approach (Single Maintenance and Reliever Therapy - MART):
    • Mild (Steps 1-2): As-needed low-dose ICS-formoterol.
    • Moderate (Step 3): Daily low-dose ICS-formoterol + as-needed ICS-formoterol for relief.
    • Severe (Steps 4-5): Increase to medium/high-dose ICS-formoterol + add-on therapy like a Long-Acting Muscarinic Antagonist (LAMA) or biologics (e.g., anti-IgE, anti-IL-5).
  • Key Principle: Pure SABA (e.g., Albuterol) alone is no longer first-line for even mild asthma. An ICS is needed to manage underlying inflammation.

2. Acute Asthma Exacerbation

  • Tx (O-SABA-S):
    • Oxygen: Maintain SpO₂ >92%.
    • SABA: High-dose Albuterol (nebulized), often with Ipratropium (a SAMA).
    • Systemic Corticosteroids: Oral Prednisone or IV Methylprednisolone are critical to reduce inflammation and prevent relapse.
  • Severe/Refractory Tx:
    • IV Magnesium Sulfate: For bronchodilation in severe attacks.
    • Mechanical Ventilation: If impending respiratory failure (↑PaCO₂, ↓mental status, silent chest).

Antileukotrienes

Leukotriene receptor antagonists (LTRAs)

  • Montelukast, Zafirlukast
  • Uses
    • Exercise-induced
    • Prevent leukotrienes from binding to their receptors (CysLT1)→ ↓ bronchoconstriction and inflammation
    • Asthma aspirin-induced asthma
    • Long-term maintenance treatment (particularly in children)

Leukotriene pathway modifiers

  • Zileuton
  • Inhibit 5-lipoxygenase → ↓ production of leukotrienes → ↓ bronchoconstriction and inflammation
  • Uses
    • Exercise-induced asthma
    • Aspirin-induced asthma

Mnemonic

Antileukotrienes → Montelukast, zafirlukast, zileuton

Long-acting muscarinic antagonists (LAMA)

  • Tiotropium bromide（噻托溴铵）
  • Long-term maintenance treatment

Mast cell stabilizers (chromones)

  • Cromolyn
  • Inhibit mast cell degranulation and prevent release of preformed chemical mediators.
  • Uses
    • Preventive treatment prior to exercise

Biologics

Anti-IgE antibodies

  • Omalizumab 单抗记忆
  • Binds to serum IgE → ↓ expression of high-affinity IgE receptors (FcεRI) on mast cells and basophils
  • Uses
    • Select cases of severe asthma

IL-5 antibodies

  • mepolizumab, reslizumab,benralizumab
  • Block the effects of IL-5 on eosinophils → ↓ chemotaxis and ↓ cell differentiation, maturation, and activation
  • Uses
    • Refractory severe eosinophilic asthma

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