Aneurysm

Definition

Localised, permanent, abnormal dilatation of a blood vessel or cardiac chamber from congenital or acquired wall weakening. Risks: thrombosis/thromboembolism, altered flow, compression, rupture.

Classification

By wall composition: true (all three layers intact — atherosclerotic, syphilitic, congenital, post-MI ventricular aneurysm) vs false/pseudoaneurysm (wall defect + free-communicating haematoma — traumatic, or ventricular free wall rupture).

By shape: saccular, fusiform, cylindrical, serpentine/varicose, racemose (arteriovenous).

By mechanism (most used clinically): atherosclerotic, syphilitic, dissecting, mycotic, berry.

Pathogenesis — general principle

Imbalance between connective tissue synthesis and degradation:

• Defective synthesis: Marfan (fibrillin defect), Ehlers-Danlos (type III collagen defect), vitamin C deficiency — all disturb TGF-β-driven matrix synthesis.
• Increased degradation: matrix metalloproteinases from plaque/vasculitis macrophages.
• Smooth muscle loss + cystic medial degeneration: ischaemia of inner media (atherosclerosis) or outer media (hypertension) → SMC death, fibrosis, proteoglycan accumulation.

Atherosclerotic aneurysm

Most common aortic aneurysm type. Men, age >50. Mechanism: plaque compresses/starves the media → ischaemic atrophy + MMP-driven elastin loss. Site: abdominal aorta (infrarenal, suprabifurcation) — assume atherosclerotic until proven otherwise. Usually fusiform, >5–6 cm, laminated mural thrombus in lumen. Effects: rupture (peritoneal/retroperitoneal, fatal), compression (ureter, vertebrae), thromboembolism (inferior mesenteric).

Syphilitic (luetic) aneurysm

Tertiary syphilis, ~40% of syphilitic aortitis. Men, >50. Mechanism: treponemes → vasa vasorum inflammation → obliterative endarteritis → medial ischaemia/scarring (syphilitic mesoaortitis). Site: ascending aorta/arch. Morphology: saccular, 3–5 cm, tree-bark intima; aortic root involvement → regurgitation → cor bovinum. Effects: rupture (pleural/pericardial/tracheal/oesophageal), compression symptoms, cardiac dysfunction.

Aortic dissection

Blood enters an intimal tear and dissects along the media, splitting the wall (true dilatation not always present — “dissecting aneurysm” is an outdated term). Men 50–70, or pregnant women.

Pathogenesis: ~90% hypertensive; remainder from connective tissue disorders (Marfan), cystic medial degeneration of ageing, iatrogenic trauma, pregnancy.

Morphology: sharp transverse/oblique intimal tear, 3–4 cm, ascending aorta (majority); dissection plane between outer and middle thirds of media; may extend proximally/distally; ~10% have a second distal tear → re-entry → double-barrelled aorta. Histology: cystic medial degeneration, no inflammation.

Classification:

Effects: sudden tearing chest pain radiating to the back (mimics MI) · rupture → pericardial tamponade (usually fatal) · aortic regurgitation · coronary obstruction → MI · branch obstruction → organ ischaemia.

Mycotic aneurysm

Misnomer — usually bacterial, not fungal. Sources: septic embolus (classically from infective endocarditis), direct extension from adjacent suppuration, or direct bloodstream invasion. Rupture of an intracranial mycotic aneurysm is a recognised cause of death in IE.

Berry aneurysm

Most important/common intracranial aneurysm; leading cause of subarachnoid haemorrhage (>85% of cases). Saccular, 2 mm–2 cm+, at arterial bifurcations of the circle of Willis. Not congenital — develops over time from an inherent medial/elastic defect at the bifurcation; increases with age through young adulthood. ~25% multiple. Associated with adult polycystic kidney disease and coarctation of the aorta.

Sites (descending frequency): anterior communicating artery > posterior communicating artery origin from ICA > MCA first bifurcation > ICA bifurcation into ACA/MCA.

Effects: silent until rupture → sudden severe headache, loss of consciousness, neurologic deficit. Chronic hypertension not established as a cause of formation/rupture, but a sudden pressure rise often precipitates rupture. First-rupture mortality ~20–25%; recurrent bleeding risk in survivors; territorial infarction post-rupture partly from vasospasm.

Clinical relevance

Site predicts type: abdominal aorta → atherosclerotic; ascending aorta/root with regurgitation → syphilis or connective tissue disorder; circle-of-Willis saccular lesion → berry aneurysm. Tearing chest pain radiating to the back → always consider dissection, since mistaking it for MI (and treating accordingly) can be fatal.

Definition

An aneurysm is a localised, permanent, abnormal dilatation of a blood vessel or of a chamber of the heart, arising from congenital or acquired weakening of the wall. Aneurysms most often affect large elastic arteries, particularly the aorta and its major branches, and can cause harm in several ways — thrombosis and distal thromboembolism, disturbance of local blood flow, compression of adjacent structures, and, most seriously, rupture.

Classification

Aneurysms are classified along several independent axes, which together describe a given lesion completely.

By Composition of the Wall

• True aneurysm: the dilated wall retains all three layers of the normal vessel — intima, media, and adventitia — or, in the heart, the thinned wall of the ventricle itself. Atherosclerotic, syphilitic, and congenital vascular aneurysms fall into this category, as does the ventricular aneurysm that complicates a transmural myocardial infarction.
• False aneurysm (pseudoaneurysm): a defect in the vascular wall through which blood escapes into a surrounding haematoma that remains in free communication with the vessel lumen — in effect, a pulsating haematoma rather than a dilated vessel. It typically follows traumatic or perforating injury, and the rupture of a ventricular free wall after myocardial infarction is a cardiac example.

By Gross Shape

• Saccular: a spherical outpouching from one side of the vessel wall.
• Fusiform: a gradual, spindle-shaped dilatation involving the full circumference over a length of vessel.
• Cylindrical: a continuous, parallel-walled dilatation.
• Serpentine (varicose): a tortuous dilatation.
• Racemose (arteriovenous): a mass of intercommunicating arteries and veins forming a direct arteriovenous connection.

By Pathogenetic Mechanism

This is the classification most often used clinically: atherosclerotic, syphilitic, dissecting (dissecting haematoma), mycotic, and berry aneurysms.

Pathogenesis

Every aneurysm reflects a loss of balance between the ongoing synthesis and the degradation of the structural connective tissue of the vessel wall — chiefly collagen and elastin.

• Defective synthesis of connective tissue. In Marfan syndrome, a defect in fibrillin weakens the elastic tissue of the aortic wall. In Ehlers-Danlos syndrome, defective type III collagen has a similar effect. Vitamin C deficiency impairs normal collagen cross-linking. In each case, TGF-β signalling — which governs smooth muscle proliferation and matrix synthesis — is disturbed.
• Increased degradation of connective tissue. Matrix metalloproteinases, secreted chiefly by macrophages within an atherosclerotic plaque or an area of vasculitis, digest collagen and elastin in the wall faster than they can be replaced.
• Loss of smooth muscle cells and cystic medial degeneration. Ischaemia of the inner media (as in atherosclerosis) or of the outer media (as in chronic hypertension) leads to smooth muscle cell death, fibrosis, and the accumulation of excess proteoglycan ground substance in the media — a nonspecific pattern termed cystic medial degeneration, seen in Marfan syndrome, scurvy, and ageing alike.

The three common types of aortic aneurysm are worked out below.

Atherosclerotic Aneurysm

The most common form of aortic aneurysm, occurring more often in men, with incidence rising sharply beyond the age of fifty as advanced, complicated atherosclerotic lesions become more common.

Pathogenesis. Severe atherosclerotic plaque in the intima thins the underlying media by compressing it and by blocking the diffusion of nutrients and oxygen from the lumen, so the media undergoes ischaemic atrophy and weakens; matrix metalloproteinases from plaque macrophages accelerate the destruction of elastic tissue.

Site. Overwhelmingly the abdominal aorta — so much so that any fusiform, cylindrical, or saccular aneurysm of the abdominal aorta should be assumed atherosclerotic until proven otherwise — typically arising below the renal arteries and above the aortic bifurcation, though it may extend into the common iliac arteries. Thoracic, iliac, and other large arteries are less commonly affected.

Morphology. Usually fusiform, often larger than five to six centimetres in diameter. The wall shows loss of normal architecture, replaced predominantly by fibrous tissue with mild chronic inflammation; the intima and inner media retain remnants of the atheromatous plaque, and the lumen frequently contains a laminated, poorly organised mural thrombus.

Effects. Rupture into the peritoneal or retroperitoneal space is the most serious and often fatal complication, with risk rising with the size of the aneurysm; compression of adjacent structures such as the ureter, or erosion of vertebral bodies, may occur; and occlusive thrombosis or thromboembolism, particularly of the inferior mesenteric artery, is common, though collateral circulation usually prevents overt ischaemia.

Syphilitic (Luetic) Aneurysm

A complication of tertiary syphilis, developing in roughly forty percent of patients who have syphilitic aortitis, typically manifesting after the age of fifty and more common in men.

Pathogenesis. Treponemal organisms have a particular affinity for the vasa vasorum of the proximal aorta. Inflammatory infiltration around these small vessels in the adventitia progresses to obliterative endarteritis, cutting off blood supply to the media and causing ischaemic destruction of its smooth muscle and elastic tissue, followed by scarring — a process termed syphilitic mesoaortitis.

Site. The ascending aorta and aortic arch, since syphilitic aortitis is most severe proximally; the aneurysm may extend into the aortic valve ring, producing aortic incompetence, or occasionally extend distally into the abdominal aorta.

Morphology. Usually saccular, three to five centimetres in diameter, with a characteristically wrinkled, “tree-bark” intimal surface reflecting the underlying medial scarring. Involvement of the aortic root, with stretching and rolling of the valve leaflets, produces aortic regurgitation and, through chronic volume overload, gross cardiac enlargement historically termed cor bovinum.

Effects. Rupture, causing massive haemorrhage into the pleural or pericardial space, trachea, or oesophagus; compression of adjacent structures, producing dyspnoea (trachea), dysphagia (oesophagus), hoarseness (recurrent laryngeal nerve), or bony erosion; and cardiac dysfunction from aortic valve involvement, sometimes compounded by narrowing of the coronary ostia.

Aortic Dissection

Aortic dissection (the older term “dissecting aneurysm” is now discouraged, since true dilatation is not always present) occurs when blood enters a tear in the intima and tracks along a plane within the media, splitting the wall into two channels rather than simply enlarging the vessel. It is an acute, catastrophic event, occurring most often in men aged fifty to seventy, and, distinctly, in women during pregnancy.

Pathogenesis. Roughly ninety percent of cases occur in the setting of hypertension, which is believed to drive degenerative change in the media in a still incompletely understood way. The remaining cases arise from an inherited or acquired connective tissue disorder weakening the aortic wall independently of blood pressure — Marfan syndrome, cystic medial degeneration of ageing, iatrogenic trauma during catheterisation or bypass surgery, or pregnancy. Once medial weakening has occurred, an intimal tear allows blood under arterial pressure to dissect along the weakest plane of the media; an alternative view holds that hemorrhage from the vasa vasorum into the media occurs first, with the intimal tear following.

Morphology. The intimal tear is sharply incised, transverse or oblique, three to four centimetres long, and located in the ascending aorta in the great majority of cases. The dissection plane runs characteristically between the outer and middle thirds of the media, so that the column of blood separates the intima and inner two-thirds of the wall from the outer third and adventitia; it may extend proximally toward the aortic valve or distally toward the abdominal aorta and its branches, and occasionally into the arteries of the neck or the coronary, renal, mesenteric, or iliac vessels. In about ten percent of cases a second, distal intimal tear allows blood to re-enter the true lumen, and if the patient survives, this false channel can become endothelialised, forming a double-barrelled aorta. Histologically the media shows cystic medial degeneration — focal separation of fibromuscular and elastic tissue, cystic spaces containing basophilic ground substance, fragmentation of elastic fibres, and increased fibrosis — without significant inflammation.

Classification. Two schemes describe the extent of involvement.

Effects. The classic presentation is sudden, excruciating, tearing chest pain that moves downward as the dissection progresses, which can mimic myocardial infarction. Rupture through the adventitia, most often into the pericardium, causes fatal haemorrhage or tamponade in the great majority of untreated cases; involvement of the aortic valve produces regurgitation; obstruction of a coronary artery can itself precipitate myocardial infarction; and obstruction of other branches produces ischaemia or infarction of the organ supplied — renal, cerebral, mesenteric, or spinal.

Mycotic Aneurysm

A weakening of the arterial wall caused by microbial infection — a misleading name, since the responsible organism is usually bacterial rather than fungal. Infection may reach the wall as a septic embolus, most often complicating infective endocarditis; by direct extension from an adjacent suppurative focus; or by circulating organisms directly invading the wall. Rupture of a mycotic aneurysm, particularly of an intracranial artery, is a recognised cause of death in infective endocarditis.

Berry Aneurysm

The most important and most common aneurysm affecting the large intracranial arteries, and the leading cause of subarachnoid haemorrhage.

Nature. A small, saccular, rounded or lobulated bulge arising at an arterial bifurcation, typically two millimetres to two centimetres or more in diameter. Berry aneurysms are rare in childhood and increase in frequency through young adult and middle life, which shows that — despite the name suggesting a birth defect — they are not truly congenital, but instead develop over time from an inherent defect in the muscular and elastic layers of the arterial wall at points of bifurcation, where the wall is structurally weakest. About a quarter are multiple, and there is a recognised association with adult polycystic kidney disease and coarctation of the aorta.

Sites, in descending order of frequency, all around the circle of Willis: the anterior communicating artery; the origin of the posterior communicating artery from the internal carotid; the first major bifurcation of the middle cerebral artery; and the bifurcation of the internal carotid into its anterior and middle cerebral branches.

Effects. Berry aneurysms remain silent until they rupture, at which point they cause more than eighty-five percent of cases of subarachnoid haemorrhage, presenting with sudden, severe generalised headache, often followed by loss of consciousness and neurological deficit. Chronic hypertension is not established as a risk factor for their formation or rupture, though a sudden rise in intravascular pressure often precipitates the rupture itself. Initial mortality from a first rupture is around twenty to twenty-five percent, and survivors remain at risk of recurrent bleeding. The affected artery’s territory frequently develops infarction after rupture, partly attributable to vasospasm triggered by the subarachnoid blood.

Clinical Relevance

• The site of an aneurysm is itself diagnostic shorthand: an abdominal aortic aneurysm is presumed atherosclerotic, an ascending aortic or aortic root aneurysm with valvular incompetence raises suspicion of syphilis or a connective tissue disorder, and a saccular aneurysm at a circle of Willis bifurcation is a berry aneurysm until proven otherwise.
• A sudden, tearing chest pain radiating to the back should always prompt consideration of aortic dissection, since it can otherwise be mistaken for myocardial infarction, with disastrous consequences if a dissection is treated as if it were an infarct.
• Because rupture is the shared final common pathway of nearly every type of aneurysm, size and rate of enlargement — tracked on serial imaging — are the key determinants of when elective repair is warranted before rupture occurs.

Diagram 1: Shapes of Aneurysms (comparative panel)

Draw a panel of five short vessel segments side by side, each showing a different aneurysm shape against a straight vessel outline for comparison.

1. Saccular: a spherical outpouching bulging from one side only, with a narrow neck connecting it to the main lumen.
2. Fusiform: the entire circumference of the vessel gradually widens and narrows again over a length, symmetric about the long axis.
3. Cylindrical: a uniform, parallel-sided widening over a length, without tapering at either end.
4. Serpentine: an S-shaped, tortuous course with dilated, winding segments.
5. Racemose: a tangled mass of small interconnecting channels linking an artery directly to a vein, with the arterial and venous sides labelled separately.

Labels required

• Each shape individually named beneath its drawing.
• The saccular aneurysm’s neck must be labelled, since this narrow connection is clinically relevant to how it is approached surgically.
• The racemose aneurysm must have its artery and vein separately labelled, since it is the one type that is fundamentally an arteriovenous communication rather than a simple dilatation.

Errors commonly made

• Confusing fusiform and cylindrical shapes — fusiform tapers at both ends, cylindrical does not; this distinction is a specific point of confusion worth drawing carefully.
• Drawing the saccular aneurysm without a narrow neck, making it indistinguishable from a fusiform lesion.

Diagram 2: Aortic Dissection — Plane and Classification

Draw two connected illustrations: a longitudinal view of the aorta from the arch to the abdominal aorta, and a cross-sectional inset at one point along its length.

Longitudinal view: draw the aortic arch, ascending aorta, descending thoracic aorta, and abdominal aorta as a continuous tube. Mark the intimal tear as a short transverse break near the aortic root, and shade a second channel running within the wall alongside the true lumen, extending along the length of the vessel to show the false lumen. Draw two vertical dividing lines on this same figure — one just distal to the subclavian artery origin — to demarcate the DeBakey/Stanford zones, and shade three variants beside the main figure: Type I/A (false lumen from root through the full descending aorta), Type II/A (false lumen confined to the ascending aorta only), Type III/B (false lumen beginning distal to the subclavian artery, sparing the ascending aorta).

Cross-sectional inset: a ring representing the aortic wall, with a line splitting it between the outer and middle thirds of the media, separating an inner segment (intima + inner two-thirds of media) from an outer segment (outer third of media + adventitia), with blood filling the space between them.

Labels required

• The cross-sectional inset must explicitly label the split as occurring “between the outer and middle thirds of the media” — this specific location is a frequently tested detail.
• Each of the three shaded variants must carry both its DeBakey number and its corresponding Stanford letter side by side, since students are asked for either system interchangeably.
• The intimal tear location must be labelled “ascending aorta, within ~10 cm of the aortic valve.”

Errors commonly made

• Splitting the media at the wrong depth (commonly drawn as splitting the full thickness, or between intima and media, rather than within the media itself) — the specific outer/middle-third plane is the anatomical detail examiners check.
• Mismatching DeBakey and Stanford labels — a very common source of error, since DeBakey I and II are both Stanford A, and only DeBakey III is Stanford B.
• Omitting the double-barrelled aorta variant (a second, re-entry tear) as an endpoint worth a small side note.

Diagram 3: Circle of Willis — Berry Aneurysm Sites

Draw the circle of Willis from below (the standard anatomical view), showing the anterior communicating artery, both anterior cerebral arteries, both internal carotid arteries, both posterior communicating arteries, both posterior cerebral arteries, and the basilar artery.

Mark four sites with numbered small circles representing berry aneurysms, in descending order of frequency:

1. On the anterior communicating artery (midline).
2. At the junction of the posterior communicating artery with the internal carotid artery, on one side.
3. At the first bifurcation of the middle cerebral artery, on one side.
4. At the bifurcation of the internal carotid into the anterior and middle cerebral arteries, on one side.

Labels required

• Each numbered site must be labelled with both its rank and its precise anatomical location, not just marked with an unlabelled dot.
• A note stating that about a quarter of berry aneurysms are multiple, alongside a small second aneurysm drawn on the opposite side to illustrate this.
• A caption noting the association with adult polycystic kidney disease and coarctation of the aorta.

Errors commonly made

• Placing the most frequent site anywhere other than the anterior communicating artery — this specific ranking is commonly asked and commonly gotten wrong.
• Drawing aneurysms in the middle of a vessel rather than precisely at a bifurcation point, which misses the anatomical basis for why these lesions form where they do.

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