Atherosclerosis — Pathogenesis

Endothelial Dysfunction and Increased Permeability

• Chronic exposure to hemodynamic shear stress (particularly at arterial branch points and curves), oxidative stress, and circulating risk factors (hyperlipidemia, hypertension, hyperglycemia, smoking) triggers dysfunction of the endothelial monolayer
• Dysfunctional endothelium exhibits reduced production of protective nitric oxide (NO) and increased expression of adhesion molecules (ICAM-1, VCAM-1, selectins) and chemokines (MCP-1/CCL2)
• Loss of the endothelium's barrier function increases transcytosis and subendothelial retention of apolipoprotein B-containing lipoproteins (LDL, VLDL), particularly in regions of hemodynamic turbulence
• Retained lipoproteins undergo oxidative modification by resident endothelial cells, smooth muscle cells, and macrophages, generating highly atherogenic oxidized LDL (oxLDL)

Monocyte Recruitment and Foam Cell Formation

• OxLDL and other danger-associated molecular patterns (DAMPs) activate pattern recognition receptors (particularly scavenger receptors SR-A and SR-B and lectin-like oxLDL receptor LOX-1) on endothelial cells and resident macrophages
• Activated endothelium upregulates adhesion molecules and releases monocyte chemoattractant protein-1 (MCP-1), facilitating recruitment of circulating monocytes into the subendothelial space via the CCL2-CCR2 axis
• Monocytes differentiate into macrophages within the intima and undergo lipid loading via scavenger receptor-mediated endocytosis of oxLDL, bypassing the normal feedback inhibition of cholesterol uptake that occurs via LDL receptors
• Lipid-laden macrophages become foam cells, the hallmark of the earliest atherosclerotic lesion (the fatty streak), which are morphologically distinguished by cytoplasmic lipid droplets visible on light microscopy with Oil Red O or Sudan Black staining
• Continued lipid accumulation leads to foam cell apoptosis and necrosis, with subsequent formation of the lipid core characteristic of advanced lesions

Inflammatory Cascade Amplification

• OxLDL directly activates innate immune receptors (TLRs, CD36, LOX-1) triggering NF-κB-dependent production of pro-inflammatory cytokines including TNF-α, IL-1β, IL-6, IL-8, and MCP-1
• These cytokines amplify endothelial activation, perpetuate monocyte recruitment, and prime local innate immune responses through activation of pattern recognition receptors (e.g., NLRP3 inflammasome)
• T cells infiltrate the lesion and are activated by antigen-presenting cells in response to oxLDL epitopes and heat shock proteins; activated T cells produce additional IFN-γ and TNF-α
• Natural killer T cells and B cells also contribute to lesional inflammation; lesional B cells produce IgM and IgG autoantibodies against oxLDL and malondialdehyde-LDL (MDA-LDL) epitopes
• This adaptive immune response represents a maladaptive response to injury, perpetuating chronic inflammation

Smooth Muscle Cell Proliferation and Extracellular Matrix Deposition

• Platelet-derived growth factor (PDGF), fibroblast growth factor (FGF), and vascular endothelial growth factor (VEGF) released by activated endothelium, macrophages, and platelets stimulate medial and intimal smooth muscle cell (SMC) proliferation and migration into the intima
• SMCs undergo a phenotypic switch from a contractile state to a synthetic, pro-inflammatory state characterized by increased production of extracellular matrix proteins (collagen types I and III, elastin, proteoglycans) and reduced expression of smooth muscle-specific genes (α-actin, SM22α, calponin)
• SMCs also contribute to the inflammatory milieu by producing MCP-1, IL-6, and other cytokines
• Progressive deposition of extracellular matrix by SMCs results in a fibrous cap that overlies the lipid-rich necrotic core, a hallmark of the advanced fibroatheromatous plaque or complicated lesion
• The fibrous cap provides mechanical stability to the plaque and consists predominantly of cross-linked collagen, particularly types I and III, along with elastin, proteoglycans, and other matrix proteins

Plaque Calcification

• Calcification represents an active process mediated by bone morphogenetic proteins (BMPs) and other osteogenic signals released during chronic inflammation
• Vascular smooth muscle cells express alkaline phosphatase, osteopontin, osteocalcin, and Runx2 transcription factor, driving osteogenic transdifferentiation
• Calcification typically occurs at the plaque-media interface and within the lipid core and is associated with plaque stability (medial calcification) but also with increased lesional complexity
• Calcified plaques may become brittle and prone to rupture, or conversely, may represent burned-out inflammatory lesions with reduced acute risk

Angiogenesis and Neovascularization

• Advanced plaques develop neovessels (microvessels) within the fibrous cap and shoulder regions, derived from vasa vasorum proliferation stimulated by hypoxia-inducible factor (HIF)-1α and VEGF
• These immature, leaky neovessels are lined by endothelium with reduced barrier function and contribute to continued recruitment of inflammatory cells and microhemorrhages within the plaque
• Intraplaque hemorrhage from fragile neovessels and erosion provides additional free iron, hemoglobin, and lipids that accelerate oxidative stress and inflammation
• Neovascularization is particularly prominent in vulnerable plaques at high risk for rupture

Non-Modifiable Risk Factors

• Age: Atherosclerosis burden increases with advancing age; men typically develop disease 10 years earlier than women (due to estrogen's protective effects before menopause)
• Sex: Males have higher prevalence of atherosclerosis and earlier disease onset; postmenopausal women have atherosclerosis prevalence approaching that of men
• Genetic predisposition: Family history of premature atherosclerosis, familial hypercholesterolemia (LDL receptor mutations, APOB mutations, PCSK9 mutations), familial combined hyperlipidemia, and familial hypoalphalipoproteinemia increase risk
• Race/ethnicity: Certain populations (African American, Hispanic, Native American) have higher prevalence and severity of atherosclerosis

Major Modifiable Risk Factors (Causal Risk Factors)

• Dyslipidemia: Elevated LDL cholesterol (>100 mg/dL optimal) is the primary causal lipid risk factor; low HDL cholesterol (<40 mg/dL in men, <50 mg/dL in women) and elevated triglycerides (>150 mg/dL) are independent risk factors; Lipoprotein(a) [Lp(a)] is an emerging causal risk factor particularly when elevated (>50 mg/dL)
• Hypertension: Systemic arterial hypertension (≥140/90 mmHg or on antihypertensive therapy) promotes endothelial dysfunction, increases shear stress, and accelerates atherosclerosis; diastolic hypertension particularly damaging
• Smoking: Active tobacco smoking dramatically increases risk through multiple mechanisms including endothelial dysfunction, oxidative stress, increased platelet aggregability, and reduced HDL; secondhand smoke exposure also increases risk
• Diabetes mellitus: Both type 1 and type 2 diabetes accelerate atherosclerosis through glycation of proteins (including LDL), increased oxidative stress, endothelial dysfunction, and increased thrombotic tendency; hyperglycemia itself is atherogenic
• Physical inactivity and obesity: Sedentary lifestyle increases risk; overweight (BMI 25-29.9) and obesity (BMI ≥30) increase risk through multiple pathways including dyslipidemia, hypertension, insulin resistance, and systemic inflammation

Emerging and Contributing Risk Factors

• Chronic inflammation: Elevated high-sensitivity C-reactive protein (hsCRP), fibrinogen, and other inflammatory markers predict atherosclerotic events independent of lipids
• Chronic kidney disease: Decreased glomerular filtration rate is associated with accelerated atherosclerosis through uremic toxins, mineral metabolism abnormalities, and inflammation
• Chronic infections: Chronic viral infections (CMV, HIV) and bacterial infections (Chlamydia pneumoniae, Helicobacter pylori) have been proposed as atherogenic cofactors, though causality remains unproven
• Metabolic syndrome: The constellation of central obesity, hypertension, dyslipidemia, and impaired fasting glucose substantially increases atherosclerotic risk
• Elevated lipoprotein(a): Lp(a) is an LDL-like particle with apo(a) covalently linked to apoB-100; elevated levels (>50 mg/dL) are associated with increased risk independent of LDL
• Autoimmune diseases: Systemic lupus erythematosus, rheumatoid arthritis, and other autoimmune conditions are associated with accelerated atherosclerosis
• Psychosocial factors: Chronic stress, depression, and social isolation increase atherosclerotic risk through neuroendocrine and inflammatory mechanisms

Asymptomatic/Silent Phase

• Atherosclerotic plaques typically cause no symptoms until they achieve a critical degree of stenosis (typically >70% diameter reduction) that significantly impairs blood flow
• Atherosclerotic disease burden may be detected incidentally on imaging performed for other indications
• Annual risk of myocardial infarction or stroke in asymptomatic patients with documented atherosclerosis is 1-2%, highlighting the unpredictability of acute events

Symptoms Related to Hemodynamically Significant Stenosis (Chronic Ischemia)

• Stable angina pectoris: Chest discomfort or pressure induced by exertion and relieved by rest, resulting from demand ischemia downstream of a stenotic coronary artery; typical character is substernal, non-radiating pressure or "heaviness" lasting 5-15 minutes
• Intermittent claudication: Leg pain with exertion (walking) and relief with rest in patients with lower extremity atherosclerosis; characteristically affects the calf, thigh, or buttock depending on lesion location
• Cerebrovascular insufficiency: Transient ischemic attacks (TIAs) with focal neurological deficits (monocular vision loss, transient speech difficulty, unilateral weakness) lasting <24 hours in patients with carotid atherosclerosis
• Mesenteric angina: Postprandial abdominal pain in patients with chronic mesenteric ischemia from atherosclerotic stenosis of mesenteric arteries; patients characteristically avoid eating due to pain (sitophobia)
• Erectile dysfunction: Loss of erectile function in men with aortoiliac atherosclerosis (Leriche syndrome when bilateral)

Acute Ischemic Events (Plaque Rupture/Erosion with Superimposed Thrombosis)

• Acute myocardial infarction (AMI): Sudden onset of severe substernal chest pain or pressure often accompanied by dyspnea, diaphoresis, nausea, and anxiety; more common in men and post-menopausal women; approximately 85% of AMIs result from atherosclerotic plaque rupture with superimposed thrombosis; the remaining 15% result from plaque erosion (more common in younger women and smokers)
• Acute ischemic stroke: Sudden focal neurological deficit (facial droop, arm weakness, speech difficulty, vision loss) resulting from thrombotic occlusion of a cerebral artery, often secondary to atherosclerotic plaque rupture in the carotid or vertebral arteries or from artery-to-artery embolism
• Acute limb ischemia: Sudden onset of pain, coldness, pallor, and numbness in a limb with loss of pulses, resulting from acute thrombotic occlusion of a peripheral artery
• Sudden cardiac death: Abrupt loss of consciousness and cardiac output due to plaque rupture triggering ventricular fibrillation; accounts for approximately 15% of atherosclerotic deaths

Physical Examination Findings

• Arterial bruits: Focal turbulent flow over atherosclerotic stenoses produces audible bruits on auscultation (carotid, femoral, abdominal aorta)
• Diminished or absent pulses: Reduced distal pulses due to severe stenosis or occlusion (femoral, dorsalis pedis, posterior tibial)
• Signs of peripheral ischemia: Cool extremities, pallor, cyanosis, rubor (reactive hyperemia), ulceration, and gangrene in advanced disease
• Xanthomas and xanthelasmas: Lipid deposits in skin and eyelids in patients with severe dyslipidemia (familial hypercholesterolemia), reflecting long-standing hypercholesterolemia
• Corneal arcus: Lipid deposition in the cornea creating a white ring; non-specific but suggests dyslipidemia when present in patients <50 years
• Signs of heart failure: Elevated jugular venous pressure, peripheral edema, hepatomegaly, and rales in patients with ischemic cardiomyopathy from prior myocardial infarctions

Laboratory and Imaging Correlates

• Elevated troponins: Cardiac troponin I or T elevation indicates myocardial necrosis; highly sensitive and specific for acute myocardial infarction
• Elevated creatine kinase (CK) and CK-MB: Less sensitive and specific than troponins but may reflect myocardial injury; CK-MB isoenzyme is cardiac-specific
• EKG changes: ST-segment elevation or depression, T-wave inversions, and new Q waves consistent with myocardial infarction (location depends on occluded coronary artery)
• Coronary angiography findings: Stenotic lesions appearing as irregular narrowing of the arterial lumen; eccentric lesions are more characteristic of atherosclerotic plaques than concentric lesions
• Computed tomography angiography (CTA): Visualization of arterial stenosis and atherosclerotic plaque burden; calcium scoring reflects atherosclerotic burden but does not distinguish stable from vulnerable plaques
• MRI findings: Characterization of plaque composition (lipid core, fibrous cap, calcification) and detection of intraplaque hemorrhage, which is associated with vulnerability

Histological Findings

• Fatty Streak (Earliest lesion, Type I-II lesion): Appears as yellowish discoloration of the intima visible on gross inspection; microscopically composed of lipid-laden macrophages (foam cells)

Immediate stabilisation (acute plaque rupture)

• Acute coronary syndrome or acute stroke is treated as a reperfusion emergency, not as a lipid problem: per the ACC/AHA chest pain and ACS pathways, obtain an ECG within 10 minutes, give aspirin (chewed, non-enteric-coated), and pursue emergent revascularization for STEMI; per AHA/ASA, IV thrombolysis and thrombectomy triage govern acute ischemic stroke.
• A new LBBB is not by itself a STEMI criterion — apply Sgarbossa criteria to the tracing.

First-line long-term therapy (risk-factor modification)

• Lifestyle: ACC/AHA primary prevention guidance places diet (Mediterranean/DASH pattern), regular aerobic activity, weight management, and complete tobacco cessation ahead of all pharmacotherapy; smoking cessation restores endothelial NO bioavailability faster than any drug.
• Statins (HMG-CoA reductase inhibitors): e.g., atorvastatin or rosuvastatin. Hepatic cholesterol synthesis blockade upregulates LDL receptors, lowering apoB-lipoprotein retention in the intima; statins also stabilise plaque by reducing macrophage content and inflammation. The ACC/AHA cholesterol guideline uses four benefit groups (clinical ASCVD, LDL ≥190 mg/dL, diabetes age 40–75, and 10-year risk by the pooled cohort equations) and high- vs moderate-intensity dosing rather than a single universal target.
• Blood pressure: ACC/AHA hypertension guidance favors thiazide-type diuretics, ACE inhibitors/ARBs, or dihydropyridine CCBs. Glycemia: ADA Standards of Care favor SGLT2 inhibitors and GLP-1 receptor agonists in type 2 diabetes with established ASCVD.

Escalation

• Ezetimibe (NPC1L1 inhibitor), then PCSK9 monoclonal antibodies (evolocumab, alirocumab) or inclisiran; bempedoic acid for statin intolerance; icosapent ethyl for residual hypertriglyceridemia. Colchicine targets the residual inflammatory (IL-1β/NLRP3) axis.

Definitive/procedural

• PCI with drug-eluting stent or CABG for obstructive CAD; carotid endarterectomy/stenting for significant symptomatic stenosis; supervised exercise therapy before revascularization in claudication.

Contraindicated/not recommended

• Statins in pregnancy and lactation are generally avoided; USPSTF advises against initiating aspirin for primary prevention in adults ≥60; antioxidant vitamins, hormone therapy, and niacin add-on are not recommended for ASCVD prevention.

Complications of the disease

• Plaque rupture with occlusive thrombosis — emergency: a thin fibrous cap degraded by macrophage matrix metalloproteinases tears, exposing tissue factor and collagen in the necrotic core to blood. Signalled by ischemic chest pain at rest with ST-segment shift and rising high-sensitivity troponin.
• Plaque erosion: endothelial denudation without cap rupture, with a proteoglycan-rich, less lipid-laden substrate; disproportionately seen in younger women and smokers.
• Intraplaque hemorrhage: rupture of fragile vasa-vasorum–derived neovessels abruptly expands plaque volume; free hemoglobin and iron drive further oxidation. Seen as a high-signal focus on plaque MRI.
• Atheroembolism (cholesterol crystal embolisation): needle-shaped cholesterol clefts lodge in arterioles, classically after catheterisation or aortic manipulation. Look for blue toe syndrome with intact pulses, livedo reticularis, Hollenhorst plaques on funduscopy, rising creatinine, transient eosinophilia, and low complement.
• Aneurysm formation: plaque-driven medial atrophy and matrix degradation weaken the wall, most often the infrarenal abdominal aorta. Rupture is an emergency — hypotension, back or flank pain, pulsatile mass.
• Acute limb ischemia — emergency: the six Ps (pain, pallor, pulselessness, paresthesia, poikilothermia, paralysis); paralysis and anesthesia mark threatened limb loss.
• Chronic sequelae: ischemic cardiomyopathy with HFrEF, ischemic nephropathy and renovascular hypertension from renal artery stenosis, chronic mesenteric ischemia with sitophobia and weight loss, and vascular cognitive impairment.
• Sudden cardiac death — emergency: ischemia-triggered ventricular fibrillation / pulseless VT, the shockable pair.

Complications of treatment

• Statins: myalgias, and rarely myopathy or rhabdomyolysis (markedly elevated CK, myoglobinuria, acute kidney injury) — risk rises with CYP3A4 inhibitors, fibrates, and higher intensities; transaminase elevation and a small excess of new-onset diabetes.
• Antiplatelet/anticoagulant therapy: gastrointestinal and intracranial bleeding.
• Post-PCI: acute stent thrombosis after premature DAPT interruption presents as STEMI and is an emergency; in-stent restenosis from neointimal smooth muscle hyperplasia causes recurrent exertional angina months later; contrast-associated nephropathy and access-site hematoma.
• Post-CABG: perioperative MI, stroke, and late saphenous vein graft atherosclerosis.

• Foam cells form through scavenger receptors (SR-A, CD36, LOX-1), not the LDL receptor: scavenger receptor uptake of oxLDL is not downregulated by intracellular cholesterol, so the macrophage engorges without feedback. This is the single most tested mechanistic point.
• Fatty streak ≠ disease endpoint: composed of foam cells and T cells, present even in children and adolescents, non-obstructive, and potentially reversible. The distractor is calling it the lesion that causes angina — it does not.
• The plaque that kills is usually not the plaque that is most stenotic: a vulnerable plaque has a thin fibrous cap, a large necrotic lipid core, dense macrophage infiltration, and neovascularisation. Most infarct-related lesions were less than 70% stenotic beforehand; high-grade stenoses produce stable angina and collaterals instead.
• The cap is a balance of synthesis versus degradation: smooth muscle cells make type I/III collagen, while Th1-derived IFN-γ inhibits collagen synthesis and macrophage MMPs digest it. Statin plaque stabilisation works on this axis, which is why event reduction precedes any change in luminal diameter.
• Distribution to memorise: abdominal aorta > coronary arteries > popliteal artery > internal carotid > circle of Willis, with lesions clustering at branch points where flow is turbulent and shear stress is low.
• **Classic distractor — *Mönckeberg medial calcific sclerosis***: calcification of the media of medium-sized muscular arteries, seen as pipestem vessels on radiograph, that does not encroach on the lumen and is not atherosclerosis. Hyaline and hyperplastic arteriolosclerosis (diabetes, hypertension, malignant hypertension with onion-skinning) are also small-vessel entities, not atheromas.
• The association examiners love: elevated Lp(a), an apo(a)-linked LDL-like particle, confers ASCVD risk independent of LDL and is largely unresponsive to statins.
• Best next step after a first ASCVD event: high-intensity statin plus antiplatelet therapy per the ACC/AHA cholesterol and chronic coronary disease guidelines — not niacin, not antioxidant vitamins, and not estrogen.