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Asthma for USMLE Step 2

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Asthma for the USMLE Step 2 Exam
Pathophysiology
  • Chronic Inflammation:
    • Asthma involves chronic inflammation of the airways, driven by immune cells like eosinophils, mast cells, and Th2 cells. This inflammation leads to airway hyperresponsiveness, bronchoconstriction, and mucus overproduction.
    • Inflammatory mediators such as histamine, leukotrienes, and prostaglandins cause bronchospasm and edema, contributing to airflow obstruction.
Asthma histopathology
  • Airway Hyperresponsiveness:
    • Asthma is marked by an exaggerated response of the airway smooth muscle to stimuli such as allergens, cold air, or exercise, resulting in reversible bronchoconstriction.
  • Airway Remodeling:
    • Chronic inflammation can cause long-term structural changes in the airways, including smooth muscle hypertrophy, goblet cell hyperplasia, and subepithelial fibrosis. This can lead to more permanent airflow limitation over time.
Risk Factors
  • Genetics:
    • A family history of asthma, allergic rhinitis, or eczema increases the risk of asthma. Atopy, the tendency to develop IgE-mediated allergic responses, is a major risk factor.
  • Environmental Exposures:
    • Common asthma triggers include allergens (e.g., pollen, dust mites, animal dander), viral infections (particularly in children), air pollution, and tobacco smoke.
  • Occupational Exposures:
    • Certain jobs expose workers to asthma triggers, such as chemical fumes, dust, and industrial allergens.
Clinical Features
  • Wheezing:
    • Expiratory wheezing is the classic symptom of asthma, often episodic and triggered by specific stimuli like exercise or allergens.
  • Dyspnea:
    • Shortness of breath is common, especially during or after physical activity or at night.
  • Cough:
    • Asthma may present with a nonproductive cough, particularly at night.
  • Chest Tightness:
    • Many patients report a sensation of tightness or discomfort in the chest, often during exacerbations.
Diagnosis
  • Spirometry:
    • Asthma diagnosis is confirmed by demonstrating reversible airflow obstruction:
    • Reduced FEV1/FVC ratio: Indicates obstructive lung disease.
    • Reversibility: An increase in FEV1 by at least 12% and 200 mL after bronchodilator use confirms reversible airway obstruction.
  • Peak Expiratory Flow (PEF):
    • PEF monitoring helps assess asthma control and detect early signs of worsening disease.
  • Methacholine Challenge Test:
    • This bronchoprovocation test is used when spirometry is inconclusive. A significant drop in FEV1 after inhaling methacholine suggests airway hyperreactivity.
Management
  • Stepwise Treatment Approach:
    • Step 1 (Intermittent Asthma):
    • Short-acting beta-agonists (SABAs) (e.g., albuterol) are used as needed for symptom relief.
    • Step 2 (Mild Persistent Asthma):
    • Low-dose inhaled corticosteroids (ICS) are the preferred maintenance therapy.
    • Step 3 (Moderate Persistent Asthma):
    • Combination of low-dose ICS + long-acting beta-agonists (LABAs) (e.g., salmeterol) or medium-dose ICS alone.
    • Step 4 (Severe Persistent Asthma):
    • High-dose ICS + LABA, with possible addition of tiotropium (a long-acting muscarinic antagonist) or biologics (e.g., omalizumab for allergic asthma).
  • Acute Exacerbation Management:
    • SABAs: First-line treatment for bronchospasm relief during exacerbations.
    • Systemic Corticosteroids: Oral or IV prednisone is used in moderate-to-severe exacerbations to reduce inflammation.
    • Oxygen: Administered to maintain SpO2 >90% in severe cases.
Complications
  • Status Asthmaticus:
    • A life-threatening asthma exacerbation that is unresponsive to standard treatment, requiring intensive care and potentially mechanical ventilation.
  • Airway Remodeling:
    • Chronic poorly controlled asthma can lead to irreversible structural changes in the airways, causing fixed airflow limitation.
  • Pneumothorax:
    • Rare but serious, this complication may occur during severe exacerbations, especially with overuse of bronchodilators.
Key Points
  • Asthma is a chronic inflammatory disorder of the airways, often triggered by allergens, respiratory infections, and environmental factors.
  • Diagnosis is confirmed by spirometry showing reversible airflow obstruction (FEV1/FVC improvement with bronchodilator use).
  • The stepwise approach to management ranges from SABAs for intermittent asthma to ICS and LABAs for persistent disease, with biologics added in severe cases.
  • Acute exacerbations are managed with SABAs, systemic corticosteroids, and oxygen if needed.
  • Complications include status asthmaticus, airway remodeling, and pneumothorax, emphasizing the need for early diagnosis and proper management.

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