Hyaline Membrane Disease (Neonatal RDS) and Acute Respiratory Distress Syndrome (ARDS)

PART 1: HYALINE MEMBRANE DISEASE (HMD) / Neonatal Respiratory Distress Syndrome (Neonatal RDS)

Description

Aetiology

1. Prematurity (most common) - type II pneumocytes are insufficiently mature to produce adequate surfactant; surfactant production accelerates after the 35th week of gestation
2. Male sex - strong association
3. Maternal diabetes - elevated fetal insulin levels counteract the surfactant-stimulating effect of corticosteroids
4. Caesarean delivery - bypasses the stress-induced cortisol surge that normally stimulates surfactant maturation
5. Genetic mutations - congenital deficiency of SP-B (SFTPB gene) or SP-C (SFTPC gene) causes severe respiratory failure through absent surfactant proteins
6. Second twin - higher risk than the first born twin

Pathogenesis

• Dipalmitoyl phosphatidylcholine (lecithin) - the main lipid component
• Phosphatidylglycerol - smaller amounts
• Hydrophilic proteins SP-A and SP-D - pulmonary host defense
• Hydrophobic proteins SP-B and SP-C - reduce surface tension at the air-liquid interface

1. Prematurity → reduced surfactant synthesis/storage/release by immature type II pneumocytes
2. Decreased alveolar surfactant → increased alveolar surface tension
3. Increased surface tension → alveolar collapse (atelectasis) with each expiration - the soft thoracic wall is pulled inward as the diaphragm descends, worsening the problem
4. Atelectasis → impaired perfusion and hypoventilation → V/Q mismatch
5. Hypoxemia and CO2 retention (acidosis)
6. Hypoxemia further impairs surfactant synthesis - a vicious cycle
7. Reduced lung compliance forces the infant to work as hard with every breath as with the first
8. Endothelial and epithelial damage → plasma leaks into the alveoli
9. Leaked protein-rich, fibrin-rich exudate + necrotic type II pneumocyte debris organize into eosinophilic hyaline membranes, lining the respiratory bronchioles and alveolar ducts
10. Hyaline membranes act as a barrier to gas exchange, completing the vicious cycle

Morphology (Gross and Microscopic)

• Lungs are normal in size but solid, airless, and reddish-purple (liver-like colour)
• Lungs typically sink in water (absent air)

• Alveoli are poorly developed and collapsed
• Early: necrotic cellular debris in terminal bronchioles and alveolar ducts
• Necrotic material organizes into eosinophilic hyaline membranes lining respiratory bronchioles, alveolar ducts, and alveoli
• Membranes composed of fibrin + debris from necrotic type II pneumocytes
• Remarkable paucity of neutrophilic infiltration
• Cuboidal epithelial lining of remaining airspaces (sign of lung immaturity)
• Lesions are NEVER seen in stillborns (requires breathing attempts)

• Alveolar epithelium proliferates under the membranes
• Membranes may detach and be phagocytosed by macrophages
• Alternatively, fibroblasts grow into membranes → incorporated into alveolar wall

Clinical Features

• Affected infants are almost always preterm with weight appropriate for gestational age
• Resuscitation may be needed at birth, but normal color/breathing often returns briefly
• Within 30 minutes, breathing becomes labored - grunting, nasal flaring, intercostal and subcostal retractions
• Within hours: cyanosis becomes evident in the untreated infant
• Fine rales over both lung fields
• Chest X-ray: uniform minute reticulogranular densities - classic "ground-glass" appearance with air bronchograms
• Full-blown condition: respiratory distress persists, cyanosis worsens; even 80% O2 via various ventilatory methods may fail
• If death is staved off for 3-4 days, the infant has an excellent chance of recovery (surfactant production matures)

• Prevention by antenatal corticosteroids (stimulate surfactant maturation)
• Amniotic fluid phospholipid analysis (lecithin:sphingomyelin ratio) assesses fetal lung maturity
• Exogenous surfactant therapy at birth for extremely premature infants (<28 weeks) has dramatically reduced mortality
• Complications include bronchopulmonary dysplasia (BPD), intraventricular hemorrhage, retinopathy of prematurity

PART 2: ACUTE RESPIRATORY DISTRESS SYNDROME (ARDS)

Description

Aetiology

Pathogenesis

• Direct insult (e.g., inhaled toxin, aspiration) injures type I and type II pneumocytes
• Indirect insult (e.g., sepsis, circulating mediators) activates pulmonary endothelium

• Pneumocyte injury is sensed by resident alveolar macrophages, which secrete TNF acting on neighboring endothelium
• Circulating inflammatory mediators (sepsis/trauma) may activate endothelium directly
• Activated endothelium upregulates adhesion molecules, procoagulant proteins, and chemokines

• Neutrophils adhere to activated endothelium and migrate into interstitium and alveoli
• Degranulation releases proteases, reactive oxygen species (ROS), and cytokines
• Neutrophil extracellular traps (NETs) contribute directly to lung damage
• This creates a feedback loop of inflammation and endothelial damage

• Endothelial injury makes pulmonary capillaries leaky → interstitial and intraalveolar edema
• Type II pneumocyte necrosis → surfactant abnormalities → further impaired gas exchange
• Inspissated protein-rich edema fluid + dead alveolar epithelial debris → organize into hyaline membranes (the hallmark histologic lesion = diffuse alveolar damage)

• Lesions are not evenly distributed - stiff, poorly aerated regions coexist with near-normal areas
• Poorly aerated regions continue to be perfused → ventilation-perfusion mismatch → refractory hypoxemia

• If triggers abate: macrophages clear debris; fibrogenic cytokines (TGF-β, PDGF) stimulate fibroblasts → fibrosis of alveolar walls (fibro-proliferative stage)
• Type II pneumocytes proliferate to replace type I; endothelial restoration occurs
• In refractory cases: progressive cardiopulmonary failure

Morphology (Gross and Microscopic)

• Lungs are heavy, firm, red, and boggy
• Congestion, interstitial and intraalveolar edema

Clinical Features

• Profound dyspnea and tachypnea are the heralding features, usually 12-48 hours after the precipitating event
• Rapidly progressive respiratory failure

• Hypoxemia - often refractory to supplemental oxygen
• Cyanosis
• Diffuse bilateral crackles
• Tachycardia

• CXR/CT: bilateral diffuse opacities/infiltrates (ground-glass and consolidation) not explained by cardiac failure
• Arterial blood gas: hypoxemia (PaO2/FiO2 ratio <300 for ALI, <200 for ARDS); respiratory alkalosis early, respiratory acidosis late
• Berlin Definition (2012): bilateral opacities, respiratory failure within 1 week, not fully explained by cardiac failure/fluid overload, PaO2/FiO2 < 300 on PEEP ≥ 5 cmH2O

• Hypoxemia may be refractory due to V/Q mismatch
• Respiratory acidosis develops as disease progresses
• Lungs become stiff (loss of functional surfactant) → requires intubation, high ventilatory pressures
• No proven disease-specific treatment; management is supportive (lung-protective ventilation with low tidal volumes, prone positioning, PEEP)
• Most deaths attributable to sepsis, multiorgan failure, or severe lung injury
• Survivors usually recover pulmonary function; a minority develop interstitial fibrosis and chronic lung disease

KEY COMPARISON: HMD vs. ARDS

• Robbins, Cotran & Kumar - Pathologic Basis of Disease, 10th ed., Chapters 10 & 15
• Robbins & Kumar - Basic Pathology

Lung Cancer: Occurrence, Classification & Etiopathogenesis

1. HOW LUNG CANCER OCCURS (Epidemiology & Overview)

• Over 2.2 million new cases globally in 2020; 1.8 million deaths per year
• In the United States: ~237,000 new cases and ~130,000 deaths annually (2022 estimates)
• Peak incidence in individuals in their fifties and sixties
• At diagnosis, >50% already have distant metastases; an additional ~25% have regional lymph node involvement
• Overall 5-year survival for all stages combined is only ~20%; even localized disease has only ~50% 5-year survival
• Since 2006, incidence has been declining ~2.5%/year in men and ~1%/year in women, attributed to reduced smoking, better screening, and improved therapies

2. CLASSIFICATION

2.1 Broad Clinical Division

2.2 WHO 2021 Classification (Histologic)

2.3 The Four Major Histologic Types (Detailed)

A. Adenocarcinoma (~40% - Most Common Overall)

• Most common subtype overall; the dominant type in women, never-smokers, and patients <45 years
• Location: Typically peripheral (subpleural); often associated with central scarring
• Gross: Peripheral gray-white nodule, often with pleural puckering; may be associated with areas of fibrosis
• Histology:
  • Glandular differentiation or mucin production
  • Patterns: acinar, papillary, micropapillary, solid, or lepidic (growth along pre-existing alveolar structures without stromal invasion)
  • Adenocarcinoma in situ (AIS): tumors ≤3 cm with pure lepidic growth, no stromal invasion (the only non-invasive adenocarcinoma pattern)
  • TTF-1 (thyroid transcription factor-1) positive - key IHC marker
• Precursor lesion: Atypical adenomatous hyperplasia → adenocarcinoma in situ
• Key molecular features: EGFR mutations (~20%), KRAS mutations (~30%), ALK fusions (4-6%), ROS1, HER2, MET - all targetable

B. Squamous Cell Carcinoma (~25-30%)

• Strongly associated with heavy cigarette smoking
• Location: Central (hilar/peribronchial); arises from the bronchial epithelium
• Gross: Central mass invading contiguous parenchyma; may cause obstructive atelectasis/pneumonia distal to the tumor
• Histology:
  • Keratinization, keratin pearls, and intercellular bridges (key features)
  • Well-differentiated tumors form obvious keratin pearls; less differentiated show sheets of cells
  • Cytology of sputum shows orange-staining, keratinized cells with hyperchromatic nuclei
• Precursor lesion: Squamous dysplasia → squamous cell carcinoma in situ
• Paraneoplastic: Produces PTH-related peptide (PTHrP) → hypercalcemia

C. Small Cell Carcinoma (~15%)

• Most aggressive lung cancer; almost universally metastatic at presentation
• Strongly related to smoking (virtually 100%)
• A neuroendocrine carcinoma - derived from neuroendocrine (Kulchitsky) cells of the bronchial epithelium
• Location: Central (perihilar)
• Gross: Soft, fleshy, pale-gray central mass with extensive necrosis
• Histology:
  • Small cells with scant cytoplasm, ill-defined borders
  • Finely granular ("salt-and-pepper") nuclear chromatin; absent/inconspicuous nucleoli
  • High mitotic rate; extensive necrosis
  • Azzopardi effect: basophilic encrustation of vascular walls by DNA from necrotic tumor cells
  • IHC markers: CD56, NCAM, synaptophysin, chromogranin, INSM1 (neuroendocrine markers)
• Paraneoplastic syndromes (most common with SCLC):
  • ACTH → Cushing's syndrome
  • ADH → SIADH
  • Gastrin-releasing peptide, calcitonin
  • Lambert-Eaton myasthenic syndrome (antibodies vs. presynaptic Ca²⁺ channels)
• Treatment: Chemotherapy + radiotherapy; but invariably recurs; median survival ~1 year; 5-year survival only ~5%

D. Large Cell Carcinoma (<10%)

• A diagnosis of exclusion - lacks features of adenocarcinoma, squamous cell carcinoma, or small cell carcinoma
• Location: Often peripheral
• Histology: Sheets of large cells with abundant cytoplasm, prominent nucleoli, no glandular or squamous differentiation; may have giant cells or spindle cells
• Large cell neuroendocrine carcinoma (LCNEC): a distinct aggressive subtype with neuroendocrine morphology but large-cell cytology; IHC positive for neuroendocrine markers

3. ETIOPATHOGENESIS

3.1 Etiological Factors

A. Tobacco Smoking (Primary Cause)

• ~90% of lung cancers occur in current or recent smokers
• Persons who smoke have a 10-fold or greater increased risk vs. never-smokers
• 60-fold higher risk in heavy smokers (2 packs/day for 20 years)
• Nearly linear correlation between pack-years and lung cancer frequency
• Smoking increases risk of all major types - squamous and small cell have the strongest association; adenocarcinoma is also related but is also the dominant type in never-smokers
• Risk decreases with cessation but never returns to baseline (pre-cancer genetic changes persist)
• Environmental tobacco smoke (ETS/second-hand smoke): ~20-30% increased risk in never-smokers married to long-term smokers
• Women are more susceptible to tobacco carcinogens than men (for unclear reasons)
• A large-scale genomic study found one genetic mutation per 15 cigarettes smoked
• ~65% of new US diagnoses are in former or never smokers - reflecting the changing demographics

B. Occupational and Environmental Carcinogens

C. Genetic Susceptibility

• P-450 monooxygenase polymorphisms: Individuals with increased capacity to activate procarcinogens from tobacco smoke are at higher risk
• Individuals whose lymphocytes show chromosomal breakage after tobacco carcinogen exposure have increased susceptibility
• GWAS (genome-wide association studies) have identified variants in inflammation and repair pathways
• Most mutations are somatic (caused by carcinogens), but familial risk is real

D. Other Factors

• Prior lung disease (COPD, pulmonary fibrosis) increases risk
• Air pollution (PM2.5, diesel exhaust)
• HIV infection (lung cancer risk increased)
• Lung transplant recipients (especially single-lung transplant for COPD/fibrosis with prior smoking)

3.2 Pathogenesis: Molecular Steps

Normal bronchial epithelium
         ↓  (Carcinogen exposure + "field effect")
Mutagenized mucosa — 3p deletion (early, universal)
         ↓
Dysplasia / atypical adenomatous hyperplasia
         ↓  (Additional mutations)
Carcinoma in situ — TP53 mutation, KRAS mutation (relatively late)
         ↓
Invasive carcinoma
         ↓
Metastatic disease

Key Molecular Events:

• Loss of putative tumor suppressor genes on the short arm of chromosome 3 (3p)
• Found even in benign bronchial epithelium of smokers without cancer → "field effect" (large areas of respiratory mucosa are mutagenized)
• Present in ~90% of SCLC; ~80% of NSCLC

• Present in ~90% of SCLC; ~50% of NSCLC
• Loss of cell cycle checkpoint → uncontrolled proliferation

• ~90% in SCLC; ~20% in NSCLC
• Loss of G1 checkpoint control

• Constitutively active RAS → continuous proliferative signaling
• Mutually exclusive with EGFR mutations (KRAS is downstream of EGFR)
• Specific KRAS variants now targetable with new drugs (e.g., sotorasib for KRAS G12C)

• Especially in non-smoking women and East Asian patients
• Activates RAS, PI3K, and other pro-growth pathways
• Targetable: gefitinib, erlotinib, osimertinib (EGFR tyrosine kinase inhibitors)
• Tumors sensitive but develop resistance

• EML4-ALK chromosomal rearrangement
• Common in non-smokers, young patients, signet-ring cell morphology
• Targetable: crizotinib, alectinib

• Each targeted by specific drugs

• ~10% SCLC; ~50% NSCLC
• Loss of CDK4/6 inhibition → unrestrained cell cycle progression

The "Field Effect" Concept:

3.3 Local Effects and Complications

• Partial bronchial obstruction → marked focal emphysema
• Total obstruction → atelectasis
• Impaired drainage → severe suppurative bronchitis, bronchiectasis, pulmonary abscess
• Superior vena cava (SVC) syndrome: compression/invasion → venous congestion, edema of head and arms, circulatory compromise
• Pancoast tumor (apical/superior sulcus tumor): invades cervical sympathetic plexus and brachial plexus → severe pain in ulnar nerve distribution + Horner syndrome (enophthalmos, ptosis, miosis, anhidrosis)
• Pericarditis or pleural effusion: extension to pericardial/pleural sacs
• Paraneoplastic syndromes (especially SCLC): SIADH, Cushing's, hypercalcemia (squamous), Lambert-Eaton

3.4 Metastatic Routes

• Lymphatics → hilar and mediastinal lymph nodes (early)
• Hematogenous → adrenal glands (most common), liver, brain, bone, kidney, and adrenal glands
• Direct spread → pleura, pericardium, great vessels, esophagus

• Brain (~30-40% of SCLC at presentation)
• Bone (especially SCLC)
• Liver and adrenals
• Contralateral lung

Summary Comparison Table: Major Subtypes

• Robbins & Kumar Basic Pathology, Chapter 11 (Lung)
• Robbins, Cotran & Kumar Pathologic Basis of Disease, 10th ed., Chapter 15
• Harrison's Principles of Internal Medicine, 22nd ed., Chapter 83