Page 1 of 34

PA26.1-2 | Atherosclerosis & Aneurysms — SDL Guide

Learning Objectives

Distinguish arteriosclerosis from atherosclerosis and classify its subtypes
Explain the response-to-injury hypothesis of atherogenesis step by step
Describe the morphology and distribution of the atheromatous plaque
List the modifiable and non-modifiable risk factors for atherosclerosis
Enumerate the complications of atherosclerosis including plaque rupture and thrombosis
Define true vs false aneurysm and classify aneurysms by shape and aetiology
Apply Laplace's Law to explain aneurysm enlargement and rupture
Describe the pathology and complications of abdominal aortic aneurysm (AAA) and aortic dissection

INSTRUCTIONS

Arterial disease kills more Indians than any other disease category — ischaemic heart disease, stroke, and aortic catastrophes all trace back to the wall of a blood vessel. Mastering atherosclerosis and aneurysms is therefore not optional pathology; it is the mechanistic backbone of clinical medicine you will apply every day as a physician. This module builds that foundation through the response-to-injury hypothesis, plaque biology, and vascular mechanics — concepts that recur in Pharmacology, Medicine, Surgery, and Cardiology.

References

Robbins & Cotran Pathologic Basis of Disease, 10th ed., Ch 11 (Blood Vessels) (textbook)
Harsh Mohan — Textbook of Pathology, 8th ed., Ch 15 (Blood Vessels) (textbook)

Version 2.0 | NMC CBUC 2024

CLINICAL SCENARIO

A 62-year-old man with a 30-year smoking history and poorly controlled hypertension presents to the emergency department with sudden, tearing chest pain radiating to the back. His blood pressure differs by 40 mmHg between his two arms. A CT angiogram reveals a Stanford Type A aortic dissection. Four hours earlier, a 58-year-old woman was brought in pulseless after a ruptured abdominal aortic aneurysm found incidentally on screening ultrasound six months ago — her family had declined surgery.

These are not rare outliers. They are the endpoint of decades-long arterial disease that started silently with a fatty streak in adolescence. Understanding why and how arteries fail is the mission of this module.

WHY THIS MATTERS

PA26.1 maps directly to the MCI/NBE long-case format: examiners ask for risk stratification, pathogenesis mechanism, and morphology description of plaques.
PA26.2 is a high-yield surgery-pathology crossover: AAA, dissecting aneurysm, and berry aneurysm appear repeatedly in USMLE-style integrated questions.
Clinico-pathological correlation: every MI, stroke, claudication, and renal artery stenosis you manage in wards has its origin in concepts you will learn today.
Year-1 foundation used here: AN vascular anatomy (aorta, circle of Willis), PY endothelial physiology, BI lipid metabolism.

RECALL

Before proceeding, mentally revisit:
1. The three layers of an arterial wall — tunica intima, media, adventitia — and the cell types in each.
2. The general structure of a lipoprotein: which lipoprotein carries cholesterol to peripheral tissues (LDL) and which carries it back to the liver (HDL)?
3. From Physiology: what is endothelial-derived nitric oxide (eNO) and why is its loss pro-thrombotic?
4. From Biochemistry: what is oxidised LDL (ox-LDL) and why is it more atherogenic than native LDL?

If any of these feel uncertain, spend two minutes with your Year-1 notes before continuing — this module builds on all four.

Arteriosclerosis: The Umbrella Term

A classification diagram compares atherosclerosis, hyaline and hyperplastic arteriolosclerosis, and Mönckeberg medial calcific sclerosis by vessel size, wall layer affected, lumen narrowing, and clinical associations. — **Panel A:** Large or medium artery, intima, fibrous cap, lipid core, atheromatous plaque, narrowed lumen, reduced blood flow, stenosis, ischaemia.. **Panel B:** Small artery or arteriole, hyaline arteriolosclerosis, pink homogeneous glassy wall thickening, plasma protein leakage, diabetes mellitus, benign hypertension, hyperplastic arteriolosclerosis, onion-skin concentric laminated thickening, malignant hypertension, luminal narrowing, fibrinoid necrosis.. **Panel C:** Muscular artery, media, dystrophic medial calcification, preserved lumen, ring-like calcification, pipe-stem artery on X-ray, femoral artery, tibial artery, radial artery, no stenosis.. **Exam Pearl Box:** Atherosclerosis affects intima and causes stenosis; Mönckeberg medial calcific sclerosis affects media and does not cause stenosis..

Arteriosclerosis (literally 'hardening of arteries') is a generic term for thickening and loss of elasticity of the arterial wall. Three distinct processes fall under this umbrella:

1. Atherosclerosis — the dominant and most clinically important form; involves large and medium elastic/muscular arteries; driven by lipid deposition and inflammation. (Covered in depth in the next sections.)

2. Arteriolosclerosis — affects small arteries and arterioles; two morphological patterns:
• Hyaline arteriolosclerosis: pink, homogeneous, glassy thickening of the vessel wall due to plasma protein leakage; associated with benign hypertension and diabetes mellitus (hyperglycaemia accelerates protein glycation and basement membrane thickening).
• Hyperplastic arteriolosclerosis (onion-skin lesion): concentric laminated thickening of the wall with luminal narrowing; seen in malignant hypertension (diastolic BP >120 mmHg). Can progress to necrotising arteriolitis with fibrinoid necrosis.

3. Mönckeberg medial calcific sclerosis — dystrophic calcification of the media of muscular arteries (typically femoral, tibial, radial); ring-like calcifications visible on X-ray ('pipe-stem' arteries); does NOT narrow the lumen and is NOT atheromatous; more common in elderly and diabetics; usually an incidental finding.

Key distinction for exams: Mönckeberg affects media, does not cause stenosis; atherosclerosis affects intima, causes stenosis and ischaemia.

Atherosclerosis: Risk Factors

A four-panel medical diagram shows how non-modifiable and modifiable risk factors converge on endothelial dysfunction and atherosclerotic plaque formation. — **Panel A:** Artery cross-section showing endothelial injury, LDL entry into intima, oxidized LDL, macrophage, foam cells, fatty streak, fibrous cap, lipid core, and narrowed lumen. **Panel B:** Non-modifiable risk factors: age, sex, post-menopausal loss of oestrogen protection, genetics, family history, familial hypercholesterolaemia, and LDL receptor mutation. **Panel C:** Major modifiable risk factors: dyslipidaemia with high LDL and low HDL, hypertension, cigarette smoking, and diabetes mellitus. **Panel D:** Converging pathway showing endothelial dysfunction, VCAM-1 mediated monocyte adhesion, foam cell formation, plaque growth, and luminal narrowing.

Atherosclerosis is a multifactorial disease. Risk factors are classically divided:

Non-modifiable risk factors:

Factor	Comment
Age	Risk rises progressively; lesions begin in adolescence but manifest clinically after 40
Sex	Men at higher risk at all ages; women 'catch up' post-menopause (loss of oestrogen's protective effect on lipid profile and endothelium)
Genetics / family history	Familial hypercholesterolaemia (FH): LDL-receptor mutations → severe premature atherosclerosis

Modifiable risk factors (MAJOR):

Factor	Mechanism
Dyslipidaemia (↑LDL, ↓HDL)	LDL enters intima, oxidised → foam cell formation; HDL exports cholesterol, is protective
Hypertension	Haemodynamic injury to endothelium; increases LDL uptake; upregulates VCAM-1
Cigarette smoking	Oxidative stress; CO damages endothelium; ↑LDL, ↓HDL; promotes thrombosis
Diabetes mellitus	Glycation of LDL (harder for receptors to clear); endothelial dysfunction; accelerates foam-cell conversion

Other contributing factors: obesity (central/android), physical inactivity, C-reactive protein (marker of inflammation), homocysteinaemia, lipoprotein(a), thrombogenic risk factors.

> Clinical pearl: The Framingham risk score integrates age, sex, LDL, HDL, BP, and smoking to estimate 10-year cardiovascular risk — a direct clinical translation of these pathology risk factors.

SELF-CHECK

A 45-year-old woman has total cholesterol 280 mg/dL, LDL 200 mg/dL, HDL 35 mg/dL, BP 155/95 mmHg, and smokes 20 cigarettes/day. Which single intervention would have the GREATEST impact on reducing her atherosclerotic risk?

A. Start a high-intensity statin to lower LDL

B. Begin antihypertensive therapy

C. Advise smoking cessation

D. Prescribe niacin to raise HDL

Reveal Answer

Answer: C. Advise smoking cessation

Smoking cessation (option C) addresses multiple simultaneous mechanisms: it eliminates oxidative endothelial injury, reduces CO-mediated damage, reverses the LDL↑/HDL↓ dyslipidaemia induced by smoking, and dramatically reduces thrombotic risk — often achieving larger net risk reduction than any single pharmacological agent. Statins and antihypertensives are important adjuncts, but cessation is the highest-yield single step in a heavy smoker.

Pathogenesis of Atherosclerosis: Response-to-Injury Hypothesis

Five-step artery-wall diagram showing endothelial injury, LDL oxidation, monocyte recruitment, foam cell formation, and smooth muscle proliferation in atherosclerosis. — **Panel A:** Endothelial injury and activation at a branch point with turbulent flow, dysfunctional endothelium, increased permeability, VCAM-1, ICAM-1, E-selectin, hypertension and smoking toxin triggers.. **Panel B:** LDL entry into the intima and oxidation by reactive oxygen species, showing LDL, ox-LDL, permeable endothelium, intima, internal elastic lamina, and media.. **Panel C:** Monocyte adhesion and migration with VCAM-1-mediated attachment, MCP-1 chemotactic gradient, circulating monocytes, and entry into the intima.. **Panel D:** Macrophage uptake of ox-LDL via SR-A and CD36 scavenger receptors, formation of foam cells, lipid droplets, and aggregation into a fatty streak.. **Panel E:** Smooth muscle cell migration and proliferation from media to intima, PDGF signaling, extracellular matrix deposition, fibrous cap formation, and early plaque growth..

The most accepted mechanistic framework is the response-to-injury hypothesis (Ross, 1990s), which unifies lipid, inflammatory, and haemodynamic inputs into a single sequence:

Step 1 — Endothelial injury and dysfunction
Triggers: haemodynamic turbulence (branch points, bifurcations), ox-LDL, hypertension, smoking toxins, homocysteine. The endothelium loses its anti-thrombotic, anti-inflammatory state — it expresses adhesion molecules (VCAM-1, ICAM-1, E-selectin) and becomes permeable.

Step 2 — LDL entry and oxidation
LDL accumulates in the intima. Local reactive oxygen species (from macrophages, smoking) oxidise it to ox-LDL. Ox-LDL is proinflammatory and cannot be cleared by the normal LDL receptor.

Step 3 — Monocyte recruitment and foam cell formation
Circulating monocytes adhere to activated endothelium (via VCAM-1/MCP-1 gradient) → migrate into intima → differentiate into macrophages → engulf ox-LDL via scavenger receptors (SR-A, CD36) → become foam cells (lipid-laden macrophages) → aggregate into fatty streaks (flat, yellow intimal deposits; seen from adolescence; reversible).

Five-panel arterial wall diagram showing endothelial activation, LDL oxidation, monocyte migration, foam cell formation, and smooth-muscle proliferation during atherosclerotic plaque development. — **Panel A:** Endothelial activation with damaged endothelium, luminal surface, intima, media, and VCAM-1 expression.. **Panel B:** LDL entry through endothelium into intima and conversion to oxidized LDL labeled as ox-LDL.. **Panel C:** Monocyte adhesion to VCAM-1 and migration into intima guided by MCP-1.. **Panel D:** Macrophages ingesting ox-LDL and becoming lipid-filled foam cells within the intima.. **Panel E:** PDGF-driven smooth-muscle cell migration and proliferation, extracellular matrix deposition, fibrous cap formation, lipid core, and narrowing arterial lumen..

Step 4 — Smooth muscle cell (SMC) migration and proliferation
Activated macrophages and platelets release PDGF (platelet-derived growth factor) and FGF → stimulate medial SMCs to migrate into the intima and proliferate. SMCs synthesise extracellular matrix (collagen, proteoglycans) → fibrous cap forms over the lipid core.

Step 5 — Progressive plaque formation
Continued lipid accumulation, SMC proliferation, collagen deposition, and inflammation → fibrous atheromatous plaque (described below). Lesion grows eccentrically into the lumen.