Asthma: A High-Yield USMLE Review
Asthma is a chronic, Th2-mediated inflammatory airway disease characterized by reversible airflow obstruction and bronchial hyperresponsiveness to triggers. It is distinguished from COPD by its reversibility and eosinophilic, allergic inflammatory basis. On the exam, asthma tests your ability to recognize reversible obstruction on spirometry, apply stepwise GINA management with inhaled corticosteroids as the cornerstone, and identify the life-threatening silent chest of status asthmaticus.
Pathophysiology
An allergen or trigger is presented by dendritic cells to T-helper cells, driving a dominant Th2 response with release of IL-4, IL-5, and IL-13. IL-4/IL-13 stimulate B cells to produce IgE (sensitizing mast cells) while IL-5 recruits and activates eosinophils; mast cell degranulation then releases histamine, leukotrienes, and prostaglandins. The net result is bronchoconstriction (smooth muscle contraction), airway wall edema, and mucus hypersecretion — the three components of obstruction. With chronic, poorly controlled disease, airway remodeling (smooth muscle hypertrophy, subepithelial fibrosis, goblet cell hyperplasia) produces partially irreversible obstruction.
Presentation
- Acute-onset (minutes to hours) episodic dyspnea, wheezing, and cough
- Symptoms often worse at night
- Signs of increased work of breathing: accessory muscle use, intercostal retractions, tripod positioning, and progressively shortened speech (phrases → single words → unable to speak)
- In severe exacerbation, a 'silent chest' with quiet or absent breath sounds and altered mental status signals impending respiratory failure
- Often associated with atopy/allergies (e.g., seasonal allergies)
Diagnosis
- Spirometry: obstructive pattern (FEV1/FVC <0.70) with significant bronchodilator reversibility — ≥12% AND ≥200 mL improvement in FEV1 after SABA
- Bronchoprovocation (methacholine challenge) when spirometry is normal but asthma is suspected: >20% fall in FEV1 at ≤4 mg/mL (PC20 ≤4); high sensitivity so a negative test rules out asthma, but lower specificity
- Supporting findings: elevated blood eosinophils, elevated IgE, positive skin prick tests to aeroallergens, and elevated FeNO (marker of eosinophilic airway inflammation)
- In severe exacerbation, ABG to assess for rising PaCO2 — a normalizing or rising CO2 in a fatiguing patient signals impending respiratory failure
Management
- Chronic (GINA stepwise) therapy: inhaled corticosteroids are the cornerstone; low-dose ICS-formoterol as needed is the preferred reliever at all steps because it delivers anti-inflammatory therapy with every rescue use. Avoid SABA-only therapy, which increases exacerbation risk and does not address inflammation
- Step up therapy for partly controlled/uncontrolled asthma (e.g., low-dose to medium-dose ICS-LABA), but first check adherence, inhaler technique, and modifiable risk factors
- Severe asthma add-ons: LAMA (tiotropium) added to ICS-LABA, biologics guided by phenotype (anti-IgE omalizumab for allergic asthma; anti-IL5/IL5R mepolizumab/benralizumab and anti-IL4R dupilumab for eosinophilic/type 2 high asthma), with oral corticosteroids as a last resort
- Acute exacerbation: supplemental oxygen (target SpO2 93–95%), frequent/continuous nebulized SABA (albuterol), ipratropium added for severe attacks, and systemic corticosteroids
- Escalation for refractory attacks: IV magnesium sulfate, consider heliox, epinephrine/terbutaline; if impending respiratory failure, intubate (ketamine for induction) using a low respiratory rate and prolonged expiratory time to prevent air trapping
High-yield
- Reversible airway obstruction + bronchial hyperresponsiveness = asthma (contrast with the largely irreversible obstruction of COPD)
- Th2 inflammation: IL-4/IL-13 → IgE → mast cell sensitization; IL-5 → eosinophils
- Bronchodilator reversibility on spirometry: ≥12% AND ≥200 mL FEV1 improvement after SABA
- ICS is the cornerstone of therapy; GINA now recommends against SABA-only treatment
- A 'silent chest' with drowsiness in severe asthma is impending respiratory failure, NOT improvement — quiet chest means too little air movement to generate wheeze
- Elevated FeNO reflects eosinophilic airway inflammation; phenotyping (eosinophilic vs allergic) guides biologic selection
Pitfalls
- Assuming 'less wheezing = better': in status asthmaticus a quieting chest with rising PaCO2 and altered mental status is deterioration toward respiratory arrest
- 'All that wheezes is not asthma' — heart failure (cardiac asthma), foreign body aspiration, anaphylaxis, and PE can all cause wheezing; always confirm with objective testing
- Relying on SABA monotherapy: it relieves bronchospasm but ignores the underlying inflammation and increases exacerbation risk
- Overlooking asthma-COPD overlap in older patients with smoking history plus atopy — these patients still need ICS and have more frequent exacerbations
- A normal or 'reassuring' PaCO2 in a fatiguing severe asthmatic can be falsely reassuring; it may signal impending failure rather than mild disease
Don't just memorize Asthma — practice reasoning through it on branching cases where your decisions shape the patient.