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Morphological Spectrum of NAFLD/NASH - MD Pathology Exam

Overview

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The Disease Spectrum (Sequential Steps)

Simple Steatosis → NASH → Bridging Fibrosis → Cirrhosis → HCC

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1. Simple Steatosis (NAFL)

• Macrovesicular steatosis (predominant): large fat droplets displacing the nucleus to the periphery - predominantly centrilobular (Zone 3)
• Microvesicular steatosis: smaller droplets, nucleus remains central - indicates more acute/severe metabolic stress
• No ballooning degeneration
• No significant fibrosis
• Generally reversible; 25-50% may develop progressive fibrosis

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2. NASH (Nonalcoholic Steatohepatitis)

Key Morphological Features

A. Hepatocyte Ballooning (pathognomonic feature)

• Enlarged hepatocytes with irregular clumped cytoplasm and optically clear non-vesiculated areas
• Cytoskeletal damage - loss of normal keratin intermediate filaments
• May contain Mallory-Denk Bodies (MDB): eosinophilic, ropey cytoplasmic inclusions of hyperphosphorylated, misfolded cytokeratin filaments
• Predominantly in Zone 3 (perivenular)

B. Lobular Inflammation

• Mixed infiltrate: lymphocytes, macrophages, and neutrophils
• Microgranulomas / lipogranulomas may be present
• Neutrophils around ballooned hepatocytes ("satellitosis") - similar to alcoholic hepatitis

C. Steatosis

• Both macrovesicular and microvesicular
• Predominantly Zone 3 (centrilobular)

D. Fibrosis (Perisinusoidal/Chicken-wire)

• Begins in Zone 3 as delicate strands of collagen in perisinusoidal distribution ("chicken-wire fibrosis")
• This is the hallmark pattern distinguishing NASH from viral hepatitis (which is portal-based)

Additional Features Seen in NASH

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3. Fibrosis Stages (NASH-CRN Classification)

Key exam point: As fibrosis progresses to Stage 3-4, steatosis, ballooning, and inflammation paradoxically decrease ("burnt-out NASH"). Cirrhosis from NASH may appear "cryptogenic" if steatohepatitis features regress.

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4. NASH Cirrhosis

• Regenerative nodules within a fibrotic network
• Steatosis and ballooning may be absent at this stage (burnt-out NASH)
• Histologically indistinguishable from other causes of cirrhosis unless residual steatohepatitis features remain
• 15-25% of NASH patients progress to cirrhosis

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NAFLD Activity Score (NAS) - NASH-CRN System

• NAS ≥5 = correlates with NASH (sensitivity 75%, specificity 83%)
• NAS ≤3 = usually not NASH
• Fibrosis staged separately (F0-F4), not included in NAS

NAS alone cannot diagnose NASH - pattern recognition is essential. 29% of biopsies with NAS ≤4 can still show definite steatohepatitis.

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SAF Scoring System (Alternative)

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NASH vs. Alcoholic Steatohepatitis (ASH) - Key Distinction

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Pediatric NAFLD - Two Distinct Patterns (Schwimmer Classification)

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Histology Images

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High-Yield Exam Summary Points

1. Minimum for NAFLD diagnosis: >5% hepatic steatosis
2. NASH triad: Steatosis + ballooning + lobular inflammation ± fibrosis
3. Earliest fibrosis in NASH: Perisinusoidal Zone 3 ("chicken-wire") - distinguishes NASH from viral hepatitis (portal fibrosis)
4. Mallory-Denk bodies: Misfolded CK-8/18, more abundant in ASH than NASH
5. Burnt-out NASH: Stage F3-F4 paradoxically shows LESS steatosis/ballooning/inflammation
6. NAS ≥5 = likely NASH; NAS ≤3 = likely NOT NASH (but diagnose by pattern, not just score)
7. Pediatric Type 2 NASH: Portal-based inflammation and fibrosis (opposite to adults)
8. Genetic risk: PNPLA3 I148M variant - strongest genetic predictor of steatosis and disease severity

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NAFLD/NASH - Pathogenesis & Clinical Features

Robbins & Kumar Basic Pathology

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Nomenclature Note

1. Steatosis (simple fatty liver)
2. Steatohepatitis (NASH)
3. Cirrhosis

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Risk Factors / Associations

• Insulin resistance and metabolic syndrome
• Type 2 diabetes (or family history)
• Obesity - primarily central/visceral
• Dyslipidemia - hypertriglyceridemia, low HDL-C, high LDL-C
• Hypertension

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Pathogenesis (Fig. 14.19, Robbins Basic Pathology)

Step 1 - Steatosis ("First Hit")

1. Insulin resistance leads to increased release of free fatty acids (FFAs) from adipocytes via overactivity of lipoprotein lipase
2. Simultaneously, reduced adiponectin production from adipocytes - adiponectin normally promotes hepatic fatty acid oxidation; its deficiency decreases FFA oxidation
3. FFAs flood the liver → triglyceride accumulation → steatosis (predominantly Zone 3/centrilobular)
4. Environmental factors compound this: dietary fat and altered gut microbiome contribute to FFA delivery to the liver

Step 2 - Hepatocyte Injury, Inflammation & Fibrosis ("Second Hit / Multiple Hits")

1. Excess intrahepatic FFAs cause lipotoxicity via two mechanisms:
   • ER (endoplasmic reticulum) stress - misfolded protein response, apoptosis
   • Mitochondrial dysfunction - impaired beta-oxidation, excess ROS generation
2. Hepatocyte injury triggers release of inflammatory mediators
3. Inflammatory mediators activate Kupffer cells (hepatic macrophages) and hepatic stellate cells
4. Activated stellate cells → collagen deposition → fibrosis

Summary Flow

Obesity/Insulin Resistance
    ↓
↑ FFA from adipose + ↓ Adiponectin
    ↓
Hepatic FFA/TG accumulation
    ↓
STEATOSIS
    ↓
Lipotoxicity → ER Stress + Mitochondrial Dysfunction
    ↓
Hepatocyte Injury
    ↓
Inflammatory Mediators
    ↓
Kupffer Cell + Stellate Cell Activation
    ↓
INFLAMMATION + FIBROSIS (NASH)
    ↓
CIRRHOSIS → HCC / Liver Failure

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Histology - Robbins (eFig. 14.2)

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Clinical Features (Robbins)

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Natural History (Fig. 14.19B, Robbins)

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Pediatric NAFLD (Robbins)

• Inflammation and scarring more prominent in portal tracts and periportal regions (vs. Zone 3 in adults)
• Mononuclear infiltrates predominate (vs. neutrophils in adult NASH)
• Becoming an increasing problem as childhood obesity and metabolic syndrome reach epidemic proportions

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Treatment (Robbins)

1. Weight reduction - most effective
2. Reversal of insulin resistance
3. Lifestyle modification (diet + exercise) - most effective form
4. Bariatric surgery - in selected cases

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High-Yield Exam Points from Robbins

1. Key initiating events: Obesity + insulin resistance → ↑ FFA + ↓ adiponectin
2. Lipotoxicity via ER stress + mitochondrial dysfunction drives the transition from simple steatosis to NASH
3. Kupffer cells + stellate cells - activated by inflammatory mediators - mediate inflammation and fibrosis respectively
4. AST:ALT <1 in NAFLD (vs. >2:1 in alcohol-related liver disease)
5. Liver biopsy - required to diagnose NASH
6. Isolated steatosis - minimal progression risk, no increased mortality
7. NASH - ~20% cirrhosis risk over 15 years
8. NASH + HCC: increasingly the leading cause of HCC, likely to surpass HCV
9. Pediatric NASH: portal-based inflammation and fibrosis, mononuclear (not neutrophilic) infiltrate
10. MAFLD - new proposed terminology emphasizing metabolic basis

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NAFLD / NASH - Complete MD Pathology Answer

Robbins & Kumar Basic Pathology + Yamada's Textbook of Gastroenterology + Goldman-Cecil Medicine

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1. Definition & Nomenclature

• Encompasses a spectrum: simple steatosis → NASH → fibrosis → cirrhosis → HCC
• NASH = overt clinical liver injury with histological features of steatohepatitis
• MAFLD (Metabolic-Associated Fatty Liver Disease) - newer proposed term emphasizing the metabolic basis
• Histologically identical to alcohol-related liver disease; distinction is by clinical history

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2. Risk Factors / Associations

• Obesity (primarily central/visceral)
• Insulin resistance and metabolic syndrome
• Type 2 diabetes mellitus (or family history)
• Dyslipidemia (↑ TG, ↓ HDL, ↑ LDL)
• Hypertension
• Also associated with: PCOS, hypothyroidism, sleep apnea, hypopituitarism

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3. Pathogenesis (Robbins, Fig. 14.19)

Step 1 - Steatosis ("First Hit")

1. Insulin resistance → ↑ FFA release from adipocytes (overactive lipoprotein lipase)
2. ↓ Adiponectin from adipose tissue → decreased hepatic FFA oxidation
3. Excess FFAs → hepatic triglyceride accumulation → steatosis (predominantly Zone 3/centrilobular)
4. Environmental cofactors: dietary fat and gut microbiome dysbiosis augment FFA delivery to liver
5. De novo lipogenesis also increased; impaired VLDL export contributes

Step 2 - Hepatocyte Injury → NASH ("Multiple Hits")

1. Excess intrahepatic FFAs → lipotoxicity via:
   • ER stress → unfolded protein response → hepatocyte apoptosis
   • Mitochondrial dysfunction → impaired β-oxidation, excess reactive oxygen species (ROS)
2. Hepatocyte injury → release of inflammatory mediators (TNF-α, IL-6, IL-1β)
3. Inflammatory mediators → Kupffer cell activation (hepatic macrophages) → amplify inflammation
4. Inflammatory signals → hepatic stellate cell activation → collagen deposition → fibrosis

Pathogenesis Summary Flow

Obesity + Insulin Resistance + Gut Microbiome/Dietary Fat
              ↓
  ↑ FFA from adipose  +  ↓ Adiponectin
              ↓
   Hepatic FFA/TG Accumulation
              ↓
         STEATOSIS
              ↓
  Lipotoxicity → ER Stress + Mitochondrial Dysfunction
              ↓
       Hepatocyte Injury (ROS, apoptosis)
              ↓
   Inflammatory Mediators (TNF-α, IL-6, IL-1β)
              ↓
  Kupffer Cell + Stellate Cell Activation
              ↓
    INFLAMMATION + FIBROSIS  =  NASH
              ↓
    CIRRHOSIS → HCC / Liver Failure

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4. Morphological Spectrum

Simple Steatosis → NASH → Bridging Fibrosis → Cirrhosis → HCC

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Stage 1 - Simple Steatosis (NAFL)

• Large single fat droplet displacing nucleus to periphery
• Predominantly centrilobular (Zone 3)
• Generally reversible with treatment

• Numerous small fat droplets, nucleus remains central
• Indicates more acute/severe metabolic stress
• Nonzonal small patches

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Stage 2 - NASH (Nonalcoholic Steatohepatitis)

Steatosis + Hepatocyte Ballooning + Lobular Inflammation ± fibrosis

A. Hepatocyte Ballooning (pathognomonic)

• Enlarged hepatocytes with irregular clumped cytoplasm and optically clear non-vesiculated areas
• Represents cytoskeletal damage - loss of keratin intermediate filaments (CK-8/18)
• Predominantly Zone 3 (perivenular)
• May contain Mallory-Denk Bodies (MDB): eosinophilic, ropey cytoplasmic inclusions of hyperphosphorylated, misfolded cytokeratin filaments

B. Lobular Inflammation

• Mixed infiltrate: lymphocytes + macrophages + neutrophils
• Neutrophils around ballooned hepatocytes ("satellitosis")
• Microgranulomas / lipogranulomas may be present

C. Steatosis

• Both macro- and microvesicular components
• Predominantly Zone 3

D. Perisinusoidal Fibrosis ("Chicken-wire") - Zone 3

• Begins as delicate strands of collagen in perisinusoidal distribution - Zone 3
• This is the hallmark pattern of NASH - distinguishes it from viral hepatitis (portal-based fibrosis)

Additional Features in NASH

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Stage 3 - Progressive Fibrosis (NASH-CRN Staging)

Key: Unlike viral hepatitis (portal → bridging), bridging fibrosis in NASH may result from extension of perisinusoidal fibrosis with retention of hepatocytes within the fibrotic network ("trapped hepatocytes").

As fibrosis progresses to F3-F4, steatosis, ballooning, and inflammation paradoxically decrease - this is "burnt-out NASH."

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Stage 4 - NASH Cirrhosis

• Regenerative nodules within a fibrotic network
• Steatohepatitis features may be absent → "cryptogenic cirrhosis"
• Robbins: NAFLD is a significant contributor to pathogenesis of cryptogenic cirrhosis
• 15-25% of NASH patients progress to cirrhosis (~20% over 15 years per Robbins)

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Stage 5 - Hepatocellular Carcinoma (HCC)

• Can arise even without cirrhosis in NASH (unlike most other causes)
• Risk: <1% per year from NASH cirrhosis
• NASH is increasingly overtaking HCV as the leading cause of HCC in the United States (Robbins)

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5. NAFLD Activity Score (NAS) - NASH-CRN System

• NAS ≥5 = correlates with NASH (sensitivity 75%, specificity 83%)
• NAS ≤3 = usually not NASH
• NAS alone cannot diagnose NASH - 29% of biopsies with NAS ≤4 can still show definite steatohepatitis

Steatosis Grading

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6. Clinical Features (Robbins)

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7. Natural History (Robbins, Fig. 14.19B)

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8. NASH vs. ASH (Alcoholic Steatohepatitis) - Differential

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9. Pediatric NAFLD - Special Pattern (Robbins + Yamada/Schwimmer)

• Type 1 pediatric NASH: Resembles adult NASH (ballooning + perisinusoidal fibrosis)
• Type 2 pediatric NASH: Steatosis + portal inflammation + portal fibrosis, NO ballooning

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10. Treatment (Robbins)

1. Weight reduction + lifestyle modification (diet + exercise) - most effective
2. Reversal of insulin resistance
3. Bariatric surgery - in selected cases

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High-Yield Exam Summary

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