Abdominal Aortic Aneurysm
last authored: April 2012, David LaPierre
last reviewed:
Introduction
An abdominal aortic aneurysm (AAA) is a dilation of the aorta below the kidneys, defined as 3cm or greater.
There are two main types of aneurysm:
- fusiform: involves the whole circumference of the aorta
- saccular: an assymetric outpouching
AAA is normally asymptomatic, and is diagnosed incidentally (if at all). However, a rupture of a AAA caries a very high mortality rate.
The Case of...
a simple case introducing clincial presentation and calling for a differential diagnosis to get students thinking.
Causes and Risk Factors
AAA is more common in men over 50 and women over 60. Men have a 5:1 ratio, as compared with women.
Risk factors include:
- older age
- men
- white
- smoking
- hypertension
- hyperlipidemia
- peripheral vascular disease
- COPD
- family history: 1st degree relative - 12x higher risk, brothers: 18x higher risk
- genetic conditions: Marfan syndrome, Ehlers-Danlos syndrome, polycystic kidney disease, tuberous sclerosis
Pathophysiology
Aneurysms are thought to arise from atherosclerosis and the resulting changes to the vessel wall. In part related to degeneration of the media, related to matrix metalloproteinases. Hypertension is also a major contributor: Laplace's law states that T (wall tension) is related to pressure x radius. A gradual expansion of the aneurysm normally follows.
The distal aorta is particularly at risk, given the lower levels of elastin, which reduces wall strength, and the smaller numbers of vasa vasorum mean that regeneration is difficult.
Other factors that may be involved include:
- inflammation
- trauma
- infection
Enzymes: risk of rupture increases following surgery, potentially due to increased blood levels of collagenase and elastinase
Screening and Prevention
Prevention is best mediated by control of cardiovascular risk factors, including smoking, hypertension, and cholesterol.
Screening is recommended:
- once for men 65-75 who have smoked
- once for men over 60 with a parent or sibling with AAA
Once identified, ongoing surveillance is recommended (Kent et al, 2004).
- 3-4 cm: screen annually
- 4-4.5 cm: screen every six months
- >4.5 cm: refer for consideration of repair
Signs and Symptoms
Clinicals should be on the lookout for the triad of shock, pulsatile mass, and abdominal pain suggesting rupture.
- history
- physical exam
History
AAA is usually asymptomatic, however may present with:
- pain in the abdomen, back, flank, groin, buttocks, or legs
- syncope
- lower extremity paralysis
Past medical history:
- atherosclerosis
- COPD
Family history
Social history
- smoking
Physical Exam
Vital signs should be obtained to look for evidence of shock.
Abdominal exam may reveal:
- palpable, pulsatile mass
- flank contusion (Gre-Turner sign)
- scrotal ecchymosis
Investigations
- lab investigations
- diagnostic imaging
Lab Investigations
If a rupture is suspected, order:
- complete blood count
- electrolytes
- BUN, creatinine
- blood type and cross-match
Diagnostic Imaging
AAA is most often identified incidentally during ultrasound or other tests done for other reasons.
As described, screening is recommended for higher risk men.
Ultrasound is the easiest and least expensive modality, and when used in someone with experience, has a very high sensitivity and specificity (Costantino et al, 2005).
During assessment of rupture, ECG is warranted.
Other imaging modalities to consider include:
- CT: not sensitive for rupture
- MRI, MRA
- abdominal X-rays (poor sensitivity; may show calcifications)
Differential Diagnosis
The differential includes
- other causes of abdominal mass, eg malignancy
- other causes of abdominal pain:
- peptic ulcer disease
- renal colic
- appendicitis
- myocardial infarction
- etc
Treatments
It is a given that cardiovascular risk factors should be optimized, including hypertension, smoking, and high cholesterol levels. Diet and exercise should be prescribed.
Heart, lung, kidney, and liver disease should also be managed appropriately.
Beta-blockers are helpful for reducing rate of expansion. Other medications that may be helpful include ASA, statins, and doxycycline.
Elective repair
The threshold for the majority of patients is 5.5 cm. However, younger patients, women, or those who are high-risk may consider repair at a smaller size (4.5-5cm).
Patients who are higher risk include:
- expansion at >0.6 cm/yeare
- ongoing smoking
- poorly-controlled hypertension
- steroid use
- strong family history
- saccular appearance
Surgery may be done with open or endovascular approaches.
Emergency treatment
If rupture is suspected, treat the ABCs with oxygen, IV access and fluid/blood resuscitation, rapid bedside ultrasound assessment, and surgical consult.
Consequences and Course
Prognosis depends on size. In general, rupture rate is as follows (Lederle et al, 2002):
- <5 cm: 1%
- 5-6 cm: 11%
- 6-7 cm: 26%
- >7 32%
Complications of AAA can include:
- rupture
- dissection
- thromboembolization
- aortoenteric fistula
- aortocaval fistula
- inflammatory aneurysm: involvement of thick inflammatory material around AAA
Rupture is a grave situation, and less than 20% of patients survive.
Resources and References
Costantino TG et al. 2005. Accuracy of emergency medicine ultrasound in the evaluation of abdominal aortic aneurysm. J Emerg Med. 29:455-60.
Kent KC et al. 2004. Screening for aortic abdominal aneurysm: a consensus statement. J Vasc Surg. 39:267-9.
Lederle FA et al. 2002. Rupture rate of large, abdominal aortic aneurysms in patients refusing or unfit for elective repair. JAMA. 287:2968-72.
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