Aneurysmal Disease
MRCS Part A & B — Comprehensive Study Notes
Definition and Pathophysiology of Aneurysms
Aneurysm Definitions
True aneurysm: A pathological dilatation of an artery involving all three layers of the arterial wall (intima, media, adventitia). The aneurysm wall is thinner than normal but retains the native anatomical structure. False aneurysm (pseudoaneurysm): A haematoma contained by fibrous tissue and surrounding structures, not by true vessel wall. Pseudoaneurysms typically result from vessel puncture (catheterisation site, knife wound, gunshot) or anastomotic disruption after surgery. The “wall” is merely compressed surrounding tissue. Saccular aneurysm: A focal outpouching or bulge from one side of the artery, resembling a sac attached to the vessel. Higher rupture risk per unit diameter because stress concentrates at the sac neck. Fusiform aneurysm: Symmetric dilatation of the entire vessel circumference; the vessel looks like a spindle or tapered tube. More stable than saccular but can still rupture.
AAA definition: An abdominal aortic aneurysm is defined as diameter ≥3.0 cm (infrarenal aorta), or ≥1.5 times the diameter of the adjacent normal aorta. The normal infrarenal aorta in men averages 1.8–2.0 cm diameter; in women it is slightly smaller (~1.6 cm). Suprarenal aneurysms (involving the aorta between the renal arteries and superior mesenteric artery) are less common and more technically demanding surgically. Thoracic aortic aneurysms (ascending, arch, descending) are distinguished separately but share similar pathophysiology.
Pathophysiology: The Laplace Law
Aortic aneurysms result from progressive medial degeneration. The media contains elastic fibres and collagen, which provide mechanical strength. Wall stress (the force per unit area that the wall experiences) is related to the pressure inside the vessel and the vessel radius by the Laplace law: Wall Stress = (Pressure × Radius) / Wall Thickness. This relationship explains why aneurysms progressively enlarge: as the aorta dilates (radius increases) and wall thickens (which increases stress; the dilated wall is paradoxically thinner per unit circumference), wall stress increases. Higher stress accelerates degradation, further weakening the wall, causing further dilatation—a vicious cycle of expansion.
At the cellular level, aneurysm formation involves: (1) Matrix metalloproteinase (MMP) activation: MMPs are enzymes that degrade elastin and collagen. In AAA, MMP-2 and MMP-9 are markedly upregulated (particularly by inflammatory macrophages), causing degradation of the extracellular matrix. (2) Chronic inflammatory infiltrate: The aneurysmal aortic wall is infiltrated with macrophages, T-lymphocytes, and mast cells, producing inflammatory cytokines (TNF-α, IL-1, IL-6) and chemokines that recruit additional immune cells. (3) Oxidative stress: Reactive oxygen species (ROS) oxidise phospholipids and proteins in the aortic wall, amplifying inflammation and matrix degradation. (4) Apoptosis: Smooth muscle cells in the media undergo programmed cell death, further weakening the wall. (5) Genetic factors: Family history is present in 15–25% of AAA patients; familial AAA may account for up to 8% of all AAAs. Mutations in genes encoding extracellular matrix proteins (COL3A1 in Ehlers-Danlos syndrome, FBN1 in Marfan) predispose to aortic aneurysm.
Risk Factors for Aneurysm Formation
Smoking: The single most modifiable risk factor. Smoking is associated with a 7-fold increased risk of AAA; the risk correlates with pack-years. Smoking increases MMP expression, oxidative stress, and inflammatory response in the aorta. Cessation reduces progression risk. Male sex: 6:1 male predominance; women develop AAA at older ages. Age: AAA is rare before age 50; incidence peaks in 70s–80s. Hypertension: Elevated blood pressure increases wall stress and progression rate. Genetics: 15–25% of AAA patients have first-degree relatives with AAA (20–30% concordance). Inverse association with diabetes: Paradoxically, diabetes mellitus is associated with LOWER AAA risk (possibly due to increased aortic wall stiffness, reduced MMP activity, altered immune response). AAA prevalence is higher in non-diabetics than diabetics. Other factors: Chronic obstructive pulmonary disease (shared smoking exposure and possibly shared matrix degradation pathways), chronic kidney disease, autoimmune diseases.
The MASS Trial and AAA Screening
The Multicentre Aneurysm Screening Study (MASS, 2002) was a landmark RCT demonstrating that one-time ultrasound screening of men aged 65 reduces AAA-related mortality by 42% at 4 years (48% at long-term follow-up). Among ~65,000 men screened, 1,333 (2%) were found to have AAA ≥3 cm. The trial established that screening is cost-effective and should be offered to all men aged 65. This led to the NHS AAA Screening Programme in the UK, which invites all men aged 65 for one-time abdominal ultrasound.