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Atherosclerosis - Complete Pathology for MBBS

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1. Definition & Overview

• A soft, friable lipid core (mainly cholesterol, cholesterol esters, necrotic debris)
• Covered by a fibrous cap

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2. Risk Factors

Non-Modifiable (Constitutional)

Modifiable (Major)

Additional Risk Factors

• Inflammation / CRP - CRP is a strong independent marker of risk for MI, stroke, and sudden cardiac death (even in apparently healthy people). CRP levels fall with statins, exercise, and weight loss.
• Hyperhomocysteinemia - Correlates with coronary, peripheral, and cerebrovascular disease
• Metabolic syndrome - Central obesity + insulin resistance + hypertension + dyslipidemia + hypercoagulability
• Lipoprotein(a) [Lp(a)] - Elevated levels independently raise coronary and cerebrovascular risk
• Hemostatic factors - Elevated plasminogen activator inhibitor-1 (PAI-1) predicts MI and stroke

Note: ~20% of cardiovascular events occur in people with none of the classic major risk factors. More than 75% of cardiovascular events in previously healthy females occur with LDL < 160 mg/dL.

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3. Pathogenesis - "Response to Injury" Hypothesis

Step-by-Step Sequence:

Step 1 - Endothelial Injury & Dysfunction

• Endothelial cell (EC) injury is the cornerstone of atherosclerosis
• Early lesions begin at sites of intact but dysfunctional endothelium
• "Dysfunction" = defective vascular tone, impaired barrier function, reduced hemostatic regulation, and enhanced inflammation
• Two most important causes of EC dysfunction:
  1. Hemodynamic disturbances - Turbulent blood flow at ostia, branch points, and posterior abdominal aorta preferentially induces EC injury. Non-turbulent laminar flow induces "atheroprotective" genes - explaining why plaques form at specific anatomical sites
  2. Hypercholesterolemia - Chronic hyperlipidemia directly impairs ECs by increasing reactive oxygen species (ROS), which accelerate nitric oxide (NO) decay, dampening vasodilator activity
• Other triggers: inflammatory cytokines (TNF-α), cigarette smoke toxins, hypertension, immune reactions

Step 2 - Lipoprotein Accumulation & Oxidation

• With chronic hyperlipidemia, LDL enters and accumulates in the intima
• Oxidized LDL (ox-LDL) forms when LDL is oxidized by free radicals from inflammatory cells - this is particularly dangerous:
  • Ox-LDL is toxic to ECs, SMCs, and macrophages
  • It is taken up by macrophages via scavenger receptors (not the normal LDL receptor) - leading to foam cell formation since it cannot be degraded
  • Stimulates release of growth factors, cytokines, and chemokines
  • Recruits more monocytes - creating a vicious cycle

Step 3 - Monocyte Recruitment & Macrophage Activation

• Dysfunctional ECs express adhesion molecules (VCAM-1, ICAM-1, E-selectin) that capture monocytes and T lymphocytes from blood
• Monocytes migrate into the intima via chemokines (e.g., MCP-1) and differentiate into macrophages
• Macrophages engulf oxidized LDL via scavenger receptors → transform into foam cells (lipid-laden macrophages)
• Macrophages also sense cholesterol crystals and free fatty acids via the inflammasome → produce IL-1 → activates ECs, recruits more mononuclear cells
• Activated macrophages produce: ROS (enhance LDL oxidation), growth factors (drive SMC proliferation), and proteases (break down ECM)

Step 4 - Platelet Adhesion

• Exposed intimal components trigger platelet adhesion and activation
• Activated platelets release platelet-derived growth factor (PDGF) and other cytokines

Step 5 - T-Cell Recruitment & Inflammatory Amplification

• T lymphocytes enter the intima and release interferon-γ (IFN-γ) which:
  • Activates macrophages further
  • Activates ECs and SMCs
  • Promotes inflammation (contributing to plaque instability)

Step 6 - Smooth Muscle Cell (SMC) Proliferation & Matrix Synthesis

• Growth factors (PDGF from platelets/macrophages, FGF, TGF-β) recruit SMCs from the media into the intima (phenotypic switch: contractile → synthetic)
• Intimal SMCs proliferate and synthesize ECM (especially collagen) - converting the fatty streak into a mature atheroma
• Collagen deposition forms and stabilizes the fibrous cap
• However, activated macrophages can also induce SMC apoptosis and ECM breakdown → unstable plaques

Step 7 - Lipid Accumulation (Extracellular)

• As foam cells die, lipids spill into the extracellular space → necrotic lipid core forms
• Cholesterol crystals become incorporated into the plaque

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4. Morphology (Lesion Types)

Stage 1 - Fatty Streak (Earliest Lesion)

• Gross: Minute, flat, yellow macules that coalesce into elongated lesions ≥1 cm long
• Microscopy: Intimal lipid-filled foam cells (macrophage-derived) - no significant flow disturbance
• Age: Can appear in the aorta of infants <1 year old; present in virtually ALL children >10 years, regardless of risk factors
• Significance: Not all progress to plaques - but coronary fatty streaks form during adolescence at the same sites that develop plaques later in life

Stage 2 - Atherosclerotic Plaque (Atheromatous Plaque / Fibrous Plaque)

• Gross: White to yellow raised lesions, 0.3-1.5 cm in diameter (may coalesce); eccentric (involves only part of the vessel wall)
• Microscopy: Three key components:
  1. Fibrous cap - Smooth muscle cells, collagen (dense), proteoglycans, elastin; relatively few inflammatory cells
  2. Cellular zone (shoulder region) - Active area with macrophages, foam cells, T lymphocytes, SMCs; most prone to rupture
  3. Necrotic lipid core - Extracellular lipids, cholesterol crystals, necrotic debris, calcification; no collagen

Distribution of Plaques (Descending Frequency):

1. Infrarenal abdominal aorta (most common/severe)
2. Coronary arteries
3. Popliteal arteries
4. Internal carotid arteries
5. Circle of Willis

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5. Stable vs. Vulnerable (Unstable) Plaque

• Macrophage-derived metalloproteinases (MMPs) that digest collagen in the fibrous cap
• T cell IFN-γ inhibiting collagen synthesis by SMCs
• SMC apoptosis from inflammatory signals

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6. Consequences of Atherosclerosis / Acute Plaque Change

1. Plaque Rupture / Erosion / Ulceration

• Exposes highly thrombogenic subendothelial components and lipid core
• Triggers thrombus formation - can cause acute coronary syndrome (unstable angina, STEMI, NSTEMI)
• Most ruptures occur at the shoulder region of the plaque (highest inflammatory cell density, thinnest fibrous cap)

2. Hemorrhage into Plaque

• Rupture of overlying fibrous cap or thin-walled neovessels within the plaque
• Sudden expansion of plaque volume → acute luminal narrowing

3. Atheroembolism

• Plaque rupture can discharge atherosclerotic debris (cholesterol crystals, etc.) into the bloodstream
• Causes microemboli downstream (e.g., "blue toe syndrome")

4. Aneurysm Formation

• Atherosclerosis-induced ischemia of the underlying media with loss of elastic tissue → wall weakening → aneurysmal dilation and rupture
• Most common: infrarenal abdominal aortic aneurysm (AAA)

Atherosclerotic Stenosis (Chronic)

• Critical stenosis: ~70% occlusion is the tipping point in coronary arteries
• At rest: adequate perfusion; with exertion: demand exceeds supply → stable angina
• Chronic arterial hypoperfusion causes: bowel ischemia, chronic IHD, ischemic encephalopathy, intermittent claudication

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7. Major Clinical Consequences

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8. Key High-Yield Points for MBBS Exams

• Response to injury hypothesis (Ross) = the core concept - endothelial injury is the initiating event
• Foam cells = lipid-laden macrophages (and SMCs) formed via scavenger receptor uptake of ox-LDL
• Fatty streak = earliest visible lesion; present in virtually all children >10 years; does NOT obstruct flow
• Fibrous cap = collagen + SMCs; protects against plaque rupture
• Vulnerable plaque = thin cap, large lipid core, heavy inflammation - responsible for ACS
• Most severe atherosclerosis = infrarenal abdominal aorta > coronary > popliteal > internal carotid
• Critical stenosis = 70% in coronary arteries = stable angina threshold
• CRP = strong independent inflammatory marker of cardiovascular risk (not causative)
• Hyperhomocysteinemia - additional risk factor (caused by vit B12/B6/folate deficiency or inborn errors)
• Diabetes = 100-fold increased risk of lower limb gangrene

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Summary Diagram of Pathogenesis (Memorization Framework)

Endothelial Injury (hyperlipidemia, HTN, smoking, turbulence)
        ↓
Endothelial Dysfunction (↑ permeability, ↑ leukocyte adhesion)
        ↓
LDL enters intima → Oxidized LDL (ox-LDL)
        ↓
Monocytes adhere (VCAM-1) → migrate → Macrophages
        ↓
Macrophages engulf ox-LDL via scavenger receptors → FOAM CELLS → FATTY STREAK
        ↓
Platelet adhesion → PDGF released
Macrophages → IL-1, TNF-α, ROS, MMPs
T cells → IFN-γ
        ↓
SMC migration (media → intima) + proliferation + collagen synthesis
        ↓
Fibrous cap forms over lipid core → ATHEROSCLEROTIC PLAQUE
        ↓
Plaque growth → stenosis OR plaque rupture → THROMBOSIS → ACS/Stroke

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