Pathology of Lung Diseases: Asthma & Bronchiectasis

I. Asthma

Asthma is a heterogeneous disease characterized by chronic airway inflammation and variable, reversible expiratory airflow obstruction. It produces episodic symptoms such as wheezing, shortness of breath, chest tightness, and cough, which vary over time and intensity. Symptomatic episodes are most likely to occur at night or in the early morning.

1. Classifications of Asthma

Special Subtypes:

• Drug-Induced Asthma (Aspirin-Sensitive): Occurs in individuals with recurrent rhinitis and nasal polyps. They are exquisitely sensitive to Aspirin and NSAIDs. Mechanism: Aspirin inhibits the Cyclooxygenase (COX) pathway, causing a drop in Prostaglandin E2. This forces all arachidonic acid to shift down the Lipoxygenase (LOX) pathway, producing massive amounts of Leukotrienes (potent bronchoconstrictors).
• Occupational Asthma: Triggered by repeated exposure to fumes (epoxy, plastics), organic dusts (wood, cotton), or chemicals (formaldehyde, penicillin). Occurs via Type I reactions or direct liberation of bronchoconstrictors.

2. Pathogenesis (The Th2 Response)

The fundamental abnormality in atopic asthma is an exaggerated Th2 CD4+ T-cell response to normally harmless environmental antigens.

• Cytokine Cascade: Th2 cells secrete cytokines that orchestrate the attack:
  • IL-4: Stimulates B-cells to switch production to IgE antibodies.
  • IL-5: Activates and recruits massive numbers of Eosinophils (the hallmark cell of asthma).
  • IL-13: Stimulates mucus secretion from submucosal glands and also promotes IgE production.
• Early-Phase Reaction (Minutes): Dominated by acute bronchoconstriction (smooth muscle spasm), increased mucus production, and vascular permeability (edema) triggered by histamine release from mast cells.
• Late-Phase Reaction (Hours): Dominated by the recruitment of leukocytes (notably eosinophils and neutrophils, partly recruited by Th17 cells releasing IL-17). This causes sustained inflammatory damage to the epithelium.

3. Morphology & Airway Remodeling

In fatal acute severe asthma attacks (Status Asthmaticus), the lungs are hyper-inflated with small areas of atelectasis. The most striking gross finding is the complete occlusion of bronchi/bronchioles by thick, tenacious mucus plugs.

Airway Remodeling (Chronic Changes): Over years, asthma permanently alters the airway structure:

1. Sub-basement membrane fibrosis (thickening due to deposition of Type I and III collagens).
2. Hypertrophy and hyperplasia of bronchial wall smooth muscle.
3. Increase in the size of submucosal glands and the number of goblet cells.
4. Increased vascularity (angiogenesis).

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II. Bronchiectasis

Bronchiectasis is defined as the abnormal and irreversible dilatation of the bronchi and bronchioles (>2mm), developing secondary to the inflammatory destruction of the muscular and elastic supporting tissues of the bronchial walls.

It is not a primary disease itself, but rather the end-stage result of other severe, chronic conditions.

1. Etiology and Associated Conditions

• Congenital / Hereditary Conditions:
  • Cystic Fibrosis: Defective CFTR ion transport leads to extremely thick, viscous mucus that blocks airways and breeds infection.
  • Primary Ciliary Dyskinesia: Autosomal recessive defect in ciliary motor proteins (dynein arms). Cilia cannot sweep mucus. Kartagener Syndrome occurs in 50% of these patients, marked by the triad of Bronchiectasis, Sinusitis, and Situs Inversus (organs flipped to the opposite side of the body).
• Infections: Severe necrotizing pneumonias (staph, klebsiella, TB).
• Bronchial Obstruction: Tumors, inhaled foreign bodies, or localized mucus impaction.
• Immune Disorders: Rheumatoid arthritis, SLE, Inflammatory Bowel Disease, or graft-versus-host disease post-transplant.
• Allergic Bronchopulmonary Aspergillosis (ABPA): A hyperimmune Th2 response to the fungus Aspergillus fumigatus in patients with asthma or CF. High IgE, intense eosinophilia, and mucus plugging lead to bronchiectasis.

2. Pathogenesis

The pathogenesis relies on a vicious cycle of two intertwined processes: Obstruction and Chronic Infection. Obstruction leads to pooling of secretions distal to the blockage. Pooled secretions act as a culture medium for bacteria. The resulting severe inflammatory response releases proteases and reactive oxygen species that destroy the smooth muscle and elastin of the airway wall, causing it to balloon outward permanently.

3. Morphology

• Gross Findings: Usually affects the lower lobes bilaterally (especially vertical air passages where gravity pools mucus). The airways are dilated up to 4 times their normal size.
  • Classic Sign: In a normal lung, bronchioles cannot be followed by the naked eye beyond 2-3 cm from the pleural surface. In bronchiectasis, the massively dilated, cystic bronchi can be followed almost to the pleura.
  • Cut surfaces show cystic spaces filled with foul, mucopurulent secretions.
• Microscopy: Intense acute and chronic exudation within the walls, leading to extensive ulceration and desquamation of the epithelium.
  • Squamous metaplasia often occurs, further destroying mucociliary clearance.
  • In severe cases, necrosis forms a lung abscess.
  • Chronic cases show dense peribronchiolar fibrosis that completely obliterates smaller airways.

Bacterial Colonization: The anaerobic, mucoid environment favors specific microbes. Haemophilus influenzae is found in ~50% of cases, and Pseudomonas aeruginosa in up to 30% (notoriously in Cystic Fibrosis patients).

4. Clinical Features & Complications