Pathophysiology of MASLD (Metabolic Dysfunction-Associated Steatotic Liver Disease)

1. Core Concept: Energy Imbalance as the Upstream Driver

• Visceral adiposity
• Insulin resistance
• Systemic metabolic dysfunction

Harrison's Principles of Internal Medicine 22E, p. 2744

2. Hepatic Steatosis - The First Step

• PNPLA3 I148M polymorphism - impairs lipolysis of lipid droplets; greatest known genetic risk factor
• TM6SF2 - influences cholesterol metabolism
• MBOAT7 - influences phospholipid metabolism
• HSD17B13 loss-of-function variants - protective against MASH, fibrosis progression, and liver cancer

Harrison's Principles of Internal Medicine 22E, p. 2744-2745

3. Transition to MASH (Steatohepatitis) - Lipotoxicity

• Dysregulated fatty acid processing produces toxic lipid intermediates (ceramides, diacylglycerols, lysophosphatidylcholine, reactive oxygen species)
• These intermediates activate endoplasmic reticulum (ER) stress, oxidative stress, and mitochondrial dysfunction
• Hepatocyte "resilience" (adaptive metabolic capacity) determines whether this resolves or progresses

4. Role of Insulin Resistance

• Adipocytes: Unrestrained lipolysis floods the portal circulation with FFAs
• Hepatocytes: Paradoxical insulin resistance - glucose homeostasis is impaired while insulin continues to drive DNL (selective insulin resistance - the lipogenic arm remains active)
• Pancreatic beta cells: Compensatory hyperinsulinemia further promotes fat synthesis and storage

Harrison's Principles of Internal Medicine 22E, p. 2745

5. Gut-Liver Axis

1. Enhanced energy harvest - altered microbiota extract more calories from dietary fiber
2. Increased intestinal permeability - dysbiosis disrupts tight junctions, allowing bacterial products (LPS/endotoxin) to enter the portal circulation via the gut-liver axis

Harrison's Principles of Internal Medicine 22E, p. 2744; Benedé-Ubieto et al., Gut Microbes 2024

6. Fibrosis - The Determinant of Prognosis

• Dying hepatocytes release DAMPs (danger-associated molecular patterns) that activate hepatic stellate cells (HSCs)
• Activated HSCs transdifferentiate into myofibroblasts and deposit collagen (fibrosis)
• Progressive fibrosis: pericellular "chicken-wire" fibrosis around zone 3 hepatocytes → bridging fibrosis (F3) → cirrhosis (F4)
• Fibrosis stage is the primary predictor of liver-related morbidity and mortality in MASLD

• Simple steatosis: ~1 stage per 14 years
• MASH: ~1 stage per 7 years
• Patients with MASH + fibrosis ≥F2 = "at-risk MASH" requiring intensive monitoring

7. Risk Factors That Amplify the Pathophysiology

8. Histopathologic Spectrum

• Macrovesicular steatosis (>5% hepatocytes)
• Hepatocyte ballooning (marker of lipotoxic injury; loss of keratin 8/18)
• Lobular inflammation (neutrophilic infiltrates)
• Pericellular fibrosis around zone 3 (centrilobular) hepatocytes - the "chicken-wire" pattern
• Note: Pediatric MASLD differs - shows more diffuse steatosis and portal (rather than central) fibrosis

9. Progression to HCC

• Sustained oxidative DNA damage
• Malignant transformation of regenerating hepatocytes under lipotoxic stress
• MASH-driven fibrosis creates a permissive oncogenic microenvironment
• Annual HCC incidence in MASLD-cirrhosis: 1-2% per year

Harrison's Principles of Internal Medicine 22E, Ch. 354

Summary Diagram of Pathogenesis

Energy excess + Sedentary lifestyle
         ↓
Visceral adiposity + Insulin Resistance
         ↓
↑ FFA flux → Hepatocyte + ↑ DNL + ↓ FA oxidation
         ↓
Triglyceride accumulation = STEATOSIS (MASL)
         ↓
Dysregulated FA processing → Toxic lipid intermediates
         ↓
LIPOTOXICITY
         ↓
Oxidative stress + ER stress + Inflammation + Gut dysbiosis (LPS/TLR4)
         ↓
Hepatocyte ballooning + Lobular inflammation = MASH
         ↓
Kupffer cell activation → Stellate cell activation → FIBROSIS
         ↓
Bridging fibrosis → CIRRHOSIS → HCC

Pathophysiology of MASLD

1. Upstream Driver: Energy Imbalance

• Visceral adiposity
• Insulin resistance
• Systemic metabolic dysfunction

Harrison's Principles of Internal Medicine 22E (2025), Ch. 354

2. Hepatic Steatosis (MASL) - Step 1

3. Transition to MASH (Steatohepatitis) - The Key Step: Lipotoxicity

Excess FFA → Dysregulated FA processing
           → Toxic lipid intermediates (ceramides, diacylglycerol,
             lysophosphatidylcholine, reactive oxygen species)
           → ER stress, mitochondrial dysfunction, oxidative stress
           → Hepatocyte injury (ballooning, apoptosis, necroptosis)

4. The Central Role of Insulin Resistance

• Adipocytes: Insulin fails to suppress lipolysis → unrestrained FFA release into portal blood
• Hepatocytes (selective/paradoxical IR): Glucose production is not suppressed (IR), but DNL remains insulin-responsive and is upregulated by compensatory hyperinsulinemia - this "selective IR" drives fat synthesis while glucose homeostasis fails
• Pancreatic beta cells: Compensatory hyperinsulinemia further promotes hepatic fat uptake, triglyceride synthesis, and fat storage

5. The Gut-Liver Axis

1. Enhanced energy harvest - altered microbiota ferment dietary fiber more efficiently, extracting additional calories
2. Increased intestinal permeability - dysbiosis disrupts epithelial tight junctions, allowing bacterial lipopolysaccharide (LPS/endotoxin) to translocate into the portal circulation

6. Genetics - Risk Modifiers

7. Fibrosis - The Determinant of Prognosis

• Dying hepatocytes release DAMPs (danger-associated molecular patterns)
• DAMPs activate hepatic stellate cells (HSCs), which transdifferentiate into myofibroblasts
• Activated HSCs deposit collagen in a characteristic pericellular "chicken-wire" pattern around zone 3 hepatocytes
• Progressive fibrosis stages: pericellular (F1) → bridging (F3) → cirrhosis (F4)

• Simple steatosis alone: ~1 fibrosis stage per 14 years
• MASH: ~1 fibrosis stage per 7 years
• MASH + fibrosis ≥F2 = "at-risk MASH" - the threshold for intensified monitoring and treatment

8. Progression to HCC

• Chronic oxidative DNA damage from sustained lipotoxic stress
• NLRP3 inflammasome activation
• Malignant transformation during dysregulated hepatocyte regeneration
• Fibrosis creating a permissive pro-oncogenic microenvironment
• Annual HCC incidence in established MASLD cirrhosis: 1-2% per year

9. Histopathologic Spectrum

• Macrovesicular steatosis (>5% of hepatocytes)
• Hepatocyte ballooning - swollen, pale cells with loss of keratin 8/18 (marker of lipotoxic injury)
• Lobular inflammation - neutrophilic and mixed infiltrates
• Pericellular fibrosis in zone 3 (centrilobular) - the hallmark "chicken-wire" pattern

Note: Pediatric MASLD differs - shows more diffuse steatosis, portal rather than centrilobular fibrosis, and ballooned hepatocytes may be absent

Summary: Pathogenesis at a Glance

Energy excess + Physical inactivity
        ↓
Visceral adiposity + Insulin Resistance
        ↓
↑ FFA flux to liver + ↑ DNL + ↓ beta-oxidation + ↓ VLDL export
        ↓
Hepatocyte triglyceride accumulation → STEATOSIS (MASL)
        ↓
Dysregulated FA metabolism → toxic lipid intermediates
(+ Gut dysbiosis → LPS/TLR4 → Kupffer cell activation)
(+ Genetic risk: PNPLA3, TM6SF2)
        ↓
LIPOTOXICITY → ER stress + ROS + Inflammation
        ↓
Hepatocyte ballooning + Lobular inflammation = MASH
        ↓
DAMP release → Hepatic stellate cell activation
        ↓
Collagen deposition → FIBROSIS
        ↓
Bridging fibrosis → CIRRHOSIS → HCC / Decompensation

Pathophysiology of ST Elevation and Depression in Myocardial Infarction

1. Normal ST Segment Physiology

• During the plateau phase (Phase 2) of the cardiac action potential, all ventricular myocytes maintain an approximately equal membrane potential (~0 mV)
• Because there is no voltage difference between adjacent cells during this phase, no net current flows between them
• The result: the ST segment sits on the isoelectric baseline

2. How Ischemia Disrupts This: The Ionic Mechanism

Harrison's Principles of Internal Medicine 22E (2025), Ch. 247; Fuster & Hurst's The Heart, 15th Ed.; Goldman-Cecil Medicine

3. The "Current of Injury" - The Key Concept

During Diastole (TQ segment / baseline)

• Ischemic cells are partially depolarized (resting membrane potential less negative, e.g., -60 mV instead of normal -90 mV)
• Normal cells are fully polarized at -90 mV
• Current flows from the ischemic zone (positive) to the normal zone during electrical diastole
• This depresses the true baseline (TQ segment)

During Systole (ST segment)

• Normal cells undergo full action potential plateau (Phase 2, ~0 mV)
• Ischemic cells have shortened action potentials - they repolarize earlier than normal cells
• So during the time the normal cells are at full plateau (~0 mV), ischemic cells have already partially repolarized (more negative)
• Current now flows from normal cells (positive) toward ischemic cells (negative)

4. ST Elevation - Transmural (Epicardial) Ischemia

• The ischemic, partially depolarized epicardial cells are electrically more positive during diastole
• Current flows outward from the ischemic epicardium
• Surface electrodes overlying the ischemic zone record this outward vector as ST elevation

1. Full occlusion of a coronary artery (total blockage)
2. All layers of myocardium are ischemic, but the epicardium is most affected
3. The ST vector points outward (toward the surface electrode)
4. Leads directly over the infarcting zone show ST elevation (STEMI)

5. ST Depression - Subendocardial Ischemia

• The ischemic, partially depolarized cells are now at the inner (endocardial) layer
• The injury current vector is directed inward toward the ventricular cavity and subendocardium
• Surface electrodes overlying the ventricle (anterior precordial leads) look away from this inward vector
• They record ST depression

Harrison's Principles of Internal Medicine 22E (2025), Ch. 247

6. The ECG Temporal Evolution in STEMI

Phase 1: Minutes (Hyperacute T Waves)

• The earliest ECG change in STEMI
• Appears within minutes of coronary occlusion
• Broad-based, tall, peaked T waves in the affected territory
• Mechanism: Acute ischemia initially shortens action potential duration, which briefly increases the T-wave gradient
• Often transient and easily missed before progressing

Phase 2: Minutes to Hours (ST Elevation)

• Current of injury fully established
• ST elevation in leads overlying the infarcted zone
• Morphology progresses: concave → flat → convex/domed → "tombstone" shape (more convex = more severe/extensive injury)
• Reciprocal ST depression appears in anatomically opposite leads (current of injury seen from the "back" side)

Phase 3: Hours (T Wave Inversion)

• As injured cells begin to die, the action potential duration difference normalizes
• Current of injury decreases, ST begins to return toward baseline
• T waves invert in infarcted leads - the repolarization sequence has been permanently reversed in the damaged zone
• Reflects post-ischemic repolarization abnormality

Phase 4: Hours to Days (Pathological Q Waves)

• Dead myocardium is electrically silent - it generates no depolarization signal
• The overlying electrode now "looks through" the scar and sees the electrical forces of the opposite wall moving away from it
• Result: a Q wave (initial negative deflection) in the leads overlying the infarct
• Pathological Q wave: ≥0.04 sec (1 mm wide) and ≥25% of the R wave height

7. Reciprocal ST Depression in STEMI

1. Mirror image of the injury vector - the same ST current of injury, viewed from the opposite electrode perspective, produces a negative deflection
2. True ischemia in the opposite territory - especially relevant in left main or proximal LAD occlusion

8. Summary: Why STEMI vs NSTEMI/UA?

Harrison's Principles of Internal Medicine 22E (2025), Ch. 247 (Goldberger, Bhatt, Goldberger); Rosen's Emergency Medicine, Ch. 64; Fuster & Hurst's The Heart 15th Ed., Ch. 37; Goldman-Cecil Medicine