ABDOMINAL AORTIC ANEURYSMS

Part 1: Pathogenesis, Diagnosis, Screening.

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Introduction to Abdominal Aortic Aneurysms.

The infra-renal aorta is the commonest site for arterial aneurysms with a prevalence of 5% (elderly males). The aorta becomes aneurysmal when its diameter exceeds 3cm. AAAs kill 1.5% of males over 55 years of age and their prevalence is increasing.

Actiology.

The AAA wall is characterised by a loss of elastin and medial smooth muscle with an increase in protein and collagen associated with an inflammatory cell infiltrate. Loss of elastin has been attributable to increased proteolytic activity systemically and locally due to infiltrating leukocytes. there are three hypotheses about why aneurysms develop.

(1) Genetic - inherited defects in structural proteins like type 3 collagen. These are rare.

(2) Atherosclerosis - where atherosclerotic plagues cause medial thinning and wall dilatation.

(3) An inflammatory response in the vessel wall producing enzymic degredation of connective tissue matrix. Workers like Tilson feel that this is akin to an auto-immune reaction similar to destruction of joints in rheumatoid arthritis.

Imaging

Imaging Modality	Use	Disadvantages
Ultrasound	Screening	Sizing of AAA is operator dependent and it is often difficult to define the relationship of the sac to the renal arteries.
CT Scan	Accurate assessment of size and will show inflammatory AAAs	Often does not show relationship of aneurysm sac to renal arteries
MRA/Spiral CT	Good for sizing; good to assess relationship of aneurysm sac to renal arteries; good to access suitability for endoluminal repair.	More expensive than above two modalities

General Work-Up

Patients with no cardiac history and a normal ECG probably need no further work-up. Patients with cardiac impairment, greater than 70 years old, or diabetic should have an echo cardiogram and probably a dipyridamole-thallium stress test. Patients with marked cardiac dysfunction need coronary catheterisation. Approximately 10% of patients will require coronary artery bypass prior to aneurysm repair.

Natural History.

Patients with AAA's have half the life expectancy of age-matched controls. Aneurysm grow at 2-8mm per year (10%) but this is not linear. Clearly aneurysm rupture rate is related to size. Five year rupture rates for AAAs of diameters 5 - 5.9cm. 6 - 6.9cm and over 7cm are 25%, 35% and 75%. However, the irregular growth pattern of AAA makes the predication of rupture difficult. Factors affecting aneurymsal expansion and rupture: aneurysm diameter, hypertension, pulse pressure, smoking, thrombus content, expansion rate and family history.

Presentation

The vast majority are asymptomatic. Development of abdominal and back pain should be cause for concern. Sudden onset back pain in an elderly patient should always carry the suspicion of a AAA and should be investigated before labeling it as degenerative spinal disease. Pain, malaise and weight loss can indicate an inflammatory AAA. Pain and hypotension usually indicates rupture.

Screening for AAA

There is good evidence that screening is beneficial. A number of studies of men over 60 years of age have reported that 8% of such patients have aneurysms of 3cm or greater. Lucarotti et al have suggested that one ultrasound scan in males of 65 years old would exclude 90% of the at risk population and reduce deaths from ruptured AAA by two thirds. Scott et al showed that screening reduces rupture by 85%. The yield from a screening programme can be increased from 8 to 10% if it targets high risk groups - males over 65 years old, co-existing atherosclerotic disease, hypertension, a first degree relative with a AAA and COPD.

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