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Define aneurysm. Classify aneurysms and describe abdominal aortic aneurysm and aortic dissection.

Pathology · 2nd Professional — Para-Clinical

Definition and Classification

An aneurysm is a localised abnormal dilatation of a blood vessel or of the heart wall.

Basis	Types
Wall structure	True aneurysm — involves all three layers, or an attenuated but intact wall. False (pseudo) aneurysm — a breach in the wall contained by surrounding tissue and haematoma; the "wall" is not vessel at all
Shape	Saccular — a spherical outpouching of part of the circumference. Fusiform — diffuse circumferential dilatation
Other	Dissecting aneurysm — blood enters the media through an intimal tear and splits it; strictly a dissection, not a true aneurysm. Berry aneurysm — congenital, at the circle of Willis. Mycotic — from infection weakening the wall

Causes

Cause	Typical site
Atherosclerosis — the commonest	Abdominal aorta, below the renal arteries
Cystic medial degeneration	Ascending aorta; MARFAN and Ehlers–Danlos syndromes
Hypertension	Aortic dissection; Charcot–Bouchard microaneurysms in the basal ganglia
Congenital	Berry aneurysm at the circle of Willis — ruptures to cause subarachnoid haemorrhage; associated with polycystic kidney disease
Infection (mycotic)	Septic emboli in infective endocarditis; salmonella; syphilis affecting the ascending aorta and arch through obliterative endarteritis of the vasa vasorum
Trauma and vasculitis	Any site; Kawasaki disease affects the coronary arteries
Post-infarction	Left ventricular aneurysm

CLINICAL PEARL

Syphilis takes the ascending aorta; atherosclerosis takes the abdominal aorta. Syphilitic aortitis destroys the vasa vasorum, which are most abundant in the thoracic aorta, producing the "tree-bark" intima and stretching the aortic ring to give regurgitation. Atherosclerosis is worst in the infrarenal abdominal aorta, which has no vasa vasorum in its media and depends on diffusion.

Abdominal Aortic Aneurysm

Men over 50, almost always atherosclerotic; below the renal arteries and above the bifurcation
Often asymptomatic and found as a pulsatile abdominal mass or incidentally on imaging
Risk of rupture relates to diameter — negligible below 4 cm, about 1% a year at 4–5 cm, and rising steeply above 6 cm
Rupture presents with abdominal or back pain, a pulsatile mass and shock, and carries a mortality above 80%
Elective repair is offered above about 5.5 cm, or for rapid expansion or symptoms

Aortic Dissection

An intimal tear allows blood to enter and split the media, creating a false lumen
Risk factors — hypertension (the commonest), Marfan syndrome, pregnancy, bicuspid aortic valve, coarctation, trauma
Stanford classification — type A involves the ascending aorta and requires surgery; type B is confined to the descending aorta and is managed medically
Severe tearing chest pain radiating to the back, with unequal pulses or blood pressures
Fatal complications — rupture into the pericardium with tamponade, aortic regurgitation, coronary or cerebral occlusion

Applied Aspects

Ultrasound screening for abdominal aortic aneurysm in older men reduces mortality and is offered in several countries
Blood pressure control is the mainstay of prevention for both aneurysm and dissection
Beta-blockers in Marfan syndrome slow aortic root dilatation
Never mistake a dissection for a myocardial infarction — thrombolysis given to a dissecting patient is rapidly fatal, and the unequal pulses and widened mediastinum are the clues
Endovascular repair has largely replaced open surgery for suitable anatomy, with lower operative mortality but a need for lifelong surveillance

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