Asthma: Pathophysiology, Clinical Presentation, and Management

Respiratory Disorders: Asthma

Learning Objectives

This section outlines the key concepts and skills students should master regarding asthma, including its pathophysiology, clinical features, diagnosis, and management.

• Th1 and Th2 Pathways: Understand how environmental exposures influence the development of Th1 and Th2 immune responses in asthma.

• Clinical Signs and Symptoms: Differentiate between signs (objective findings) and symptoms (subjective complaints) of asthma.

• Diagnosis: Use laboratory data, diagnostic tests, physical exam findings, and clinical presentation to diagnose asthma.

• Key Mediators: Identify the main cellular and molecular mediators involved in asthma pathophysiology.

• Abnormal Findings: Explain abnormal laboratory, diagnostic, and physical exam findings using physiological and pathophysiological principles.

• Airway Obstruction: Describe the processes leading to airway obstruction in asthma.

• Acute Severe Asthma: Recognize signs and symptoms of acute severe asthma.

• Asthma vs. COPD: Differentiate key mechanisms, signs, and disease characteristics between asthma and chronic obstructive pulmonary disease (COPD).

Asthma Defined

Characteristics of Asthma

Asthma is a heterogeneous disease primarily characterized by chronic airway inflammation and variable airflow obstruction.

• Acute/Chronic Airway Inflammation: Persistent inflammation of the bronchial mucosa.

• Bronchial Hyper-responsiveness: Airways are overly sensitive to various stimuli.

• Intermittent Bronchospasm: Sudden constriction of bronchial smooth muscle.

• Mucus Hypersecretion: Excessive production of mucus by goblet cells.

• Airway Obstruction: Blockage of airflow due to inflammation, bronchospasm, and mucus.

• Airway Remodeling: Long-term structural changes in the airway wall.

Asthma Phenotypes

Types of Asthma

Asthma presents in several phenotypes, each with distinct pathophysiological features and clinical implications.

• Allergic (High Type 2):

  • Eosinophilic airway inflammation.

  • Associated with atopy (eczema, rhinitis, food/drug allergy).

  • Common in childhood; may remit with age.

• Non-allergic (Low Type 2, Neutrophilic):

  • Variable inflammatory mediators.

  • Poor response to inhaled corticosteroids.

• Late-onset:

  • More common in adults, especially females.

  • Non-allergic, poor response to corticosteroids.

  • Unlikely to remit.

• Exercise-induced:

  • Triggered by physical activity.

• Asthma with Obesity:

  • Limited eosinophilic involvement.

Asthma Risk Factors

Host and Environmental Factors

Asthma risk is influenced by genetic predisposition and environmental exposures.

• Host Factors:

  • Genetic: Family history of asthma or atopy.

  • Atopy: Allergic rhinitis, eczema (atopic dermatitis), allergies.

• Environmental Factors:

  • Exposure to allergens, pollution, tobacco smoke, occupational irritants.

Protective and Risk Factors Table

The following table summarizes household, environmental, and microbiological factors that influence asthma risk.

Protective Factors	Risk Factors
Being the younger sibling Birth and nursing: natural birth, breastfeeding Farm living: agriculture, animal consumption Microbiological exposure: parasitic and viral infections Higher socioeconomic status: better access to resources Other environmental factors: pet exposure	Asthma history in family Birth and nursing: formula feeding Farm living: urban lifestyle Microbiological exposure: lower infection rates Lower socioeconomic status: increased risk Other environmental factors: use of antibiotics

Th1 vs. Th2 Phenotype Table

Factors Favoring Th1 Phenotype	Factors Favoring Th2 Phenotype
Presence of older siblings Early exposure to daycare Tuberculosis, measles, hepatitis A infection Rural environment	Widespread use of antibiotics Western lifestyle Urban environment Diet Sensitization to house-dust mites and cockroaches

Th1 phenotype is associated with protective immunity, while Th2 phenotype is linked to allergic diseases including asthma.

Pathophysiology

Mechanisms of Airway Narrowing and Obstruction

Asthma involves complex interactions between immune cells and airway structures, leading to reversible and irreversible changes.

• Bronchospasm (Bronchoconstriction): Sudden contraction of airway smooth muscle.

• Airway Hyper-responsiveness: Increased sensitivity to stimuli.

• Bronchial Inflammation/Edema: Swelling and infiltration of inflammatory cells.

• Mucus Secretion: Goblet cell hyperplasia increases mucus production.

• Key Mediators:

  • IgE: Immunoglobulin E, central to allergic responses.

  • Mast Cells: Release histamine and leukotrienes, causing bronchoconstriction and hyper-responsiveness.

  • Eosinophils: Contribute to inflammation and tissue damage.

  • T Helper Cells (Th2 > Th1): Th2 cells promote allergic inflammation.

Normal vs. Asthmatic Airway Table

Normal Airway	Asthmatic Airway
Open lumen Thin smooth muscle Minimal mucus Intact epithelium	Narrowed lumen Thickened smooth muscle Excess mucus Damaged epithelium Collagen deposition

Airway Remodeling

Structural Changes in Asthma

Chronic inflammation leads to permanent structural changes in the airway, known as airway remodeling.

• Goblet Cell Enlargement and Proliferation: Increases mucus production.

• Thickened Basement Membrane: Due to collagen deposition.

• Increased Smooth Muscle Cells: Contributes to airway narrowing.

• Angiogenesis: Formation of new blood vessels.

• Non-reversible Component: Remodeling leads to fixed airflow limitation.

Clinical Presentation

General Features

Asthma is marked by chronic inflammation with periods of exacerbation and remission. Severity and triggers vary among individuals.

• Chronic Inflammation: Persistent airway inflammation.

• Exacerbations and Remissions: Fluctuating symptoms.

• Variable Severity: Different triggers produce different responses in patients.

Respiratory Symptoms and Signs

• Dyspnea: Shortness of breath.

• Expiratory Wheeze: Most common symptom; heard on physical exam.

• Chest Tightness: Sensation of constriction.

• Cough: Often non-productive.

• Symptom Intensity: Varies over time; more than one symptom usually present.

• Commonly Worse at Night/Early Morning: Circadian variation in airway responsiveness.

• Triggers: Emotions, exercise, allergens, cold air, respiratory infections.

Physical Exam and Pulmonary Function Tests

• Physical Exam: Expiratory wheezing (not always present).

• Pulmonary Function Tests:

  • Forced Expiratory Volume (FEV1): Measures volume of air exhaled in one second.

  • FEV1/FVC Ratio: Used to indicate airflow limitation. Normal values: > 0.75–0.80 in adults, > 0.90 in children.

Variability and Reversibility

• Variability: Lung function changes hour-to-hour, day-to-day, or seasonally.

• Reversibility:

  • Improved lung function after bronchodilator (e.g., albuterol): $\Delta FEV_1 > 12\%$ and $> 200$ ml over baseline

  • Improved after exercise: $\Delta FEV_1 > 10\%$ and $> 200$ ml

  • Provocation tests (methacholine, histamine, hypertonic saline): $\Delta FEV_1 > 15-20\%$

Acute Severe Asthma

Clinical Features

Acute severe asthma is a medical emergency characterized by rapid progression of symptoms and signs.

• Symptoms:

  • Severe dyspnea

  • Distress; unable to complete full sentences

• Signs:

  • Tachycardia (increased heart rate)

  • Tachypnea (increased respiratory rate)

  • Hyperinflated chest (air trapping)

  • Use of accessory breathing muscles

  • FEV1 < 40% predicted

  • Low oxygen saturation (O2), low PaO2

  • Pale/cyanotic skin (lips, nailbeds)

Asthma Medications

Pharmacological Management

Asthma is managed with a combination of medications targeting different aspects of the disease.

• β2 Agonists:

  • Bronchodilation

  • Short acting (albuterol, levalbuterol)

  • Long acting (salmeterol, formoterol)

• Anticholinergics:

  • Reverse cholinergic-induced bronchoconstriction

  • Short acting (ipratropium)

  • Long acting (tiotropium)

• Inhaled/Systemic Corticosteroids:

  • Reduce airway inflammation

• Leukotriene Receptor Antagonists:

  • Montelukast

• Cromolyn Sodium:

  • Mast cell stabilizer

• Biologics:

  • Omalizumab (IgE antibody)

  • Reslizumab, mepolizumab, benralizumab (IL5 antibody)

  • Dupilumab (IL4 antibody)

  • Tezepelumab (TSLP antibody; downregulates multiple inflammatory mediators)

Summary Table: Asthma Pathophysiology and Management

Pathophysiology	Clinical Presentation	Management
Bronchospasm Airway inflammation Mucus hypersecretion Airway remodeling	Dyspnea Wheeze Cough Chest tightness Variable FEV1/FVC	β2 agonists Anticholinergics Corticosteroids Leukotriene antagonists Biologics

Additional info: Asthma is a chronic inflammatory disorder of the airways involving multiple cell types and mediators. Its hallmark features are variable and reversible airflow obstruction, airway hyper-responsiveness, and airway remodeling. Diagnosis is based on clinical presentation and objective measures of lung function, with management tailored to phenotype and severity.