Abdominal aortic aneurysm (AAA) – causes and methods of repair.

“An abdominal aortic aneurysm is a bulge/weakening that has formed in the largest artery in the body. As it grows in size, the risk of rupture generally increases.”

 

An abdominal aortic aneurysm (AAA) forms in three stages – initiation, progression and then rupture. Most aneurysms are described as “degenerative” ie the general quality of the aortic wall is worsening.

The processes involved in the formation of degenerative AAAs include

    • up-regulation of proteolytic pathways
    • apoptosis
    • oxidative stress
    • inflammation
    • loss of arterial wall matrix.

Historically there was a belief that AAAs were a consequence of advanced atherosclerosis. However, there is an evolving evidence base that they are a local representation of a systemic disease process which includes

    • Increased levels of matrix metalloproteinases (MMPs)
    • Increased collagenase

Once the aneurysm reaches 5cm in maximum diameter in women, or 5.5cm in men repair is then considered.

  • Incidence of AAA in patients over 65 
    • 1.3% of women over 65
    • 4- 7.5% of men over 65
    • Lower in Asian populations
  • Risk factors
    • Family history
    • More frequent in  causasians
    • Smoking 7.6x increase, and 15 fold increase if you smoke more than 25/day
  • Lipids
    • Elevated HDL is probably protective
    • Low LDL not convincingly proven to lead to a difference
    • Statins may delay growth
  • Hypertension
    • Association is weak
    • More so of a risk in women
  • Diabetes
    • Protective of AAA development
  • Obsesity
    • Obesity likely associated with AAA
  • Other factors
    • Alcohol likely plays a part in development of growth and rupture
  • Likely a hereditary component given the findings from studies on twins

 

 

Risk factors for rupture

Rupture is a multifaceted process due to systemic factors within the patient and local factors related to the aneurysm itself. Risk factors include:

    • Increased peak wall stress
    • Size
    • Growth >1cm/year
    • Genetics
    • Maybe connective tissue disease
    • Mean blood pressure in women
    • Female gender (at least 3 times more likely than men to rupture)
    • Saccular aneurysms have not convincingly been shown to portend an increase rate of rupture
    • Pathogenesis of rupture
      • Likely related to localised elevations in proteolyic enzymes leading to ^^extracellular matrix proteolysis^^
      • Local elevations of MMP8 and MMP9
      • Intraluminal thrombus is a site of proteolytic enzyme release

Repair

If repair is planned, generally we consider the size and shape of the aneurysm. There is a general trend within Australia for repair with a stent rather than traditional open surgery. In this form of repair, the aorta is religned and reinforced internally with a graft that is introduced via the femoral arteries in the groin. Aneurysms that involve the arteries to the kidneys require side holes (fenestrations) to maintain blood flow to these important arteries. For some patients, the anatomy is not suitable for a stent, and in this situation open repair may sometimes be recommended.

 

References

Mechanisms of Vascular Disease. Rob Fitridge.

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© Sam Farah 2021