Abdominal Aortic Aneurysm Classification

Abdominal Aortic Aneurysm Classification

Abdominal aortic aneurysms are commonly divided according to their size and symptomatology. An aneurysm is usually defined as an outer aortic diameter over 3 cm (normal diameter of aorta is around 2 cm). However, if the AAA is surgically repaired before rupture, the post-operative mortality rate is substantially lower: approximately 1-6%.

The vast majority of aneurysms are asymptomatic. However, as abdominal aortic aneurysms expand, they may become painful and lead to pulsating sensations in the abdomen or pain in the chest, lower back, or scrotum. The risk of rupture is high in a symptomatic aneurysm, which is therefore considered an indication for surgery. The complications include rupture, peripheral embolisation, acute aortic occlusion, and aortocaval (between the aorta and inferior vena cava) or aortoduodenal (between the aorta and the duodenum) fistulae. On physical examination, a palpable abdominal mass can be noted. Bruits can be present in case of renal or visceral arterial stenosis. The bleeding can be retroperitoneal or intraperitoneal, or the rupture can create an aortocaval or aortointestinal (between the aorta and intestine) fistula.

* Genetic influences: The influence of genetic factors is highly probable. The high familial prevalence rate is most notable in male individuals. There are many theories about the exact genetic disorder that could cause higher incidence of AAA among male members of the affected families. Some presumed that the influence of alpha 1-antitrypsin deficiency could be crucial, some experimental works favored the theory of X-linked mutation, which would explain the lower incidence in heterozygous females. Other theories of genetic etiology have also been formulated.
* Atherosclerosis: The AAA was long considered to be caused by atherosclerosis, because the walls of the AAA are frequently affected heavily. However, this theory cannot be used to explain the initial defect and the development of occlusion, which is observed in the process.

Hemodynamics affect the development of AAA. It has a predilection for the infrarenal aorta. The histological structure and mechanical characteristics of infrarenal aorta differ from those of the thoracic aorta. The diameter decreases from the root to the bifurcation, and the wall of the abdominal aorta also contains a lesser proportion of elastin. The mechanical tension in abdominal aortic wall is therefore higher than in the thoracic aortic wall. The elasticity and distensibility also decline with age, which can result in gradual dilatation of the segment. Higher intraluminal pressure in patients with arterial hypertension markedly contributes to the progression of the pathological process. This is a grade B recommendation. A re-analysis of the meta-analysis estimated a number needed to screen of approximately 850 patients.

The largest of the randomized controlled trials on which this guideline was based studied a screening program that consisted of:

Screening men ages 65–74 years (not restricted to ever smokers). 'Men in whom abdominal aortic aneurysms (> or =3 cm in diameter) were detected were followed-up... Patients with an aortic diameter of 3·0–4·4 cm were rescanned at yearly intervals, whereas those with an aortic diameter of 4·5–5·4 cm were rescanned at 3-monthly intervals ... Surgery was considered on specific criteria (diameter > or =5.5 cm, expansion > or =1 cm per year, symptoms)'.

This trial reported significant short) and long term (number needed to screen after 7 years of approximately 280 to prevent nonfatal ruptured AAA plus AAA-related deaths) benefit and cost effectiveness. Subsequent randomized controlled trials also found benefit:

* number needed to screen after 4 years of 300
* number needed to screen after and after 7 years of 563 (calculation).

In the U.S., effective January 1, 2007, provisions of the SAAAVE Act (Screening Abdominal Aortic Aneurysm Very Efficiently) now provide a free, one-time, ultrasound AAA screening benefit for those qualified seniors. Men who have smoked at least 100 cigarettes during their life, and men and women with a family history of AAA qualify for the one-time ultrasound screening. Enrollees must visit their healthcare professional for their Welcome to Medicare physical within six months of enrolment in order to qualify for the free screening. The Welcome to Medicare Physical Exam must be completed within the first six months of Medicare eligibility, but there is no published time limit thereafter for completion of the AAA screening. Providers who perform the physical and order the AAA screening need to document the AAA risk factors.