Emphysema (Robbins Pathology)

"Rhobbin" = Robbins - the classic pathology textbook series (Robbins & Cotran / Robbins & Kumar Basic Pathology).

Definition

• Robbins & Cotran Pathologic Basis of Disease, p. 634
• Robbins & Kumar Basic Pathology, p. 448

Types of Emphysema

1. Centriacinar (Centrilobular) Emphysema

• Most common type - >95% of clinically significant cases
• The central/proximal parts of the acinus (respiratory bronchioles) are destroyed; distal alveoli are spared
• Both emphysematous and normal airspaces coexist within the same acinus/lobule
• Upper lobe predominant, especially apical segments
• Strongly associated with cigarette smoking

2. Panacinar (Panlobular) Emphysema

• The entire acinus is uniformly enlarged - from respiratory bronchiole to terminal alveoli
• Lower lobe predominant, especially at the bases
• Associated with α1-antitrypsin (α1-AT) deficiency and exacerbated by smoking
• About 1% of emphysema patients have α1-AT deficiency; >80% of homozygous Z-allele individuals develop panacinar emphysema

3. Distal Acinar (Paraseptal) Emphysema

• The distal portion of the acinus is affected; proximal portion is spared
• Occurs near pleura, along lobular connective tissue septa, adjacent to fibrosis/scarring
• More severe in upper half of lungs
• Enlarged airspaces range from <0.5 cm to >2.0 cm - can form cystic structures/bullae
• Classic cause of spontaneous pneumothorax in young adults

4. Irregular (Paracicatricial) Emphysema

• The acinus is irregularly involved
• Almost always associated with scarring
• Usually occurs in small foci and is clinically insignificant

Only centriacinar and panacinar types cause clinically significant airflow obstruction and COPD.

Pathogenesis

Mechanisms (Robbins & Cotran, p. 635-636):

• Cigarette smoke and noxious particles damage respiratory epithelium and trigger chronic inflammation
• Inflammatory mediators released: LTB4, IL-8, TNF, and others
• Macrophages and resident epithelial cells recruit more neutrophils and amplify inflammation
• T and B cells accumulate in affected lung (role of adaptive immunity still uncertain)

• Inflammatory cells release proteases (especially elastase) that break down connective tissue (elastin, collagen)
• Normally, α1-antitrypsin inhibits these proteases - but in smokers or those with genetic deficiency, this protection is overwhelmed
• Loss of elastic tissue in alveolar walls reduces radial traction on small airways → respiratory bronchioles collapse during expiration → functional airflow obstruction without mechanical obstruction
• α1-AT is encoded at the Pi locus on chromosome 14; ~0.01% of the U.S. population is homozygous for the Z allele (very low serum α1-AT)

• Tobacco smoke and inflammatory cells produce oxidants causing tissue damage, endothelial dysfunction, and further inflammation
• NRF2 (encoded by NFE2L2) is a key transcription factor protecting against oxidant damage; genetic variants in NRF2 pathways are linked to smoking-related lung disease

• Bacterial/viral infections are not thought to initiate emphysema but can acutely exacerbate existing disease

Morphology

• Advanced emphysema → voluminous, pale lungs that overlap the heart and flatten the diaphragm (especially panacinar)
• Centriacinar emphysema: lungs deeper pink, less voluminous; upper 2/3 more affected
• Large alveoli visible on cut surface of fixed lungs
• Apical blebs or bullae may appear (especially paraseptal/irregular emphysema)

• Destruction of alveolar walls without fibrosis → enlarged airspaces
• Focal centriacinar fibrosis may be seen
• Loss of attachments between alveoli and small airway walls
• Pores of Kohn become so large that septa appear to "float" or protrude blindly into airspaces with a club-shaped end
• Decreased capillary bed area (leads to pulmonary hypertension in severe disease)
• Bronchiolar inflammation and submucosal fibrosis in advanced disease

Key Summary Table

Chronic Bronchitis (Robbins Pathology)

Definition

Persistent productive cough for at least 3 consecutive months in at least 2 consecutive years, in the absence of any other identifiable cause.

• Robbins & Cotran Pathologic Basis of Disease, p. 637
• Robbins & Kumar Basic Pathology, p. 448

Etiology

• Cigarette smoking is the primary cause - 90% of affected individuals are smokers
• Other inhaled irritants: sulfur dioxide, nitrogen dioxide, grain dust, cotton dust, silica
• Cigarette smoke damages airways in two key ways:
  1. Causes direct cellular damage and chronic inflammation
  2. Impairs ciliary action of respiratory epithelium, preventing mucus clearance and increasing infection risk

Pathogenesis

1. Mucus Hypersecretion (the earliest and most distinctive feature)

• Enlargement of submucosal glands in the trachea and bronchi (measured by the Reid index)
• Goblet cell hyperplasia in small bronchi and bronchioles - causes mucus production that obstructs small airways
• Mediators involved: histamine and IL-13 from T cells
• Both gland enlargement and goblet cell increase are considered protective reactions against tobacco smoke/pollutants

2. Acquired CFTR Dysfunction

• Smoking leads to acquired CFTR (cystic fibrosis transmembrane conductance regulator) dysfunction
• This causes secretion of abnormal, dehydrated mucus that worsens disease severity

3. Inflammation

• Inhaled irritants cause cellular damage → both acute and chronic inflammatory responses
• Inflammatory cells: neutrophils, lymphocytes, macrophages (notably, NO eosinophils - unlike asthma)
• Long-standing inflammation causes small airway fibrosis (bronchi and bronchioles <2-3 mm diameter) → chronic airway obstruction

4. Infection

• Does not initiate chronic bronchitis
• Plays a role in maintaining the disease and triggering acute exacerbations

Morphology

Gross

• Mucosal lining of larger airways: hyperemic, swollen, edematous, covered by mucinous or mucopurulent secretions
• Smaller bronchi and bronchioles filled with secretions
• Heavy casts of secretions and pus can fill bronchi/bronchioles in severe cases

Microscopic

• Chronic inflammation of airways (lymphocytes and macrophages predominate)
• Enlargement of mucus-secreting submucosal glands - the hallmark diagnostic feature
• Goblet cell hyperplasia in small airways
• Thickening of bronchiolar wall - smooth muscle hypertrophy, ECM deposition, peribronchial fibrosis
• Mucus plugging of bronchiolar lumens
• In severe cases: bronchiolitis obliterans - complete lumen obliteration by fibrosis
• Emphysematous changes frequently coexist

The Reid Index

Reid index = thickness of submucosal gland layer / thickness of bronchial wall (epithelium to cartilage)

Clinical Features

• Most patients have a smoking history of ≥40 pack-years
• Presents insidiously with:
  • Chronic cough with sputum production (cardinal symptom)
  • Increasing dyspnea on exertion
  • Wheezing/bronchospasm in some (asthmatic bronchitis)
• Symptoms are often worse in the morning
• Key diagnostic test: spirometry showing FEV1/FVC < 0.7

"Blue Bloater" vs "Pink Puffer"

Most real patients fall somewhere in the middle, with a mix of bronchitic and emphysematous changes.

Complications and Prognosis

• Pulmonary hypertension due to chronic hypoxemia-induced vasoconstriction + loss of capillary beds
• Cor pulmonale (right heart failure) - more common in bronchitis-predominant COPD
• Acute respiratory failure from superimposed infections
• Obstructive sleep apnea - ~10-30% of patients
• Patients with severe chronic bronchitis have more frequent exacerbations, faster progression, and worse outcomes than those with pure emphysema

Chronic Bronchitis - Quick Reference (Robbins)

Definition

Etiology

• Cigarette smoking (#1 cause - 90% of cases)
• Air pollutants: SO₂, NO₂, grain/cotton/silica dust

Pathogenesis

Reid Index

Submucosal gland thickness / Bronchial wall thickness

• Normal: ≤ 0.4
• Chronic bronchitis: > 0.4

Morphology

• Gross: Hyperemic, edematous mucosa; mucopurulent secretions; mucus plugs
• Micro: Submucosal gland enlargement, goblet cell hyperplasia, chronic inflammation (lymphocytes/macrophages), bronchiolar wall fibrosis
• Severe cases: bronchiolitis obliterans (lumen obliteration by fibrosis)

Clinical

• Cough with sputum production (worse in morning)
• FEV1/FVC < 0.7 on spirometry
• Classic picture: "Blue Bloater" - cyanotic, hypercapnic, overweight, recurrent infections, cor pulmonale

Complications

Asthma (Robbins Pathology)

Definition

1. Intermittent, reversible airway obstruction
2. Chronic bronchial inflammation with eosinophils
3. Bronchial smooth muscle hypertrophy and hyperreactivity
4. Increased mucus secretion

• Robbins & Cotran Pathologic Basis of Disease, p. 638
• Robbins & Kumar Basic Pathology, p. 451

Types of Asthma

1. Atopic Asthma (most common)

• Classic type I IgE-mediated hypersensitivity reaction
• Begins in childhood; positive family history of atopy
• Associated with: allergic rhinitis, urticaria, eczema
• Triggers: dust, pollen, animal dander, food, viral infections
• Skin test with offending allergen → immediate wheal-and-flare reaction
• Diagnosis: elevated total serum IgE; RAST (radioallergosorbent test) identifies specific IgE antibodies

2. Nonatopic Asthma

• No evidence of allergen sensitization; skin tests usually negative
• Less frequent family history
• Triggers: viral respiratory infections (rhinovirus, parainfluenza, RSV), air pollutants (tobacco smoke, SO₂, ozone, NO₂), cold air, exercise

3. Drug-Induced Asthma

• Aspirin-sensitive asthma: associated with recurrent rhinitis and nasal polyps
• Aspirin/NSAIDs inhibit COX → ↓ prostaglandin E₂ → ↑ leukotrienes (LTB4, LTC4, LTD4, LTE4) → bronchoconstriction
• Triggers urticaria AND asthmatic attacks

4. Occupational Asthma

• Triggered by workplace fumes, organic/chemical dusts, gases, or chemicals (epoxy resins, wood dust, toluene, formaldehyde, penicillin products)
• Only minute quantities needed; attacks occur after repeated exposure
• Mechanisms: type I reactions, direct bronchoconstriction, or unknown hypersensitivity

Pathogenesis

Step-by-Step Mechanism

• Inhaled allergens are processed by dendritic cells → presented to naive T cells → differentiation into Th2 cells
• Th2 cytokines drive the response:
  • IL-4 → stimulates B cells to produce IgE
  • IL-5 → recruits and activates eosinophils
  • IL-13 → stimulates mucus secretion from submucosal glands; promotes IgE production
• IgE binds to Fc receptors on submucosal mast cells (sensitization complete)
• ILC2 (group 2 innate lymphoid cells) also produce Th2-type cytokines, activated by damaged epithelial cytokines (IL-33, TSLP) - this is called type 2 immunity

• Re-exposure to allergen → cross-linking of IgE on mast cells → degranulation
• Mediators released:
  • Histamine - bronchoconstriction
  • Prostaglandin D₂ - bronchoconstriction + vasodilation
  • Leukotrienes C4, D4, E4 - prolonged bronchoconstriction, ↑ vascular permeability, ↑ mucus secretion
• Also activated: vagal (parasympathetic) afferent/efferent reflexes → smooth muscle contraction via muscarinic receptors
• Net result: bronchoconstriction, mucus production, vasodilation, edema

• Inflammatory mediators from mast cells + epithelial cells release chemokines (including eotaxin)
• Recruitment of: eosinophils, neutrophils, Th2 cells, basophils, monocytes
• Eosinophils release:
  • Major basic protein (MBP)
  • Eosinophil cationic protein (ECP) - both damage the epithelium
  • Galectin-10 (GAL10) → forms Charcot-Leyden crystals → pro-inflammatory, induces mucus production
• IL-5 antagonists are effective in severe eosinophilic asthma → confirms IL-5's central role

• Hypertrophy and hyperplasia of bronchial smooth muscle
• Subepithelial fibrosis (type I and III collagen deposition)
• Increased vascularity
• Mucous gland enlargement + goblet cell hyperplasia
• Thickened subepithelial basement membrane
• May contribute to chronic, irreversible airway obstruction over time

Genetics

• Chromosome 5q locus near cytokine cluster: IL-3, IL-4, IL-5, IL-9, IL-13 and IL-4 receptor
  • IL-13 gene polymorphisms: strongest, most consistent association
  • IL-4 receptor variants: linked to atopy, elevated IgE, asthma
• HLA class II alleles - linked to IgE production against specific antigens (e.g., ragweed pollen)
• IL-33 / ST2 receptor variants - induce Th2 cytokine production
• TSLP (thymic stromal lymphopoietin) gene variants - epithelial cytokine that initiates allergic reactions

Hygiene Hypothesis

Morphology

Gross (status asthmaticus / fatal cases)

• Lungs overinflated, small areas of atelectasis
• Bronchi and bronchioles occluded by thick, tenacious mucus plugs (often contain shed epithelium)

Microscopic - Airway Remodeling

Key Mediators Summary

Clinical Features

• Classic acute attack: chest tightness, dyspnea, wheezing, cough - lasts up to several hours
• Prolonged expiration (wheeze on expiration)
• Diagnosis: spirometry showing reversible airflow obstruction; peripheral eosinophilia in atopic asthma; Curschmann spirals and Charcot-Leyden crystals in sputum
• Acute severe asthma (status asthmaticus): unrelenting attack lasting days or weeks; can be fatal with marked cyanosis

Treatment

• Bronchodilators (β₂-agonists, anticholinergics)
• Glucocorticoids (inhaled/systemic)
• Leukotriene antagonists (montelukast)
• Biologics for severe/refractory disease:
  • Anti-IL-4R (dupilumab)
  • Anti-IL-5 (mepolizumab, reslizumab)
  • Anti-TSLP (tezepelumab)
• ~50% of childhood asthma remits in adolescence; may return in adulthood

Bronchiectasis (Robbins Pathology)

Definition

• Robbins & Cotran Pathologic Basis of Disease, p. 642
• Robbins & Kumar Basic Pathology, p. 455

Conditions That Predispose to Bronchiectasis

1. Congenital / Hereditary Conditions

2. Bronchial Obstruction

• Causes: tumors, foreign bodies, mucus impaction
• Result: localized bronchiectasis in the obstructed segment
• May also complicate atopic asthma and chronic bronchitis

3. Necrotizing / Suppurative Pneumonia

• Single severe episode or recurrent infections
• Virulent organisms: Staphylococcus aureus, Klebsiella spp.
• Post-tuberculosis bronchiectasis remains significant in endemic areas
• Advanced bronchiectasis has also been reported after SARS-CoV-2 pneumonia

4. Immune / Inflammatory Disorders

• Rheumatoid arthritis, SLE, inflammatory bowel disease
• Post-transplant: chronic rejection after lung transplant; chronic graft-versus-host disease after HSCT

5. Allergic Bronchopulmonary Aspergillosis (ABPA)

• Occurs in patients with asthma or cystic fibrosis
• Hyperimmune Th2 response to Aspergillus fumigatus
• High serum IgE, serum anti-Aspergillus antibodies, intense eosinophilic airway inflammation, mucus plugs → bronchiectasis

6. Idiopathic

• Up to 50% of cases - dysfunctional host immunity to infectious agents → chronic inflammation

Pathogenesis

Obstruction ↔ Chronic Infection (vicious cycle)

Obstruction → impaired secretion clearance → infection
             ↑                                    ↓
    peribronchial fibrosis ←— inflammatory damage to bronchial wall
             ↑                                    ↓
       traction dilation ←——— accumulating exudate distends airways

• Either process can be the initiator
• Obstruction (e.g., foreign body) → impaired drainage → bacterial superinfection → inflammatory wall destruction → irreversible dilation
• Infection (e.g., necrotizing pneumonia) → poor secretion clearance → obstruction → peribronchial fibrosis and traction → full-blown bronchiectasis
• In cystic fibrosis: CFTR defect → thick viscous secretions → obstructed mucociliary clearance → chronic bacterial infections → bronchial wall destruction + smaller bronchioles obliterated by fibrosis (bronchiolitis obliterans)
• In primary ciliary dyskinesia: dynein mutations → ciliary dysfunction → same sequence of obstruction and recurrent infection

Morphology

Gross

• Lower lobes bilaterally - particularly vertical (gravity-dependent) air passages
• Airways dilated up to 4 times normal size
• Dilated bronchi visible almost to pleural surface (normally bronchioles cannot be seen beyond 2-3 cm from pleura)
• Cut surface: dilated bronchi appear cystic, filled with mucopurulent secretions
• Localized when due to tumor or foreign body

Microscopic

Microbiology of Sputum Cultures

Clinical Features

• Severe persistent cough with expectoration of mucopurulent, foul-smelling sputum (sometimes blood-stained)
• Hemoptysis - may occasionally be massive
• Dyspnea, orthopnea, rhinosinusitis
• Paroxysms of cough are worst in the morning on rising (change in position causes pooled pus/secretions to drain into bronchi)
• Symptoms are often episodic, precipitated by upper respiratory infections or new pathogens

Complications

With modern antibiotics and physical therapy (postural drainage), life expectancy has almost doubled, and severe complications (cor pulmonale, brain abscess, amyloidosis) occur far less frequently than in the past.

Key Concept Summary