Respiratory Pathology I: Bronchial Asthma & ARDS

Bronchial Asthma: Definition and Clinical Hallmarks

Bronchial asthma is defined as a clinico-pathologic chronic inflammatory disorder of the airways. It is characterized by an increased responsiveness of the tracheobronchial tree, which results in widespread but variable bronchoconstriction and airflow limitation. This limitation is typically partially reversible, occurring either spontaneously or through medical treatment. Clinically, asthma is an episodic disease manifested by recurrent episodes of wheezing, breathlessness, chest tightness, and coughing, which are particularly prevalent at night and in the early morning. The four major hallmarks of the disease include increased airway responsiveness to various stimuli, bronchoconstriction, inflammation of the bronchial walls, and increased mucus secretion.

Clinical Classifications and Types of Asthma

Asthma can be classified clinically into three primary types. The most common type is Extrinsic asthma, also known as Allergic or Atopic asthma. The second type is Intrinsic asthma, also referred to as Idiosyncratic or Non-atopic asthma. The third is the Mixed type, where clinical features do not clearly fit into the extrinsic or intrinsic categories. Pathogenetically, asthma is divided into Extrinsic (Allergic/Immune) and Intrinsic (Non-immune) types. Extrinsic varieties include Atopic (IgE-mediated), Occupational (IgE or rarely IgG), and Allergic Bronchopulmonary Aspergillosis (IgE-mediated). Intrinsic varieties include those induced by aspirin or infections.

Characteristics of Atopic vs. Non-Atopic Asthma

Atopic asthma is the most common form and is characterized by an $IgE$-mediated hypersensitivity reaction. It typically begins in childhood and is triggered by environmental allergens. Patients often have a positive family history and show a positive wheal-and-flare reaction on skin tests. In contrast, Non-atopic asthma shows no evidence of allergen sensitization, and skin test results are usually negative. Family history is less common, and it most often affects adults. Common triggers for non-atopic asthma include respiratory viral infections (such as rhinovirus, parainfluenza virus, and respiratory syncytial virus) and inhaled air pollutants (tobacco smoke, sulfur dioxide, ozone, and nitrogen dioxide). Some cases are triggered by cold exposure or exercise.

Comparative Features of Extrinsic and Intrinsic Asthma

Key differences between the two major types include: 1. Age at onset: Extrinsic occurs in childhood, while Intrinsic occurs in adults. 2. History: Extrinsic commonly has a personal/family history of atopy, which is absent in Intrinsic. 3. Allergic illness: Preceding conditions like rhinitis, urticaria, or eczema are present in Extrinsic cases but absent in Intrinsic. 4. Allergens: Dust, pollens, and danders trigger Extrinsic asthma, whereas none are identified for Intrinsic. 5. Drug hypersensitivity: Absent in Extrinsic, while aspirin hypersensitivity is often present in Intrinsic. 6. Serum $IgE$ levels: Elevated in Extrinsic, but normal in Intrinsic. 7. Associated conditions: Chronic bronchitis and nasal polyps are present in Intrinsic asthma but absent in Extrinsic. 8. Emphysema: Unusual in Extrinsic but common in Intrinsic.

Pathogenesis of Atopic Asthma

The pathogenesis involves a $Th2$-mediated $IgE$ response to environmental allergens in genetically predisposed individuals. Central to the pathophysiology is airway inflammation and bronchial hyper-responsiveness. Airway dysfunction occurs through the release of inflammatory mediators and airway wall remodeling. Type 2 helper (CD4) T cells ($Th2$ cells) play a fundamental role by exhibiting an exaggerated response to harmless antigens. These cells secrete interleukins $IL-3$, $IL-4$, and $IL-5$. $IL-4$ stimulates B cells to produce $IgE$, while $IL-5$ recruits eosinophils. Both $IgE$ and eosinophils are key in Type I hypersensitivity reactions, which consist of an acute (immediate) phase and a late phase.

Cellular Mediators and Reactions

In the immediate phase (minutes), allergen exposure triggers mast cells to release granule contents, including histamine, prostaglandins ($PGD_2$), and leukotrienes ($LTB$). This results in bronchoconstriction, increased vascular permeability, edema, and mucus production. The late phase (hours) is characterized by the recruitment of leukocytes, including eosinophils, neutrophils, and more T cells. Mast cells also release enzymes (tryptase, chymase) and cytokines ($IL-4, IL-5, IL-6$, and $TNF-\alpha$). Eosinophils release granule-derived basic proteins such as Major Basic Protein ($MBP$), Eosinophil Cationic Protein ($ECP$), Eosinophil Derived Neurotoxin ($EDN$), and Eosinophil Peroxidase ($EPO$), alongside leukotriene $C_4$ and Platelet Activating Factor. Genetic susceptibility is linked to chromosome $5q$, near a cytokine gene cluster ($IL-3, IL-4, IL-5, IL-9, IL-13$), with $IL-13$ polymorphisms being most consistently associated with the disease.

Morphology and Airway Remodeling

Grossly, in cases of status asthmaticus (acute severe asthma), the lungs are overinflated with small areas of atelectasis. Bronchi and bronchioles are occluded by thick, tenacious mucus plugs containing shed epithelium. Sputum may contain Curschmann spirals (mucus plug extrusions) and Charcot-Leyden crystals composed of the protein galectin-10. Microscopically, "airway remodeling" is observed, including: 1. Thickening of the airway wall; 2. Sub-basement membrane fibrosis (Type I and III collagen deposition); 3. Increased vascularity; 4. Hypertrophy/hyperplasia of bronchial wall muscle and submucosal glands; and 5. Goblet cell hyperplasia. This remodeling contributes to chronic irreversible airway obstruction.

Environmental Influence and Clinical Progression

Asthma is associated with industrialized societies. The "Hygiene Hypothesis" suggests that limited exposure to microbial antigens in early childhood in urban environments may predispose children to asthma. Repeated allergen exposure lead to structural remodeling. Infections are co-factors; children with aeroallergen sensitization who get viral infections (rhinovirus type C, RSV) have a 10- to 30-fold increased risk of severe asthma. A subset of steroid-refractory patients shows neutrophil-enriched infiltrates, possibly driven by a $Th17$ T-cell response to bacterial colonization. Clinical diagnosis involves showing increased airflow obstruction, prolonged expiration, wheezing, and eosinophilia. Therapy uses bronchodilators, glucocorticoids, and leukotriene antagonists.

Complications and Specific Mediators

The most common complication is emphysema. Others include severe hypoxemia, cardiac arrhythmias, atelectasis, pneumothorax, cor pulmonale, respiratory failure, and psychological issues. Critical pharmacologic targets include Leukotrienes $C_4, D_4, E_4$ (prolonged bronchoconstriction), Acetylcholine (parasympathetic muscle constriction), and $IL-5$ (eosinophil recruitment). Historically considered mediators like Histamine, Prostaglandin $D_2$, and Platelet Activating Factor are present but have more minor contributions to the disease state compared to the primary targets.

ALI and ARDS: Definitions and Causes

Acute Lung Injury (ALI) is the abrupt onset of hypoxemia and bilateral pulmonary edema without cardiac failure (non-cardiogenic). Acute Respiratory Distress Syndrome (ARDS) is the severe form of ALI. Both involve increased vascular permeability, edema, epithelial cell death, and diffuse alveolar damage (DAD). ALI can result from pulmonary or systemic disorders. Causes include infections (sepsis, diffuse viral/Pneumocystis pneumonia), physical injury (trauma, near-drowning, burns), inhaled irritants (oxygen toxicity, smoke), chemical injury (heroin or aspirin overdose, paraquat), and hematologic conditions (TRALI, DIC).

Pathogenesis and Morphology of ARDS

ARDS is initiated by injury to pneumocytes and pulmonary endothelium. This leads to endothelial activation and neutrophil extravasation. Characteristics include the accumulation of intra-alveolar fluid and the formation of hyaline membranes (composed of fibrin-rich edema and necrotic cell remnants), caused by damage to type II alveolar pneumocytes and subsequent surfactant abnormalities. In the acute exudative stage, lungs are heavy, firm, red, and boggy. In the proliferative/organizing stage, type II pneumocytes proliferate and granulation tissue forms. This may resolve or lead to fibrotic thickening (scarring) of alveolar septa, driven by fibrogenic cytokines like $TGF-\beta$ and platelet-derived growth factor.

Clinical Phases and Complications of ARDS

Symptoms include profound dyspnea, tachypnea, and cyanosis with bilateral radiographic infiltrates. Hypoxemia may be refractory to oxygen due to ventilation-perfusion mismatch. Progression occurs in 4 phases: 1. Acute Injury (tachycardia, tachypnea, respiratory alkalosis); 2. Latent Period (6-48 hours, clinically stable but hyperventilating); 3. Acute Respiratory Failure (decreased lung compliance, diffuse infiltrates, high-pitched crackles); 4. Severe Abnormalities (severe hypoxemia, metabolic/respiratory acidosis, lethargic/obtunded). Complications include nosocomial infections, pulmonary emboli, oxygen toxicity, GI hemorrhage, sepsis, DIC, and permanent disability from pulmonary fibrosis.