Aneurysm — MRCS Part B Surgical pathology

A ruptured abdominal aortic aneurysm on CT, then the pathology of aneurysms and atheroma.

Acute management

Two large cannulae and cross-matched blood, permissive hypotension with a systolic around 90 mmHg while the patient is conscious, no delay for fluid, and straight to a vascular unit for open repair or, if the anatomy allows and the unit can, endovascular repair. Rupture kills about four in five, half of them before hospital.

Definitions

An aneurysm is a permanent, localised dilatation of an artery to more than one and a half times its normal diameter; for the infrarenal aorta that means 3 cm. A true aneurysm involves all three layers of the wall. A false aneurysm (pseudoaneurysm) is a haematoma outside the wall, contained by the adventitia or surrounding tissue, communicating with the lumen through a hole: after arterial puncture at the groin, a stab wound, an infected graft anastomosis, or pancreatitis eroding the splenic artery.

The arterial wall

Tunica intima: endothelium on a basement membrane with a little subendothelial connective tissue, bounded by the internal elastic lamina. Tunica media: smooth muscle and elastic lamellae, thick in arteries, bounded by the external elastic lamina. Tunica adventitia: collagen, the vasa vasorum and nerves.

Morphology and causes

Fusiform (the whole circumference, the usual aortic pattern), saccular (a pouch from one side, typical of mycotic and berry aneurysms) and dissecting, in which blood enters a tear in the intima and tracks along the media, which is a different disease from a true aneurysm. Causes: degenerative (atherosclerosis and loss of elastin and collagen from the media, the usual abdominal cause), infection (mycotic aneurysms from endocarditis, salmonella; syphilis of the thoracic aorta), inflammatory arteritis, trauma, connective tissue disease (Marfan, Ehlers-Danlos type IV), and congenital (berry aneurysms of the circle of Willis, associated with polycystic kidneys). Risk factors: age, male sex, smoking, hypertension, a family history and chronic lung disease; diabetes, oddly, is protective.

Presentation and screening

Most are found by chance or by screening, which in England offers every man an ultrasound at 65. Otherwise: a pulsatile mass, back or abdominal pain, distal embolism, or rupture presenting as collapse with abdominal and back pain. Under 5.5 cm an aneurysm is surveilled by ultrasound (yearly under 4.5 cm, three-monthly above), with the risk factors treated. Repair is offered at 5.5 cm, at growth of more than 1 cm a year, or with symptoms; the risk of rupture rises steeply with diameter, from around 1% a year at 5 cm to more than 10% a year over 6 cm.

Atheroma and atherosclerosis

An atheroma is a plaque in the intima of a large or medium artery with a core of lipid and cell debris under a fibrous cap. Atherosclerosis is the disease of arteries characterised by these plaques. It begins with endothelial injury from turbulent flow, hypertension, smoking, glycation and cholesterol; low-density lipoprotein enters the intima and is oxidised; monocytes follow, become macrophages, take up the lipid and become foam cells (the fatty streak); smooth muscle cells migrate in from the media and lay down collagen to form the cap; the core grows as foam cells die. The consequences are narrowing with ischaemia, plaque rupture with thrombosis (infarction, stroke, acute limb ischaemia), embolism of plaque debris, weakening of the wall with aneurysm, and calcification. Risk factors are the same as for aneurysm, with diabetes and hyperlipidaemia added.