Abdominal Aortic Aneurysm

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OnSurg thanks collaborative partner Dr Anubhav Mittal at SurgeryNZ.com for making this valuable content available. Dr Mittal tweets at @anubhavnz.

Differentials

Renal colic, diverticulitis and severe lower abdominal and testicular pain.
(Ruptured AAA an "acute abdomen" condition).

Examination Findings/Signs

May be none - especially in overweight people.
Expansile abdominal mass
Signs of shock (hypotension, tachycardia, poor peripheral perfusion, tachypnoea).
"Trash Feet" - dusky discolouration of the digits secondary to emboli from aortic thrombus.

Prognosis/Other

Mortality rate of a ruptured aneurysm is about 50% (even with those who have surgery).
Elective AAA repair is less risky (< 5% mortality).
Annual risk of rupture of an asymptomatic AAA:
- 3.0 - 3.9cm = very rare
- 4.0 - 4.9cm = 0.5 - 5%
- 5.0 - 5.9cm = 3 -15%
- 6.0 - 6.9cm = 10 - 20%
- 7.0cm + = 20 - 50%

Epidemiology and Aetiology

EPIDEMIOLOGY:
Most commonly infrarenal
Incidence increases with age. 5% > 60yrs.
M > F (5:1)

AEITIOLOGY: (no clear cut aetiology)
Familial relationship - a son of an affected individual has a 25% chance of developing AAA.
Atherosclerosis
Smoking
Other: infection (syphilis, E.coli, Salmonella), trauma, genetic (Marfan's syndrome).

Investigations / Tests

USS - used to determine the size and to screen for aneurysms. Used for surveillance.
CT scan - nearly 100% sensitivity for aneurysm and is also useful in preoperative planning, detailing the anatomy and possibility for endovascular repair. In the case of suspected rupture, it can also reliably detect retroperitoneal fluid.
MRI and angiography are raremy used.

Pathophysiology

Degradation of tunica media by proteolytic processes seems to be the basic pathophysiologic mechanism of the AAA development.
- Increased MMP expression leading to elimination of elastin from the media, making the aortic wall more susceptible to aneurysm.
- Reduced amount of vasa vasorum in the abdominal aorta vs thoracic aorta; consequently, the tunica media relies mostly on diffusion for nutrition which makes it susceptible to damage.
Hemodynamics affect the development of AAA, this is why most AAA are infrarenal (turbulent flow after the branching of the renal arteries).

Definition

An aneurysm is a permanent dilatation of an artery to twice the normal diameter.
True aneurysm - arterial wall forms the wall.
False aneurysm - surrounding tissues form the wall e.g. haematoma after arterial puncture.

Management/Treatment

Management depends on operative risk vs risk of a ruptured AAA.
In general - an operation if AAA >= 5.5cm diameter, expanding >1cm/yr or if the AAA is symptomatic.
MEDICAL: (AAA < 5.5cm)
Smoking cessation, HTN medication and lipid lowering. Regular USS surveillance.
SURGICAL/RADIOLOGICAL:
Open Repair - supported by long-term data therefore good in young, fit patients.
Endovascular Repair - less invasive but not supported by long-term data.

Presenting Complaint / Symptoms

Majority are asymptomatic and found incidentally (via radiology or routine examination).

RAPID EXPANSION AND/OR RUPTURE:
Abdominal pain
- Intermittent or continuous.
- Radiation to back, iliac fossae or groins.
- Commonly dismissed as renal colic.
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